Balance Quality Grade (G-Grade) \u2014 ISO 1940-1 \/ ISO 21940-11 \u2014 Vibromera<\/title>\n<meta name=\"description\" content=\"Complete guide to Balance Quality Grades (G-Grades) per ISO 1940-1 and ISO 21940-11. Interactive calculator for permissible residual unbalance. G0.4 to G4000 reference tables, worked examples, and industry recommendations for pumps, fans, turbines.\">\n<meta name=\"keywords\" content=\"balance quality grade, G-grade, ISO 1940, ISO 21940-11, permissible residual unbalance, balancing tolerance, G 6.3, G 2.5, G 1.0, rotor balancing, dynamic balancing, vibration, Balanset, balancing calculator\">\n<meta name=\"author\" content=\"Vibromera\">\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large\">\n\n\n\n\n\n\n\n\n\n\n\n<meta property=\"og:type\" content=\"article\">\n<meta property=\"og:title\" content=\"Balance Quality Grade (G-Grade) \u2014 ISO 1940 \/ ISO 21940-11 \u2014 Complete Guide\">\n<meta property=\"og:description\" content=\"What is a Balance Quality Grade? Interactive calculator, full ISO G-grade table (G0.4\u2013G4000), formulas for permissible residual unbalance, worked examples, and industry recommendations.\">\n<meta property=\"og:url\" content=\"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/\">\n<meta property=\"og:site_name\" content=\"Vibromera \u2014 Vibration Analysis & Balancing Equipment\">\n<meta property=\"og:locale\" content=\"en_US\">\n<meta property=\"og:image\" content=\"https:\/\/vibromera.eu\/wp-content\/uploads\/balance-quality-grade-og.jpg\">\n<meta property=\"article:publisher\" content=\"https:\/\/vibromera.eu\/\">\n<meta property=\"article:section\" content=\"Glossary\">\n<meta property=\"article:tag\" content=\"Balance Quality Grade\">\n<meta property=\"article:tag\" content=\"ISO 1940\">\n<meta property=\"article:tag\" content=\"Rotor Balancing\">\n\n\n<meta name=\"twitter:card\" content=\"summary_large_image\">\n<meta name=\"twitter:title\" content=\"Balance Quality Grade (G-Grade) \u2014 ISO 1940 \/ ISO 21940-11\">\n<meta name=\"twitter:description\" content=\"Interactive calculator for permissible residual unbalance per ISO 21940-11. Full G-grade table, formulas, worked examples.\">\n\n\n<meta name=\"geo.region\" content=\"EU\">\n<meta name=\"geo.placename\" content=\"Porto, Portugal\">\n<meta name=\"geo.position\" content=\"41.1579;-8.6291\">\n<meta name=\"ICBM\" content=\"41.1579, -8.6291\">\n\n\n<script type=\"application\/ld+json\">\n{\n \"@context\": \"https:\/\/schema.org\",\n \"@graph\": [\n {\n \"@type\": \"TechArticle\",\n \"@id\": \"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/#article\",\n \"headline\": \"Balance Quality Grade (G-Grade): ISO 1940-1 \/ ISO 21940-11 \u2014 Complete Guide\",\n \"description\": \"Comprehensive guide to Balance Quality Grades (G-Grades) per ISO 1940-1 and ISO 21940-11. Interactive calculator, reference tables, formulas, and worked examples.\",\n \"author\": {\n \"@type\": \"Organization\",\n \"name\": \"Vibromera\",\n \"url\": \"https:\/\/vibromera.eu\/\"\n },\n \"publisher\": {\n \"@type\": \"Organization\",\n \"name\": \"Vibromera\",\n \"url\": \"https:\/\/vibromera.eu\/\",\n \"logo\": {\n \"@type\": \"ImageObject\",\n \"url\": \"https:\/\/vibromera.eu\/wp-content\/uploads\/vibromera-logo.png\"\n }\n },\n \"mainEntityOfPage\": \"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/\",\n \"datePublished\": \"2024-01-15\",\n \"dateModified\": \"2026-02-07\",\n \"inLanguage\": \"en\",\n \"about\": [\n {\"@type\": \"Thing\", \"name\": \"Balance Quality Grade\"},\n {\"@type\": \"Thing\", \"name\": \"ISO 1940-1\"},\n {\"@type\": \"Thing\", \"name\": \"ISO 21940-11\"},\n {\"@type\": \"Thing\", \"name\": \"Rotor Balancing\"},\n {\"@type\": \"Thing\", \"name\": \"Residual Unbalance\"}\n ],\n \"isPartOf\": {\n \"@type\": \"WebPage\",\n \"url\": \"https:\/\/vibromera.eu\/glossary\/\"\n }\n },\n {\n \"@type\": \"FAQPage\",\n \"@id\": \"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/#faq\",\n \"mainEntity\": [\n {\n \"@type\": \"Question\",\n \"name\": \"What is a Balance Quality Grade (G-Grade)?\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"A Balance Quality Grade (G-Grade) is a standardized classification per ISO 21940-11 (formerly ISO 1940-1) that defines the maximum permissible residual unbalance for a rigid rotor. The G number represents the maximum velocity of the rotor's center-of-gravity displacement in mm\/s. For example, G 6.3 limits this velocity to 6.3 mm\/s at maximum service speed. Lower G values mean tighter tolerance and higher precision.\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"How do you calculate permissible residual unbalance from a G-Grade?\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"Use the formula: U_per (g\u00b7mm) = (9549 \u00d7 G \u00d7 m) \/ n, where G is the grade in mm\/s, m is rotor mass in kg, and n is maximum service speed in RPM. For example, a 25 kg rotor at 3000 RPM with G 6.3: U_per = (9549 \u00d7 6.3 \u00d7 25) \/ 3000 = 502 g\u00b7mm total, or 251 g\u00b7mm per plane for two-plane balancing.\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"What is the difference between ISO 1940-1 and ISO 21940-11?\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"ISO 21940-11:2016 supersedes ISO 1940-1:2003. Both define the same G-grade system with identical values and application tables. ISO 21940-11 is part of the comprehensive ISO 21940 series covering all aspects of rotor balancing. The G-grade values (G 0.4 through G 4000) and their recommended applications remain unchanged.\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"What G-Grade should I use for a pump or fan?\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"For most industrial pumps, fans, blowers, and general electric motors, G 6.3 is the standard recommendation per ISO 21940-11. For high-speed or critical service pumps (API 610), G 2.5 is often specified. HVAC fans in noise-sensitive environments may also require G 2.5.\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"What is the most commonly used Balance Quality Grade?\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"G 6.3 is the most widely specified balance quality grade worldwide. It applies to general industrial machinery including electric motors, pump impellers, fans, blowers, flywheels, and process plant equipment. It provides a good balance between manufacturing cost and vibration performance.\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"Does G-Grade equal machine vibration level?\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"No. The G value is a property of the rotor alone, not the installed machine. G 6.3 does NOT mean the machine will vibrate at 6.3 mm\/s. Installed vibration depends on many additional factors: bearing condition, alignment, natural frequencies, structural stiffness, and damping. 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}\n\n.page-footer { background: var(--beige); border-top: 1px solid var(--beige-dark); padding: 32px 0; text-align: center; }\n.page-footer a { color: var(--blue); text-decoration: none; font-weight: 500; }\n.page-footer p { font-size: 14px; color: var(--text-muted); }\n\n@media print { .quick-nav, .shop-cta { display: none; } body { min-width: auto; } }\n<\/style>\n<\/head>\n<body>\n\n\n<header class=\"hero\">\n <div class=\"hero-inner\">\n <div class=\"breadcrumb\">\n <a href=\"https:\/\/vibromera.eu\/\">Home<\/a> \u2192 <a href=\"https:\/\/vibromera.eu\/glossary\/\">Glossary<\/a> \u2192 Balance Quality Grade\n <\/div>\n <h1>Balance Quality Grade <span>(G-Grade)<\/span><\/h1>\n <div style=\"display:inline-flex;align-items:center;gap:6px;padding:4px 12px;background:rgba(37,99,235,0.2);border:1px solid rgba(37,99,235,0.3);border-radius:20px;font-size:12px;font-weight:600;letter-spacing:0.5px;color:rgba(255,255,255,0.85);margin-bottom:12px;text-transform:uppercase;\">\n <span style=\"font-size:14px;\">\ud83d\udccc<\/span> Canonical Reference Article \u2014 vibromera.eu\n <\/div>\n <p class=\"subtitle\">The international standard for rotor balancing precision \u2014 how ISO 1940-1 and ISO 21940-11 G-grades define permissible residual unbalance, why they matter for bearing life and machine reliability, and how to calculate tolerances for any rotor.<\/p>\n <\/div>\n<\/header>\n\n\n<nav class=\"quick-nav\">\n <div class=\"quick-nav-inner\">\n <a href=\"#calculator\">\u2699 Tolerance Calculator<\/a>\n <a href=\"#grade-overview\">\ud83d\udcca G-Grade Overview<\/a>\n <a href=\"#iso-table\">\ud83d\udccb Full ISO Table<\/a>\n <a href=\"#definition\">\ud83d\udcd0 Definition<\/a>\n <a href=\"#purpose\">\ud83c\udfaf Purpose<\/a>\n <a href=\"#formulas\">\ud83d\udd22 Formulas<\/a>\n <a href=\"#selection\">\ud83d\udd0d Grade Selection<\/a>\n <a href=\"#two-plane\">\u2696 Two-Plane Balancing<\/a>\n <a href=\"#examples\">\ud83d\udcdd Worked Examples<\/a>\n <a href=\"#standards\">\ud83d\udcda Standards Comparison<\/a>\n <a href=\"#mistakes\">\u26a0 Common Mistakes<\/a>\n <a href=\"#faq\">\u2753 FAQ<\/a>\n <\/div>\n<\/nav>\n\n\n<section class=\"summary-dashboard\" id=\"calculator\">\n <div class=\"container\">\n <div class=\"dashboard-grid\">\n\n <div class=\"calc-panel\">\n <h2 class=\"panel-title\">Balancing Tolerance Calculator<\/h2>\n <p class=\"panel-subtitle\">Calculate permissible residual unbalance per ISO 21940-11 \/ ISO 1940-1<\/p>\n <div class=\"calc-form\">\n <div class=\"form-group full-width\">\n <label>Balance Quality Grade (G)<\/label>\n <select id=\"gGrade\" onchange=\"calculate()\">\n <option value=\"0.4\">G 0.4 \u2014 Gyroscopes, spindles, HDD<\/option>\n <option value=\"1.0\">G 1.0 \u2014 Grinding spindles, turbochargers<\/option>\n <option value=\"2.5\" selected>G 2.5 \u2014 Turbines, turbocompressors, high-speed motors<\/option>\n <option value=\"6.3\">G 6.3 \u2014 Pumps, fans, motors, general machinery<\/option>\n <option value=\"16\">G 16 \u2014 Drive shafts, crushers, agricultural<\/option>\n <option value=\"40\">G 40 \u2014 Car wheels, crankshafts (slow)<\/option>\n <option value=\"100\">G 100 \u2014 Crankshaft assemblies (rigid)<\/option>\n <option value=\"250\">G 250 \u2014 Slow cement machinery<\/option>\n <option value=\"630\">G 630 \u2014 Coarse reciprocating<\/option>\n <option value=\"4000\">G 4000 \u2014 Crank drives (inherently unbalanced)<\/option>\n <\/select>\n <\/div>\n <div class=\"form-group\">\n <label>Rotor Mass <span class=\"unit\">(kg)<\/span><\/label>\n <input type=\"number\" id=\"rotorMass\" value=\"25\" min=\"0.001\" step=\"0.1\">\n <\/div>\n <div class=\"form-group\">\n <label>Max Operating Speed <span class=\"unit\">(RPM)<\/span><\/label>\n <input type=\"number\" id=\"rotorRPM\" value=\"3000\" min=\"1\" step=\"1\">\n <\/div>\n <div class=\"form-group\">\n <label>Number of Correction Planes<\/label>\n <select id=\"numPlanes\">\n <option value=\"1\">1 plane (static balancing)<\/option>\n <option value=\"2\" selected>2 planes (dynamic balancing)<\/option>\n <\/select>\n <\/div>\n <div class=\"form-group\">\n <label>Correction Radius <span class=\"unit\">(mm, optional)<\/span><\/label>\n <input type=\"number\" id=\"corrRadius\" value=\"\" min=\"0.1\" step=\"0.1\" placeholder=\"e.g. 150\">\n <\/div>\n <button class=\"calc-btn\" onclick=\"calculate()\">Calculate Tolerance \u2192<\/button>\n <\/div>\n <\/div>\n\n <div class=\"results-panel\">\n <h2 class=\"panel-title\">Results<\/h2>\n <p class=\"panel-subtitle\">Permissible residual unbalance and balancing targets<\/p>\n <div id=\"resultsArea\">\n <div class=\"results-empty\">\n <span class=\"icon\">\u2696\ufe0f<\/span>\n Enter rotor parameters and click Calculate<br>to see balancing tolerances\n <\/div>\n <\/div>\n <\/div>\n\n <\/div>\n <\/div>\n<\/section>\n\n\n<section class=\"grades-section\" id=\"grade-overview\">\n <div class=\"container\">\n <div class=\"section-header\">\n <h2>Balance Quality Grades at a Glance<\/h2>\n <p>From ultra-precision gyroscopes (G 0.4) to coarse reciprocating engines (G 4000) \u2014 the complete ISO classification<\/p>\n <\/div>\n\n <div class=\"grade-cards\">\n <div class=\"grade-card ultra\">\n <div class=\"gc-grade\">G 0.4<\/div>\n <div class=\"gc-vel\">\u2264 0.4 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>Ultra-precision:<\/strong> Gyroscopes, precision spindles, HDD platters, satellite components, microelectronics manufacturing equipment<\/div>\n <\/div>\n <div class=\"grade-card ultra\">\n <div class=\"gc-grade\">G 1.0<\/div>\n <div class=\"gc-vel\">\u2264 1.0 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>Precision:<\/strong> Grinding machine drives, small high-speed motors, automotive turbochargers, dental\/medical equipment spindles<\/div>\n <\/div>\n <div class=\"grade-card high\">\n <div class=\"gc-grade\">G 2.5<\/div>\n <div class=\"gc-vel\">\u2264 2.5 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>High-quality:<\/strong> Gas\/steam turbines, turbocompressors, high-speed electric motors, machine tool drives, centrifuge rotors<\/div>\n <\/div>\n <div class=\"grade-card standard\">\n <div class=\"gc-grade\">G 6.3<\/div>\n <div class=\"gc-vel\">\u2264 6.3 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>Standard:<\/strong> Pump impellers, fans, blowers, general electric motors, flywheels, process machinery \u2014 the most commonly specified grade<\/div>\n <\/div>\n <div class=\"grade-card standard\">\n <div class=\"gc-grade\">G 16<\/div>\n <div class=\"gc-vel\">\u2264 16 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>Medium:<\/strong> Drive shafts (cardan), crushers, agricultural machinery, parts of process plant with moderate requirements<\/div>\n <\/div>\n <div class=\"grade-card general\">\n <div class=\"gc-grade\">G 40<\/div>\n <div class=\"gc-vel\">\u2264 40 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>General:<\/strong> Car wheels\/rims, crankshafts for slow marine engines, non-critical rotating assemblies<\/div>\n <\/div>\n <div class=\"grade-card general\">\n <div class=\"gc-grade\">G 100<\/div>\n <div class=\"gc-vel\">\u2264 100 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>Coarse:<\/strong> Complete crankshaft assemblies (rigidly mounted), slow agricultural machinery<\/div>\n <\/div>\n <div class=\"grade-card general\">\n <div class=\"gc-grade\">G 630+<\/div>\n <div class=\"gc-vel\">\u2264 630\u20134000 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>Very coarse:<\/strong> Crankshaft drives of inherently unbalanced, slow reciprocating engines on rigid mounts<\/div>\n <\/div>\n <\/div>\n\n \n <div class=\"table-wrap\" id=\"iso-table\">\n <div class=\"table-title\">\ud83d\udccb Complete ISO 21940-11 \/ ISO 1940-1 \u2014 Balance Quality Grade Table<\/div>\n <table>\n <thead>\n <tr>\n <th>G-Grade<\/th>\n <th>e\u00b7\u03c9 (mm\/s)<\/th>\n <th>Precision Class<\/th>\n <th>Typical Rotor Types \/ Applications<\/th>\n <\/tr>\n <\/thead>\n <tbody>\n <tr><td class=\"mono\">G 4000<\/td><td class=\"mono\">4000<\/td><td><span class=\"tag coarse\">Very Coarse<\/span><\/td><td>Crankshaft drives of inherently unbalanced, rigidly mounted slow marine diesel engines<\/td><\/tr>\n <tr><td class=\"mono\">G 1600<\/td><td class=\"mono\">1600<\/td><td><span class=\"tag coarse\">Very Coarse<\/span><\/td><td>Crankshaft drives, rigidly mounted<\/td><\/tr>\n <tr><td class=\"mono\">G 630<\/td><td class=\"mono\">630<\/td><td><span class=\"tag coarse\">Coarse<\/span><\/td><td>Crankshaft drives of inherently unbalanced, elastically mounted engines<\/td><\/tr>\n <tr><td class=\"mono\">G 250<\/td><td class=\"mono\">250<\/td><td><span class=\"tag coarse\">Coarse<\/span><\/td><td>Crankshaft drives of fast 4-cylinder engines, elastically mounted<\/td><\/tr>\n <tr><td class=\"mono\">G 100<\/td><td class=\"mono\">100<\/td><td><span class=\"tag general\">General<\/span><\/td><td>Complete engines (gasoline\/diesel) for cars, trucks; crankshafts for rigidly mounted 6+ cylinder engines<\/td><\/tr>\n <tr><td class=\"mono\">G 40<\/td><td class=\"mono\">40<\/td><td><span class=\"tag general\">General<\/span><\/td><td>Car wheels; wheel rims; drive shafts; crankshafts, elastically mounted, of fast 4-cylinder engines<\/td><\/tr>\n <tr><td class=\"mono\">G 16<\/td><td class=\"mono\">16<\/td><td><span class=\"tag standard\">Standard<\/span><\/td><td>Drive shafts (cardan); parts of crushing machinery; parts of agricultural machinery; crankshafts, elastically mounted, of 6+ cylinder engines<\/td><\/tr>\n <tr><td class=\"mono\">G 6.3<\/td><td class=\"mono\">6.3<\/td><td><span class=\"tag standard\">Standard<\/span><\/td><td>Fans; flywheels; pump impellers; general machinery parts; normal electric motor rotors; process plant machinery<\/td><\/tr>\n <tr><td class=\"mono\">G 2.5<\/td><td class=\"mono\">2.5<\/td><td><span class=\"tag precision\">Precision<\/span><\/td><td>Gas and steam turbines; turbo-generators; turbocompressors; machine tool drives; medium and large electric motor rotors with special requirements<\/td><\/tr>\n <tr><td class=\"mono\">G 1.0<\/td><td class=\"mono\">1.0<\/td><td><span class=\"tag precision\">Precision<\/span><\/td><td>Grinding machine drives; small high-speed electric motors; turbochargers<\/td><\/tr>\n <tr><td class=\"mono\">G 0.4<\/td><td class=\"mono\">0.4<\/td><td><span class=\"tag ultra\">Ultra-precision<\/span><\/td><td>Gyroscopes; precision spindles; hard disk drives; ultra-high-speed spindles for microelectronics<\/td><\/tr>\n <\/tbody>\n <\/table>\n <\/div>\n\n \n <div class=\"table-wrap\">\n <div class=\"table-title\">\ud83d\udcca Quick-Reference \u2014 Pre-Calculated Tolerances for Common Scenarios<\/div>\n <table>\n <thead>\n <tr>\n <th>Rotor Type<\/th>\n <th>Mass (kg)<\/th>\n <th>Speed (RPM)<\/th>\n <th>Grade<\/th>\n <th>U<sub>per<\/sub> Total (g\u00b7mm)<\/th>\n <th>U<sub>per<\/sub> per Plane (g\u00b7mm)<\/th>\n <th>e<sub>per<\/sub> (\u00b5m)<\/th>\n <\/tr>\n <\/thead>\n <tbody>\n <tr><td>Small electric motor<\/td><td class=\"mono\">8<\/td><td class=\"mono\">2900<\/td><td class=\"mono\">G 6.3<\/td><td class=\"mono\">166<\/td><td class=\"mono\">83<\/td><td class=\"mono\">20.7<\/td><\/tr>\n <tr><td>Pump impeller<\/td><td class=\"mono\">12<\/td><td class=\"mono\">2950<\/td><td class=\"mono\">G 6.3<\/td><td class=\"mono\">245<\/td><td class=\"mono\">122<\/td><td class=\"mono\">20.4<\/td><\/tr>\n <tr><td>Industrial fan<\/td><td class=\"mono\">85<\/td><td class=\"mono\">1480<\/td><td class=\"mono\">G 6.3<\/td><td class=\"mono\">3459<\/td><td class=\"mono\">1730<\/td><td class=\"mono\">40.7<\/td><\/tr>\n <tr><td>Large motor rotor<\/td><td class=\"mono\">350<\/td><td class=\"mono\">1500<\/td><td class=\"mono\">G 2.5<\/td><td class=\"mono\">5578<\/td><td class=\"mono\">2789<\/td><td class=\"mono\">15.9<\/td><\/tr>\n <tr><td>Steam turbine<\/td><td class=\"mono\">1200<\/td><td class=\"mono\">3600<\/td><td class=\"mono\">G 2.5<\/td><td class=\"mono\">7958<\/td><td class=\"mono\">3979<\/td><td class=\"mono\">6.6<\/td><\/tr>\n <tr><td>Turbocharger<\/td><td class=\"mono\">0.8<\/td><td class=\"mono\">90000<\/td><td class=\"mono\">G 1.0<\/td><td class=\"mono\">0.085<\/td><td class=\"mono\">0.042<\/td><td class=\"mono\">0.11<\/td><\/tr>\n <tr><td>Grinding spindle<\/td><td class=\"mono\">5<\/td><td class=\"mono\">12000<\/td><td class=\"mono\">G 1.0<\/td><td class=\"mono\">3.98<\/td><td class=\"mono\">1.99<\/td><td class=\"mono\">0.80<\/td><\/tr>\n <tr><td>Crusher flywheel<\/td><td class=\"mono\">500<\/td><td class=\"mono\">600<\/td><td class=\"mono\">G 16<\/td><td class=\"mono\">127,320<\/td><td class=\"mono\">63,660<\/td><td class=\"mono\">254.6<\/td><\/tr>\n <tr><td>Drive shaft (cardan)<\/td><td class=\"mono\">15<\/td><td class=\"mono\">4500<\/td><td class=\"mono\">G 16<\/td><td class=\"mono\">509<\/td><td class=\"mono\">255<\/td><td class=\"mono\">33.9<\/td><\/tr>\n <tr><td>HVAC blower<\/td><td class=\"mono\">45<\/td><td class=\"mono\">1750<\/td><td class=\"mono\">G 6.3<\/td><td class=\"mono\">1546<\/td><td class=\"mono\">773<\/td><td class=\"mono\">34.4<\/td><\/tr>\n <tr><td>Car wheel assembly<\/td><td class=\"mono\">20<\/td><td class=\"mono\">900<\/td><td class=\"mono\">G 40<\/td><td class=\"mono\">8488<\/td><td class=\"mono\">4244<\/td><td class=\"mono\">424.4<\/td><\/tr>\n <tr><td>Centrifuge<\/td><td class=\"mono\">30<\/td><td class=\"mono\">6000<\/td><td class=\"mono\">G 2.5<\/td><td class=\"mono\">119<\/td><td class=\"mono\">60<\/td><td class=\"mono\">3.98<\/td><\/tr>\n <\/tbody>\n <\/table>\n <\/div>\n\n \n <div class=\"table-wrap\" id=\"standards-comparison\">\n <div class=\"table-title\">\ud83d\udcda Balancing Standards Comparison \u2014 ISO vs. API vs. ANSI vs. VDI<\/div>\n <table>\n <thead>\n <tr>\n <th>Standard<\/th>\n <th>Scope<\/th>\n <th>G-Grade System?<\/th>\n <th>Key Difference<\/th>\n <th>Status<\/th>\n <\/tr>\n <\/thead>\n <tbody>\n <tr><td>ISO 21940-11:2016<\/td><td>All rigid rotors \u2014 general procedures<\/td><td>Yes (primary)<\/td><td>Current international standard; replaces ISO 1940-1<\/td><td><span class=\"tag standard\">Current<\/span><\/td><\/tr>\n <tr><td>ISO 1940-1:2003<\/td><td>All rigid rotors<\/td><td>Yes (original)<\/td><td>Established the G-grade system; still widely referenced<\/td><td><span class=\"tag general\">Superseded<\/span><\/td><\/tr>\n <tr><td>ISO 21940-12<\/td><td>Balancing procedures and tolerances<\/td><td>Yes (references Part 11)<\/td><td>Practical balancing procedures, correction plane allocation<\/td><td><span class=\"tag standard\">Current<\/span><\/td><\/tr>\n <tr><td>API 610 \/ 617 \/ 611<\/td><td>Pumps \/ compressors \/ turbines (petroleum industry)<\/td><td>References ISO; adds stricter limits<\/td><td>Often specifies 4W\/N (\u2248 G 1.0) for API 617 rotors; more conservative<\/td><td><span class=\"tag standard\">Current<\/span><\/td><\/tr>\n <tr><td>ANSI S2.19<\/td><td>US-adopted version of ISO 1940<\/td><td>Yes (identical)<\/td><td>Direct adoption of ISO G-grade system for US market<\/td><td><span class=\"tag standard\">Current<\/span><\/td><\/tr>\n <tr><td>VDI 2060<\/td><td>German standard (pre-ISO)<\/td><td>Equivalent system<\/td><td>Historical predecessor to ISO 1940; still referenced in German industry<\/td><td><span class=\"tag general\">Superseded by ISO<\/span><\/td><\/tr>\n <tr><td>MIL-STD-167-1<\/td><td>US military \u2014 shipboard equipment<\/td><td>No (vibration limits)<\/td><td>Specifies vibration amplitude limits, not unbalance tolerances<\/td><td><span class=\"tag precision\">Active<\/span><\/td><\/tr>\n <\/tbody>\n <\/table>\n <\/div>\n\n <\/div>\n<\/section>\n\n\n<main class=\"main-content\" id=\"definition\">\n <div class=\"container\">\n <div class=\"content-layout\">\n <article class=\"article-content\">\n\n <h2>What is a Balance Quality Grade (G-Grade)?<\/h2>\n\n <div class=\"info-box\" style=\"border-left-color: var(--navy); background: var(--beige-light);\">\n <div class=\"box-title\" style=\"font-size: 16px;\">Quick Answer<\/div>\n <p style=\"font-size: 15px;\"><strong>A Balance Quality Grade (G-Grade)<\/strong> is an international standard classification per <strong>ISO 21940-11<\/strong> (formerly ISO 1940-1) that defines the maximum permissible residual <a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\">unbalance<\/a> for a rigid rotor. The G number represents the maximum velocity of the rotor's center-of-gravity displacement in mm\/s. Common grades: <strong>G 6.3<\/strong> for general machinery (pumps, fans, motors), <strong>G 2.5<\/strong> for turbines and precision equipment, <strong>G 1.0<\/strong> for grinding spindles and turbochargers. The formula for permissible unbalance: <strong>U<sub>per<\/sub> = 9549 \u00d7 G \u00d7 m \/ n<\/strong> (g\u00b7mm), where m = mass (kg), n = speed (RPM).<\/p>\n <\/div>\n\n <p>A <strong>Balance Quality Grade<\/strong>, commonly called a \"G-Grade,\" is a standardized classification defined in <strong>ISO 21940-11<\/strong> (which superseded ISO 1940-1) that specifies the maximum permissible residual <a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\">unbalance<\/a> for a rigid rotor. The G-grade defines how precisely a rotor must be balanced \u2014 not a vibration measurement in the installed machine, but a quality specification for the rotor itself based on its mass and maximum service speed.<\/p>\n <p>The number following the letter \"G\" represents the maximum permissible velocity of the rotor's center-of-mass displacement, expressed in millimeters per second (mm\/s). For example, G 6.3 means the product of the specific eccentricity (e<sub>per<\/sub>) and the angular velocity (\u03c9) must not exceed 6.3 mm\/s. G 2.5 limits this velocity to 2.5 mm\/s. The lower the G number, the tighter the balancing tolerance \u2014 meaning higher precision and less permissible residual unbalance.<\/p>\n\n <div class=\"info-box\">\n <div class=\"box-title\">What the G Number Physically Means<\/div>\n <p>The G value represents the maximum permissible velocity of the rotor's center of gravity relative to the geometric rotation axis, at the maximum service speed. G 6.3 means the center of gravity may move at no more than 6.3 mm\/s relative to the spin axis. Since centrifugal force is proportional to this velocity squared, even small reductions in G-grade produce significant reductions in dynamic bearing loads.<\/p>\n <\/div>\n\n <h2 id=\"purpose\">The Purpose of the G-Grade System<\/h2>\n <p>Before the G-grade system was established, balancing specifications were vague \u2014 \"balance as well as possible\" or \"balance until smooth.\" The ISO G-grade system replaced this ambiguity with a universal, verifiable standard. It provides a common language for manufacturers, service engineers, and end users worldwide. The main objectives are:<\/p>\n\n <h3>1. Limiting Unbalance-Induced Vibration to Acceptable Levels<\/h3>\n <p><a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\">Unbalance<\/a> produces centrifugal forces that increase with the square of rotational speed. These forces cause vibration, noise, fatigue loading, and ultimately mechanical failure. By specifying a G-grade, the engineer limits these forces to levels the machine's bearings, seals, and structure can safely tolerate throughout the intended service life.<\/p>\n\n <h3>2. Minimizing Dynamic Loads on Bearings<\/h3>\n <p>Bearings are the components most directly affected by unbalance. The cyclic radial load from residual unbalance acts as a fatigue load on rolling elements and raceways. Bearing life (L<sub>10<\/sub>) is inversely proportional to the cube of the applied load \u2014 so even a modest reduction in unbalance force can dramatically extend bearing service life. Balancing a motor rotor from G 16 to G 6.3 typically doubles bearing L<sub>10<\/sub> life; balancing to G 2.5 can quadruple it.<\/p>\n\n <h3>3. Ensuring Safe Operation at Maximum Design Speed<\/h3>\n <p>Centrifugal force from unbalance is proportional to \u03c9\u00b2 \u2014 doubling the speed quadruples the force from the same unbalance. A rotor that is acceptably balanced at 1500 RPM may produce dangerous vibration at 3000 RPM. The G-grade system accounts for this by incorporating speed into the tolerance calculation, ensuring the rotor is safe at its maximum rated speed.<\/p>\n\n <h3>4. Providing a Clear, Measurable Acceptance Criterion<\/h3>\n <p>The G-grade converts \"balance quality\" from a subjective judgment into an objective, measurable pass\/fail criterion. After balancing, the residual unbalance is compared against the calculated tolerance. If the measured value is below the limit, the rotor passes. This is essential for manufacturing quality control, contractual specifications, warranty claims, and regulatory compliance.<\/p>\n\n <h2 id=\"formulas\">Calculating Permissible Residual Unbalance<\/h2>\n <p>The core of the G-grade system is the ability to calculate a specific, numerical unbalance tolerance for any rotor. Two key quantities are derived from the G-grade:<\/p>\n\n <h3>Specific Unbalance (Permissible Eccentricity)<\/h3>\n <div class=\"formula-box\">\n <div class=\"formula-label\">Permissible Specific Unbalance (Eccentricity)<\/div>\n <div class=\"formula-main\">e<sub>per<\/sub> = (9549 \u00d7 G) \/ n<\/div>\n <div class=\"formula-note\">e<sub>per<\/sub> in \u00b5m (micrometers), G in mm\/s, n in RPM. Constant 9549 = 60\u00d71000\/(2\u03c0)<\/div>\n <\/div>\n <p>The specific unbalance (e<sub>per<\/sub>) represents the maximum permissible displacement of the rotor's center of gravity from the rotation axis, in micrometers. It depends only on the G-grade and the speed \u2014 not on the rotor mass. This makes it useful for comparing the balance quality of rotors of different sizes.<\/p>\n\n <h3>Total Permissible Residual Unbalance<\/h3>\n <div class=\"formula-box\">\n <div class=\"formula-label\">Total Permissible Residual Unbalance<\/div>\n <div class=\"formula-main\">U<sub>per<\/sub> = e<sub>per<\/sub> \u00d7 m = (9549 \u00d7 G \u00d7 m) \/ n<\/div>\n <div class=\"formula-note\">U<sub>per<\/sub> in g\u00b7mm, G in mm\/s, m in kg, n in RPM<\/div>\n <\/div>\n <p>The total permissible residual unbalance (U<sub>per<\/sub>) is the actual target the balancing technician must achieve. It is expressed in g\u00b7mm (gram-millimeters) \u2014 the product of the residual unbalance mass times its distance from the rotation axis. This is the number displayed on the balancing machine and compared against the tolerance.<\/p>\n\n <h3>Centrifugal Force from Residual Unbalance<\/h3>\n <div class=\"formula-box\">\n <div class=\"formula-label\">Centrifugal Force at Tolerance Limit<\/div>\n <div class=\"formula-main\">F = m \u00d7 e<sub>per<\/sub> \u00d7 \u03c9\u00b2 = U<sub>per<\/sub> \u00d7 \u03c9\u00b2 \/ 10\u2076<\/div>\n <div class=\"formula-note\">F in Newtons, e<sub>per<\/sub> in meters, \u03c9 = 2\u03c0\u00d7n\/60 in rad\/s. Divide by 10\u2076 when U<sub>per<\/sub> in g\u00b7mm<\/div>\n <\/div>\n <p>This formula shows the actual dynamic force the bearings must withstand from the permissible residual unbalance at operating speed. It is useful for verifying that the bearing load rating is adequate and for understanding the real-world impact of the G-grade specification.<\/p>\n\n <h3>Variables Reference<\/h3>\n <table class=\"article-table\">\n <thead><tr><th>Symbol<\/th><th>Name<\/th><th>Unit<\/th><th>Description<\/th><\/tr><\/thead>\n <tbody>\n <tr><td class=\"mono\">G<\/td><td>Balance quality grade<\/td><td>mm\/s<\/td><td>Product e<sub>per<\/sub>\u00b7\u03c9; defines the ISO grade (e.g. 6.3, 2.5, 1.0)<\/td><\/tr>\n <tr><td class=\"mono\">e<sub>per<\/sub><\/td><td>Permissible specific unbalance<\/td><td>\u00b5m<\/td><td>Maximum CG offset from rotation axis<\/td><\/tr>\n <tr><td class=\"mono\">U<sub>per<\/sub><\/td><td>Permissible residual unbalance<\/td><td>g\u00b7mm<\/td><td>Total unbalance tolerance = e<sub>per<\/sub> \u00d7 mass<\/td><\/tr>\n <tr><td class=\"mono\">m<\/td><td>Rotor mass<\/td><td>kg<\/td><td>Total mass of the rotor being balanced<\/td><\/tr>\n <tr><td class=\"mono\">n<\/td><td>Maximum service speed<\/td><td>RPM<\/td><td>Highest speed at which the rotor will operate<\/td><\/tr>\n <tr><td class=\"mono\">\u03c9<\/td><td>Angular velocity<\/td><td>rad\/s<\/td><td>= 2\u03c0 \u00d7 n \/ 60<\/td><\/tr>\n <tr><td class=\"mono\">F<\/td><td>Centrifugal force<\/td><td>N<\/td><td>Dynamic force from residual unbalance at speed<\/td><\/tr>\n <\/tbody>\n <\/table>\n\n <h2 id=\"selection\">How to Select the Right G-Grade<\/h2>\n <p>The ISO standard provides recommendations for hundreds of rotor types, but in practice the selection depends on several interrelated factors:<\/p>\n\n <h3>Machine Type and Application<\/h3>\n <p>The standard groups rotors by application and recommends a G-grade for each group (see the ISO table above). A high-speed turbine needs much tighter balance (G 2.5 or G 1.0) than a slow-speed agricultural mechanism (G 16 or G 40). The designer considers how sensitive the machine is to vibration and what the consequences of unbalance-induced failure would be.<\/p>\n\n <h3>Rotor Speed<\/h3>\n <p>Speed is the single most important factor. For the same G-grade, permissible unbalance (U<sub>per<\/sub>) decreases linearly with speed. A rotor at 6000 RPM has half the tolerance of the same rotor at 3000 RPM. For high-speed rotors (turbines, turbochargers, grinding spindles), the tolerance becomes extremely small, requiring specialized balancing equipment and procedures.<\/p>\n\n <h3>Bearing Type and Support Stiffness<\/h3>\n <p>A rotor mounted on flexible (elastic) supports typically requires tighter balance than one on a rigid foundation, because the flexible system transmits vibration more readily. The same crankshaft may require G 16 on elastic mounts but G 40 on rigid mounts. Similarly, rotors on fluid-film bearings may tolerate more unbalance than those on rolling-element bearings due to the damping effect of the oil film.<\/p>\n\n <h3>Environmental and Safety Requirements<\/h3>\n <p>Equipment operating near personnel (HVAC, medical devices), in noise-sensitive environments, or in safety-critical applications (power generation, aviation, offshore) may require tighter balance than the standard recommends for the rotor type. Some industries (petrochemical, power generation) have their own standards (API, IEEE) that specify tighter limits than ISO.<\/p>\n\n <h3>Industry-Specific Recommendations<\/h3>\n <table class=\"article-table\">\n <thead><tr><th>Industry \/ Application<\/th><th>Typical G-Grade<\/th><th>Notes<\/th><\/tr><\/thead>\n <tbody>\n <tr><td>Power generation (turbines)<\/td><td class=\"mono\">G 1.0 \u2013 G 2.5<\/td><td>API 612\/617 often specifies even tighter than ISO<\/td><\/tr>\n <tr><td>Petroleum \/ chemical (pumps, compressors)<\/td><td class=\"mono\">G 2.5 \u2013 G 6.3<\/td><td>API 610 pumps often G 2.5 or tighter<\/td><\/tr>\n <tr><td>HVAC (fans, blowers, AHU)<\/td><td class=\"mono\">G 6.3<\/td><td>Noise-sensitive installations may require G 2.5<\/td><\/tr>\n <tr><td>Pulp & paper (rollers, dryers)<\/td><td class=\"mono\">G 6.3 \u2013 G 16<\/td><td>Large slow rollers; high mass compensates for lower precision<\/td><\/tr>\n <tr><td>Mining & minerals (crushers, screens)<\/td><td class=\"mono\">G 16 \u2013 G 40<\/td><td>Harsh environment; moderate precision acceptable<\/td><\/tr>\n <tr><td>Automotive (wheels, driveshafts)<\/td><td class=\"mono\">G 16 \u2013 G 40<\/td><td>NVH requirements may tighten beyond ISO minimum<\/td><\/tr>\n <tr><td>Machine tools (spindles, drives)<\/td><td class=\"mono\">G 1.0 \u2013 G 2.5<\/td><td>Surface finish quality depends on spindle balance<\/td><\/tr>\n <tr><td>Marine (propeller shafts, engines)<\/td><td class=\"mono\">G 6.3 \u2013 G 40<\/td><td>Classification society rules (DNV, Lloyd's, ABS) apply<\/td><\/tr>\n <tr><td>Wind energy (rotor hubs, generators)<\/td><td class=\"mono\">G 6.3<\/td><td>Blade pitch imbalance handled separately from hub balance<\/td><\/tr>\n <tr><td>Aerospace (turbofan, gyros)<\/td><td class=\"mono\">G 0.4 \u2013 G 2.5<\/td><td>Extremely tight; military standards (MIL-STD) may override ISO<\/td><\/tr>\n <\/tbody>\n <\/table>\n\n <h2 id=\"two-plane\">Two-Plane Balancing \u2014 Distributing the Tolerance<\/h2>\n <p>The total permissible unbalance U<sub>per<\/sub> calculated from the G-grade formula is for the <em>entire rotor<\/em>. In practice, most rotors are balanced in two correction planes (dynamic balancing), so the tolerance must be apportioned between the planes.<\/p>\n\n <h3>ISO Guidance for Tolerance Distribution<\/h3>\n <ul>\n <li><strong>Symmetric rotors<\/strong> (CG approximately at midspan): Divide U<sub>per<\/sub> equally between the two planes. Each plane gets U<sub>per<\/sub>\/2.<\/li>\n <li><strong>Asymmetric rotors<\/strong> (CG offset toward one end): Distribute proportionally to the bearing distances from the CG. The plane closest to the CG receives the larger share of the tolerance.<\/li>\n <li><strong>Single-plane balancing:<\/strong> The entire U<sub>per<\/sub> applies to the single correction plane. This is appropriate for narrow disc-shaped rotors (L\/D < 0.5) where couple unbalance is negligible.<\/li>\n <\/ul>\n\n <div class=\"info-box warning\">\n <div class=\"box-title\">Important: Don't Double the Tolerance<\/div>\n <p>A common error is to calculate U<sub>per<\/sub> and then apply this value to <em>each<\/em> plane, effectively doubling the total tolerance. The correct approach: U<sub>per<\/sub> is the total; divide it between planes. Each plane receives U<sub>per<\/sub>\/2 for a symmetric rotor.<\/p>\n <\/div>\n\n <h2 id=\"examples\">Worked Examples<\/h2>\n\n <div class=\"example-block\">\n <div class=\"example-title\">Example 1: Centrifugal Pump Impeller<\/div>\n <p><strong>Given:<\/strong> Pump impeller, mass = 12 kg, operating speed = 2950 RPM, required grade G 6.3.<\/p>\n <p><strong>Step 1 \u2014 Specific unbalance:<\/strong> e<sub>per<\/sub> = 9549 \u00d7 6.3 \/ 2950 = <strong>20.4 \u00b5m<\/strong><\/p>\n <p><strong>Step 2 \u2014 Total tolerance:<\/strong> U<sub>per<\/sub> = 20.4 \u00d7 12 = <strong>245 g\u00b7mm<\/strong><\/p>\n <p><strong>Step 3 \u2014 Per plane (symmetric):<\/strong> 245 \/ 2 = <strong>122 g\u00b7mm per plane<\/strong><\/p>\n <p><strong>Step 4 \u2014 Correction weight:<\/strong> At correction radius R = 100 mm: weight = 122 \/ 100 = <strong>1.22 grams<\/strong> per plane maximum<\/p>\n <p><strong>Step 5 \u2014 Centrifugal force:<\/strong> \u03c9 = 2\u03c0 \u00d7 2950\/60 = 308.9 rad\/s. F = 245 \u00d7 10\u207b\u2076 \u00d7 308.9\u00b2 = <strong>23.4 N<\/strong> \u2014 well within bearing capacity.<\/p>\n <\/div>\n\n <div class=\"example-block\">\n <div class=\"example-title\">Example 2: Large Industrial Fan<\/div>\n <p><strong>Given:<\/strong> Fan rotor, mass = 85 kg, operating speed = 1480 RPM, required grade G 6.3.<\/p>\n <p><strong>Step 1 \u2014 Specific unbalance:<\/strong> e<sub>per<\/sub> = 9549 \u00d7 6.3 \/ 1480 = <strong>40.6 \u00b5m<\/strong><\/p>\n <p><strong>Step 2 \u2014 Total tolerance:<\/strong> U<sub>per<\/sub> = 40.6 \u00d7 85 = <strong>3,455 g\u00b7mm<\/strong><\/p>\n <p><strong>Step 3 \u2014 Per plane:<\/strong> 3,455 \/ 2 = <strong>1,728 g\u00b7mm per plane<\/strong><\/p>\n <p><strong>Step 4 \u2014 Correction weight:<\/strong> At R = 400 mm: weight = 1728 \/ 400 = <strong>4.3 grams<\/strong> per plane maximum.<\/p>\n <p><strong>Practical note:<\/strong> This fan can be balanced in the field using a <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> portable balancer with the rotor installed. The device automatically calculates the G 6.3 tolerance based on rotor mass and speed.<\/p>\n <\/div>\n\n <div class=\"example-block\">\n <div class=\"example-title\">Example 3: Automotive Turbocharger<\/div>\n <p><strong>Given:<\/strong> Turbine wheel, mass = 0.8 kg, max speed = 90,000 RPM, required grade G 1.0.<\/p>\n <p><strong>Step 1 \u2014 Specific unbalance:<\/strong> e<sub>per<\/sub> = 9549 \u00d7 1.0 \/ 90000 = <strong>0.106 \u00b5m<\/strong> \u2014 about 100 nanometers!<\/p>\n <p><strong>Step 2 \u2014 Total tolerance:<\/strong> U<sub>per<\/sub> = 0.106 \u00d7 0.8 = <strong>0.085 g\u00b7mm<\/strong><\/p>\n <p><strong>Step 3 \u2014 Correction weight:<\/strong> At R = 20 mm: weight = 0.085 \/ 20 = <strong>0.004 grams<\/strong> (4 milligrams!) per plane maximum.<\/p>\n <p><strong>Practical note:<\/strong> This extremely tight tolerance requires specialized high-speed balancing machines with sub-milligram resolution. Material removal (grinding\/drilling) is typically used rather than adding weights at this precision level.<\/p>\n <\/div>\n\n <h2 id=\"standards\">Historical Context \u2014 ISO 1940-1 to ISO 21940-11<\/h2>\n <p>The G-grade system has evolved through several iterations:<\/p>\n <ul>\n <li><strong>VDI 2060 (1966):<\/strong> The original German standard that established the concept of balance quality grades. Developed by the Verein Deutscher Ingenieure (Association of German Engineers).<\/li>\n <li><strong>ISO 1940 (1973, rev. 1986, 2003):<\/strong> International adoption of the VDI 2060 concept. ISO 1940-1:2003 \"Mechanical vibration \u2014 Balance quality requirements for rotors in a constant (rigid) state\" became the worldwide reference for G-grades.<\/li>\n <li><strong>ISO 21940-11:2016:<\/strong> The current standard. Part of the comprehensive ISO 21940 series covering all aspects of rotor balancing. Part 11 specifically covers balance quality requirements and replaces ISO 1940-1. The G-grade values and application tables remain essentially the same; the main changes are editorial and structural.<\/li>\n <\/ul>\n <p>Despite the formal supersession, \"ISO 1940\" remains the most commonly used reference in industry conversations, purchase specifications, and equipment manuals. Both designations refer to the same G-grade system.<\/p>\n\n <h2 id=\"mistakes\">Common Mistakes in Applying G-Grades<\/h2>\n\n <h3>Mistake 1: Using Balancing Speed Instead of Service Speed<\/h3>\n <p>The G-grade tolerance must be calculated using the <em>maximum service speed<\/em> (operating speed), not the balancing machine speed. Many rotors are balanced at a lower RPM than their service speed. Using the balancing speed in the formula produces a tolerance that is too loose for the actual operating conditions. The <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> software allows you to enter the service speed separately from the balancing speed to avoid this error.<\/p>\n\n <h3>Mistake 2: Confusing G-Grade with Vibration Level<\/h3>\n <p>G 6.3 does NOT mean the installed machine will vibrate at 6.3 mm\/s. The G value is a property of the <em>rotor alone<\/em>, measured or calculated as a free-body tolerance. The vibration of the installed machine depends on many additional factors: bearing condition, <a href=\"https:\/\/vibromera.eu\/glossary\/misalignment\/\">alignment<\/a>, structural <a href=\"https:\/\/vibromera.eu\/glossary\/natural-frequency\/\">natural frequencies<\/a>, damping, and more. A rotor balanced to G 6.3 may produce 1 mm\/s vibration in one machine and 4 mm\/s in another, depending on the installation.<\/p>\n\n <h3>Mistake 3: Over-Specifying the Grade<\/h3>\n <p>Specifying G 1.0 for a slow-speed fan that only needs G 6.3 wastes time and money. Tighter grades require more balancing iterations, more precise equipment, and longer balancing times. Specify the grade appropriate to the application \u2014 better balance than needed provides diminishing returns while increasing cost.<\/p>\n\n <h3>Mistake 4: Applying Total Tolerance to Each Plane<\/h3>\n <p>As noted above, U<sub>per<\/sub> is the <strong>total<\/strong> tolerance for the rotor. For two-plane balancing, divide by 2 (or distribute proportionally for asymmetric rotors). Applying U<sub>per<\/sub> to each plane doubles the actual total tolerance, potentially exceeding the intended grade.<\/p>\n\n <h3>Mistake 5: Ignoring Temperature and Assembly Changes<\/h3>\n <p>Some rotors change balance state between cold (ambient) and hot (operating) conditions due to thermal distortion, centrifugal growth, or fit changes. A rotor that meets G 2.5 on the balancing machine at room temperature may exceed this tolerance at operating temperature. For critical rotors, high-speed balancing at or near operating conditions is recommended.<\/p>\n\n <h3>Mistake 6: Neglecting Key and Keyway Convention<\/h3>\n <p>ISO 21940-11 specifies that the half-key convention should be used when balancing a rotor with a keyway (add a half-key to the keyway during balancing to approximate the installed condition). Using a full key, no key, or ignoring this convention introduces an initial unbalance error that may be significant for tight G-grades.<\/p>\n\n <h2>Why G-Grades Matter \u2014 The Business Case<\/h2>\n <p>Proper application of G-grades delivers measurable benefits:<\/p>\n <ul>\n <li><strong>Bearing life:<\/strong> Bearing L<sub>10<\/sub> life is proportional to (C\/P)\u00b3 where P includes the unbalance force. Reducing unbalance by half can increase bearing life by up to 8\u00d7 (2\u00b3 = 8). This translates directly to reduced maintenance costs and downtime.<\/li>\n <li><strong>Energy efficiency:<\/strong> <a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\">Unbalance<\/a>-induced vibration dissipates energy as heat in bearings, seals, and dampers. Well-balanced rotors run cooler and consume less power \u2014 typically 1\u20133% energy savings on industrial motors.<\/li>\n <li><strong>Noise reduction:<\/strong> Vibration from unbalance transmits through the structure and radiates as noise. Meeting the correct G-grade is often the most cost-effective way to comply with workplace noise regulations.<\/li>\n <li><strong>Standardization and interoperability:<\/strong> The G-grade system ensures that a rotor balanced by Manufacturer A meets the same quality standard as one balanced by Manufacturer B \u2014 essential for global supply chains and interchangeable components.<\/li>\n <li><strong>Regulatory compliance:<\/strong> Many industries require documented evidence of balance quality for insurance, warranty, and safety certification. The G-grade provides a universally recognized documentation standard.<\/li>\n <\/ul>\n\n <div class=\"info-box success\">\n <div class=\"box-title\">Practical Balancing Equipment for G-Grade Compliance<\/div>\n <p>The <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> portable balancer includes a built-in ISO 1940 \/ ISO 21940-11 tolerance calculator. Enter the rotor mass, service speed, and desired G-grade \u2014 the software automatically calculates U<sub>per<\/sub>, distributes the tolerance between planes, and provides a clear pass\/fail indication after each balancing run. The <a href=\"https:\/\/vibromera.eu\/product\/balanset-4\/\">Balanset-4<\/a> extends this capability to four-channel measurement for complex balancing setups.<\/p>\n <\/div>\n\n <hr style=\"margin: 48px 0 24px; border: none; border-top: 1px solid var(--border-light);\">\n <p><a href=\"https:\/\/vibromera.eu\/glossary\/\">\u2190 Back to Glossary Index<\/a><\/p>\n\n <\/article>\n\n \n <aside class=\"toc-sidebar\">\n <div class=\"toc-box\">\n <h3>On This Page<\/h3>\n <a href=\"#calculator\">Tolerance Calculator<\/a>\n <a href=\"#grade-overview\">G-Grade Overview Cards<\/a>\n <a href=\"#iso-table\">Full ISO Table<\/a>\n <a href=\"#definition\">Definition<\/a>\n <a href=\"#purpose\">Purpose of G-Grades<\/a>\n <a href=\"#formulas\">Formulas<\/a>\n <a class=\"sub\" href=\"#formulas\">e<sub>per<\/sub> (eccentricity)<\/a>\n <a class=\"sub\" href=\"#formulas\">U<sub>per<\/sub> (tolerance)<\/a>\n <a class=\"sub\" href=\"#formulas\">Centrifugal force<\/a>\n <a href=\"#selection\">Grade Selection<\/a>\n <a class=\"sub\" href=\"#selection\">Industry recommendations<\/a>\n <a href=\"#two-plane\">Two-Plane Distribution<\/a>\n <a href=\"#examples\">Worked Examples<\/a>\n <a class=\"sub\" href=\"#examples\">Pump impeller<\/a>\n <a class=\"sub\" href=\"#examples\">Industrial fan<\/a>\n <a class=\"sub\" href=\"#examples\">Turbocharger<\/a>\n <a href=\"#standards\">Standards History<\/a>\n <a href=\"#mistakes\">6 Common Mistakes<\/a>\n <a href=\"#faq\">FAQ (8 Questions)<\/a>\n <\/div>\n\n <div class=\"toc-box\" style=\"margin-top: 24px; background: var(--navy); border-color: var(--navy);\">\n <h3 style=\"color: var(--white);\">Vibromera Equipment<\/h3>\n <p style=\"color: rgba(255,255,255,0.6); font-size: 13px; margin-bottom: 12px;\">Portable balancing with built-in ISO G-grade tolerance calculator.<\/p>\n <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\" style=\"display: block; padding: 8px 14px; background: var(--blue); color: white; border-radius: 6px; text-align: center; font-weight: 600; font-size: 14px; text-decoration: none; margin-bottom: 8px; border-left: none;\">Balanset-1A \u2192<\/a>\n <a href=\"https:\/\/vibromera.eu\/product\/balanset-4\/\" style=\"display: block; padding: 8px 14px; background: rgba(255,255,255,0.1); color: white; border-radius: 6px; text-align: center; font-weight: 600; font-size: 14px; text-decoration: none; border: 1px solid rgba(255,255,255,0.2); border-left: none;\">Balanset-4 \u2192<\/a>\n <\/div>\n <\/aside>\n <\/div>\n <\/div>\n<\/main>\n\n\n<section class=\"grades-section\" id=\"faq\" style=\"background: var(--white); border-top: 1px solid var(--border-light);\">\n <div class=\"container\" style=\"max-width:900px;\">\n <div class=\"section-header\">\n <h2>Frequently Asked Questions \u2014 Balance Quality Grades<\/h2>\n <p>Common questions about G-grades, ISO 1940, and balancing tolerances<\/p>\n <\/div>\n\n <div style=\"display:flex;flex-direction:column;gap:16px;\">\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is the most commonly used Balance Quality Grade?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n <strong>G 6.3<\/strong> is by far the most widely specified grade worldwide. It applies to the majority of general industrial machinery: electric motors, pump impellers, fans, blowers, flywheels, and process plant equipment. It provides a practical balance between manufacturing cost and vibration performance for equipment operating at typical industrial speeds (750\u20133600 RPM).\n <\/div>\n <\/details>\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is the difference between ISO 1940-1 and ISO 21940-11?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n <strong>ISO 21940-11:2016<\/strong> supersedes <strong>ISO 1940-1:2003<\/strong>. Both define exactly the same G-grade system \u2014 the G values and application tables are identical. ISO 21940-11 is part of the comprehensive ISO 21940 series covering all aspects of rotor balancing (terminology, procedures, tolerances, machines, instrumentation). The change is primarily organizational and editorial, not technical. In practice, most engineers still say \"ISO 1940\" when referring to G-grades, and both references are universally understood.\n <\/div>\n <\/details>\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> Does the G-Grade equal machine vibration level?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n <strong>No.<\/strong> This is one of the most common misconceptions. The G value is a property of the <em>rotor alone<\/em> \u2014 it defines the permissible unbalance of the free rotor, not the vibration of the installed machine. G 6.3 does NOT mean the machine will vibrate at 6.3 mm\/s. The actual installed vibration depends on bearing condition, <a href=\"https:\/\/vibromera.eu\/glossary\/misalignment\/\">alignment<\/a>, structural <a href=\"https:\/\/vibromera.eu\/glossary\/natural-frequency\/\">natural frequencies<\/a>, foundation stiffness, and damping. A rotor balanced to G 6.3 might produce anywhere from 0.5 to 5 mm\/s in different machines.\n <\/div>\n <\/details>\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> How do you calculate permissible residual unbalance?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n Use the formula: <strong>U<sub>per<\/sub> (g\u00b7mm) = (9549 \u00d7 G \u00d7 m) \/ n<\/strong>, where G is the grade (mm\/s), m is rotor mass (kg), and n is maximum service speed (RPM). Example: a 25 kg rotor at 3000 RPM, grade G 6.3 \u2192 U<sub>per<\/sub> = (9549 \u00d7 6.3 \u00d7 25) \/ 3000 = <strong>502 g\u00b7mm<\/strong> total. For two-plane balancing, each plane gets 502 \/ 2 = <strong>251 g\u00b7mm<\/strong>. The <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> software performs this calculation automatically.\n <\/div>\n <\/details>\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What G-Grade for pumps, fans, and electric motors?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n ISO 21940-11 recommends <strong>G 6.3<\/strong> for standard industrial pumps, fans, blowers, and general electric motors. Critical service pumps per API 610 often require <strong>G 2.5<\/strong>. HVAC fans in noise-sensitive locations (hospitals, offices) may also warrant G 2.5. Large, slow fans (<600 RPM) can sometimes use G 16. Turbine-driven pumps and high-speed compressors: G 2.5 or G 1.0.\n <\/div>\n <\/details>\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> Should I use balancing speed or operating speed in the formula?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n <strong>Always use the maximum operating (service) speed<\/strong> \u2014 the highest speed at which the rotor will run in actual service. This is a common and dangerous mistake: many rotors are balanced on machines at lower speeds than they will operate in the field. Using the balancing speed produces a tolerance that is too loose. For example, if a rotor operates at 3600 RPM but is balanced at 600 RPM, the tolerance calculated at 600 RPM would be 6\u00d7 too generous.\n <\/div>\n <\/details>\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> Can I balance in the field to an ISO G-Grade?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n Yes. Modern portable balancing equipment like the <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> enables field balancing to ISO G-grade standards without removing the rotor from the machine. The device measures vibration amplitude and phase, and its built-in software calculates the required correction weights for single-plane or two-plane <a href=\"https:\/\/vibromera.eu\/glossary\/rotor-balancing\/\">dynamic balancing<\/a>. It includes an automatic ISO 1940 \/ ISO 21940-11 tolerance calculator with pass\/fail indication.\n <\/div>\n <\/details>\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What about balancing quality for flexible rotors?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n The G-grade system (ISO 21940-11, formerly ISO 1940-1) applies specifically to <strong>rigid rotors<\/strong> \u2014 rotors that operate well below their first <a href=\"https:\/\/vibromera.eu\/glossary\/critical-speed\/\">critical speed<\/a>. Flexible rotors (those operating above or near their first critical speed, such as large turbogenerators) require modal balancing techniques covered by <strong>ISO 21940-12<\/strong>. The G-grade can still be applied as an initial low-speed balance target, but additional high-speed balancing runs at or near operating speed are necessary.\n <\/div>\n <\/details>\n\n <\/div>\n <\/div>\n<\/section>\n\n\n<section style=\"padding:32px 0;background:var(--beige-light);border-top:1px solid var(--border-light);\">\n <div class=\"container\" style=\"max-width:900px;\">\n <h3 style=\"font-family:'DM Serif Display',serif;font-size:22px;color:var(--navy);margin-bottom:20px;\">Related Glossary Articles<\/h3>\n <div style=\"display:grid;grid-template-columns:repeat(3,1fr);gap:16px;\">\n <a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\" style=\"display:block;padding:18px;background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius-sm);text-decoration:none;transition:all 0.2s;box-shadow:var(--shadow-sm);\">\n <div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Unbalance (Imbalance)<\/div>\n <div style=\"font-size:13px;color:var(--text-secondary);\">Definition, types (static, couple, dynamic), causes, and measurement methods<\/div>\n <\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/rotor-balancing\/\" style=\"display:block;padding:18px;background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius-sm);text-decoration:none;transition:all 0.2s;box-shadow:var(--shadow-sm);\">\n <div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Rotor Balancing<\/div>\n <div style=\"font-size:13px;color:var(--text-secondary);\">Single-plane vs. two-plane dynamic balancing procedures and equipment<\/div>\n <\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/natural-frequency\/\" style=\"display:block;padding:18px;background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius-sm);text-decoration:none;transition:all 0.2s;box-shadow:var(--shadow-sm);\">\n <div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Natural Frequency<\/div>\n <div style=\"font-size:13px;color:var(--text-secondary);\">Resonance, critical speeds, and why they matter for balanced rotors<\/div>\n <\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/bearing-fault-frequencies\/\" style=\"display:block;padding:18px;background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius-sm);text-decoration:none;transition:all 0.2s;box-shadow:var(--shadow-sm);\">\n <div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Bearing Fault Frequencies<\/div>\n <div style=\"font-size:13px;color:var(--text-secondary);\">BPFO, BPFI, BSF, FTF calculator \u2014 how unbalance damages bearings<\/div>\n <\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/harmonics-vibration\/\" style=\"display:block;padding:18px;background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius-sm);text-decoration:none;transition:all 0.2s;box-shadow:var(--shadow-sm);\">\n <div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Harmonics in Vibration<\/div>\n <div style=\"font-size:13px;color:var(--text-secondary);\">1\u00d7, 2\u00d7, 3\u00d7 patterns and what they reveal about machinery faults<\/div>\n <\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/fft\/\" style=\"display:block;padding:18px;background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius-sm);text-decoration:none;transition:all 0.2s;box-shadow:var(--shadow-sm);\">\n <div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">FFT Spectrum Analysis<\/div>\n <div style=\"font-size:13px;color:var(--text-secondary);\">How frequency analysis is used to verify balance quality and diagnose faults<\/div>\n <\/a>\n <\/div>\n <\/div>\n<\/section>\n\n\n<section class=\"shop-cta\">\n <div class=\"container\">\n <h2>Achieve ISO Balance Quality \u2014 In the Field<\/h2>\n <p>Vibromera's portable balancing devices calculate G-grade tolerances automatically and guide you to precise correction weights \u2014 no rotor removal required.<\/p>\n <a href=\"https:\/\/vibromera.eu\/shop\/\" class=\"cta-btn\">Browse Balancing Equipment \u2192<\/a>\n <\/div>\n<\/section>\n\n\n<footer class=\"page-footer\">\n <div class=\"container\">\n <p><a href=\"https:\/\/vibromera.eu\/glossary\/\">\u2190 Back to Glossary<\/a> | <a href=\"https:\/\/vibromera.eu\/\">vibromera.eu<\/a><\/p>\n <\/div>\n<\/footer>\n\n<script>\nfunction calculate() {\n const G = parseFloat(document.getElementById('gGrade').value);\n const m = parseFloat(document.getElementById('rotorMass').value);\n const n = parseFloat(document.getElementById('rotorRPM').value);\n const planes = parseInt(document.getElementById('numPlanes').value);\n const R = parseFloat(document.getElementById('corrRadius').value) || 0;\n\n if (!m || !n || m <= 0 || n <= 0) return alert('Enter valid rotor mass and speed.');\n\n const e_per = (9549 * G) \/ n; \/\/ \u00b5m\n const U_per = e_per * m; \/\/ g\u00b7mm\n const U_plane = U_per \/ planes; \/\/ g\u00b7mm per plane\n const omega = 2 * Math.PI * n \/ 60; \/\/ rad\/s\n const F = (U_per \/ 1e6) * omega * omega; \/\/ N (U in kg\u00b7m: g\u00b7mm \/ 1e6)\n\n let corrWeight = 0;\n if (R > 0) corrWeight = U_plane \/ R; \/\/ grams\n\n const area = document.getElementById('resultsArea');\n area.innerHTML = `\n <div class=\"result-primary\">\n <div class=\"res-label\">Total Permissible Unbalance (U<sub>per<\/sub>)<\/div>\n <div class=\"res-value\">${U_per >= 100 ? U_per.toFixed(0) : U_per >= 1 ? U_per.toFixed(1) : U_per.toFixed(3)}<\/div>\n <div class=\"res-unit\">g\u00b7mm<\/div>\n <\/div>\n <div class=\"results-secondary\">\n <div class=\"res-card\">\n <div class=\"rc-label\">Specific Unbalance (e<sub>per<\/sub>)<\/div>\n <div class=\"rc-value\">${e_per >= 10 ? e_per.toFixed(1) : e_per >= 0.1 ? e_per.toFixed(2) : e_per.toFixed(3)}<\/div>\n <div class=\"rc-unit\">\u00b5m<\/div>\n <\/div>\n <div class=\"res-card\">\n <div class=\"rc-label\">Centrifugal Force<\/div>\n <div class=\"rc-value\">${F >= 10 ? F.toFixed(1) : F >= 0.1 ? F.toFixed(2) : F.toFixed(3)}<\/div>\n <div class=\"rc-unit\">N at ${n} RPM<\/div>\n <\/div>\n <div class=\"res-card\">\n <div class=\"rc-label\">Grade<\/div>\n <div class=\"rc-value\" style=\"font-size:18px\">G ${G}<\/div>\n <div class=\"rc-unit\">ISO 21940-11<\/div>\n <\/div>\n <div class=\"res-card\">\n <div class=\"rc-label\">Rotor<\/div>\n <div class=\"rc-value\" style=\"font-size:14px; font-family:'Source Sans 3',sans-serif\">${m} kg @ ${n} RPM<\/div>\n <div class=\"rc-unit\">${planes} plane${planes > 1 ? 's' : ''}<\/div>\n <\/div>\n <\/div>\n <div class=\"plane-note\">\n <div class=\"pn-label\">Per Plane Tolerance (${planes === 1 ? 'single plane' : 'each of ' + planes + ' planes'})<\/div>\n <div class=\"pn-value\">${U_plane >= 100 ? U_plane.toFixed(0) : U_plane >= 1 ? U_plane.toFixed(1) : U_plane.toFixed(3)}<\/div>\n <div class=\"pn-unit\">g\u00b7mm per plane${corrWeight > 0 ? ' = ' + (corrWeight >= 0.1 ? corrWeight.toFixed(2) : corrWeight.toFixed(3)) + ' g at R=' + R + ' mm' : ''}<\/div>\n <\/div>\n `;\n}\n\ncalculate();\n<\/script>\n\n<\/body>\n<\/html><\/div><\/div><\/div><\/div><\/div>","protected":false},"excerpt":{"rendered":"<p>ISO 1940-1 \u09a6\u09cd\u09ac\u09be\u09b0\u09be \u09b8\u0982\u099c\u09cd\u099e\u09be\u09af\u09bc\u09bf\u09a4 \u09ad\u09be\u09b0\u09b8\u09be\u09ae\u09cd\u09af \u09ae\u09be\u09a8\u09c7\u09b0 \u0997\u09cd\u09b0\u09c7\u09a1 (\u099c\u09bf-\u0997\u09cd\u09b0\u09c7\u09a1) \u09ac\u09cd\u09af\u09be\u0996\u09cd\u09af\u09be\u0964 G2.5 \u098f\u09ac\u0982 G6.3 \u098f\u09b0 \u09ae\u09a4\u09cb \u0997\u09cd\u09b0\u09c7\u09a1\u0997\u09c1\u09b2\u09bf \u0995\u09c0\u09ad\u09be\u09ac\u09c7 \u09b0\u09cb\u099f\u09b0 \u09ad\u09be\u09b0\u09b8\u09be\u09ae\u09cd\u09af \u09b8\u09b9\u09a8\u09b6\u09c0\u09b2\u09a4\u09be \u09a8\u09bf\u09b0\u09cd\u09a6\u09bf\u09b7\u09cd\u099f \u0995\u09b0\u09a4\u09c7 \u09ac\u09cd\u09af\u09ac\u09b9\u09c3\u09a4 \u09b9\u09af\u09bc \u09a4\u09be \u09b6\u09bf\u0996\u09c1\u09a8\u0964<\/p>","protected":false},"featured_media":0,"template":"","meta":{"ai_generated_summary":"","footnotes":""},"categories":[109,112],"tags":[],"class_list":["post-20631","glossary","type-glossary","status-publish","hentry","category-glossary","category-iso-standards"],"_links":{"self":[{"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/glossary\/20631","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/glossary"}],"about":[{"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/types\/glossary"}],"version-history":[{"count":6,"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/glossary\/20631\/revisions"}],"predecessor-version":[{"id":101729,"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/glossary\/20631\/revisions\/101729"}],"wp:attachment":[{"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/media?parent=20631"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/categories?post=20631"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/tags?post=20631"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}

\n\n\n\n\nBalance Quality Grade (G-Grade) \u2014 ISO 1940-1 \/ ISO 21940-11 \u2014 Vibromera<\/title>\n<meta name=\"description\" content=\"Complete guide to Balance Quality Grades (G-Grades) per ISO 1940-1 and ISO 21940-11. Interactive calculator for permissible residual unbalance. G0.4 to G4000 reference tables, worked examples, and industry recommendations for pumps, fans, turbines.\">\n<meta name=\"keywords\" content=\"balance quality grade, G-grade, ISO 1940, ISO 21940-11, permissible residual unbalance, balancing tolerance, G 6.3, G 2.5, G 1.0, rotor balancing, dynamic balancing, vibration, Balanset, balancing calculator\">\n<meta name=\"author\" content=\"Vibromera\">\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large\">\n\n\n\n\n\n\n\n\n\n\n\n<meta property=\"og:type\" content=\"article\">\n<meta property=\"og:title\" content=\"Balance Quality Grade (G-Grade) \u2014 ISO 1940 \/ ISO 21940-11 \u2014 Complete Guide\">\n<meta property=\"og:description\" content=\"What is a Balance Quality Grade? Interactive calculator, full ISO G-grade table (G0.4\u2013G4000), formulas for permissible residual unbalance, worked examples, and industry recommendations.\">\n<meta property=\"og:url\" content=\"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/\">\n<meta property=\"og:site_name\" content=\"Vibromera \u2014 Vibration Analysis & Balancing Equipment\">\n<meta property=\"og:locale\" content=\"en_US\">\n<meta property=\"og:image\" content=\"https:\/\/vibromera.eu\/wp-content\/uploads\/balance-quality-grade-og.jpg\">\n<meta property=\"article:publisher\" content=\"https:\/\/vibromera.eu\/\">\n<meta property=\"article:section\" content=\"Glossary\">\n<meta property=\"article:tag\" content=\"Balance Quality Grade\">\n<meta property=\"article:tag\" content=\"ISO 1940\">\n<meta property=\"article:tag\" content=\"Rotor Balancing\">\n\n\n<meta name=\"twitter:card\" content=\"summary_large_image\">\n<meta name=\"twitter:title\" content=\"Balance Quality Grade (G-Grade) \u2014 ISO 1940 \/ ISO 21940-11\">\n<meta name=\"twitter:description\" content=\"Interactive calculator for permissible residual unbalance per ISO 21940-11. Full G-grade table, formulas, worked examples.\">\n\n\n<meta name=\"geo.region\" content=\"EU\">\n<meta name=\"geo.placename\" content=\"Porto, Portugal\">\n<meta name=\"geo.position\" content=\"41.1579;-8.6291\">\n<meta name=\"ICBM\" content=\"41.1579, -8.6291\">\n\n\n<script type=\"application\/ld+json\">\n{\n \"@context\": \"https:\/\/schema.org\",\n \"@graph\": [\n {\n \"@type\": \"TechArticle\",\n \"@id\": \"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/#article\",\n \"headline\": \"Balance Quality Grade (G-Grade): ISO 1940-1 \/ ISO 21940-11 \u2014 Complete Guide\",\n \"description\": \"Comprehensive guide to Balance Quality Grades (G-Grades) per ISO 1940-1 and ISO 21940-11. Interactive calculator, reference tables, formulas, and worked examples.\",\n \"author\": {\n \"@type\": \"Organization\",\n \"name\": \"Vibromera\",\n \"url\": \"https:\/\/vibromera.eu\/\"\n },\n \"publisher\": {\n \"@type\": \"Organization\",\n \"name\": \"Vibromera\",\n \"url\": \"https:\/\/vibromera.eu\/\",\n \"logo\": {\n \"@type\": \"ImageObject\",\n \"url\": \"https:\/\/vibromera.eu\/wp-content\/uploads\/vibromera-logo.png\"\n }\n },\n \"mainEntityOfPage\": \"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/\",\n \"datePublished\": \"2024-01-15\",\n \"dateModified\": \"2026-02-07\",\n \"inLanguage\": \"en\",\n \"about\": [\n {\"@type\": \"Thing\", \"name\": \"Balance Quality Grade\"},\n {\"@type\": \"Thing\", \"name\": \"ISO 1940-1\"},\n {\"@type\": \"Thing\", \"name\": \"ISO 21940-11\"},\n {\"@type\": \"Thing\", \"name\": \"Rotor Balancing\"},\n {\"@type\": \"Thing\", \"name\": \"Residual Unbalance\"}\n ],\n \"isPartOf\": {\n \"@type\": \"WebPage\",\n \"url\": \"https:\/\/vibromera.eu\/glossary\/\"\n }\n },\n {\n \"@type\": \"FAQPage\",\n \"@id\": \"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/#faq\",\n \"mainEntity\": [\n {\n \"@type\": \"Question\",\n \"name\": \"What is a Balance Quality Grade (G-Grade)?\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"A Balance Quality Grade (G-Grade) is a standardized classification per ISO 21940-11 (formerly ISO 1940-1) that defines the maximum permissible residual unbalance for a rigid rotor. The G number represents the maximum velocity of the rotor's center-of-gravity displacement in mm\/s. For example, G 6.3 limits this velocity to 6.3 mm\/s at maximum service speed. Lower G values mean tighter tolerance and higher precision.\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"How do you calculate permissible residual unbalance from a G-Grade?\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"Use the formula: U_per (g\u00b7mm) = (9549 \u00d7 G \u00d7 m) \/ n, where G is the grade in mm\/s, m is rotor mass in kg, and n is maximum service speed in RPM. For example, a 25 kg rotor at 3000 RPM with G 6.3: U_per = (9549 \u00d7 6.3 \u00d7 25) \/ 3000 = 502 g\u00b7mm total, or 251 g\u00b7mm per plane for two-plane balancing.\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"What is the difference between ISO 1940-1 and ISO 21940-11?\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"ISO 21940-11:2016 supersedes ISO 1940-1:2003. Both define the same G-grade system with identical values and application tables. ISO 21940-11 is part of the comprehensive ISO 21940 series covering all aspects of rotor balancing. The G-grade values (G 0.4 through G 4000) and their recommended applications remain unchanged.\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"What G-Grade should I use for a pump or fan?\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"For most industrial pumps, fans, blowers, and general electric motors, G 6.3 is the standard recommendation per ISO 21940-11. For high-speed or critical service pumps (API 610), G 2.5 is often specified. HVAC fans in noise-sensitive environments may also require G 2.5.\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"What is the most commonly used Balance Quality Grade?\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"G 6.3 is the most widely specified balance quality grade worldwide. It applies to general industrial machinery including electric motors, pump impellers, fans, blowers, flywheels, and process plant equipment. It provides a good balance between manufacturing cost and vibration performance.\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"Does G-Grade equal machine vibration level?\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"No. The G value is a property of the rotor alone, not the installed machine. G 6.3 does NOT mean the machine will vibrate at 6.3 mm\/s. Installed vibration depends on many additional factors: bearing condition, alignment, natural frequencies, structural stiffness, and damping. A rotor balanced to G 6.3 may produce 1 mm\/s vibration in one machine and 4 mm\/s in another.\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"How do you distribute unbalance tolerance between two correction planes?\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"For symmetric rotors (center of gravity at midspan), divide U_per equally: each plane gets U_per\/2. For asymmetric rotors, distribute proportionally to bearing distances from the center of gravity. 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solid var(--blue); border-radius: 0 var(--radius-sm) var(--radius-sm) 0; padding: 20px 24px; margin: 24px 0; }\n.info-box.warning { background: #fffbeb; border-left-color: var(--warning); }\n.info-box.success { background: #ecfdf5; border-left-color: var(--success); }\n.info-box.danger { background: #fef2f2; border-left-color: var(--danger); }\n.info-box .box-title { font-weight: 700; font-size: 15px; margin-bottom: 6px; color: var(--navy); }\n.info-box p { margin-bottom: 8px; font-size: 14px; }\n.info-box p:last-child { margin-bottom: 0; }\n\n.example-block { background: var(--beige-light); border: 1px solid var(--beige-dark); border-radius: var(--radius); padding: 28px; margin: 24px 0; }\n.example-block .example-title { font-family: 'DM Serif Display', serif; font-size: 18px; color: var(--navy); margin-bottom: 16px; }\n\n.formula-box {\n background: var(--navy); color: var(--white); border-radius: var(--radius); padding: 26px 32px; margin: 24px 0;\n text-align: center; position: 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var(--navy-light) 100%); padding: 48px 0; color: var(--white);\n text-align: center; position: relative; overflow: hidden;\n}\n.shop-cta::before { content: ''; position: absolute; inset: 0; background: radial-gradient(circle at 50% 50%, rgba(37,99,235,0.15) 0%, transparent 60%); pointer-events: none; }\n.shop-cta h2 { font-family: 'DM Serif Display', serif; font-size: 32px; margin-bottom: 12px; position: relative; }\n.shop-cta p { font-size: 17px; color: rgba(255,255,255,0.7); margin-bottom: 28px; max-width: 640px; margin-left: auto; margin-right: auto; position: relative; }\n.shop-cta .cta-btn {\n display: inline-flex; align-items: center; gap: 8px; padding: 14px 36px; background: var(--blue); color: var(--white);\n text-decoration: none; border-radius: var(--radius-sm); font-weight: 600; font-size: 16px; transition: all 0.2s; position: relative;\n}\n.shop-cta .cta-btn:hover { background: var(--blue-light); transform: translateY(-2px); box-shadow: 0 8px 24px rgba(37,99,235,0.3); }\n\n.page-footer { background: var(--beige); border-top: 1px solid var(--beige-dark); padding: 32px 0; text-align: center; }\n.page-footer a { color: var(--blue); text-decoration: none; font-weight: 500; }\n.page-footer p { font-size: 14px; color: var(--text-muted); }\n\n@media print { .quick-nav, .shop-cta { display: none; } body { min-width: auto; } }\n<\/style>\n<\/head>\n<body>\n\n\n<header class=\"hero\">\n <div class=\"hero-inner\">\n <div class=\"breadcrumb\">\n <a href=\"https:\/\/vibromera.eu\/\">Home<\/a> \u2192 <a href=\"https:\/\/vibromera.eu\/glossary\/\">Glossary<\/a> \u2192 Balance Quality Grade\n <\/div>\n <h1>Balance Quality Grade <span>(G-Grade)<\/span><\/h1>\n <div style=\"display:inline-flex;align-items:center;gap:6px;padding:4px 12px;background:rgba(37,99,235,0.2);border:1px solid rgba(37,99,235,0.3);border-radius:20px;font-size:12px;font-weight:600;letter-spacing:0.5px;color:rgba(255,255,255,0.85);margin-bottom:12px;text-transform:uppercase;\">\n <span style=\"font-size:14px;\">\ud83d\udccc<\/span> Canonical Reference Article \u2014 vibromera.eu\n <\/div>\n <p class=\"subtitle\">The international standard for rotor balancing precision \u2014 how ISO 1940-1 and ISO 21940-11 G-grades define permissible residual unbalance, why they matter for bearing life and machine reliability, and how to calculate tolerances for any rotor.<\/p>\n <\/div>\n<\/header>\n\n\n<nav class=\"quick-nav\">\n <div class=\"quick-nav-inner\">\n <a href=\"#calculator\">\u2699 Tolerance Calculator<\/a>\n <a href=\"#grade-overview\">\ud83d\udcca G-Grade Overview<\/a>\n <a href=\"#iso-table\">\ud83d\udccb Full ISO Table<\/a>\n <a href=\"#definition\">\ud83d\udcd0 Definition<\/a>\n <a href=\"#purpose\">\ud83c\udfaf Purpose<\/a>\n <a href=\"#formulas\">\ud83d\udd22 Formulas<\/a>\n <a href=\"#selection\">\ud83d\udd0d Grade Selection<\/a>\n <a href=\"#two-plane\">\u2696 Two-Plane Balancing<\/a>\n <a href=\"#examples\">\ud83d\udcdd Worked Examples<\/a>\n <a href=\"#standards\">\ud83d\udcda Standards Comparison<\/a>\n <a href=\"#mistakes\">\u26a0 Common Mistakes<\/a>\n <a href=\"#faq\">\u2753 FAQ<\/a>\n <\/div>\n<\/nav>\n\n\n<section class=\"summary-dashboard\" id=\"calculator\">\n <div class=\"container\">\n <div class=\"dashboard-grid\">\n\n <div class=\"calc-panel\">\n <h2 class=\"panel-title\">Balancing Tolerance Calculator<\/h2>\n <p class=\"panel-subtitle\">Calculate permissible residual unbalance per ISO 21940-11 \/ ISO 1940-1<\/p>\n <div class=\"calc-form\">\n <div class=\"form-group full-width\">\n <label>Balance Quality Grade (G)<\/label>\n <select id=\"gGrade\" onchange=\"calculate()\">\n <option value=\"0.4\">G 0.4 \u2014 Gyroscopes, spindles, HDD<\/option>\n <option value=\"1.0\">G 1.0 \u2014 Grinding spindles, turbochargers<\/option>\n <option value=\"2.5\" selected>G 2.5 \u2014 Turbines, turbocompressors, high-speed motors<\/option>\n <option value=\"6.3\">G 6.3 \u2014 Pumps, fans, motors, general machinery<\/option>\n <option value=\"16\">G 16 \u2014 Drive shafts, crushers, agricultural<\/option>\n <option value=\"40\">G 40 \u2014 Car wheels, crankshafts (slow)<\/option>\n <option value=\"100\">G 100 \u2014 Crankshaft assemblies (rigid)<\/option>\n <option value=\"250\">G 250 \u2014 Slow cement machinery<\/option>\n <option value=\"630\">G 630 \u2014 Coarse reciprocating<\/option>\n <option value=\"4000\">G 4000 \u2014 Crank drives (inherently unbalanced)<\/option>\n <\/select>\n <\/div>\n <div class=\"form-group\">\n <label>Rotor Mass <span class=\"unit\">(kg)<\/span><\/label>\n <input type=\"number\" id=\"rotorMass\" value=\"25\" min=\"0.001\" step=\"0.1\">\n <\/div>\n <div class=\"form-group\">\n <label>Max Operating Speed <span class=\"unit\">(RPM)<\/span><\/label>\n <input type=\"number\" id=\"rotorRPM\" value=\"3000\" min=\"1\" step=\"1\">\n <\/div>\n <div class=\"form-group\">\n <label>Number of Correction Planes<\/label>\n <select id=\"numPlanes\">\n <option value=\"1\">1 plane (static balancing)<\/option>\n <option value=\"2\" selected>2 planes (dynamic balancing)<\/option>\n <\/select>\n <\/div>\n <div class=\"form-group\">\n <label>Correction Radius <span class=\"unit\">(mm, optional)<\/span><\/label>\n <input type=\"number\" id=\"corrRadius\" value=\"\" min=\"0.1\" step=\"0.1\" placeholder=\"e.g. 150\">\n <\/div>\n <button class=\"calc-btn\" onclick=\"calculate()\">Calculate Tolerance \u2192<\/button>\n <\/div>\n <\/div>\n\n <div class=\"results-panel\">\n <h2 class=\"panel-title\">Results<\/h2>\n <p class=\"panel-subtitle\">Permissible residual unbalance and balancing targets<\/p>\n <div id=\"resultsArea\">\n <div class=\"results-empty\">\n <span class=\"icon\">\u2696\ufe0f<\/span>\n Enter rotor parameters and click Calculate<br>to see balancing tolerances\n <\/div>\n <\/div>\n <\/div>\n\n <\/div>\n <\/div>\n<\/section>\n\n\n<section class=\"grades-section\" id=\"grade-overview\">\n <div class=\"container\">\n <div class=\"section-header\">\n <h2>Balance Quality Grades at a Glance<\/h2>\n <p>From ultra-precision gyroscopes (G 0.4) to coarse reciprocating engines (G 4000) \u2014 the complete ISO classification<\/p>\n <\/div>\n\n <div class=\"grade-cards\">\n <div class=\"grade-card ultra\">\n <div class=\"gc-grade\">G 0.4<\/div>\n <div class=\"gc-vel\">\u2264 0.4 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>Ultra-precision:<\/strong> Gyroscopes, precision spindles, HDD platters, satellite components, microelectronics manufacturing equipment<\/div>\n <\/div>\n <div class=\"grade-card ultra\">\n <div class=\"gc-grade\">G 1.0<\/div>\n <div class=\"gc-vel\">\u2264 1.0 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>Precision:<\/strong> Grinding machine drives, small high-speed motors, automotive turbochargers, dental\/medical equipment spindles<\/div>\n <\/div>\n <div class=\"grade-card high\">\n <div class=\"gc-grade\">G 2.5<\/div>\n <div class=\"gc-vel\">\u2264 2.5 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>High-quality:<\/strong> Gas\/steam turbines, turbocompressors, high-speed electric motors, machine tool drives, centrifuge rotors<\/div>\n <\/div>\n <div class=\"grade-card standard\">\n <div class=\"gc-grade\">G 6.3<\/div>\n <div class=\"gc-vel\">\u2264 6.3 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>Standard:<\/strong> Pump impellers, fans, blowers, general electric motors, flywheels, process machinery \u2014 the most commonly specified grade<\/div>\n <\/div>\n <div class=\"grade-card standard\">\n <div class=\"gc-grade\">G 16<\/div>\n <div class=\"gc-vel\">\u2264 16 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>Medium:<\/strong> Drive shafts (cardan), crushers, agricultural machinery, parts of process plant with moderate requirements<\/div>\n <\/div>\n <div class=\"grade-card general\">\n <div class=\"gc-grade\">G 40<\/div>\n <div class=\"gc-vel\">\u2264 40 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>General:<\/strong> Car wheels\/rims, crankshafts for slow marine engines, non-critical rotating assemblies<\/div>\n <\/div>\n <div class=\"grade-card general\">\n <div class=\"gc-grade\">G 100<\/div>\n <div class=\"gc-vel\">\u2264 100 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>Coarse:<\/strong> Complete crankshaft assemblies (rigidly mounted), slow agricultural machinery<\/div>\n <\/div>\n <div class=\"grade-card general\">\n <div class=\"gc-grade\">G 630+<\/div>\n <div class=\"gc-vel\">\u2264 630\u20134000 mm\/s<\/div>\n <div class=\"gc-apps\"><strong>Very coarse:<\/strong> Crankshaft drives of inherently unbalanced, slow reciprocating engines on rigid mounts<\/div>\n <\/div>\n <\/div>\n\n \n <div class=\"table-wrap\" id=\"iso-table\">\n <div class=\"table-title\">\ud83d\udccb Complete ISO 21940-11 \/ ISO 1940-1 \u2014 Balance Quality Grade Table<\/div>\n <table>\n <thead>\n <tr>\n <th>G-Grade<\/th>\n <th>e\u00b7\u03c9 (mm\/s)<\/th>\n <th>Precision Class<\/th>\n <th>Typical Rotor Types \/ Applications<\/th>\n <\/tr>\n <\/thead>\n <tbody>\n <tr><td class=\"mono\">G 4000<\/td><td class=\"mono\">4000<\/td><td><span class=\"tag coarse\">Very Coarse<\/span><\/td><td>Crankshaft drives of inherently unbalanced, rigidly mounted slow marine diesel engines<\/td><\/tr>\n <tr><td class=\"mono\">G 1600<\/td><td class=\"mono\">1600<\/td><td><span class=\"tag coarse\">Very Coarse<\/span><\/td><td>Crankshaft drives, rigidly mounted<\/td><\/tr>\n <tr><td class=\"mono\">G 630<\/td><td class=\"mono\">630<\/td><td><span class=\"tag coarse\">Coarse<\/span><\/td><td>Crankshaft drives of inherently unbalanced, elastically mounted engines<\/td><\/tr>\n <tr><td class=\"mono\">G 250<\/td><td class=\"mono\">250<\/td><td><span class=\"tag coarse\">Coarse<\/span><\/td><td>Crankshaft drives of fast 4-cylinder engines, elastically mounted<\/td><\/tr>\n <tr><td class=\"mono\">G 100<\/td><td class=\"mono\">100<\/td><td><span class=\"tag general\">General<\/span><\/td><td>Complete engines (gasoline\/diesel) for cars, trucks; crankshafts for rigidly mounted 6+ cylinder engines<\/td><\/tr>\n <tr><td class=\"mono\">G 40<\/td><td class=\"mono\">40<\/td><td><span class=\"tag general\">General<\/span><\/td><td>Car wheels; wheel rims; drive shafts; crankshafts, elastically mounted, of fast 4-cylinder engines<\/td><\/tr>\n <tr><td class=\"mono\">G 16<\/td><td class=\"mono\">16<\/td><td><span class=\"tag standard\">Standard<\/span><\/td><td>Drive shafts (cardan); parts of crushing machinery; parts of agricultural machinery; crankshafts, elastically mounted, of 6+ cylinder engines<\/td><\/tr>\n <tr><td class=\"mono\">G 6.3<\/td><td class=\"mono\">6.3<\/td><td><span class=\"tag standard\">Standard<\/span><\/td><td>Fans; flywheels; pump impellers; general machinery parts; normal electric motor rotors; process plant machinery<\/td><\/tr>\n <tr><td class=\"mono\">G 2.5<\/td><td class=\"mono\">2.5<\/td><td><span class=\"tag precision\">Precision<\/span><\/td><td>Gas and steam turbines; turbo-generators; turbocompressors; machine tool drives; medium and large electric motor rotors with special requirements<\/td><\/tr>\n <tr><td class=\"mono\">G 1.0<\/td><td class=\"mono\">1.0<\/td><td><span class=\"tag precision\">Precision<\/span><\/td><td>Grinding machine drives; small high-speed electric motors; turbochargers<\/td><\/tr>\n <tr><td class=\"mono\">G 0.4<\/td><td class=\"mono\">0.4<\/td><td><span class=\"tag ultra\">Ultra-precision<\/span><\/td><td>Gyroscopes; precision spindles; hard disk drives; ultra-high-speed spindles for microelectronics<\/td><\/tr>\n <\/tbody>\n <\/table>\n <\/div>\n\n \n <div class=\"table-wrap\">\n <div class=\"table-title\">\ud83d\udcca Quick-Reference \u2014 Pre-Calculated Tolerances for Common Scenarios<\/div>\n <table>\n <thead>\n <tr>\n <th>Rotor Type<\/th>\n <th>Mass (kg)<\/th>\n <th>Speed (RPM)<\/th>\n <th>Grade<\/th>\n <th>U<sub>per<\/sub> Total (g\u00b7mm)<\/th>\n <th>U<sub>per<\/sub> per Plane (g\u00b7mm)<\/th>\n <th>e<sub>per<\/sub> (\u00b5m)<\/th>\n <\/tr>\n <\/thead>\n <tbody>\n <tr><td>Small electric motor<\/td><td class=\"mono\">8<\/td><td class=\"mono\">2900<\/td><td class=\"mono\">G 6.3<\/td><td class=\"mono\">166<\/td><td class=\"mono\">83<\/td><td class=\"mono\">20.7<\/td><\/tr>\n <tr><td>Pump impeller<\/td><td class=\"mono\">12<\/td><td class=\"mono\">2950<\/td><td class=\"mono\">G 6.3<\/td><td class=\"mono\">245<\/td><td class=\"mono\">122<\/td><td class=\"mono\">20.4<\/td><\/tr>\n <tr><td>Industrial fan<\/td><td class=\"mono\">85<\/td><td class=\"mono\">1480<\/td><td class=\"mono\">G 6.3<\/td><td class=\"mono\">3459<\/td><td class=\"mono\">1730<\/td><td class=\"mono\">40.7<\/td><\/tr>\n <tr><td>Large motor rotor<\/td><td class=\"mono\">350<\/td><td class=\"mono\">1500<\/td><td class=\"mono\">G 2.5<\/td><td class=\"mono\">5578<\/td><td class=\"mono\">2789<\/td><td class=\"mono\">15.9<\/td><\/tr>\n <tr><td>Steam turbine<\/td><td class=\"mono\">1200<\/td><td class=\"mono\">3600<\/td><td class=\"mono\">G 2.5<\/td><td class=\"mono\">7958<\/td><td class=\"mono\">3979<\/td><td class=\"mono\">6.6<\/td><\/tr>\n <tr><td>Turbocharger<\/td><td class=\"mono\">0.8<\/td><td class=\"mono\">90000<\/td><td class=\"mono\">G 1.0<\/td><td class=\"mono\">0.085<\/td><td class=\"mono\">0.042<\/td><td class=\"mono\">0.11<\/td><\/tr>\n <tr><td>Grinding spindle<\/td><td class=\"mono\">5<\/td><td class=\"mono\">12000<\/td><td class=\"mono\">G 1.0<\/td><td class=\"mono\">3.98<\/td><td class=\"mono\">1.99<\/td><td class=\"mono\">0.80<\/td><\/tr>\n <tr><td>Crusher flywheel<\/td><td class=\"mono\">500<\/td><td class=\"mono\">600<\/td><td class=\"mono\">G 16<\/td><td class=\"mono\">127,320<\/td><td class=\"mono\">63,660<\/td><td class=\"mono\">254.6<\/td><\/tr>\n <tr><td>Drive shaft (cardan)<\/td><td class=\"mono\">15<\/td><td class=\"mono\">4500<\/td><td class=\"mono\">G 16<\/td><td class=\"mono\">509<\/td><td class=\"mono\">255<\/td><td class=\"mono\">33.9<\/td><\/tr>\n <tr><td>HVAC blower<\/td><td class=\"mono\">45<\/td><td class=\"mono\">1750<\/td><td class=\"mono\">G 6.3<\/td><td class=\"mono\">1546<\/td><td class=\"mono\">773<\/td><td class=\"mono\">34.4<\/td><\/tr>\n <tr><td>Car wheel assembly<\/td><td class=\"mono\">20<\/td><td class=\"mono\">900<\/td><td class=\"mono\">G 40<\/td><td class=\"mono\">8488<\/td><td class=\"mono\">4244<\/td><td class=\"mono\">424.4<\/td><\/tr>\n <tr><td>Centrifuge<\/td><td class=\"mono\">30<\/td><td class=\"mono\">6000<\/td><td class=\"mono\">G 2.5<\/td><td class=\"mono\">119<\/td><td class=\"mono\">60<\/td><td class=\"mono\">3.98<\/td><\/tr>\n <\/tbody>\n <\/table>\n <\/div>\n\n \n <div class=\"table-wrap\" id=\"standards-comparison\">\n <div class=\"table-title\">\ud83d\udcda Balancing Standards Comparison \u2014 ISO vs. API vs. ANSI vs. VDI<\/div>\n <table>\n <thead>\n <tr>\n <th>Standard<\/th>\n <th>Scope<\/th>\n <th>G-Grade System?<\/th>\n <th>Key Difference<\/th>\n <th>Status<\/th>\n <\/tr>\n <\/thead>\n <tbody>\n <tr><td>ISO 21940-11:2016<\/td><td>All rigid rotors \u2014 general procedures<\/td><td>Yes (primary)<\/td><td>Current international standard; replaces ISO 1940-1<\/td><td><span class=\"tag standard\">Current<\/span><\/td><\/tr>\n <tr><td>ISO 1940-1:2003<\/td><td>All rigid rotors<\/td><td>Yes (original)<\/td><td>Established the G-grade system; still widely referenced<\/td><td><span class=\"tag general\">Superseded<\/span><\/td><\/tr>\n <tr><td>ISO 21940-12<\/td><td>Balancing procedures and tolerances<\/td><td>Yes (references Part 11)<\/td><td>Practical balancing procedures, correction plane allocation<\/td><td><span class=\"tag standard\">Current<\/span><\/td><\/tr>\n <tr><td>API 610 \/ 617 \/ 611<\/td><td>Pumps \/ compressors \/ turbines (petroleum industry)<\/td><td>References ISO; adds stricter limits<\/td><td>Often specifies 4W\/N (\u2248 G 1.0) for API 617 rotors; more conservative<\/td><td><span class=\"tag standard\">Current<\/span><\/td><\/tr>\n <tr><td>ANSI S2.19<\/td><td>US-adopted version of ISO 1940<\/td><td>Yes (identical)<\/td><td>Direct adoption of ISO G-grade system for US market<\/td><td><span class=\"tag standard\">Current<\/span><\/td><\/tr>\n <tr><td>VDI 2060<\/td><td>German standard (pre-ISO)<\/td><td>Equivalent system<\/td><td>Historical predecessor to ISO 1940; still referenced in German industry<\/td><td><span class=\"tag general\">Superseded by ISO<\/span><\/td><\/tr>\n <tr><td>MIL-STD-167-1<\/td><td>US military \u2014 shipboard equipment<\/td><td>No (vibration limits)<\/td><td>Specifies vibration amplitude limits, not unbalance tolerances<\/td><td><span class=\"tag precision\">Active<\/span><\/td><\/tr>\n <\/tbody>\n <\/table>\n <\/div>\n\n <\/div>\n<\/section>\n\n\n<main class=\"main-content\" id=\"definition\">\n <div class=\"container\">\n <div class=\"content-layout\">\n <article class=\"article-content\">\n\n <h2>What is a Balance Quality Grade (G-Grade)?<\/h2>\n\n <div class=\"info-box\" style=\"border-left-color: var(--navy); background: var(--beige-light);\">\n <div class=\"box-title\" style=\"font-size: 16px;\">Quick Answer<\/div>\n <p style=\"font-size: 15px;\"><strong>A Balance Quality Grade (G-Grade)<\/strong> is an international standard classification per <strong>ISO 21940-11<\/strong> (formerly ISO 1940-1) that defines the maximum permissible residual <a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\">unbalance<\/a> for a rigid rotor. The G number represents the maximum velocity of the rotor's center-of-gravity displacement in mm\/s. Common grades: <strong>G 6.3<\/strong> for general machinery (pumps, fans, motors), <strong>G 2.5<\/strong> for turbines and precision equipment, <strong>G 1.0<\/strong> for grinding spindles and turbochargers. The formula for permissible unbalance: <strong>U<sub>per<\/sub> = 9549 \u00d7 G \u00d7 m \/ n<\/strong> (g\u00b7mm), where m = mass (kg), n = speed (RPM).<\/p>\n <\/div>\n\n <p>A <strong>Balance Quality Grade<\/strong>, commonly called a \"G-Grade,\" is a standardized classification defined in <strong>ISO 21940-11<\/strong> (which superseded ISO 1940-1) that specifies the maximum permissible residual <a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\">unbalance<\/a> for a rigid rotor. The G-grade defines how precisely a rotor must be balanced \u2014 not a vibration measurement in the installed machine, but a quality specification for the rotor itself based on its mass and maximum service speed.<\/p>\n <p>The number following the letter \"G\" represents the maximum permissible velocity of the rotor's center-of-mass displacement, expressed in millimeters per second (mm\/s). For example, G 6.3 means the product of the specific eccentricity (e<sub>per<\/sub>) and the angular velocity (\u03c9) must not exceed 6.3 mm\/s. G 2.5 limits this velocity to 2.5 mm\/s. The lower the G number, the tighter the balancing tolerance \u2014 meaning higher precision and less permissible residual unbalance.<\/p>\n\n <div class=\"info-box\">\n <div class=\"box-title\">What the G Number Physically Means<\/div>\n <p>The G value represents the maximum permissible velocity of the rotor's center of gravity relative to the geometric rotation axis, at the maximum service speed. G 6.3 means the center of gravity may move at no more than 6.3 mm\/s relative to the spin axis. Since centrifugal force is proportional to this velocity squared, even small reductions in G-grade produce significant reductions in dynamic bearing loads.<\/p>\n <\/div>\n\n <h2 id=\"purpose\">The Purpose of the G-Grade System<\/h2>\n <p>Before the G-grade system was established, balancing specifications were vague \u2014 \"balance as well as possible\" or \"balance until smooth.\" The ISO G-grade system replaced this ambiguity with a universal, verifiable standard. It provides a common language for manufacturers, service engineers, and end users worldwide. The main objectives are:<\/p>\n\n <h3>1. Limiting Unbalance-Induced Vibration to Acceptable Levels<\/h3>\n <p><a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\">Unbalance<\/a> produces centrifugal forces that increase with the square of rotational speed. These forces cause vibration, noise, fatigue loading, and ultimately mechanical failure. By specifying a G-grade, the engineer limits these forces to levels the machine's bearings, seals, and structure can safely tolerate throughout the intended service life.<\/p>\n\n <h3>2. Minimizing Dynamic Loads on Bearings<\/h3>\n <p>Bearings are the components most directly affected by unbalance. The cyclic radial load from residual unbalance acts as a fatigue load on rolling elements and raceways. Bearing life (L<sub>10<\/sub>) is inversely proportional to the cube of the applied load \u2014 so even a modest reduction in unbalance force can dramatically extend bearing service life. Balancing a motor rotor from G 16 to G 6.3 typically doubles bearing L<sub>10<\/sub> life; balancing to G 2.5 can quadruple it.<\/p>\n\n <h3>3. Ensuring Safe Operation at Maximum Design Speed<\/h3>\n <p>Centrifugal force from unbalance is proportional to \u03c9\u00b2 \u2014 doubling the speed quadruples the force from the same unbalance. A rotor that is acceptably balanced at 1500 RPM may produce dangerous vibration at 3000 RPM. The G-grade system accounts for this by incorporating speed into the tolerance calculation, ensuring the rotor is safe at its maximum rated speed.<\/p>\n\n <h3>4. Providing a Clear, Measurable Acceptance Criterion<\/h3>\n <p>The G-grade converts \"balance quality\" from a subjective judgment into an objective, measurable pass\/fail criterion. After balancing, the residual unbalance is compared against the calculated tolerance. If the measured value is below the limit, the rotor passes. This is essential for manufacturing quality control, contractual specifications, warranty claims, and regulatory compliance.<\/p>\n\n <h2 id=\"formulas\">Calculating Permissible Residual Unbalance<\/h2>\n <p>The core of the G-grade system is the ability to calculate a specific, numerical unbalance tolerance for any rotor. Two key quantities are derived from the G-grade:<\/p>\n\n <h3>Specific Unbalance (Permissible Eccentricity)<\/h3>\n <div class=\"formula-box\">\n <div class=\"formula-label\">Permissible Specific Unbalance (Eccentricity)<\/div>\n <div class=\"formula-main\">e<sub>per<\/sub> = (9549 \u00d7 G) \/ n<\/div>\n <div class=\"formula-note\">e<sub>per<\/sub> in \u00b5m (micrometers), G in mm\/s, n in RPM. Constant 9549 = 60\u00d71000\/(2\u03c0)<\/div>\n <\/div>\n <p>The specific unbalance (e<sub>per<\/sub>) represents the maximum permissible displacement of the rotor's center of gravity from the rotation axis, in micrometers. It depends only on the G-grade and the speed \u2014 not on the rotor mass. This makes it useful for comparing the balance quality of rotors of different sizes.<\/p>\n\n <h3>Total Permissible Residual Unbalance<\/h3>\n <div class=\"formula-box\">\n <div class=\"formula-label\">Total Permissible Residual Unbalance<\/div>\n <div class=\"formula-main\">U<sub>per<\/sub> = e<sub>per<\/sub> \u00d7 m = (9549 \u00d7 G \u00d7 m) \/ n<\/div>\n <div class=\"formula-note\">U<sub>per<\/sub> in g\u00b7mm, G in mm\/s, m in kg, n in RPM<\/div>\n <\/div>\n <p>The total permissible residual unbalance (U<sub>per<\/sub>) is the actual target the balancing technician must achieve. It is expressed in g\u00b7mm (gram-millimeters) \u2014 the product of the residual unbalance mass times its distance from the rotation axis. This is the number displayed on the balancing machine and compared against the tolerance.<\/p>\n\n <h3>Centrifugal Force from Residual Unbalance<\/h3>\n <div class=\"formula-box\">\n <div class=\"formula-label\">Centrifugal Force at Tolerance Limit<\/div>\n <div class=\"formula-main\">F = m \u00d7 e<sub>per<\/sub> \u00d7 \u03c9\u00b2 = U<sub>per<\/sub> \u00d7 \u03c9\u00b2 \/ 10\u2076<\/div>\n <div class=\"formula-note\">F in Newtons, e<sub>per<\/sub> in meters, \u03c9 = 2\u03c0\u00d7n\/60 in rad\/s. Divide by 10\u2076 when U<sub>per<\/sub> in g\u00b7mm<\/div>\n <\/div>\n <p>This formula shows the actual dynamic force the bearings must withstand from the permissible residual unbalance at operating speed. It is useful for verifying that the bearing load rating is adequate and for understanding the real-world impact of the G-grade specification.<\/p>\n\n <h3>Variables Reference<\/h3>\n <table class=\"article-table\">\n <thead><tr><th>Symbol<\/th><th>Name<\/th><th>Unit<\/th><th>Description<\/th><\/tr><\/thead>\n <tbody>\n <tr><td class=\"mono\">G<\/td><td>Balance quality grade<\/td><td>mm\/s<\/td><td>Product e<sub>per<\/sub>\u00b7\u03c9; defines the ISO grade (e.g. 6.3, 2.5, 1.0)<\/td><\/tr>\n <tr><td class=\"mono\">e<sub>per<\/sub><\/td><td>Permissible specific unbalance<\/td><td>\u00b5m<\/td><td>Maximum CG offset from rotation axis<\/td><\/tr>\n <tr><td class=\"mono\">U<sub>per<\/sub><\/td><td>Permissible residual unbalance<\/td><td>g\u00b7mm<\/td><td>Total unbalance tolerance = e<sub>per<\/sub> \u00d7 mass<\/td><\/tr>\n <tr><td class=\"mono\">m<\/td><td>Rotor mass<\/td><td>kg<\/td><td>Total mass of the rotor being balanced<\/td><\/tr>\n <tr><td class=\"mono\">n<\/td><td>Maximum service speed<\/td><td>RPM<\/td><td>Highest speed at which the rotor will operate<\/td><\/tr>\n <tr><td class=\"mono\">\u03c9<\/td><td>Angular velocity<\/td><td>rad\/s<\/td><td>= 2\u03c0 \u00d7 n \/ 60<\/td><\/tr>\n <tr><td class=\"mono\">F<\/td><td>Centrifugal force<\/td><td>N<\/td><td>Dynamic force from residual unbalance at speed<\/td><\/tr>\n <\/tbody>\n <\/table>\n\n <h2 id=\"selection\">How to Select the Right G-Grade<\/h2>\n <p>The ISO standard provides recommendations for hundreds of rotor types, but in practice the selection depends on several interrelated factors:<\/p>\n\n <h3>Machine Type and Application<\/h3>\n <p>The standard groups rotors by application and recommends a G-grade for each group (see the ISO table above). A high-speed turbine needs much tighter balance (G 2.5 or G 1.0) than a slow-speed agricultural mechanism (G 16 or G 40). The designer considers how sensitive the machine is to vibration and what the consequences of unbalance-induced failure would be.<\/p>\n\n <h3>Rotor Speed<\/h3>\n <p>Speed is the single most important factor. For the same G-grade, permissible unbalance (U<sub>per<\/sub>) decreases linearly with speed. A rotor at 6000 RPM has half the tolerance of the same rotor at 3000 RPM. For high-speed rotors (turbines, turbochargers, grinding spindles), the tolerance becomes extremely small, requiring specialized balancing equipment and procedures.<\/p>\n\n <h3>Bearing Type and Support Stiffness<\/h3>\n <p>A rotor mounted on flexible (elastic) supports typically requires tighter balance than one on a rigid foundation, because the flexible system transmits vibration more readily. The same crankshaft may require G 16 on elastic mounts but G 40 on rigid mounts. Similarly, rotors on fluid-film bearings may tolerate more unbalance than those on rolling-element bearings due to the damping effect of the oil film.<\/p>\n\n <h3>Environmental and Safety Requirements<\/h3>\n <p>Equipment operating near personnel (HVAC, medical devices), in noise-sensitive environments, or in safety-critical applications (power generation, aviation, offshore) may require tighter balance than the standard recommends for the rotor type. Some industries (petrochemical, power generation) have their own standards (API, IEEE) that specify tighter limits than ISO.<\/p>\n\n <h3>Industry-Specific Recommendations<\/h3>\n <table class=\"article-table\">\n <thead><tr><th>Industry \/ Application<\/th><th>Typical G-Grade<\/th><th>Notes<\/th><\/tr><\/thead>\n <tbody>\n <tr><td>Power generation (turbines)<\/td><td class=\"mono\">G 1.0 \u2013 G 2.5<\/td><td>API 612\/617 often specifies even tighter than ISO<\/td><\/tr>\n <tr><td>Petroleum \/ chemical (pumps, compressors)<\/td><td class=\"mono\">G 2.5 \u2013 G 6.3<\/td><td>API 610 pumps often G 2.5 or tighter<\/td><\/tr>\n <tr><td>HVAC (fans, blowers, AHU)<\/td><td class=\"mono\">G 6.3<\/td><td>Noise-sensitive installations may require G 2.5<\/td><\/tr>\n <tr><td>Pulp & paper (rollers, dryers)<\/td><td class=\"mono\">G 6.3 \u2013 G 16<\/td><td>Large slow rollers; high mass compensates for lower precision<\/td><\/tr>\n <tr><td>Mining & minerals (crushers, screens)<\/td><td class=\"mono\">G 16 \u2013 G 40<\/td><td>Harsh environment; moderate precision acceptable<\/td><\/tr>\n <tr><td>Automotive (wheels, driveshafts)<\/td><td class=\"mono\">G 16 \u2013 G 40<\/td><td>NVH requirements may tighten beyond ISO minimum<\/td><\/tr>\n <tr><td>Machine tools (spindles, drives)<\/td><td class=\"mono\">G 1.0 \u2013 G 2.5<\/td><td>Surface finish quality depends on spindle balance<\/td><\/tr>\n <tr><td>Marine (propeller shafts, engines)<\/td><td class=\"mono\">G 6.3 \u2013 G 40<\/td><td>Classification society rules (DNV, Lloyd's, ABS) apply<\/td><\/tr>\n <tr><td>Wind energy (rotor hubs, generators)<\/td><td class=\"mono\">G 6.3<\/td><td>Blade pitch imbalance handled separately from hub balance<\/td><\/tr>\n <tr><td>Aerospace (turbofan, gyros)<\/td><td class=\"mono\">G 0.4 \u2013 G 2.5<\/td><td>Extremely tight; military standards (MIL-STD) may override ISO<\/td><\/tr>\n <\/tbody>\n <\/table>\n\n <h2 id=\"two-plane\">Two-Plane Balancing \u2014 Distributing the Tolerance<\/h2>\n <p>The total permissible unbalance U<sub>per<\/sub> calculated from the G-grade formula is for the <em>entire rotor<\/em>. In practice, most rotors are balanced in two correction planes (dynamic balancing), so the tolerance must be apportioned between the planes.<\/p>\n\n <h3>ISO Guidance for Tolerance Distribution<\/h3>\n <ul>\n <li><strong>Symmetric rotors<\/strong> (CG approximately at midspan): Divide U<sub>per<\/sub> equally between the two planes. Each plane gets U<sub>per<\/sub>\/2.<\/li>\n <li><strong>Asymmetric rotors<\/strong> (CG offset toward one end): Distribute proportionally to the bearing distances from the CG. The plane closest to the CG receives the larger share of the tolerance.<\/li>\n <li><strong>Single-plane balancing:<\/strong> The entire U<sub>per<\/sub> applies to the single correction plane. This is appropriate for narrow disc-shaped rotors (L\/D < 0.5) where couple unbalance is negligible.<\/li>\n <\/ul>\n\n <div class=\"info-box warning\">\n <div class=\"box-title\">Important: Don't Double the Tolerance<\/div>\n <p>A common error is to calculate U<sub>per<\/sub> and then apply this value to <em>each<\/em> plane, effectively doubling the total tolerance. The correct approach: U<sub>per<\/sub> is the total; divide it between planes. Each plane receives U<sub>per<\/sub>\/2 for a symmetric rotor.<\/p>\n <\/div>\n\n <h2 id=\"examples\">Worked Examples<\/h2>\n\n <div class=\"example-block\">\n <div class=\"example-title\">Example 1: Centrifugal Pump Impeller<\/div>\n <p><strong>Given:<\/strong> Pump impeller, mass = 12 kg, operating speed = 2950 RPM, required grade G 6.3.<\/p>\n <p><strong>Step 1 \u2014 Specific unbalance:<\/strong> e<sub>per<\/sub> = 9549 \u00d7 6.3 \/ 2950 = <strong>20.4 \u00b5m<\/strong><\/p>\n <p><strong>Step 2 \u2014 Total tolerance:<\/strong> U<sub>per<\/sub> = 20.4 \u00d7 12 = <strong>245 g\u00b7mm<\/strong><\/p>\n <p><strong>Step 3 \u2014 Per plane (symmetric):<\/strong> 245 \/ 2 = <strong>122 g\u00b7mm per plane<\/strong><\/p>\n <p><strong>Step 4 \u2014 Correction weight:<\/strong> At correction radius R = 100 mm: weight = 122 \/ 100 = <strong>1.22 grams<\/strong> per plane maximum<\/p>\n <p><strong>Step 5 \u2014 Centrifugal force:<\/strong> \u03c9 = 2\u03c0 \u00d7 2950\/60 = 308.9 rad\/s. F = 245 \u00d7 10\u207b\u2076 \u00d7 308.9\u00b2 = <strong>23.4 N<\/strong> \u2014 well within bearing capacity.<\/p>\n <\/div>\n\n <div class=\"example-block\">\n <div class=\"example-title\">Example 2: Large Industrial Fan<\/div>\n <p><strong>Given:<\/strong> Fan rotor, mass = 85 kg, operating speed = 1480 RPM, required grade G 6.3.<\/p>\n <p><strong>Step 1 \u2014 Specific unbalance:<\/strong> e<sub>per<\/sub> = 9549 \u00d7 6.3 \/ 1480 = <strong>40.6 \u00b5m<\/strong><\/p>\n <p><strong>Step 2 \u2014 Total tolerance:<\/strong> U<sub>per<\/sub> = 40.6 \u00d7 85 = <strong>3,455 g\u00b7mm<\/strong><\/p>\n <p><strong>Step 3 \u2014 Per plane:<\/strong> 3,455 \/ 2 = <strong>1,728 g\u00b7mm per plane<\/strong><\/p>\n <p><strong>Step 4 \u2014 Correction weight:<\/strong> At R = 400 mm: weight = 1728 \/ 400 = <strong>4.3 grams<\/strong> per plane maximum.<\/p>\n <p><strong>Practical note:<\/strong> This fan can be balanced in the field using a <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> portable balancer with the rotor installed. The device automatically calculates the G 6.3 tolerance based on rotor mass and speed.<\/p>\n <\/div>\n\n <div class=\"example-block\">\n <div class=\"example-title\">Example 3: Automotive Turbocharger<\/div>\n <p><strong>Given:<\/strong> Turbine wheel, mass = 0.8 kg, max speed = 90,000 RPM, required grade G 1.0.<\/p>\n <p><strong>Step 1 \u2014 Specific unbalance:<\/strong> e<sub>per<\/sub> = 9549 \u00d7 1.0 \/ 90000 = <strong>0.106 \u00b5m<\/strong> \u2014 about 100 nanometers!<\/p>\n <p><strong>Step 2 \u2014 Total tolerance:<\/strong> U<sub>per<\/sub> = 0.106 \u00d7 0.8 = <strong>0.085 g\u00b7mm<\/strong><\/p>\n <p><strong>Step 3 \u2014 Correction weight:<\/strong> At R = 20 mm: weight = 0.085 \/ 20 = <strong>0.004 grams<\/strong> (4 milligrams!) per plane maximum.<\/p>\n <p><strong>Practical note:<\/strong> This extremely tight tolerance requires specialized high-speed balancing machines with sub-milligram resolution. Material removal (grinding\/drilling) is typically used rather than adding weights at this precision level.<\/p>\n <\/div>\n\n <h2 id=\"standards\">Historical Context \u2014 ISO 1940-1 to ISO 21940-11<\/h2>\n <p>The G-grade system has evolved through several iterations:<\/p>\n <ul>\n <li><strong>VDI 2060 (1966):<\/strong> The original German standard that established the concept of balance quality grades. Developed by the Verein Deutscher Ingenieure (Association of German Engineers).<\/li>\n <li><strong>ISO 1940 (1973, rev. 1986, 2003):<\/strong> International adoption of the VDI 2060 concept. ISO 1940-1:2003 \"Mechanical vibration \u2014 Balance quality requirements for rotors in a constant (rigid) state\" became the worldwide reference for G-grades.<\/li>\n <li><strong>ISO 21940-11:2016:<\/strong> The current standard. Part of the comprehensive ISO 21940 series covering all aspects of rotor balancing. Part 11 specifically covers balance quality requirements and replaces ISO 1940-1. The G-grade values and application tables remain essentially the same; the main changes are editorial and structural.<\/li>\n <\/ul>\n <p>Despite the formal supersession, \"ISO 1940\" remains the most commonly used reference in industry conversations, purchase specifications, and equipment manuals. Both designations refer to the same G-grade system.<\/p>\n\n <h2 id=\"mistakes\">Common Mistakes in Applying G-Grades<\/h2>\n\n <h3>Mistake 1: Using Balancing Speed Instead of Service Speed<\/h3>\n <p>The G-grade tolerance must be calculated using the <em>maximum service speed<\/em> (operating speed), not the balancing machine speed. Many rotors are balanced at a lower RPM than their service speed. Using the balancing speed in the formula produces a tolerance that is too loose for the actual operating conditions. The <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> software allows you to enter the service speed separately from the balancing speed to avoid this error.<\/p>\n\n <h3>Mistake 2: Confusing G-Grade with Vibration Level<\/h3>\n <p>G 6.3 does NOT mean the installed machine will vibrate at 6.3 mm\/s. The G value is a property of the <em>rotor alone<\/em>, measured or calculated as a free-body tolerance. The vibration of the installed machine depends on many additional factors: bearing condition, <a href=\"https:\/\/vibromera.eu\/glossary\/misalignment\/\">alignment<\/a>, structural <a href=\"https:\/\/vibromera.eu\/glossary\/natural-frequency\/\">natural frequencies<\/a>, damping, and more. A rotor balanced to G 6.3 may produce 1 mm\/s vibration in one machine and 4 mm\/s in another, depending on the installation.<\/p>\n\n <h3>Mistake 3: Over-Specifying the Grade<\/h3>\n <p>Specifying G 1.0 for a slow-speed fan that only needs G 6.3 wastes time and money. Tighter grades require more balancing iterations, more precise equipment, and longer balancing times. Specify the grade appropriate to the application \u2014 better balance than needed provides diminishing returns while increasing cost.<\/p>\n\n <h3>Mistake 4: Applying Total Tolerance to Each Plane<\/h3>\n <p>As noted above, U<sub>per<\/sub> is the <strong>total<\/strong> tolerance for the rotor. For two-plane balancing, divide by 2 (or distribute proportionally for asymmetric rotors). Applying U<sub>per<\/sub> to each plane doubles the actual total tolerance, potentially exceeding the intended grade.<\/p>\n\n <h3>Mistake 5: Ignoring Temperature and Assembly Changes<\/h3>\n <p>Some rotors change balance state between cold (ambient) and hot (operating) conditions due to thermal distortion, centrifugal growth, or fit changes. A rotor that meets G 2.5 on the balancing machine at room temperature may exceed this tolerance at operating temperature. For critical rotors, high-speed balancing at or near operating conditions is recommended.<\/p>\n\n <h3>Mistake 6: Neglecting Key and Keyway Convention<\/h3>\n <p>ISO 21940-11 specifies that the half-key convention should be used when balancing a rotor with a keyway (add a half-key to the keyway during balancing to approximate the installed condition). Using a full key, no key, or ignoring this convention introduces an initial unbalance error that may be significant for tight G-grades.<\/p>\n\n <h2>Why G-Grades Matter \u2014 The Business Case<\/h2>\n <p>Proper application of G-grades delivers measurable benefits:<\/p>\n <ul>\n <li><strong>Bearing life:<\/strong> Bearing L<sub>10<\/sub> life is proportional to (C\/P)\u00b3 where P includes the unbalance force. Reducing unbalance by half can increase bearing life by up to 8\u00d7 (2\u00b3 = 8). This translates directly to reduced maintenance costs and downtime.<\/li>\n <li><strong>Energy efficiency:<\/strong> <a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\">Unbalance<\/a>-induced vibration dissipates energy as heat in bearings, seals, and dampers. Well-balanced rotors run cooler and consume less power \u2014 typically 1\u20133% energy savings on industrial motors.<\/li>\n <li><strong>Noise reduction:<\/strong> Vibration from unbalance transmits through the structure and radiates as noise. Meeting the correct G-grade is often the most cost-effective way to comply with workplace noise regulations.<\/li>\n <li><strong>Standardization and interoperability:<\/strong> The G-grade system ensures that a rotor balanced by Manufacturer A meets the same quality standard as one balanced by Manufacturer B \u2014 essential for global supply chains and interchangeable components.<\/li>\n <li><strong>Regulatory compliance:<\/strong> Many industries require documented evidence of balance quality for insurance, warranty, and safety certification. The G-grade provides a universally recognized documentation standard.<\/li>\n <\/ul>\n\n <div class=\"info-box success\">\n <div class=\"box-title\">Practical Balancing Equipment for G-Grade Compliance<\/div>\n <p>The <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> portable balancer includes a built-in ISO 1940 \/ ISO 21940-11 tolerance calculator. Enter the rotor mass, service speed, and desired G-grade \u2014 the software automatically calculates U<sub>per<\/sub>, distributes the tolerance between planes, and provides a clear pass\/fail indication after each balancing run. The <a href=\"https:\/\/vibromera.eu\/product\/balanset-4\/\">Balanset-4<\/a> extends this capability to four-channel measurement for complex balancing setups.<\/p>\n <\/div>\n\n <hr style=\"margin: 48px 0 24px; border: none; border-top: 1px solid var(--border-light);\">\n <p><a href=\"https:\/\/vibromera.eu\/glossary\/\">\u2190 Back to Glossary Index<\/a><\/p>\n\n <\/article>\n\n \n <aside class=\"toc-sidebar\">\n <div class=\"toc-box\">\n <h3>On This Page<\/h3>\n <a href=\"#calculator\">Tolerance Calculator<\/a>\n <a href=\"#grade-overview\">G-Grade Overview Cards<\/a>\n <a href=\"#iso-table\">Full ISO Table<\/a>\n <a href=\"#definition\">Definition<\/a>\n <a href=\"#purpose\">Purpose of G-Grades<\/a>\n <a href=\"#formulas\">Formulas<\/a>\n <a class=\"sub\" href=\"#formulas\">e<sub>per<\/sub> (eccentricity)<\/a>\n <a class=\"sub\" href=\"#formulas\">U<sub>per<\/sub> (tolerance)<\/a>\n <a class=\"sub\" href=\"#formulas\">Centrifugal force<\/a>\n <a href=\"#selection\">Grade Selection<\/a>\n <a class=\"sub\" href=\"#selection\">Industry recommendations<\/a>\n <a href=\"#two-plane\">Two-Plane Distribution<\/a>\n <a href=\"#examples\">Worked Examples<\/a>\n <a class=\"sub\" href=\"#examples\">Pump impeller<\/a>\n <a class=\"sub\" href=\"#examples\">Industrial fan<\/a>\n <a class=\"sub\" href=\"#examples\">Turbocharger<\/a>\n <a href=\"#standards\">Standards History<\/a>\n <a href=\"#mistakes\">6 Common Mistakes<\/a>\n <a href=\"#faq\">FAQ (8 Questions)<\/a>\n <\/div>\n\n <div class=\"toc-box\" style=\"margin-top: 24px; background: var(--navy); border-color: var(--navy);\">\n <h3 style=\"color: var(--white);\">Vibromera Equipment<\/h3>\n <p style=\"color: rgba(255,255,255,0.6); font-size: 13px; margin-bottom: 12px;\">Portable balancing with built-in ISO G-grade tolerance calculator.<\/p>\n <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\" style=\"display: block; padding: 8px 14px; background: var(--blue); color: white; border-radius: 6px; text-align: center; font-weight: 600; font-size: 14px; text-decoration: none; margin-bottom: 8px; border-left: none;\">Balanset-1A \u2192<\/a>\n <a href=\"https:\/\/vibromera.eu\/product\/balanset-4\/\" style=\"display: block; padding: 8px 14px; background: rgba(255,255,255,0.1); color: white; border-radius: 6px; text-align: center; font-weight: 600; font-size: 14px; text-decoration: none; border: 1px solid rgba(255,255,255,0.2); border-left: none;\">Balanset-4 \u2192<\/a>\n <\/div>\n <\/aside>\n <\/div>\n <\/div>\n<\/main>\n\n\n<section class=\"grades-section\" id=\"faq\" style=\"background: var(--white); border-top: 1px solid var(--border-light);\">\n <div class=\"container\" style=\"max-width:900px;\">\n <div class=\"section-header\">\n <h2>Frequently Asked Questions \u2014 Balance Quality Grades<\/h2>\n <p>Common questions about G-grades, ISO 1940, and balancing tolerances<\/p>\n <\/div>\n\n <div style=\"display:flex;flex-direction:column;gap:16px;\">\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is the most commonly used Balance Quality Grade?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n <strong>G 6.3<\/strong> is by far the most widely specified grade worldwide. It applies to the majority of general industrial machinery: electric motors, pump impellers, fans, blowers, flywheels, and process plant equipment. It provides a practical balance between manufacturing cost and vibration performance for equipment operating at typical industrial speeds (750\u20133600 RPM).\n <\/div>\n <\/details>\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is the difference between ISO 1940-1 and ISO 21940-11?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n <strong>ISO 21940-11:2016<\/strong> supersedes <strong>ISO 1940-1:2003<\/strong>. Both define exactly the same G-grade system \u2014 the G values and application tables are identical. ISO 21940-11 is part of the comprehensive ISO 21940 series covering all aspects of rotor balancing (terminology, procedures, tolerances, machines, instrumentation). The change is primarily organizational and editorial, not technical. In practice, most engineers still say \"ISO 1940\" when referring to G-grades, and both references are universally understood.\n <\/div>\n <\/details>\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> Does the G-Grade equal machine vibration level?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n <strong>No.<\/strong> This is one of the most common misconceptions. The G value is a property of the <em>rotor alone<\/em> \u2014 it defines the permissible unbalance of the free rotor, not the vibration of the installed machine. G 6.3 does NOT mean the machine will vibrate at 6.3 mm\/s. The actual installed vibration depends on bearing condition, <a href=\"https:\/\/vibromera.eu\/glossary\/misalignment\/\">alignment<\/a>, structural <a href=\"https:\/\/vibromera.eu\/glossary\/natural-frequency\/\">natural frequencies<\/a>, foundation stiffness, and damping. A rotor balanced to G 6.3 might produce anywhere from 0.5 to 5 mm\/s in different machines.\n <\/div>\n <\/details>\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> How do you calculate permissible residual unbalance?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n Use the formula: <strong>U<sub>per<\/sub> (g\u00b7mm) = (9549 \u00d7 G \u00d7 m) \/ n<\/strong>, where G is the grade (mm\/s), m is rotor mass (kg), and n is maximum service speed (RPM). Example: a 25 kg rotor at 3000 RPM, grade G 6.3 \u2192 U<sub>per<\/sub> = (9549 \u00d7 6.3 \u00d7 25) \/ 3000 = <strong>502 g\u00b7mm<\/strong> total. For two-plane balancing, each plane gets 502 \/ 2 = <strong>251 g\u00b7mm<\/strong>. The <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> software performs this calculation automatically.\n <\/div>\n <\/details>\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What G-Grade for pumps, fans, and electric motors?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n ISO 21940-11 recommends <strong>G 6.3<\/strong> for standard industrial pumps, fans, blowers, and general electric motors. Critical service pumps per API 610 often require <strong>G 2.5<\/strong>. HVAC fans in noise-sensitive locations (hospitals, offices) may also warrant G 2.5. Large, slow fans (<600 RPM) can sometimes use G 16. Turbine-driven pumps and high-speed compressors: G 2.5 or G 1.0.\n <\/div>\n <\/details>\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> Should I use balancing speed or operating speed in the formula?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n <strong>Always use the maximum operating (service) speed<\/strong> \u2014 the highest speed at which the rotor will run in actual service. This is a common and dangerous mistake: many rotors are balanced on machines at lower speeds than they will operate in the field. Using the balancing speed produces a tolerance that is too loose. For example, if a rotor operates at 3600 RPM but is balanced at 600 RPM, the tolerance calculated at 600 RPM would be 6\u00d7 too generous.\n <\/div>\n <\/details>\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> Can I balance in the field to an ISO G-Grade?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n Yes. Modern portable balancing equipment like the <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> enables field balancing to ISO G-grade standards without removing the rotor from the machine. The device measures vibration amplitude and phase, and its built-in software calculates the required correction weights for single-plane or two-plane <a href=\"https:\/\/vibromera.eu\/glossary\/rotor-balancing\/\">dynamic balancing<\/a>. It includes an automatic ISO 1940 \/ ISO 21940-11 tolerance calculator with pass\/fail indication.\n <\/div>\n <\/details>\n\n <details style=\"background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius);padding:0;overflow:hidden;\">\n <summary style=\"padding:18px 24px;font-weight:700;font-size:16px;cursor:pointer;color:var(--navy);list-style:none;display:flex;align-items:center;gap:10px;\">\n <span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What about balancing quality for flexible rotors?\n <\/summary>\n <div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">\n The G-grade system (ISO 21940-11, formerly ISO 1940-1) applies specifically to <strong>rigid rotors<\/strong> \u2014 rotors that operate well below their first <a href=\"https:\/\/vibromera.eu\/glossary\/critical-speed\/\">critical speed<\/a>. Flexible rotors (those operating above or near their first critical speed, such as large turbogenerators) require modal balancing techniques covered by <strong>ISO 21940-12<\/strong>. The G-grade can still be applied as an initial low-speed balance target, but additional high-speed balancing runs at or near operating speed are necessary.\n <\/div>\n <\/details>\n\n <\/div>\n <\/div>\n<\/section>\n\n\n<section style=\"padding:32px 0;background:var(--beige-light);border-top:1px solid var(--border-light);\">\n <div class=\"container\" style=\"max-width:900px;\">\n <h3 style=\"font-family:'DM Serif Display',serif;font-size:22px;color:var(--navy);margin-bottom:20px;\">Related Glossary Articles<\/h3>\n <div style=\"display:grid;grid-template-columns:repeat(3,1fr);gap:16px;\">\n <a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\" style=\"display:block;padding:18px;background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius-sm);text-decoration:none;transition:all 0.2s;box-shadow:var(--shadow-sm);\">\n <div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Unbalance (Imbalance)<\/div>\n <div style=\"font-size:13px;color:var(--text-secondary);\">Definition, types (static, couple, dynamic), causes, and measurement methods<\/div>\n <\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/rotor-balancing\/\" style=\"display:block;padding:18px;background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius-sm);text-decoration:none;transition:all 0.2s;box-shadow:var(--shadow-sm);\">\n <div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Rotor Balancing<\/div>\n <div style=\"font-size:13px;color:var(--text-secondary);\">Single-plane vs. two-plane dynamic balancing procedures and equipment<\/div>\n <\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/natural-frequency\/\" style=\"display:block;padding:18px;background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius-sm);text-decoration:none;transition:all 0.2s;box-shadow:var(--shadow-sm);\">\n <div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Natural Frequency<\/div>\n <div style=\"font-size:13px;color:var(--text-secondary);\">Resonance, critical speeds, and why they matter for balanced rotors<\/div>\n <\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/bearing-fault-frequencies\/\" style=\"display:block;padding:18px;background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius-sm);text-decoration:none;transition:all 0.2s;box-shadow:var(--shadow-sm);\">\n <div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Bearing Fault Frequencies<\/div>\n <div style=\"font-size:13px;color:var(--text-secondary);\">BPFO, BPFI, BSF, FTF calculator \u2014 how unbalance damages bearings<\/div>\n <\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/harmonics-vibration\/\" style=\"display:block;padding:18px;background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius-sm);text-decoration:none;transition:all 0.2s;box-shadow:var(--shadow-sm);\">\n <div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Harmonics in Vibration<\/div>\n <div style=\"font-size:13px;color:var(--text-secondary);\">1\u00d7, 2\u00d7, 3\u00d7 patterns and what they reveal about machinery faults<\/div>\n <\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/fft\/\" style=\"display:block;padding:18px;background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius-sm);text-decoration:none;transition:all 0.2s;box-shadow:var(--shadow-sm);\">\n <div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">FFT Spectrum Analysis<\/div>\n <div style=\"font-size:13px;color:var(--text-secondary);\">How frequency analysis is used to verify balance quality and diagnose faults<\/div>\n <\/a>\n <\/div>\n <\/div>\n<\/section>\n\n\n<section class=\"shop-cta\">\n <div class=\"container\">\n <h2>Achieve ISO Balance Quality \u2014 In the Field<\/h2>\n <p>Vibromera's portable balancing devices calculate G-grade tolerances automatically and guide you to precise correction weights \u2014 no rotor removal required.<\/p>\n <a href=\"https:\/\/vibromera.eu\/shop\/\" class=\"cta-btn\">Browse Balancing Equipment \u2192<\/a>\n <\/div>\n<\/section>\n\n\n<footer class=\"page-footer\">\n <div class=\"container\">\n <p><a href=\"https:\/\/vibromera.eu\/glossary\/\">\u2190 Back to Glossary<\/a> | <a href=\"https:\/\/vibromera.eu\/\">vibromera.eu<\/a><\/p>\n <\/div>\n<\/footer>\n\n<script>\nfunction calculate() {\n const G = parseFloat(document.getElementById('gGrade').value);\n const m = parseFloat(document.getElementById('rotorMass').value);\n const n = parseFloat(document.getElementById('rotorRPM').value);\n const planes = parseInt(document.getElementById('numPlanes').value);\n const R = parseFloat(document.getElementById('corrRadius').value) || 0;\n\n if (!m || !n || m <= 0 || n <= 0) return alert('Enter valid rotor mass and speed.');\n\n const e_per = (9549 * G) \/ n; \/\/ \u00b5m\n const U_per = e_per * m; \/\/ g\u00b7mm\n const U_plane = U_per \/ planes; \/\/ g\u00b7mm per plane\n const omega = 2 * Math.PI * n \/ 60; \/\/ rad\/s\n const F = (U_per \/ 1e6) * omega * omega; \/\/ N (U in kg\u00b7m: g\u00b7mm \/ 1e6)\n\n let corrWeight = 0;\n if (R > 0) corrWeight = U_plane \/ R; \/\/ grams\n\n const area = document.getElementById('resultsArea');\n area.innerHTML = `\n <div class=\"result-primary\">\n <div class=\"res-label\">Total Permissible Unbalance (U<sub>per<\/sub>)<\/div>\n <div class=\"res-value\">${U_per >= 100 ? U_per.toFixed(0) : U_per >= 1 ? U_per.toFixed(1) : U_per.toFixed(3)}<\/div>\n <div class=\"res-unit\">g\u00b7mm<\/div>\n <\/div>\n <div class=\"results-secondary\">\n <div class=\"res-card\">\n <div class=\"rc-label\">Specific Unbalance (e<sub>per<\/sub>)<\/div>\n <div class=\"rc-value\">${e_per >= 10 ? e_per.toFixed(1) : e_per >= 0.1 ? e_per.toFixed(2) : e_per.toFixed(3)}<\/div>\n <div class=\"rc-unit\">\u00b5m<\/div>\n <\/div>\n <div class=\"res-card\">\n <div class=\"rc-label\">Centrifugal Force<\/div>\n <div class=\"rc-value\">${F >= 10 ? F.toFixed(1) : F >= 0.1 ? F.toFixed(2) : F.toFixed(3)}<\/div>\n <div class=\"rc-unit\">N at ${n} RPM<\/div>\n <\/div>\n <div class=\"res-card\">\n <div class=\"rc-label\">Grade<\/div>\n <div class=\"rc-value\" style=\"font-size:18px\">G ${G}<\/div>\n <div class=\"rc-unit\">ISO 21940-11<\/div>\n <\/div>\n <div class=\"res-card\">\n <div class=\"rc-label\">Rotor<\/div>\n <div class=\"rc-value\" style=\"font-size:14px; font-family:'Source Sans 3',sans-serif\">${m} kg @ ${n} RPM<\/div>\n <div class=\"rc-unit\">${planes} plane${planes > 1 ? 's' : ''}<\/div>\n <\/div>\n <\/div>\n <div class=\"plane-note\">\n <div class=\"pn-label\">Per Plane Tolerance (${planes === 1 ? 'single plane' : 'each of ' + planes + ' planes'})<\/div>\n <div class=\"pn-value\">${U_plane >= 100 ? U_plane.toFixed(0) : U_plane >= 1 ? U_plane.toFixed(1) : U_plane.toFixed(3)}<\/div>\n <div class=\"pn-unit\">g\u00b7mm per plane${corrWeight > 0 ? ' = ' + (corrWeight >= 0.1 ? corrWeight.toFixed(2) : corrWeight.toFixed(3)) + ' g at R=' + R + ' mm' : ''}<\/div>\n <\/div>\n `;\n}\n\ncalculate();\n<\/script>\n\n<\/body>\n<\/html><\/div><\/div><\/div><\/div><\/div>","protected":false},"excerpt":{"rendered":"<p>ISO 1940-1 \u09a6\u09cd\u09ac\u09be\u09b0\u09be \u09b8\u0982\u099c\u09cd\u099e\u09be\u09af\u09bc\u09bf\u09a4 \u09ad\u09be\u09b0\u09b8\u09be\u09ae\u09cd\u09af \u09ae\u09be\u09a8\u09c7\u09b0 \u0997\u09cd\u09b0\u09c7\u09a1 (\u099c\u09bf-\u0997\u09cd\u09b0\u09c7\u09a1) \u09ac\u09cd\u09af\u09be\u0996\u09cd\u09af\u09be\u0964 G2.5 \u098f\u09ac\u0982 G6.3 \u098f\u09b0 \u09ae\u09a4\u09cb \u0997\u09cd\u09b0\u09c7\u09a1\u0997\u09c1\u09b2\u09bf \u0995\u09c0\u09ad\u09be\u09ac\u09c7 \u09b0\u09cb\u099f\u09b0 \u09ad\u09be\u09b0\u09b8\u09be\u09ae\u09cd\u09af \u09b8\u09b9\u09a8\u09b6\u09c0\u09b2\u09a4\u09be \u09a8\u09bf\u09b0\u09cd\u09a6\u09bf\u09b7\u09cd\u099f \u0995\u09b0\u09a4\u09c7 \u09ac\u09cd\u09af\u09ac\u09b9\u09c3\u09a4 \u09b9\u09af\u09bc \u09a4\u09be \u09b6\u09bf\u0996\u09c1\u09a8\u0964<\/p>","protected":false},"featured_media":0,"template":"","meta":{"ai_generated_summary":"","footnotes":""},"categories":[109,112],"tags":[],"class_list":["post-20631","glossary","type-glossary","status-publish","hentry","category-glossary","category-iso-standards"],"_links":{"self":[{"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/glossary\/20631","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/glossary"}],"about":[{"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/types\/glossary"}],"version-history":[{"count":6,"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/glossary\/20631\/revisions"}],"predecessor-version":[{"id":101729,"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/glossary\/20631\/revisions\/101729"}],"wp:attachment":[{"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/media?parent=20631"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/categories?post=20631"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/vibromera.eu\/bn\/wp-json\/wp\/v2\/tags?post=20631"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}