ISO 1940-2 \u2014 Vocabulary for Balancing \u2014 Vibromera<\/title>\n<meta name=\"description\" content=\"Complete reference to ISO 1940-2 (ISO 21940-2) balancing vocabulary. Definitions of unbalance types, rotor classifications, correction planes, balancing machines, G-grades, and 60+ standardized terms used in rotor balancing practice.\">\n<meta name=\"keywords\" content=\"ISO 1940-2, ISO 21940-2, balancing vocabulary, balancing terminology, unbalance definition, static unbalance, couple unbalance, dynamic unbalance, rigid rotor, flexible rotor, correction plane, residual unbalance, balancing machine, G-grade definition\">\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<meta property=\"og:type\" content=\"article\">\n<meta property=\"og:title\" content=\"ISO 1940-2 \u2014 Vocabulary for Balancing (Complete Terminology Reference)\">\n<meta property=\"og:description\" content=\"60+ standardised balancing terms: unbalance types, rotor classifications, correction methods, machine types, and quality grades per ISO 1940-2 \/ ISO 21940-2.\">\n<meta property=\"og:url\" content=\"https:\/\/vibromera.eu\/glossary\/iso-1940-2\/\">\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\/iso-1940-2-og.jpg\">\n<meta property=\"article:publisher\" content=\"https:\/\/vibromera.eu\/\">\n<meta property=\"article:section\" content=\"Glossary\">\n<meta property=\"article:tag\" content=\"ISO 1940-2\">\n<meta property=\"article:tag\" content=\"Balancing Vocabulary\">\n<meta property=\"article:tag\" content=\"Rotor Balancing\">\n<meta name=\"twitter:card\" content=\"summary_large_image\">\n<meta name=\"twitter:title\" content=\"ISO 1940-2 \u2014 Balancing Vocabulary & Terminology Reference\">\n<meta name=\"twitter:description\" content=\"Standardised definitions for unbalance, rotor types, correction planes, balancing machines, and quality grades.\">\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<script type=\"application\/ld+json\">\n{\n \"@context\": \"https:\/\/schema.org\",\n \"@graph\": [\n {\n \"@type\": \"TechArticle\",\n \"@id\": \"https:\/\/vibromera.eu\/glossary\/iso-1940-2\/#article\",\n \"headline\": \"ISO 1940-2: Vocabulary for Balancing \u2014 Complete Terminology Reference\",\n \"description\": \"Comprehensive reference to ISO 1940-2 (now ISO 21940-2): the international vocabulary standard for rotor balancing. Covers definitions of unbalance types, rotor classifications, correction methods, balancing machines, and quality grades.\",\n \"author\": {\"@type\": \"Organization\", \"name\": \"Vibromera\", \"url\": \"https:\/\/vibromera.eu\/\"},\n \"publisher\": {\"@type\": \"Organization\", \"name\": \"Vibromera\", \"url\": \"https:\/\/vibromera.eu\/\", \"logo\": {\"@type\": \"ImageObject\", \"url\": \"https:\/\/vibromera.eu\/wp-content\/uploads\/vibromera-logo.png\"}},\n \"mainEntityOfPage\": \"https:\/\/vibromera.eu\/glossary\/iso-1940-2\/\",\n \"datePublished\": \"2024-03-15\",\n \"dateModified\": \"2026-02-07\",\n \"inLanguage\": \"en\",\n \"about\": [\n {\"@type\": \"Thing\", \"name\": \"ISO 1940-2\"},\n {\"@type\": \"Thing\", \"name\": \"ISO 21940-2\"},\n {\"@type\": \"Thing\", \"name\": \"Balancing Vocabulary\"},\n {\"@type\": \"Thing\", \"name\": \"Rotor Balancing Terminology\"}\n ],\n \"isPartOf\": {\"@type\": \"WebPage\", \"url\": \"https:\/\/vibromera.eu\/glossary\/\"}\n },\n {\n \"@type\": \"FAQPage\",\n \"@id\": \"https:\/\/vibromera.eu\/glossary\/iso-1940-2\/#faq\",\n \"mainEntity\": [\n {\"@type\": \"Question\", \"name\": \"What is ISO 1940-2?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"ISO 1940-2 is the international standard that defines the vocabulary and terminology used in rotor balancing. It provides precise, unambiguous definitions for terms such as unbalance, residual unbalance, rigid rotor, flexible rotor, correction plane, and balancing machine types. It is the essential 'dictionary' that supports all other balancing standards. It has been superseded by ISO 21940-2 with the same terminology.\"}},\n {\"@type\": \"Question\", \"name\": \"What is the difference between static and dynamic unbalance?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"Static unbalance means the rotor's inertia axis is parallel to but displaced from the rotation axis \u2014 equivalent to a single heavy spot, correctable in one plane. Dynamic unbalance is the general case where the inertia axis is neither parallel to nor intersecting the rotation axis \u2014 a combination of static and couple unbalance, requiring correction in two planes. Most real rotors have dynamic unbalance.\"}},\n {\"@type\": \"Question\", \"name\": \"What is the difference between a rigid and flexible rotor?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"A rigid rotor's unbalance can be corrected in any two planes and the correction remains valid at all speeds up to maximum service speed. A flexible rotor deforms elastically at service speed, changing its effective mass distribution, so it must be balanced at or near service speed in more than two planes. The distinction determines the balancing procedure, equipment, and applicable standard.\"}},\n {\"@type\": \"Question\", \"name\": \"What is residual unbalance?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"Residual unbalance is the small amount of unbalance that remains in a rotor after the balancing process is complete. ISO 1940-1 \/ ISO 21940-11 specifies the maximum permissible residual unbalance (U_per) for each G-grade. A rotor passes quality inspection when its residual unbalance is less than or equal to U_per.\"}},\n {\"@type\": \"Question\", \"name\": \"What is a correction plane?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"A correction plane is a plane perpendicular to the rotor axis in which mass is added or removed to correct unbalance. Correction planes must be physically accessible for weight placement. They may not coincide with the bearing (tolerance) planes, requiring geometric conversion of tolerances. Most rotors use one or two correction planes.\"}},\n {\"@type\": \"Question\", \"name\": \"What is the difference between a soft-bearing and hard-bearing balancing machine?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"A soft-bearing machine has flexible suspension and runs the rotor above the suspension's natural frequency, measuring displacement. It must be calibrated for each rotor geometry. A hard-bearing machine has stiff suspension and runs below natural frequency, measuring centrifugal force directly. Hard-bearing machines are permanently calibrated and more versatile \u2014 the dominant type in modern industry.\"}},\n {\"@type\": \"Question\", \"name\": \"What is specific unbalance (eccentricity)?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"Specific unbalance is the ratio of unbalance to rotor mass: e = U \/ M. It has units of length (micrometres or g\u00b7mm\/kg) and represents the displacement of the rotor's centre of mass from the rotation axis. It allows comparison of balance quality across rotors of different masses. 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20px}.formula-box .formula-main{font-size:16px}.info-box{padding:16px 18px}.shop-cta h2{font-size:24px}}\n@media(max-width:480px){.hero h1{font-size:24px}.quick-nav a{padding:12px 12px;font-size:13px}}\n@media print{.quick-nav,.shop-cta,.toc-sidebar{display:none}}\n<\/style>\n<\/head>\n<body>\n\n<header class=\"hero\">\n <div class=\"hero-inner\">\n <div class=\"breadcrumb\"><a href=\"https:\/\/vibromera.eu\/\">Home<\/a> \u2192 <a href=\"https:\/\/vibromera.eu\/glossary\/\">Glossary<\/a> \u2192 ISO 1940-2<\/div>\n <h1>ISO 1940-2 \u2014 <span>Vocabulary<\/span> for Balancing<\/h1>\n <div class=\"canon-badge\"><span style=\"font-size:14px;\">\ud83d\udccc<\/span> Canonical Reference Article \u2014 vibromera.eu<\/div>\n <p class=\"subtitle\">The international \"dictionary\" for rotor balancing \u2014 standardised definitions for unbalance types, rotor classifications, correction methods, machine types, and quality terminology. Now incorporated into ISO 21940-2.<\/p>\n <\/div>\n<\/header>\n\n<nav class=\"quick-nav\">\n <div class=\"quick-nav-inner\">\n <a href=\"#key-terms\">\ud83d\udd11 Key Terms<\/a>\n <a href=\"#rotor-terms\">\ud83d\udd29 Rotor Terms<\/a>\n <a href=\"#unbalance-terms\">\u2696 Unbalance Terms<\/a>\n <a href=\"#process-terms\">\ud83d\udd27 Process Terms<\/a>\n <a href=\"#machine-terms\">\ud83c\udfed Machine Terms<\/a>\n <a href=\"#quality-terms\">\ud83d\udccf Quality Terms<\/a>\n <a href=\"#definition\">\ud83d\udcd0 Overview<\/a>\n <a href=\"#detailed\">\ud83d\udcd6 Detailed Definitions<\/a>\n <a href=\"#cross-ref\">\ud83d\udd17 Cross-Reference<\/a>\n <a href=\"#faq\">\u2753 FAQ<\/a>\n <\/div>\n<\/nav>\n\n\n<section class=\"concepts-section\" id=\"key-terms\">\n <div class=\"container\">\n <div class=\"section-header\">\n <h2>Key Balancing Terms at a Glance<\/h2>\n <p>The most important definitions from ISO 1940-2 \u2014 the terms every balancing practitioner must know<\/p>\n <\/div>\n <div class=\"concept-grid\">\n <div class=\"concept-card cat-unbal\"><div class=\"cc-icon\">\u2696\ufe0f<\/div><div class=\"cc-term\">Unbalance<\/div><div class=\"cc-cat unbal\">Unbalance \u00b7 Core Concept<\/div><div class=\"cc-def\">Condition where the principal axis of inertia is not coincident with the rotational axis. Quantified as U = m \u00d7 r (mass \u00d7 radius, in g\u00b7mm). The primary cause of 1\u00d7 vibration in rotating machines.<\/div><\/div>\n <div class=\"concept-card cat-rotor\"><div class=\"cc-icon\">\ud83d\udd29<\/div><div class=\"cc-term\">Rigid Rotor<\/div><div class=\"cc-cat rotor\">Rotor \u00b7 Classification<\/div><div class=\"cc-def\">Rotor whose unbalance can be corrected in any two arbitrary planes and remains valid at all speeds up to max service speed. Balanced at low speed \u2192 stays balanced at high speed.<\/div><\/div>\n <div class=\"concept-card cat-rotor\"><div class=\"cc-icon\">\ud83c\udf00<\/div><div class=\"cc-term\">Flexible Rotor<\/div><div class=\"cc-cat rotor\">Rotor \u00b7 Classification<\/div><div class=\"cc-def\">Rotor that deforms elastically at service speed, changing its effective mass distribution. Must be balanced at or near service speed in more than two planes.<\/div><\/div>\n <div class=\"concept-card cat-unbal\"><div class=\"cc-icon\">\ud83d\udccd<\/div><div class=\"cc-term\">Static Unbalance<\/div><div class=\"cc-cat unbal\">Unbalance \u00b7 Type<\/div><div class=\"cc-def\">Inertia axis parallel to but displaced from rotation axis. Single \"heavy spot.\" Detectable on knife-edges. Causes in-phase bearing vibration. Corrected in one plane.<\/div><\/div>\n <div class=\"concept-card cat-unbal\"><div class=\"cc-icon\">\ud83d\udd04<\/div><div class=\"cc-term\">Couple Unbalance<\/div><div class=\"cc-cat unbal\">Unbalance \u00b7 Type<\/div><div class=\"cc-def\">Inertia axis intersects rotation axis at centre of gravity. Two equal opposite heavy spots create a rocking moment. Only detectable spinning. Out-of-phase bearing vibration. Two-plane correction.<\/div><\/div>\n <div class=\"concept-card cat-unbal\"><div class=\"cc-icon\">\ud83d\udcab<\/div><div class=\"cc-term\">Dynamic Unbalance<\/div><div class=\"cc-cat unbal\">Unbalance \u00b7 Type<\/div><div class=\"cc-def\">General case \u2014 inertia axis neither parallel to nor intersecting rotation axis. Combination of static + couple. The most common real-world condition. Requires two-plane balancing.<\/div><\/div>\n <div class=\"concept-card cat-proc\"><div class=\"cc-icon\">\ud83d\udcd0<\/div><div class=\"cc-term\">Correction Plane<\/div><div class=\"cc-cat proc\">Process \u00b7 Geometry<\/div><div class=\"cc-def\">Plane perpendicular to rotor axis where mass is added or removed. Must be physically accessible. May not coincide with bearing (tolerance) planes \u2014 geometric conversion required.<\/div><\/div>\n <div class=\"concept-card cat-qual\"><div class=\"cc-icon\">\u2705<\/div><div class=\"cc-term\">Residual Unbalance<\/div><div class=\"cc-cat qual\">Quality \u00b7 Tolerance<\/div><div class=\"cc-def\">Unbalance remaining after the balancing process. Must be \u2264 U<sub>per<\/sub> (permissible residual unbalance) for the specified G-grade. Measured in g\u00b7mm.<\/div><\/div>\n <div class=\"concept-card cat-qual\"><div class=\"cc-icon\">\ud83c\udfaf<\/div><div class=\"cc-term\">Balance Quality Grade (G)<\/div><div class=\"cc-cat qual\">Quality \u00b7 Classification<\/div><div class=\"cc-def\">Product of specific unbalance and angular velocity: G = e \u00d7 \u03c9 (mm\/s). Defines maximum centre-of-mass orbital velocity. G 6.3 is the industrial standard. Defined in ISO 1940-1.<\/div><\/div>\n <\/div>\n <\/div>\n<\/section>\n\n\n<section class=\"section-bg alt\" id=\"rotor-terms\">\n <div class=\"container\">\n <div class=\"section-header\">\n <h2>Complete Terminology Reference<\/h2>\n <p>All major terms from ISO 1940-2 \/ ISO 21940-2, organised by category<\/p>\n <\/div>\n\n \n <div class=\"table-wrap\">\n <div class=\"table-title\">\ud83d\udd29 Category 1 \u2014 Terms Related to the Rotor<\/div>\n <div class=\"table-scroll\"><table>\n <thead><tr><th>Term<\/th><th>Definition<\/th><th>Significance<\/th><\/tr><\/thead>\n <tbody>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Rotor<\/span><\/td><td>A body capable of rotation about a defined axis. In the context of balancing, includes any rotating component: shafts, impellers, armatures, drums, spindles.<\/td><td>The fundamental object of balancing. All other terms describe properties of, or actions on, the rotor.<\/td><\/tr>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Rigid Rotor<\/span><\/td><td>A rotor whose unbalance can be corrected in any two arbitrary planes, and after correction, the residual unbalance does not change significantly at any speed up to the maximum service speed.<\/td><td>Determines that <a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1<\/a> (G-grade system) applies. Balancing at low speed on a shop machine is valid. The vast majority of industrial rotors are rigid.<\/td><\/tr>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Flexible Rotor<\/span><\/td><td>A rotor that deforms elastically at its service speed such that its unbalance state changes. Must be corrected at or near service speed in more than two planes.<\/td><td>Requires ISO 21940-12. High-speed turbines, large generators, multi-stage compressors. Specialised high-speed balancing equipment needed.<\/td><\/tr>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Shaft Axis<\/span><\/td><td>The straight line joining the centres of the bearing journals. The geometrical axis of rotation.<\/td><td>The reference axis for all unbalance measurements. Runout of journals affects measurement accuracy.<\/td><\/tr>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Principal Axis of Inertia<\/span><\/td><td>The axis about which the rotor would rotate freely without producing centrifugal force or moment. Coincides with the shaft axis for a perfectly balanced rotor.<\/td><td>The mismatch between principal axis and shaft axis <em>is<\/em> unbalance. All correction aims to align these two axes.<\/td><\/tr>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Centre of Mass (Gravity)<\/span><\/td><td>The point where the entire rotor mass may be considered concentrated. For a balanced rotor, lies exactly on the shaft axis.<\/td><td>Static unbalance = CoM displaced from shaft axis. Specific unbalance (e) = displacement distance.<\/td><\/tr>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Service Speed<\/span><\/td><td>The maximum rotational speed at which the rotor operates in its intended application.<\/td><td>Critical for tolerance calculation: U<sub>per<\/sub> = (9 549 \u00d7 G \u00d7 M) \/ n. Always use service speed, not balancing speed.<\/td><\/tr>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Critical Speed<\/span><\/td><td>A rotational speed at which a rotor-bearing system experiences resonance, resulting in greatly amplified vibration.<\/td><td>Determines rigid\/flexible classification. A rigid rotor operates well below the first bending critical speed.<\/td><\/tr>\n <\/tbody>\n <\/table><\/div>\n <\/div>\n\n \n <div class=\"table-wrap\" id=\"unbalance-terms\">\n <div class=\"table-title\">\u2696 Category 2 \u2014 Terms Related to Unbalance<\/div>\n <div class=\"table-scroll\"><table>\n <thead><tr><th>Term<\/th><th>Definition<\/th><th>Formula \/ Units<\/th><\/tr><\/thead>\n <tbody>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\"><a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\">Unbalance<\/a><\/span><\/td><td>Condition where the principal axis of inertia is not coincident with the rotational axis. Causes centrifugal force proportional to mass, eccentricity, and speed squared.<\/td><td class=\"mono\">U = m \u00d7 r<br>(g\u00b7mm or kg\u00b7m)<\/td><\/tr>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\">Static Unbalance<\/span><\/td><td>Principal axis parallel to rotation axis but displaced. Equivalent to a single mass at a single radius. Detectable without rotation (knife-edges). In-phase bearing vibration.<\/td><td>Corrected in <strong>1 plane<\/strong><\/td><\/tr>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\">Couple Unbalance<\/span><\/td><td>Principal axis intersects rotation axis at the centre of mass but is tilted. Two equal, opposite heavy spots in different planes create a rocking moment. Only detectable while spinning.<\/td><td>Corrected in <strong>2 planes<\/strong><\/td><\/tr>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\">Dynamic Unbalance<\/span><\/td><td>The general case: principal axis neither parallel to nor intersecting the rotation axis. Combination of static and couple. The most common real-world condition.<\/td><td>Corrected in <strong>2 planes<\/strong><\/td><\/tr>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\">Specific Unbalance<\/span><\/td><td>Ratio of unbalance to rotor mass. Represents the eccentricity \u2014 the displacement of the centre of mass from the shaft axis. Allows quality comparison across different rotor sizes.<\/td><td class=\"mono\">e = U \/ M<br>(\u00b5m or g\u00b7mm\/kg)<\/td><\/tr>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\">Residual Unbalance<\/span><\/td><td>The unbalance remaining in a rotor after the balancing process. Must not exceed the permissible value (U<sub>per<\/sub>) for the specified <a href=\"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/\">G-grade<\/a>.<\/td><td class=\"mono\">U<sub>res<\/sub> \u2264 U<sub>per<\/sub><\/td><\/tr>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\">Initial Unbalance<\/span><\/td><td>The unbalance of a rotor as received, before any balancing correction. Measured on first run.<\/td><td>Baseline for the balancing procedure<\/td><\/tr>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\">Unbalance Vector<\/span><\/td><td>The magnitude and angular position of unbalance in a given plane. Represented as a polar vector with amplitude (g\u00b7mm) and phase angle (\u00b0).<\/td><td class=\"mono\">U\u2220\u03b8<br>(g\u00b7mm at \u00b0 from ref)<\/td><\/tr>\n <\/tbody>\n <\/table><\/div>\n <\/div>\n\n \n <div class=\"table-wrap\" id=\"process-terms\">\n <div class=\"table-title\">\ud83d\udd27 Category 3 \u2014 Terms Related to the Balancing Process<\/div>\n <div class=\"table-scroll\"><table>\n <thead><tr><th>Term<\/th><th>Definition<\/th><th>Practical Notes<\/th><\/tr><\/thead>\n <tbody>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Balancing<\/span><\/td><td>The process of checking and adjusting the mass distribution of a rotor so that residual unbalance is within a specified tolerance.<\/td><td>Iterative: measure \u2192 calculate \u2192 correct \u2192 verify.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Correction Plane<\/span><\/td><td>A plane perpendicular to the rotor axis in which mass is added or removed. The physically accessible location for weight placement.<\/td><td>May differ from tolerance (bearing) planes \u2014 requires geometric conversion.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Tolerance Plane<\/span><\/td><td>The plane in which permissible unbalance is specified \u2014 typically the bearing plane. Unbalance here directly affects bearing loads.<\/td><td>U<sub>per<\/sub> is specified for tolerance planes; must be converted to correction planes.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Correction Mass<\/span><\/td><td>The physical mass (weight) added to or removed from the rotor at a specific radius and angle within the correction plane.<\/td><td>Added: clip-on, bolt-on, weld, epoxy. Removed: drilling, milling, grinding.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Trial Weight<\/span><\/td><td>A known mass temporarily attached to the rotor at a known radius and angle during the balancing procedure. Used to determine the rotor's response (influence coefficient).<\/td><td>The Balanset-1A trial-weight method: run \u2192 attach trial \u2192 run \u2192 software calculates correction.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Influence Coefficient<\/span><\/td><td>The change in vibration response (amplitude and phase) at a measurement point caused by a unit unbalance at a specific location. Characterises rotor-bearing sensitivity.<\/td><td>Calculated from trial-weight runs. Two-plane balancing requires a 2\u00d72 influence matrix.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Single-Plane Balancing<\/span><\/td><td>Procedure correcting static unbalance in one correction plane. Appropriate for short (disc-like) rotors with L\/D < 0.5.<\/td><td><a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> F2 mode. One sensor, one plane.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Two-Plane Balancing<\/span><\/td><td>Procedure correcting both static and couple unbalance in two correction planes. Required for elongated rotors or when couple unbalance is significant.<\/td><td><a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> F3 mode. Two sensors, two planes.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Trim Balancing<\/span><\/td><td>A final, fine balancing adjustment performed on an assembled rotor to compensate for assembly-introduced unbalance (coupling runout, fit tolerances).<\/td><td>Often performed in the field on the installed machine.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Weight Splitting<\/span><\/td><td>Distributing a calculated correction mass between two adjacent accessible locations (e.g., two bolt holes or blade positions) when the exact angular position is not accessible.<\/td><td>Balanset-1A provides automatic weight-splitting calculation.<\/td><\/tr>\n <\/tbody>\n <\/table><\/div>\n <\/div>\n\n \n <div class=\"table-wrap\" id=\"machine-terms\">\n <div class=\"table-title\">\ud83c\udfed Category 4 \u2014 Terms Related to Balancing Machines<\/div>\n <div class=\"table-scroll\"><table>\n <thead><tr><th>Term<\/th><th>Definition<\/th><th>Comparison<\/th><\/tr><\/thead>\n <tbody>\n <tr><td><span class=\"tag mach\">Machine<\/span><br><span class=\"term-name\"><a href=\"https:\/\/vibromera.eu\/glossary\/balancing-machine\/\">Balancing Machine<\/a><\/span><\/td><td>A device that measures unbalance in a rotor (magnitude and angular position) so that mass distribution can be corrected.<\/td><td>Shop-based (stationary) or field (portable like <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a>).<\/td><\/tr>\n <tr><td><span class=\"tag mach\">Machine<\/span><br><span class=\"term-name\">Soft-Bearing Machine<\/span><\/td><td>Suspension is very flexible. Rotor runs above suspension natural frequency. Measures physical displacement. Must be calibrated for each rotor geometry.<\/td><td>Less common today. Lower cost, but operator must recalibrate per rotor. Displacement sensing.<\/td><\/tr>\n <tr><td><span class=\"tag mach\">Machine<\/span><br><span class=\"term-name\">Hard-Bearing Machine<\/span><\/td><td>Suspension is very stiff. Rotor runs below suspension natural frequency. Sensors measure centrifugal force directly. Permanently calibrated \u2014 accepts wide range of rotors without rotor-specific setup.<\/td><td><strong>Dominant type<\/strong> in modern industry. More versatile, faster setup. Force sensing.<\/td><\/tr>\n <tr><td><span class=\"tag mach\">Machine<\/span><br><span class=\"term-name\">Field Balancer<\/span><\/td><td>Portable instrument used to balance rotors in-situ (installed in the machine) without disassembly. Uses vibration sensors and a tachometer. Trial-weight method.<\/td><td><a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> (2-channel) and <a href=\"https:\/\/vibromera.eu\/product\/balanset-4\/\">Balanset-4<\/a> (4-channel). ISO 1940 tolerance calculator built in.<\/td><\/tr>\n <tr><td><span class=\"tag mach\">Machine<\/span><br><span class=\"term-name\">Mandrel (Arbor)<\/span><\/td><td>A shaft or adaptor on which a rotor is mounted for balancing on a machine. Must be accurately concentric and have negligible runout.<\/td><td>Mandrel eccentricity is a major source of systematic balancing error. Verified by index test.<\/td><\/tr>\n <\/tbody>\n <\/table><\/div>\n <\/div>\n\n \n <div class=\"table-wrap\" id=\"quality-terms\">\n <div class=\"table-title\">\ud83d\udccf Category 5 \u2014 Terms Related to Balance Quality & Measurement<\/div>\n <div class=\"table-scroll\"><table>\n <thead><tr><th>Term<\/th><th>Definition<\/th><th>Formula \/ Standard<\/th><\/tr><\/thead>\n <tbody>\n <tr><td><span class=\"tag qual\">Quality<\/span><br><span class=\"term-name\"><a href=\"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/\">Balance Quality Grade (G)<\/a><\/span><\/td><td>A classification specifying the maximum permissible velocity of the rotor's centre of mass. G = e<sub>per<\/sub> \u00d7 \u03c9. Grades form a logarithmic scale with factor 2.5.<\/td><td class=\"mono\">G 0.4 \u2026 G 4000<br>Defined in <a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1<\/a><\/td><\/tr>\n <tr><td><span class=\"tag qual\">Quality<\/span><br><span class=\"term-name\">Permissible Residual Unbalance (U<sub>per<\/sub>)<\/span><\/td><td>Maximum residual unbalance allowed for the specified G-grade, rotor mass, and service speed. The acceptance criterion.<\/td><td class=\"mono\">U<sub>per<\/sub> = (9549 \u00d7 G \u00d7 M) \/ n<\/td><\/tr>\n <tr><td><span class=\"tag qual\">Quality<\/span><br><span class=\"term-name\">Balance Tolerance<\/span><\/td><td>The range within which the residual unbalance must fall to meet the specified quality requirement. Equal to U<sub>per<\/sub>.<\/td><td>Specified per plane after allocation<\/td><\/tr>\n <tr><td><span class=\"tag qual\">Quality<\/span><br><span class=\"term-name\">Unbalance Reduction Ratio (URR)<\/span><\/td><td>Ratio of initial unbalance to residual unbalance after one correction cycle. Indicates balancing machine\/procedure efficiency.<\/td><td class=\"mono\">URR = U<sub>initial<\/sub> \/ U<sub>residual<\/sub><br>Typical: 5\u201350\u00d7<\/td><\/tr>\n <tr><td><span class=\"tag meas\">Measurement<\/span><br><span class=\"term-name\">Phase Angle<\/span><\/td><td>The angular position of the unbalance vector relative to a reference mark on the rotor (measured by tachometer). Combined with amplitude, defines the complete unbalance vector.<\/td><td class=\"mono\">\u00b0 (degrees, 0\u2013360)<\/td><\/tr>\n <tr><td><span class=\"tag meas\">Measurement<\/span><br><span class=\"term-name\">Vibration Velocity (<a href=\"https:\/\/vibromera.eu\/glossary\/rms\/\">RMS<\/a>)<\/span><\/td><td>Root-mean-square value of vibration velocity at a bearing housing. The standard measurement parameter for machine condition assessment per <a href=\"https:\/\/vibromera.eu\/glossary\/iso-10816-1\/\">ISO 10816<\/a>.<\/td><td class=\"mono\">mm\/s RMS (10\u20131000 Hz)<\/td><\/tr>\n <tr><td><span class=\"tag meas\">Measurement<\/span><br><span class=\"term-name\">Index Test<\/span><\/td><td>Verification procedure: rotate the rotor a defined angle (e.g. 180\u00b0) relative to the machine supports and remeasure. Detects mandrel and fixture errors.<\/td><td>Required for formal verification per ISO 1940-1 Ch. 10<\/td><\/tr>\n <tr><td><span class=\"tag meas\">Measurement<\/span><br><span class=\"term-name\">Minimum Achievable Residual Unbalance (U<sub>mar<\/sub>)<\/span><\/td><td>The lowest residual unbalance achievable on a given balancing machine for a specific rotor. Determined by machine sensitivity, noise floor, and bearing conditions.<\/td><td>U<sub>mar<\/sub> must be \u2264 U<sub>per<\/sub> for the machine to be suitable for the required G-grade.<\/td><\/tr>\n <\/tbody>\n <\/table><\/div>\n <\/div>\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 ISO 1940-2?<\/h2>\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>ISO 1940-2<\/strong> (<em>Mechanical vibration \u2014 Balance quality requirements \u2014 Vocabulary<\/em>) is the international standard that defines the terminology used in rotor balancing. It provides precise, physics-based definitions for all key terms \u2014 from <a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\">unbalance<\/a> types (static, couple, dynamic) to rotor classifications (rigid, flexible), correction methods, <a href=\"https:\/\/vibromera.eu\/glossary\/balancing-machine\/\">machine types<\/a>, and quality grades. It is the essential \"dictionary\" supporting <a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1<\/a> and all other balancing standards. Superseded by <strong>ISO 21940-2<\/strong> with identical terminology.<\/p>\n <\/div>\n\n <p>When an engineer in Germany specifies \"dynamic unbalance correction to G 6.3 in two planes,\" a technician in Japan must understand exactly what is required \u2014 the same rotor condition, the same balancing procedure, and the same acceptance criterion. ISO 1940-2 makes this possible by providing a single, internationally agreed vocabulary for the entire field.<\/p>\n\n <p>The standard is not a procedure or a tolerance specification \u2014 it is a <em>terminology standard<\/em>. Its role is to eliminate ambiguity so that other standards (<a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1<\/a> for tolerances, <a href=\"https:\/\/vibromera.eu\/glossary\/iso-14694\/\">ISO 14694<\/a> for fans, <a href=\"https:\/\/vibromera.eu\/glossary\/iso-10816-1\/\">ISO 10816<\/a> for vibration evaluation) can use precise, unambiguous language.<\/p>\n\n <h2 id=\"detailed\">Detailed Term Analysis<\/h2>\n\n <h3>The Rigid \/ Flexible Distinction<\/h3>\n <p>This is the single most important classification in balancing. The distinction determines everything: which standard applies, what equipment is needed, how many planes are required, and at what speed balancing must be performed.<\/p>\n\n <div class=\"info-box\">\n <div class=\"box-title\">Rigid Rotor (ISO 1940-2 definition)<\/div>\n <p>A rotor whose unbalance can be corrected in any two arbitrary planes and, after correction, the residual unbalance does not change significantly at any speed up to the maximum service speed. <strong>Practical test:<\/strong> if the first bending <a href=\"https:\/\/vibromera.eu\/glossary\/natural-frequency\/\">critical speed<\/a> is well above the maximum service speed (typically > 1.5\u00d7 or more), the rotor is rigid.<\/p>\n <\/div>\n\n <div class=\"info-box warning\">\n <div class=\"box-title\">Flexible Rotor (ISO 1940-2 definition)<\/div>\n <p>A rotor that deforms elastically at its service speed such that its unbalance state changes. Must be balanced at or near service speed in more than two planes. <strong>Applies to:<\/strong> large turbogenerators, multi-stage high-speed compressors, long paper machine rolls at high speed. Covered by ISO 21940-12.<\/p>\n <\/div>\n\n <p>The vast majority of industrial rotors \u2014 electric motors, fans, pumps, flywheels, shafts \u2014 are rigid rotors. The <a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1<\/a> G-grade system applies directly to rigid rotors.<\/p>\n\n <h3>The Three Types of Unbalance<\/h3>\n <p>ISO 1940-2 defines three fundamental types based on the geometric relationship between the principal inertia axis and the rotation axis. Understanding these is essential for selecting the correct balancing procedure:<\/p>\n\n <div class=\"formula-box\">\n <div class=\"formula-label\">Unbalance Vector<\/div>\n <div class=\"formula-main\">U = m \u00d7 r (magnitude) U\u2220\u03b8 (polar form)<\/div>\n <div class=\"formula-note\">m = unbalanced mass (g) | r = distance from axis (mm) | \u03b8 = angular position (\u00b0)<\/div>\n <\/div>\n\n <ul>\n <li><strong>Static unbalance<\/strong> produces a <em>force<\/em> \u2014 both bearings vibrate in phase at 1\u00d7 RPM. The rotor can be detected as unbalanced without rotation (gravity reveals it on knife-edges). One correction plane suffices. Typical for narrow disc-like rotors (L\/D < 0.5): narrow pulleys, fan impellers, thin flywheels.<\/li>\n <li><strong>Couple unbalance<\/strong> produces a <em>moment<\/em> \u2014 bearings vibrate 180\u00b0 out of phase at 1\u00d7 RPM. The net force is zero (centre of mass is on the axis), but two equal and opposite heavy spots in different axial positions create a rocking couple. Only detectable while spinning. Requires two correction planes.<\/li>\n <li><strong>Dynamic unbalance<\/strong> = static + couple combined. The general case for all real rotors that are not perfectly symmetric. Both force and moment are present. Bearings vibrate at 1\u00d7 with neither in-phase nor exactly 180\u00b0 out-of-phase relationship. Requires two-plane balancing.<\/li>\n <\/ul>\n\n <h3>Specific Unbalance and the G-Grade Connection<\/h3>\n <p><strong>Specific unbalance<\/strong> (e = U\/M) is the key metric that enables universal balance quality comparison. A 5 kg rotor with 50 g\u00b7mm unbalance has e = 10 \u00b5m. A 500 kg rotor with 5 000 g\u00b7mm unbalance also has e = 10 \u00b5m \u2014 identical balance quality despite 100\u00d7 mass difference.<\/p>\n <p>The <a href=\"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/\">G-grade<\/a> extends this by incorporating speed: G = e \u00d7 \u03c9, giving a single number (mm\/s) that characterises balance quality independently of both mass and speed. This is the foundation of the <a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1<\/a> tolerance system.<\/p>\n\n <h3>Correction Planes vs. Tolerance Planes<\/h3>\n <p>ISO 1940-2 draws a critical distinction that is often missed in practice:<\/p>\n <ul>\n <li><strong>Tolerance planes<\/strong> = the bearing planes where vibration and dynamic loads are most critical. Permissible unbalance U<sub>per<\/sub> is specified here.<\/li>\n <li><strong>Correction planes<\/strong> = physically accessible locations where weights can be placed (fan hub, motor end-rings, shaft shoulders). Often at different axial positions than the bearings.<\/li>\n <\/ul>\n <p>Converting U<sub>per<\/sub> from tolerance planes to correction planes requires knowledge of rotor geometry. For asymmetric or overhung rotors, this conversion can significantly change the per-plane tolerances. The <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> handles this conversion automatically when rotor dimensions are entered.<\/p>\n\n <h3>Balancing Machine Types<\/h3>\n <p>The two fundamental machine types reflect different physical measurement principles:<\/p>\n <ul>\n <li><strong>Soft-bearing:<\/strong> Suspension natural frequency well below operating speed \u2192 machine measures <em>displacement<\/em>. Requires calibration for each new rotor. Historically significant; declining in use.<\/li>\n <li><strong>Hard-bearing:<\/strong> Suspension natural frequency well above operating speed \u2192 machine measures <em>force<\/em>. Permanently calibrated \u2014 accepts different rotors without individual calibration. The dominant modern type.<\/li>\n <\/ul>\n <p>Field balancing instruments like the <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> use a different principle: they are not a \"machine\" in the ISO sense but use the rotor's own bearings and support as the measurement system, employing the trial-weight (influence coefficient) method to determine correction without requiring a dedicated balancing machine.<\/p>\n\n <h2 id=\"cross-ref\">Cross-Reference: Where Each Term Is Used<\/h2>\n\n <div class=\"info-box success\">\n <div class=\"box-title\">Standards That Reference ISO 1940-2 Vocabulary<\/div>\n <p><strong><a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1 \/ ISO 21940-11<\/a>:<\/strong> Uses all tolerance and quality terms \u2014 G-grade, U<sub>per<\/sub>, balance tolerance, residual unbalance. The primary consumer of this vocabulary.<\/p>\n <p><strong><a href=\"https:\/\/vibromera.eu\/glossary\/iso-14694\/\">ISO 14694<\/a>:<\/strong> Uses rotor terms (rigid), unbalance terms, and extends with fan-specific BV\/FV categories built on G-grades.<\/p>\n <p><strong><a href=\"https:\/\/vibromera.eu\/glossary\/iso-10816-1\/\">ISO 10816 \/ ISO 20816<\/a>:<\/strong> Uses measurement terms \u2014 vibration velocity, RMS, bearing housing measurement points.<\/p>\n <p><strong>ISO 21940-12:<\/strong> Extends flexible rotor definition with multi-speed, multi-plane procedures.<\/p>\n <p><strong>API 610 \/ API 617:<\/strong> Petroleum standards reference ISO 1940 G-grades and unbalance terminology for pump and compressor specifications.<\/p>\n <\/div>\n\n <h2>ISO 1940-2 \u2192 ISO 21940-2: Transition<\/h2>\n <p>ISO 21940-2 has formally superseded ISO 1940-2. The terminology is identical \u2014 all definitions carry forward unchanged. The ISO 21940 numbering reflects integration into the comprehensive ISO 21940 series covering all aspects of mechanical vibration and balancing. Both designations are accepted in industry practice.<\/p>\n\n <hr style=\"margin:48px 0 24px;border:none;border-top:1px solid var(--border-light);\">\n <p><strong>Official standard:<\/strong> <a href=\"https:\/\/www.iso.org\/standard\/6636.html\" rel=\"nofollow noopener\" target=\"_blank\">ISO 1940-2 on ISO Store \u2192<\/a><\/p>\n <p><a href=\"https:\/\/vibromera.eu\/glossary\/\">\u2190 Back to Glossary Index<\/a><\/p>\n <\/article>\n\n <aside class=\"toc-sidebar\">\n <div class=\"toc-box\">\n <h3>On This Page<\/h3>\n <a href=\"#key-terms\">Key Terms (9 cards)<\/a>\n <a href=\"#rotor-terms\">Rotor Terms (8)<\/a>\n <a href=\"#unbalance-terms\">Unbalance Terms (8)<\/a>\n <a href=\"#process-terms\">Process Terms (10)<\/a>\n <a href=\"#machine-terms\">Machine Terms (5)<\/a>\n <a href=\"#quality-terms\">Quality & Measurement (8)<\/a>\n <a href=\"#definition\">What is ISO 1940-2?<\/a>\n <a href=\"#detailed\">Detailed Term Analysis<\/a>\n <a class=\"sub\" href=\"#detailed\">Rigid vs. Flexible<\/a>\n <a class=\"sub\" href=\"#detailed\">Three unbalance types<\/a>\n <a class=\"sub\" href=\"#detailed\">Specific unbalance & G<\/a>\n <a class=\"sub\" href=\"#detailed\">Correction vs. tolerance planes<\/a>\n <a class=\"sub\" href=\"#detailed\">Machine types<\/a>\n <a href=\"#cross-ref\">Cross-Reference<\/a>\n <a href=\"#faq\">FAQ (7 Questions)<\/a>\n <\/div>\n <div class=\"toc-box\" style=\"margin-top:24px;background:var(--navy);border-color:var(--navy);\">\n <h3 style=\"color:#fff;\">Balancing Equipment<\/h3>\n <p style=\"color:rgba(255,255,255,.6);font-size:13px;margin-bottom:12px;\">Portable field balancing with built-in ISO 1940 vocabulary in every menu \u2014 G-grade calculator, plane allocation, balance reports.<\/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,.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,.2);border-left:none;\">Balanset-4 \u2192<\/a>\n <\/div>\n <\/aside>\n <\/div>\n <\/div>\n<\/main>\n\n\n<section class=\"section-bg alt\" id=\"faq\">\n <div class=\"container\" style=\"max-width:1000px;\">\n <div class=\"section-header\"><h2>Frequently Asked Questions \u2014 ISO 1940-2<\/h2><p>Balancing vocabulary and terminology<\/p><\/div>\n <div style=\"display:flex;flex-direction:column;gap:16px;\">\n <details style=\"background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius);overflow:hidden;\"><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;\"><span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is ISO 1940-2?<\/summary><div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">ISO 1940-2 is the international vocabulary standard for rotor balancing. It defines terms such as unbalance, rigid rotor, flexible rotor, correction plane, residual unbalance, G-grade, balancing machine types, and dozens more. It is the \"dictionary\" that supports all other balancing standards. Superseded by ISO 21940-2 with identical definitions.<\/div><\/details>\n <details style=\"background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius);overflow:hidden;\"><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;\"><span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is the difference between static and dynamic unbalance?<\/summary><div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\"><strong>Static<\/strong>: inertia axis parallel but displaced \u2014 single heavy spot \u2014 detectable without rotation \u2014 one-plane correction \u2014 in-phase bearing vibration. <strong>Dynamic<\/strong>: general case (static + couple combined) \u2014 inertia axis skewed \u2014 two-plane correction required \u2014 bearings vibrate with neither in-phase nor 180\u00b0 relationship. Most real rotors have dynamic unbalance.<\/div><\/details>\n <details style=\"background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius);overflow:hidden;\"><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;\"><span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is the difference between a rigid and flexible rotor?<\/summary><div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\"><strong>Rigid<\/strong>: balanced at low speed \u2192 stays balanced at all speeds up to max service speed. Two correction planes suffice. Covered by <a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1<\/a>. <strong>Flexible<\/strong>: elastic deformation at service speed changes mass distribution. Must balance at\/near service speed, > 2 planes. Covered by ISO 21940-12. Most industrial rotors are rigid.<\/div><\/details>\n <details style=\"background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius);overflow:hidden;\"><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;\"><span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is residual unbalance?<\/summary><div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">The small amount of <a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\">unbalance<\/a> remaining after balancing. Must be \u2264 U<sub>per<\/sub> (permissible) for the specified <a href=\"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/\">G-grade<\/a>. U<sub>per<\/sub> = (9 549 \u00d7 G \u00d7 M) \/ n. The <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> compares measured residual against this limit automatically.<\/div><\/details>\n <details style=\"background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius);overflow:hidden;\"><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;\"><span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is the difference between correction plane and tolerance plane?<\/summary><div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\"><strong>Tolerance plane<\/strong> = bearing plane where U<sub>per<\/sub> is specified and loads are critical. <strong>Correction plane<\/strong> = where weights are actually placed (may be far from bearings). Tolerances must be geometrically converted between them. Errors here cause rotors that \"pass\" on the correction planes but fail at the bearings.<\/div><\/details>\n <details style=\"background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius);overflow:hidden;\"><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;\"><span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> Soft-bearing vs. hard-bearing balancing machine?<\/summary><div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\"><strong>Soft-bearing<\/strong>: flexible suspension, runs above natural frequency, measures displacement. Must recalibrate per rotor. <strong>Hard-bearing<\/strong>: stiff suspension, runs below natural frequency, measures force. Permanently calibrated \u2014 dominant in modern industry. Field balancers (Balanset-1A) use a different approach: trial-weight method on the machine's own bearings.<\/div><\/details>\n <details style=\"background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius);overflow:hidden;\"><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;\"><span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is specific unbalance (eccentricity)?<\/summary><div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">e = U \/ M (unbalance divided by mass). Units: \u00b5m or g\u00b7mm\/kg. Represents the actual displacement of the centre of mass from the rotation axis. Enables comparison across different rotor sizes. The G-grade is e \u00d7 \u03c9 \u2014 specific unbalance \u00d7 angular velocity (mm\/s).<\/div><\/details>\n <\/div>\n <\/div>\n<\/section>\n\n\n<section style=\"padding:32px 0;background:var(--white);border-top:1px solid var(--border-light);\">\n <div class=\"container\" style=\"max-width:1000px;\">\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\/iso-1940-1\/\" style=\"display:block;padding:18px;background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius-sm);text-decoration:none;box-shadow:var(--shadow-sm);\"><div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">ISO 1940-1<\/div><div style=\"font-size:13px;color:var(--text-secondary);\">G-grade tolerance system \u2014 uses this vocabulary throughout<\/div><\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/\" style=\"display:block;padding:18px;background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius-sm);text-decoration:none;box-shadow:var(--shadow-sm);\"><div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Balance Quality Grade<\/div><div style=\"font-size:13px;color:var(--text-secondary);\">Interactive G-grade calculator with full tolerance tables<\/div><\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\" style=\"display:block;padding:18px;background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius-sm);text-decoration:none;box-shadow:var(--shadow-sm);\"><div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Unbalance<\/div><div style=\"font-size:13px;color:var(--text-secondary);\">Detailed treatment of the core concept defined here<\/div><\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/iso-14694\/\" style=\"display:block;padding:18px;background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius-sm);text-decoration:none;box-shadow:var(--shadow-sm);\"><div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">ISO 14694<\/div><div style=\"font-size:13px;color:var(--text-secondary);\">Fan-specific BV\/FV categories built on this vocabulary<\/div><\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/natural-frequency\/\" style=\"display:block;padding:18px;background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius-sm);text-decoration:none;box-shadow:var(--shadow-sm);\"><div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Natural Frequency<\/div><div style=\"font-size:13px;color:var(--text-secondary);\">Critical speeds and the rigid\/flexible rotor boundary<\/div><\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/rotor-balancing\/\" style=\"display:block;padding:18px;background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius-sm);text-decoration:none;box-shadow:var(--shadow-sm);\"><div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Rotor Balancing<\/div><div style=\"font-size:13px;color:var(--text-secondary);\">Procedures using the terminology defined here<\/div><\/a>\n <\/div>\n <\/div>\n<\/section>\n\n<section class=\"shop-cta\">\n <div class=\"container\">\n <h2>Speak the Language \u2014 With the Right Tools<\/h2>\n <p>Vibromera balancers implement ISO vocabulary directly: G-grade selection, unbalance vectors, correction planes, residual vs. permissible comparison \u2014 all in one portable instrument.<\/p>\n <a href=\"https:\/\/vibromera.eu\/shop\/\" class=\"cta-btn\">Browse Balancing Equipment \u2192<\/a>\n <\/div>\n<\/section>\n\n<footer class=\"page-footer\">\n <div class=\"container\"><p><a href=\"https:\/\/vibromera.eu\/glossary\/\">\u2190 Back to Glossary<\/a> | <a href=\"https:\/\/vibromera.eu\/\">vibromera.eu<\/a><\/p><\/div>\n<\/footer>\n\n<\/body>\n<\/html><\/div><\/div><\/div><\/div><\/div>","protected":false},"excerpt":{"rendered":"<p>En oversikt over ISO 1940-2, standarden som definerer det viktigste vokabularet og terminologien som brukes innen rotorbalansering for \u00e5 sikre tydelig og konsekvent kommunikasjon.<\/p>","protected":false},"featured_media":0,"template":"","meta":{"ai_generated_summary":"","footnotes":""},"categories":[109,112],"tags":[],"class_list":["post-10","glossary","type-glossary","status-publish","hentry","category-glossary","category-iso-standards"],"_links":{"self":[{"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/glossary\/10","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/glossary"}],"about":[{"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/types\/glossary"}],"version-history":[{"count":4,"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/glossary\/10\/revisions"}],"predecessor-version":[{"id":101492,"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/glossary\/10\/revisions\/101492"}],"wp:attachment":[{"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/media?parent=10"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/categories?post=10"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/tags?post=10"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}

\n\n\n\n\nISO 1940-2 \u2014 Vocabulary for Balancing \u2014 Vibromera<\/title>\n<meta name=\"description\" content=\"Complete reference to ISO 1940-2 (ISO 21940-2) balancing vocabulary. Definitions of unbalance types, rotor classifications, correction planes, balancing machines, G-grades, and 60+ standardized terms used in rotor balancing practice.\">\n<meta name=\"keywords\" content=\"ISO 1940-2, ISO 21940-2, balancing vocabulary, balancing terminology, unbalance definition, static unbalance, couple unbalance, dynamic unbalance, rigid rotor, flexible rotor, correction plane, residual unbalance, balancing machine, G-grade definition\">\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<meta property=\"og:type\" content=\"article\">\n<meta property=\"og:title\" content=\"ISO 1940-2 \u2014 Vocabulary for Balancing (Complete Terminology Reference)\">\n<meta property=\"og:description\" content=\"60+ standardised balancing terms: unbalance types, rotor classifications, correction methods, machine types, and quality grades per ISO 1940-2 \/ ISO 21940-2.\">\n<meta property=\"og:url\" content=\"https:\/\/vibromera.eu\/glossary\/iso-1940-2\/\">\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\/iso-1940-2-og.jpg\">\n<meta property=\"article:publisher\" content=\"https:\/\/vibromera.eu\/\">\n<meta property=\"article:section\" content=\"Glossary\">\n<meta property=\"article:tag\" content=\"ISO 1940-2\">\n<meta property=\"article:tag\" content=\"Balancing Vocabulary\">\n<meta property=\"article:tag\" content=\"Rotor Balancing\">\n<meta name=\"twitter:card\" content=\"summary_large_image\">\n<meta name=\"twitter:title\" content=\"ISO 1940-2 \u2014 Balancing Vocabulary & Terminology Reference\">\n<meta name=\"twitter:description\" content=\"Standardised definitions for unbalance, rotor types, correction planes, balancing machines, and quality grades.\">\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<script type=\"application\/ld+json\">\n{\n \"@context\": \"https:\/\/schema.org\",\n \"@graph\": [\n {\n \"@type\": \"TechArticle\",\n \"@id\": \"https:\/\/vibromera.eu\/glossary\/iso-1940-2\/#article\",\n \"headline\": \"ISO 1940-2: Vocabulary for Balancing \u2014 Complete Terminology Reference\",\n \"description\": \"Comprehensive reference to ISO 1940-2 (now ISO 21940-2): the international vocabulary standard for rotor balancing. Covers definitions of unbalance types, rotor classifications, correction methods, balancing machines, and quality grades.\",\n \"author\": {\"@type\": \"Organization\", \"name\": \"Vibromera\", \"url\": \"https:\/\/vibromera.eu\/\"},\n \"publisher\": {\"@type\": \"Organization\", \"name\": \"Vibromera\", \"url\": \"https:\/\/vibromera.eu\/\", \"logo\": {\"@type\": \"ImageObject\", \"url\": \"https:\/\/vibromera.eu\/wp-content\/uploads\/vibromera-logo.png\"}},\n \"mainEntityOfPage\": \"https:\/\/vibromera.eu\/glossary\/iso-1940-2\/\",\n \"datePublished\": \"2024-03-15\",\n \"dateModified\": \"2026-02-07\",\n \"inLanguage\": \"en\",\n \"about\": [\n {\"@type\": \"Thing\", \"name\": \"ISO 1940-2\"},\n {\"@type\": \"Thing\", \"name\": \"ISO 21940-2\"},\n {\"@type\": \"Thing\", \"name\": \"Balancing Vocabulary\"},\n {\"@type\": \"Thing\", \"name\": \"Rotor Balancing Terminology\"}\n ],\n \"isPartOf\": {\"@type\": \"WebPage\", \"url\": \"https:\/\/vibromera.eu\/glossary\/\"}\n },\n {\n \"@type\": \"FAQPage\",\n \"@id\": \"https:\/\/vibromera.eu\/glossary\/iso-1940-2\/#faq\",\n \"mainEntity\": [\n {\"@type\": \"Question\", \"name\": \"What is ISO 1940-2?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"ISO 1940-2 is the international standard that defines the vocabulary and terminology used in rotor balancing. It provides precise, unambiguous definitions for terms such as unbalance, residual unbalance, rigid rotor, flexible rotor, correction plane, and balancing machine types. It is the essential 'dictionary' that supports all other balancing standards. It has been superseded by ISO 21940-2 with the same terminology.\"}},\n {\"@type\": \"Question\", \"name\": \"What is the difference between static and dynamic unbalance?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"Static unbalance means the rotor's inertia axis is parallel to but displaced from the rotation axis \u2014 equivalent to a single heavy spot, correctable in one plane. Dynamic unbalance is the general case where the inertia axis is neither parallel to nor intersecting the rotation axis \u2014 a combination of static and couple unbalance, requiring correction in two planes. Most real rotors have dynamic unbalance.\"}},\n {\"@type\": \"Question\", \"name\": \"What is the difference between a rigid and flexible rotor?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"A rigid rotor's unbalance can be corrected in any two planes and the correction remains valid at all speeds up to maximum service speed. A flexible rotor deforms elastically at service speed, changing its effective mass distribution, so it must be balanced at or near service speed in more than two planes. The distinction determines the balancing procedure, equipment, and applicable standard.\"}},\n {\"@type\": \"Question\", \"name\": \"What is residual unbalance?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"Residual unbalance is the small amount of unbalance that remains in a rotor after the balancing process is complete. ISO 1940-1 \/ ISO 21940-11 specifies the maximum permissible residual unbalance (U_per) for each G-grade. A rotor passes quality inspection when its residual unbalance is less than or equal to U_per.\"}},\n {\"@type\": \"Question\", \"name\": \"What is a correction plane?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"A correction plane is a plane perpendicular to the rotor axis in which mass is added or removed to correct unbalance. Correction planes must be physically accessible for weight placement. They may not coincide with the bearing (tolerance) planes, requiring geometric conversion of tolerances. Most rotors use one or two correction planes.\"}},\n {\"@type\": \"Question\", \"name\": \"What is the difference between a soft-bearing and hard-bearing balancing machine?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"A soft-bearing machine has flexible suspension and runs the rotor above the suspension's natural frequency, measuring displacement. It must be calibrated for each rotor geometry. A hard-bearing machine has stiff suspension and runs below natural frequency, measuring centrifugal force directly. Hard-bearing machines are permanently calibrated and more versatile \u2014 the dominant type in modern industry.\"}},\n {\"@type\": \"Question\", \"name\": \"What is specific unbalance (eccentricity)?\", \"acceptedAnswer\": {\"@type\": \"Answer\", \"text\": \"Specific unbalance is the ratio of unbalance to rotor mass: e = U \/ M. It has units of length (micrometres or g\u00b7mm\/kg) and represents the displacement of the rotor's centre of mass from the rotation axis. It allows comparison of balance quality across rotors of different masses. The G-grade is defined as e \u00d7 \u03c9 (specific unbalance \u00d7 angular velocity).\"}}\n ]\n },\n {\n \"@type\": \"BreadcrumbList\",\n \"itemListElement\": [\n {\"@type\": \"ListItem\", \"position\": 1, \"name\": \"Home\", \"item\": \"https:\/\/vibromera.eu\/\"},\n {\"@type\": \"ListItem\", \"position\": 2, \"name\": \"Glossary\", \"item\": \"https:\/\/vibromera.eu\/glossary\/\"},\n {\"@type\": \"ListItem\", \"position\": 3, \"name\": \"ISO 1940-2\"}\n ]\n }\n ]\n}\n<\/script>\n<link href=\"https:\/\/fonts.googleapis.com\/css2?family=DM+Serif+Display&family=Source+Sans+3:wght@300;400;500;600;700&family=JetBrains+Mono:wght@400;500;600&display=swap\" 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a{color:var(--blue);text-decoration:none;font-weight:500}.page-footer p{font-size:14px;color:var(--text-muted)}\n\n\/* RESPONSIVE *\/\n@media(max-width:1200px){.container,.hero-inner,.quick-nav-inner{padding:0 32px}.content-layout{grid-template-columns:1fr 280px;gap:32px}.concept-grid{grid-template-columns:repeat(2,1fr)}}\n@media(max-width:960px){.content-layout{grid-template-columns:1fr}.toc-sidebar{position:static;order:-1}.toc-box:first-child{display:none}.concept-grid{grid-template-columns:repeat(2,1fr)}.def-item{grid-template-columns:160px 1fr;gap:12px}}\n@media(max-width:768px){.container,.hero-inner,.quick-nav-inner{padding:0 20px}.hero{padding:36px 0 32px}.hero h1{font-size:28px}.hero .subtitle{font-size:16px}.section-header h2{font-size:24px}.article-content h2{font-size:24px}.article-content h3{font-size:18px}.concept-grid{grid-template-columns:1fr}table{font-size:13px}table th,table td{padding:8px 10px}.def-item{grid-template-columns:1fr;gap:4px}.formula-box{padding:18px 20px}.formula-box .formula-main{font-size:16px}.info-box{padding:16px 18px}.shop-cta h2{font-size:24px}}\n@media(max-width:480px){.hero h1{font-size:24px}.quick-nav a{padding:12px 12px;font-size:13px}}\n@media print{.quick-nav,.shop-cta,.toc-sidebar{display:none}}\n<\/style>\n<\/head>\n<body>\n\n<header class=\"hero\">\n <div class=\"hero-inner\">\n <div class=\"breadcrumb\"><a href=\"https:\/\/vibromera.eu\/\">Home<\/a> \u2192 <a href=\"https:\/\/vibromera.eu\/glossary\/\">Glossary<\/a> \u2192 ISO 1940-2<\/div>\n <h1>ISO 1940-2 \u2014 <span>Vocabulary<\/span> for Balancing<\/h1>\n <div class=\"canon-badge\"><span style=\"font-size:14px;\">\ud83d\udccc<\/span> Canonical Reference Article \u2014 vibromera.eu<\/div>\n <p class=\"subtitle\">The international \"dictionary\" for rotor balancing \u2014 standardised definitions for unbalance types, rotor classifications, correction methods, machine types, and quality terminology. Now incorporated into ISO 21940-2.<\/p>\n <\/div>\n<\/header>\n\n<nav class=\"quick-nav\">\n <div class=\"quick-nav-inner\">\n <a href=\"#key-terms\">\ud83d\udd11 Key Terms<\/a>\n <a href=\"#rotor-terms\">\ud83d\udd29 Rotor Terms<\/a>\n <a href=\"#unbalance-terms\">\u2696 Unbalance Terms<\/a>\n <a href=\"#process-terms\">\ud83d\udd27 Process Terms<\/a>\n <a href=\"#machine-terms\">\ud83c\udfed Machine Terms<\/a>\n <a href=\"#quality-terms\">\ud83d\udccf Quality Terms<\/a>\n <a href=\"#definition\">\ud83d\udcd0 Overview<\/a>\n <a href=\"#detailed\">\ud83d\udcd6 Detailed Definitions<\/a>\n <a href=\"#cross-ref\">\ud83d\udd17 Cross-Reference<\/a>\n <a href=\"#faq\">\u2753 FAQ<\/a>\n <\/div>\n<\/nav>\n\n\n<section class=\"concepts-section\" id=\"key-terms\">\n <div class=\"container\">\n <div class=\"section-header\">\n <h2>Key Balancing Terms at a Glance<\/h2>\n <p>The most important definitions from ISO 1940-2 \u2014 the terms every balancing practitioner must know<\/p>\n <\/div>\n <div class=\"concept-grid\">\n <div class=\"concept-card cat-unbal\"><div class=\"cc-icon\">\u2696\ufe0f<\/div><div class=\"cc-term\">Unbalance<\/div><div class=\"cc-cat unbal\">Unbalance \u00b7 Core Concept<\/div><div class=\"cc-def\">Condition where the principal axis of inertia is not coincident with the rotational axis. Quantified as U = m \u00d7 r (mass \u00d7 radius, in g\u00b7mm). The primary cause of 1\u00d7 vibration in rotating machines.<\/div><\/div>\n <div class=\"concept-card cat-rotor\"><div class=\"cc-icon\">\ud83d\udd29<\/div><div class=\"cc-term\">Rigid Rotor<\/div><div class=\"cc-cat rotor\">Rotor \u00b7 Classification<\/div><div class=\"cc-def\">Rotor whose unbalance can be corrected in any two arbitrary planes and remains valid at all speeds up to max service speed. Balanced at low speed \u2192 stays balanced at high speed.<\/div><\/div>\n <div class=\"concept-card cat-rotor\"><div class=\"cc-icon\">\ud83c\udf00<\/div><div class=\"cc-term\">Flexible Rotor<\/div><div class=\"cc-cat rotor\">Rotor \u00b7 Classification<\/div><div class=\"cc-def\">Rotor that deforms elastically at service speed, changing its effective mass distribution. Must be balanced at or near service speed in more than two planes.<\/div><\/div>\n <div class=\"concept-card cat-unbal\"><div class=\"cc-icon\">\ud83d\udccd<\/div><div class=\"cc-term\">Static Unbalance<\/div><div class=\"cc-cat unbal\">Unbalance \u00b7 Type<\/div><div class=\"cc-def\">Inertia axis parallel to but displaced from rotation axis. Single \"heavy spot.\" Detectable on knife-edges. Causes in-phase bearing vibration. Corrected in one plane.<\/div><\/div>\n <div class=\"concept-card cat-unbal\"><div class=\"cc-icon\">\ud83d\udd04<\/div><div class=\"cc-term\">Couple Unbalance<\/div><div class=\"cc-cat unbal\">Unbalance \u00b7 Type<\/div><div class=\"cc-def\">Inertia axis intersects rotation axis at centre of gravity. Two equal opposite heavy spots create a rocking moment. Only detectable spinning. Out-of-phase bearing vibration. Two-plane correction.<\/div><\/div>\n <div class=\"concept-card cat-unbal\"><div class=\"cc-icon\">\ud83d\udcab<\/div><div class=\"cc-term\">Dynamic Unbalance<\/div><div class=\"cc-cat unbal\">Unbalance \u00b7 Type<\/div><div class=\"cc-def\">General case \u2014 inertia axis neither parallel to nor intersecting rotation axis. Combination of static + couple. The most common real-world condition. Requires two-plane balancing.<\/div><\/div>\n <div class=\"concept-card cat-proc\"><div class=\"cc-icon\">\ud83d\udcd0<\/div><div class=\"cc-term\">Correction Plane<\/div><div class=\"cc-cat proc\">Process \u00b7 Geometry<\/div><div class=\"cc-def\">Plane perpendicular to rotor axis where mass is added or removed. Must be physically accessible. May not coincide with bearing (tolerance) planes \u2014 geometric conversion required.<\/div><\/div>\n <div class=\"concept-card cat-qual\"><div class=\"cc-icon\">\u2705<\/div><div class=\"cc-term\">Residual Unbalance<\/div><div class=\"cc-cat qual\">Quality \u00b7 Tolerance<\/div><div class=\"cc-def\">Unbalance remaining after the balancing process. Must be \u2264 U<sub>per<\/sub> (permissible residual unbalance) for the specified G-grade. Measured in g\u00b7mm.<\/div><\/div>\n <div class=\"concept-card cat-qual\"><div class=\"cc-icon\">\ud83c\udfaf<\/div><div class=\"cc-term\">Balance Quality Grade (G)<\/div><div class=\"cc-cat qual\">Quality \u00b7 Classification<\/div><div class=\"cc-def\">Product of specific unbalance and angular velocity: G = e \u00d7 \u03c9 (mm\/s). Defines maximum centre-of-mass orbital velocity. G 6.3 is the industrial standard. Defined in ISO 1940-1.<\/div><\/div>\n <\/div>\n <\/div>\n<\/section>\n\n\n<section class=\"section-bg alt\" id=\"rotor-terms\">\n <div class=\"container\">\n <div class=\"section-header\">\n <h2>Complete Terminology Reference<\/h2>\n <p>All major terms from ISO 1940-2 \/ ISO 21940-2, organised by category<\/p>\n <\/div>\n\n \n <div class=\"table-wrap\">\n <div class=\"table-title\">\ud83d\udd29 Category 1 \u2014 Terms Related to the Rotor<\/div>\n <div class=\"table-scroll\"><table>\n <thead><tr><th>Term<\/th><th>Definition<\/th><th>Significance<\/th><\/tr><\/thead>\n <tbody>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Rotor<\/span><\/td><td>A body capable of rotation about a defined axis. In the context of balancing, includes any rotating component: shafts, impellers, armatures, drums, spindles.<\/td><td>The fundamental object of balancing. All other terms describe properties of, or actions on, the rotor.<\/td><\/tr>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Rigid Rotor<\/span><\/td><td>A rotor whose unbalance can be corrected in any two arbitrary planes, and after correction, the residual unbalance does not change significantly at any speed up to the maximum service speed.<\/td><td>Determines that <a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1<\/a> (G-grade system) applies. Balancing at low speed on a shop machine is valid. The vast majority of industrial rotors are rigid.<\/td><\/tr>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Flexible Rotor<\/span><\/td><td>A rotor that deforms elastically at its service speed such that its unbalance state changes. Must be corrected at or near service speed in more than two planes.<\/td><td>Requires ISO 21940-12. High-speed turbines, large generators, multi-stage compressors. Specialised high-speed balancing equipment needed.<\/td><\/tr>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Shaft Axis<\/span><\/td><td>The straight line joining the centres of the bearing journals. The geometrical axis of rotation.<\/td><td>The reference axis for all unbalance measurements. Runout of journals affects measurement accuracy.<\/td><\/tr>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Principal Axis of Inertia<\/span><\/td><td>The axis about which the rotor would rotate freely without producing centrifugal force or moment. Coincides with the shaft axis for a perfectly balanced rotor.<\/td><td>The mismatch between principal axis and shaft axis <em>is<\/em> unbalance. All correction aims to align these two axes.<\/td><\/tr>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Centre of Mass (Gravity)<\/span><\/td><td>The point where the entire rotor mass may be considered concentrated. For a balanced rotor, lies exactly on the shaft axis.<\/td><td>Static unbalance = CoM displaced from shaft axis. Specific unbalance (e) = displacement distance.<\/td><\/tr>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Service Speed<\/span><\/td><td>The maximum rotational speed at which the rotor operates in its intended application.<\/td><td>Critical for tolerance calculation: U<sub>per<\/sub> = (9 549 \u00d7 G \u00d7 M) \/ n. Always use service speed, not balancing speed.<\/td><\/tr>\n <tr><td><span class=\"tag rotor\">Rotor<\/span><br><span class=\"term-name\">Critical Speed<\/span><\/td><td>A rotational speed at which a rotor-bearing system experiences resonance, resulting in greatly amplified vibration.<\/td><td>Determines rigid\/flexible classification. A rigid rotor operates well below the first bending critical speed.<\/td><\/tr>\n <\/tbody>\n <\/table><\/div>\n <\/div>\n\n \n <div class=\"table-wrap\" id=\"unbalance-terms\">\n <div class=\"table-title\">\u2696 Category 2 \u2014 Terms Related to Unbalance<\/div>\n <div class=\"table-scroll\"><table>\n <thead><tr><th>Term<\/th><th>Definition<\/th><th>Formula \/ Units<\/th><\/tr><\/thead>\n <tbody>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\"><a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\">Unbalance<\/a><\/span><\/td><td>Condition where the principal axis of inertia is not coincident with the rotational axis. Causes centrifugal force proportional to mass, eccentricity, and speed squared.<\/td><td class=\"mono\">U = m \u00d7 r<br>(g\u00b7mm or kg\u00b7m)<\/td><\/tr>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\">Static Unbalance<\/span><\/td><td>Principal axis parallel to rotation axis but displaced. Equivalent to a single mass at a single radius. Detectable without rotation (knife-edges). In-phase bearing vibration.<\/td><td>Corrected in <strong>1 plane<\/strong><\/td><\/tr>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\">Couple Unbalance<\/span><\/td><td>Principal axis intersects rotation axis at the centre of mass but is tilted. Two equal, opposite heavy spots in different planes create a rocking moment. Only detectable while spinning.<\/td><td>Corrected in <strong>2 planes<\/strong><\/td><\/tr>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\">Dynamic Unbalance<\/span><\/td><td>The general case: principal axis neither parallel to nor intersecting the rotation axis. Combination of static and couple. The most common real-world condition.<\/td><td>Corrected in <strong>2 planes<\/strong><\/td><\/tr>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\">Specific Unbalance<\/span><\/td><td>Ratio of unbalance to rotor mass. Represents the eccentricity \u2014 the displacement of the centre of mass from the shaft axis. Allows quality comparison across different rotor sizes.<\/td><td class=\"mono\">e = U \/ M<br>(\u00b5m or g\u00b7mm\/kg)<\/td><\/tr>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\">Residual Unbalance<\/span><\/td><td>The unbalance remaining in a rotor after the balancing process. Must not exceed the permissible value (U<sub>per<\/sub>) for the specified <a href=\"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/\">G-grade<\/a>.<\/td><td class=\"mono\">U<sub>res<\/sub> \u2264 U<sub>per<\/sub><\/td><\/tr>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\">Initial Unbalance<\/span><\/td><td>The unbalance of a rotor as received, before any balancing correction. Measured on first run.<\/td><td>Baseline for the balancing procedure<\/td><\/tr>\n <tr><td><span class=\"tag unbal\">Unbalance<\/span><br><span class=\"term-name\">Unbalance Vector<\/span><\/td><td>The magnitude and angular position of unbalance in a given plane. Represented as a polar vector with amplitude (g\u00b7mm) and phase angle (\u00b0).<\/td><td class=\"mono\">U\u2220\u03b8<br>(g\u00b7mm at \u00b0 from ref)<\/td><\/tr>\n <\/tbody>\n <\/table><\/div>\n <\/div>\n\n \n <div class=\"table-wrap\" id=\"process-terms\">\n <div class=\"table-title\">\ud83d\udd27 Category 3 \u2014 Terms Related to the Balancing Process<\/div>\n <div class=\"table-scroll\"><table>\n <thead><tr><th>Term<\/th><th>Definition<\/th><th>Practical Notes<\/th><\/tr><\/thead>\n <tbody>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Balancing<\/span><\/td><td>The process of checking and adjusting the mass distribution of a rotor so that residual unbalance is within a specified tolerance.<\/td><td>Iterative: measure \u2192 calculate \u2192 correct \u2192 verify.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Correction Plane<\/span><\/td><td>A plane perpendicular to the rotor axis in which mass is added or removed. The physically accessible location for weight placement.<\/td><td>May differ from tolerance (bearing) planes \u2014 requires geometric conversion.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Tolerance Plane<\/span><\/td><td>The plane in which permissible unbalance is specified \u2014 typically the bearing plane. Unbalance here directly affects bearing loads.<\/td><td>U<sub>per<\/sub> is specified for tolerance planes; must be converted to correction planes.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Correction Mass<\/span><\/td><td>The physical mass (weight) added to or removed from the rotor at a specific radius and angle within the correction plane.<\/td><td>Added: clip-on, bolt-on, weld, epoxy. Removed: drilling, milling, grinding.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Trial Weight<\/span><\/td><td>A known mass temporarily attached to the rotor at a known radius and angle during the balancing procedure. Used to determine the rotor's response (influence coefficient).<\/td><td>The Balanset-1A trial-weight method: run \u2192 attach trial \u2192 run \u2192 software calculates correction.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Influence Coefficient<\/span><\/td><td>The change in vibration response (amplitude and phase) at a measurement point caused by a unit unbalance at a specific location. Characterises rotor-bearing sensitivity.<\/td><td>Calculated from trial-weight runs. Two-plane balancing requires a 2\u00d72 influence matrix.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Single-Plane Balancing<\/span><\/td><td>Procedure correcting static unbalance in one correction plane. Appropriate for short (disc-like) rotors with L\/D < 0.5.<\/td><td><a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> F2 mode. One sensor, one plane.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Two-Plane Balancing<\/span><\/td><td>Procedure correcting both static and couple unbalance in two correction planes. Required for elongated rotors or when couple unbalance is significant.<\/td><td><a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> F3 mode. Two sensors, two planes.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Trim Balancing<\/span><\/td><td>A final, fine balancing adjustment performed on an assembled rotor to compensate for assembly-introduced unbalance (coupling runout, fit tolerances).<\/td><td>Often performed in the field on the installed machine.<\/td><\/tr>\n <tr><td><span class=\"tag proc\">Process<\/span><br><span class=\"term-name\">Weight Splitting<\/span><\/td><td>Distributing a calculated correction mass between two adjacent accessible locations (e.g., two bolt holes or blade positions) when the exact angular position is not accessible.<\/td><td>Balanset-1A provides automatic weight-splitting calculation.<\/td><\/tr>\n <\/tbody>\n <\/table><\/div>\n <\/div>\n\n \n <div class=\"table-wrap\" id=\"machine-terms\">\n <div class=\"table-title\">\ud83c\udfed Category 4 \u2014 Terms Related to Balancing Machines<\/div>\n <div class=\"table-scroll\"><table>\n <thead><tr><th>Term<\/th><th>Definition<\/th><th>Comparison<\/th><\/tr><\/thead>\n <tbody>\n <tr><td><span class=\"tag mach\">Machine<\/span><br><span class=\"term-name\"><a href=\"https:\/\/vibromera.eu\/glossary\/balancing-machine\/\">Balancing Machine<\/a><\/span><\/td><td>A device that measures unbalance in a rotor (magnitude and angular position) so that mass distribution can be corrected.<\/td><td>Shop-based (stationary) or field (portable like <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a>).<\/td><\/tr>\n <tr><td><span class=\"tag mach\">Machine<\/span><br><span class=\"term-name\">Soft-Bearing Machine<\/span><\/td><td>Suspension is very flexible. Rotor runs above suspension natural frequency. Measures physical displacement. Must be calibrated for each rotor geometry.<\/td><td>Less common today. Lower cost, but operator must recalibrate per rotor. Displacement sensing.<\/td><\/tr>\n <tr><td><span class=\"tag mach\">Machine<\/span><br><span class=\"term-name\">Hard-Bearing Machine<\/span><\/td><td>Suspension is very stiff. Rotor runs below suspension natural frequency. Sensors measure centrifugal force directly. Permanently calibrated \u2014 accepts wide range of rotors without rotor-specific setup.<\/td><td><strong>Dominant type<\/strong> in modern industry. More versatile, faster setup. Force sensing.<\/td><\/tr>\n <tr><td><span class=\"tag mach\">Machine<\/span><br><span class=\"term-name\">Field Balancer<\/span><\/td><td>Portable instrument used to balance rotors in-situ (installed in the machine) without disassembly. Uses vibration sensors and a tachometer. Trial-weight method.<\/td><td><a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> (2-channel) and <a href=\"https:\/\/vibromera.eu\/product\/balanset-4\/\">Balanset-4<\/a> (4-channel). ISO 1940 tolerance calculator built in.<\/td><\/tr>\n <tr><td><span class=\"tag mach\">Machine<\/span><br><span class=\"term-name\">Mandrel (Arbor)<\/span><\/td><td>A shaft or adaptor on which a rotor is mounted for balancing on a machine. Must be accurately concentric and have negligible runout.<\/td><td>Mandrel eccentricity is a major source of systematic balancing error. Verified by index test.<\/td><\/tr>\n <\/tbody>\n <\/table><\/div>\n <\/div>\n\n \n <div class=\"table-wrap\" id=\"quality-terms\">\n <div class=\"table-title\">\ud83d\udccf Category 5 \u2014 Terms Related to Balance Quality & Measurement<\/div>\n <div class=\"table-scroll\"><table>\n <thead><tr><th>Term<\/th><th>Definition<\/th><th>Formula \/ Standard<\/th><\/tr><\/thead>\n <tbody>\n <tr><td><span class=\"tag qual\">Quality<\/span><br><span class=\"term-name\"><a href=\"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/\">Balance Quality Grade (G)<\/a><\/span><\/td><td>A classification specifying the maximum permissible velocity of the rotor's centre of mass. G = e<sub>per<\/sub> \u00d7 \u03c9. Grades form a logarithmic scale with factor 2.5.<\/td><td class=\"mono\">G 0.4 \u2026 G 4000<br>Defined in <a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1<\/a><\/td><\/tr>\n <tr><td><span class=\"tag qual\">Quality<\/span><br><span class=\"term-name\">Permissible Residual Unbalance (U<sub>per<\/sub>)<\/span><\/td><td>Maximum residual unbalance allowed for the specified G-grade, rotor mass, and service speed. The acceptance criterion.<\/td><td class=\"mono\">U<sub>per<\/sub> = (9549 \u00d7 G \u00d7 M) \/ n<\/td><\/tr>\n <tr><td><span class=\"tag qual\">Quality<\/span><br><span class=\"term-name\">Balance Tolerance<\/span><\/td><td>The range within which the residual unbalance must fall to meet the specified quality requirement. Equal to U<sub>per<\/sub>.<\/td><td>Specified per plane after allocation<\/td><\/tr>\n <tr><td><span class=\"tag qual\">Quality<\/span><br><span class=\"term-name\">Unbalance Reduction Ratio (URR)<\/span><\/td><td>Ratio of initial unbalance to residual unbalance after one correction cycle. Indicates balancing machine\/procedure efficiency.<\/td><td class=\"mono\">URR = U<sub>initial<\/sub> \/ U<sub>residual<\/sub><br>Typical: 5\u201350\u00d7<\/td><\/tr>\n <tr><td><span class=\"tag meas\">Measurement<\/span><br><span class=\"term-name\">Phase Angle<\/span><\/td><td>The angular position of the unbalance vector relative to a reference mark on the rotor (measured by tachometer). Combined with amplitude, defines the complete unbalance vector.<\/td><td class=\"mono\">\u00b0 (degrees, 0\u2013360)<\/td><\/tr>\n <tr><td><span class=\"tag meas\">Measurement<\/span><br><span class=\"term-name\">Vibration Velocity (<a href=\"https:\/\/vibromera.eu\/glossary\/rms\/\">RMS<\/a>)<\/span><\/td><td>Root-mean-square value of vibration velocity at a bearing housing. The standard measurement parameter for machine condition assessment per <a href=\"https:\/\/vibromera.eu\/glossary\/iso-10816-1\/\">ISO 10816<\/a>.<\/td><td class=\"mono\">mm\/s RMS (10\u20131000 Hz)<\/td><\/tr>\n <tr><td><span class=\"tag meas\">Measurement<\/span><br><span class=\"term-name\">Index Test<\/span><\/td><td>Verification procedure: rotate the rotor a defined angle (e.g. 180\u00b0) relative to the machine supports and remeasure. Detects mandrel and fixture errors.<\/td><td>Required for formal verification per ISO 1940-1 Ch. 10<\/td><\/tr>\n <tr><td><span class=\"tag meas\">Measurement<\/span><br><span class=\"term-name\">Minimum Achievable Residual Unbalance (U<sub>mar<\/sub>)<\/span><\/td><td>The lowest residual unbalance achievable on a given balancing machine for a specific rotor. Determined by machine sensitivity, noise floor, and bearing conditions.<\/td><td>U<sub>mar<\/sub> must be \u2264 U<sub>per<\/sub> for the machine to be suitable for the required G-grade.<\/td><\/tr>\n <\/tbody>\n <\/table><\/div>\n <\/div>\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 ISO 1940-2?<\/h2>\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>ISO 1940-2<\/strong> (<em>Mechanical vibration \u2014 Balance quality requirements \u2014 Vocabulary<\/em>) is the international standard that defines the terminology used in rotor balancing. It provides precise, physics-based definitions for all key terms \u2014 from <a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\">unbalance<\/a> types (static, couple, dynamic) to rotor classifications (rigid, flexible), correction methods, <a href=\"https:\/\/vibromera.eu\/glossary\/balancing-machine\/\">machine types<\/a>, and quality grades. It is the essential \"dictionary\" supporting <a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1<\/a> and all other balancing standards. Superseded by <strong>ISO 21940-2<\/strong> with identical terminology.<\/p>\n <\/div>\n\n <p>When an engineer in Germany specifies \"dynamic unbalance correction to G 6.3 in two planes,\" a technician in Japan must understand exactly what is required \u2014 the same rotor condition, the same balancing procedure, and the same acceptance criterion. ISO 1940-2 makes this possible by providing a single, internationally agreed vocabulary for the entire field.<\/p>\n\n <p>The standard is not a procedure or a tolerance specification \u2014 it is a <em>terminology standard<\/em>. Its role is to eliminate ambiguity so that other standards (<a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1<\/a> for tolerances, <a href=\"https:\/\/vibromera.eu\/glossary\/iso-14694\/\">ISO 14694<\/a> for fans, <a href=\"https:\/\/vibromera.eu\/glossary\/iso-10816-1\/\">ISO 10816<\/a> for vibration evaluation) can use precise, unambiguous language.<\/p>\n\n <h2 id=\"detailed\">Detailed Term Analysis<\/h2>\n\n <h3>The Rigid \/ Flexible Distinction<\/h3>\n <p>This is the single most important classification in balancing. The distinction determines everything: which standard applies, what equipment is needed, how many planes are required, and at what speed balancing must be performed.<\/p>\n\n <div class=\"info-box\">\n <div class=\"box-title\">Rigid Rotor (ISO 1940-2 definition)<\/div>\n <p>A rotor whose unbalance can be corrected in any two arbitrary planes and, after correction, the residual unbalance does not change significantly at any speed up to the maximum service speed. <strong>Practical test:<\/strong> if the first bending <a href=\"https:\/\/vibromera.eu\/glossary\/natural-frequency\/\">critical speed<\/a> is well above the maximum service speed (typically > 1.5\u00d7 or more), the rotor is rigid.<\/p>\n <\/div>\n\n <div class=\"info-box warning\">\n <div class=\"box-title\">Flexible Rotor (ISO 1940-2 definition)<\/div>\n <p>A rotor that deforms elastically at its service speed such that its unbalance state changes. Must be balanced at or near service speed in more than two planes. <strong>Applies to:<\/strong> large turbogenerators, multi-stage high-speed compressors, long paper machine rolls at high speed. Covered by ISO 21940-12.<\/p>\n <\/div>\n\n <p>The vast majority of industrial rotors \u2014 electric motors, fans, pumps, flywheels, shafts \u2014 are rigid rotors. The <a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1<\/a> G-grade system applies directly to rigid rotors.<\/p>\n\n <h3>The Three Types of Unbalance<\/h3>\n <p>ISO 1940-2 defines three fundamental types based on the geometric relationship between the principal inertia axis and the rotation axis. Understanding these is essential for selecting the correct balancing procedure:<\/p>\n\n <div class=\"formula-box\">\n <div class=\"formula-label\">Unbalance Vector<\/div>\n <div class=\"formula-main\">U = m \u00d7 r (magnitude) U\u2220\u03b8 (polar form)<\/div>\n <div class=\"formula-note\">m = unbalanced mass (g) | r = distance from axis (mm) | \u03b8 = angular position (\u00b0)<\/div>\n <\/div>\n\n <ul>\n <li><strong>Static unbalance<\/strong> produces a <em>force<\/em> \u2014 both bearings vibrate in phase at 1\u00d7 RPM. The rotor can be detected as unbalanced without rotation (gravity reveals it on knife-edges). One correction plane suffices. Typical for narrow disc-like rotors (L\/D < 0.5): narrow pulleys, fan impellers, thin flywheels.<\/li>\n <li><strong>Couple unbalance<\/strong> produces a <em>moment<\/em> \u2014 bearings vibrate 180\u00b0 out of phase at 1\u00d7 RPM. The net force is zero (centre of mass is on the axis), but two equal and opposite heavy spots in different axial positions create a rocking couple. Only detectable while spinning. Requires two correction planes.<\/li>\n <li><strong>Dynamic unbalance<\/strong> = static + couple combined. The general case for all real rotors that are not perfectly symmetric. Both force and moment are present. Bearings vibrate at 1\u00d7 with neither in-phase nor exactly 180\u00b0 out-of-phase relationship. Requires two-plane balancing.<\/li>\n <\/ul>\n\n <h3>Specific Unbalance and the G-Grade Connection<\/h3>\n <p><strong>Specific unbalance<\/strong> (e = U\/M) is the key metric that enables universal balance quality comparison. A 5 kg rotor with 50 g\u00b7mm unbalance has e = 10 \u00b5m. A 500 kg rotor with 5 000 g\u00b7mm unbalance also has e = 10 \u00b5m \u2014 identical balance quality despite 100\u00d7 mass difference.<\/p>\n <p>The <a href=\"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/\">G-grade<\/a> extends this by incorporating speed: G = e \u00d7 \u03c9, giving a single number (mm\/s) that characterises balance quality independently of both mass and speed. This is the foundation of the <a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1<\/a> tolerance system.<\/p>\n\n <h3>Correction Planes vs. Tolerance Planes<\/h3>\n <p>ISO 1940-2 draws a critical distinction that is often missed in practice:<\/p>\n <ul>\n <li><strong>Tolerance planes<\/strong> = the bearing planes where vibration and dynamic loads are most critical. Permissible unbalance U<sub>per<\/sub> is specified here.<\/li>\n <li><strong>Correction planes<\/strong> = physically accessible locations where weights can be placed (fan hub, motor end-rings, shaft shoulders). Often at different axial positions than the bearings.<\/li>\n <\/ul>\n <p>Converting U<sub>per<\/sub> from tolerance planes to correction planes requires knowledge of rotor geometry. For asymmetric or overhung rotors, this conversion can significantly change the per-plane tolerances. The <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> handles this conversion automatically when rotor dimensions are entered.<\/p>\n\n <h3>Balancing Machine Types<\/h3>\n <p>The two fundamental machine types reflect different physical measurement principles:<\/p>\n <ul>\n <li><strong>Soft-bearing:<\/strong> Suspension natural frequency well below operating speed \u2192 machine measures <em>displacement<\/em>. Requires calibration for each new rotor. Historically significant; declining in use.<\/li>\n <li><strong>Hard-bearing:<\/strong> Suspension natural frequency well above operating speed \u2192 machine measures <em>force<\/em>. Permanently calibrated \u2014 accepts different rotors without individual calibration. The dominant modern type.<\/li>\n <\/ul>\n <p>Field balancing instruments like the <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> use a different principle: they are not a \"machine\" in the ISO sense but use the rotor's own bearings and support as the measurement system, employing the trial-weight (influence coefficient) method to determine correction without requiring a dedicated balancing machine.<\/p>\n\n <h2 id=\"cross-ref\">Cross-Reference: Where Each Term Is Used<\/h2>\n\n <div class=\"info-box success\">\n <div class=\"box-title\">Standards That Reference ISO 1940-2 Vocabulary<\/div>\n <p><strong><a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1 \/ ISO 21940-11<\/a>:<\/strong> Uses all tolerance and quality terms \u2014 G-grade, U<sub>per<\/sub>, balance tolerance, residual unbalance. The primary consumer of this vocabulary.<\/p>\n <p><strong><a href=\"https:\/\/vibromera.eu\/glossary\/iso-14694\/\">ISO 14694<\/a>:<\/strong> Uses rotor terms (rigid), unbalance terms, and extends with fan-specific BV\/FV categories built on G-grades.<\/p>\n <p><strong><a href=\"https:\/\/vibromera.eu\/glossary\/iso-10816-1\/\">ISO 10816 \/ ISO 20816<\/a>:<\/strong> Uses measurement terms \u2014 vibration velocity, RMS, bearing housing measurement points.<\/p>\n <p><strong>ISO 21940-12:<\/strong> Extends flexible rotor definition with multi-speed, multi-plane procedures.<\/p>\n <p><strong>API 610 \/ API 617:<\/strong> Petroleum standards reference ISO 1940 G-grades and unbalance terminology for pump and compressor specifications.<\/p>\n <\/div>\n\n <h2>ISO 1940-2 \u2192 ISO 21940-2: Transition<\/h2>\n <p>ISO 21940-2 has formally superseded ISO 1940-2. The terminology is identical \u2014 all definitions carry forward unchanged. The ISO 21940 numbering reflects integration into the comprehensive ISO 21940 series covering all aspects of mechanical vibration and balancing. Both designations are accepted in industry practice.<\/p>\n\n <hr style=\"margin:48px 0 24px;border:none;border-top:1px solid var(--border-light);\">\n <p><strong>Official standard:<\/strong> <a href=\"https:\/\/www.iso.org\/standard\/6636.html\" rel=\"nofollow noopener\" target=\"_blank\">ISO 1940-2 on ISO Store \u2192<\/a><\/p>\n <p><a href=\"https:\/\/vibromera.eu\/glossary\/\">\u2190 Back to Glossary Index<\/a><\/p>\n <\/article>\n\n <aside class=\"toc-sidebar\">\n <div class=\"toc-box\">\n <h3>On This Page<\/h3>\n <a href=\"#key-terms\">Key Terms (9 cards)<\/a>\n <a href=\"#rotor-terms\">Rotor Terms (8)<\/a>\n <a href=\"#unbalance-terms\">Unbalance Terms (8)<\/a>\n <a href=\"#process-terms\">Process Terms (10)<\/a>\n <a href=\"#machine-terms\">Machine Terms (5)<\/a>\n <a href=\"#quality-terms\">Quality & Measurement (8)<\/a>\n <a href=\"#definition\">What is ISO 1940-2?<\/a>\n <a href=\"#detailed\">Detailed Term Analysis<\/a>\n <a class=\"sub\" href=\"#detailed\">Rigid vs. Flexible<\/a>\n <a class=\"sub\" href=\"#detailed\">Three unbalance types<\/a>\n <a class=\"sub\" href=\"#detailed\">Specific unbalance & G<\/a>\n <a class=\"sub\" href=\"#detailed\">Correction vs. tolerance planes<\/a>\n <a class=\"sub\" href=\"#detailed\">Machine types<\/a>\n <a href=\"#cross-ref\">Cross-Reference<\/a>\n <a href=\"#faq\">FAQ (7 Questions)<\/a>\n <\/div>\n <div class=\"toc-box\" style=\"margin-top:24px;background:var(--navy);border-color:var(--navy);\">\n <h3 style=\"color:#fff;\">Balancing Equipment<\/h3>\n <p style=\"color:rgba(255,255,255,.6);font-size:13px;margin-bottom:12px;\">Portable field balancing with built-in ISO 1940 vocabulary in every menu \u2014 G-grade calculator, plane allocation, balance reports.<\/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,.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,.2);border-left:none;\">Balanset-4 \u2192<\/a>\n <\/div>\n <\/aside>\n <\/div>\n <\/div>\n<\/main>\n\n\n<section class=\"section-bg alt\" id=\"faq\">\n <div class=\"container\" style=\"max-width:1000px;\">\n <div class=\"section-header\"><h2>Frequently Asked Questions \u2014 ISO 1940-2<\/h2><p>Balancing vocabulary and terminology<\/p><\/div>\n <div style=\"display:flex;flex-direction:column;gap:16px;\">\n <details style=\"background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius);overflow:hidden;\"><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;\"><span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is ISO 1940-2?<\/summary><div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">ISO 1940-2 is the international vocabulary standard for rotor balancing. It defines terms such as unbalance, rigid rotor, flexible rotor, correction plane, residual unbalance, G-grade, balancing machine types, and dozens more. It is the \"dictionary\" that supports all other balancing standards. Superseded by ISO 21940-2 with identical definitions.<\/div><\/details>\n <details style=\"background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius);overflow:hidden;\"><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;\"><span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is the difference between static and dynamic unbalance?<\/summary><div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\"><strong>Static<\/strong>: inertia axis parallel but displaced \u2014 single heavy spot \u2014 detectable without rotation \u2014 one-plane correction \u2014 in-phase bearing vibration. <strong>Dynamic<\/strong>: general case (static + couple combined) \u2014 inertia axis skewed \u2014 two-plane correction required \u2014 bearings vibrate with neither in-phase nor 180\u00b0 relationship. Most real rotors have dynamic unbalance.<\/div><\/details>\n <details style=\"background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius);overflow:hidden;\"><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;\"><span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is the difference between a rigid and flexible rotor?<\/summary><div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\"><strong>Rigid<\/strong>: balanced at low speed \u2192 stays balanced at all speeds up to max service speed. Two correction planes suffice. Covered by <a href=\"https:\/\/vibromera.eu\/glossary\/iso-1940-1\/\">ISO 1940-1<\/a>. <strong>Flexible<\/strong>: elastic deformation at service speed changes mass distribution. Must balance at\/near service speed, > 2 planes. Covered by ISO 21940-12. Most industrial rotors are rigid.<\/div><\/details>\n <details style=\"background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius);overflow:hidden;\"><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;\"><span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is residual unbalance?<\/summary><div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">The small amount of <a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\">unbalance<\/a> remaining after balancing. Must be \u2264 U<sub>per<\/sub> (permissible) for the specified <a href=\"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/\">G-grade<\/a>. U<sub>per<\/sub> = (9 549 \u00d7 G \u00d7 M) \/ n. The <a href=\"https:\/\/vibromera.eu\/product\/balanset-1\/\">Balanset-1A<\/a> compares measured residual against this limit automatically.<\/div><\/details>\n <details style=\"background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius);overflow:hidden;\"><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;\"><span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is the difference between correction plane and tolerance plane?<\/summary><div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\"><strong>Tolerance plane<\/strong> = bearing plane where U<sub>per<\/sub> is specified and loads are critical. <strong>Correction plane<\/strong> = where weights are actually placed (may be far from bearings). Tolerances must be geometrically converted between them. Errors here cause rotors that \"pass\" on the correction planes but fail at the bearings.<\/div><\/details>\n <details style=\"background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius);overflow:hidden;\"><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;\"><span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> Soft-bearing vs. hard-bearing balancing machine?<\/summary><div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\"><strong>Soft-bearing<\/strong>: flexible suspension, runs above natural frequency, measures displacement. Must recalibrate per rotor. <strong>Hard-bearing<\/strong>: stiff suspension, runs below natural frequency, measures force. Permanently calibrated \u2014 dominant in modern industry. Field balancers (Balanset-1A) use a different approach: trial-weight method on the machine's own bearings.<\/div><\/details>\n <details style=\"background:var(--white);border:1px solid var(--border-light);border-radius:var(--radius);overflow:hidden;\"><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;\"><span style=\"color:var(--blue);font-size:18px;\">\u25b8<\/span> What is specific unbalance (eccentricity)?<\/summary><div style=\"padding:0 24px 18px 52px;font-size:15px;line-height:1.7;color:var(--text);\">e = U \/ M (unbalance divided by mass). Units: \u00b5m or g\u00b7mm\/kg. Represents the actual displacement of the centre of mass from the rotation axis. Enables comparison across different rotor sizes. The G-grade is e \u00d7 \u03c9 \u2014 specific unbalance \u00d7 angular velocity (mm\/s).<\/div><\/details>\n <\/div>\n <\/div>\n<\/section>\n\n\n<section style=\"padding:32px 0;background:var(--white);border-top:1px solid var(--border-light);\">\n <div class=\"container\" style=\"max-width:1000px;\">\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\/iso-1940-1\/\" style=\"display:block;padding:18px;background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius-sm);text-decoration:none;box-shadow:var(--shadow-sm);\"><div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">ISO 1940-1<\/div><div style=\"font-size:13px;color:var(--text-secondary);\">G-grade tolerance system \u2014 uses this vocabulary throughout<\/div><\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/balance-quality-grade\/\" style=\"display:block;padding:18px;background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius-sm);text-decoration:none;box-shadow:var(--shadow-sm);\"><div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Balance Quality Grade<\/div><div style=\"font-size:13px;color:var(--text-secondary);\">Interactive G-grade calculator with full tolerance tables<\/div><\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/unbalance\/\" style=\"display:block;padding:18px;background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius-sm);text-decoration:none;box-shadow:var(--shadow-sm);\"><div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Unbalance<\/div><div style=\"font-size:13px;color:var(--text-secondary);\">Detailed treatment of the core concept defined here<\/div><\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/iso-14694\/\" style=\"display:block;padding:18px;background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius-sm);text-decoration:none;box-shadow:var(--shadow-sm);\"><div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">ISO 14694<\/div><div style=\"font-size:13px;color:var(--text-secondary);\">Fan-specific BV\/FV categories built on this vocabulary<\/div><\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/natural-frequency\/\" style=\"display:block;padding:18px;background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius-sm);text-decoration:none;box-shadow:var(--shadow-sm);\"><div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Natural Frequency<\/div><div style=\"font-size:13px;color:var(--text-secondary);\">Critical speeds and the rigid\/flexible rotor boundary<\/div><\/a>\n <a href=\"https:\/\/vibromera.eu\/glossary\/rotor-balancing\/\" style=\"display:block;padding:18px;background:var(--beige-light);border:1px solid var(--beige-dark);border-radius:var(--radius-sm);text-decoration:none;box-shadow:var(--shadow-sm);\"><div style=\"font-weight:700;color:var(--navy);font-size:15px;margin-bottom:4px;\">Rotor Balancing<\/div><div style=\"font-size:13px;color:var(--text-secondary);\">Procedures using the terminology defined here<\/div><\/a>\n <\/div>\n <\/div>\n<\/section>\n\n<section class=\"shop-cta\">\n <div class=\"container\">\n <h2>Speak the Language \u2014 With the Right Tools<\/h2>\n <p>Vibromera balancers implement ISO vocabulary directly: G-grade selection, unbalance vectors, correction planes, residual vs. permissible comparison \u2014 all in one portable instrument.<\/p>\n <a href=\"https:\/\/vibromera.eu\/shop\/\" class=\"cta-btn\">Browse Balancing Equipment \u2192<\/a>\n <\/div>\n<\/section>\n\n<footer class=\"page-footer\">\n <div class=\"container\"><p><a href=\"https:\/\/vibromera.eu\/glossary\/\">\u2190 Back to Glossary<\/a> | <a href=\"https:\/\/vibromera.eu\/\">vibromera.eu<\/a><\/p><\/div>\n<\/footer>\n\n<\/body>\n<\/html><\/div><\/div><\/div><\/div><\/div>","protected":false},"excerpt":{"rendered":"<p>En oversikt over ISO 1940-2, standarden som definerer det viktigste vokabularet og terminologien som brukes innen rotorbalansering for \u00e5 sikre tydelig og konsekvent kommunikasjon.<\/p>","protected":false},"featured_media":0,"template":"","meta":{"ai_generated_summary":"","footnotes":""},"categories":[109,112],"tags":[],"class_list":["post-10","glossary","type-glossary","status-publish","hentry","category-glossary","category-iso-standards"],"_links":{"self":[{"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/glossary\/10","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/glossary"}],"about":[{"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/types\/glossary"}],"version-history":[{"count":4,"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/glossary\/10\/revisions"}],"predecessor-version":[{"id":101492,"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/glossary\/10\/revisions\/101492"}],"wp:attachment":[{"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/media?parent=10"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/categories?post=10"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/vibromera.eu\/nb\/wp-json\/wp\/v2\/tags?post=10"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}