Dynamic Shaft Balancing Instruction: Static vs Dynamic, Field Procedure & ISO 21940 Grades
Everything a field engineer needs to balance rotors on-site — from the physics of unbalance to the final verification run. Seven-step procedure, trial weight formulas, correction angle measurement, and ISO tolerance tables. Based on Vibromera field work across fans, mulchers, crushers, and shafts.
¿Qué es el balanceo dinámico?
Balanceo dinámico is the process of measuring and correcting the uneven mass distribution of a rotating body (rotor) while it spins at operating speed. Unlike static balancing, which corrects mass offset in a single plane, dynamic balancing addresses imbalance in two or more planes simultaneously, eliminating both centrifugal force and rocking couple that cause bearing vibration.
Every rotating part — from a 200 kg mulcher rotor to a 5 g dental drill spindle — has some residual unbalance. Manufacturing tolerances, material inconsistencies, corrosion, and accumulated deposits shift the mass centre away from the geometric rotation axis. The result is a centrifugal force that grows with the square of speed: double the RPM and the force quadruples.
A rotor spinning at 3,000 RPM with just 10 g of unbalance at a 150 mm radius generates roughly 150 N of rotating force — enough to destroy bearings in weeks. Dynamic balancing reduces this force to a level specified by international standards (ISO 21940‑11, formerly ISO 1940), extending bearing life from months to years and cutting vibration‑related downtime.
Balanceo estático vs dinámico
The rotor's centre of gravity is offset from the rotation axis in un plano. Al colocarse sobre soportes de cuchilla, el lado pesado rueda hacia abajo: esto se puede detectar sin girar.
Corrección: agregar o retirar masa en una sola posición angular opuesta al punto pesado. Un solo plano de corrección es suficiente.
Aplica a: narrow disc-shaped parts where L/D is below about 0.5 - flywheels, grinding wheels, single-disc impellers, saw blades, brake discs.
Dos (o más) desplazamientos de masa se encuentran en planos diferentes a lo largo del rotor. Pueden cancelarse entre sí estáticamente: el rotor permanece quieto sobre los soportes de cuchilla, pero generan un par de fuerzas oscilante al girar. Este par no puede detectarse ni corregirse sin rotación.
Corrección: dos pesos de compensación en dos planos separados. El instrumento calcula la masa y el ángulo para cada plano a partir de la matriz de coeficientes de influencia.
Aplica a: rotores alargados: ejes, ventiladores con impulsores anchos, rotores de mulcher, rodillos, impulsores de bombas multietapa, turbinas.
Cuatro tipos de desbalance
La norma ISO 21940-11 distingue cuatro patrones fundamentales de desbalance. Comprender cuál predomina ayuda a elegir la estrategia de balanceo correcta.
En la práctica, casi todos los rotores que se encuentran en campo tienen desbalance dinámico: una combinación de componentes de fuerza y par. Por eso, el balanceo en dos planos es el procedimiento predeterminado para cualquier rotor que no sea un disco delgado.
When to Use Single‑Plane vs Two‑Plane Balancing
The deciding factor is the rotor's geometry ratio L/D (axial length to outer diameter) combined with its operating speed.
| Criterio | Single‑Plane (1 sensor) | Two‑Plane (2 sensors) |
|---|---|---|
| L/D ratio | L/D < 0.5 (narrow disc-like rotor) | L/D >= 0.5, or significant axial mass distribution |
| Typical parts | Grinding wheel, flywheel, single‑disc impeller, pulley, brake disc, saw blade | Fan rotor, mulcher, shaft, roller, multi‑stage pump, turbine, crusher |
| Unbalance types corrected | Static only (force) | Static + couple + dynamic (force + moment) |
| Planos de corrección | 1 | 2 |
| Measurement runs | 2 (initial + 1 trial) | 3 (initial + 2 trials, one per plane) |
| Time on site | 15–20 min | 30–45 min |
ISO 21940‑11 Balance Quality Grades
ISO 21940‑11 (the successor to ISO 1940‑1) assigns each class of rotating machinery a balance quality grade G, defined as the maximum permissible velocity of the rotor's centre of gravity in mm/s. The permissible residual specific unbalance epor (in g·mm/kg) is derived from the grade and the operating speed:
G — balance quality grade (e.g. 6.3 means 6.3 mm/s)
ω — angular velocity, rad/s
RPM — operating speed, rev/min
| Grado | e·ω, mm/s | Tipos de máquina |
|---|---|---|
G 0.4 |
0.4 | Gyroscopes, spindles of precision grinding machines |
G 1.0 |
1.0 | Turbochargers, gas turbines, small electric armatures with special requirements |
G 2.5 |
2.5 | Electric motors, generators, medium/large turbines, pumps with special requirements |
G 6.3 |
6.3 | Fans, pumps, process machinery, flywheels, centrifuges, general industrial machinery |
G 16 |
16 | Agricultural machinery, crushers, drive shafts (cardan), parts of crushing machines |
G 40 |
40 | Passenger car wheels, crankshaft assemblies (series production) |
G 100 |
100 | Fast diesel engine crankshaft assemblies with six or more cylinders |
Worked Example: Fan Rotor
A centrifugal fan rotor weighs 80 kg, operates at 1,450 RPM, and the correction radius is 250 mm. Required grade: G 6.3.
At correction radius 250 mm: max residual mass = 3320 / 250 = 13.3 g total residual mass
For a two-plane job, distribute that total tolerance between planes; a simple equal split gives about 6.6 g per plane.
Related standards: ISO 21940‑11 (rigid rotors), ISO 21940‑12 (flexible rotors), ISO 10816‑3 (vibration severity limits), ISO 1940 (legacy predecessor).
Seven‑Step Field Balancing Procedure
This is the influence coefficient method for two‑plane field balancing, applied with a portable instrument such as the Balanset‑1A. The same logic works with any two‑channel balancing analyser.
Mt = Mr × K / (Rt × (N/100)²) where Mr = rotor mass (g), K = support stiffness coefficient (1–5, use 3 for average), Rt = installation radius (cm), N = RPM. Or use our online trial weight calculator — enter your rotor parameters and get the recommended mass instantly.
Trial Weight Calculation
The trial weight must be heavy enough to produce a noticeable vibration change, but light enough not to overload bearings or create a dangerous condition. The standard empirical formula accounts for rotor mass, correction radius, operating speed, and support stiffness:
Mr — rotor mass, grams
K — support stiffness coefficient (1 = soft mounts, 3 = average, 5 = rigid foundation)
Rt — trial weight installation radius, cm
N — operating speed, RPM
Don't want to do the maths by hand? Use our online trial weight calculator ↗ — enter your rotor parameters, support type, and vibration level, and get the recommended mass instantly.
Worked Examples (K = 3, average stiffness)
| Máquina | Masa del rotor | RPM | Radio | Trial weight (K = 3) |
|---|---|---|---|---|
| Rotor de moldeadora | 120 kg | 2,200 | 30 cm | 360,000 / (30 × 484) ≈ 25 g |
| Ventilador industrial | 80 kg | 1,450 | 40 cm | 240,000 / (40 × 210.25) ≈ 29 g |
| Centrifuge drum | 45 kg | 3,000 | 15 cm | 135,000 / (15 × 900) = 10 g |
| Crusher shaft | 250 kg | 900 | 25 cm | 750,000 / (25 × 81) ≈ 370 g |
Medición del ángulo de corrección
The balancing instrument outputs two numbers per plane: masa (how much weight) and ángulo (where to place it). The angle is always referenced to the trial weight position.
How to Measure the Angle
- Reference point (0°): the angular position where you placed the trial weight. Mark it clearly on the rotor before the trial run.
- Measurement direction: always in the direction of rotor rotation.
- Reading the angle: the instrument displays angle f₁ for Plane 1 and f₂ for Plane 2. From the trial weight mark, count that many degrees in the rotation direction — that is where the correction weight goes.
- If removing mass: place the correction at 180° opposite the indicated "add" position.
Weight Splitting to Fixed Positions
When the rotor has pre‑drilled holes or fixed mounting positions (e.g. fan blade bolts), you may not be able to place a weight at the exact calculated angle. The Balanset‑1A includes a weight splitting function: you enter the angles of the two nearest available positions, and the software decomposes the single correction vector into two smaller weights at those positions. The combined effect matches the original vector.
Correction Planes & Sensor Placement
El plano de corrección es la posición axial en el rotor donde se agrega o retira masa. El sensor mide la vibración en el rodamiento más cercano. Algunas reglas clave:
- El sensor se coloca en el soporte del rodamiento — lo más cerca posible de la línea central del rodamiento, en dirección radial (horizontal preferible).
- El plano 1 corresponde al sensor 1, el plano 2 al sensor 2. Mantenga la numeración consistente o el software intercambiará los planos de corrección.
- Maximise plane separation: the further apart the two correction planes, the better the couple resolution. Minimum practical separation is ⅓ of the bearing span.
- Elija posiciones accesibles: el plano de corrección debe ser una ubicación donde pueda fijar físicamente los pesos: el borde de una brida, un círculo de pernos, un aro o una superficie de soldadura.
En la foto anterior, un rotor de mulcher está preparado para balanceo en dos planos. Las marcas azules 1 y 2 indican las posiciones de los sensores en los soportes de los rodamientos. Las marcas rojas 1 y 2 muestran los planos de corrección: en este caso, los extremos con brida del cuerpo del rotor donde se soldarán los pesos.
Rotor en voladizo (sobresaliente)
Los rotores en voladizo: impulsores de ventiladores, volantes montados fuera del tramo de rodamientos, impulsores de bombas, requieren un diseño diferente de sensores y planos. Ambos planos de corrección están del mismo lado de los rodamientos, y la colocación de los sensores debe tener en cuenta que la masa en voladizo amplifica el desbalance de par.
Aplicaciones por tipo de máquina
Métodos de fijación de pesos
| Método | Attachment | Ideal para | Límites |
|---|---|---|---|
| Welding | Steel washers or plates tack‑welded to rotor rim | Mulchers, crushers, heavy industrial rotors | Permanent. Cannot use on aluminium or stainless without special rod |
| Bolts & nuts | Bolts through pre‑drilled holes with locknuts | Fan impellers, flywheels, coupling flanges | Requires existing holes or new drilling |
| Hose clamps | Stainless‑steel hose clamp with weight sandwiched | Shafts, rollers, cylindrical rotors in the field | Temporary or semi‑permanent. Verify clamp torque |
| Set‑screw clip‑on | Pre‑made clip‑on weights (like tyre weights) | Fan blades, thin rims, light rotors | Limited mass range. May slip at high RPM |
| Adhesive (epoxy) | Weight glued to surface | Precision rotors, clean environments | Requires clean dry surface. Temperature limit ~120°C |
| Retiro de material | Drilling or grinding material away from heavy side | Turbochargers, high‑speed spindles, impellers | Permanent and precise but irreversible. Use when adding weight is not safe |
Errores comunes en el balanceo en campo
| # | Error | Consequence | Fix |
|---|---|---|---|
| 1 | Sensor mounted on a guard or cover | Resonance of the cover distorts amplitude and phase readings → wrong correction | Always mount on the bearing housing metal surface |
| 2 | Peso de prueba demasiado ligero | Phase and amplitude change is within noise → influence coefficients are unreliable | Ensure 20-30% amplitude change or 20-30 degrees of phase shift at least one sensor |
| 3 | Speed variation between runs | Vibration at 1× changes with RPM² — even 5% speed change corrupts the data | Use a tachometer for precise RPM tracking. Wait for speed to stabilise |
| 4 | Forgetting to remove the trial weight | Correction calculation includes trial weight effect → result is meaningless | Follow a strict routine: remove trial weight before installing correction weights |
| 5 | Mixing up Plane 1 and Plane 2 | Correction weights go in the wrong planes → vibration increases | Label sensors and planes clearly. Sensor 1 → Plane 1, Sensor 2 → Plane 2 |
| 6 | Measuring angle opposite to rotation | Correction goes 360° − f instead of f → opposite side of rotor | Confirm rotation direction before starting. Always measure in rotation direction |
| 7 | Thermal growth during runs | Bearing clearance changes between cold start runs → drifting measurements | Either warm up to steady state before run 0, or complete all runs quickly (<5 min apart) |
| 8 | Using single‑plane on a long rotor | Couple unbalance remains uncorrected → vibration may even increase at the far bearing | Use two-plane balancing for any rotor where L/D >= 0.5, plane separation is significant, or single-plane correction affects the far bearing |
Field Report: Mulcher Rotor Balancing
Máquina: Maschio Bisonte 280 flail mulcher, 165 kg rotor, 2,100 RPM PTO speed. Client reported severe vibration after replacing 8 flails.
Configuración: Two accelerometers on bearing housings, laser tachometer on PTO shaft. Balanset-1A two-plane mode.
Run 0: Sensor 1 = 12.4 mm/s @ 47°, Sensor 2 = 8.9 mm/s @ 213°. ISO 10816-3 zone D (danger).
Trial runs: 500 g trial weight used in both planes. Clear response — amplitude change >60% at both sensors.
Corrección: Plane 1: 340 g welded at 128°. Plane 2: 215 g welded at 276°.
Verificación: Sensor 1 = 0.8 mm/s, Sensor 2 = 0.6 mm/s. ISO zone A (good). No trim run needed.
Two‑Plane Dynamic Balancing of a Fan
Industrial fans — centrifugal, axial, and mixed‑flow — are among the most common rotors balanced in the field. The procedure below walks through a real two‑plane job on a radial fan using the Balanset‑1A.
Determinación de planos e instalación de sensores
Clean the surfaces for sensor installation from dirt and oil. Sensors must fit snugly to the metal surface of the bearing housing — never mount on covers, guards, or unsupported sheet‑metal panels.
- Sensor 1 (red): Install closer to the front of the fan (Plane 1 side).
- Sensor 2 (green): Install closer to the rear of the fan (Plane 2 side).
- Plane 1 (red zone): Correction plane on the impeller disc, closer to the front.
- Plane 2 (green zone): Correction plane closer to the back plate or hub.
Connect both vibration sensors and the laser tachometer to the Balanset‑1A. Attach reflective tape to the shaft or hub for RPM reference.
Proceso de balanceo
Start the fan and take initial vibration measurements (Run 0). Install a trial weight of known mass on Plane 1 at an arbitrary point, run the fan, and record the vibration change (Run 1). Move the trial weight to Plane 2 at an arbitrary point, run the fan again, and record (Run 2). The Balanset‑1A software uses all three measurements to calculate the correction mass and angle for each plane.
Angle Measurement for Fan Correction Weights
The angle is measured from the trial weight position in the direction of fan rotation — exactly as described in the Medición del ángulo de corrección section above. Mark where the trial weight was placed (0° reference), then count the indicated angle along the rotation direction to find the correction weight position.
Based on the angles and masses calculated by the software, install the correction weights on Plane 1 and Plane 2. Run the fan once more and verify that vibration has dropped to an acceptable level per ISO 21940‑11 (typically G 6.3 for general‑purpose fans). If residual vibration is still above target, perform one trim run.
Preguntas frecuentes
Equipo para balanceo en campo
El Balanset‑1A is a two‑channel portable instrument that handles single‑plane and two‑plane dynamic balancing, plus vibration analysis (overall velocity, spectra, waveform). It ships as a complete kit:
- 2x MEMS vibration sensors (ADXL335-based accelerometers) with magnetic mounts
- Laser tachometer (non‑contact RPM sensor) with reflective tape
- USB measuring unit (connects to any Windows laptop)
- Software: balancing wizard, vibration meter, spectrum analyser
- Carrying case with all cables and accessories
RPM range: 250-90,000. Vibration range: 0.2-80 mm/s RMS. Frequency range: 5-1000 Hz. Phase accuracy: ?1?. Weight splitting, trim runs, tolerance checking, and report generation included in the software. Full kit weighs approximately 4 kg.
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