Rotor Balancing — Procedures, Types & Standards
The complete guide to balancing rotating machinery: static vs. dynamic (single-plane and two-plane), the influence coefficient method, ISO 21940 tolerances, field balancing, and correction techniques.
This guide reflects the field balancing methodology used by Vibromera, whose Balanset two-plane instruments implement the influence coefficient method described below.
For a buying-decision view of the same topic, see how to choose a portable balancing machine.
Static vs. Dynamic Balancing
The two fundamental balancing types — determined by rotor geometry and the type of unbalance present
| Criterion | Single-Plane | Two-Plane |
|---|---|---|
| Unbalance type corrected | Static only | Static + couple (dynamic) |
| Rotor geometry | L/D < 0.5 (disc-like) | L/D > 0.5 (elongated) |
| Number of runs | 2 (initial + trial) | 3–4 (initial + 2 trials, or cross-coupling) |
| Sensors required | 1 accelerometer + tacho | 2 accelerometers + tacho |
| Bearing vibration pattern | In-phase at 1× | Phase varies (not in-phase, not 180°) |
| Typical rotors | Fan impellers, pulleys, grinding wheels | Motors, pumps, rolls, turbines, shafts |
| ISO plane recommendation | Narrow rotors per ISO 1940-1 §4.3 | Standard for all elongated rotors |
| Balanset-1A mode | F2 | F3 |
The Balancing Procedure
Influence coefficient (trial weight) method — the standard approach for field and shop balancing
Why Balance? — The Benefits
Unbalance is the #1 source of vibration in rotating machinery. Correction delivers measurable returns.
What is Rotor Balancing?
Rotor balancing is the process of improving the mass distribution of a rotating body so that its centre of mass coincides with the geometric axis of rotation. This minimises centrifugal forces, reducing vibration, bearing loads, noise, and energy consumption. Correction is done by adding or removing weight at specific locations and angles, guided by vibration measurements and phase analysis. The acceptance criterion is defined by ISO 1940-1 (ISO 21940-11) G-grades. The two types are static (single-plane) for disc-like rotors and dynamic (two-plane) for elongated rotors.
Unbalance is the most common source of vibration in rotating machinery. When mass distribution is imperfect — due to manufacturing tolerances, material non-homogeneity, corrosion, deposit buildup, or damage — centrifugal forces are generated that increase with the square of speed. A small unbalance at low speed can become destructive at high speed.
Balancing addresses this by iteratively measuring vibration response and adjusting mass distribution until residual unbalance is within tolerance. It is both a manufacturing process (on shop balancing machines) and a maintenance process (field balancing on installed equipment).
The Influence Coefficient Method
Modern balancing — both on dedicated machines and in the field — uses the influence coefficient (trial weight) method. The physical principle: if we know how a known mass at a known position changes the vibration, we can calculate the mass and position needed to cancel the original unbalance.
For two-plane balancing, the system becomes a 2×2 matrix (four influence coefficients accounting for cross-coupling between planes), but the principle is identical. The Balanset-1A solves this automatically — the operator just runs the machine and attaches trial weights.
Trial Weight Selection
The trial weight should produce a noticeable change in vibration (ideally 10–30% of the initial level) without creating dangerous loads. A useful starting estimate:
When to Balance — Vibration Signature
How do you know vibration is caused by unbalance rather than misalignment, looseness, or bearing defects?
Frequency: Dominant peak at exactly 1× RPM (running speed) in the FFT spectrum.
Direction: Primarily radial (horizontal and vertical). Axial component is small.
Phase: Stable, repeatable phase angle at 1×. Phase does not drift over time.
Speed dependence: Amplitude increases with the square of speed (proportional to ω²).
Contrast with misalignment: Misalignment produces significant 2× and/or axial 1× components. Bearing defects produce non-synchronous frequencies.
Before balancing, always verify the diagnosis. The Balanset-1A spectrum analyser (F1 mode) shows the full FFT spectrum, allowing confirmation that 1× dominates before proceeding to balance.
Correction Methods
Adding Mass
- Clip-on weights: Spring-clip zinc or steel weights. Common for fans, wheels. Quick, non-permanent.
- Bolt-on weights: Precision weights secured with bolts in tapped holes or T-slots. Standard for large rotors, turbines.
- Weld-on weights: Steel plates or rods tack-welded to the rotor. Permanent. Common for heavy industrial fans and crusher rotors.
- Epoxy/putty: Two-part adhesive with metal filler. Good for irregular surfaces. Limited to moderate temperatures.
- Set screws: Threaded into radial holes. Common on coupling hubs and spindles. Adjustable.
Removing Mass
- Drilling: Remove material from the heavy spot. Precise control of mass removed (mass = density × volume). Irreversible.
- Milling/grinding: Remove material from the rim or face. Common on turbine wheels, brake rotors.
Weight Splitting
When the exact calculated angle falls between accessible positions (e.g., between bolt holes on a coupling), the correction is split between the two adjacent positions using vector decomposition. The Balanset-1A includes an automatic weight-splitting calculator.
Field Balancing (In-Situ)
Field balancing means balancing a rotor without removing it from the machine. This eliminates disassembly downtime and accounts for the actual operating conditions (alignment, bearing preload, foundation effects) that shop balancing cannot replicate.
The Balanset-1A is a complete portable field balancing system: 2-channel vibration analyser, laser tachometer, built-in ISO 1940 tolerance calculator, single-plane (F2) and two-plane (F3) balancing modes, automatic weight splitting, and formal balance report generation (F6). Measurement accuracy: ±5% velocity, ±1° phase. Suitable for G 16 through G 2.5.
The Balanset-4 extends to 4 channels for complex multi-bearing rotors or simultaneous monitoring of multiple machines.
Advantages of Field Balancing
- No disassembly: Saves hours or days of downtime for large machines.
- Real operating conditions: Includes alignment, bearing preload, thermal state, foundation effects.
- Trim balancing: Corrects assembly-introduced unbalance that shop balancing cannot address.
- Post-maintenance verification: Quick check after impeller replacement, coupling change, or bearing overhaul.
Standards and Tolerances
Balancing is not "as good as possible" — it is "within tolerance." The tolerance is defined by international standards:
| Standard | Subject | Key Content |
|---|---|---|
| ISO 1940-1 / ISO 21940-11 | Balance quality grades (G-grades) | G 0.4–G 4000 scale. Formula: Uper = (9 549×G×M)/n. G 6.3 = standard for fans, pumps, motors. |
| ISO 1940-2 / ISO 21940-2 | Vocabulary | Definitions: unbalance types, rotor classifications, machine types, quality terms. |
| ISO 14694 | Industrial fans | BV fan-application categories, balance grades and vibration-limit guidance specific to industrial fans. |
| ISO 10816 / ISO 20816 | Machine vibration evaluation | Measures the operational result of balance quality. Zone A/B/C/D classification. |
| ISO 21940-12 | Flexible rotors | Multi-speed, multi-plane procedures for rotors above first bending critical speed. |
| ISO 21940-14 | Balancing procedures | General procedures for balancing in several planes. |
| API 610 / API 617 | Petroleum pumps / compressors | Reference ISO 1940 G-grades for rotor balance requirements. |
Worked Examples
Machine: 22 kW centrifugal supply fan, 1 460 RPM, impeller mass 38 kg. Excessive vibration: 8.2 mm/s RMS on drive-end bearing. FFT confirms dominant 1× peak with stable phase.
Setup: Balanset-1A sensor on DE bearing, laser tachometer on shaft. Mode F2 (single-plane — L/D < 0.4).
Step 1: Initial run: 8.2 mm/s at 47°.
Step 2: Trial weight: 15 g at 0° on fan hub, R = 200 mm.
Step 3: Trial run: 5.9 mm/s at 112°.
Step 4: Software calculates: correction = 22 g at 198°, R = 200 mm.
Step 5: Install weld-on weight 22 g at 198°. Remove trial weight.
Step 6: Verification: 0.9 mm/s. ISO tolerance G 6.3 → Uper = 1 570 g·mm. Achieved: ~180 g·mm. ✅ Pass.
Machine: 45 kW motor + centrifugal pump, 2 950 RPM, rotor mass 55 kg. Vibration: DE bearing 6.1 mm/s, NDE bearing 4.8 mm/s. Phase difference ~140° → dynamic unbalance.
Setup: Balanset-1A two sensors (DE + NDE), mode F3. Correction planes: coupling hub (plane 1) and motor fan end (plane 2).
Runs: Initial → trial plane 1 (10 g at 0°) → trial plane 2 (8 g at 0°).
Result: Software solves 2×2 matrix. Correction: plane 1 = 18 g at 245°, plane 2 = 12 g at 68°.
Verification: DE: 0.7 mm/s, NDE: 0.5 mm/s. G 6.3 limit: 1 122 g·mm. ✅ Both planes well within tolerance.
Machine: Hammer mill crusher, 980 RPM, rotor mass 420 kg. After hammer replacement, vibration increased to 14.5 mm/s.
Specification: G 16 (heavy-duty, severe conditions). Uper = 9 549 × 16 × 420 / 980 = 65 500 g·mm.
Procedure: Single-plane (disc-like rotor). Trial 150 g at 0° on rim. Correction: 280 g at 315°. Weld-on steel plate.
Result: 2.8 mm/s. Residual ~5 600 g·mm. ✅ Well within G 16 limit.
Related Topics
- ISO 1940-1: G-grade tolerance system — the acceptance criterion for balancing results.
- ISO 1940-2: Vocabulary — definitions of all balancing terms.
- Balance Quality Grade: Interactive G-grade calculator.
- Unbalance: The physical condition that balancing corrects.
- ISO 14694: Fan-specific BV categories and vibration limits.
- Harmonics: Distinguishing 1× (unbalance) from 2× (misalignment) and other orders.
- Natural Frequency: Rigid/flexible rotor boundary — critical for balancing approach.
Frequently Asked Questions — Rotor Balancing
Procedures, types, diagnosis, and standards
▸ What is rotor balancing?
▸ Static vs. dynamic balancing?
▸ How does the trial weight method work?
▸ Single-plane or two-plane?
▸ What ISO standard for tolerances?
▸ Can I balance in-situ (without removing the rotor)?
▸ What are common correction methods?
▸ How do I know it's unbalance and not misalignment?
Related Glossary Articles
Balance Any Rotor — In the Field
Single-plane and two-plane modes, ISO 1940 tolerance calculator, spectrum analyser for diagnosis, automatic weight splitting, and formal balance reports — all in one portable instrument.
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