What is the difference between static and dynamic balancing?

Static (single-plane) balancing corrects a single heavy spot — the centre of mass is displaced from the axis but the inertia axis stays parallel. Suitable for narrow disc-like rotors (L/D < 0.5). Dynamic (two-plane) balancing corrects both force and couple unbalance — the general case where the inertia axis is skewed. Required for elongated rotors. Most real rotors need dynamic balancing.

How does the trial weight (influence coefficient) method work?

Step 1: Measure initial vibration (amplitude + phase). Step 2: Attach a known trial weight at a known angle. Step 3: Measure new vibration. Step 4: The vector difference between runs reveals how that location influences vibration — the influence coefficient. Step 5: Software calculates the exact correction mass and angle to cancel the original unbalance. Step 6: Remove trial weight, install correction, verify.

When should I use single-plane vs. two-plane balancing?

Single-plane: rotors with L/D 0.5 (elongated) — motor armatures, multi-stage pump shafts, paper rolls, cardan shafts. When in doubt, use two-plane — it handles both static and couple unbalance.

What ISO standard applies to rotor balancing tolerances?

ISO 21940-11 (formerly ISO 1940-1) defines the G-grade balance quality system. G 6.3 is the standard for most industrial fans, pumps, and motors. G 2.5 for turbines and critical machinery. The formula U_per = (9549 × G × M) / n gives permissible residual unbalance in g·mm. ISO 14694 extends this to fan-specific BV categories.

Can I balance a rotor without removing it from the machine?

Yes — this is called field balancing or in-situ balancing. A portable balancer like the Balanset-1A uses vibration sensors and a tachometer mounted on the installed machine. The trial-weight influence coefficient method determines correction without needing a dedicated balancing machine. This saves hours of disassembly/reassembly downtime.

How do I know if vibration is caused by unbalance?

Unbalance produces vibration at exactly 1× RPM (the running speed) in the radial direction. In an FFT spectrum, a dominant 1× peak with stable phase angle is the classic signature. If vibration is at 2×, 3×, or other multiples, other faults (misalignment, looseness, bearing defects) are more likely. A Balanset-1A spectrum analyser can confirm the diagnosis before balancing.

Rotor Balancing — Procedures, Types & Standards — Vibromera

Rotor Balancing — Procedures, Types & Standards

Q: 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 its 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.

Q: How does the trial weight (influence coefficient) method work?

Step 1: Measure initial vibration (amplitude + phase). Step 2: Attach a known trial weight at a known angle. Step 3: Measure new vibration. Step 4: The vector difference between runs reveals how that location influences vibration — the influence coefficient. Step 5: Software calculates the exact correction mass and angle to cancel the original unbalance. Step 6: Remove trial weight, install correction, verify.

Q: When should I use single-plane vs. two-plane balancing?

Single-plane: rotors with L/D 0.5 (elongated) — motor armatures, multi-stage pump shafts, paper rolls, cardan shafts. When in doubt, use two-plane — it handles both static and couple unbalance.

Q: What ISO standard applies to rotor balancing tolerances?

ISO 21940-11 (formerly ISO 1940-1) defines the G-grade balance quality system. G 6.3 is the standard for most industrial fans, pumps, and motors. G 2.5 for turbines and critical machinery. The formula U_per = (9549 × G × M) / n gives permissible residual unbalance in g·mm. ISO 14694 extends this to fan-specific BV categories.

Q: Can I balance a rotor without removing it from the machine?

Yes — this is called field balancing or in-situ balancing. A portable balancer like the Balanset-1A uses vibration sensors and a tachometer mounted on the installed machine. The trial-weight influence coefficient method determines correction without needing a dedicated balancing machine. This saves hours of disassembly/reassembly downtime.

Q: What are common correction methods?

Adding mass: clip-on weights, bolt-on weights, weld-on weights, epoxy/putty, set screws. Removing mass: drilling, milling, grinding. The choice depends on rotor design, operating environment, and whether the correction is permanent or for trim balancing. Weight splitting distributes correction between adjacent accessible positions when the exact angle is not reachable.

Q: How do I know if vibration is caused by unbalance?

Unbalance produces vibration at exactly 1× RPM (the running speed) in the radial direction. In an FFT spectrum, a dominant 1× peak with stable phase angle is the classic signature. If vibration is at 2×, 3×, or other multiples, other faults (misalignment, looseness, bearing defects) are more likely. A Balanset-1A spectrum analyser can confirm the diagnosis before balancing.

Devices

Portable balancer & Vibration analyzer Balanset-1A

Parts

Vibration sensor

Parts

Optical Sensor (Laser Tachometer)

Vibromera rotor balancing kit: carry case, multi-channel signal conditioner, handheld analyzer, magnetic base, vibration sensors, and coiled cables.

Devices

Balanset-4

Parts

Magnetic Stand Insize-60-kgf

Parts

Reflective tape

Balanset-1A. Portable balancer , vibration analyzer

Devices

Dynamic balancer “Balanset-1A” OEM

📌 Canonical Reference Article — vibromera.eu

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.

Static vs. Dynamic Balancing

The two fundamental balancing types — determined by rotor geometry and the type of unbalance present

📍

Static (Single-Plane)

One correction plane

Corrects static unbalance — the rotor's centre of mass is displaced from the rotation axis but the inertia axis remains parallel. Equivalent to a single "heavy spot." Detectable without rotation (gravity reveals it on knife-edges).

Planes1 correction plane

Sensors1 vibration sensor + 1 tachometer

Rotor shapeDisc-like, L/D < 0.5

ExamplesFan impellers, pulleys, grinding wheels, narrow flywheels

Balanset modeF2 — Single-Plane

💫

Dynamic (Two-Plane)

Two correction planes

Corrects dynamic unbalance — the general case combining static and couple components. The inertia axis is neither parallel to nor intersecting the rotation axis. Only detectable while spinning. Creates both force and rocking-moment vibration.

Planes2 correction planes

Sensors2 vibration sensors + 1 tachometer

Rotor shapeElongated, L/D > 0.5

ExamplesMotor armatures, pump shafts, paper rolls, cardan shafts

Balanset modeF3 — Two-Plane

📊 Single-Plane vs. Two-Plane — Decision Guide

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

📊

Initial Measurement

Run the machine at operating speed. Measure vibration amplitude (mm/s) and phase angle (°) at each bearing. This is the baseline — the unbalance signature. Record in Balanset-1A.

⚖️

Attach Trial Weight

Stop the machine. Attach a known trial weight at a known angular position in the correction plane. Mass should produce a noticeable but not dangerous vibration change (10–30% of initial).

📈

Trial Run

Run again at the same speed. Measure new vibration amplitude and phase. The vector difference between initial and trial runs is caused solely by the trial weight.

🧮

Calculate Correction

Software computes the influence coefficient — how much vibration change per unit mass at that location. Then calculates exact correction mass (g) and angle (°) to cancel the original unbalance.

🔩

Install Correction

Remove trial weight. Attach the calculated permanent correction weight at the prescribed angle. For two-plane: repeat steps 2–4 for the second plane (Balanset-1A solves both simultaneously).

✅

Verify

Final run to confirm residual vibration is within tolerance. Compare measured residual against ISO 1940-1 U_per limit. Balanset-1A displays pass/fail and generates a balance report.

Why Balance? — The Benefits

Unbalance is the #1 source of vibration in rotating machinery. Correction delivers measurable returns.

⚙️

Bearing Life

Unbalance forces go directly to bearings. 50% vibration reduction → up to 8× bearing life extension.

🛡️

Reliability

Lower vibration → less fatigue on seals, shafts, couplings, and foundations. Fewer breakdowns.

🔇

Noise

Vibration is the primary noise source. Balanced machines run significantly quieter.

⚡

Energy

Energy wasted on vibration and heat is recovered as useful work. Measurable efficiency gain.

🛑

Safety

Severe unbalance can cause catastrophic failure. Balancing prevents vibration-induced accidents.

What is Rotor Balancing?

Quick Answer

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.

Influence Coefficient

α = (V_trial − V_initial) / T

α = influence coefficient (vibration per unit unbalance) | V = vibration vector (amplitude∠phase) | T = trial weight vector (mass∠angle)

Correction Calculation

C = −V_initial / α

C = correction weight vector (mass∠angle) — the weight that produces vibration equal and opposite to V_initial

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:

Trial Weight Estimate

m_trial ≈ (10 × M) / (R × (n/1000)²)

m in grams | M = rotor mass (kg) | R = trial radius (mm) | n = RPM — rule of thumb for approximately 10% of G 6.3 unbalance

When to Balance — Vibration Signature

How do you know vibration is caused by unbalance rather than misalignment, looseness, or bearing defects?

Unbalance Vibration Signature

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.

Balanset-1A Field Balancing Kit

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:

📏 Key Balancing Standards

Standard	Subject	Key Content
ISO 1940-1 / ISO 21940-11	Balance quality grades (G-grades)	G 0.4–G 4000 scale. Formula: U_per = (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 categories (balance) and FV categories (vibration) specific to fan impellers.
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.

ISO 1940-1 Tolerance Formula

U_per = (9 549 × G × M) / n

U_per = permissible residual unbalance (g·mm) | G = grade (mm/s) | M = mass (kg) | n = max RPM

Worked Examples

Case 1: Centrifugal Fan — Single-Plane Field Balancing

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 → U_per = 1 570 g·mm. Achieved: ~180 g·mm. ✅ Pass.

Case 2: Motor-Pump Assembly — Two-Plane

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.

Case 3: Crusher Rotor — Coarse G 16

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). U_per = 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.

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/FV categories.
Harmonics: Distinguishing 1× (unbalance) from 2× (misalignment) and other orders.
Natural Frequency: Rigid/flexible rotor boundary — critical for balancing approach.

← Back to Glossary Index

Frequently Asked Questions — Rotor Balancing

Procedures, types, diagnosis, and standards

▸ What is rotor balancing?

The process of improving mass distribution of a rotating body so that its centre of mass coincides with the geometric rotation axis. Minimises centrifugal forces → reduces vibration, bearing loads, noise, energy loss. Correction: adding/removing weight at specific locations/angles. Acceptance: ISO 1940-1 G-grades.

▸ Static vs. dynamic balancing?

Static (1-plane): corrects displaced CoM — single heavy spot — disc-like rotors (L/D < 0.5). Dynamic (2-plane): corrects both force + couple — general case — elongated rotors. Most real machines need dynamic. When in doubt, use 2-plane.

▸ How does the trial weight method work?

Measure initial vibration → attach known trial weight → measure again → vector difference = effect of trial. Software calculates influence coefficient, then determines the exact correction mass and angle to cancel the original unbalance. Remove trial, install correction, verify. Balanset-1A automates the calculation.

▸ Single-plane or two-plane?

Single-plane: fan impellers, pulleys, grinding wheels, flywheels (L/D < 0.5). Two-plane: motors, shafts, pumps, rolls, turbines (L/D > 0.5). Balanset-1A: F2 = single, F3 = two-plane. If bearings vibrate out of phase at 1×, couple unbalance is present → two-plane needed.

▸ What ISO standard for tolerances?

ISO 21940-11 (ISO 1940-1): G-grade system. G 6.3 = standard for fans/pumps/motors. G 2.5 = turbines/compressors. U_per = (9 549 × G × M) / n. ISO 14694 extends to fan-specific BV categories. ISO 10816 evaluates vibration in operation.

▸ Can I balance in-situ (without removing the rotor)?

Yes — field balancing. A portable Balanset-1A uses vibration sensors + tachometer on the installed machine. Trial-weight method determines correction without a dedicated balancing machine. Saves hours of disassembly. Accounts for actual operating conditions (alignment, temperature, foundation).

▸ What are common correction methods?

Adding: clip-on, bolt-on, weld-on weights; epoxy/putty; set screws. Removing: drilling, milling, grinding. Choice depends on rotor design, temperature, permanence needs. Weight splitting distributes correction between adjacent accessible positions when the exact angle is blocked.

▸ How do I know it's unbalance and not misalignment?

Unbalance: dominant 1× peak, radial, stable phase, amplitude ∝ speed². Misalignment: significant 2× and/or axial 1×, phase relationship between bearings, amplitude less speed-dependent. Use Balanset-1A spectrum analyser (F1) to confirm before balancing.

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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