Balancing services › Two-Plane (Dynamic) Balancing
Two-Plane (Dynamic) Balancing — Method, Physics and Field Procedure
When a rotor is wide enough that unbalance differs at each end, a single correction plane is not enough. Two-plane dynamic balancing corrects both the static and couple components simultaneously — using the influence-coefficient method — so the rotor runs smoothly across its full length, not just at its centre.
In short: Two-plane (dynamic) balancing is required whenever a rotor carries both static unbalance and a couple component — meaning the unbalance is distributed along the shaft axis rather than concentrated at one disc. A vibration sensor at each bearing housing and a laser tachometer on the shaft are used to measure the rotor’s response to trial weights placed in each plane in turn; the Balanset-1A then solves for the exact correction mass and angle in both planes simultaneously. No removal from the machine is needed — the entire four-run procedure is completed at operating speed, in the rotor’s own bearings, in under one hour for most rotors.
Signs your rotor needs two-plane balancing
A single-plane correction can quiet one bearing while the other still shakes. If you see any of these patterns, two-plane treatment is the right answer:
Single-plane vs two-plane: when do you need two planes?
The choice between one and two correction planes depends on the geometry of the rotor and the nature of its unbalance. Understanding the three types of unbalance helps you decide immediately.
The three types of unbalance
Static unbalance — the centre of mass sits off the rotation axis but the principal inertia axis is parallel to it. One correction plane suffices: add mass on the heavy side and the rotor is balanced. Typical rotors: thin pulleys, narrow grinding wheels, single-plane fan discs.
Couple unbalance — the centre of mass is on the axis but the principal inertia axis is tilted. The rotor rocks rather than wobbles. This cannot be corrected in one plane; two equal and opposite masses 180° apart in two separated planes are needed to cancel the rocking moment. Typical rotors: long cylindrical drums, motor armatures, shaft assemblies.
Dynamic (combined) unbalance — the general case: both static and couple components are present. Correction requires two planes chosen arbitrarily along the shaft. All real production rotors fall into this category.
| Factor | Single-plane (static) | Two-plane (dynamic) |
|---|---|---|
| Rotor shape | Thin disc; axial width much less than diameter | Wide rotor; axial width comparable to or larger than diameter |
| Unbalance type | Static unbalance only | Couple or combined (dynamic) unbalance |
| L/D ratio (axial length / diameter) | L/D < 0.5 (approx.) | L/D ≥ 0.5, or rotor exceeds its first critical speed |
| Number of sensors | 1 vibration sensor + 1 laser tacho | 2 vibration sensors + 1 laser tacho |
| Number of measurement runs | 3 runs (baseline + trial + correction) | 4 runs (baseline + plane-1 trial + plane-2 trial + correction) |
| Correction planes | 1 | 2 |
| Typical equipment | Narrow fan impellers, pulleys, single-stage discs | Drums, driveshafts, wide impellers, multi-stage rotors, motor rotors |
| Standard reference | ISO 21940-11 (1-plane rigid rotor) | ISO 21940-11 (2-plane rigid rotor) |
Rule of thumb: if the rotor vibration measured at one bearing changes in opposite direction from the vibration at the other bearing when you move a trial weight, you have a couple component and two planes are required.
Why wide rotors lose dynamic balance — and what it costs
When a rotor is manufactured or repaired, mass is rarely distributed symmetrically along its axis. Erosion chews one end of an impeller faster than the other; weld repairs add material at a single axial station; product build-up accumulates non-uniformly along a drum. The result is not just static unbalance but also a couple component that creates a rocking moment. Only simultaneous correction in two planes eliminates both. Because centrifugal force grows with the square of rotational speed, a modest couple unbalance at 500 RPM becomes a destructive force at 3,000 RPM.
Ignoring the couple component means both bearings carry elevated dynamic loads every revolution. Bearing fatigue accumulates, seals fail, fasteners loosen, and structural cracks propagate from the mounting feet outward. The economic loss — bearings, seals, lost production, emergency labour — typically exceeds the cost of a proper two-plane job many times over.
Why halving vibration multiplies bearing life
Two-plane balancing — step-by-step field procedure
The Balanset-1A applies the influence-coefficient method. Two vibration sensors and one laser tacho characterise the rotor fully and solve for both correction planes in a single on-site session:
- Mount the sensors. Fix a vibration accelerometer to each bearing housing (Planes 1 and 2) and aim the laser tachometer at a reflective strip on the shaft. No disassembly is needed — the rotor runs under normal operating conditions throughout the procedure.
- Measure the baseline. One run at full operating speed records the vibration amplitude and phase angle simultaneously at both bearing locations, giving the starting 1× RPM vectors that define the initial unbalance state in both planes.
- Add a trial weight in Plane 1. A known mass is clamped at a marked angular position in the first correction plane. A second run captures how this weight influences vibration at both bearing locations, yielding two of the four influence coefficients.
- Move the trial weight to Plane 2. The same mass is repositioned to the second correction plane and another run records the cross-influence on both sensors. The device now has all four influence coefficients needed for the 2×2 system.
- Let the device calculate. The Balanset-1A solves the two-plane influence-coefficient equations and outputs the exact correction mass and angular position for each plane simultaneously — no manual arithmetic required.
- Fit corrections and verify. Correction weights are placed at the calculated positions on both planes. A final run confirms residual unbalance is within the ISO 21940-11 tolerance for the specified G-grade, and the Balanset-1A saves a documented balancing report.
What we balance in two planes
- Wide centrifugal fan impellers and double-inlet blowers
- Combine-harvester threshing and chopping drums
- Driveshafts and cardan shafts
- Multi-stage pump rotors and compressor impeller stacks
- Paper-machine rolls and printing / coating cylinders
- Screw conveyors and augers longer than ~500 mm
- Motor rotors and generator rotors with significant axial length
- Turbocharger rotors and steam-turbine rotors (field vibration verification)
- Any rotor where single-plane correction leaves one bearing still shaking
Tolerances & standards
ISO 21940-11 (formerly ISO 1940-1) defines balance quality grades G0.4 through G4000 for rigid rotors. Two-plane balancing is the required method whenever the rotor axial-length-to-diameter ratio exceeds roughly 0.5, or when the rotor operates above its first critical speed. The permissible residual unbalance per plane is calculated as:
Uper (g·mm) = eper × m / 2, where eper = G × 9549 / n (mm/s × rpm → μm eccentricity), m is the rotor mass in kg, and the factor 2 distributes the tolerance between the two planes.
Fan rotors are commonly balanced to G6.3 or G2.5 per ISO 14694; precision machine-tool spindles and high-speed turbo equipment target G1.0 or finer. Use our residual-unbalance calculator to find the permissible tolerance for your G-grade, rotor mass and service speed before starting the job.
The Balanset-1A — your complete field-balancing kit
Two-plane dynamic balancing of any rigid rotor — fans, drums, driveshafts, multi-stage pump assemblies — is done with one portable instrument: the Балансет-1А. It is a two-channel dynamic balancer and vibration analyzer that balances rotors in their own bearings, at operating speed, using the influence-coefficient method — one plane in three runs, two planes in four. The software calculates the exact correction mass and angle for both planes and saves a report.
What’s in the Full Kit
€1,975 · Full Kit, in stock, VAT invoice
- Interface measurement unit (USB, 2 channels)
- Two vibration accelerometers (4 m cable, 10 m optional)
- Laser tachometer / optical phase sensor (50–500 mm)
- Magnetic stand for the sensor
- Digital scale for trial & correction weights
- Windows balancing & analysis software
- Plastic transport case
Full Kit
Unit · 2 sensors · laser tachometer · magnetic stand · digital scale · software · transport case. Everything needed to start two-plane balancing out of the box.
OEM set
Unit · 2 sensors · laser tachometer · software. For integrators who already have a stand, scale and case, or who embed the unit into a dedicated balancing machine.
| Parameter | Value |
|---|---|
| Measurement channels | 2 (single- & two-plane balancing) |
| Vibration velocity range | 0.05–100 mm/s |
| Frequency range | 5–300 Hz |
| Measurement accuracy | ±5% of full scale |
| Method | 3-run influence-coefficient (1 or 2 planes) |
| Analysis | Amplitude & phase at 1×, FFT spectrum & waveform, saved reports |
| Laptop | Not included (Windows PC, available on request) |
Real two-plane balancing cases
Combine drum (2-plane)
Both correction planes balanced in one field session on an agricultural combine harvester.
Driveshaft (2-plane)
Dynamic balancing of a long driveshaft with correction weight at each end flange.
Wide exhauster impeller
Two-plane correction on a wide industrial exhauster impeller balanced in situ.
Two-plane balancing — from the field
Influence-coefficient setup
Two sensors and one laser tacho positioned to characterise both correction planes simultaneously.
Balanced in place
The rotor stays in its own bearings and is corrected at operating speed — no removal required.
Both planes solved
Correction mass and angle computed for Plane 1 and Plane 2 simultaneously in one session.
Verified result
The final run confirms residual unbalance within the ISO 21940-11 tolerance at both planes.
Free calculators for two-plane balancing
Two-plane balancing FAQ
When is single-plane balancing enough?
How does the influence-coefficient method work for two planes?
How many measurement runs does a two-plane job require?
Do I need to remove the rotor from the machine?
What balance quality grade should I target for my rotor?
Can our maintenance team do two-plane balancing with the Balanset-1A?
Learn the theory
Solve both planes in one visit — at operating speed, no removal
The Balanset-1A guides you through the full two-plane influence-coefficient procedure: baseline, Plane 1 trial, Plane 2 trial, correction and verification — all at running speed, in the rotor’s own bearings. Documented residual unbalance to ISO 21940-11, ISO 14694 and API 610. Ready to ship.
