How to balance a fan, mulcher or crusher rotor on site: a step-by-step field balancing guide

Unbalance is one of the most common reasons fans, mulchers, crushers and pumps vibrate. To fix it you don't always have to pull the rotor out and send it to a balancing machine. A portable two-channel balancer lets you balance it in place, in its own bearings, at its real operating speed.

Below is the routine Vibromera teaches with Balanset-1A. The logic is the same with any two-channel field balancer, so it's useful even if you use a different instrument.

1. Make sure it really is unbalance

Balancing fixes one thing: mass that's unevenly spread around the rotor. It won't fix a cracked weld, a loose bearing housing, a bent shaft or misalignment. If you balance on top of those problems, the result won't last.

Quick checks before you start:

• Look at the spectrum. Unbalance shows up as a dominant peak at 1× running speed. Large peaks at 2× or 3× point more toward misalignment or looseness.
• Watch the phase. At constant speed, amplitude and phase should repeat from run to run (the Balanset-1A manual says within 10 to 15%). If they wander, check for looseness, an unsteady speed or a resonance before you go further.
• Clean the rotor. Dirt built up on fan blades is a common cause of unbalance, and cleaning alone sometimes fixes it.
• Check the mechanics. Bolts, bearing housings, the base frame. On mulchers and crushers, look for missing, broken or unevenly worn knives and hammers.

2. What you need

• A two-channel balancer with two vibration sensors and a tachometer (a laser tacho plus reflective tape)
• Trial weights and scales to weigh them
• A way to fix weights: bolts, clamps or welding
• Lockout gear, and guards you can close for every test run

3. Safety first

Lock out and tag out the machine every time you touch the rotor. Fix the trial weight so it can't come off: at 2,000 rpm a loose weight becomes a projectile. Here is what that looks like when it goes wrong: a trial weight that flew off and hit a car and a wall. Nobody stands in the plane of rotation during runs, and guards stay closed.

4. Mount the sensors and the tachometer

• Put the sensors on the bearing housings, as close to the bearings as you can, perpendicular to the shaft. Horizontal usually gives the highest reading.
• Use solid metal. Not a guard, not a thin cover, not a loose bracket.
• Sensor 1 goes on the bearing next to correction plane 1, sensor 2 next to plane 2. Point both in the same direction.
• Stick reflective tape on the shaft or pulley and aim the laser tacho at it from a stand. The tape is your phase reference, so don't touch it until the job is done.

5. Initial run

Run the machine at its normal speed and keep that speed constant. Record vibration amplitude and phase on both channels, plus the rpm. This is your baseline. Check the spectrum once more: 1× should dominate.

6. Pick the trial weight

If the trial weight is too light, the vibration hardly changes and the calculation gets noisy. Too heavy, and you shake the machine even harder than it already shakes.

The Balanset-1A manual gives this rule of thumb:

Mt = Mr × Ksupport × Kvibration / (Rt × (N/100)²)

• Mt: trial weight, g
• Mr: rotor mass, g
• Ksupport: 1 for very soft supports (rubber dampers), 2 to 3 for standard bearings, 4 to 5 for rigid supports on a massive foundation
• Kvibration: 0.5 up to 5 mm/s, 1.0 for 5 to 10 mm/s, 1.5 for 10 to 20 mm/s, 2.0 for 20 to 40 mm/s, 2.5 above 40 mm/s
• Rt: radius where you fit the weight, cm
• N: rotor speed, rpm

Example: a 100 kg fan rotor (100,000 g) on standard bearings (Ksupport 2), vibrating at 8 mm/s (Kvibration 1), weight fitted at 30 cm, running at 1,500 rpm: 100,000 × 2 × 1 / (30 × 15²) = 200,000 / 6,750 ≈ 30 g.

Treat it as a starting estimate, not a guarantee: watch how the machine responds on the first trial run. There's also an online trial weight calculator.

7. Trial run in plane 1

Fix the trial weight in plane 1. Write down its mass, its radius and where it sits relative to the reflective tape. Run at the same speed as before. The manual's target: the amplitude changes by more than 20 to 30%, or the phase shifts by more than 20 to 30°. If the change is smaller, the weight is too light: go heavier or move it to a larger radius.

8. Trial run in plane 2

Take the weight off plane 1, fix it in plane 2, run again.

Skip this step if you're balancing in one plane. Rough rule: narrow, disc-shaped rotors (many fan impellers, pulleys, grinding wheels) can often be done in one plane. Long rotors (mulcher and chopper drums, crusher rotors, rolls, double-inlet fans) usually need two. Speed and the tolerance you need matter too, so if one plane doesn't get you there, go to two.

This whole method assumes a rigid rotor, one that doesn't bend noticeably at running speed. Flexible rotors need a different approach.

9. Fit the correction weights

The software now calculates the mass and angle of the correction weight for each plane. Three things to get right:

• Angles. In Balanset-1A the angle is counted from the spot where the trial weight sat, in the direction the rotor turns. Other instruments may use another convention, so check your manual.
• Radius. Fit the correction weights at the same radius as the trial weights. If you can't, the mass has to be recalculated for the new radius.
• Remove the trial weights unless you told the software you'd leave them on.

If you can only fit weights at fixed spots (blades, bolt holes), use the split-weight function: it divides the correction between the two nearest positions. If you remove material instead of adding it, the drill calculator gives you the hole depth. Weld or bolt the weights properly.

10. Check run and trim

Run the machine again. If the residual vibration is still above your target, the software suggests a trim correction. Fit it and run again.

11. Is it good enough?

Two ways to judge the result:

• Residual unbalance against ISO 21940-11 (formerly ISO 1940-1). Balanset-1A has a built-in tolerance calculator. Examples of balance grades from the standard: G 6.3 for fans and pump impellers, G 16 for parts of crushing machines and agricultural machinery. Pick the grade for your specific machine from the standard's table.
• Vibration at the bearings against ISO 20816-3 (formerly ISO 10816-3) for industrial machines. For example, for medium machines (15 to 300 kW) on rigid foundations, below 1.4 mm/s rms is zone A (newly commissioned machines) and above 4.5 mm/s is zone D (vibration that can cause damage).

12. Save the influence coefficients

If you balance the same type of rotor often (a fleet of identical mulchers, a row of identical fans), save the influence coefficients. Next time you skip the trial runs: one run, fit the correction, check. This only works when everything matches: same speed, same supports, same sensor positions, same correction radius and phase mark. After a major repair, do the trial runs again. And always finish with a check run.

Common mistakes

• Speed changes between runs. A VFD ramping, or a tractor-driven mulcher at a different throttle. Use the same rpm every run.
• Sensor on a guard or a loose bracket. You measure the bracket, not the bearing.
• Reflective tape moved or peeled off between runs. All your phase readings are now wrong.
• Trial weight too light. Less than about a 20 to 30% (or 20 to 30°) change gives a poor calculation.
• Correction weight at a different radius than the trial weight, without recalculating the mass.
• Balancing a rotor with mechanical faults. Loose, cracked or with a bad bearing, it won't stay balanced.
• Uneven knives or hammers on mulchers and crushers. Replace them in matched sets before balancing.

About the instrument in this guide

Balanset-1A: two channels, one or two planes, speed measurement from 100 to 100,000 rpm, laser tachometer, two accelerometers on 5 m cables, scales and a hard case. You need your own Windows laptop. €1,975 excl. VAT on vibromera.eu.

More: how Balanset-1A compares with other portable balancers, a mulcher case study and the FAQ.