Every field balancer eventually meets the session from hell: the procedure was followed, the weights went exactly where the software said, and the vibration did not drop. Sometimes it got worse. The reflex is to blame the instrument and run the procedure again — and again. The actual fix is almost never another run; it is finding which assumption of the balancing method your machine is violating.
This guide is the diagnostic tree for that moment. It assumes you know the basic procedure (if not, start with the fan or mulcher walk-throughs).
First: is the problem even imbalance?
Balancing corrects exactly one defect — uneven mass distribution. Its signature is specific: vibration dominated by the 1× component (once per revolution), with a stable phase, growing roughly with the square of speed. Before any weights, take a minute in vibrometer/spectrum mode:
| What you see | What it suggests |
|---|---|
| Dominant 1×, stable phase | Imbalance — balance it |
| Strong 2× (and axial vibration) | Shaft misalignment — align the coupling, then re-measure |
| Harmonics series (1×, 2×, 3×…), clipped waveform | Mechanical looseness — find and torque the joint |
| Raised broadband floor, no clean peaks | Bearing wear — replace, then balance if needed |
| Vibration at blade-pass or mesh frequencies | Aerodynamic/gear sources — not a balancing problem |
A machine can carry two defects at once — a worn bearing and imbalance. Fix the structural defect first: weights cannot compensate a bearing, and the bearing’s noise corrupts the balancing measurement.
Unstable readings: the session-killer
The influence-coefficient method has one hard requirement: repeatability. The same rotor state must produce the same amplitude and phase. If "Run 0 again" gives a different answer than Run 0, no calculation can succeed. When readings float, walk this list in order:
- Speed. Is every run at the same rpm? A drifting drive (PTO, hydraulics, a loaded motor) changes both amplitude and phase. Hold the speed; let readings settle before recording.
- Tachometer. Did the stand get nudged? Is the reflective mark clean and unique (one mark only)? Is direct sunlight or a work lamp hitting the optics? Any of these scrambles the phase reference.
- Sensors. Flat, clean, paint-free mounting spot; magnet seated firmly; cable not slapping against the machine; sensor not touching a moving part.
- Loose mass. Liquid or debris inside the rotor (sand in a mulcher tube, fluid in a hollow shaft) relocates on every start — the phase wanders run to run. Spin the rotor several times: a phase that won’t repeat with no other changes is the classic symptom.
- Resonance. If the working speed sits near a structural resonance, tiny speed differences produce huge reading swings. Shift the test speed 10–15% and watch: if amplitude collapses or phase jumps, you are balancing on a resonance — move the test speed off it, or stiffen/isolate the structure.
- Thermal drift. Some machines need to reach working temperature before behaving repeatably. Balance warm if the machine runs warm.
The weights went on, vibration went up
Three causes account for nearly all of these:
- Angle counted the wrong way. The correction angle is measured from the trial-weight position, in the direction of rotation. Counting against rotation mirrors the position — the new weight reinforces the imbalance instead of cancelling it. This is the single most common field error.
- The machine changed between runs. A support placed under the frame, a removed guard, loosened then retightened mounts, a knife replaced mid-session — any change of mass or stiffness invalidates the influence coefficients measured two runs ago. Change something structural → start the session over.
- Trial weight too small. If Run 1 changed the reading by less than ~20% in amplitude or phase, the influence coefficient is computed from noise. The correction it produces is fiction. Double the trial mass and redo the run.
Convergence that stalls
Vibration drops, but each trim gains less, and the level plateaus above target:
- You are balancing the residual of another defect. A bent shaft or eccentric seat produces genuine 1× vibration that mimics imbalance but cannot be fully weighted away. The tell: corrections converge to a stubborn floor. Check runout with an indicator.
- The weights are not where the math thinks. Welded weight placed "about there", or the correction radius entered wrong. Angle and radius precision matter — a few degrees off costs real percentage points.
- Weight attachment is moving. A clip-on weight that shifted, a weld that cracked. Re-verify mass positions before the next trim.
- One plane was enough — until it wasn’t. Wide rotors corrected in a single plane can hit a floor where the remaining vibration is a moment imbalance. Switch to two-plane.
A useful rule from field practice: one trim run is routine; three or more means stop and re-diagnose.
When it is genuinely not balanceable in place
Some situations are out of scope for field balancing, and recognising them early saves a day: cracked rotor bodies (balancing a cracked rotor is dangerous, full stop), bearing seats hammered beyond repair, rotors operating through a resonance with no speed flexibility, and flexible-shaft machines that need balancing at multiple speeds on a machine designed for it. In those cases the honest output of your session is a diagnosis, not a balance — and the before/after report documenting the defect is what you hand over.
Real cases where the diagnosis was the job: a crusher drive shaft and a vibrating-screen motor. And if you are still choosing the instrument that does both the measuring and the explaining, start with Choosing a Field Balancer.