Imbalance (a heavy spot off the rotation axis) throws a once-per-rev force F = m·r·ω² — which grows with the square of speed. Balancing removes it; ISO 21940 G-grades say how fine is fine enough.
Misalignment (shafts not collinear) forces the coupling and bearings twice per rev — the 2× peak — and bleeds energy and life. Precision alignment (today, laser) fixes it; tolerances tighten as speed rises.
Both are proactive tasks: do them right at install and after any work, and you remove the loads that cause a large share of bearing and seal failures before they ever start.
1 · Balancing: the 1× fault
A rotor is balanced when its mass is distributed symmetrically about the rotation axis. Any residual heavy spot — a casting variation, a corroded vane, a lost balance weight, product build-up on an impeller — sits at a radius from the centre and gets flung outward as the rotor turns. The resulting centrifugal force rotates with the shaft, so the machine is pushed once per revolution: the 1× vibration.
That square law is the whole reason balancing matters. Double the speed and the unbalance force quadruples; the bearings feel every newton of it, every revolution, and the cube-law from bearings & lubrication turns that extra load into dramatically shorter life. The model shows it:
Interactive — Unbalance force
Live modelUnbalance force vs speed
F = m·r·ω² with ω = 2πN/60. "Weight of" is F/g for intuition. Acceptable residual unbalance is set by ISO 21940 balance grades (G), which fix an allowable mass·radius per unit rotor mass that tightens as speed rises — fine balancing (low G) for high-speed machines.Balance grades, and one plane or two
ISO 21940 (formerly ISO 1940) defines balance quality as G grades — e.g. G6.3 for general pumps and motors, G2.5 for higher-speed machine tools and turbines, finer still for grinding spindles. The grade pins down the permissible residual unbalance for a given rotor mass and speed. Thin, disc-like rotors usually need single-plane balancing; longer rotors need two-plane balancing because a couple (heavy spots at each end, opposite sides) can be balanced statically yet still rock dynamically.
2 · Alignment: the 2× fault
When the driver and driven shafts aren't collinear, the flexible coupling has to absorb the error on every turn, and it does so by forcing the shafts and their bearings — typically twice per revolution, the 2× peak, often with strong axial vibration that distinguishes it from imbalance. There are two pure forms, usually mixed:
- Offset (parallel) misalignment — the shaft centrelines are parallel but displaced.
- Angular misalignment — the centrelines meet at an angle.
The costs are real and continuous: accelerated bearing and seal wear, coupling fatigue, higher power draw, and loosening of bolted joints. A surprising amount of "bad bearings" are really bad alignment.
Soft foot, thermal growth, and tolerances
Two traps catch the unwary. Soft foot — one machine foot not sitting flat — distorts the casing when you bolt it down, so the machine is bent before you even start aligning; always check and shim it out first. Thermal growth — a hot pump or turbine rises as it warms — means a machine aligned cold can be misaligned running; you align cold with deliberate offsets (targets) so it grows into alignment.
How precise is precise enough? Tolerances tighten with speed, because the dynamic effect of a given error scales with rpm:
| Speed | Offset (acceptable) | Angularity |
|---|---|---|
| 1500 rpm | ~0.07 mm | ~0.07 mm / 100 mm |
| 3000–3600 rpm | ~0.03 mm | ~0.05 mm / 100 mm |
| > 6000 rpm | tighter still | tighter still |
(Indicative "excellent" tolerances; follow the coupling/OEM spec.) The methods have evolved from straightedge → dial indicators (rim-and-face, reverse-dial) → laser shaft alignment, which is now standard: fast, accurate, and it computes the exact shim and move for each foot.
This is where condition monitoring closes the loop. The vibration spectrum tells you which fault you have — a tall 1× says balance it, a tall 2× with axial energy says align it. Precision alignment and balancing are the corrective tasks that turn that diagnosis into a fix, and a post-job vibration check confirms it worked.
Key takeaways
- Imbalance throws F = m·r·ω² — once per rev (1×), growing with the square of speed; balance to an ISO 21940 G-grade.
- Misalignment forces the coupling twice per rev (2×, axial) — offset and angular; fix with precision (laser) alignment.
- Mind soft foot and thermal growth — and tighten tolerances as speed rises.
- They're the cure for the vibration faults — diagnose with the spectrum, fix with alignment/balancing, verify with a re-check.