Published 2026-09-05 · By Chen Guoqiang
Commutator turning: why runout decides brush life
What runout is, how it is measured, what causes it upstream of the turning station, and what a servo turning machine can and cannot correct.
Brushes ride on the commutator at several thousand revolutions per minute. Any deviation of the surface from a true circle makes the brush lift and land on every turn, which causes arcing, pitting and accelerated brush wear. That is why the turning station is the process most closely tied to motor life.
Where runout comes from
- Commutator pressed off-centre or tilted at the pressing station
- Shaft bent during handling or pressing
- Bar movement after welding heat input
- Tool wear and spindle error at the turning machine itself
What the turning machine can correct
Turning removes stock relative to the shaft centres, so it corrects commutator eccentricity and tilt as long as enough copper is available. It cannot correct a bent shaft: the surface will be round about the bent axis and still run out in the motor. Upstream control is therefore part of the turning specification.
Figures
On the three-servo machine the turned surface holds ≤0.008 mm TIR between centres at Ra ≤0.8 μm; the automatic machine holds ≤0.012 mm at Ra ≤1.2 μm. Both figures are measured on the machine, not on the finished motor, for the reason above.
