Normal Test Trace

Normal test – A “normal” test, like beauty, is in the eye of the beholder. The M15 is a very sensitive device and a perfectly made part may show many small variations during rotation. The important thing is to decide what is important to the application. The test shown here is typical of a disk drive pivot, showing even rotation over 360°. Small irregularities and the grease hash do not indicate any problems. In a typical ball bearing the inner race must rotate 3-4 complete revolutions relative to the outer race before the balls have moved around the raceways once. This means that consecutive 360° tests of the same part actually have different starting positions for the balls, causing apparent torque variation.

Brinelling Test Trace

Brinelling – Dented raceways (brinelling) are shown as a series of evenly-spaced spikes with the leading
edge falling (the ball falls into the hole in the raceway before climbing out). Very severe brinelling may cause a rippling of the raceway, seen as multiple spikes. At this point, the damage can usually be felt by hand.