Ball-bearing-like specimens are prepared with different groove factors (GF) to simulate a ball screw operating in dry contact and grease lubrication. The fluctuations of frictional coefficient μ are identified as instabilities in the running-in process and in the tribo contacts through the model developed for the threshold value μthreshold. The thermoelastic instability in the running-in is dominated by the surface roughness (Ra) and the GF, and is thus governed by the slip ratio (SR). The inception of running-in instability is the earliest when SR is the highest. With a fixed GF, an increase in Ra results in a SR rise. Wear coefficient increases by raising SR. The time for instability inception of base material is shortened by increasing SR.
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