I have recently noticed quite a lot of bearings (C3 and C4) skidding in the non drive end of large motors. My main question is what is the cause of skidding is it linked to grease types, cross contamination, heat or lack there of? In some instances I will get 6g's overall in a 10K acceleration spectrum which is quite high as normal can be .8 - 1g. I feel as though if visually inspected there will be no defect / markings to the naked eye as the acceleration waveform is ok. Is this state Ok? The common fix is adding grease but the skidding remains slightly? So the "problem" if it is one persists. Any help will be greatly apperciated.