A Study on a Mechanism of Grease Leakage from Rolling Bearings

This paper proposes a mechanism of grease leakage from rolling bearings which was never reported before. In shielded rolling bearings called ZZ-type, a lump of grease deposited astride the inner race and shield plate ring is forcibly rotated by a moment of force generated by adhesion to both the inner race and shield plate ring in passing each other. When the lump of grease is relatively rigid and easily slips on the surface of the inner race or shield plate ring, the lump actually rotates and some grease is squeezed into the gap between the shield plate ring and inner race, resulting in grease leakage. PTFE (polytetrafluoroethylene) grease deposited on a mineral oil grease coating was found to easily slip on the coating and tends to leak from the bearings due to this mechanism. This result explains the large grease leakage from bearings packed with PTFE grease without sufficient removal of previously packed mineral oil grease.