Tribo-Characteristics of Self-Lubricating Ball Bearings for the LE-7 Liquid Hydrogen Rocket-Turbopump

The tribo characteristics of self-lubricating 40-mm-bore ball bearings with a retainer of glass cloth-polytetrafluoroelhylene (PTFE) laminate, which has elliptical pockets with a large pocket clearance, were tested under thrust loads at speeds up to 50000 rpm, 2 million DN, in liquid hydrogen (LH2) and in liquid nitrogen (LN2). During testing, the bearing torque, outer-race temperature, and electric resistance between the inner and outer races were monitored to verify the formation and rupture of a PTFE transfer film. Testing showed that the bearings having the elliptical retainer pockets were superior to the conventional bearings with circular pockets. It was determined that at the maximum inner race spinning velocity of about 5 mls a PTFE transfer film could sustain the maximum Hertz stress, up to about 2000 N/mm2, in LH2, without severe film rupture resulting in bearing seizure. In LN2, the critical load capacity of PTFE transfer film with bearing seizure was about 2700 N/mm2.