Improvement of Durability of Hybrid Ceramic Ball Bearings in Liquid Hydrogen at 3 Million DN (120,000 rpm)

Abstract

The durability of a hybrid ceramic 25-mm-bore ball bearing having a single outer land-guided retainer was evaluated and improved in liquid hydrogen at speeds up to 120,000 rpm (120,000 min-1) (3 million DN) by comparing the propagation of superficial thermal microcracks on Si3N4 balls at various speeds. To select a tough Si3N4 ball capable of restraining crack propagation, bearing tests were conducted for three kinds of the Si3N4 balls with different jet-cooling systems. The test results showed that the Si3N4 ball having high fracture toughness as well as high thermal shock resistance could restrain the propagation of wide-ditch microcracks. With regard to the jet-cooling system, four nozzles were better than two nozzles with increasing jet speed. Furthermore, the jet flow aimed at the retainer was superior to that aimed at the inner race, and its result indicated that microcracks might be generated at the trace in contact with the outer raceway under insufficient cooling conditions with a large centrifugal force. Thus, decreasing the maximum contact stress of the outer race to 2.0 GPa by a limited race curvature of 0.51 could restrain the propagation of superficial microcracks.

Keywords

• cryogenic tribology
• solid-lubrication
• ball bearing
• hybrid ceramic bearing
• si3n4 ball
• thermal shock
• crush load
• thermal cracks
• jet cooling
• bearing acceleration