Apparatus for Studying Ball Spinning Friction

An experimental apparatus was designed and constructed at the NASA Lewis Research Center to study one aspect of spinning friction based on interfacial slip over the entire ball-race contact area. The apparatus is capable of measuring spinning moments of less than 0. Ol inch-pound at maximum Hertz contact stresses to over 400 000 psi, speeds to 3500 rpm, and under varying contact configurations and conditions. From the torque measurement, a coefficient of spinning friction can be calculated. The apparatus comprises a drive assembly, a dead-weight load assembly, a spherical upper test specimen, a cylindrically grooved lower test specimen of varying conformity, a lower test specimen housing assembly incorporating a hydrostatic air-bearing assembly, and a torque-measuring system. In operation, the upper test specimen is loaded against the lower test specimen through the drive shaft assembly by the dead- weight load assembly. As the drive assembly is rotated, the upper test specimen rotates against the stationary lower test specimen actuating the torque-measuring system. Preliminary tests were conducted under varying Hertz stress to 136 000-psi maximum Hertz stress at a spinning speed of 950 rpm, with a 51 percent ball-groove conformity using a polyphenyl ether (5P4E), a highly purified naphthenic mineral oil, and a di-2-ethylhexyl sebacate (MIL-7808). It was found that the coefficient of spinning friction decreased with increasing maximum Hertz stress to an intermediate stress level and had a minimum value of 0. 122, 0. 089, and 0. 050, respectively, for the lubricants listed previously.