Polyimide Composite Retainer Materials Optimized for Minimal Wear and Film Transfer

To reduce torque and torque noise, the designers of sputtered MoS₂ film-lubricated precision ball bearings must avoid the use of self-lubricating composite retainers which exhibit high wear and film transfer rates. To develop an essentially benign ball separator with minimal, ball pocket wear and polymer transfer film formation, the tribological behavior of two chemically homologous polyimides were optimized by blending with solid lubricant additives, as guided by a Taguchi design of experiments approach. The study employed an L9 fractional factorial lest matrix design with analysis of variance calculations. A fluorinated and a non-fluorinated version of a commercially available polyimide were compounded with two types of powdered solid lubricants, using three levels of filler content. The results of flat-on-flat oscillatory wear tests performed with these composites sliding against 440C bearing steel were analyzed and compared with the wear rates of two commercially available polymeric composite retainer materials. The results of the optimization study indicated that the composite with the lowest wear consisted of the fluorinated Polyimide B filled with 7.5%, by volume, of molybdenum disulfide (MoS₂). Its wear rate was substantially below that of the two commercial retainer materials. Analysis of variance calculations showed that all three factors, i.e., polyimide type, filler type, and filler content, proved to be significant in reducing the wear of these retainer candidates.