Tribology and lubrication of constant velocity joints

Constant velocity joints (CVJs), as used to transmit torque on vehicles, are complex tribological systems. This paper explains the tribology of CVJs and discusses the lubrication of their concentrated contacts. Ball type CVJs, both fixed and plunging, consisting of six balls held in the homokinetic plane by a cage, running in curved or straight tracks are described. It is explained that the concentrated contacts are elliptical in shape and are highly elongated with relative dimensions above the level at which Hertzian contact mechanics can normally be applied. Contact pressures under normal operating conditions are 2-4GPa. Track surfaces may be hard machined after heat treatment or left unaltered with typical Pa values of 0.4 to 4 mu m. Plunging tripod type CVJs consist of three rollers, on needle bearings running in straight tracks. The track-roller contacts are also highly conformal and are approximated as line contacts. In the straight ahead position, the track-roller contacts are pure rolling, but at angle, an element of sliding is introduced. Advanced multi-element roller tripod joints eliminate this sliding at higher angles. The as-formed surfaces of the outer raceway tracks have typical Pa values of 0.4 to 0.8 mu m, the rollers having typical rolling bearing finishes. The motions of the contacts in CVJs are low speed variable sliding rolling with sinusoidal oscillation of velocity. Using steady-state approximations, very low lambda-ratios have been calculated. CVJs operate in the mixed elastohydrodynamic and boundary lubrication regimes. The various concentrated contacts will be described together with an analysis of grease characteristics which provide effective lubrication under these conditions.