Mixed-lubrication analysis of planetary roller screw

The mixed-lubrication model of the planetary roller screw is proposed considering contact load, contact geometry of thread, surface roughness and transient effects. The influences of rotational speed of the screw, axial load and surface roughness on lubrication are discussed. Increasing rotational speed has a significant effect on the lubrication of contact areas. As the speed increases, the film thickness increases while the friction coefficient and contact-area ratio decrease, and the lubrication mode changes from mixed to full-film. As the load increases, the film-thickness ratio decreases while the friction coefficient and maximum subsurface von Mises stress increase. High-precision machining can improve lubrication by properly reducing the roughness of the machined surface, which would increase film thickness and decrease the contact-area ratio.