Film Thickness and Traction in Soft EHL with Grease

Engineering plastic materials like polyamide and polyacetal are widely used for worm wheels and other tribological elements under EHL conditions with grease; typical examples are found in electrical power steering systems and power window mechanisms. Low elastic modulus of those materials makes their contact pressure much lower than steel-to-steel contact, and EHL of those contacts is called “soft EHL”. The current concern of this technology is higher reliability and lower friction losses, which mean maintaining sufficient EHL film thickness and reducing traction in tribological terms. The present work tries to measure soft EHL film thickness and traction of oil and grease by an optical interferometry EHL system, in which a steel ball was made contact with a transparent polycarbonate disk having a similar elasticity as the above-cited engineering plastic materials. The results show that prediction is possible by using the analyses by Dong and Qian for the film thickness and by Muraki and Kimura for the traction coefficient.