Experimental investigation of friction in entrapped elastohydrodynamic contacts

Start-up friction is a performance-limiting aspect of hydraulic motor operation. This study was conducted to gain a better understanding of the roles played by contact pressure, speed, and oil type to start-up friction behavior for contacts containing trapped pockets of highly pressurized oil, also known as elastohydrodynamic lubrication (EHL) entrapments. An apparatus was built to measure the start up friction response for ball-on-plane sliding contact with simultaneous observation of the contact region by optical interferometry. Baseline trials for all cases were conducted in the absence of any entrapment and then repeated after forming an entrapment. An impact, activated by solenoids, was used to create a small separation whereby oil would fill the gap and then become trapped as the load rapidly brought the surfaces back into contact.In all cases, entrapment substantially decreased the start-up friction. Additionally, the short-lived entrapments provide the greatest reduction in start-up friction. Therefore, the method of entrapment that may be implemented with least delay before the initiation of sliding has the greatest potential.