Simplified Elasto-Hydrodynamic Friction Model of the Cam-Tappet Contact
The paper analyses the particularities of the lubricating conditions at the contact between the cam and a flat tappet in the valve train of an internal combustion engine and develops a method for the calculation of the friction force. The existing lubrication models show the predominance of the entraining speed and oil viscosity on the thickness of the oil film entrapped between cam and tappet, predicting a very small value (less than 0.1 μm) of the oil film thickness (OFT). The oil viscosity increases exponentially with pressure in the Hertzian contact, determining non-Newtonian behavior of the oil in the contact zone. Using the model developed by Greenwood and Tripp  for the contact of two rough surfaces and the Eyring model  for the oil it is shown that non-Newtonian behavior of the oil prevails and that the OFT plays a secondary role on the friction force.