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 [1¹] for the contact of two rough surfaces and the Eyring model [2] 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. The simplified friction model developed according to these assumptions is experimentally validated by measurements made on a single cylinder diesel engine. An original technique was developed to permit the measurement of the friction forces on the tappet. This technique is based on a modified design of the tappet, which is equipped with strain gauges that measure the forces and moments applied on the tappet. The processing of the measured data permits to evaluate the friction force cam-tappet and tappet-bore and, to determine the angular rotation of the tappet.