Experimental Study on Tribological Characteristics of Cam and Roller Tappet : 7th Report : Influence of Cam Surface Finish Roughness(DEPARTMENT OF MECHANICAL ENGINEERING)

The friction characteristics of the cam and roller tappet were studied for the design of engine valve train elements. The influence of the cam surface finish roughness on the friction was examined by measuring the contact load and the friction force between cam and roller tappet with the test rig. The sintered cam lobes are finished by grinding or blasted by the shot peening finish. The traction effect between rough faces under the rolling contact was considered as a reason why the friction characteristics change. The change of the bearing rate of solid contact between cam and roller with the surface roughness was predicted under the elastohydrodynamic and mixed lubrication theory. Some study results are summarized as follows, (1) The influence of the cam surface finish and roughness on the contact bad and friction diagrams is quite small, but the tappet clearance should be adequate to the deviation of cam profile. (2) The influence of the oil temperature or the cam surface finish and roughness on the mean friction torque and the mean drive torque is also small. (3) Under the high rotational speed of camshaft, however, the mean friction coefficient decreases as the surface roughness becomes large. (4) The rough surface of cam lobe prevents the occurrence of skidding under the rolling contact between cam and roller because the bearing rate of solid contact increases as the surface roughness becomes large. (5) The surface roughness should be larger than one micron and smaller than two microns in center line average values. (6) Therefore the shot peening finish is more desirable than the grinding finish for the cam of the roller follower valve train