The Effect of Novel Surface Textures on Tappet Shims on Valvetrain Friction and Wear

In an engine, the valvetrain contributes about 6–10 % of the total frictional loss depending on architecture. The cam and the tappet contact in a direct acting mechanical bucket type valvetrain offers opportunities for friction reduction. Work has been done in the past to reduce frictional loss at the cam and tappet contact through use of lightweight materials to reduce reciprocating mass, improved surface finish, and low friction thin film coatings. This investigation explored the potential for additional friction reduction through the use of novel surface textures on tappet shims. Surface textures were produced on tappet shims using two techniques: (a) regular patterns like parallel line V-grooves, and square grooves, circular V-grooves and spiral V-grooves using a diamond tool, and (b) random irregular dimples using either ceramic peening or steel shot peening. The friction performance of these shims was compared with standard production shims and isotropic finish on production shims. Friction was measured using a motored valvetrain rig using a 3.0L cylinder head. The friction response was different on each type of groove and dependent on speed and oil temperature. The shims with parallel line V-grooves showed the highest friction reduction (up to 35%) compared to production shims. There appears to be no significant difference in wear pattern on cam lobes tested against production shims and shims with parallel line V-grooves. Also, the wear rate of a shim with parallel line V-grooves was lower than the production shims. The random dimples created by peening did not offer any friction reduction, probably due to increased surface roughness.