APPLYING HARD THIN COATINGS TO TAPPETS TO REDUCE FRICTION

In a motorcycle engine, the tappets convert the rotating motion of the cams to the pitching motion of the valves, and they slide with the cams at a high speed under high pressure. Reducing the friction between the tappets and the cams would increase the power of the engine.

We studied a way to reduce the coefficient of friction between tappets and cams by applying a hard thin coating of DLC, MoS₂, CrN, or TiN to the tappets. We investigated the adhesion of the coatings with a scratch test and their tribological properties with a ball-on-disk test. We then examined their durability with an engine stand test. We concluded that a tappet coated with DLC effectively reduces friction at the cam and tappet interface and is sufficiently durable to be applied to actual motorcycle engines.