There are many components in the internal combustion engine which are subject to sliding contact, and which have to transmit substantial forces through that sliding contact. Some contacts are conformal - that is, the contacting pieces have similar and mating geometry - and hence have low levels of contact stress. A crankshaft operating within its bearings is a good example of this. Other contacts will have no such conformal aspects, and these non-conformal contacts are subject to high levels of contact stress (also known as Hertz or Hertzian stresses), even under some modest loads. Good examples of this are cam lobes and their followers, and gears.
Where there is sliding involved in a loaded non-conformal contact, it is common to find parts with a very fine surface finish. This is not simply for decoration; there are very good reasons for achieving a good finish, and the cam-to-follower contact is an excellent example here. While gears are a non-conformal contact pair, there is a lot of rolling contact in a gear, while the cam-to-follower contact is predominantly one of sliding.
The lubrication of these parts is very important to ensure sufficient service life and to mitigate the effects of friction. The lubrication regime is fundamentally hydrodynamic, and relies on oil being swept into the contact. There are two points during the opening and closing cycle of the valve where the entrainment velocity - a measure of the speed at which oil is swept into the contact - becomes zero. Where entrainment velocities are zero, or close to it, the oil film will tend to become much thinner. We can sometimes see the point at which the entrainment velocity is low by examining the cam lobe, as it can show signs of distress before any other points. If you go through the calculations, which are beyond the scope of this article, you can often correlate this point against real cam lobe damage.
As the oil film becomes very thin, the height of the asperities (high points) on the surfaces of the contacting components becomes significant. At the point where these can begin to touch because the oil film isn't thick enough to keep them apart, friction starts to increase rapidly. In this situation, the lubrication regime is called 'mixed' or 'boundary' lubrication, where the applicable friction coefficient is a function of both the lubricated friction coefficient and the dynamic dry coefficient of friction. As the film thickness ratio - the ratio of oil film thickness to the 'combined' surface finish of the pairs of surfaces in contact - decreases, a greater proportion of the contact area comes into solid contact.
By improving surface finish, the oil film can be much thinner before boundary lubrication applies. This mitigates frictional losses and can prevent wear from occurring.
There are several possible wear mechanisms in such a contact. Even if adhesive or abrasive wear does not apply, subsurface fatigue may become a problem, and another effect of rising friction coefficient is to increase the level of subsurface stress.
This is why it is common to find very highly polished camshafts and followers.
Fig. 1 - Cams are often polished, and there are very good reasons for doing so
Written by Wayne Ward