What Are Entrainment Velocity And Hydrodynamic Lubricity.
In engines that do not use a roller cam follower, oil entrainment velocity is created across the contact face, between the cam lobe and the cam follower. It is the speed, or velocity, that the contact point of the two parts is moving across surfaces of the two parts.
As the two surface move there are two distinctly different motions that can create entrainment velocity and the total entrainment velocity can be considered the sum of the velocity created by the two different motions.
The Hydrodynamic Lubricity of an oil is its willingness to travel between two parts, from a higher pressure zone on one side to a lower pressure zone on the other.
Wiping Motion
Wiping is probably the most obvious motion. We can use the example of when the cam lobe to follower contact is on the cam base circle. Although with a mechanical cam there is no forced contact at this point it is the easiest to explain.
During this motion the contact point stays at the same place on the follower and the cam lobe wipes or slide past this point. Friction is created at this point, and the higher the load and the smaller the contact patch the higher the friction.
The oil surrounding the contact area develops a high pressure zone on the side where it is being drawn towards the contact point, and a low pressure zone on the side where the two parts are separating after contact. The ability of the oil to travel between the two parts, from the high pressure side to the low pressure side, is the oils hydrodynamic lubricity.
Rolling Motion
During cam lift the contact patch between the cam lobe and follower is most often moving across the face of the cam follower. If the contact patch is moving across the face of the follower at the same velocity that the surface of the cam lobe is rotating at, then the motion is the same as that of a roller, with no sliding friction created at the contact point. On the side of the contact patch which the parts are rolling towards a high pressure zone forms in the surrounding oil, and on the side the parts are rolling away from a low pressure zone exists. Again a good oil will travel between the two parts driven by the difference in pressure on either side.
Troublesome Cancellations
At some stage during cam lift all non-roller valve trains will have wiping motion and rolling motion entrainment velocities that cancel each other out. That means no high and low pressure zones to encourage oil to get between the cam and follower.
If this happens at a time when the load on the parts is sufficiently high, and the contact area is sufficiently low, the oil film will break down and have a problem. There is still friction caused by the wiping motion, but there is no reason for the oil get between the two parts to where the friction is.
Hopefully friction modifiers in the oil will have left some deposits on the surface of the parts which will give some protection during this time, but in any case these periods of low or zero entrainment velocity must be kept short in duration so lubrication can be resumed before catastrophic damage is done.
Is Entrainment Velocity Likely A Problem In My Engine
Flat tappet push-rod and bucket style OHC valve trains do not typically have big problems with this, but if you are getting lobe wear that is worse either side of the lobe nose causing flats that give the nose a pointed triangle shape it may need looking in to.
Rocker style OHC engines on the other hand it needs careful consideration. Because the rocker has a curved surface rather than a flat surface for the lobe to run against, contact patch is reduced and the oil film is more likely to break down. Further more because of the asymmetric motion, rockers which have the cam rotating towards the valve end are likely to see longer periods of low entrainment velocity.
Pinto engines have the oil spray bar on the intake side to counter this, SR Nissan engines are hard on intake cams when the oiling becomes marginal, and OHC motorcycles typically wear the exhaust rocker worst for the same reason.
Keep good oil in your engine and use well designed cams and they will live long and prosper.