‘External’ lash caps for inverted bucket cam followers
In engines with overhead camshafts, the choice of cylindrical translating followers is very common, both in passenger car and bespoke race engines. These are in the form of ‘inverted buckets’ (also called tappet buckets), with the flat ‘bottom’ of the bucket being the contact surface between cam and follower.
Other types of cam followers, such as finger followers, may offer a number of advantages – including lower reciprocating mass, more latitude with valve lift curves and possibly also decreased friction – but there are significant practical advantages with inverted bucket followers. They are essentially cylindrical components that can be produced by conventional turning on a lathe, and can be finished by grinding on conventional grinding machinery. Finger followers are much more complicated to manufacture, with a great deal of complex milling, and it is difficult to achieve the required accuracy between the pivot and the faces that contact the follower and the lash cap.
The correct valve clearances are generally achieved by selecting a lash cap (often called a tappet shim) that sits between the top of the valve and the inside of the bucket. However, there are engines which have used inverted bucket followers but which have taken a different approach to the adjustment of clearances, choosing instead to fit a larger shim on the top of the follower, between the follower and the cam lobe.
In order to be able to use the maximum possible valve opening and closing velocities, the shim needs to be as large as possible. The need to retain the shim means the follower top needs to have a wall, which is responsible for keeping the shim in its correct position. This means that, for a given follower outside diameter, the shim diameter must be smaller; as this represents the maximum contact diameter, the valve opening velocity is limited (valve opening velocity for a translating flat-faced follower is proportional to the maximum contact eccentricity from zero opening). This can represent a significant disadvantage.
This approach does however allow the use of a low-density bucket, providing that the contact stresses are maintained within material limits. If we take the simple example of an 80 g reciprocating mass being accelerated at 1000 g and 1000 N spring force, the force required is 1000 + (0.08 x 1000 x 9.81) = 63,784 N.
If we were to use a titanium bucket with a yield stress of 900 N/mm2, we would need an area of 69.76 mm to resist yielding. This represents a circle of just less than 9.5 mm diameter. So, if we were to use something similar to a lash cap on the underside of the follower and a large lash disc on the top, we can use a much lower density material for the cylindrical body of the follower assembly. This would reduce the reciprocating mass of the valvetrain and thus lower the requirement for contact area.
As passenger cars use much more modest valve acceleration rates and lower spring forces than are usual in race engines, they can make even larger steps in the use of low-density materials. I have seen cam followers made from anodised aluminium for the follower body with a hardened and ground steel lash disc between the follower and camshaft. A steel ‘lash cap equivalent’ would provide sufficient contact area beneath the follower to prevent yield.
Written by Wayne Ward