Choices for single-piece pushrods
If we look at the types of engines used in various car racing series worldwide, there is a very definite split between the US and elsewhere. The US has a hugely successful racing structure that predominantly uses pushrod (overhead valve/OHV) engines. From those racing at the local tracks right through to NASCAR Sprint Cup at the very top of the closed-wheel racing tree, the engines are based on a very similar architecture, namely that of production pushrod engines. Even the bespoke Sprint Cup engines are very similar to the previous generation of engines, which were based on production blocks.
The pushrod valvetrain is the most striking difference between OHV engines and overhead cam (OHC) engines used elsewhere in the world. The pushrod is one of the most critical components in the engine – not only does its design have a huge effect on the stiffness and resonant frequencies of the valvetrain, it also has to run very reliably in spherical sockets at each end of the pushrod, where conditions for lubrication aren’t great.
The main reason that lubrication conditions aren’t ideal is that the pushrod is constantly moving relative to its pivot, but only at a very low velocity. It is difficult therefore to generate a satisfactory oil film that can reliably separate the components under all operating conditions. In general, race engines place higher loads and stresses on components and contacts between them. This is especially true of valvetrains, where high rates of valve acceleration are one of the development engineer’s tools when producing improved valve lift profiles.
As such, the materials from which pushrods are made is critical. Some engine suppliers choose to use three-piece pushrods, where the main ‘tube’ and the ends can be made from different materials with very different properties. However, a number of engine suppliers, right through to some of those supplying winning engines for Sprint Cup, choose to use single-piece pushrods.
The material choice for such components is critical. If we look at NASCAR engines, the use of steels is mandatory, but it may not be wise simply to choose the first very hard steel that springs to mind for a single-piece pushrod for various reasons. First, some very hard steels are also very abrasive, especially where the surface is not machined to a very fine finish; this may promote wear of the sockets at each end of the pushrod. Second, for the main part of the pushrod, we want something with a degree of toughness in order to cope with high cyclic loads and shocks.
However, choosing a steel with high toughness is also not without risk. Many high-toughness steels may not be capable of being made sufficiently hard to withstand the contact stresses. In specifying the steel for a pushrod, the engineer needs to be sure that he or she has the correct combination of toughness and strength. They also need to ensure that the material is not too abrasive in its finished condition.
A popular choice for single-piece pushrods are tool steels. There are a number of such steels which, when suitably heat treated, are suitable for high-spec pushrods. For example, some companies offer single-piece pushrods in H13.
Written by Wayne Ward