The tip of the racing poppet valve is seldom considered, despite having to withstand relatively high contact stresses. Whilst we can say that the contact stresses concerned are generally low compared to other valve train applications, the relative movement between the valve tip and the adjacent component can be low, especially in the case of overhead cam engines where a lash cap or, more rarely, a bucket follower applies the load to the valve.
Even in the case of those engines where rockers are used, the sliding velocity of the adjacent parts, relative to the valve tip, is low. The consequence of this is that there is little or no chance that a significant and stable oil film can be formed. The oil films associated with hydrodynamic lubrication rely on significant and continued relative movement (or more precisely they depend on there generally being a non-zero entrainment velocity for those of your familiar with the technical terms).
Therefore, what scant quantity of lubricant finds its way into the valve/lash cap contact probably relies to a great degree on squeeze-film lubrication for any useful effect. Where rockers act directly on the valve, there may be some operation within the hydrodynamic lubrication regime, but this will be intermittent and transient, with a fully developed oil film being present only momentarily.
There can be a tendency for the lash cap to shuffle about on the top of the valve and this may cause some wear problems. The poor lubrication situation of engines using rockers is probably of greater concern, as there is a reasonable amount of relative movement with little protection offered by a fully developed oil film.
Whilst the frictional losses here are negligible, the problem of wear can be significant, leading not only to damaged components, but also to increased valve clearances if wear is significant. Where extra clearance is present, the result may be loss of performance, but there is a genuine risk of premature and catastrophic component failure owing to high valve seating velocities.
It is for these reasons that people have over the years tended to specify various processes to prevent damage to the tip of the valve from contact loads. Hardened tip materials such as ‘stellite’ (a cobalt based hard alloy) have been specified for many years, and the process of brazing a tip of a different material from that used for the rest of the part to the tip of the valve has been used with success. For instance, austenitic steel valves have a low level of general hardness and this cannot really be improved greatly by heat treatment. However, it is possible to add a tip of a hardenable steel material to give increased resistance to surface damage. There are many suitable candidates for the tip material. A steel such as 4340/EN24 is a popular choice and can be hardened to a level above 50HRc. Some people prefer even to go a step beyond this level of hardness and specify a high-carbon steel which can be hardened beyond 60HRc.
Written by Wayne Ward.