Rocker stands and shafts
The pushrod valvetrain, while having great advantages for engine packaging, has the distinct disadvantage of not having a very direct connection between cam and valve, as is the case with overhead cam (OHC) engines. Even in the case where finger followers with roller bearings are used, the stiffness of the system is higher than is typically the case with an overhead valve (pushrod) system. As has been stated in previous RET-Monitor articles on pushrods, efforts are always being made to enhance the stiffness of the components in pushrod valvetrains.
As the use of valvetrain dynos such as the Spintron rig has become common, so people have realised the importance of component stiffness. Pushrod dimensions have increased markedly as a result in the search for stiffness, and a great deal of effort has also been put into augmenting the stiffness of rockers, with these parts commonly made in steel.
The more anonymous components can have an important effect on system stiffness, and here we must include rocker shafts and stands.
In order to be kept stiff, rocker shafts have to be short and well supported. In cases where they are bolted directly to their rocker stands, this is the reason why the bolts are kept as close as possible to the rocker. With the deflection of a simply supported beam being proportional to the cube - that is, the third power - of the distance between the supports, we can see the reason why it is important to keep the supports close to the rocker.
The diameter of the shaft is also important, the deflection of the shaft being inversely proportional to the cube of the shaft diameter. Therefore, even a small increase in shaft diameter is a very potent way of increasing shaft stiffness and minimising deflection in service.
The rocker stand has an important part to play in the pushrod valvetrain, and the structure supporting the rocker shaft needs to be designed with stiffness in mind. The stands are commonly made from aluminium alloys, but also steel can be used where stiffness is of particular importance. Titanium would be another acceptable material for this application, being intermediate in terms of both stiffness and mass when compared to aluminium and steel.
There is, however, little point in fastening a very stiff structure to one that is very flimsy in comparison, because we will find that the dynamic behaviour of the system is dictated by the stiffness of the least stiff component. We therefore need to ensure that both the rocker stand and the structure to which it is mounted are sufficiently stiff. Where a stiff rocker shaft is mounted to a well-designed, stiff rocker stand, but the rocker stand is mounted to a thin, poorly supported section of the cylinder head, the full benefit of the stiff shaft and stand won't be fully apparent. That is why we sometimes see some really bulky rocker stands which extend well beyond the area apparently required to mount it to the cylinder head. This is an attempt to transfer the valvetrain reaction loads to a stiff part of the cylinder head structure.
Fig. 1 - This rocker and stand assembly is very stiff. The steel shafts are very short, using reduced flange 12-point nuts to secure them, and the rocker stands are made of steel (Courtesy of T&D Machine Products)
Written by Wayne Ward