LiftersTags : valvetrain
The overhead valvetrain is no longer widely used in much of the world, although it retains a firm grip on the North American market and engines based on it also appeal to the Australian motorist. Perhaps the advantage of having historically had low fuel prices compared to Europe, combined with long and deserted roads between places of note, makes such engines appealing.
In racing, the overhead valve (pushrod) engine has a mandated stranglehold in NASCAR, and where large-capacity engines really count – in drag racing – they reign supreme. Where pushrod engines compete on even terms, such as Le Mans, they are more than capable of holding their own.
Each component in the racing pushrod valvetrain is very specialised, and the lifter (cam follower) is no exception. Apart from the Mercedes-Ilmor 500I Indy engine, people have generally stuck to using a translating cylindrical follower. Where the design engineer has the freedom to use a lifter with a bearing, he will generally do so. It allows him to open the valves with higher velocity and get more gas flow through the engine. Airflow means power, and if a good job is made of the tuning, an engine using roller followers will always beat one equipped with flat-faced followers. Almost without exception, roadcar manufacturers have also adopted the roller follower.
There are variations on this basis design, with one of the main distinguishing differences being the type of bearing used for the roller. The simplest bearing is of course the plain bearing. In the plain bearing lifter, the roller can be bushed or bushless, but the principle remains the same – the roller and shaft are separated by a thin film of oil, as is the case with the bearings for a typical crankshaft. This arrangement has the advantage of being very stiff – the roller has a thick section and the shaft can also be stiff.
The roller bearing variant uses a needle roller bearing between the shaft and roller. The roller and shaft surfaces must both be very hard in order to resist rolling contact fatigue, and the stresses involved are very high compared with the plain bearing variant. The load is carried by a small number of rollers which are essentially line contacts. Although elastic deformation provides some increase in actual contact area, it requires that the components are very specialised, and the material limitations of the follower shaft and roller bore need to be borne in mind when designing new cam profiles; for plain bearings contact stresses are hardly a concern. The stiffness of the roller bearing arrangement is also lower than the plain bearing type. For a given follower roller outside diameter, the section is diminished by the space taken by the bearing. An increased shaft diameter can help improve stiffness and reduce contact stresses for a given follower load.
Additional to the above considerations are friction and wear. The roller bearing is felt to offer lower friction, and at start-up we have a rolling contact scenario, where the plain bearing variant is in sliding contact until the oil film is fully established. The roller bearing type is probably less prone to failure due to lack of lubrication at start-up.
Think carefully when specifying the type of follower, as friction, wear and stress all need to be considered.
Written by Wayne Ward