Design of spring retainers
Valvetrains are often the key to improving engine performance. Given a free hand, most engine development engineers will strive for ever higher engine speeds as a route to increased power output, if friction can be prevented from overcoming the extra power released by the increase in speed. Before the FIA reined in the Formula One engine suppliers, the engine speeds of the V8s were more than 20,000 rpm and heading for 21,000, which would have put the 2.4 litre engines into the territory Honda was in with its six-cylinder 297 cc bikes 40 years ago.
Controlling the valvetrain at high speed is much easier when the valvetrain components are stiff and light. One advantage of pneumatic valve return systems (PVRS), as found in MotoGP and Formula One, is that there is no valve spring mass, and the ‘piston’ that in effect replaces the spring retainer is very light indeed. Many of the diagrams showing PVRS though have the ‘piston’ as a far larger and more complex component than is actually the case in a modern system.
The valve spring retainer has to be as light as possible. A previous article on this subject looked at materials options, but in this article I want to look at some of the design features that make for a low-mass and reliable retainer.
The retainer’s main function is to retain the valve in its compressed state, providing the correct level of pre-load and reliably coping with the stress cycles imposed on it. The valve retention involves initially compressing the valve and then locking a pair of collets in place so that the valve remains loaded against the seat when the valve is closed or, when the valve is open, the follower should (in most circumstances) remain in contact with the cam lobe.
Collets are tapered on the outside, and in order to provide a reliable seat the cone angle on the retainer must closely match the collet angle. It is also important that the surface finish in the collet bore is good, otherwise there may be some wear and slight loss of pre-load. A certain amount of material around the collet bore is needed to prevent the collet being pulled through the retainer – any movement of the collet subtracts from the spring load, so we need to guard against this by providing a sufficiently stiff ring around the collet. It can be tapered though to reduce mass.
The springs are retained in a concentric relationship with the valve by having the spring location (or multiple locations if nested springs are used) machined concentrically to the collet bore. If the springs are not concentric then the loads with the valve won’t be either, and bending stresses will be increased. The transition between the compression face of the spring location and the radial location surface also needs to have a generous radius. This is the point of highest stress in most circumstances, and you should be sure your valve spring has a large enough chamfer on its upper end turn to fit this radius on the retainer without interference.
The top of the retainer represents a large mass, and sometimes there is a tendency to make this too flexible – remember that there will be a loss of spring load compared to that expected for a given valve lift as the retainer deflects. This can’t be prevented, but it can be mitigated.
Written by Wayne Ward