From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Tappet spring performance, valve bounce speeds, and a comparison of Halford and G.M. type automatic tappets.
| Identifier | ExFiles\Box 17\6\ Scan299 | |
| Date | 18th July 1934 | |
| -4- Whereas in the first two cases the tappet spring is more than strong enough to control the tappet at the valve bouncing speed, for the 20/25 this is not so. The same bouncing speed could be obtained on this engine by reducing the valve spring loading and increasing the tappet load, thus reducing the valve gear noise. The Bentley tappet spring has twice the load of the 20/25 with the valve closed, so that no difficulty with lubrication of the base circle of the cam is to be expected from this modification. In order to raise the valve bounce speeds, the possibility of controlling the push rod with the tappet spring might be looked into. Automatic tappets. Our experience with hydraulic tappets has been reported on elsewhere, and may be summarised as follows :- Halford Type. Owing to the high leakage factor and small oil pressures in the plungers, this type of tappet does not tend to hold the valve off its seat as does the G.M. type. The mechanism, however, adds largely to the reciprocating weight of the valve gear, and considerably reduces the valve bounce speed. G.M. Type. This type, whilst not lowering the valve bounce speed, causes the engine to be more sensitive to bounce. The mechanism lifts the valves off their seats at the occurrence of the least amount of bounce, and hence at a speed lower than that at which the bounce is sufficiently violent to cause a drop in power with standard valve gear. With both these types, then, every effort must be made to lighten the valve gear. Increasing the valve spring loading makes the gear more noisy and hence invalidates the purpose for which the tappets were designed. | ||