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).
Comparison of poppet versus ball valves in G.M. hydraulic tappets, focusing on performance at high speeds.
| Identifier | ExFiles\Box 17\6\ Scan302 | |
| Date | 4th September 1934 | |
| To E.{Mr Elliott - Chief Engineer} c. to By.{R.W. Bailey - Chief Engineer} Hs{Lord Ernest Hives - Chair}/Smth.11/KW.4.9.34. G.M. Hydraulic Tappets Poppet versus Ball Valves. 1. High Speeds. It is found that with a certain valve gear arrangement, embodying hydraulic tappets, on Ph.II, a speed of 3500 r.p.m. can be reached before the valves are held off their seats. With ball valves in the hydraulic units, a minimum clearance of .0025 must be provided between the plunger and its casing in order to attain this speed. With smaller clearances the critical speeds are correspondingly lower, thus :- | Clearance | Critical Speed | |-----------|----------------| | .0005 | 2800 | | .0010 | 3000 | | .00175 | 3300 | | .0025 | 3500 | With poppet valves 3500 r.p.m. can be obtained with .00075 clearance only, and, owing to the low oil leakage in this case, the plungers do not dance up and down in their casings at low speeds, as they do with greater clearances, and less oil is required to operate the mechanism. Experience gained during the development of the tappets has shown that at certain instants during the valve lifting operation positive contact between the push rod and the bottom tappet may break down at speeds much below the "bouncing speed". By bouncing speed we mean the lowest speed at which, with standard valve gear, the valves bounce on their seats sufficiently violently to cause a drop in power. | ||