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).
Crankshaft balance weights and critical speeds for the 25 HP and Phantom 11 engines.
| Identifier | ExFiles\Box 182\M19\ img173 | |
| Date | 16th May 1931 | |
| -2- In the case of the 25 HP. with the big pin crankshaft with twelve balance weights on, the master period is at 4,200 R.P.M. which can begin to be felt at 51 m.p.h. on third gear. Therefore we would say that with eight balance weights we shall be entirely clear of the main crankshaft period. We are not in such a good position on the Phantom. On Phantom 11 with twelve balance weights the critical speed is 3,200 R.P.M. or 82 m.p.h. and can be felt three or four m.p.h. below this. Grys. calculates that if we use six units or eight balance weights scheme the critical speed will be 3,500 R.P.M. or 90 M.P.H. We should consider this was still within the range of engine speeds. The present shaft without any balance weights is 3,900 R.P.M. Grys. suggestion is that on Phantom 11 we may have to modify the design of the balance weight so as to have a small inertia with a larger mass. We attach herewith a sketch which illustrates this point. (The sketch supplied is for 25 HP. but same applies to P.2). In connection with the eight balance weight scheme Grys. point out the difficulties that may arise where four weights are one unit and four are half a unit, in making the half unit a mass multiplied by the distance of the centre of gravity of journal centre line correct, otherwise the crankshaft will not be in balance. Hs.{Lord Ernest Hives - Chair} See file copy for blue prints | ||