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).
Analysis of engine balance, inertia loads, and vibration characteristics at high speeds.
| Identifier | ExFiles\Box 182\M16\ img009 | |
| Date | 2nd June 1930 guessed | |
| ) - 2 - at high speeds, and the inertia loads on the shaft are more balanced than in the usual arrangements. This should help to give smoother running. The normal 2-4-2 or 4-4 arrangements would have resultant inertia loads on all five bearings. It would appear a good arrangement for a high speed engine. The maximum permissible R.P.M. is 5,500, at which speed the car is geared to do about 120 M.P.H. A previous owner is reputed to have captured the Hour record at Brooklands with it at 117. The shaft was very tight when we took it down, but had been fretting slightly on most of the taper wedges etc., presumably during C.T.V. periods. The wedge-shaped dowels that grip the crank-pin extensions are not actually flat as shown in my drawing, but very slightly radiused lengthwise (and also hardened) so that they bed definitely at either end of the flat on the crankpin extension. (3) BALANCE AND GENERAL SMOOTHNESS. - I have worked out the balance as follows:- Piston Acceleration Forces. As in normal 4-4 arrangement, e.g. 4th, 8th, 12th, etc., harmonics occur. Piston Couples. Again as in normal 4-4. -2nd, 6th, 10th, etc., harmonics occur. Big End Couple. There is a long couple for each half of the engine, and they combine to produce a small couple of magnitude 2w^2/g x a x centrifugal force on one big end, where 'a' is the least distance between adjacent cylinders. This is special to this crank arrangement, and is balanceable, though no effort is made to balance it on my engine. The couple is in a plane at 45° to the two planes of cranks. Conn. Rod Couple. does not occur. Inertia Torque. As in normal 8 cylinder, 4th, 8th, 12th, harmonics. A normal 4-cylinder has 2nd, 4th, 6th, etc., harmonics. There is a distinct vibration period and movement of the whole car, at about 1200 R.P.M. running light, but it cannot be noticed under load. There is no other vibration period in the speed range which could be blamed to out-of-balance. The engine is uniformly smooth, though not dead smooth, up to maximum R.P.M. | ||