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).
Shaft distortions, vibrations, and how engine components resist dynamic forces.
Identifier | ExFiles\Box 25\3\ Scan378 | |
Date | 9th January 1929 guessed | |
contd :- -2- distortions. It should be realised that just to provide bearings to any shaft rotating under load is no guarantee of smooth vibrationless running. In order to carry any load the bearings must yield, and if the loads be large and rapidly applied and removed then the yields will be large and the smoothness will vanish. Prof. Morley says (Strength of Materials): "A rotating shaft, when laterally disturbed, has its elastic righting forces reduced by the centrifugal force arising from its own inertia, hence its stiffness and frequency of transverse vibrations are reduced". He also says that the application of a 'live' load (one applied suddenly) to an elastic material will start a vibration of amplitude equal to twice the displacement under the same load applied slowly. The engine parts (e.g. crankcase and crankshaft) have to provide equal and opposite reaction to resist the dynamic forces, and they can only do it in two ways: (a) partly by "mass-acceleration" reaction, or inertia-resistance to being vibrated. {handwritten} This proportion increases as the square of the frequency or speed (b) partly by their stiffness, i.e., by elastic yield until stresses are built up to resist the loads. This takes time and becomes of less and less importance at high revs. It is owing to there being these two ways of taking the loads that the stresses in a high speed crankshaft bear only a faint relationship to the loads applied as calculated. It is only by method (b) that loads are transmitted to the main bearings; i.e. loads applied at the crankpin say, and resisted on the spot by inertia do not reach the crankcase, in the same way that the contd :- | ||