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).
Engine vibration, damping, and damper design, comparing the Phantom and 20 HP models.
Identifier | ExFiles\Box 25\3\ Scan177 | |
Date | 9th November 1926 guessed | |
contd :- -3- the influence this normally exerts does not enter into the question as to the reason for the vibration on the larger engine being less severe than on the 20 HP. Now the rigidity of the Phantom crankshaft is 2.68 times greater than the 20 HP. but the disturbing force due to the piston inertia torque has increased approximately 2.7 times and while from theoretical considerations we should expect the amplitude of the vibration to increase in proportion to the energy supplied we can only surmise that the reason why the vibration does not build up to the same extent as the 20 HP. is that the internal damping must be greater. Exactly what factors influence the internal damping which must be present in these shafts in order that they avoid fracture when resonance occurs - we have at present no information. Therefore, working on this supposition, it seems that internal damping exerts a greater influence than can be obtained by the present external damping of reasonable proportions. We think we could direct our further investigation towards elucidating these points. In the case of the Phantom not only do we find that the critical amplitude is less than the 20 HP. but the lower vibrations - or harmonics - are correspondingly less severe and on the engine investigated we failed to get a diagram shewing any appreciable amplitude at the half critical speed. In a R.A.E. report No.3039/A - which is a theoretical investigation of viscous-friction dampers - it is concluded that where viscous friction dampers of a finite damping co-efficient are rigidly driven, the damper requires to be undesirably heavy to be effective. Also it is shewn that "a much smaller damper can be made effective by adopting an elastic drive of suitable "rigidity instead of an infinitely rigid drive". This latter arrangement seems to be similar to the original damper employed on the first Goshawk and we therefore think it would be worth while to test this again and carry out further investigation on it. It was suggested by Mr. Royce that a centrifugal damper would be preferable to the constant friction type at present employed. We know that if the damper friction is increased until the damper virtually becomes part of the shaft, the resonant speed falls to a lower value, the range between the speeds depending on the polar inertia of the damper. In the case of the 20 HP. the damper when locked solid, reduces the resonant speed from 3200 RPM. to 2400 RPM. Therefore if the damper became virtually solid, or the friction became very high, after the 2400 RPM. had been exceeded, we should expect to be free from resonance effects at both speeds. contd :- | ||