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).
Technical analysis of shaft vibration and the operation of harmonic balancers.
| Identifier | WestWitteringFiles\Q\December1926-January1927\ 98 | |
| Date | 15th December 1926 guessed | |
| contd :- -2- vibration in the shaft the vibration excited in the balancer opposes the movement of the shaft. In practice this works out best when the rate of oscillation of the balancer alone is approximately two thirds that of the shaft. These figures vary of course for different assemblies and mass distribution. Considerable study has been made of the phase relations between the motion of the shaft and balancer. These studies were conducted by artificially exciting the shaft and by means of a light beam reflected from mirrors oscillated by the shaft and the balancer respectively. The beams were carried along the screen by rotating octagonal mirrors shewing the phase relation between the movements of the shaft and balancer under various conditions of operation. When the tuning is correct the balancer lags the shaft by approx. 90°. Our studies indicate that the detailed operation is as follows: When the shaft makes a slight movement it compresses the balancer springs but the mass of the balancer resists the motion thereby applying a resistance to the shaft. The persistance of the pressure however accelerates the balancer in the direction of the shaft's movement but owing to lag of the balancer the shaft reaches the end of its vibration just as the balancer is moving at its maximum velocity. The balancer overruns and through its springs exerts a resistance to the shaft in its movement in the opposite direction. The cycle is repeated. Since the balancer operates actively on its springs a relatively light mass controls a shaft of many times its weight. When in operation the movements of the balancer are considerably greater than those of the shaft, the vibration of the shaft being limited to a very small range. In any shaft to which we have supplied the harmonic balancer we have kept the fundamental period above the operating speed. I am not certain whether a harmonic balancer could be built properly to control the fundamental period. This would only be necessary in a shaft which is long and perhaps equipped with counterweights so the fundamental period occurs at a rather low speed. The addition of a harmonic balancer to a shaft tends to bring down the fundamental period. In the case of Buick the addition of the harmonic balancer and counterweights lowered the fundamental period from 4800 to 3700 r.p.m. Since 3700 r.p.m. is still above the driving range, this period does not interfere with operation. We find there is practically no difference in the operation of the balancer when equipped with leaf springs or the coil springs. The beneficial action of the balancer is not through frictional dissipation of the energy. In fact, friction is a hindrance to its operation. It controls vibration effectively through superimposing upon one vibration system another system of a different frequency so there is interference. contd :- | ||