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 discussion on crankshaft torsional resonance and the implications of using cast iron versus steel.
| Identifier | ExFiles\Box 140\3\ scan0132 | |
| Date | 1st January 1938 | |
| -2- 1.1.38. Generally speaking, with a reasonable size of flywheel and normal damper on the crank nose, this resonance is so violent that the torsional deflection of the shaft each cycle may exceed 5° and the crank soon breaks. It is, therefore, general practice to design shafts so that their frequency lies outside the speed range of the engine. Even when this is done there is still a torsional resonance to be faced at half the critical speed, i.e., in the case cited at 2,000 R.P.M., an explosion impulse will excite the shaft when it makes every other torsional oscillation. This, owing to the natural damping due to piston and bearing friction, is not nearly so violent as the vibration at 4,000 R.P.M., and in most cases can be taken care of with an ordinary crankshaft damper. Where the explosion pressures are exceedingly high, however, as in a diesel, the ordinary damper may be unable to completely eliminate this vibration, and in consequence the shaft may be in danger of failure. You are probably very familiar with all this but I suggest it has a bearing on the failure of the cast crank in the oil engine. I understand from your Experimental people that the crank was run on a 6-cylinder, which has the six per rev torsional at just below 2,000 R.P.M., and to avoid this the max. engine revs were kept down to 1,800. I was told that this torsional period was much worse on the oil than on the petrol engine, which is what I should expect. Now when you put a cast crank in this engine of the same dimensions as the steel one removed, the inertia of the system would remain unaltered, but the rigidity of the shaft would be reduced in proportion to the reduction of the modulus of torsional rigidity of cast iron as compared with steel. I understand that the modulus of torsional rigidity of iron lies between 5.5 and 8 x 10^6 steel 12 x 10^6 which, if correct, would result in the speed of the six per rev. torsional being reduced from about 2,000 R.P.M. to about 1,500 R.P.M. (i.e. as the square root of the stiffness). If this actually happened, then your iron shaft brought the six per rev. torsional vibration into the speed range of the engine, and due to the high max. explosion pressures of the diesel, this would appear to have been disastrous for the life of the crank. | ||