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 harmonics and crankshaft stiffness, proposing solutions involving reduction gear design.
| Identifier | ExFiles\Box 137\4\ scan0038 | |
| Date | 6th October 1929 guessed | |
| -2- In twin-4 car engines they succeed in keeping this period up by means of the short stiff shafts. In our case a reduction gear is fitted and we find the crankshaft stiffness is reduced thereby to about half. The "F" engine on a 6 cyl. would have a 3-per-rev. period at 2100 R.P.M. we believe. We have calculated this figure but have never checked it, which we hope to do shortly. It would have a 4-per-rev. period at 1580 R.P.M. if it could. Actually it cannot, as the 6th harmonics which are present in a V-8 cancel out in a twin-6. In a twin-4 the short shaft should tend to raise the period above 1580, but the necessity for balance weights will tend to lower it, so we think the period will be between 2000 and 2500 R.P.M. It would be possible to calculate more closely to the period after seeing the designs, but we would not hope to get more than +- 15% accuracy on our limited experience with aero engines. The two alternatives to ameliorate the trouble are (1) To aim at terrific stiffness in the reduction gear and crankshaft, and raise the period. (2) Introduce a damped spring drive and reduce the period to lower engine R.P.M. With regard to the reduction gear which we find to be so flexible, we could never trace the cause of this. The only two shafts, namely the prop. shaft and the splined connection shaft together, only account for about 30% of the flexibility. Therefore we do not at present | ||