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 crankshaft torsional periods and distortion of moving parts as causes of high-speed engine vibration.
| Identifier | ExFiles\Box 25\3\ Scan359 | |
| Date | 13th November 1928 guessed | |
| contd :- -3- However on other cars we find that the main torsional period when reached is very much more sudden and sharply defined. We have noticed recently on two cars with smooth engines that we have tested, namely, the Stutz and an Alfa Romeo, that you could run up to within a few r.p.m. of the main period and not know it was there. We think that this is more typical of crankshaft torsional periods, namely, that they are sharp and well defined and extend only over a few r.p.m., as we find on all the lower periods on our own engines. It appears therefore that proximity to the torsional period should not in itself be a cause of high speed roughness, and that there is still some other cause which, if eliminated, would enable us to run very much closer to the main period on the 20 HP. than we can already. Crankshafts stiffer by 40% have been standardised now on the 20 HP. since June, yet careful enquiries amongst the Test Dept. show that they have detected no reduction in high speed roughness. Yet the torsional period must have been raised by about 15%. DISTORTION OF THE MOVING PARTS. We think that at high speeds this is the most serious of the possible causes of vibration, due to the nature of the loads, and to the nature of our engine parts and materials. In the vertical plane of each cylinder there are very large inertia forces acting on the crankshaft due to the rotation of the big end, pin and webs, and to the reciprocation of the piston and little end. The magnitude of these loads is a 20 HP. engine at 3500 r.p.m. is shewn on fig:1. attached. contd :- | ||