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 oscillation, nodal points, and critical speeds in different engine models.
| Identifier | ExFiles\Box 25\3\ Scan158 | |
| Date | 8th November 1926 guessed | |
| CONTD :- -2- Having determined the stiffness of the shaft and the inertia of the parts, we were able to estimate the fundamental or free period of oscillation of the system, also the position of the nodal point - by calculation. The nodal point depends upon the disposition and inertia of the various masses and in this particular case works out at 1.6" from the flywheel end. As the node is situated so near to the flywheel we can, without introducing any serious error, assume the total inertia of the crankshaft to be replaced by an equivalent mass at the front end. This equivalent mass would then have one-third the total inertia of the crankshaft and would be added to the inertia of the spring drive. Working this out on the lines given in our first report, we find that the free p w period would be 158 oscillations per second or 9480 oscillations per minute. We know from the 20 HP. results that the disturbing force has three impulses per revolution and should therefore expect the critical speed to be 9480/3 = 3160 r.p.m. This agrees with the actual vibration we experienced at 3150 r.p.m. and tends to confirm that this is the critical speed of the Phantom crankshaft. In practice we should seldom expect to run to the critical speed because this represents a top gear speed of 88 m.p.h. and again the valves start bouncing about this speed. The results of these tests and the conclusions drawn therefrom shew that although the crankshaft is considerably heavier than the 20 HP. the increase of stiffness counteracts the greater inertia and keeps the critical speed approx. the same as the 20 HP. At the first opportunity we shall make a torsional deflection test on a Silver Ghost. ------------------------ | ||