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).
Calculation and discussion of engine critical speed based on component inertia and stiffness.
| Identifier | ExFiles\Box 25\3\ Scan317 | |
| Date | 29th February 1928 guessed | |
| contd :- -7- The crankshaft stiffness Cs = 16,350 lbs.ft/radian. Crankshaft inertia (M k^2 / g) = .01682 slugs ft^2. Big ends (M x r^2 / g) = .00778 " " Pistons (M x r^2 / g) = .004710 " " Total distributed inertia = .02931 " " One-third of this = .00977 " " Plus I of spring drive hub = .00582 " " Total effective inertia = Ic = .01559 slugs ft^2. Inertia of flywheel = If = .6911 " " So that critical r.p.m. = 60 * 1/2π * √[16350(1/.6911 + 1/.01559)] = 3296 r.p.m. Actual measured speed is 3300 r.p.m. Fig.4. attached is a table shewing the calculated and observed critical speed on all engines to which we have had access. In the case of the Stutz, neither the big end or little end weights or the crankshaft inertia were known. The total weight of the conn. rod was known, and the crankshaft inertia had to be calculated from its dimensions. It appears that starting from nothing but the drawings of a car engine, it should be possible to predict the critical speed within about 6% as outlined above; the max. error attainable being about 10%. The method given is of greater accuracy in predicting the effect on the critical speed of any modification that may be made. Hs{Lord Ernest Hives - Chair}/S.S.Tresilian. | ||