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 document detailing the calculation of crankshaft vibration frequency and critical speed.
| Identifier | ExFiles\Box 25\3\ Scan316 | |
| Date | 7th February 1928 guessed | |
| contd :- -6- Ic consists of the sum of : (a) any inertia on the nose itself of the shaft. (b) One-third of all inertia distributed along the shaft. The distributed inertia is the sum of the following : (1) Crankshaft. (2) The big ends (having a k² = r²) (3) 1/2 the reciprocating weight (with k² = r²) The reason for taking the crank radius as the radius of gyration for the last two items is that they may be considered as rigidly attached to, and part of the crankpins. So that Ic consists of : any interia on the nose plus one-third of the sum of the crank-shaft, big ends, and reciprocating weight inertias. Calling the crankshaft stiffness Cs, in lbs.ft. per radian, and the inertia units being slugs-ft² (M k² / g) the critical frequency of crankshaft vibration is given by : Frequency = 1/(2π) ⋅ √ (Cs (1/If + 1/Ic) ) per sec. Therefore, if n be the number of vibrations per revolution of the crankshaft at the critical speed, then the critical speed is given in r.p.m. by : Critical r.p.m. = 60/n ⋅ 1/(2π) ⋅ √ (Cs (1/If + 1/Ic) ) n = 3 in a 6 or twin-6 engine ; 4 in a 4, 8, twin-4, twin-8. As an example of calculating the critical speed, the 20 HP. std. engine will be taken :- contd :- | ||