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).
Letter from the Air Ministry discussing engine damping, torque variation, and resonance in response to a note from Mr. Lovesey.
| Identifier | ExFiles\Box 25\3\ Scan188 | |
| Date | 30th November 1926 | |
| TEL. No. HOLBORN 3434. Any communications on the subject of this letter should be addressed to :- THE SECRETARY, AIR MINISTRY, ADRASTAL HOUSE, KINGSWAY, LONDON, W.C.2. and the following number quoted:- 632745/25/R.D.3.1. [Handwritten Note]: Jun 12y. Rg{Mr Rowledge} 9/12/26 [Stamp]: RECEIVED DEC 4 1926 [Handwritten Note]: X634 AIR MINISTRY, LONDON, W.C.2. 30th November, 1926. [Handwritten Note and Signature]: W. Hives I have not ack. this yet. Ry. Dear Mr.Rowledge, Many thanks for your letter of the 23rd instant enclosing Mr.Lovesey's note, which is very interesting. So many points arise out of this note and the previous ones that I am not attempting to cover them in this letter but will wait until my next visit to Derby. There are one or two observations I should like to make however, - (1) On the question of relative damping of the large and small engines I am not clear that the angle of twist of the shaft is a true basis of comparison, it may be possible to get a truer basis on the following lines:- 20 h.p. engine with stiffer crankshaft. synchronous speed about 3500 r.p.m. inertia torque variation 3000 r.p.m. is ± 240 lb.ft. therefore inertia torque variation at 3500 r.p.m. is ± 335 lb.ft. On an average this acts at mid length of shaft and the twist that would be produced statically by the inertia torque variation would be half that corresponding to 335 lb.ft. on the front end of the shaft. This gives ± 1/2 degree. The actual shaft twist on the front end as measured with the torsiograph was ± 3.3o or 6.6 times the average static figure. I do not know whether the ± 3.3o was measured with the engine running under power or with motoring, but in either case the torque variation would probably be of the same order of magnitude and thus the "Resonance Factor" appears to be in the region of 6.6 at the synchronous speed or Δ = 1 / 6.6 roughly, which seems reasonable. Worked out on the same basis the 40/50 h.p. engine might be found to give about the same value but I have not the inertia torque to work from for this. (2) On the question of the peak width I note that indefinite runs have been made at 83% of the synchronous speed, but the question of the shaft failure is wrapped up with the shaft strength and does not throw direct light on the width of the resonance peak. If the curve of the "Resonance Factor" plotted against "Frequency ratio" could be obtained with the torsiograph, the danger zone could be decided for a shaft of known strength and there would be a check on theory, for the present the safe course appears to be to stress crankshafts on the basis of the theoretical resonance curve. (3) Mr. A.J.Rowledge, Messrs Rolls-Royce Ltd., D E R B Y. | ||