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 to Mr. Pilkington of Leyland Motors regarding gearbox design and gear ratios.
| Identifier | ExFiles\Box 140\3\ scan0154 | |
| Date | 29th March 1938 | |
| 117S. March 29th.1938 Rm{William Robotham - Chief Engineer}/Gry.{Shadwell Grylls}2/AP. V.W.Pilkington Esq., Messrs Leyland Motors Limited, Leyland, LANCS. Dear Mr.Pilkington, I had not realised from your last letter that you wanted a ratio of nearly 5 to 1 between the highest and lowest gear. I thought that a lower 3rd. gear of 1.5 to 1.6 would meet your requirements. You will notice on the drawing of the box you have that there are immediate limitations to the smallness of the 3rd. speed driven gear owing to the coupling dogs having to pass underneath the synchro cone. Without altering the design of coupling and retaining the present constant mesh ratio of 25/34 you could not get an overdrive ratio higher than .9. In our overdrive box we only got .83 by altering the constant mesh and re-obtained 1st. speed by increasing the shaft centres to 4.050 and putting a few more teeth in the 1st. speed wheel. If we were out to get the ratios you want and were prepared to use a geared up highest speed, we should increase the length of the box about .5" so that the engaging dogs of the 3rd. speed wheel did not limit the 3rd. speed ratios. The smallest possible 3rd. speed wheel still retaining the brass bushes would provide a ratio for this train at 3.750 shaft centres of 1:2 which would not give you quite a high enough overdrive at 25/34 constant mesh. 4.050 shaft centres would provide exactly the ratios of your GB.9 after doing the necessary redesign round the overdrive coupling dogs. As we told you when you were here it is essential to keep the shaft centres as small as possible to obtain light operation of the synchromesh. It is for this reason that we continued | ||