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).
Comparison of crankshaft inertia and stiffness, particularly between four-bearing and seven-bearing types.
| Identifier | ExFiles\Box 25\3\ Scan249 | |
| Date | 22th April 1927 guessed | |
| contd :- -8- .04322 16220 ------- X ------- = 1.47 23350 .016323 or 47% greater than the 20 HP. RR. This relatively large increase in inertia to stiffness must be partly due to the longer stroke as we should expect the inertia of the shaft to increase roughly as the square of the stroke. The stroke of the Lorraine is 5.125" which on the above assumption would account for 30% of the increase. Extending the comparison still further we find that the polar inertia of the Phantom crankshaft is proportionately very nearly the same as the Lorraine. From these considerations of crankshafts we have tested we should imagine there is little reason to expect a four bearing crankshaft should possess lower inertia for stiffness than the seven bearing type provided the cyl. spacing remained the same. With a four bearing crankshaft advantage could [struck] probably be taken for of its construction to allow closer grouping of the cyls. and so, by shortening the shaft, increase its stiffness at the same time as reducing its inertia. Under these conditions only does it seem that benefit in this particular respect - i.e. low inertia for stiffness - could be derived from a four bearing crankshaft. We admit we have had experience only with this one case of four bearing crankshaft but these above notes on crankshafts are put forward in an endeavour to explain the reason for the higher 'master period' in the case of the Lorraine. contd :- | ||