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).
Investigation into crankshaft stiffness, comparing 4-bearing and 7-bearing shaft designs and the effect of disc webs.
| Identifier | ExFiles\Box 82\4\ scan0084 | |
| Date | 19th January 1935 | |
| -2- We next further investigated why the 4 bearing shaft is only 17% stiffer than the 7 bearing shaft. It will be remembered that the 4 bearing shaft had to suit existing cylinder centres and that therefore there are three spans of two crankpins longer than would be the case if the cylinder centres were altered to suit a 4 bearing shaft. Model centre spans to Hs.{Lord Ernest Hives - Chair}1580/1 were made to determine this point. Torsion tests are attached. It will be seen that the stiffness of a crankpin is proportional to its length, and that in comparison with the rest of the shaft it is very weak. If the engine were redesigned for a 4 bearing shaft the cylinders would be in groups of two, the short centre distances of the cylinders being about 3.750. In the long centre distances the crankshaft consists of a stiff journal of negligible inertia. A saving of 4.650 + (2x 4.150) - (3x 3.750) = 1.7 in crankpin length would result, and the crankshaft stiffness would rise from 50,000 to approximately 54,500. The reduction in inertia would be about 5%. The critical speed, therefore, of an engine designed to suit a 4 bearing crankshaft should be 6.5% higher than for our experimental crankshaft. At present our model cranks have circular disc webs. These we shall machine down to the ordinary shape to determine what advantage in stiffness is gained by disc webs. | ||