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 Studebaker Corporation discussing the comparative merits of nine-bearing and five-bearing crankshafts.
| Identifier | ExFiles\Box 128\1\ scan0315 | |
| Date | 21th March 1938 | |
| COPY 1104. THE STUDEBAKER CORPORATION, SOUTH BEND, INDIANA. March 21st.1938. Mr.W.A.Robotham, Rolls-Royce Ltd., Derby, ENGLAND. Dear Mr.Robotham, With regard to the relative merits of the nine-bearing and five-bearing crankshafts, our experience has been that the advantage of the nine-bearing shaft consists primarily in that it eliminates, or greatly reduces, the tendency toward 'main bearing bumps.' We have found that with engines in which there are two throws between bearings, unless extreme care is used in bearing fits, there is a likelihood of obtaining a main bearing noise at moderate car speeds. This noise is usually most pronounced at the rear main bearing and comes at the speed when the inertia of the reciprocating parts is sufficient to lift the rear end of the crank. Although we have been able to make engines passable when this condition prevailed, we have always had periods in the course of the engine's career when this trouble would develop due to errors which would have been comparatively harmless in a nine-bearing job. As for your questions, I believe that the nine-bearing engine is usually slightly heavier than the five-bearing design, altho in the particular instance where we changed from a five to a nine-bearing installation, there was no attempt made to reduce the crankcase to maintain the same stiffness and hence the increase in weight was undoubtedly accomplished by a definite increase in stiffness. As regards torsional vibrations, we have found very little difference in either the magnitude of the torsional periods or in the engine speeds at which they appeared. As regards general smoothness, the only difference that has been noticeable is that mentioned above, namely; an improvement as regards freedom from main bearing noise. The friction with the nine-bearing crank is slightly higher than with the five-bearing shaft. However, this difference is not large and the increase in oil temperature should be comparatively small. As a matter of fact, the effect of the crankshaft friction upon oil temperature depends to a considerable extent upon whether or not there is the possibility of sufficient deflection to momentarily permit the film to be broken down. When this occurs temperatures are much higher than is the case when the heat input is due solely to the shearing of the oil film. Hence, it is quite possible that the added stiffness of the nine-bearing crank might prove to be an advantage. continued | ||