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).
Article discussing the theory and calculation of torsional vibration in Diesel engine crankshafts.
| Identifier | ExFiles\Box 132\1\ scan0109 | |
| Date | 18th March 1939 | |
| 372 1116 Torsional Vibration of Diesel By O.{Mr Oldham} MALYCHEVITCH, A.E. THE theory of vibration as applied to the calculation of multi-cylinder crankshafts has occupied the attention of engineers for the past 40 years, especially that of engineers in the field of heavy Diesel engines. As a result of analyses and test work by Frahm, Holzer, Gümbel, Lewis, Timoshenko, Carter, Hartog, Dorey, Wilson, Porter and others, whose publications have been of great assistance and are duly acknowledged in the bibliography, a method of determining the risks of shaft breakage and of assuring smooth performance while the engine is still in the design stage, has been developed. When the speeds of gasoline engines reached the figure of 5000 r.p.m., it was found desirable to provide the crankshaft with dampers, especially in the case of long crankshafts such as those of eight-in-line engines. The recent rapid development of the high-speed automotive-type of Diesel engine, and many shaft failures and troubles in actual operation, have brought the torsional-vibration problem to the front in connection with this type of engine. In the following are given a series of calculations of crankshafts, preceded by brief explanations intended to clarify the meaning and application of the calculations. For a detailed study of the subject the reader is referred to the bibliography appended to this article. The crankshafts referred to are for four- and six-cylinder high-speed automotive Diesel engines of the same bore and stroke, compression ratio, i.m.e.p., and weights of reciprocating and rotating parts. The crankshafts of both engines are identical in every respect except that of length, but the flywheel of the four is heavier than that of the six. The crankshafts are of the five- and seven-bearing types, respectively, and their proportions are representative of the practices of leading U. S. and European Diesel-engine builders. * * * Every rotating elastic shaft carrying heavy masses and subjected to periodic torque variations is liable to develop torsional vibration. If the shaft d₂, Fig. 1, is subjected to turning moments at both ends, one moment being clockwise, the other counter-clockwise, and is then released, torsional vibration will be produced; the shaft will move alternately to opposite sides of its position of rest, and the movement will be repeated again and again, each time with a slightly decreased amplitude, until finally the vibration ceases. Experiment has shown that at certain speeds such vibration will produce very high stresses in the shaft. There will be a point O, which will not move during the oscillation, and this is called the nodal point, or nodal section, or simply the node of the vibration. The assembly represented by Fig. 1 is called a two-disc or two-mass system with single-node vibration, and for any definite system this vibration has a definite frequency called the natural frequency of the system. In the case of a three-mass system as illustrated in Fig. 2, if the two end discs are subjected to turning moments in the same direction and the center disc to a turning moment in the opposite direction, there will be two nodes, and the natural frequency of the system will be different, being much higher than for a similarly dimensioned two-disc system with single-node vibration. In shafts carrying a plurality of attached masses there may be vibration with three, four, or more nodes, depending upon the number of lengths of shafting between attached masses. The lines cd and df in Fig. 2 are Figure 1 Figure 2 Figure 3 March 18, 1939 Automotive Industries. | ||