The Axial Vibration of Diesel Engine Crankshafts

Abstract

The paper describes an investigation carried out to determine the magnitude of axial vibration of engine crankshafts, with the object of establishing that axial resonance may occur within the normal running range of an engine. The author gives the results of tests on various makes of engine using a portable vibrograph to record the axial movement of the crankshaft. In some cases the torsional osculation was measured simultaneously by means of a universal vibrograph. The vibrograph records show that at certain speeds some engines are subject to a very marked increase in axial vibration of a magnitude consistent with a condition of axial resonance. The author describes a test in which the natural frequency of axial vibration of a crankshaft is determined experimentally by using an a.c. electromagnet as a means of applying an alternating force. Expressions are derived for calculating the axial deflexion of a crankshaft due to the piston load, and also for calculating the axial deflexion under an axial load. It is suggested that the relation between these two deflexions determines the magnitude of the axial force set up by the piston loading. From a series of load-deflexion tests, carried out on a number of crankshafts, the author determines the value of empirical factors to be used in estimating the axial stiffness.