In a recent paper, Yates has shown that it is convenient to distinguish between two main types of stimulus which induce torsional vibration in geared-shaft systems. The first, which is dealt with in detail in the literature, is “force excitation” by which periodic torques act upon the system and throw it into oscillation; for the purpose of this note, it will be convenient to refer to a previous article as dealing with this problem.

The second type of excitation to which Yates refers is “displacement excitation.” It may be thought of as arising from errors in the cutting of gears which cause periodic variations in the velocity ratio of mating components. A Hooke's joint which couples two rotating systems which are capable of torsional oscillation would give rise to this form of stimulus.