TABLE OF CONTENTS Anyone designing a piece of heavy machinery is likely to perform both a lateral (bending) and a torsional natural-frequency analysis of the potential design. This is done to verify that the machine will operate properly in its design range without resonating to destruction. Such calculations are particularly important in systems involving reciprocating equipment and / or a number of flexible couplings or gears.
Although both the lateral- and torsional-vibration characteristics of a machine train are considered to be important in the design stages, hardly anyone ever takes the trouble to actually measure the torsional vibration of the installed machine for either acceptance determination or as an ongoing predictive-maintenance activity. Lateral-vibration-monitoring programs, on the other hand, are extremely popular.
The reason for this obvious inconsistancy between the efforts to measure lateral versus torsional vibration is quite simple. Modern transducers make it easy to measure lateral vibration at each bearing housing of each machine in question. Measuring torsional vibration is more difficult.
In this appendix we will briefly cover the basic mathematics of torsional vibration and then discuss some potential important applications of torsional-vibration measurements. Finally, the more complicated problem of the instrumentation necessary to measure torsional vibration will then be covered.
There is a direct analogy between lateral- and torsional-vibration analysis. This can be seen by comparing the two equations of motion below:
where F is the motive force applied to the system, m is the mass (or resistance to motion), and a is the acceleration...
