- 13 December 2005 -

Taming vibration demons with flexible couplings

Unless it is dealt with effectively, torsional vibration can wreck a power transmission system and the pump it is driving. Alan Dean of Renold Hi-Tec Couplings examines the problem and highlights the advantages of rubber-in-compression couplings in providing a solution.

Like any other energy-expending mechanical device, pumps need a power source to drive them. They require mechanical energy in order to carry out the task of moving liquids from one place to another and that energy is provided either by an internal combustion engine or an electric motor. The power source drives the pump through a power transmission system, which is usually made up of an arrangement of gearboxes, drive-shafts and couplings. That might all seem rather straightforward, but there are inherent demons in the system that need to be tamed if the pump is to be driven safely and efficiently.

The first of these demons takes the form of a rather insidious phenomenon known as torsional vibration. Unless it is dealt with effectively torsional vibration can wreck a power transmission system and the pump it is driving to boot. It can literally shatter solid steel drive shafts, cause pumps to wrench themselves loose from their mountings and possibly even pose an injury threat to anyone in the vicinity. So what is torsional vibration and how do we deal with it?

In the early part of the twentieth century torsional vibration was a power transmission engineer’s worst nightmare. It was responsible for plane crashes, for cars breaking down and for an epidemic of failures in drive systems across industry. It is caused by the almost imperceptible pulses in torque that are inherent to an internal combustion engine. Each time the compressed fuel-and-air mixture ignites it rapidly expands driving a piston down a cylinder on its power stroke. Each power stroke produces a pulse or peak in torque that causes an indiscernible twisting of the drive shaft.

One would think that something as strong as a steel drive shaft would not twist significantly but any piece of metal will deflect when a force is applied, and, when large amounts of power are generated the forces involved can be colossal. What's more, the effects of torsional vibration can be amplified by another demon in the system, a related phenomenon known as torsional resonance.

If we consider the dynamics of a diesel driven pumping system each diesel engine has its own natural resonance, a bit like the note of a ringing bell or the sound of a vibrating guitar string. If this torsional resonance coincides with the natural frequency of the pump then the results can be catastrophic or, at the very least, will significantly reduce the lifetime of the system and necessitate an increased level of maintenance.

During the first quarter of the twentieth century this torsional vibration was unobservable to the crude instrumentation that was available at the time because the twisting vibration of the drive shaft was superposed over the steady average rate of rotation. An observation made by Professor Edward Miller, one time head of the Mechanical Engineering department at MIT, demonstrated the sheer destructive power of torsional vibration. During the initial testing of a pump driven by a diesel engine he noted that the shaft connecting the engine to the pump began glowing ‘cherry red’ before it failed completely. This phenomenal amount of destructive energy is simply the result of torsional vibration that at the time was virtually undetectable until failure occurred.

Engineering knowledge has advanced considerably since then and torsional vibration is no longer the mystery it once was. Manufacturers provide detailed information on the torsional resonance of each engine and design engineers are able to ensure that levels of torsional vibration in a system remain within acceptable limits. The modern solution is to fit a flexible, or torsionally soft, coupling in-between the diesel engine and the drive shaft to isolate the engine’s harmonics from the rest of the system. There are two types of flexible coupling that are commonly used for this purpose, one known as rubber-in-compression and the other as rubber-in-shear.

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