Understanding Coupling Elastomer Element Failure Due to Torsional Vibration

Torsional vibration is an occurrence that can have a negative effect on coupling elastomer elements. It can also have a catastrophic effect on couplings and drive systems. The graph shows a calculated torque spike in a drive, at the point the spike goes off the chart at around 1200rpm, the value is infinite and will go on adding energy to the system until something breaks. In other words, if the machine that this chart represents were allowed to run continuously at any of the speeds shown, the coupling elastomer would melt and fail. If the elastomer was sized well enough to survive, the failure point would be the next weakest point, perhaps a shaft with a machined sharp corner or stress raiser.

Consider an electric motor driving a device such as a high pressure piston water pump using a flexible shaft coupling. As the water in each cylinder opens the outlet valve, it can deliver a torque spike or torque reversal back into the drivetrain. The greater the number of cylinders and the higher the speed, the greater the quantity of torque spikes. We now don’t have a shaft rotating at a steady speed. Every time there is a torque spike it slows momentarily then speeds up again only to be hit by another spike and so on.

This continuous pulsing in the drive train will usually be damped by the coupling element however if the level increases above the thermal capability of the elastomer then problems result.

Think of the elastomer as a plastic spring; at a molecular level when the plastic is compressed the molecules subjected to this compression rub together generating heat. The element has a thermal limit of the amount of heat it can dissipate into the air and coupling drive material in contact. If this heat input value is within the element’s capability all is well, however, if the heat input exceeds this thermal rating the plastic inside the affected areas will melt, as the coupling is rotating centrifugal force causes this melted plastic to extrude from the outer surfaces, when the molten plastic comes into contact with cooler air it solidifies into the string like tails you can see in the picture.

As the surfaces of the affected areas are in contact with the coupling drive mechanism they remain cool and may not melt which can produce hollow sections.

There are many circumstances that can generate torsional vibration, not just piston water pumps. Engine drives can be susceptible especially if there is a large inertia differential either side of the coupling, or perhaps an engine misfiring.

Torsional vibration is the responsibility of the machine builder, however, we are able to advise and examine systems by the use of one of our many specialist partners but be warned, the amount of data required to perform a torsional vibration analysis is considerable. For engine drives a mass elastic diagram will be essential, driven equipment inertia and stiffness values to name a few more.

Critical speed due to resonance:

Select coupling stiffness so that the system does not run at high resonance as well as the normal running and idle speeds are not at or near critical speeds.

Critical speeds are related to the system natural frequency and the number of pulses or excitations generated per revolution ‘i’ (order). For analysis, if possible, reduce the application to a 2-mass system and apply the following equations shown in figure 1.

The coupling will be modelled as the spring controlling torsional oscillations of the engine and the flywheel on one side and the driven equipment on the other, figure 2.

Use the dynamic torsional stiffness values (C_Tdyn) from the performance data tables which are found in representative coupling catalogues. Mass moment of inertia values may be obtained from the respective engine and equipment manufacturers.

Note: System steady-state operating speeds should be 1.5 to 2 times the major critical speed for safe, low-resonance operation.

For links to relevant catalogue information please visit: https://www.jbj.co.uk/couplings.html#understanding-coupling-elastomer-element-failure-due-to-torsional-vibration

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