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As with all mechanical components and mechanisms, drive couplings must be carefully selected to ensure that they are suited to their proposed application, including any specific operational parameters or environmental conditions that they may encounter in service.

Failing to spend sufficient time at the design stage, to ensure correct coupling selection, is likely to lead to premature failure, either of the coupling itself, or potentially other components in the drive train.

Unexpected failure of an incorrectly specified coupling in service, whilst more than just an inconvenience, can in some cases be rectified quite quickly, especially if the coupling is designed with easy maintenance in mind. The main issues will often be a combination of the cost of the replacement coupling and the additional costs associated with unplanned downtime.

However, when replacing the coupling, it is important to establish the cause of the original failure, rather than succumb to the temptation to just install the same type or variant again. This is highly likely to lead to a subsequent premature failure, probably for exactly the same reasons. A coupling that is under specified for the application may appear to operate normally for some time. During this period however, it is likely that the coupling will be under stress from one or more influences such as heat, over torque, misalignment, higher than anticipated speeds, or even just the environment within which it is operating.

An incorrectly specified coupling will eventually begin to exhibit signs of the excessive stresses that it is under. It may begin to heat up, start to show signs of vibration or even become noisy in use. The visible or audible symptoms will vary depending upon the operating criteria outside of the couplings design specification. A coupling that has been incorrectly specified for misalignment may produce axial and radial vibration, whilst one which is under specified for torque or start – stop frequency, may induce fatigue or failure within any elastomeric element.

A further consequence of incorrectly specified drive couplings is likely to be an increase in maintenance time associated with that particular drive train. Once the coupling begins to show signs of wear, it is likely that maintenance engineers will devote disproportionate time and effort in trying to keep the drive train operational, resulting in unpredictable and excessive downtime.

In some cases, couplings are incorrectly selected based upon just a few simple criteria – it will fit and it’s less expensive than the alternative. That initial cost saving will often incur additional costs many times the value of the perceived saving, to replace the damaged coupling and get the drive train operational again.

Another factor that can cause drive couplings to begin to operate outside of their design parameters is when changes or upgrades are made to the original design configuration. Increases in power or speed, or adding a power take off for an auxiliary drive component should be the signal for a re-appraisal of the drive coupling being used. Upgrading the drive coupling if necessary at this stage is a much more cost effective approach than hoping that all will be well and then having to deal with a catastrophic failure.

Avoiding these pit falls and unnecessary costs is simple. Spending sufficient time at the design stage ensuring that the coupling to be used is able to meet or exceed all of the technical and application specific criteria, means that engineers can rest assured that the coupling will provide long and trouble free service, with no unexpected consequences.

Reich Drive Systems UK General Manager David Proud, author of this article, also authored two other complementary articles: “A Smooth Path To Drive Coupling Selection” and “Fit and Forget – Just One Of The Benefits Of Correct Coupling Selection”. For copies of these articles or expert advice on drive coupling selection, please contact Reich Drive Systems UK.

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