The consequences of incorrect coupling selection

A drive coupling looks like a simple component. It connects two shafts, transmits torque and accommodates a degree of misalignment between them when operating within the drive train. But selecting the wrong one is one of the most common avoidable causes of unplanned downtime in rotating machinery and why engineers need to spend time at the design stage to select the right coupling for application.

Why coupling selection matters

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 during their lifetime.

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, along with increased maintenance time and costs.

How incorrect coupling selection leads to failure?

Incorrect coupling selection rarely causes immediate failure. Instead, it operates under stress, often for months before symptoms become impossible to ignore.

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 the exact same reasons.

Why are couplings mis-specified?

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 perceived cost saving will often incur additional costs many times over the value of replacing the coupling and get the drive train operational again.

Another factor that can cause drive couplings to operate outside of their design parameters is when changes or upgrades are made to the original design configuration. For example, 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.

Common signs of an incorrect coupling

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. An incorrectly specified coupling may be exposed to:

• Heat from slip or misalignment
• Over-torque, including starting torque or shock loads not accounted for at selection
• Misalignment beyond the coupling's rated capacity and may produce axial and radial vibration
• Operating at speeds higher than the coupling was rated for
• Environmental exposure or operational loads which the coupling material wasn't designed to withstand

The coupling may also 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 coupling's design specification.

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

Avoiding these pit falls and unnecessary costs is simple. Design engineers must spend sufficient time at the design stage ensuring that the specified coupling is able to meet or exceed all the technical and application specific criteria for which it is going to be used. This will ensure that engineers remain confident that the coupling will provide long and trouble-free service, with no unexpected consequences, or costs.