Simple steps to selecting the right coupling

Modern industrial couplings usually do much more than simply joining two shafts and are vital, hi-tech components that ensure the optimum performance of the power transmission system and the driven equipment. They can provide features such as, fail-safe operation, reduced maintenance and downtime, vibration damping, elimination of backlash and torque amplification, misalignment solutions, soft starting, protection against shock loading, torsional vibration and resonance.

They are essential components in a power transmission system that are typically located between the prime mover, usually an electric motor or diesel engine, the gearbox and the driven machine. There is a wide range of coupling types available to provide solutions that are often unique for different applications and so correct selection and installation is essential to get the perfect choice and ensure years of trouble free service.

Incorrect coupling selection is likely to result in increased maintenance, more frequent downtime and higher operating costs. It could also result in coupling failure – leading to lost production, unplanned maintenance, and in some cases could have catastrophic consequences.

When selecting the right coupling for an application, it's highly likely that more than one type will be suitable, so it's worth thinking in advance about what you want or need the coupling to do. For instance, if the couplings are located somewhere that is difficult to access then low or zero maintenance couplings would be beneficial and should be considered even if there is a lower-cost type that is also suitable.

It's essential to ensure that the coupling selected is the right size for the application in terms of drive power, speed of operation and suitable for the shaft sizes of the motor and driven equipment. An incorrectly sized coupling will cause problems, whether too big or too small, and it's important to consider all of the forces the coupling will be exposed to during normal operation, including peak torques and shock loads.

Misalignment

Another factor to consider is misalignment and, if present, the type of misalignment conditions that the coupling will need to accommodate. Typically this includes, Angular, Parallel Offset and End Float. A wide range of flexible couplings is available that provide perfect solutions for these different misalignment conditions, and are essentially of two basic types – elastomeric and metallic.

Elastomeric couplings incorporate non-metallic elements, such as rubber or plastic, through which the coupling is driven, and they have the benefit of being either low maintenance or maintenance free as they do not normally require lubrication. This type of coupling also has the benefit of providing excellent vibration damping, noise reduction,  protection against shock loads and backlash free operation.

Metallic couplings derive their flexibility through components that slide or move against each other, or through the flexing of a membrane or disc. They are torsionally stiff and generally well suited to high temperature applications, they can be selected with good resistance to chemicals and are suited to high speed and high power applications with large shaft sizes.

When considering alternative coupling types it's always worth being aware of their weaknesses and modes of failure as well as the strengths and benefits they offer. For instance, elastomeric couplings are not usually suitable for high temperature applications and can break down as a result of chemical contamination or when the internal temperature of the elastomeric element exceeds its limits. High temperature elastomers and elastomers that offer chemical resistance are now available but do not come as standard and so it's important to discuss your requirements with your coupling supplier.

Failure in metallic couplings is due primarily to wear, as a result of the moving parts and they therefore require regular lubrication. Overloads in the system shorten their life as a result of increased wear rather than sudden failure. Couplings with membrane technology are generally more expensive but require no lubrication and little if any maintenance. Their mode of failure is fatigue and they can fail quickly if overloaded.

Due to the fact that there is such a wide range of coupling types available there is almost certainly an off-the-shelf solution for every application that will provide excellent performance and years of trouble free service.

The basic information that needs to be known before the most suitable coupling can be selected includes: the power to be transmitted, whether normal or high, continuous or intermittent; the characteristics of the drive, the type of prime mover and the characteristics of the driven equipment; speed, including the speed at which maximum power is transmitted, as well as the maximum speed; and finally the dimensions of the shafts to be connected including keyway details.