Disc couplings make the world go round
21 November 2012
As with most of the components that keep high power industrial transmissions running smoothly and efficiently, most of us probably don’t notice the spinning cylinder mounted between the torque supply and the driven device.
While high quality couplings are often to be found in the ‘fit-and-forget’ category, knowledge of how they work and how designs differ is useful when the time comes time to replace them; after all, a disc coupling’s job is to protect an expensive asset - the connected equipment.
Disc couplings are used to transmit torque efficiently, while accommodating misalignment and compensating for any end float between the driving and driven equipment. Indeed, disc couplings can transmit torque with over 99 percent efficiency, and they are also maintenance free, even in rough or reversing applications. There are two main styles – ‘traditional’ and ‘drop-in’.
These designs differ in their approach to absorbing the flex that is produced by torque transfer. Traditional disc couplings feature a five piece construction - two hubs, one spacer and two flex discs – and are lower in terms of initial cost and the lighter of the two. The drop-in coupling is a three piece construction comprising two hubs and a factory assembled spacer that includes flex elements. Drop-in couplings are quicker to install and they occupy a higher balance class.
As well as transmitting torque with virtually no losses, disc couplings excel at compensating for both angular and parallel offset misalignments. Unlike common elastomeric coupling types, where a single flexing element accommodates all types of misalignment, within a disc coupling a single disc only accommodates angular misalignment; double-flexing couplings are needed to compensate for parallel misalignment. Free end float rating is also an important consideration when specifying a coupling in relation to its ability to compensate for end movement.
The bolting arrangement for couplings is one of the most important design features, an equal number of holes are bolted to the driver as are bolted to the driven hub. Torque is transmitted in tension through half of the links of the flex discs located between the hubs and the spacers; the remaining links operate in compression and are generally not considered in the ratings. Compressed links tend to buckle under load, so flex discs are commonly pre-stretched at installation to allow the compressed sections to relax to neutral under load as they are compressed.
Straight sided and scalloped flex discs are the two most commonly used blade types (though round discs are still commonly used in many older coupling styles). Straight sided discs are stiffer torsionally so require less pre-stretch and are somewhat easier to assemble. Scalloped discs are ‘stress optimised’ making them lighter and less stiff, though the edge stresses are greater, which requires a better edge surface finish. They tend to involve costly EDM machining and are typically best suited to custom/high-speed applications.
FEA analysis has shown that material can be removed from the area in the middle of the span between bolt holes without affecting the disc’s ability to handle axial and angular misalignment, so straight sided and scalloped discs perform equally well. However, straight sided discs are better suited to fluctuating and reversing loads commonly found in general purpose applications, so torque requirements are often a deciding factor in disc style selection.
Applications requiring disc couplings are found through general industry, wherever the transfer of torque from driving to driven equipment is necessary. Typical applications include pumps, compressors, machine tools, engine driving equipment, food processing equipment, blowers and fans and test equipment.
While couplings should be a ‘fit-and-forget’ component, there are still many design details that should be considered at the point of specification to ensure many years of maintenance free running and the protection of other, rotating connected equipment.
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