What readers will learn in this article.
- The purpose of shaft couplings and how they work.
- What to consider when selecting shaft couplings.
- Maintenance issues with using couplings.
Couplings are used between shafts to connect them so power can be transmitted and to allow for axial expansion. Shafts can either be aligned or intentionally inclined at an angle to each other. When at an angle a universal joint is used to connect the shafts. When aligned a shaft coupling is fitted.
The selection of a shaft coupling requires knowledge of – the power being transmitted, the speed of rotation (RPM), the environment (temperature, chemical vapours, oil, etc), the inertia (mass) of components and the duty it must sustain (steady or sudden fluctuating loads, vibration, load reversal, etc). But how do you pick a coupling if many coupling designs satisfy these requirements?
The decision should consider three ‘friendliness’ factors – the ease with which they permit shaft alignment, the simplicity of doing maintenance on them and the ease with which they allow maintenance of the equipment either side.
Alignments require the use of a ruler across the coupling and feeler gauges to determine the necessary equipment adjustments. A coupling design with wide flanges on both halves is preferred. If precision alignment is required the coupling size and shape must still permit mounting of dial gauges or laser aligning emitters and reflectors on the remaining section of shaft.
Eventually a coupling will need to be removed, for example a motor may require changing-out. The coupling should be easy to disassemble and to remove from the shaft. Couplings shaft-keyed and shrunk-fit on a shaft should be avoided. Often there is insufficient room to mount a puller so the only option left is to heat the coupling with a flame torch and knock it off the shaft. Taper lock mountings are best where possible.
Any unnecessary time lost during repairs must be avoided. The couplings preferred by maintainers are those that permit removal of the damaged item while leaving the undamaged item in-place. Many couplings can have a spacer fitted between the shaft-mounted ends to provide space to work.
If removal of a coupling requires separating it into parts it needs to allow easy access for hands and tools. Its design should be easy to understand and let the maintainer see that every part is being assembled correctly.
Mike Sondalini – Maintenance Engineer
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