There are many kinds of flexible couplings available today, each of which has certain characteristics that make it more effective in one application than another. So how do you go about selecting the ideal coupling?
Flexible shaft couplings connect two shafts, end to end. They transmit rotation from one to the other and accommodate the slight, unintentional misalignment that is present when two independently mounted shafts are presented for connection.
One of the first factors that must be considered in determining the best for the job is whether it can provide adequate misalignment protection. The particular mechanism used – bellows, membrane, flexible beam or sliding disc – determines the performance characteristic of the coupling including its tolerance of misalignment or axial motion.
Sliding disc and universal/lateral couplings can, for example, tolerate large misalignments, but with reduced backlash-free life. Bellows-type couplings can absorb a high degree of axial motion with a possible reduction in misalignment capacity. Membrane couplings can be damaged beyond repair if axial motion exceeds the specification, although they can withstand large misalignments with little or no reduction in life expectancy.
Where misalignment is incidental – caused by manufacturing tolerances – a more realistic measure is the effective radial error. This is the radial distance between the shafts’ axes measured midway along the length of the coupling. Sometimes called the composite error, this can be crucial when determining a value for the maximum permissible misalignment.
Axial motion is often created as a result of axial clearances in the shaft bearings, or through thermal expansion. While it is usual to absorb this with a suitable coupling, in some cases it may be more beneficial to resist the motion, particularly if it has a positioning function. Couplings such as the universal/lateral type can be useful in such circumstances.
Flexible couplings are designed to protect shaft support bearings from destructive radial and thrust loads arising from misalignment and axial motion. In effect, all couplings resist these properties, therefore the conclusion is that those with least resistance will better protect the bearings.
Load torque, inertia and stiffness
In applications where couplings are used to drive frictional loads – such as pumps, shutter doors and machinery – the torsional stiffness is not a major factor as the angular synchronisation of the shafts is not an issue. However, when resonance is a problem, it is possible to reduce the coupling’s torsional stiffness and avoid conflict with the natural resonance of the machine.
This does not apply when the loads are inertial – typically positioning and velocity control systems where registration of input and output shafts is critical throughout the operating cycle. Here, the three elements of motor, coupling and load combine to create a resonant system. The frequency of the system is controlled by the load inertia and the coupling’s torsional stiffness. Increasing the inertia, or lowering the torsional stiffness, results in a lower resonant frequency.
In order to control a resonant system you must work well below its resonant frequency. Imagine supporting a weight on an elastic band. You can control the weight’s vertical movement if you move your hand slowly; increase the speed and the weight barely moves. To improve responsiveness you require less elasticity or need to reduce the weight. If you now substitute the elastic band with a coupling and the weight with an inertial load, you have an analogy of an inertial system.
When the emphasis is on performance, you therefore require a stiffer coupling in order to reduce setting times, improve the positional accuracy and raise the upper limit of dynamic performance.
Selection
There are a number of guidelines to consider when selecting a coupling.
? Does the coupling provide adequate misalignment protection?
? Can it transmit the load torque?
? Do I need axial motion or axial stiffness?
? Can it sustain the required speed of rotation?
? Will it fit within the available space envelope?
? Can it operate at the designated ambient temperature?
? Does it provide the torsional stiffness required for positional accuracy?
? Does it provide electrical isolation between the shafts?
? Will it have the required life expectancy?
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