Flexible Coupler Design Assistance

Up to 10 issues may need consideration, not necessarily prioritized in this order, when selecting a flexible coupler:

Does it provide adequate misalignment capability?

Can it transmit the load torque?

Do I need axial compliance?

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 required torsional stiffness?

Does it provide electrical isolation between shafts?

Will it have the required life expectancy?

Will it meet my cost expectations?

The issues of misalignment and torsional stiffness will be discussed here.

Misalignment Compensation & Axial Motion

These properties differentiate a flexible coupler from a solid sleeve type. The nature of the enabling mechanism (i.e., bellows, membrane, sliding disc, etc.) determines almost every other performance characteristic of the coupler, including its tolerance of misalignment and/or axial motion.

Sliding disc and universal/lateral types can tolerate large misalignments but their backlash-free life may reduce as a result; bellows types can absorb significant axial motion but their misalignment capacity may suffer accordingly; membrane couplers are irrevocably damaged if axial motion exceeds the catalogue specification, but can accommodate large misalignments with no reduction in life expectancy if the distance between membrane centers is increased, typically by linking a pair of single-stage couplers with an intermediate shaft.

Incidental misalignment is caused by manufacturing tolerances, thermal expansion, wear, fitting difficulties and structural settlement. The resultant errors are small, generally in the range 0º - 1/2º angular and 0-0.2mm parallel, and are difficult to predict. Be aware that a 0.2mm (0.008") parallel error can grow substantially due to adverse interaction with the angular component.

When misalignment is incidental, it is more realistic to consider the effective radial error, being the radial distance between shaft center lines measured midway along the length of the coupler. In effect, this is the composite error and is what matters when determining a value for maximum misalignment. Only a radial value need be specified.

Axial motion can result from axial clearances in the shaft bearings, or from shaft growth due to thermal expansion. It is usually beneficial to absorb this with a suitable coupler. In some cases, however, it may be preferable to resist the axial motion of an unrestricted shaft, particularly if this has a positioning function, and anchor it to a stable motor shaft. Couplers such as the universal/lateral can be useful in these cases.

The reason we use flexible couplers is to protect the shaft support bearings from destructive radial and thrust loads due to misalignment and axial motion, respectively. Since all couplings resist misalignment and axial motion, it follows that those with least resistance can better protect the bearings. Fig. 1 compares the radial bearing loads of a number of popular couplers. Excluding the 30mm (1.125") jaw coupler, all results were obtained with couplers of nominal outside 25mm (1").

Fig. 1

Load, Torque, Inertia and Torsional Stiffness

Applications in which couplers are used for driving so-called frictional loads, for example pumps, shutter doors, textile machinery, and so on, are not generally sensitive to coupler torsional stiffness because angular synchronization of the shafts is not an issue. Where resonance is a problem, it is possible to reduce the coupler s torsional stiffness and thus avoid conflict with the natural resonant frequency of the machine which is most likely operating at constant speed.

This is not a solution when the loads are inertial, typified by position and velocity control systems, where registration of input and output shafts is critical throughout the operating cycle.

In these systems motor, coupler, and load form a resonant system. Its resonant frequency depends on the load inertia and on the coupler s torsional stiffness. Increasing the load inertia, or decreasing the coupler s torsional stiffness, lowers the resonant frequency.

To control a resonant system you have to be working well below its resonant frequency. Imagine you are holding an elastic band with a weight suspended from it. You can control the vertical movement of the weight provided you move your hand slowly. Speed up the movement and the weight barely moves.

To improve response, you need a less elastic band, or you need to reduce the weight at the end of it Substitute a coupler for the elastic band, and an inertial load for the weight, and you have a good analogy for an inertial system. 

When the focus is on performance, a stiffer coupler reduces settling times, improves positional accuracy, and raises the upper limit of dynamic performance.

Fig. 2 compares torsional deflection (the inverse of torsional stiffness) for a number of popular couplers tested with 8mm shafts. Excluding the 30mm (1.125") jaw coupler, all results were obtained with couplers of nominal outside 25mm (1").

Fig. 2

This article was written and provided by Huco Engineering Industries, Ltd, manufacturers of the world's most comprehensive range of precision couplers.  For more information about Huco, please visit www.huco.com