This article by Michael Neale was first published in the Power Transmissions section of the June 1999 edition of Plant Services magazine.

Choosing flexible shaft couplings for long-term reliability

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Consider operating conditions and the effect the coupling has on the machinery.

Some machines, such as large turbo-alternators, use rigidly coupled rotors. This is possible because both the turbine and alternator are fabricated and installed by the same manufacturer on a common base. Also, since these are large machines with plain bearings, the bearing clearances are proportionately large, allowing some flexibility for transient misalignments.

With the majority of coupled machines, however, each is generally supplied by a different manufacturer and installed at a site where the mounting conditions may vary. In these circumstances, it is sensible to use a flexible coupling between the two machines, so that they can move relative to each other. Such movement can occur as a result of temperature changes, pressure changes or any form of relative foundation movement.

Types of flexible coupling

The most rugged of the displacement-type couplings, is the gear coupling with matching male and female gears in a continuous concentric mesh. However, using only one gear coupling in a drive train will not eliminate angular misalignment. In practice, the shafts are likely to also be laterally displaced while remaining broadly parallel to each other. That is why a pair of gear couplings axially separated from each other, is the typical gear coupling arrangement.

Flexure-based couplings use the same principle - two flexible connections axially separated from each other - to allow for the necessary lateral misalignment. When the application demands high torsional rigidity, the flexible links are usually made from stacks of thin metallic connecting strips, stiff along their length, but flexible laterally. Alternatively, discs can serve as the flexible members. These are generally contoured to a thinner section at their outer diameters to give uniform stress under load.

In some cases, it may be desirable to connect the machines with a degree of torsional flexibility as well as lateral flexibility. Couplings for this purpose generally use rubber components or metal springs. These have the advantage that the rubber or spring stiffness is easily adjustable to give a range of torsional flexibility.

Each of these couplings has some degree of mechanical complication and in some cases it may be possible to use a flexible quill shaft instead. This device allows a small amount of angular and lateral misalignment, with some torsional flexibility, but with a rigid axial connection. If appropriately designed it can provide a high degree of reliability

Figure 1. Power and speed limits of flexible couplings

Coupling selection

It is, however, useful to have some overall guidance regarding the comparative performance given in the graph and tables used in the selection process. The typical graph shows the power capacity to the approximate performance limit as a function of speed for the various coupling types (see Figure 1). Use the envelopes on the diagram and discussion with the manufacturer to select the optimum size of coupling.

The tables show the relative advantages and disadvantages of the various types and give general guidance on the misalignments that the various couplings can endure. This information should enable making a broad selection of an appropriate type of coupling for a particular installation.

Coupling effects

The couplings that work by component displacement, such as gear couplings, apply unidirectional forces to the associated machines. It is fairly well known that gear couplings apply an axial load to a machine, if they continue to transmit torque while a machine warms up or cools down. This arises because the teeth are in close mutual contact while relative thermal movement occurs. The teeth, therefore, must slip axially under their interacting torque loads. They then provide some frictional resistance that gives rise to an axial force. Provided the teeth are in good condition and relatively unworn, these forces can be moderate, corresponding to a maximum coefficient of friction of 0.15. If the coupling teeth are worn or damaged, the coefficient of friction could rise to about 0.30 - a value that can overload the machine's thrust bearings. This need not be a problem in service provided that the phenomenon is understood and avoided. In extreme cases, it may be a reason why a gear coupling cannot be used.

Another rather more subtle unidirectional force that gear couplings apply to machines is a radial or lateral force. This arises from the cyclical sliding interaction between the gear teeth. These interactions produce moments in the coupling sleeve and machine shafts. If the misalignment pattern is in the form of a shallow letter C, the moments largely balance and generate only small sideways forces. However, in the more common case of Z shaped, or parallel offset misalignment, the radial forces generated can be quite high, The forces are sometimes sufficient to cause adjacent bearings to fail.

Figure 2. Effect of couplings on critical speeds.

Since couplings are fitted at the end of machine shafts, they constitute an overhung mass. Overhung masses reduce the lateral critical speed of rotors. If a machine is operating near its critical speeds, the overhung mass of the coupling needs to be considered.

It might be assumed from these effects, that gear couplings are less desirable and that some form of flexure coupling is always more desirable.

However, gear couplings have the advantage of being relatively small in diameter, lighter in weight and easier to dismantle than some of the other types of coupling.

The mass of the coupling can be quite critical in some machines, particularly when operating near a lateral critical speed. The problem arises because the coupling becomes an overhung mass, which tends to reduce the natural lateral resonant frequency of the rotor, which, in turn, reduces the lateral critical speed (see figure 2). This effect can be forgotten easily when you are working only with the critical speed information for the machines. This may suggest that there is an ample margin when the critical speed is adequately greater than the running speed. However, the addition of a relatively heavy flexible coupling can bring the critical speed too close to the operating range.

If, however, problems arise from a torsional critical frequency, a torsional flexible coupling, such as the elastomeric element type or the convoluted axial spring type, can drop the frequency below the running speed. These couplings also have an advantage: The flexibility of the elastic members can be modified on site, for detuning the system in a controlled experimental manner.

Michael J Neale, Chairman, Neale Consulting Engineers Limited.

The illustrations shown in this article are taken from The Tribology Handbook by Michael Neale, and also Couplings and Shaft Alignment by Michael Neale, Paul Needham and Roger Horrell.