Types of coupling

Non-disengaging couplings maintain, after assembly, a more or less flexible but continuous transmission of the rotational movement. The connection is only broken for disassembly, re­pair, etc. Flexible couplings of one form or another, which are capable of absorbing residual misalignment, are most com­mon; although solid couplings do have their areas of use, see Figure 12.1.

Owe coupling Figure 12.1 Examples of solid shaft couplings

One example is the split muff coupling, the main advantage be­ing its ease of assembly. It is best used for low speed applica­tions due to the difficulties in balancing. The sleeve coupling is mounted and removed by oil-injection; being almost symmetri­cal, balancing is easy.

In the early days of fan engineering rigid couplings were fre­quently used, as witness the Keith mine fan in Figure 1.21 in Chapter 1. However, extremely careful alignment was neces­sary if additional loads were not to be imposed on the fan or motor bearings.

It did, however, give the possibility of using only one fan bear­ing. Reference to Chapter 9, Figure 9.3 show that rigid cou­plings were used in arrangements 5 and 6 of the NAFM (USA) Bulletin 105. It is not without significance that these arrange­ments are now withdrawn. Fitters would nowadays have apo­plexy if called upon to align three or four bearings!

Torsionally-rigid flexible couplings consist of various types of di­aphragm and gear couplings, shown in Figure 12.2. Couplings with a single functional element have the ability to take up angu­lar and axial misalignment. Couplings with two functioning ele­ments separated by a fixed “spacer”, are also able to cope with radial misalignment, whereby the magnitude of the radial mis­alignment is determined by the angular misalignment multiplied by the distance between the coupling elements.

Torsionally-flexible shaft couplings usually consist of flexible rubber, plastic or even steel elements, as in Figure 12.3. The first mentioned coupling elements require somewhat larger

Figure 12.2 Examples of torsionally-rigid flexible couplings

Figure 12.4 Shaft coupling examples

Figure 12.3 Examples of torsionally flexible couplings

Coupling diameters because of their lower load carrying capac­ity. Single element couplings can accommodate radial mis­alignment as well as angular and axial. The flexible spring cou­pling is interesting because it is designed to have a variable torque/deflection characteristic. Together with dampening pro­vided by the grease lubricant, the variable torque/deflection characteristic provides a powerful torsional vibration damp — ener.

The torsionally-flexible couplings shown can be built with two working elements and a spacer to allow additional radial mis­alignment. In order to simplify disassembly and service of some machines, spacer couplings can be used. An example of these is shown in Figure 12.4 b.

Removal of the spacer enables the rotating elements to be ser­viced without necessitating the removal of the whole machine. A limited end float feature is available for driving or driven ma­chines not fitted with an axially located bearing as shown in Fig­ure 12.4 a.

Cardan shaft couplings with rubber end stops as shown in Fig­ure 12.4 c are also available.

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