Selecting for True Angular Misalignment
The common causes of true angular misalignment are when one of the connected shafts is compliantly mounted; for example, when it is located by a self-aligning bearing (See figure 18). Alternatively, it could be that an unsupported intermediate shaft is placed between the driver and the load (See figure 19). Because the shafts are not mounted conventionally, they will self-align to intersect at the centre of the coupling, which acts as a hinge and, to a degree, a radial bearing. As the coupling is locating the shafts on a stable axis of rotation, it should be of the single-stage type due to the fact that any radial compliance in the coupling is counter-productive.
While couplings based on the flexible shaft can be used in these circumstances, there is a possibility that the coupled system may go into lateral oscillation. This is best described by visualising the effect of a belt and pulley drive mounted on the compliant shaft. Having a lateral compliance capacity, the coupling responds to fluctuating tension in the belt by allowing lateral oscillation of the shaft (See figure 20). The shafts in figure 18 are described as semi-floating, while those in figure 19 are fully-floating. This is an important point as under no circumstances should a coupling with lateral displacement be used with floating shafts. The reason is that this type of coupling has no self-centring action and its use would allow the shafts to orbit in an uncontrolled way.
Couplings capable of overcoming true angular misalignment include the single universal joint with its capacity to handle large offsets, torsional damping, water resistance and lubrication-free operation. Single-stage disc couplings are also ideal, thanks to their near-infinite life and built-in dynamically balanced properties. Similarly, single-stage bellows with their high torsional stiffness are a good choice in this application.
Fig.18 A semi-floating shaft located by a self -aligning bearing at one end should be supported with a single-stage coupling at the other.
Fig.19 Two single-stage couplings locate a fully-floating shaft on a stable axis of rotation.
Fig.20 A semi-floating shaft located by a self-aligning bearing at one end and a multi-stage bellows at the other. The coupling reacts to fluctuating tension in the belt by allowing lateral oscillations in the shaft.