10 - Couplings

Abstract

Couplings are designed to provide two functions: to transmit torsional power between a power source and driven unit, and to absorb torsional variations in the drive train. There are two types of couplings—rigid and flexible. A rigid coupling permits neither axial nor radial relative motion between the shafts of the driver and driven unit. When the two shafts are connected solidly and properly, they operate as a single shaft. A rigid coupling is primarily used for vertical applications. Types of rigid couplings discussed in this chapter are flanged, split, and compression. On other hand, flexible couplings, which are classified as mechanical flexing, material flexing, or combination, allow the coupled shafts to slide or move relative to each other. Although clearances are provided to permit movement within specified tolerance limits, flexible couplings are not designed to compensate for major misalignments. Significant misalignment creates a whipping movement of the shaft, adds thrust to the shaft and bearings, causes axial vibrations, and leads to premature wear or failure of equipment. Periodically, worn or broken couplings must be replaced. One of the most important steps in performing this maintenance procedure is to ensure that the correct replacement parts are used. After having determined the cause of failure, it is crucial to identify the correct type and size of coupling needed. Even if practically identical in appearance to the original, a part still may not be an adequate replacement. Coupling selection should be application specific, and therefore it is important to consider the type of equipment that it connects. Couplings must be installed properly if they are to operate satisfactorily.