As the name suggests, a shaft coupling allows for the joining of two shafts end to end. There are many different types depending on the application, but they generally fall into two categories: rigid and flexible.
Flexible shaft couplings are more common. They offer the advantage of being able to compensate for minor shaft misalignments. There are numerous designs, but the majority feature two hubs (usually metal – one for each shaft) and a cushioning element that fits between them. Because the element is pliable, it is this that compensates for the misalignment in the shafts, as well as absorbing the backlash from when rotation is started or stopped.The hubs can be pilot-bored, or often they are also available to take a taper lock bush, which lets shafts of different diameters to be coupled together.The most common types of flexible couplings include HRC, Tyre, Beam, and Essex type.
Rigid shaft couplings are usually only available pilot bored, so machining will be necessary to enable them to fit the required shaft diameter. Also it is important that the shafts are perfectly aligned. However, a rigid shaft coupling allows for torque limiting as a safety factor. This can be from friction plates that will slip when a certain torque is reached, or shear pins that will break at a certain torque. The advantage of this is that, for example, if you have an electric motor driving a conveyor system, and the conveyor jams or otherwise becomes overloaded, when the preset torque is reached the connection between the two shafts will be severed, preventing the motor from damage as well.