Rigid coupling

Rigid coupling is used only whenever the misalignment between equipment shafts is very small or no misalignment is present in the system. When the equipment is appropriate to carry the reactionary load produced from trying to flex rigid couplings. This type of coupling mostly prefers for vertical applications such as vertical pump. In those cases, rigid couplings are a very effective means of connecting machine shafts. But the rigid couplings used to connect two shafts of two rotating equipment and that allow transmitting power and torque from one machine to another machine. That also can be used if machines that have shafts of different sizes.

That couplings should be used only when the both equipment shaft does not have  misalignment and when the shafts of the machine and the rigid coupling are long enough that  accept the forces and moments produced by the mechanical shaft deflection on these.

Rigid coupling dimensions, the dimension is also a measure factor during the selection of best suited of coupling for your machinery. Because the rigid coupling used where very less dbse required because spacer does not use with this type of arrangement. The shaft alignment is very less. For rotating machinery, when two machine shafts are connected rigidly and properly, then both machines operate as a single shaft.

Rigid couplings are used when precise shaft alignment is required (shafts are already aligned); any shaft misalignment will affect the coupling's performance as well as its life, because rigid couplings do not have the ability to compensate misalignments. Due to this, their application is limited, and they're typically used in applications involving vertical drivers.