Flexible couplings connect rotating elements such as motors, gearboxes and drive shafts to transmit torque while permitting some radial, axial and angular misalignment.  These specific couplings can accomodate for the misalignment of those elements without the need for additional alignment equipment.   Special tip when ordering a flexible coupling would be to know the misalignment tolerance for the angular, parallel and axial motion of your application

Rigid couplings are used when precise shaft alignment is required.  It does not allow for misalignment while connecting the various rotating elements of motors, gearboxes or drive shafts. Shaft misalignment will effect the couplings performance as well as its life.  Special tip when ordering would be the alignment and motion parameters.