The need for couplings arises from the limited length of shaft which it is possible to forge in one piece and from the frequent need to use different materials for the various conditions of temperature and stress. The multicylinder construction of large turbine-generators necessitates the use of a coupled shaft system.
Couplings are essentially devices for transmitting torque but they may also have to allow relative angular misalignment, transmit axial thrust and ensure axial location or allow relative axial movement. They may be classified as flexible, semiflexible or rigid. On smaller turbine-generators (i.e., up to 120 MW) semi-flexible and flexible couplings were commonly used, but for large turbine-generators it is now common practice to use rigid couplings.