A gear coupling is a mechanical device for transmitting torque between two shafts that are not coaxial. It consists of a flexible joint fixed to each shaft. The two joints are connected by a third shaft, called the spindle.
Each joint consists of a 1-to-1 ratio internal/external gear pair. The tooth flanks and outer diameter of the external gear are crowned to allow for angular displacement between the two gears. Mechanically, the gears are equivalent to splines with modified profiles. They are called gears because of the relatively large size of the teeth.
Gear couplings and universal joints are used in similar applications. Gear couplings have higher torque densities than universal joints designed for the same physical envelope while universal joints induce lower vibrations. The limit on torque density in universal joints is due to the limited cross sections of the cross and yoke. The gear teeth in a gear coupling have high backlash to allow for angular misalignment. The excess backlash can contribute to vibration.
Single joint gear couplings are also used to connected two nominally coaxial shafts. In this application the device is called a gear-type flexible, or flex coupling . The single joint allows for minor misalignments such as installation errors and changes in shaft alignment due to operating conditions.