A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power

Gear couplings are torsionally rigid and are supplied to two designs:-
The gear couplings are selected based on.

Horesepower or Torque
Running RPM
Physical Space limitations
Shaft diameter
Shaft gaps
Special bore or finish information and type of fit.
Service factor
Application or type of equipment to be connected
Based on the application the appropriate service factor needs to be determined.

The maximum degree of misalignment permissible in gear couplings is from 0.5 Degrees to 1.5 Degrees. This varies with the size of the coupling being used.