One of the steps in properly sizing a coupling is to understand the application. Continuous operation vs stop-start, high inertia loading, excessive vibration, etc… The severity of the application is used to modify the calculated application torque to truly represent the usage of the system. A handy reference guide is the Driven Machine Service Factor charts that coupling manufacturers publish. This will typically give a number between 1.0 and 3.0.
An item that is often overlooked but should be considered for specialized applications (infrequently, but particularly pertaining to gear and disc couplings) is the driver. When a Variable Speed Electric Motor or Internal Combustion Engine is used as the driver within a coupled system, the torque variations experienced by the coupling are more severe than with a standard electric motor system. This variation is included in the formula:
Application Torque x [Service Factor (Driven) + Service Factor (Driver)] = Selection Torque
The Driver Service Factor can vary from 0.0 (standard electric motor) to 1.5 (low cylinder count internal combustion engines).
As you can see, in specific specialized applications, the Driver can potential have a greater impact on sizing the coupling than the Driven unit!