1. Horsepower -- Almost every coupling is sized based on the horsepower to be transmitted. The exceptions would be chain and rigid designs which are selected based on the shaft diameters to be coupled.
2. Type of Drive -- If the power source is a reciprocating engine, there will usually be an adder to the service factor. If the engine is less than four cylinders you may have to consult the factory for design assistance. The problem here is torsional vibration in a continuous steady form, that can be transmitted directly into the driven machine. The elastomeric design coupling can help to dampen this vibration.
3. Type of Driven Equipment -- Driven machines are classified according to their load characteristics, and a service factor is assigned based on those characteristics. Multiplication of the service factor by the input horsepower gives you the design horsepower upon which most couplings are sized.
4. Speed at Which Couplings Operate -- Most selections are based on the H P at a certain RPM. As always, as the speed goes down the torque goes up. The other thing you must check is that you don?t exceed the maximum speed of the coupling selected.
5. Shaft Sizes -- You have to make sure the coupling selected will accept the bore required. In many instances you may have to go to a larger coupling than design would indicate in order to get the bore size you need.
6. Space Limitations -- Make sure that the coupling?s length and diameter will fit within the necessary confines.
7. Environment -- If it is over 170 degrees F(77 degrees C) or below -30 degrees F(-34 degrees C) it can change the operating characteristics of a elastomeric coupling or effect the lubrication of a mechanically flexible coupling. If the environment is wet or oily, it limits the types of couplings you can consider. There are many things that occur in environments that can affect the service life of a coupling.

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