The FH is a coupling with progressive characteristics where the dynamic torsional stiffness is enhanced with increasing coupling load. This type of coupling gives a distinct advantage in installations where a wide speed range is required, and where the torque increases with speed. The resulting characteristics provide the FH coupling with a relatively low torsional stiffness that places the critical speed below the operational speed of the system. This design is intended for higher inertia loads such as a compressor, centrifugal pump, blower, and generator. The coupling will fit all SAE flywheels from SAE 6.5 through SAE 18. The variable durometer inserts can provide torque capacities up to 9999.1570125NM (88,500 in-lbs).