These clutches or hubs are used if the torque must only be transmitted up to a certain, adjustable value. 

If the set maximum value is exceeded the coupling device starts slipping. If the torque falls below the limit again, the slipping stops. 

Thus for safety reasons a separate stop mechanism for the drive unit might be required. 

Friction clutches usually serve to connect two shafts. 

Sliding hubs usually serve to mount a drive wheel (chain wheel, drive pulley, spur gear, friction wheel, or similar ) on a shaft. 

Some types can be used for both applications as, e.g., either a drive wheel or a shaft connection can be flange mounted. 

Combinations of elastic coupling and friction clutch can also be supplied