A releasable pneumatically operated coupling for an industrial drive such as for a grinding mill. A variable speed motor has its output shaft provided with a flange rotating therewith on which are mounted a plurality of radially moveable arcuate elements having friction pads on their radially inner surfaces. An inflatable ring surrounds the arcuate elements and, upon inflation, urges the pads into driving contact with an annular friction surface on a coupling output shaft. The arcuate elements have U-shaped undercuts in the axial torque bar passages therethrough for stress relief. Separate speed sensors on the motor shaft and coupling output shaft provide signals to a comparator which, upon sensing a predetermined speed difference, provide a signal to an electrically operated valve which exhausts air pressure from the inflatable ring to release the coupling.
