Fail-safe actuator device

In a fail-safe apparatus of the type which comprises a housing defining a cylindrical bore having a port in one end, a piston member mounted for axial movement in said bore toward and away from said one end, between a charged limiting position whereat said piston is spaced a predetermined distance from said one end and a discharged limiting position whereat said piston lies adjacent to said one end, spring means in said bore and biased between said piston and the other end of said bore and urging said piston toward said one end, a flexible cable member operatively connected at one end to said piston member, a drive shaft mounted in said housing for rotation about an axis normal to the axis of said bore, said shaft including a yoke having an outwardly open arcuate track concentric with the axis of said shaft, the other end of said cable member being secured to said yoke, said cable being arrayed in said track, the improvement which comprises said piston defining with said bore an annular clearance space, a rolling elastomeric diaphragm member mounted in said bore in the area between said one end and said piston, said diaphragm including an outer annular portion adapted to line said bore, an inner annular portion extending into said clearance space and adapted to line the outer periphery of said piston, and a roll portion linking said inner and outer annular portions and disposed within said clearance space, an annulus surrounding said piston and movably mounted in said clearance space, said annulus including a stop face directed toward said one end of said bore, and spring means biased against said annulus and urging said stop face into engagement with said roll portion to prevent extrusion of said roll portion of said diaphragm away from said one end and through said clearance space.