Fail-safe actuator and hydraulic system incorporating the same

A fail-safe rotary actuator member for imparting rotary movement to the shaft of a rotary valve responsive to a failure condition and for enabling unimpeded conventional operation of said valve during normal operating conditions comprising, in combination, a casing having a cylindrical bore formed therein and having a port at one end thereof, a piston mounted for reciprocal movement within said bore between extended and contracted positions and defining with said bore a variable volume fluid pressure chamber, spring means in said casing biased between said piston and the other end of said casing for urging said piston to said extended position whereat said piston lies adjacent said port end of said chamber, pilot valve means connected with said port for selectively introducing and bleeding fluid from said chamber, thereby to control the position of said piston in said chamber, a housing on said casing, a drive shaft journalled in said housing and connected to said valve shaft for rotation between first and second positions about an axis of rotation perpendicular to the axis of said bore, a quadrant-shaped yoke member mounted on said drive shaft and including an outwardly open peripheral guide track coaxially arranged with respect to said drive shaft, a flexible cable having one end operatively connected to said piston, said cable being disposed in said guide track of said yoke member, the other end of said cable being operatively connected to said yoke member at a position to induce rotation of said drive shaft from said first to said second position responsive to movement of said piston from said retracted to said extended position, said cable, when said drive shaft is in said second position, being in a slack condition when said piston is in said retracted position and in a tautened condition when said piston is in said extended position, drive handle means operatively associated with said drive shaft for imparting rotary movement thereto whereby said drive shaft may be rotated by said handle means between said positions without interference from said cable when said piston is in said retracted position, and said cable is effective to rotate said drive shaft to said second position when said piston is shifted by said spring means to said extended condition as a result of outward flow of fluid from said chamber through said port.