Hydraulic servo-motor for a regulating valve having a hydraulic closing mechanism

A valve actuator comprising:

• a piston mounted in a cylinder for reciprocating movement;
• supply conduit means for supplying pressure fluid to one side of said piston to urge said piston in a first direction;
• spring means for urging said piston in a second direction opposed to said first direction;
• said piston including a piston rod movable with said piston;
• said supply conduit means including a housing;
• said supply conduit means also including control valve means for selectively supplying pressure fluid to said one side of said piston;
• cam means for transmitting a motion of said piston rod to said control valve means;
• said control valve means including a valve sleeve, which valve sleeve is movable with respect to said housing;
• and
• said control valve means further including a regulating valve slidably arranged within said valve sleeve and movable with respect to said housing, movement of said regulating valve being responsive to movement of said cam means and movement of said valve sleeve being responsive to movement of said regulating valve, said valve sleeve being movable to close said control valve means.