Abstract: A brake actuator comprising: a piston-cylinder arrangement having a piston member and a cylinder member arranged for sliding motion relative to one another by fluid pressure medium; an elongate plunger longitudinally movable by one said member and comprising two mutually screw-threaded parts arranged to vary the effective or overall length of the plunger upon relative rotation of said two parts; a resilient spring torque motor for effecting said relative rotation, the torque motor being coupled to one of the parts for driving that part relative to the other part; and rotation preventing means comprising friction clutch or brake members coupled to said one part and releasable when the stroke of the plunger exceeds a pre-set or predetermined amount to permit torque transmission from the torque motor to said one part in a direction tending to increase the effective or overall length of the plunger.

Abstract: An air operated brake actuator has an air pressure operated brake force applying member, the application to and removal of pressure from which member determines the brake force, another such member which must be separately pressurised to follow the first member to lock in a brake force, and yet another member which requires to be additionally pressurized to effect unlocking of the brake force.