Patent ID: 6776356
Filing Date: 2004-08-17
Classification: A01M

Abstract:
A hydraulic actuator for controlling relative movement between first and second structures between which the actuator is mounted, the actuator comprising:a cylinder having a mounting end for coupling to one of the structures and an open end opposite the mounting end; a cushion piston slidably mounted within the cylinder which is sealed with respect to the cylinder to define a cushion chamber between the cushion piston and the mounting end at the cylinder; a stop member arranged to restrict displacement of the cushion piston away from the mounting end of the cylinder beyond a prescribed spacing from the mounting end of the cylinder in a static position of the cushion piston; a rod slidably mounted through the open end of the cylinder with the open end of the cylinder being sealed with respect to the rod, the rod having a piston end slidably mounted in sealing engagement within the cylinder between a retracted position adjacent the cushion piston and an extended position adjacent the open end of the cylinder and having a mounting end for coupling to the other structure; the cylinder including: a retraction port adjacent the open end of cylinder in communication with a first side of the piston end of the rod to retract the rod into the retracted position when receiving fluid under pressure through the retraction port, an extension port adjacent the mounting end of the cylinder in communication with a second side of the piston end of the rod to extend the rod into the extended position when receiving fluid under pressure through the extension port, and a cushion port adjacent mounting end of the cylinder in communication with the cushion chamber for urging the cushion piston away from the mounting end of the cylinder towards the static position adjacent the stop member; hydraulic fluid circuitry selectively coupling both the retraction port and the cushion port to common fluid under pressure in the retracted position; and a pressure relief mechanism coupled to the cushion chamber whereby the rod is movable with the cushion piston towards the mounting end of the cylinder beyond the retracted position when subjected to a force which exceeds a prescribed holding force of the pressure relief mechanism.