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A double-acting hydraulic actuator with an axial locking device has a cylinder chamber defined by two coaxial cylindrical walls of different diameters interconnected by a frustoconical wall. A guide piston sliding in the smaller-diameter cylindrical wall is fixed to an end of an actuator rod. The ac

A double-acting hydraulic actuator with an axial locking device has a cylinder chamber defined by two coaxial cylindrical walls of different diameters interconnected by a frustoconical wall. A guide piston sliding in the smaller-diameter cylindrical wall is fixed to an end of an actuator rod. The actuator rod has an annular protuberance of diameter equal to the diameter of the cylindrical wall. An annular stepped piston providing sealing between the two cylinder chambers is slidably mounted in a middle portion of the actuator rod to slide between the piston and the annular protuberance. When the actuator is extended, a narrowed end of the annular piston bears against conical surfaces of two half-rings and urges them outwards. When the half-rings are pushed outwards and the narrowed end of the annular piston becomes interposed between them and the middle portion of the actuator rod.

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1. A double-acting hydraulic actuator comprising a cylinder body ( 2 ) closed at one end by an end wall ( 3 ) and at the other end by a cylinder head ( 4 ), a guide piston ( 7 ) slidably mounted in the cylinder body ( 2 ), an actuator rod ( 9 ) secured to the guide piston ( 7 ) and passing in leakti

1. A double-acting hydraulic actuator comprising a cylinder body ( 2 ) closed at one end by an end wall ( 3 ) and at the other end by a cylinder head ( 4 ), a guide piston ( 7 ) slidably mounted in the cylinder body ( 2 ), an actuator rod ( 9 ) secured to the guide piston ( 7 ) and passing in leaktight manner through the cylinder head ( 4 ), hydraulic means for actuating the actuator rod ( 9 ), and mechanical locking means for locking the actuator rod ( 9 ) in a predetermined extended position, in which:a) the cylinder body ( 2 ) presents two coaxial cylindrical walls ( 26 , 27 ) of different inside diameters and interconnected by a frustoconical annular section ( 25 ), the larger-diameter cylindrical wall ( 27 ) being placed beside the cylinder head ( 4 ) and the smaller-diameter cylindrical wall ( 26 ) serving to guide the guide piston ( 7 );b) the actuator rod ( 9 ) presents an annular protuberance ( 33 ) between the guide piston ( 7 ) and the cylinder head ( 4 ), the diameter of the protuberance being not more than the diameter of the smaller-diameter cylindrical wall ( 26 ), said protuberance ( 33 ) being disposed in register with the larger-diameter cylindrical wall ( 27 ) when the actuator rod ( 9 ) is in the predetermined extended position;c) the guide piston ( 7 ) has means ( 30 ) putting both sides of said guide piston ( 7 ) into fluid communication;d) an annular piston is placed on portion ( 34 ) of the actuator rod that is situated between the guide piston ( 7 ) and the annular protuberance ( 33 ), the annular piston ( 40 ) being capable of sliding in leaktight manner against the smaller-diameter cylindrical wall ( 26 ) and being capable of sliding in leaktight manner against said portion ( 34 ) of the actuator rod under drive of pressure agent between a position in which the actuator rod ( 9 ) is released in which it bears against the guide piston ( 7 ), and a locked position in which it bears against the annular protuberance ( 33 ), said annular piston ( 40 ) presenting a narrowed end section ( 44 ) adjacent to the annular protuberance ( 33 ); ande) the locking means have locking members ( 41 a , 41 b ) and means for urging said locking members ( 41 a , 41 b ) to move radially, which locking members, in the released position of the actuator rod ( 9 ), are received axially between a radial face ( 36 ) of the protuberance ( 33 ) and the annular piston ( 40 ) and are housed radially between the portion ( 34 ) of the actuator rod and the smaller-diameter cylindrical wall ( 26 ), and in the locked position, are received radially between the larger-diameter cylindrical wall ( 27 ) and the narrowed end section ( 44 ) of the annular piston ( 40 ) and are housed axially between said radial face ( 36 ) of the protuberance ( 33 ) and the frustoconical section ( 25 ) of the cylinder body ( 2 );the hydraulic actuator being characterized by the fact that the narrowed end section ( 44 ) of the annular piston ( 40 ) is cylindrical and is terminated by a conical surface ( 48 ) for co-operating with a conical surface ( 46 ) of the locking members ( 41 a , 41 b ) so as to urge them radially outwards when the pressure agent acts on said annular piston ( 40 ) in the direction for extending the actuator rod ( 9 ). 2. An actuator according to claim 1, characterized by the fact that the locking members ( 41 a , 41 b ) have conical surfaces ( 47 ) facing radially outwards, which surfaces, in the locked position, co-operate with the frustoconical section ( 25 ) of the cylinder body ( 2 ). 3. An actuator according to claim 2, characterized by the fact that the locking members are constituted by two half-rings ( 41 a , 41 b ). 4. An actuator according to claim 1, characterized by the fact that the actuator rod ( 9 ) is made up of two separable cylindrical parts ( 9 a , 9 b ) connected together by screw-fastening, one of the parts ( 9 b ) having the annular protuberance ( 33 ) and the portion ( 34 ) of the piston r od on which the annular piston ( 40 ) is mounted, and the other part ( 9 a ) sliding in the cylinder head ( 4 ). 5. An actuator according to claim 1, characterized by the fact that the cylinder body ( 2 ) is constituted by two coaxial portions ( 2 a , 2 b ), of which an outer portion ( 2 a ) includes the cylinder head ( 4 ), means for fixing to a structure, couplings ( 17 , 18 , 19 ) for coupling to a fluid source, and a cylindrical outer wall ( 11 ) having inside tapping ( 12 ), and an inner other portion ( 2 b ) comprises the end wall ( 3 ) of the cylinder body ( 2 ) and the body of the cylinder chamber, and presents an outside thread ( 14 ) for co-operating with said inside tapping ( 12 ) so as to enable the predetermined extended position to be adjusted relative to the structure, internal ducts ( 20 , 22 , 23 , 24 ) being provided in said two portions ( 2 a , 2 b ) between the couplings ( 17 , 18 , 19 ) and the ends of the cylinder chamber.