This invention relates to a hydraulic piston cylinder device comprising a cylinder, a piston dividing the cylinder into two cylinder spaces, and valve elements between the cylinder spaces and the respective pipes for pressure medium.

When this kind of piston cylinder device is being used and the extern feed pressure stops, both valve elements are closed by springs whereby the piston cannot be moved by any extern force into either direction because liquid cannot escape from either of the cylinder spaces. It would be sometimes necessary, however, that the compressing or tractive force of the cylinder is maintained even when the extern pressure stops. For this purpose it is known to use a pressure accumulator in the hydraulic system. In most cases a separate pressure accumulator connected with hoses is sufficient, but sometimes such accumulator does not provide the required reliability. For example breaking of the hose between the piston cylinder and the accumulator stops the function of the accumulator. There are some events where such uncertain factor cannot be allowed, e.g.:

- A retaining centralizer of the drill rod whereby the pressure accumu¬ lator prevents the dropping of drill rods into the hole if the external working, pressure stops for some reason,

- Different kinds of support devices, for instance the top supports of wagon drills,

- Some brake systems.

The object of the present invention is an improved hydraulic piston cylinder device and such cooperation of valve elements incorporated therewith, that the above-mentioned uncertain factor is eliminated.

This object is achieved through the present invention so that the piston is elastically compressible by means of the pressure of the cylinder and that the valve elements are coupled so that when one of them is closed,

the other cannot be closed. If under these conditions the movement of the piston is stopped by an external obstacle before the piston has reached the end surface of the cylinder, the piston is elastically compressed, whereby it absorbs pressure energy. When the external feed pressure stops and the valve member prevents the escape of pressure from the corresponding cylinder space, there prevails in the cylinder space the pressure caused by the compressible piston. The pressure causes upon the piston rod a power which has the same direction as the power caused by the external pressure before stopping. To make this power effective, it is necessary that the pressure can escape from the cylinder space on the other side of the piston. This is facilitated by the above-mentioned coupling of the valve elements, allowing only one of the valve members to be closed at a time.

One embodiment of the invention will be described in the following with reference to the accompanying drawing which shows a partial crosscut of a piston cylinder device according to the invention, and of valve elements incorporated with it.

The piston cylinder device comprises a cylinder 1 which is divided by a piston 2 into two cylinder spaces A and B.- The piston 2 is divided in the axial direction into two successive parts 21 and 22 which are separated by a space 6 and between which there is elastic material 3, e.g. rubber, plastic, a steel spring, or the like. The piston rod 5 is connected to the part 22, while the other part 21 is movable in relation thereto within the limits of the space 6 against the elastic material 3. The piston is moved to and fro inside the cylinder by means of pressure liquid which is fed by pipes 11 and 12 via a pilot controlled non-return valve 4. The pilot controlled non-return valve 4 comprises a movable poppet 8 in the middle of which there is piston 8' to move the poppet 8 according to which side of the piston 8' is affacted by pressure. The ends of the poppet 8 cooperate with balls 7 and 9 acting as valve elements which are pressed by springs 10 against valve seats 7 ^f and 9' to close the valves, if not the pressure or the poppet 8 is pushing the ball 7 or 9 apart from the valve seat 7' or 9'. According to the invention the length of the poppet 8 relative to the distance between the valve seats 7', 9' is such that only one ball, either 7 or 9, at a time can close the flow between the respective cylinder spaces A and B and the pipes 11 and 12.

The device functions as follows. For example, when pressure liquid is fed into pipe 11, the piston 8' moves the poppet 8 to the right whereby ball 9 moves apart from valve seat 9' and the return channel from cylinder space B into pipe 12 is opened. At the same- time pressure has caused ball 7 to move apart from valve seat 7', whereby pressure has free access into cylinder space A, and moves the piston 2 to the right until the movement is stopped by an external obstacle. Thereby the elastic element 3 is compressed within the space 6, whereby it absorbs pressure energy. When the external pressure affecting pipe 11 stops, ball 7 of the pilot controlled non-return valve 4 is pressed against valve seat 7' thus preventing the escape of pressure from cylinder space A, whereby the pressure caused by the elastic element 3 remains in the cylinder space A. This pressure affects the piston 2 and the piston rod 5 with a force which is parallel to the force which was caused by the external pressure inside pipe 11 before this pressure stopped. Since the spring 10 and the pressure in cylinder space A keep ball 7 closed, the extended poppet 8 prevents the closing of ball 9 (situation shown in the drawing) , whereby the flow from cylinder space B into pipe 12 is free. Thereby the pressure energy reserved in the elastic element 3 can affect with a force parallel to the piston rod 5. Exactly similar function in reversed direction is caused by leading pressure into the pilot controlled non-return valve 4 through pipe 12.

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