Previously known from the published application 970621 is a damper, where inside the cylinder there are two pistons so that in the space between the pistons a gas space for the damper and between the spaces of the cylinder ends in their turn spaces for the cylinder fluid. So it is then possible to damp two separate hydraulic circles with the damper. The damper is an independent unit adaptive to a hydraulic circle and needing piping. In a damper like this it is impossible to carry out gas pressure control, pressure rise or pressure relief while damper in operating conditions.

In order to solve these problems a new damper and a hydraulic system adaptive to the damper have been developed. For the invention is characteristic what is presented in the cha- racterizing parts of the claims 1 and 4. Moreover the advantageous embodiments for the invention has been presented in the depended claims.

The advantage of the invention is that a separate damper is not necessarily needed in connection with a linear motion cylinder, because the cylinder has an in-built damper muffling at least the motion in one direction. No piping needed as for a separate damper. The damper gas pressure can be controlled while damper in operating conditions and, if needed, the damper/cylinder can be furnished with a device detecting the piston rod position. Damper/cylinder can be easily formed in a system with control, where the damped cylinder can be controlled according to its load, e. g. control the degree of control or, on the other hand, control the piston rod position instead of changing the load.

In the following the invention is disclosed with reference to the enclosed drawing, where, Fig. 1 shows a side view of the damper/cylinder partly cut.

Fig. 2 shows a hydraulic system with control.

Fig. 3 shows a cylinder, the motion of which in both direct- ions is damped by a through piston rod solution.

Fig. 4 shows a conventional cylinder damped in both direct- ions.

Figure 1 shows a cylinder 1, furnished with a first end piece 29 and a second end piece 6, inside of which cylinder there is a first piston 3 fixed to piston rod 2 and a second piston 4 which in regard to rod piston 2 can slide about it. Pistons 3, 4 and piston rod 2 have necessary sliding faces and packings 10-18. In piston rod a drill hole has been done, whereat there is from valve 21 in the rod base a connection along the drill hole to channel 22, whereat it is possible to fill, add or reduce gas in the space between pistons 3,4.

Further, there is in the centre of piston rod 2 a drill hole, into which the sensing element 23 of device 8 measuring the piston rod 2 position is placed. The sensing element 23 detects the position of element 24 moving along with piston rod 2 and, accordingly, the measuring device 8 can transmit to the control centre 25 (figure 2) information about piston rod 2 position.

So the cylinder of figure 1 is a double-acting cylinder making linear motion and working as damper at the same time. However, the gas space between pistons 3,4 damps only the shortening motion of the cylinder, whereat in order to produce damping for the lengthening motion a pressure accumulator 27 connected to the hydraulic fluid line, for instance, or a solution as per figure 4 is required.

In figure 2 a hydraulic system is illustrated, where a damper/cylinder as per figure 1 works as actuator. The system includes a control centre 25 receiving pressure information from the space between pistons 3,4 and along line 28 informat- ion about piston rod 2 position, possibly about accumulator 27 pressure and, for instance, information into which position piston rod 2 shall be steered. The control centre 25 shall have a control system, known per se, to control valve 26 in order to reach a required piston rod position and to steer the pressure of gas through valves 30 or 21 into the space between pistons or in the accumulator 27.

The system can have been given the function to change the degree of damping which takes place in changing the gas pressure in accumulator 27 or in the space between pistons 3,4. Further, the control centre can have the function to retain the piston rod position in spite of the cylinder loading and its variations. Then the control centre mainly controls valve 26 in order to retain piston rod position. One application for the damper/cylinder is, for instance, imple- mentation of suspension of a vehicle by means of the system as per figure 2. The system comprises then adjustment of ground clearance and as well as the"hardness"of suspension.

Thanks to the damper construction the gas pressure in the space between pistons 3 and 4 can be, for instance, conti- nually measured from the piping. Pressure can be increased or decreased, while piston rod yet moving, using a flexible pressure pipe between valve 30 and piston rod connection 21, for instance.

Figure 3 shows a cylinder 1 furnished with a through piston rod 2', a piston 3 fixed and pistons 4 sliding in regard to piston rod 2'. The device has a structural symmetry in regard to piston 3. The gas spaces are between pistons 3 and 4 and the hydraulic fluid is conveyed to spaces between cylinder ends and pistons 4. Connection to the gas spaces along channels is formed in the piston rod. The cylinder presented in figure 3 damps the motions in both directions. Most advantageously, in this case the sensor measuring the piston rod position shall be placed outside the cylinder.

In figure 4 there is a conventional cylinder damping in both directions. The first gas pressure space is formed between the first piston 3 and second piston 4. The second gas pressure space is formed between the first piston 3 and the second piston 4'moving freely in cylinder 1. There is a channel to both gas pressure spaces over piston rod 2. The first space of hydraulic fluid is between the end and piston 4, the second space of hydraulic fluid is between the end and piston 4'.

The invention is not restricted to the presented embodiment but many modifications are possible within the limits of the inventional concept disclosed in the claims.