The invention relates to a driving device comprising a cylinder, at least one displaceable driving piston arranged in said cylinder, feeding means for feeding pressure medium into the cylinder on one side of the driving piston reset 5 means having energy storing members such as a spring for exerting a reset force on the driving piston.

In know driving means of the kind set forth the energy storing means formed by one or more springs directly engage the driving piston. When the driving device becomes opera- 10 tive, the piston is displaced against the springs. This re¬ sults in that with an increasing displacement of the driving piston the effective driving force decreases since the force exerted by the springs increases. In order to have still suf ficient effective force near the end of the stroke of the 15 driving piston available, the piston must consequently have larger diameter than would be required in the absence of reset m ans. -

The object of the present invention is to provide a dri¬ ving device of the kind set forth above, which is free of the 20 above-mentioned isadvantage.

This object is reached in that the reset means comprise a reset member transferring forces and arranged between the energy storing means and the driving piston and retaining means.holding the reset member by a pressure medium against 25 the effect of the energy storing means in a non-operative position out of contac with the driving piston. It is thus ensured that the reset means become operative not until the medium pressure is obviated so that the driving piston can % operate without being hindered by the reset means as long as 30 the resent means are subjected to the pressure medium.

The invention furthermore provides an additional advan¬ tage in that when the energy storing means are formed by springs, these springs need not be stretched and relaxed with every moment of the driving piston, which materially 35 lengthens the lifetime of the springs and hence the interval between two inspection turns of the driving device.

In an advantageous embodiment, of the invention the reset member comprises a reset piston and the retaining means com- ^" prise a reset cylinder receiving the reset piston and an in¬ let for allowing pressure medium to enter the reset cylinder. Preferably the feeding means communicate through a con- trolvalve with a pressure-medium source, whilst the inlet directly communicates with the pressure-medium source. As a result the reset means become operative solely and directly when the pressure of the pressure-medium source is failing. As long as the press-ure-medium source provides pressure,* the driving device operates as if there were no reset means.

.The invention will now be described more fully with re¬ ference to embodiments of the driving device in accordance with the invention shown in the Figures. Fig.1 is a longitudinal sectional view of a driving de¬ vice for rotational drive.

Fig.2 is a cross—sectional view taken on the line II-II in Fig.1.

Fig.3 is a longitudinal ^' sectional view of a further dri- ving evice embodying the invention.

The device. 1 for rotational drive comprises a cylinder 2 having two pistons 3, 4 movable therein in opposite senses. Each piston 3, 4 has three extensions 5 and 6 respectively serving to satisfactorily guide the pistons in the cylinder 2. Two opposite extensions 5,6 are provided with toothed racks 7 and. 8 respectively, which engage a pinion 12 of a transverse shaft 9 rotatably mounted in the device 1. At one end the cylinder 2 is closed by a cover TO and at the other end by a cover 11. The device 1 furthermore comprises means for feeding pressure medium to the cylinder from a pressure-medium source 28. The space bounded by the pistons 3,4 in the cylinder 2 communicates through a channel 16 with a port 15 in a cover 10. The two distal sides of the pistons 3,4 bounding the spaces in the cylinder 2 are connected with oneanother through a channel 17. The space on the side of the cover 10 communicates with the port 18. The ducts 27 connected with the ports 15,18 include a valve 14 by means of which pressure medium can be fed at will from the pressure-medium source 28

to one of the ports 15,18. At the ^* same time pressure medium is conducted away from the other source towards a reservoir 30. -In the position of the valve 14 shown in Figs. 1 and 2 pressure medium is fed from the source 28 to the port 15. The pressure medium flows through the channel 16 into the space between the pistons 3,4 and pushes these pistons away from one another so that the transverse shaft 9 turns in the di¬ rection of the arrow 31. The displaced oil flows through channel 17 and the port 18 to the reservoir 30. The pistons 2 _r 3 are provided with sealing rings 13 establishing sealing relationship between the pistons and the cylinder.

This device, known per se as described so far, comprises reset means 20 embodying the invention. The reset means 20 comprise a cylinder 21 connected with the cylinder 2 of the driving device. This cylinder comprises a slidable piston 22. On the side of the reset piston 22 remote from the driving pistons 3, 4 energy storing means formed by springs 23 are bearing on the head wall 29 of the cylinder 21. With the re¬ set piston 22 is connected a piston rod 24 extending through an orifice in the cover 11 provided with sealing rings 25. In the position shown in Fig.1 the piston rod 24 is not in con¬ tact with the driving piston 3 since pressure medium is in¬ troduced through the inlet 19 into the cylinder 21. The pis¬ ton 22 has therefore moved to the right against the force of the springs 23. The air thus displaced by the piston 22 has escaped through the vent channel 26 in the head wall 29. As long as the pressure medium introduced via the inlet 19 is pressurized, the reset piston 22 remains in the position shown in Fig.1. When the pressure-medium source 28 drops out so that the pressure medium does no longer exert pressure on the reset piston 22, the springs 23 urge the reset piston 22 to the right. Thus the piston rod 24 comes into contact with the driving piston 3 and urges the same back into its rest posi- tion.

Instead of being formed by springs 23, the energy sto¬ ring means may be constituted by pressurized gas or a pres¬ sure medium from a pressure accumulator.

The driving device shown in Fig.3 is a device for linear* ⁵ drive. The right-hand part of Fig. shows a conventional pis¬ ton—cylinder assembly. The piston 37 is movable in the cylin¬ der 36 and can linearly drive a member connected herewith. For the inlet and outlet of pressure medium ports 39,40 are provided. In the position shown of the control-valve 41 pres¬ sure medium is fed from the pressure-medium source 42 to the port 39. The piston 37 is thus compelled to move in the di¬ rection of the arrow 49. The displaced pressure medium is conducted away via the port 40 to a reservoir 50.

On its side remote from the piston rod 38 the piston 37 is provided with a control-rod 43, which co-operates with the reset means 51. The-reset means 51 comprise a reset cylinder 44 holding a reset piston 46. On its side remote from the piston 37 the reset piston 46 is loaded by a compression spring 47 forming the energy storing means. The inlet 51 of the reset cylinder 44 communicates with the pressure-medium source 42. When the medium source 42 is supplying pressure, the piston 46 is held against the effort of the spring 47 in a position shifted to the left. As ^" soon as the pressure of the pressure—medium source 42 fails, the spring 47 moves the piston 46 to the right. The piston 46 then moves along the control-rod 43 and- hence the piston 37, since the piston 46 contacts a shoulder 45 of the control-rod 43. It is thus en- sured that, when the pressure of the pressure-medium source 42 is lacking, the member driven by the piston rod 38 is automatically reset in its rest position, whilst in normal operation the driving piston 37 is not hindered by the coun¬ teracting force of the spring 47. In the figures. the connections of the driving cylinder with the pressure-medium source are shown schematically. It will be obvious that these connections are always designed so that the operation of the reset means is not hindered.

In the event the reset means embodying the invention serve as safety means, the inlet is preferably connected each time directly with the pressure-medium source so that at a drop-out of the pressure the reset means become operative. However, if desired, as an alternative the inlet may be con¬ nected with the pressure-medium source so that the reset means can be switched on independently of the drop-out of -