A hydropneumatic spring usable as steplessly blockable setting device comprises a by-pass connection in its piston. In this by-pass connection there is arranged a shut-off valve actuatable from the exterior. Furthermore the by-pass connection can be opened by excess pressure in each of the two working chambers. In this way the setting of length of the hydropneumatic spring can be altered by predetermined inward and/or outward thrust forces even when the shut-off valve is closed.

BACKGROUND OF THE INVENTION 
The invention relates to a steplessly blockable device comprising a 
cylinder with a cylinder axis, a cavity, a cylinder inner surface and two 
end walls, a piston rod conducted in sealing manner through at least one 
of the end walls and movable in the direction of the cylinder axis in 
relation to the cylinder, a piston unit connected within the cylinder with 
the piston rod, which unit rests in sealing manner on the cylinder inner 
surface and divides the cavity within the cylinder into two working 
chambers, a fluid filling in the working chambers, a by-pass connection 
connecting the two working chambers with one another, and a shut-off valve 
arrangement, actuatable from the exterior of the cylinder, in the by-pass 
connection. 
Such setting devices are used for example for setting doors or flaps into 
desired open positions. In order to effect the setting, the shut-off valve 
is opened. When the desired setting is reached, the shut-off valve is 
closed again so that then the door or flap is made fast. It can occur that 
a closing or opening force is exerted upon the door or flap without the 
shut-off valve being opened. Then extremely high forces occur on the 
setting device and on the mounting of the door or flap. In that case 
deformations can easily occur to the setting device and/or to the mounting 
of the door or flap. Under some circumstances the piston rod can be torn 
violently out of the piston. 
EXPLANATION OF THE PRIOR ART 
From Fed. German P.S. No. 1,554,479 a steplessly blockable setting device 
is known which comprises a fluid-filled cylinder on the inner wall of 
which a piston connected with a piston rod is guided in sealing manner. 
The cylinder interior is divided by the piston into two working chambers 
and these two working chambers are connectable with one another by means 
of a blocking valve actuatable from the exterior. The fluid present in the 
cylinder interior is in this case under pressure and exerts an outward 
thrust force upon the piston rod when the blocking valve is opened. 
A further steplessly blockable setting device is known from Fed. German 
Utility Model No. 8 500855, in which no outward thrust force of any kind 
is exerted upon the piston rod. For the compensation of the temperature 
caused fluid expansion at least one working chamber has a limitation by an 
axially movable partition which is arranged resting on an abutment face of 
a resilient compensation body. 
From Fed. German O.S. No. 2,659,491 a gas spring is known which comprises 
an arresting system acting pressure-dependently in the inward thrust 
movement of the piston rod into the cylinder. A blocking valve actuatable 
according to choice from the exterior is not present in this gas spring 
which serves for the partial compensation of the weight of a motor vehicle 
boot lid. 
OBJECT OF THE INVENTION 
The object of the present invention is to produce a steplessly blockable 
setting device which avoids damage to the components even in the case of 
an error in the operation of the shut-off valve for actuation from the 
exterior; at the same time the setting device is to be simple in 
construction and to permit simple assembly. 
THE ESSENCE OF THE INVENTION 
The invention is based upon a steplessly blockable setting device 
comprising a cylinder with a cylinder axis, a cavity, a cylinder inner 
surface and two end walls, a piston rod conducted in sealing manner 
through at least one of the end walls and movable in relation to the 
cylinder in the direction of the cylinder axis, a piston unit connected 
within the cylinder with the piston rod, which unit rests in sealing 
manner on the cylinder inner surface and divides the cavity within the 
cylinder into two working chambers, a fluid filling in the working 
chambers, a by-pass connection connecting the two working chambers with 
one another and a shut-off valve arrangement, actuatable from the exterior 
of the cylinder, in the by-pass connection. With regard to the object as 
formulated above it is here provided that the by-pass connection is 
openable in response to excess pressure in at least one of the working 
chambers. By reason of the opening of the by-pass connection in the case 
of excess pressure a high functional reliability is achieved, since if the 
shut-off valve is not opened, as from a predetermined pressure difference 
between the working chambers of the setting device the effect is obtained 
that the forces acting upon the piston rod cannot rise further. Especially 
when such a setting device is used as door fixer for motor vehicles the 
setting device can be made as small as possible and with low weight. 
A setting device according to the invention is preferably formed in a 
manner in which a valve chamber with a valve chamber axis and a valve 
chamber circumferential surface is provided, coaxially with the valve 
chamber a valve body is accommodated displaceably therein, this valve body 
comprises a valve head with an external circumferential surface of larger 
diameter and a prolongation with an external circumferential surface of 
smaller diameter, a sealing ring unit resting in sealing manner on the 
valve chamber circumferential surface is arranged radially between the 
valve body in the valve chamber circumferential surface, the valve body is 
axially biased into a basic position determined by a valve stop, the 
sealing ring unit is axially biased into a basic position determined by a 
sealing stop, when the valve body and the sealing ring unit are in their 
basic positions the valve head rests with its external circumferential 
surface in sealing manner on the sealing ring unit, the valve body is 
displaceable out of its basic position into an open position in which the 
external circumferential surface of the prolongation has approached the 
sealing ring unit and the sealing ring unit is displaceable out of its 
basic position into an open position in which it has approached the 
external circumferential surface of the prolongation. In this form of 
embodiment the valve body can be shifted out of its basic position firstly 
by external force action and secondly by excess pressure in the one of the 
working chambers. It here acts as a first safety valve. For the other part 
the sealing ring unit can be shifted out by excess pressure in the other 
of the working chambers. It then acts as second safety valve. Thus a 
safety valve is provided for each direction of movement of the piston rod 
in relation to the cylinder, which valve opens as from a predetermined 
force action even if the shut-off valve is closed. 
The setting device of such formation is especially simple in construction 
because the functions of a shut-off valve and two safety valves are taken 
over by one single valve body and one single sealing ring unit. The 
biasing action on the valve body and the sealing ring unit in the 
direction towards their respective basic positions can be taken over 
wholly or partially by the pressures in the working chambers. However it 
is also possible to allocate a valve spring to the valve body and a 
sealing spring to the sealing ring unit, which load the valve body and the 
sealing ring unit respectively in the direction towards their respective 
basic positions. By suitable selection of this valve spring and this 
sealing spring it is possible easily to set the response pressures, at 
which the by-pass connection opens automatically, to the value desired in 
each case. 
The shut-off valve can be accommodated especially within the piston unit, 
so that a setting device of small construction and simple external form is 
obtained. 
The various features of the invention are discussed especially in the 
accompanying claims which form a part of the disclosure. For the better 
understanding of the invention, its working advantages and specific 
effects reference is now made to the accompanying drawings and the 
description, in which preferred forms of embodiment of the invention are 
discussed.

DESCRIPTION OF THE PREFERRED FORM OF EMBODIMENT 
In FIG. 1 the steplessly blockable setting device 1 is pivotably secured by 
means of a joint eye fast with the cylinder on a bearing bolt 3 connected 
with the bodywork 2, while a bearing bolt 5 firmly connected with a door 4 
forms the pivot bearing for a component secured on the piston rod and 
accommodating a release device 7 for a shut-off valve actuatable from the 
exterior. This release device 7 can be actuated through a draw cable or 
other suitable means. The door 4 of the motor vehicle is mounted by means 
of a door hinge 6 in the bodywork 2 and on actuation pivots about a 
vertical or nearly vertical axis. In the opening and closing of the door 
the release device 7 is actuated by the door handle and opens the shut-off 
valve, while after the door handle is released the shut-off valve closes 
and the door 4 is held fast in the desired position. 
FIG. 2 shows the steplessly blockable setting device 1 which consists of a 
cylinder 8 on the cylindrical inner surface 8a of which a piston unit 10 
connected with a piston rod 9 is guided in sealing manner. This piston 
unit 10 divides the liquid filled interior of the cylinder 8 into the 
working chamber 13 on the piston rod side and the working chamber 14 
remote from the piston rod, the piston rod 9 being guided by means of a 
piston rod guide 11 and sealed to the exterior by a piston rod seal 12 at 
the piston rod exit end of the cylinder 8. In the interior of the cylinder 
8 there is also a spring chamber 16 provided with a gas filling under 
pressure, which is sealed off from the working chamber 14 remote from the 
piston rod by a separator piston 15, and the liquid filling in the working 
chambers 13 and 14 is loaded by the gas pressure in the spring chamber 16. 
The piston rod 9 and the piston unit 10 form a construction group 9, 10. 
Within this construction group 9, 10 a valve chamber 19 is accommodated 
which comprises a valve chamber end face 19a, a valve chamber 
circumferential surface 19b and a valve chamber opening 19c. A valve body 
18 comprising a valve head 25 and a valve shank 27 is accommodated in the 
valve chamber 19. The valve head 25 possesses a cylindrical external 
circumferential surface of larger diameter and the valve shank 27 
possesses a cylindrical external circumferential surface of smaller 
diameter. The valve head 25 and the valve shank 27 merge into one another 
by way of a conical connection surface 26. The valve shank 27 is conducted 
through the valve chamber end face 19a through the intermediary of a seal 
27a into the bore 9a of the piston rod 9. The bore 9a of the piston rod 9 
is in communication with atmospheric pressure. 
A stop ring 29 penetrated by the valve head 25 is inserted into the valve 
chamber opening 19c. The valve head 25 is equipped with a slotted ring 28 
which rests against the stop ring 29. A sealing ring 21 is accommodated 
between the external circumferential surface of the valve head 25 and the 
valve chamber inner surface 19b. This sealing ring 21 rests in sealing 
manner on the external circumferential surface of the valve head 25 and 
likewise in sealing manner on the valve chamber circumferential surface 
19b. The sealing ring 21 is pressed against the stop ring 29 by a sealing 
spring 22 which bears on the valve chamber end face 19a, and by a presser 
washer 24. 
The upper end of the valve head 25 is subjected to the pressure in the 
working chamber 14. For this purpose the ring 28 is provided with a 
relatively wide slot. The valve chamber 19 is connected by a radial bore 
20 with the working chamber 13 so that the same pressure prevails in the 
valve chamber 19 as in the working chamber 13. The pressure in the working 
chamber 14 acts upon the cross-section of the valve head 25. The pressure 
in the working chamber 13 acts upon the conical transition surface 26. 
Since the cross-section of the valve head 25 is larger than the 
cross-section of the valve shank 27, the valve body 18 is pressed against 
the stop ring 29--as long as the pressures in the working chambers 13 and 
14 are approximately equal. Thus the by-pass connection 19c, 19, 20 
between the two working chambers 13 and 14 is closed. If no external force 
acts upon the cylinder 8 and the piston rod 9, the piston unit 10 is held 
in a position of equilibrium where the pressure in the working chamber 13 
is greater than the pressure in the working chamber 14 in accordance with 
the cross-sectional difference between the working chamber 14 and the 
working chamber 13 which is determined by the cross-section of the piston 
rod 9. If the setting of the piston rod 9 in relation to the cylinder 8 is 
to be changed, the valve body 18 is pressed upwards by the valve plunger 
17 until the conical transition surface 26 comes into the region of the 
sealing ring 21. Then the by-pass connection 19c, 19, 20 is opened and the 
piston rod 9 can be shifted in relation to the cylinder 8, the position of 
the separator piston 15 being varied in accordance with the variation of 
the volume of the piston rod 9 within the cylinder 8. 
If with the shut-off valve in the position according to FIG. 3 the piston 
rod 9 is pushed into the cylinder 8, a pre-determined force being exerted, 
the pressure in the working chamber 14 rises in such a way that the 
sealing ring 21 is displaced downwards against the action of the sealing 
spring 22 in FIG. 3 and comes into the region of the conical transition 
surface 26. Then the by-pass connection 19c, 19, 20 is likewise opened and 
the piston rod 9 can be displaced in relation to the cylinder 8. 
If with the shut-off valve in the position according to FIG. 3 the piston 
rod 9 is drawn with a predetermined force out of the cylinder 8, the 
pressure in the working chamber 13 rises in relation to the pressure in 
the working chamber 14. At a predetermined pressure difference the 
pressure prevailing in the working chamber 13 acts upon the differential 
area between the valve head 25 and the valve stem 27 with such force that 
the valve body 18 is displaced upwards until its conical transition 
surface 26 comes into the region of the sealing ring 21. Then the by-pass 
connection 19c, 19, 20 is again open and the piston rod 9 can be drawn out 
of the cylinder. 
In application to the construction according to FIG. 1, this signifies that 
1. on actuation of the release device 7 the door can easily be displaced 
and made fast again at any time; 
2. if without actuation of the release device 7 the door is pivoted with 
great force in the one or the other direction, the by-pass connection 19c, 
19, 20 is likewise opened, so that damage cannot occur either to the 
setting device nor to the door. If no more force is applied to the door, 
the condition according to FIG. 3 automatically re-establishes itself. 
By way of example the release device 7 can be coupled with the door handle 
so that the by-pass connection 19c, 19, 20 is necessarily opened on action 
upon the door handle (not shown). 
In the upper end region of the cylinder 8 longitudinal grooves 23 are 
provided, as visible from FIG. 2, which can by-pass the piston unit 10 as 
soon as the piston unit approaches the upper end position, that is that 
end position which corresponds to an approximately closed door according 
to FIG. 1. Then the piston unit 10 is displaceable in relation to the 
cylinder 8 even without opening of the by-pass connection 19c, 19, 20. The 
action of the setting device is therefore substantially eliminated when 
the door is closed. 
In FIG. 4 a further form of embodiment is shown in which a valve spring 32 
acts upon the ring 28 and thus upon the valve head 25. This valve spring 
32 is supported on the end piece 33a of a cage 33. The cage 33 possesses 
axially directed tongues 34 with radial projections 35. These radial 
projections 35 engage in a groove 40 of the valve unit 10 so that the cage 
33 for one part and the stop ring 29 for the other part are secured on the 
valve unit 10. With the aid of the valve spring 32 the response pressure 
in the working chamber 13 at which the by-pass connection 19c, 19, 20 is 
opened by upward displacement of the valve body 18, can be varied. For 
this purpose it is necessary only to use different valve springs 32. 
Moreover the construction according to FIG. 4, thanks to the valve spring 
32, is especially suitable for those cases where the pressure in the 
working chambers 13 and 14 is relatively slight, so that the biasing 
action downward upon the valve body 18 is slight. 
The sealing spring 22 can also be selected as desired. Thus it is also 
possible to adjust the response pressure in the working chamber 14 at 
which the by-pass connection 19c, 19, 20 is opened by downward 
displacement of the sealing ring 21. 
Specific forms of embodiment of the invention have been represented and 
described in order to illustrate the use of the principles of the 
invention. Of course the invention can also be realised in other ways 
without departing from these principles. 
The references in the Claims serve only for facilitation of understanding 
and are not to be understood as a limitation.