GB2065827A

GB2065827A - Gas spring

Info

Publication number: GB2065827A
Application number: GB8039775A
Authority: GB
Original assignee: Stabilus GmbH
Current assignee: Stabilus GmbH
Priority date: 1979-12-19
Filing date: 1980-12-11
Publication date: 1981-07-01
Also published as: DE2951093C2; FR2473145A1; JPS5694042A; JPS6331014B2; FR2473145B1; GB2065827B; BR8008329A; AU538023B2; AU5874180A; DE2951093A1

Abstract

A gas spring for opening and supporting a vehicle includes means for arresting the piston rod in one terminal position. These arresting means comprise pressure relief valve means 12 which are closable by a prestressing member 15 and can be opened by a pressure differential existing between the two compartments 4, 5. The prestressing force exerted by the prestressing means is responsive to the axial position of the piston rod in such a way that the prestressing force is increased when the piston approaches that end of the cylinder member at which the piston rod member is to be arrested. Due to this design the prestressing force decreases when the piston rod is moved from its arrested terminal position towards the other terminal position so that a substantial manual force for overcoming the arresting action, is only necessary during a short section of the total path of the piston rod with respect to the cylinder. <IMAGE>

Description

SPECIFICATION Gas spring This invention relates to gas springs of the cylinder-and-piston type and particularly concerns improved means for arresting the piston rod in one of its terminal positions relative to the cylinder, in view of using this gas spring in a structure for connecting a support of this structure and a mass movably mounted on this support, so as to arrest this mass with respect to this support in a terminal position.

A gas spring of this type is known from German Offenlegungsschrift 2 659 491. This known gas spring comprises a cylinder member having an axis and two ends, namely a first end and a second end and bounding a sealed cavity therein. A piston rod member extends outward and inward of said cavity and is axially movable with respect to said cylinder member. A piston unit is mounted in said cavity on said piston rod member for movement therewith. This piston unit axially separates the cavity into a first compartment adjacent said first end, and a second compartment adjacent said second end. A body of fluid under pressure higher than atmospheric pressure is sealed in said cavity for biassing said piston rod member axially with respect to said cylinder member. First passage means are provided for connecting said first and said second compartment.First valve means are associated to said first passage means. The first valve means are responsive to the direction of the axial movement of said piston rod member with respect to said cavity, so as to close said first passage means when said piston rod member moves towards said second end, and to open said first passage means when said piston rod member moves towards said first end. Second passage means are also provided for connecting said first and said second compartments. Pressure relief valve means are associated to said second passage means for closing said second passage means in the absence of a fluid pressure differential between said first and said second compartments, and for opening said second passage means in the presence of such fluid pressure differential between a higher pressure in said second compartment and a lower pressure in said first compartment.In the open condition of said pressure relief valve means fluid can flow from said second compartment towards said first compartment. The pressure relief valve means incorporates a valve member and a prestressing member exerting a prestressing force onto said valve member towards a closing position thereof with respect to said second passage means. The prestressing force is independent of the axial position of the piston rod member with respect to the cylinder member.

This gas spring as known from German Offenlegungsschrift 2 659 491 is commonly used in motor vehicle boot lids and serves to facilitate operation, in that the outward biassing force of the pressurized gas is adapted to the weight of the boot lid. In the known construction the outward biassing force of the fluid under pressure is selected so that it is slightly less than the weight of the boot lid, so that to open the boot lid, it is merely necessary for the operator to exert a slight force, and after the boot lid is released it is held arrested in the open position. This arresting is due to the construction of the first and second valve means.The prestressing force exerted onto the valve member of the second valve means is such that it overcomes the opening force exerted onto said valve member by the pressure differential, which pressure differential is due to the weight of the boot lid. So the second passage means can only be opened for fluid flow when an additional force by hand is exerted onto the boot lid so as to increase the pressure differential and thereby the opening force exerted by said pressure differential on said valve member beyond the prestressing force exerted onto said valve member by said prestressing member.

From German Offenlegungsschrift 23 45 503 a further embodiment of a gas spring is known, in which the arresting means are mechanical arresting means responsive to small axial movement of the piston rod member with respect to the cylinder member, such that closing of the boot lid from the arrested open condition can be achieved by additionally raising the boot lid beyond the open arrested position and thereafter allowing it to sink.

It is further commonly known in the art to use a gas spring in motor vehicle boot lids in which gas spring the biassing force exerted by the fluid under pressure, is sufficient to overcome the weight of the boot lid, so that the boot lid is automatically raised towards its open position when a lock mechanism locking the boot lid in the closed position has been unlocked. For closing the boot lid it is necessary in this case to exert a downward directed force by hand onto the boot lid for assisting the weight of the boot lid in overcoming the biassing force of the fluid under pressure in the gas spring. In order to keep the force to be exerted by hand in closing the boot lid very small, it is common practice to adjust the pressure of said fluid such that the biassing force exerted by the fluid is nearly compensated for by the weight of the boot lid.However, the biassing force exerted by the fluid under pressure is variable according to the temperature condition under which the motor vehicle is used. Further the pressure of the fluid in the gas spring may be reduced by a leakage after long periods of operation. So it may occur that the biassing force of the gas spring becomes smaller than the weight of the boot lid to be compensated for. If this situation occurs the boot lid can't further be arrested in its open position.

The present invention overcomes this and other disadvantages of the prior art.

It is the aim of this invention to provide a gas spring of a simple design, particularly of simple design as far as the arresting means are concerned, which gas spring allows under normal conditions opening and closing of a boot lid with minimum force to be exerted by hand and even under abnormal condition warrants arresting of the boot lid in the open position.

In accordance with the invention an improvement is provided in a gas spring of the type known from German Offenlegungsschrift 2 659 491, which improvements consists in that said prestressing means are responsive to the axial position of said piston rod member with respect to said cylinder member in such a way that said prestressing force is increased when said piston unit approaches said first end.

Due to this improvement improved operational conditions are achieved: When the biassing force exerted onto the piston rod member by said fluid under pressure is not sufficient to overcome the weight of the boot lid in the opened condition thereof, it is - in order to arrest the boot lid in the opened position -- only necessary to open the boot lid to such an extent that the prestressing force exerted by the prestressing member overcomes the opening force exerted by the pressure differential between the two compartments, which pressure differential is due to the weight of the boot lid resting on the gas spring. When it is hereupon desired to close the boot lid again, a small additional closing force is to be exerted onto the boot lid by hand.By this the pressure differential between the two compartments is increased and the opening force acting onto the valve member of the second passage means is also increased to as to overcome the prestressing force onto said valve member exerted by said prestressing member.

After a small movement of the boot lid towards the closed position the prestressing force of the prestressing member is decreased so that the pressure differential resulting from the weight of the boot lid is sufficient for overcoming the prestressing force acting onto the valve member and no further additional force is to be exerted by hand onto the boot lid for completely closing the boot lid.

It is to be noted that no additional overflow means are to be provided between the compartments on both sides of the piston unit for suppressing the arresting effect after an initial movement of the boot lid towards its closed position. The spring has therefore a most simple and economic design.

According to a further feature of the invention the prestressing means exerts a prestressing force onto said valve member only after said piston unit has passed a predetermined axial position during its movement towards said first end.

According to a preferred embodiment of the invention which is of particular interest in connection with boot lids of motor vehicles, the piston rod member passes through a guiding and sealing unit provided adjacent said first end of said cylinder member.

This invention is further directed to a structure comprising a support and a mass mounted on said support for movement in a direction having a vertical component between a low position and a raised position, whereby the force of gravity acting on said mass biasses said mass towards said low position. Compensating means are provided for compensating at least a portion of said force of gravity, said compensating means including at least one gas spring. The gas spring comprises a cylinder member having an axis and two ends, namely a first end and a second end and bounding a sealed cavity therein. A piston rod member extends outward and inward of said cavity and is axially movable with respect to said cylinder member. A piston unit is mounted onto said piston rod member within said cavity for movement with said piston rod member.This piston unit axially separates the cavity into a first compartment adjacent said first end, and a second compartment adjacent said second end. A body of fluid under pressure higher than atmospheric pressure is sealed in said cavity for biassing said piston rod member axially with respect to said cylinder member. First passage means are provided for connecting said first and said second compartments. First valve means are associated to said first passage means. Said first valve means are responsive to the direction of the axial movement of said piston rod member with respect to said cavity so as to close said first passage means when said piston rod member moves towards said second end, and as to open said first passage means when said piston unit moves towards said first end. Second passage means are provided for connecting said first and said second compartments.Pressure relief valve means are associated to said second passage means so as to close said second passage means in the absence of a fluid pressure differential between said first and said second compartments, and as to open said second passage means in the presence of such fluid pressure differential between a higher pressure in said second compartment and a lower pressure in said first compartment. So a fluid flow is allowed from said second compartment towards said first compartment. The said pressure relief valve means comprises a valve member and a prestressing member exerting a prestressing force onto said valve member towards a closing position with respect to said second passage means. The biassing force of said fluid in said at least one gas spring is sufficient for overcoming said force of gravity under normal conditions in said raised position of said mass. Said force of gravity generates a pressure differential between said second and said first compartment effecting an opening force onto said valve member under abnormal conditions under which said force of gravity is sufficient for overcoming the biassing force of said pressurized fluid in said at least one gas spring in said raised position of said mass. The prestressing force of said prestressing means overcomes said opening force of said pressure differential at least when said prestressing force has achieved its maximum value.

The support may be for example the body-work of a motor vehicle, whereas the movable mass may be the boot lid of such motor vehicle. One gas spring may be sufficient. It is however possible to use a plurality of gas spring in parallel arrangement between the support and the movable mass. In case of a motor vehicle with a boot lid one gas spring may be arranged adjacent both side edges of the boot lid, which are offset in a horizontal transverse direction with respect to the longitudinal axis of the motor vehicle.

Further objects and advantages of the invention will be apparent from the following detailed description of exemplary embodiments thereof taken in conjunction with the accompanying drawings in which: Figure 1 shows a first embodiment of a gas spring of this invention in longitudinal section with the piston rod member in the outermost position; Figure 2 shows the gas spring as illustrated in figure 1 during inward movement of the piston rod member; Figure 3 shows the gas spring during further inward movement of the piston rod member; Figure 4 shows a longitudinal section of a second embodiment of a gas spring of this invention; Figure 5 shows a third embodiment of a gas spring in longitudinal section; and Figure 6 shows a structure incorporating a gas spring of this invention.

In the embodiments of figures 1 to 3, the gas spring comprises a cylinder member 1. The cylinder member 1 is provided with a bottom 6 and a piston rod guiding and sealing unit 11. A piston rod member 2 is introduced into the cavity 20 defined within the cylinder member 1. At the inner end of the piston rod member 2 there is provided a piston unit 3. The bottom 6 of the cylinder member 1 is provided with a joint eye 21, and also the outer end of the piston rod member 2 is provided with a joint eye 21.

The piston unit 3 is provided with an annular groove 7. Annular gaps 22 and 23 are defined between the outer circumferential faces of the piston unit 3 and the inner circumferential face of the cylinder member 1. A first group of axial bores 9 extends between the upper face 24 of the piston unit 3 and the groove 7. A second group of axial bores 10 of restricted diameter extends between the lower face 25 of the piston unit 3 and the groove 7. A piston ring 8 is provided within the groove 7 of the piston unit 3. This piston ring 8 is in frictional engagement with the inner circumferential face of the cylinder member 1 and is capable of limited axial movement with respect to the piston unit 3.

The upper end of the axial bores 9 is closable by a valve member 12. The valve member 12 is mounted in a peripheral groove 26 of an annular bottom 13 which is part of a tubular valve member supporting member 16. The valve member supporting member 16 defines an annular gap 27 with the inner circumferential face of the cylinder member 1. The annular bottom 13 and the tubular valve member supporting member 1 6 are integral and are axially movable with respect to the piston rod member 2. A helical compression spring member 1 5 is housed in the annular gap defined between the piston rod member 2 and the valve member supporting member 16.The lower end of the helical compression spring member 1 5 is in engagement with the upper face of the annular bottom 13, whereas the upper end of the helical compression spring member 15 is in engagement with the lower face of the piston rod guiding and sealing unit 11.

The piston unit 3 defines within the cavity 20 a lower compartment 4 and an upper compartment 5.

The cavity 20 is filled with a gas under superatmospheric pressure, for example, with nitrogen.

In the position as shown in Figure 1 the helical compression spring member 1 5 is prestressed between the bottom 1 3 of the tubular valve member supporting member 16 and the guiding and sealing unit 11. When the piston rod member 2 is moved downward from the position of Figure 1 towards the position of Figures 2 and 3, the piston ring 8 due to its frictional engagement with the inner circumferential face of the cylinder member 1 is adjacent the upper end face of the groove 7 so that the annular gap 22 is closed.

Simultaneously in the position of Figure 1 the axial bores 9 are closed by the valve member 12.

Consequently the compartments 4 and 5 are completely separated from each other. During the downward movement of the piston rod member 2 from the position of Figure 1 towards the position of Figure 2, the pressure in compartment 4 is raised as compared with the pressure in compartment 5. The pressure differential between the higher pressure in compartment 4 and the lower pressure in compartment 5 generates an opening force acting onto the valve member 12.

As soon as this opening force overcomes the prestressing force exerted by the helical compression spring member 15, the valve member 12 is lifted from the upper end of the axial bores 12, so that gas can escape from the compartment 4 into the compartment 5.

As can be seen from Figure 2 the prestressing force exerted by the helical compression spring member 1 5 decreases when the piston rod member 2 moves downward. This means that in the position of Figure 2 the valve member 12 is lifted from the upper ends of the axial bores 9 at a smaller pressure differential between the compartments 4 and 5 as compared with the condition in Figure 1.

In the position of the piston rod member as shown in Figure 3 the helical compression spring member 1 5 is out of contact with the piston rod guiding and sealing unit 11 so that no further prestressing force is exerted onto the valve member 12. Therefore the gas can flow from the compartment 4 to the compartment 5 through the annular gas 23, the axial bores 10 and the axial bores 9 without pressure drop at the valve member 12. So the force necessary for moving the piston rod member 2 further downward from the position as shown in Figure 3 results only from the pressure of the gas in cavity 20 acting onto the cross-sectional area of the piston rod member 2.

When the piston rod member 2 is moved from the position as shown in Figure 3 towards the position as shown in Figures 2 and 1 by the biassing force exerted by the pressurized gas in the cavity 20 onto the cross-sectional area of the piston rod member 2, the piston ring 8 is positioned at the lower end of the groove 7 so as to close the gap 23. Under these circumstances the gas can flow from compartment 5 through the annular gap 22 and the restricted bore 10 into the compartment 4. Due to the restricted crosssection of the axial bores 10 the outward movement of the piston rod member from the position as shown in Figure 3 to the position as shown in Figures 2 and 1 is damped. The combination of the annular gap 22, the axial bores 10 and the groove 7 may be regarded as first passage means.The piston ring 8 may be regarded as first valve means associated to the first passage means 22, 7, 10. The first valve means 8 are responsive to the direction of movement of the piston rod member 2 with respect to the cylinder 1 in so far as the first passage means 22, 7, 10 are closed when the piston rod member 2 moves from the position as shown in Figure 1 towards the position as shown in Figures 2 and 3, whereas the first passage means 22, 7, 10 ar open when the piston rod member 2 moves from the position as shown in Figure 3 towards the position as shown in Figures 2 and 1.

The combination of the axial bore 9, the axial bore 10, the groove 7 and the annular gap 23 may be regarded as second passage means, and the valve member 12 may be regarded as pressure relief valve means associated to the second passage means 10, 7, 23, 9, said pressure relief valve means 12 being opened only when the force exerted onto the valve member 12 by a pressure differential between compartment 4 and compartment 5 overcomes a prestressing action exerted onto the valve member 12 by the helical compression spring member 1 5 as shown in Figures 1 and 2.

In Figure 6 the gas spring of Figures 1,2 and 3 is shown as part of a motor vehicle structure. The body-work of the motor vehicle is designated by 29. A boot lid is designated by 30. The boot lid 30 is pivotally mounted by link means 31 on the body-work 29. In the closed position as shown in Figure 3 the boot lid 30 is locked by a locking mechanism 32. The joint eye 21 of the cylinder member 1 is connected by a pivot pin 33 to the body-work 29. The joint eye 21 of the piston rod 2 is connected by a pivot pin 34 to the boot lid 30.

In the position as shown in full lines in Figure 6 the boot lid 30 is closed and the piston rod member 2 is in its most inward position with respect to the cylinder 1. In dotted lines there is shown in Figure 6 the open position of the boot lid 30, in which the piston rod member 2 is in its outermost position with respect to the cylinder member 1. It is assumed now that the biassing force of the pressurized gas contained in the cavity 20 is just sufficient for raising the boot lid 30 from the position as shown in full lines in Figure 6 to the position as shown in dotted lines in Figure 6 when the locking mechanism 32 is unlocked. So the boot lid 30 is maintained in its uppermost position as shown in Figure 6 by the biassing force exerted by the pressure of gas in cavity 20 onto the crosssectional area of the piston rod member 2.

For closing the boot lid from the dotted position to the full line position as shown in Figure 6 it is necessary to exert by hand a small downward force onto the boot lid. It is assumed that the so far described situation corresponds to normal ambient temperature. When the ambient temperature falls below a predetermined value the pressure of the gas in the cavity 20 is decreased.

This may have the effect that the biassing force exerted by the pressure of the gas is not further sufficient for overcoming the weight of the boot lid. As a consequence thereof the boot lid will not further be opened by the gas spring when the locking mechanism 32 is unlocked. It is necessary to exert an additional opening force by hand onto the boot lid for raising the boot lid from the position as shown in full lines to the position as shown in dotted lines in Figure 6. If the raising force exerted by hand ceases during the path section A as shown in Figure 6 the weight of the boot lid 30 overcomes the biassing force of the gas pressure exerted onto the piston rod member 2 and the boot lid 30 falls back into the closed position.It is assumed that in the position of the boot lid indicated by 36 the helical compression spring member 15 as shown in Figures 1 to 3, comes into engagement with the piston rod sealing and guiding unit 11. As a consequence thereof the helical compression spring member 1 5 is compressed when the boot lid is further raised from the position 36 as shown in Figure 6 to the position as indicated in dotted lines.This means that the additional raising force to be exerted by hand onto the boot lid 30 for raising the boot lid from the position as indicated at 36 to the position as indicated in dotted lines must be increased for overcoming the action of the helical compression spring member 1 5. During the path section B as shown in Figure 6 the helical compression spring member 1 5 is progressively compressed up to the condition as shown in Figure 1. When in the position as shown in Figure 1 the raising force exerted by hand onto the boot lid 30 is ceased, the boot lid 30 is arrested in the position as shown in dotted lines in Figure 6. This is due to the prestress exerted now by the helical compression spring member 1 5 onto the valve member 12 as shown in Figure 1. When the raising force exerted by hand onto the boot lid 30 ceases the boot lid 30 falls downward for a very small path until the piston ring 8 arrives in the position as shown in Figure 1. Now the compartments 4 and 5 are completely separated. A pressure differential is generated between the compartments 4 and 5 due to the weight of the boot lid acting on the gas spring. This pressure differential is, however, not sufficient for overcoming the prestressing force exerted by the helical compression spring member 1 5 onto the valve member 1 2 in closing direction.

Therefore when the boot lid 30 is to be closed again starting from the position as shown in dotted lines in Figure 6 it is necessary to exert a small additional force by hand onto the boot lid 30 in downward direction so that the pressure differential between the compartments 4 and 5 is increased until it overcomes the closing force exerted by the prestressed helical compression spring member onto the valve member 1 2. Then the second passage means 10, 23, 7, 9 is opened by lifting the valve member 12 from the upper face 24 of the piston unit 3 and the piston member 2 can be pushed inward of the cylinder member 1.When the position as indicated at 36 in Figure 6 has been achieved the prestressing force of the helical compression spring member 1 5 has ceased so that the additional closing force exerted onto the boot lid 30 may be ceased and the boot lid 30 falls back into its closed position, because the biassing force of the pressurized gas acting onto the piston rod member 2 is overcome by the weight component of the boot lid 30 acting onto the piston rod member 2. It is to ba noted that the boot lid 3 can be opened and closed under all ambient temperature conditions with small additional forces exerted by hand. Further the boot lid can be maintained in its opened condition under all temperature conditions. No mechanical arresting means are necessary. The pneumatical arresting means as described are of very simple design.No additional overflow means between the two compartments 4 and 5 are necessary beyond the above described passage means.

In the embodiment of Figure 4 analogous parts are designated by the same reference numbers as in the embodiment of Figures 1 to 3 increased by 100. The differences with respect to the embodiment of Figures 1 to 3 are as follows: The upper end of the helical compression spring member 11 5 engages an abutment member 118 which is fixed at the lower end of the piston rod guiding and sealing member 111. The tubular valve member supporting member 11 6 has an inner diameter substantially equal to the outer diameter of the piston rod member 102 and the helical compression spring member 11 5 is positioned in an annular chamber defined between the annular valve member supporting member 116 and the inner circumferential face of the cylinder member 101.The upward movement of the tubular valve member supporting member 11 6 is limited by a circlip 117 which engages an annular groove in the piston rod member 102. So the stroke of the valve member 11 2 with respect to the piston rod member 102 is limited.

The operation of this embodiment is identical to the operation of the embodiment as shown in Figures 1 to 3.

In the embodiment of Figure 5 analogous parts are designated by the same reference numbers as in Figures 1 to 3 increased by 200. The differences with respect to the embodiment of Figures 1 to 3 are as follows: The helical compression spring member 21 5, the valve member supporting member 21 6 and the valve member 212 are made as an integral part of an elastic plastic material.

This integral part has such a configuration as to define flow channels along the inner and/or outer circumferential face of the integral path even in compressed condition.

Instead of filling the cavity with a pressurized gas exclusively it is also possible that a small amount of oil is contained within the cavity besides the pressurized gas. If it is desired that the arresting action is obtained by the oil it is necessary that the piston rod member 2 is downwardly directed in its outermost position.

Consequently in the structure of Figure 6 the cylinder member 1 should be pivotally connected to the boot lid 30 whereas the piston rod member 2 should be pivotally connected to the bodywork 29.

Although the invention has been described with respect to specific embodiments thereof, many modifications and variations of such embodiments may be made by those skilled in the art without departing from the inventive concepts disclosed.

Accordingly all such modifications and variations are intended to be included within the spirit and scope of the appended claims.

Claims

1. In a gas spring comprising: a) a cylinder member (1) having an axis and two ends, namely a first end (11) and a second end (6) and bounding a sealed cavity (20) therein; b) a piston rod member (2) extending outward and inward of said cavity (20) and being axially movable with respect to said cylinder member (1); c) a piston unit (3) mounted in said cavity (20) on said piston rod member (2) for movement therewith and axially separating said cavity (20) into a first compartment (5) adjacent said first end (11) and a second compartment (4) adjacent said second end (6); d) a body of fluid under a pressure higher than atmospheric pressure, sealed in said cavity (20) for biassing said piston rod member (2) axially with respect to said cylinder member (1);; e) first passage means (10,7,22) for connecting said first (5) and said second (4) compartments, said first passage means (10,7,22) comprising first valve means (8) responsive to the direction of the axial movement of said piston rod member (2) with respect to said cavity (20), said first valve means (8) closing said first passage means (1 0, 7, 22) when said piston rod member (2) moves towards said second end (6) and opening said first passage means (10,7,22), when said piston rod member (2) moves towards said first end (11);; second passage means (10,23,7,9) for connecting said first (5) and said second (4) compartments, said second passage means (10, 23, 7, 9) comprising pressure relief valve means (12, 1 5) for closing said second passage means (10, 23, 7, 9) in the absence of a fluid pressure differential between said first (5) and said second (4) compartments and opening said second passage means (10, 23, 7, 9) in the presence of such fluid pressure differential between a higher pressure in said second compartment (4) and a lower pressure in said first compartment (5), so as to allow fluid flow from said second compartment (4) towards said first compartment (5), said pressure relief valve means incorporating a valve member(12) and a prestressing member (15) exerting a prestressing force onto said valve member (12) towards a closing position thereof with respect to said second passage means (10, 23, 7, 9); the improvement which consists in that said prestressing means (15) are responsive to the axial position of said piston rod member (2) with respect to said cylinder member (1) in a way that said prestressing force is increased when said piston unit (3) approaches said first end (11).

2. A gas spring as set forth in claim 1, wherein said prestressing means (1 5) exert a prestressing force onto said valve member (12) only after said piston unit (3) has passed a predetermined axial position (Fig. 2) during its movement towards said first end (11).

3. A gas spring as set forth in claim 1 or 2, wherein said prestressing means (15) comprises a spring member (15) engaging said valve member (12) on the one hand and abutment means (11) fixed with respect to said cylinder member (1) on the other hand.

4. A gas spring as set forth in claim 3, wherein said spring member (15) is a compression spring member (15) and said abutment means (11) are provided adjacent said first end (11) of said cylinder member (1).

5. A gas spring as set forth in one of claims 4, wherein said piston rod member (2) passes through a guiding and sealing unit (11) provided adjacent said first end (11) of said cylinder member (1).

6. A gas spring as set forth in claim 4, wherein said valve member (12) is connected to a tubular valve member supporting member (16) surrounding said piston rod member (2), said tubular valve member supporting member (1 6) accommodating part of the axial length of said compression spring member (15) and providing an engagement face for that end of said compression spring member (15) which is remote from said first end (11) of said cylinder member (1).

7. A gas spring as set forth in claim 6, wherein said tubular valve member supporting member (16) is provided with an annular bottom (13) at its end remote from said first end (11) of said cylinder member (1), said annular bottom (13) having a first front face directed towards said first end (11) and defining said engagement face and a second front face directed towards said piston unit (3) and defining a recess (26) accommodating said valve member (12).

8. A gas spring as set forth in claim 6 or 7, wherein said tubular valve member supporting member (16) has an inner diameter substantially equal to the outer diameter of said compression spring member (15), said compression spring member (15) being housed over at least part of its axial length in an annular chamber defined between said tubular valve member supporting member (16) and said piston rod member (2).

9. A gas spring as set forth in claim 6 or 7, wherein said tubular valve member supporting member (116) has an outer diameter substantially equal to the inner diameter of said compression spring member (115) and said compression spring member (115) is housed over at least part of its axial length in an annular chamber defined between said tubular valve member supporting member (116) and the inner circumferential face of said cylinder member (101).

10. A gas spring as set forth in one of claims 6-9, wherein said tubular valve member supporting member (16) is made of plastic material.

11. A gas spring as set forth in claim 4, wherein said compression spring member (215) is an axially compressive member of plastic or rubber like material, said axially compressive member (215) being integral with said valve member (212).

12. A gas spring as set forth in claim 1 , wherein the axial movement of said valve member (112) with respect to said piston rod member (102) away from said closing position, is limited by valve member abutment means (117) provided on said piston rod member(102).

13. In a structure comprising: a) a support (29); b) a mass (30) mounted on said support (29) for movement in a direction having a vertical component between a low position and a raised position, whereby the force of gravity acting on said mass (30) biasses said mass (30) towards said low position; compensating means for compensating at least a portion of said force of gravity, said compensating means including at least one gas spring (1, 2), said gas spring (1, 2) comprising: aa) a cylinder member (1) having an axis and two ends, namely a first end (11) and a second end (6) and bounding a sealed cavity (20) therein; bb) a piston rod member (2) extending outward and inward of said cavity (20) and being axially movable with respect to said cylinder member (1); cc) a piston unit (3) mounted in said cavity (20) on said piston rod member (2) for movement therewith and axially separating said cavity (20) into a first compartment (5) adjacent said first end (11) and a second compartment (4) adjacent said second end (6); ; dd) a body of fluid under a pressure higher than atmospheric pressure, sealed in said cavity (20) for biassing said piston rod member (2) axially with respect to said cylinder member (1); ee) first passage means (10,7,22) for connecting said first (5) and said second (4) compartments, said first passage means (10, 7, 22) comprising first valve means (8) responsive to the direction of the axial movement of said piston rod member (2) with respect to said cavity (20), said first valve means (8) closing said first passage means (10,7,22) when said piston rod member (2) moves towards said second end (6), and opening said first passage means (10,7,22) when said piston unit (3) moves towards said first end (11);; ff) second passage means (10,23,7,9) for connecting said first (5) and said second (4) compartments, said second passage means (10, 23, 7, 9) comprising pressure relief valve means (12, 1 5) for closing said second passage means (10, 23, 7, 9) in the absence of a fluid pressure differential between said first (5) and said second (4) compartments, and opening said second passage means (10,23,7,9) in the presence of such fluid pressure differential between a higher pressure in said second compartment (4) and a lower pressure in said first compartment (5), so as to allow fluid flow from said second compartment (4) towards said first compartment (5); said pressure relief valve means (12, 15) comprising a valve member (12) and a prestressing member (15) exerting a prestressing force onto said valve member (12) towards a closing position with respect to said second passage means (10, 23, 7, 9), said prestressing force of said prestressing means (15) being responsive to the axial position of said piston unit (3) with respect to said cylinder member (1) in such a way that said prestressing force is increased when said piston unit (3) approaches said first end (11); 1) the biassing force of said fluid in said at least one gas spring (1, 2) being sufficient for overcoming said force of gravity under normal conditions in said raised position of said mass (30); ; 2) said force of gravity generating a pressure differential between said second (4) and said first (5) compartment effecting an opening force onto said valve member (12) under abnormal conditions under which said force of gravity is sufficient for overcoming the biassing force of said pressurized fluid in said at least one gas spring in said raised position of said mass (30); 3) the prestressing force of said prestressing means overcoming said opening force of said pressure differential at least when said prestressing force has achieved its maximum value.

14. A gas spring substantially as described herein with reference to the accompanying drawings.