GB2241046A

GB2241046A - Pneumatic spring

Info

Publication number: GB2241046A
Application number: GB9103208A
Authority: GB
Inventors: Rolf Mintgen; Stephan Enders
Original assignee: Stabilus GmbH
Current assignee: Stabilus GmbH
Priority date: 1990-02-15
Filing date: 1991-02-15
Publication date: 1991-08-21
Anticipated expiration: 2011-02-15
Also published as: GB2241046B; GB9103208D0; FR2658255A1; FR2658255B1; DE4004714A1

Abstract

A pneumatic spring has a stroke-dependent intermediate stoppage provided by a constriction 15 which extends over the periphery and has a diameter which is smaller than the internal diameter of the cylinder. The external diameter of a cavity dividing module 3 is equal to the internal diameter of the constriction so that, on extension of the spring. the module can seal against this constriction and thereby separate, the work chambers 6, 7 of the cylinder from one another. <IMAGE>

Description

1 - 0 1 PNEUMATIC SPRING 1- 0,4 CS The invention relates to a pneumatic

spring comprising a cylinder with an axis, two ends, an internal peripheral face and a cavity defined by the two ends and the internal peripheral face, a piston rod guiding and sealing module in the region of at least one of these ends, a piston rod which is introduced through the piston rod guiding and sealing module into the cavity of the cylinder and is axially movable, and a volume of gas under excess pressure within the cavity, a cavity dividing module dividing the cavity into two work chambers being arranged on the piston rod within the cavity.

DE-OS 25 13 302 discloses a pneumatic spring of this design which provides an intermediate stoppage for the pneumatic spring between the completely retracted position of the piston rod and the completely extended position of the piston rod. In the embodiment according to Figures 2 and 3 thereof there is provided a ram which is rigidly connected to the cylinder base, projects into the tubular piston rod and is provided with at least one portion of greater diameter. The portion of greater diameter co-operates with a seal arranged in a central aperture in the piston and thus allows closure of channels provided in the piston for hydraulic connection of the work chambers separated by the piston. A disadvantage of.this arrangement is the complex and necessarily very exact fixing of the ram in the cylinder base so that the ram runs exactly centrally in the piston and in the piston rod bore. This necessitates complicated assembly to implement and check the exact alignment of the ram.

It is also known from said document and from DE-OS 3 301 544 to provide bypass channels for a piston ring co-operating 2 with the internal face of the cylinder by means of grooves arranged in the cylinder. Reliability of operation of the pneumatic spring cannot be guaranteed over a long period of time as the groove edges of the cylinder grooves can cause wear of the piston ring.

The object of the present invention is to provide a pneumatic spring having at least one stroke-dependent intermediate stoppage, which is simple in construction, allows unproblematic assembly and guarantees high reliability of operation over a long period of time.

To achieve this object it is proposed that the internal peripheral face has an axial portion of reduced diameter at least at one region, preferably between the two ends, and that the cavity dividing module is adapted with respect to its external diameterto the axial portion of reduced diameter such that the two work chambers are separated from one another for the through-flow of fluid in at least one direction when the cavity dividing module is positioned in the axial portion of reduced diameter while the two work chambers are connected to one another by an overflow connection between the internal peripheral face of the cylinder and the cavity dividing module when the cavity dividing module is positioned outside the axial portion.

The pneumatic spring according to the invention is intended, in particular, to simplify the raising of a rear flap or of an engine bonnet of motor vehicles and thus to allow the rear flap or engine bonnet to be positioned not only in a completely raised position but also in an intermediate position. It may happen that, in normal operation, the bonnet or flap is raised only to the intermediate position and only beyond the intermediate position if bulky goods are to be introduced into the boot space or the engine is to be removed from the engine space.

3 The invention can be employed regardless of whether or not the pneumatic spring, optionally a plurality of pneumatic springs, is adequate to overcome the force of gravity on the bonnet or flap. In the former case, the respective intermediate stoppage becomes effective automatically when the pneumatic spring causes the bonnet or flap to rise. In the latter case, the intermediate stoppage becomes effective when the bonnet or flap has been raised manually with the assistance of the pneumatic spring and the bonnet or flap is then allowed to descend again.

With the design of the pneumatic spring according to the invention, careful treatment of the cavity dividing module is ensured because the cavity dividing module in any case enters a central axial portion of reduced diameter which is circular in cross section. Sharp edges cannot come into contact with the cavity dividing module. Construction of the pneumatic spring and assembly are unproblematic, and high reliability of operation is guaranteed over a long period of time.

To dampen the movement of the piston rod, the cavity dividing module can comprise a damping sealing element which rests on the internal peripheral face of the cylinder over the majority of the entire stroke of the piston rod. The opening movement of the flap or bonnet can therefore be damped if the opening force of the pneumatic spring outweighs the weight of the bonnet or flap. On the other hand, the closing movement.of the bonnet or flap can be damped if the force of the pneumatic spring is lower than the gravityinduced closing force of the flap or bonnet. It is therefore advisable for the damping sealing element to deliver different damping forces depending on the direction of movement of the piston rod relative to the cylinder.

4 The directionally different damping effect can be achieved, for example, in that the damping sealing element has axial play relative to the piston rod and assumes two different damping positions relative to the piston rod as a function of the direction of movement of the piston rod with respect to the cylinder, the damping sealing element forming damping orifices of different cross section between the two work chambers in the two damping positions. If the opening force of the pneumatic spring outweighs the weight of the flap or bonnet, the damping orifice can have a greater cross section in a damping position of the damping sealing element adjusted during the retraction of the piston rod into the cavity and the damping orifice can have a smaller cross section in a damping position adjusted during extension of the piston rod from the cylinder.

If the opening force of the pneumatic spring is lower than the weight of the flap or bonnet, it is recommended that the damping orifice have a smaller cross section in a damping position of the damping sealing element adjusted during retraction of the piston rod into the cavity and the damping orifice have a greater cross section in a damping position adjusted during extension of the piston rod from the cylinder.

The damping device can be constructed, for exampler such that the damping sealing element is received in a radial groove of the cavity dividing module which is open toward the internal peripheral face of the cylinder and forms radially directed damping orifices with lateral limiting faces of this radial groove. The damping sealing element can be arranged axially next to a work chamber separating element, cooperating with the axial portion of reduced diameter, of the cavity dividing module.

j To simplify the over-running of the intermediate stoppage in the position in which the intermediate stop is not to be effective, the cavity dividing module can comprise a piston-like separating element carrier which receives a work chamber separating element with axial play, this work chamber separating element, when positioned inside the axial portion of reduced diameter, assuming, as a function of the direction of the relative movement between piston rod and cylinder, two different axial positions relative to the separating element carrier, in one of which the work chambers are separated from one another and in the other of which the work chambers are connected to one another. In one embodiment, the work chamber separating element is formed by an annular groove of the separating element carrier, which annular groove is open toward the internal peripheral face of the cylinder and allows axial play for the work chamber separating element., recesses which bridge over the separating element close to this end being provided at one axial end of this annular groove. The work chamber separating element can be formed by an 0-ring.

Another way of simplifying the over-running of the intermediate stoppage in the direction in which the intermediate stoppage is not to be effective is to bridge over the cavity dividing module using a bridging channel which connects the two work chambers and has a nonreturn valve which opens or closes as a function of the direction of the pressure difference between the two work chambers. The nonreturn valve.can be formed by an 0-ring which covers a radial portion of the bridging channel at its radially outer end.

To allow an intermediate stoppage to become effective, it is necessary for the cavity dividing module and the axial portion of reduced diameter to engage with one another, in a work chamber separating manner, over such travel of the 6 piston rod that a considerable pressure difference builds up between the two work chambers when the cavity dividing module is moved away axially over the axial portion of reduced diameter. The pressure difference to be built up depends on the value of the flap weight or bonnet weight, and the length of said travel is consequently dimensioned such that it, on the one hand, reliably brings about the intermediate stoppage but, on the other hand, can relatively easily be overcome by manually applied force.

The travel in which the cavity dividing module engages, in a work chamber separating manner, with the axial portion of reduced diameter, can be determined either by a corresponding axial length of the axial portion of reduced diameter or by a corresponding axial length of a work chamber separating element of the cavity dividing module.

According to a first embodiment, the axial portion of reduced diameter is formed by an elongate constriction of the cylinder. With this embodiment, the work chamber separating element can be short in the axial direction. To guarantee careful running-over of the axial portion of reduced diameter, it is recommended that the elongate constriction of the cylinder pass at least at one end continuously into adjacent portions of the cylinder.

According to a further embodiment, the axial portion of reduced diameter is formed by at least one radially inwardly directed furrow pf the cylinder. In this embodimentp a relatively long work chamber separating element will have to be provided to guarantee engagement between work chamber separating element and furrow over sufficiently long travel.

It is also conceivable that two axially spaced furrows of the cylinder are provided, of which the axial spacing is 7 smaller than or equal to the axial length of a work chamber separating element of the cavity dividing module.

In the pneumatic spring according to the invention, additional guidance of the piston rod is not guaranteed by a piston of conventional design over the majority of the stroke of the piston rod in any case. It is therefore proposed that the piston rod guiding and sealing module be constructed with such a guide length that it alone guides the piston rod in the cylinder without the additional effect of a guiding piston. Such a great guide length can be obtained, for example, if the piston rod guiding and sealing module is formed by two axially spaced guiding elements which receive a piston rod seal between themselves.

The invention also relates to a constructional unit, in particular motor vehicle constructional unit, with a basic framework and a constructional part mounted pivotally round a substantially horizontal axis on this basic framework, wherein this constructional part can be raised against the effect of gravity and can be lowered with the aid of gravity and wherein at least one pneumatic spring of the type described hereinbefore is provided to assist the raising movement of the constructional part. According to a first variation, the lifting force exerted on the constructional part by the pneumatic spring, of which there is at least ones is lower than the force required to raise the constructional part, and the cavity dividing module is constructed such that it separates thetwo work chambersi at least during the descent of the constructional part. In this variation, the axial portion of reduced diameter and the cavity dividing module also engage over adequate travel of the piston rod to produce between the two work chambers a pressure difference which brings about the stoppage of the constructional part during the descent of the constructional part. According to a further variation of the abovementioned constructional 8 unit, the lifting force exerted on the constructional part by the pneumatic spring, of which there is at least one, is greater than the force required to raise the constructional part; and the cavity dividing module is constructed such that it separates the two work chambers at least during the raising of the constructional part. In this variation, the dimensioning rule applies, according to which the axial portion of reduced diameter and the cavity dividing module engage over adequate travel of the piston rod to produce, between the two work chambers, a pressure difference which brings about the stoppage of the constructional part during the ascent of the constructional part.

The invention is described in more detail below with reference to the embodiments illustrated in the drawings.

Figure 1 showt a longitudinal section through a pneumatic spring with a stroke-dependent intermediate stoppage.

Figure 2 shows an embodiment of a pneumatic spring in a longitudinal section, a constriction being formed by two encircling furrows.

Figure 3 shows an embodiment corresponding to the one in Figure 2 with a bypass cross section which can be closed by a nonreturn valve.

The pneumatic springs shown in the Figures are preferably employed as counterweights for motor vehicle rear flaps or engine bonnets as well as other flaps or bonnets constructed pivotally round a horizontally arranged axis of rotation. These pneumatic springs have the object of keeping the respective flaps in the open position, and it is desirable for the flap to be held at various opening angles by these pneumatic springs. The pneumatic spring can be 4 9 designed such that it does not completely compensate the flap weight and the flap therefore has to be raised manually, unintentional closure of the flap being prevented from a certain opening angle. The stroke- dependent intermediate stoppage serves for this purpose, fixing also being desirable in the completely extended pneumatic spring position. The subject of the invention is also suitable for such a pneumatic spring design.

However, the Figures show constructions of the pneumatic spring which are designed such that the expulsion force is greater than the flap weight to be compensated. The pneumatic spring according to Figure 1 has a cylinder 1 in which a piston rod 2 is guided and sealed, and a cavity dividing module 3 with a piston 3a is fixed on a piston rod projection. This piston 3a has a radial groove 4 in which a damping sealing element 5 is axially and radially movably arranged. The cylinder 1 filled with a gas filling under pressure is divided by the cavity dividing package 3 into the work chamber 6 and the piston rod-side work chamber 7. The piston rod 2 is sealed from the exterior by a piston rod seal 8. The piston rod 2 is guided in the cylinder 1 by two guide rings 9 and 10 between which the piston rod seal 8 and a spacer ring 11 are arranged so that the guide means has a relatively great guide length. An annular groove 12 is provided with recesses 13 toward the sealing element 5 so that a work chamber separating element in the form of an 0ring 14, arranged in the annular groove 12, can vacate a bypass channel as a function of the piston rod movement when this 0-ring 14 comes to rest in a constriction formed by a cylindrical portion 15 and the piston rod 2 is pushed in by a closing force exerted on the flap. During the expulsion movement of the piston rod 2 from the cylinder lt the 0-ring 14 rests in the groove 12 and on the constriction 15 so that the work chambers 6 and 7 are separated from one another. The expulsion movement of the piston rod 2 is damped by apertures 16 in the sealing element 5 as this sealing element 5 rests, during the extension movement, against an end face of the radial groove 4 into which the recesses 13 open. As the sealing element 5 is preferably radially movably arranged in the radial groove 4, the radial groove 4 forms an orifice cross section so that the extension movement until the cylindrical portion is reached allows the passage of fluid from the work chamber 7 into the work chamber 6. During the push-in movement of the piston rod 2, i.e. when a closing force is exerted on the rear flap (not shown), the sealing element 5 rests against the end face of a piston disc 30 so that the apertures 17 form an orifice. These apertures 17 are usually selected substantially greater than the apertures 16 so that the push-in movement of the piston rod 2 into the cylinder 1 is opposed by no resistance or at least by no significant resistance due to damping in the apertures 17.

The use of such pneumatic springs as counterweights for flaps or bonnets of motor vehicles which are pivotal round horizontal axes is possible, for example, since the cylinder is articulated to the vehicle body by the connecting member 31 fixed on the cylinder base, while the connecting member 32 fixed on the end of the piston rod 2 acts in an articulated manner on the flap. In order to keep the pneumatic spring as free as possible from transverse forces, ball-and-socket joints are preferably used as connecting joints so the joint parts 31, 32 arranged on the cylinder 1 and on the piston rod 2 are constructed as ball-and-socket joint sockets. Figure 1 shows the completely extended position of the piston rod 2 from the cylinder 1. The maximum opening of the rear flap or engine bonnet corresponds to this position. To close the rear flap, a closing force is exerted thereon so that the piston rod 2 is pushed into the cylinder 1 against the effect of the push-on force. The sealing element 5 rests against the piston disc 30 so that the apertures 17 in this direction of movement form the orifice cross section for the fluid flowing z 4 11 out of the work chamber 6 into the piston rod-side work chamber 7. When the cylindrical portion 15 forming the constriction is reached, the 0ring 14 comes to rest on the internal face of this cylindrical portion 15 and is shifted in the annular groove 12 such that the recesses 13 are vacated and a bypass channel is thus formed round the 0-ring 14, through which the fluid displaced from the work chamber 6 can flow without significant forces opposing the push-in movement. The piston or separating element carrier 3a is dimensioned in its external diameter such that it can be guided without difficulty by the internal face of the cylindrical portion 15, and the sealing element 5 is constructed resiliently such that it can also run over the cylindrical portion 15.

As the flap is opened, the piston rod 2 exerts, owing to the internal pressure inside the cylinder 1, a force on the rear flap which is designed such that the further opening movement takes place automatically after the lock has been opened and optionally after a slight opening movement. This lasts until the sealing element 5 has run over the cylindrical portion 15 and the 0-ring 14 located in the annular groove 12 comes to rest in a sealing manner on the internal face of the cylindrical portion 15. The expulsion movement of the piston rod is limited in this way as an exchange of fluid from the piston rod-side work chamber 7 into the.work chamber 6 no longer takes place. The first open position of the rear flap or engine bonnet is achieved' automatically in. this way. If further opening of the rear flap or engine bonnet is desired, the separating element carrier 3a has to be guided away with the 0-ring 14 over the cylindrical portion 15 by application of an opening force on the rear flap or engine bonnet. Further opening of the rear flap then takes place automatically until it reaches the completely open position or optionally until it co-operates with a second cylindrical portion not shown in Figure 1. TO 0 12 guarantee unproblematic operation of the pneumatic spring over a long period of time, the transition from the internal diameter of the cylinder to the internal diameter of the cylindrical portion 15 is formed by inclined faces. On the other hand, the illustrated intermediate stoppage can be used directly even in pneumatic springs which are designed such that the expulsion force of the piston rod is not capable of compensating the entire weight of the flap. For this purpose it is merely necessary for the separating element carrier 3a together with the piston disc 30 and the sealing element 5 as well as the O-ring 14 to be mounted in reverse on the journal of the piston rod 2. As the rear flap is opened, an opening force is then exerted thereon until the separating element carrier 3 has run over the first cylindrical portion 15, the rear flap is then released and arresting takes place automatically by blocking of the work chambers from one another by the seal formed by the O-ring 14 and the internal face of the cylindrical portion 15. Such a design of the expulsion force of the pneumatic spring relative to the flap weight again demands a corresponding constriction on the cylinder 1 in the region of the base-side cylinder end for the completely extended position of the piston rod 2. To close the flap, the flap merely has to be guided away over the cylindrical portions 15 - by a closing force exerted on the flap -, and the flap then automatically falls into the lock as the expulsion force of the pneumatic spring is lower than the force exerted thereon by the flap.

The embodiment according to Figure 2 differs from the one according to Figure 1 essentially in that the cavity dividing module 3 is formed by a piston disc 22 and a piston tube 23 as well as a separating element carrier 20 with a separating element 21, the damping sealing element resting movably on the piston sleeve 23. These parts are fixed on a projection of the piston rod 2 such that a radial groove 4 is provided for the damping sealing element 5 while the 13 separating element carrier 20 carries a work chamber separating element 21 which is substantially rectangular in a longitudinal section and has a relatively large axial dimension. In the present case, the constriction is formed by two encircling furrows 18 and 19 which are easy to produce and co-operate with the work chamber separating element 21. In this embodiment also, the piston rod 2 is guided in the cylinder by the two guide rings 9 and 10 and the piston rod seal 8 arranged therebetween and the spacer ring 11 and is sealed from the exterior. When the rear flap is closed, a closing force has to be exerted thereon which increases by the resultant holding force as the work chamber separating element 21 runs over the encircling furrows 18 and 19. During the push-in movement, the apertures 17 of the sealing element 5 also in this embodiment become effective for the flowing of the fluid from the work chamber 6 into the work chamber 7. To open the flap, the flap is moved by the expulsion force of the piston rod 2 in the opening direction until the work chamber separating element 21 comes to rest on the encircling furrow 19 and - if the expulsion force is substantially greater than the flap weight - also on the encircling furrow 18. The flap opening which is usually to be approached is now achieved. If - for example for loading bulky goods - the flap is opened further, the work chamber separating element 21 has to be guided over the two encircling furrows 18 and 19 by application of an opening force, the apertures 16 again being effective as a damping cross section for the further opening movement.

The embodiment according to Figure 3 differs from that according to Figure 2 essentially in that the separating element carrier 20 comprising the separating element 21 has a bypass cross section provided with a nonreturn valve 34 and consisting of an axially extending channel 24 and a radial bore 25. The nonreturn valve 34 is formed by an 0-ring 26 which closes the radial bore 25 externally and is received in 14 an annular aperure 27. This nonreturn valve 34 for the channels 24 and 25 opens as a function of pressure only during the push-in movement of the piston rod into the z cylinder. During the expulsion movement of the piston rod from the cylinder, the 0-ring 26 reliably closes the bore 25.

Claims

CLAIMS:

1. Pneumatic spring comprising a cylinder (1) with an axis, two ends, an internal peripheral face and a cavity (6, 7) defined by the two ends and the internal peripheral face, a piston rod guiding and sealing module (9, 10, 11) in the region of at least one of these ends, a piston rod (2) which is introduced through the piston rod guiding and sealing module (9, 10, 11) into the cavity (6, 7) of the cylinder (1) and is axially movable, and a volume of gas under excess pressure within the cavity (6, 7), a cavity dividing module (3) dividing the cavity (6, 7) into two work chambers (6, 7) being arranged on the piston rod (2) within the cavity (6, 7), characterised in that the internal peripheral face has an axial portion of reduced diameter at least at one region (15), and that the cavity dividing module (3) is adapted with respect to its external diameter to the axial portion (15) of reduced diameter such that the two work chambers (6, 7) are separated from one another for the through-flow of fluid in at least one direction when the cavity dividing module (3) is positioned in the axial portion (15) of reduced diameter while the two work chambers (6, 7) are connected to one another by an overflow connection between the internal peripheral face of the cylinder (1) and the cavity dividing module (3) when the cavity dividing module (3) is positioned outside the axial portion (15).

2. Pneumatic spring according to claim lr characterised in that the cavity dividing module (3) comprises a damping sealing element (5) which rests 'on the internal peripheral face of the'cylinder (1) over the majority of the entire stroke of the piston rod (2).

3. Pneumatic spring according to claim 1 or 2, characterised in that the damping sealing element (5) is 16 radially resilient such that it can run over the axial portion (15) of reduced diameter without significant resistance to movement.

4. Pneumatic spring according to one of claims 2 and 3, characterised in that the damping sealing element (5) delivers different damping forces depending on the direction of movement of the piston rod (2) relative to the cylinder (1).

5. Pneumatic spring according to claim 4, characterised in that the damping sealing element (5) has axial play relative to the piston rod (2) and assumes two different damping positions relative to the piston rod (2) as a function of the direction of movement of the piston rod (2) with respect to the cylinder (1), the damping sealing element (5) forming damping orifices (16, 17) of different cross section between the two work chambers (6, 7) in the two damping positions.

6. Pneumatic spring according to claim 5, characterised in that the damping orifice (17) has a greater cross section in a damping position of the damping sealing element (5) adjusted during manually assisted movement of the piston rod (2) and the damping orifice (16) has a smaller cross section in a damping position adjusted during free movement of the piston rod (2) or vice versa.

7. Pneumatic spring according to one of claims 2 to 6, characterised in that the damping sealing element (5) is received in a radial groove (4) of the cavity dividing module (3) which is open toward the internal peripheral face of the cylinder (1) and forms radially directed damping orifices (16, 17) with lateral limiting faces of this radial groove (4).

17

8. Pneumatic spring according to one of claims 2 to 7, characterised in that the damping sealing element (5) is arranged axially next to a work chamber separating element (14), co-operating with the axial portion (15) of reduced diameter, of the cavity dividing module (3).

9. Pneumatic spring according to one of claims 1 to 8, characterised in that the cavity dividing module (3) comprises a piston-like separating element carrier (3a) which receives a work chamber separating element (14) with axial play, this work chamber separating element (14), when positioned inside the axial portion (15) of reduced diameter, assuming, as a function of the direction of the relative movement between piston rod (2) and cylinder (1), two different axial positions relative to the separating element carrier (3a), in one of which the work chambers (6, 7) are separated from one another and in the other of which the work chambers are connected to one another.

10. Pneumatic spring according to claim 9, characterised in that the work chamber separating element (14) is formed by an annular groove (12) of the separating element carrier (3a), which annular groove is open toward the internal peripheral face of the cylinder (1) and allows axial play for the work chamber separating element (14)y recesses (13) which bridge over the separating element (14) close to this end being provided at one axial end of this annular groove (12).

11. Pneumatic spring according to one of claims 9 and 10, characterised in that the work chamber separating element (14) is formed by an 0-ring.

12. Pneumatic spring according to one of claims 1 to 8, characterised in that the cavity dividing module (3) is bridged over by a bridging channel (24, 25) which connects the two work chambers (6r 7) and has a nonreturn valve (34) 18 which opens or closes as a function of the direction of the pressure difference between the two work chambers (6, 7).

13. Pneumatic spring according to claim 121 characterised in that the nonreturn valve (34) is formed by an 0-ring (26) which covers a radial portion (25) of the bridging channel (24, 25) at its radially outer end.

14. Pneumatic spring according to one of claims 1 to 13, characterised in that the cavity dividing module (3) and the axial portion (15) of reduced diameter engage with one another, in a work chamber separating manner, over such travel of the piston rod (2) that a considerable pressure difference builds up between the two work chambers (6# 7) when the cavity dividing module (3) is moved away axially over the axial portion (15) of reduced diameter.

15. Pneumatic spring according to claim 141 characterised in that the travel in which the cavity dividing module (3) engages, in a work chamber separating manner, with the axial portion (15) of reduced diameter is defined by a corresponding axial length of the axial portion (15) of reduced diameter.

16. Pneumatic spring according to claim 14, characterised in that the travel in which the cavity dividing module (3) engages, in a work chamber separating manner, with the axial portion (18, 19) of reduced diameter is defined by a corresponding axial length of a work chamber separating element (21) of the cavity dividing module (3).

17. Pneumatic spring according to one of claims 1 to 16, characterised in that the axial portion (15) of reduced diameter is formed by an elongate constriction of the cylinder (1).

p 19

18. Pneumatic spring according to claim 17, characterised in that the elongate constriction (15) of the cylinder (1) passes at least at one end continuously into adjacent, portions of the cylinder (1).

19. Pneumatic spring according to one of claims 1 to 16, characterised in that the axial portion (18, 19) of reduced diameter is formed by at least one radially inwardly directed furrow of the cylinder (1).

20. Pneumatic spring according to claim 19, characterised in that two axially spaced furrows,(18, 19) of the cylinder (1) are provided, of which the axial spacing is smaller than or equal to the axial length of a work chamber separating element (21) of the cavity dividing module (3).

21. Pneumatic spring according to one of claims 1 to 20, characterised in that the piston rod guiding and sealing module (8, 9, 10, 11) is constructed with such a guide length that it alone guides the piston rod (2) in the cylinder (1) without the additional effect of a guiding piston.

22. Pneumatic spring according to claim 21, characterised in that the piston rod guiding and sealing module (8, 9, 10, 11) is formed by two axially spaced guiding elements (9, 10) which receive a piston rod seal (8) between themselves.

23. Constructional unit, in particular motor vehicle constructional unit, with a basic framework and a constructional part mounted pivotally round a substantially horizontal axis on this basic framework, wherein this constructional part can be raised against the effect of gravity and can be lowered with the aid of gravity and wherein at least one pneumatic spring according to one of claims 1 to 22 is provided to assist the raising movement of the constructional part, characterised in that the lifting force exerted on the constructional part by the pneumatic spring, of which there is at least one, is lower than the force required to raise the constructional part and in that the cavity dividing module (3) is constructed such that it separates the two work chambers (6, 7) at least during the descent of the constructional part.

24. Constructional unit according to claim 23, characterised in that the axial portion (15) of reduced diameter and the cavity dividing module (3) engage over adequate travel of the piston rod (2) to produce between the two work chambers (6r 7) a pressure difference which brings about the stoppage of the constructional part during the descent of the constructional part.

25. Constructional unit, in particular motor vehicle constructional unit, with a basic framework and a constructional part mounted pivotally round a substantially horizontal axis on this basic framework, wherein this constructional part can be raised against the effect of gravity and can be lowered with the aid of gravity and wherein at least one pneumatic spring according to one of claims 1 to 22 is provided to assist the raising movement of the constructional party characterised in that the lifting force exerted on the constructional part by the pneumatic spring, of which there is at least one, is greater than the force required to raise the constructional part and in that the cavity dividing module (3) is constructed such that it separates the two work chambers (6, 7) at least during the raising of the constructional part.

26. Constructional unit according to claim 25, characterised in that the axial portion (15) of reduced diameter and the cavity dividing module (3) engage over adequate travel of the piston rod (2) to produce between the two work chambers (6r 7) a pressure difference which brings about the stoppage of i 21 the constructional part during the ascent of the constructional part.

c

27. Pneumatic spring as claimed in claim 1 substantially as described with reference to Figure 1, 2 or 3 of the accompanying drawings.

Published 1991 at The Patent Office. State House. 66171 High Holborn. London WC1R41P. Further copies rnay be obtained from Sales Branch. Unit 6. Nine Mile Point Chnfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques ltd. St Mary Cray. Kent.