WO2015185125A1

WO2015185125A1 - Gas-spring for balancing heavy loads

Info

Publication number: WO2015185125A1
Application number: PCT/EP2014/061598
Authority: WO
Inventors: Frans Verheijen
Original assignee: Multidocker Cargo Handling Ab
Priority date: 2014-06-04
Filing date: 2014-06-04
Publication date: 2015-12-10

Abstract

A gas-spring (1) has a hollow piston-rod (2) carrying a piston inside a first diameter cylinder (5). A variable length first annular space (8) is formed along the piston-rod (2) between the piston (3) and a first end cap (6). A second diameter cylinder (9) is arranged concentrically over the first cylinder (5). A first end (9a) is sealed by the first end cap (6) and a second end (9b) sealed by a second end cap (10). The second diameter is larger than the first diameter such that a second annular space (11) is formed along the external periphery of the first cylinder (5). The second cylinder (9) is longer than the first cylinder (5) such that a gap (12) is formed between the first cylinder (5) and the second end cap (10). A cooling fluid connection (13) to the first annular space (8) is provided. The space formed by the second annular space (11), the gap (12) and the second end cap (12), and the hollow piston-rod (2) is arranged to contain pressurised gas.

Description

GAS-SPRING FOR BALANCING HEAVY LOADS TECHNICAL FIELD

Embodiments herein relate to a gas-spring for balancing heavy loads, as well as a material handling lifting machine that comprises such a gas-spring.

BACKGROUND OF THE INVENTION

Special gas spring cylinders are known that are used for lifting applications where large own weights of construction parts, such as lifting arms, need to be balanced to increase lifting capacity and/or to reduce energy consumption of an associated lifting machine.

The idea of using a gas spring for compensation of own weight is already old, e.g. you will find such gas springs in any back door of a hatchback or station wagon car, the use in lifting machines, especially in material handling machines, however, is relatively new. US 2012291429 relates to an implement, in particular an excavator or a machine for material handling, with an element movable via at least one working drive, wherein at least one energy recovery cylinder is provided for energy recovery from the movement of the movable element, which includes a chamber filled with gas. The gas-filled chamber of the energy recovery cylinder is compressed when lowering the movable element and thus stores the potential energy, in order to release the same again during an upward movement of the movable element for supporting the working drive. As it was found that the gas in the gas-filled chamber can heat up as the gas is compressed during operation the energy recovery cylinder has been equipped with a heat exchanger by which the energy recovery cylinder and/or the gas in the gas-filled chamber can be cooled. The heat exchanger surrounds the cylinder jacket of the energy recovery cylinder. This provides a large surface for heat exchange whilst the internal structure of the energy recovery cylinder need not be interfered with.

Although partially addressing the problems relating to the heating of the gas in a gas- spring of the aforementioned kind, the present invention is based on the realization that there is room for further improvement in the area of gas-springs for balancing heavy loads. SUMMARY OF THE INVENTION

Embodiments herein aim to provide an improved gas-spring for balancing heavy loads, especially suitable for use in material handling lifting machines.

This is provided through a gas-spring for balancing heavy loads comprising: a hollow piston-rod having a first closed end and a second open end carrying a piston having a piston-seal for sealingly supporting the piston slideably at an inner periphery of a first diameter cylinder having at a first end thereof a first end cap with a first opening having a piston-rod-seal for sealingly receiving the piston-rod slideably concentrically within the first diameter cylinder such that the first closed end of the piston-rod protrudes through the first opening and such that a variable length first annular space is formed along the hollow piston-rod between the piston and the first end cap; which gas-spring further comprises: a second diameter cylinder arranged concentrically over the first diameter cylinder which second diameter cylinder at a first end is sealed by the first end cap and at a second end sealed by a second end cap; where the second diameter is larger than the first diameter such that a second annular space is formed along the external periphery of the first diameter cylinder between the first and the second ends thereof; and where the second diameter cylinder further has a longitudinal extension which exceeds the longitudinal extension of the first diameter cylinder such that a gap is formed between the second end of the first diameter cylinder and the second end cap; a cooling fluid connection to the variable length first annular space; and where the space formed by the second annular space, the gap between the second end of the first diameter cylinder and the second end cap, and the hollow piston-rod is arranged to contain pressurised gas.

The provision of a cooling fluid connection to the variable length first annular space provides for an automatic adaptation of the cooling surface to the degree of compression of the gas contained in the gas-spring whilst the placement of the piston-seal and the piston-rod-seal such that one side of the respective seals always are exposed to cooling fluid ensures efficient cooling and lubrication of the respective seals which provides for an increased service life thereof.

According to a second aspect is provided that the cooling fluid connection to the variable length first annular space is arranged at the first end of the first diameter cylinder.

The provision of the cooling fluid connection at the first end of the first diameter cylinder provides for an unobstructed connection to the variable length first annular space that is unaffected by the extension of the gas-spring. According to a third aspect is provided that the cooling fluid connection further comprises an inlet with a first check valve arranged to allow only fluid flow into the variable length first annular space and an outlet with a second check valve arranged to allow only fluid flow out from the variable length first annular space. The provision of a first and a second check valve in this way provides for an automatic replenishment of cooling fluid into the variable length first annular space for each compression of the gas-spring as the first check valve will allow only fluid flow into the variable length first annular space upon compression of the gas-spring and the second check valve will allow only fluid flow out from the variable length first annular space upon extension of the gas-spring. This automatic replenishment ensures that any heated cooling fluid is ejected from the variable length first annular space and subsequently replaced with fresh cooling fluid which ensures efficient cooling of the gas-spring.

According to a fourth aspect is provided that the second diameter cylinder and the respective first and second end caps are designed as a pressure vessel. The provision of designing the second diameter cylinder and the respective first and second end caps as a pressure vessel ensures that the gas-spring will have high structural integrity and makes it possible to use a more-thin walled cylinder as the first diameter cylinder whereby the overall weight of the gas-spring can be reduced.

According to a fifth aspect is provided that the gas-spring further comprises an hydraulic connection which opens into the hollow piston-rod at an inner-seal of a stationary hydraulic piston within the hollow piston-rod, whereby hydraulic fluid can be urged into a chamber formed between the hydraulic piston and the first closed end of the hollow piston-rod to force the hollow piston-rod to move through the first opening.

The provision of an hydraulic connection in this way enables a combination of load balancing gas-spring and hydraulic lifting cylinder in one integral compact unit.

Furthermore, the placement of the inner-seal of the stationary hydraulic piston such that one side of this seal always will be exposed to hydraulic oil ensures efficient lubrication of the inner-seal while the proximity to the cooling fluid filled variable length first annular space ensures cooling thereof, which two factors provides for an increased service life of the inner-seal.

According to a sixth aspect is provided that the hydraulic piston is arranged in close proximity to the first end of the second diameter cylinder. The provision of the hydraulic piston in close proximity to the first end of the second diameter cylinder ensures cooling of the inner-seal of the stationary hydraulic piston as the variable length first annular space will always contain some cooling fluid adjacent to the first end of the second diameter cylinder. According to a seventh aspect is provided that the hydraulic connection enters through the second end cap.

The provision of the hydraulic connection through the second end cap facilitates attachment of an hydraulic supply line at the second end of the gas spring, which in most applications will be attached to a base of an associated material handling lifting machine, and thus exposed to less movement than the other end of the gas spring which protects the hydraulic supply line from being subject to excessive bending and twitching which will have a positive effect on the service life thereof.

According to an eight aspect is provided a material handling lifting machine that comprises a gas-spring as above. A material handling lifting machine that comprises a gas-spring as above will have an improved service life as efficient cooling and lubrication is ensured whilst providing for an increase in lifting capacity and/or a reduction in energy consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, embodiments herein will be described in greater detail by way of example only with reference to attached drawings, in which

Fig. 1 is a schematic illustration of an extended gas-spring for balancing heavy loads according to some embodiments herein.

Fig. 2 is a schematic illustration of the gas-spring for balancing heavy loads according to figure 1 in a compressed state. Fig. 3 is a schematic illustration of the force to extension relationship of the gas-spring according to figures 1 and 2.

Fig. 4 is a schematic illustration of an extended combined gas-spring/hydraulic cylinder according to other embodiments herein. Fig. 5 is a schematic illustration of the combined gas-spring/hydraulic cylinder according to figure 4 in a compressed state.

Fig. 6 is a schematic illustration of the force to extension relationship of the combined gas- spring/hydraulic cylinder according to figures 4 and 5. Fig. 7 is a schematic illustration of a material handling lifting machine comprising a gas- spring according to any embodiment disclosed herein.

Fig. 8 is a schematic illustration of a cooling fluid connection with an inlet and an outlet with respective check valves providing for automatic replenishment of cooling fluid.

Still other objects and features of embodiments of the invention presented herein will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits hereof, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The figures are schematic and simplified for clarity, and they merely show details which are essential to the understanding of the embodiments, while other details have been left out.

In overview, embodiments herein aim to provide an improved gas-spring 1 for balancing heavy loads, especially suitable for use in material handling lifting machines 18. As illustrated in figures 1 and 2, this is provided through a gas-spring 1 for balancing heavy loads comprising: a hollow piston-rod 2 having a first closed end 2a and a second open end 2b. A piston-rod-side bearing point 2c is arranged at the closed end 2a of the hollow piston-rod 2, for e.g. articulated attachment of the gas-spring 1 to an implement, such as a boom of a material handling lifting machine. The second open end 2b carries a piston 3 having a piston-seal 4 for sealingly supporting the piston 3 slideably at an inner periphery of a first diameter cylinder 5. The first diameter cylinder 5 at a first end 5a thereof has a first end cap 6 with a first opening 6a having a piston-rod-seal 7 for sealingly receiving the piston-rod 2 slideably concentrically within the first diameter cylinder 5 such that the first closed end 2a of the piston-rod 2 protrudes through the first opening 6a and such that a variable length first annular space 8 is formed along the hollow piston-rod 2 between the piston 3 and the first end cap 6.

The gas-spring further comprises a second diameter cylinder 9 arranged concentrically over the first diameter cylinder 5, which second diameter cylinder at a first end 9a is sealed by the first end cap 6 and at a second end 9b is sealed by a second end cap 10. A bottom side bearing point 10a is arranged at the second end cap 10, for e.g. articulated attachment of the gas-spring 1 to an implement, such as a base of a material handling lifting machine.

The second diameter is larger than the first diameter such that a second annular space 1 1 is formed along the external periphery of the first diameter cylinder 5 between the first end 5a and the second end 5b thereof. The second diameter cylinder 9 further has a longitudinal extension which exceeds the longitudinal extension of the first diameter cylinder 5, such that a gap 12 is formed between the second end 5b of the first diameter cylinder 5 and the second end cap10.

Further, there is a cooling fluid connection 13 to the variable length first annular space 8. The space formed by the second annular space 1 1 , the gap 12 between the second end 5b of the first diameter cylinder 5 and the second end cap 10, and the hollow piston-rod 2 is arranged to contain pressurised gas, such as e.g. nitrogen.

The provision of a cooling fluid connection 13 to the variable length first annular space 8 provides for an automatic adaptation of the cooling surface to the degree of compression of the gas contained in the gas-spring 1 , which can be seen through comparing the extended gas-spring 1 of figure 1 with the compressed gas-spring 1 of figure 2.

Due to extreme heating of the gas while compressing it, the first annular space 8 has been designed in such a way, that oil or another cooling fluid (dotted) can flow in this extra chamber between the gas containing second annular space 1 1 and the hollow piston-rod 2 cooling the walls directly bordered by the compressed gas, thus maximizing the surface available for cooling of the adjacent gas at both sides of the first annular space 8.

The placement of the piston-seal 4 and the piston-rod-seal 7, such that one side of the respective seals 4, 7 always will be exposed to cooling fluid ensures efficient cooling and lubrication of the respective seals 4, 7, which provides for an increased service life thereof. Lubrication is suitably ensured through using a cooling-fluid having good lubricating properties, such as e.g. oil.

The cooling fluid connection 13 to the variable length first annular space 8 is suitably arranged at the first end 5a of the first diameter cylinder 5. Having the cooling fluid connection 13 at the first end 5a of the first diameter cylinder 5 provides for an unobstructed connection to the variable length first annular space 8 that is unaffected by the extension of the gas-spring 1 .

Furthermore, the cooling fluid connection 13 further, as illustrated in figure 8, suitably comprises an inlet 13a with a first check valve 19, arranged to allow only fluid flow into the variable length first annular space 8, and an outlet 13b with a second check valve 20, arranged to allow only fluid flow out from the variable length first annular space 8. Having a first and a second check valve 19, 20 in this way provides for automatic replenishment of cooling fluid into the variable length first annular space 8 during each compression of the gas-spring 1. This in particular as the first check valve 19 will allow only fluid flow into the variable length first annular space 8 upon compression of the gas-spring 1 and the second check valve 20 will allow only fluid flow out from the variable length first annular space 8 upon extension of the gas-spring 1 . Through this automatic replenishment is ensured that any heated cooling fluid is ejected from the variable length first annular space 8 and subsequently replaced with fresh cooling fluid. Hereby efficient cooling of the gas-spring 1 is ensured.

The second diameter cylinder 9 and the respective first and second end caps 6, 10 are suitably designed as an integrated pressure vessel. Hereby is ensured that the gas-spring 1 will have high structural integrity which makes it possible to use a more-thin walled cylinder as the first diameter cylinder 5, whereby the overall weight of the gas-spring can be reduced. It also facilitates compliance with rules for pressure vessels, which makes it easier to obtain requisite approvals and certifications.

Thus, the outer hull of the construction i.e. the second diameter cylinder 9 and the respective first and second end caps 6, 10, can be built as strong as required to provide the required safety factors for pressure vessels whilst the inner walls, e.g. the first diameter cylinder 5, can be relatively thin as they are not exposed to the surrounding environment. Hereby a relatively light weight design is obtainable. Having a gas-tank provided as an outer lining through the second annular space 1 1 , which is directly connected, as an integral unit, with the hollow piston-rod 2 and communicates therewith via the annular gap 12 adjacent the second end cap 12 facilitates an approval as a pressure vessel, as there is no requirement for a separate gas-tank connected by tubes/pipes and thus no sealing's therefore either. Instead the outer shell, i.e. the second diameter cylinder 9 and the two end caps 6, 10, forms a sealed integral unit.

Furthermore, having the gas-spring exterior 1 designed as an integrated pressure vessel provides for high resistance to influences from the often harsh environment, e.g. when used in a lifting machine such as a machine for material handling.

Figure 3 illustrates schematically the force F to extension E relationship of the gas-spring according to figures 1 and 2.

The gas contained inside the gas-spring 1 is pre-pressurized to reach a force required while the hollow piston-rod 2 is completely extended out of the first opening 6a. The force F1 of the extended gas-spring 1 is equal to the gas pressure multiplied with the combined surface of the piston 3 and the first closed end 2a of the hollow piston rod 2. The force F2, while the hollow piston-rod 2 is completely compressed into the first opening 6a is equal to the compression factor (division between the gas volume while the piston-rod 2 is compressed to the gas volume while the piston-rod 2 is completely extended) multiplied with the combined surface of the piston 3 and the first closed end 2a of the hollow piston rod 2. However, some energy is lost due to the hysteresis of compressed gas, this loss is however relatively low. Thus, smaller hydraulic lift cylinders can be chosen when combined with the gas-spring 1 herein described for performing the same lifting work than would have been possible without it. In many cases it may not be possible to bring extra gas-cylinders into a design of a lifting arrangement, therefore the following design, as illustrated in figures 4 and 5, of a combined gas-spring/hydraulic cylinder can be used. Thus, the gas-spring 1 may in these embodiments comprise an hydraulic connection 14, which opens into the hollow piston- rod 2 at an inner-seal 15 of a stationary hydraulic piston 16 within the hollow piston-rod 2. Hereby hydraulic fluid (dashed) can be urged into a chamber 17, formed between the hydraulic piston 16 and the first closed end 2a of the hollow piston-rod 2, to force the hollow piston-rod 2 to move through the first opening 6a. The surface area acted upon by the hydraulic fluid will not have to be as big as on a normal hydraulic cylinder as the pre- pressurized gas will assist in pushing the hollow piston-rod 2 outwards of the first opening 6a. Having an hydraulic connection 14 in this way enables a combination of load balancing gas-spring and hydraulic lifting cylinder in one integral compact unit.

However, the flow of cooling fluid, e.g. oil, on the piston-rod 2 side, i.e. in the first annular space 8, does normally not require much pressurization. This as the force created by an associated lifting arrangement and possible loads will push the hollow piston-rod 2 into the first opening 6a, but is however required for cooling.

In certain applications however, the cooling fluid in the first annular space 8 can be pressurized to achieve a pulling force on the piston-rod 2, this can be useful for works which require pulling forces, such as e.g. excavating or drilling. Depending on the pressurizing of the cooling fluid in the first annular space 8 the pulling force can be extended over the stroke of the gas-spring 1 , however always against the rising gas pressure while the while the hollow piston-rod 2 is completely compressed into the first opening 6a. Furthermore, the placement of the inner-seal 15 of the stationary hydraulic piston 16, such that one side of this inner seal 15 always will be exposed to hydraulic oil, ensures efficient lubrication of the inner-seal 15. The proximity to the cooling fluid filled variable length first annular space 8 furthermore ensures cooling of the inner-seal 15. Lubrication and cooling in this way provides for an increased service life of the inner-seal 15. The hydraulic piston should suitably be arranged in close proximity to the first end of the second diameter cylinder, to ensure cooling of the inner-seal 15 of the stationary hydraulic piston 16. Cooling thereof is ensured as the variable length first annular space 8 during normal operation always will contain some cooling fluid adjacent to the first end 9a of the second diameter cylinder 9. Still further, the hydraulic connection 14 should suitably enter through the second end cap 10. Having the hydraulic connection 14 enter through the second end cap 10 facilitates attachment of an hydraulic supply line (not shown) at an end of the gas spring 1 , which in most applications will be attached to a base of an associated lifting machine, and thus exposed to less movement than the other, piston-rod, end of the gas spring, which often will be attached to a more moveable lifting arm. This location of the hydraulic connection 14 serves to protect the hydraulic supply line from being subject to excessive bending and twitching, which protection will have a positive effect on the service life thereof. Figure 6 illustrates schematically the force F to extension E relationship of the combined gas-spring/hydraulic cylinder according to figures 4 and 5.

The gas contained inside the gas-spring 1 is pre-pressurized to reach the force required when the hollow piston-rod 2 is completely extended out of the first opening 6a. The force F1 of the extended gas-spring 1 is equal to the gas pressure multiplied with the surface of the piston 3 plus oil pressure multiplied with the surface of the first closed end 2a of the hollow piston rod 2.

The force F2 (solid line), while the hollow piston-rod 2 is completely compressed into the first opening 6a is equal to the compression factor (division between the gas volume while the piston-rod 2 is compressed to the gas volume while the piston-rod 2 is completely extended) multiplied with the combined surface of the piston 3 and the first closed end 2a of the hollow piston rod 2. The force F3 (dashed) is the force due to the hydraulic pressure on the chamber (17) formed between the hydraulic piston (16) and the first closed end (2a) of the hollow piston-rod (2). F2 + F3 (dotted) is the combination of forces F2 and F3. The lowermost force (dash-dotted) illustrates the force due to hydraulic pressure on the piston (3) in applications where the cooling fluid in the first annular space 8 has been pressurized to achieve a pulling force on the piston-rod 2, above of which this force combined with the force due to gas pressure (F2) is illustrated (dash-double-dotted).

In accordance with the present invention is also envisaged a material handling lifting machine 18, as illustrated schematically in figure 7, comprising a gas-spring 1 as described in the foregoing. Other details of the illustrated material handling lifting machine 18 have been left out for brevity. A material handling lifting machine that comprises a gas- spring 1 as above will have an improved service life as efficient cooling and lubrication is ensured whilst at the same time providing for an increase in lifting capacity and/or a reduction in energy consumption. The term material handling lifting machine, as used herein, is intended to cover all types of material lifting and handling machines including but not limited to: earth moving equipment, such as track and wheel excavators; cargo handlers for ports & terminals, e.g. for loading or unloading sawn wood, round timber, chips, pellets, bags, scrap, rocks, gravel, and several other sorts of bulk and general cargo; forest machines and forwarders for loading and removing heavy loads from the forest; hydraulic mining shovels; knuckleboom loaders; track and wheel material handlers; track and wheel loaders. The gas-spring 1 with the gas-filled chamber itself serves as energy accumulator for energy recovery from the movement of a movable element. The space formed by the second annular space 1 1 , the gap 12 between the second end 5b of the first diameter cylinder 5 and the second end cap 12, and the hollow piston-rod (2) advantageously is filled with pressurized gas, which is compressed during a movement of the piston-rod 2 into the first opening 6a. The energy stored then is available again during movement of the piston-rod 2 outwards of the first opening 6a for supporting a working drive, in particular a working hydraulic cylinder. The movable element of the material handling lifting machine according to the invention advantageously is pivotally attached to the machine about a vertical axis of rotation and pivotable in a vertical swivel plane via one or more working drives. In particular, the movable element is the arm of an excavator or the boom of a machine for material handling. Furthermore advantageously, the machine includes an undercarriage with traveling gear and an upper carriage rotatably arranged thereon about a vertical axis of rotation, to which the movable element is articulated.

On the movable element a working tool, for example a shovel or a grab, can be arranged. When lowering the movable element, the potential energy of the movable element and of the working tool is stored by the energy recovery gas-spring 1 , in order to at least partly compensate the equipment weight again during the upward movement of the movable element. As a result, less energy must be spent via a working drive, in order to move the movable element upwards. As a result, the energy balance of the machine is improved, since less installed engine power is required and the fuel consumption thereof is lowered.

Claims

A gas-spring (1 ) for balancing heavy loads comprising: a hollow piston-rod (2) having a first closed end (2a) and a second open end (2b) carrying a piston (3) having a piston seal (4) for sealingly supporting the piston (3) slideably at an inner periphery of a first diameter cylinder (5) having at a first end thereof (5a) a first end cap (6) with a first opening (6a) having a piston-rod-seal (7) for sealingly receiving the piston-rod (2) slideably concentrically within the first diameter cylinder (5) such that the first closed end (2a) of the piston-rod (2) protrudes through the first opening (6a) and such that a variable length first annular space (8) is formed along the hollow piston-rod

(2) between the piston

(3) and the first end cap (6);

characterized in that it further comprises: a second diameter cylinder (9) arranged concentrically over the first diameter cylinder (5) which second diameter cylinder (9) at a first end (9a) is sealed by the first end cap (6) and at a second end (9b) sealed by a second end cap (10); where the second diameter is larger than the first diameter such that a second annular space (1 1 ) is formed along the external periphery of the first diameter cylinder (5) between the first and the second ends (5a, 5b) thereof; and where the second diameter cylinder (9) further has a longitudinal extension which exceeds the longitudinal extension of the first diameter cylinder (5) such that a gap (12) is formed between the second end (5b) of the first diameter cylinder (5) and the second end cap (10); a cooling fluid connection (13) to the variable length first annular space (8); and where the space formed by the second annular space (1 1 ), the gap (12) between the second end (5b) of the first diameter cylinder (5) and the second end cap (12), and the hollow piston-rod (2) is arranged to contain pressurised gas.

A gas-spring (1 ) according to claim 1 , characterized in that the cooling fluid connection (13) to the variable length first annular space (8) is arranged at the first end (5a) of the first diameter cylinder (5).

A gas-spring (1 ) according to any one of claims 1 or 2, characterized in that the cooling fluid connection (13) further comprises an inlet with a first check valve arranged to allow only fluid flow into the variable length first annular space (8) and an outlet with a second check valve arranged to allow only fluid flow out from the variable length first annular space (8).

4. A gas-spring (1 ) according to any one of claims 1 to 3, characterized in that the second diameter cylinder (9) and the respective first and second end caps (6, 10) are designed as a pressure vessel.

A gas-spring (1 ) according to any one of claims 1 to 4, characterized in that it further comprises an hydraulic connection (14) which opens into the hollow piston- rod (2) at an inner-seal (15) of a stationary hydraulic piston (16) within the hollow piston-rod (2), whereby hydraulic fluid can be urged into a chamber (17) formed between the hydraulic piston (16) and the first closed end (2a) of the hollow piston- rod (2) to force the hollow piston-rod (2) to move through the first opening (6a).

A gas-spring (1 ) according to any one of claims 1 to 5, characterized in that the hydraulic piston (16) is arranged in close proximity to the first end (9a) of the second diameter cylinder (9).

A gas-spring (1 ) according to any one of the preceding claims, characterized in that the hydraulic connection (14) enters through the second end cap (10).

A material handling lifting machine, characterized in that it comprises a gas- spring (1 ) according to any one of claims 1 to 7.