GB2328999A

GB2328999A - A gas pressure shock absorber

Info

Publication number: GB2328999A
Application number: GB9818658A
Authority: GB
Inventors: Dr Dieter Ammon
Original assignee: Daimler Benz AG
Current assignee: Daimler Benz AG
Priority date: 1997-09-04
Filing date: 1998-08-26
Publication date: 1999-03-10
Anticipated expiration: 2018-08-26
Also published as: ITRM980567A0; GB2328999B; IT1302308B1; DE19738617C2; DE19738617A1; FR2767887B1; FR2767887A1; ITRM980567A1; GB9818658D0

Abstract

A gas pressure shock absorber has a working piston 4 which can move in a working cylinder 2 and is mounted on a piston rod 3 and which divides the working cylinder into two chambers 5,6 filled with hydraulic fluid. The chambers 5, 6 are interconnected via at least one overflow duct 7, 8 of an overflow valve 15, 16. Furthermore, a gas-filled pressure chamber 20is provided between the working piston 4 and one of the end faces of the working cylinder 2. The pressure chamber 20 is constructed as a pressure-tight gas reservoir designed as a separate component and with a elastic wall 11. Furthermore, bypass flow ducts 25, 26, controlled via the piston ring 24 position, are provided in the working piston 4, and an elastic axial bearing 13,14 of the working piston is provided in order to permit frequency-selective tuning of damping.

Description

1 Gas pressure shock absorber 2328999 The invention relates to a gas

pressure shock absorber for vehicles, having a working piston movable in a working cylinder, and mounted on a piston rod which piston divides the working cylinder into two chambers which are filled with hydraulic fluid and are interconnected via at least one overflow duct of an overflow valve.

Such a gas pressure shock absorber has been disclosed in DE 39 13 912 AI. The shock absorber of this printed publication has a working cylinder in which a working piston mounted on a piston rod is supported in an axially displaceable fashion. Located in the interior of the working cylinder is a working chamber filled with hydraulic oil which is divided into two chambers by the working piston. In the event of an axial movement of the working piston, the hydraulic oil flows through an overflow duct between the two chambers, the flow resistance of the hydraulic oil opposing the movement of the working piston.

Adjacent to the working chamber, between one of the chambers and the end face of the cylinder, there is arranged in the interior a gas-filled pressure chamber which is required for volume compensation and is separated by a displaceable separating piston from the directly neighbouring chamber, the piston chamber. A change in pressure in the piston chamber, caused by a movement of the working piston, also affects the displacement of the separating piston until pressure equalization takes place between the gas-filled pressure chamber and the piston chamber.

So that the shock absorber is fully functional, the pressure chamber must he isolated in a gas-tight and liquid-tight fashion from the piston chamber over the entire pressure range occurring during the working operation. In the region of the separating piston, this requires a seal which is expensive to produce and must bear against the inner wall of the working cylinder with a high frictional grip in order to achieve the required tightness. In order to be able to displace the separating piston axially, it is firstly necessary to overcome the frictional grip of the separating piston seal; relatively weak forces do not displace the separating piston, and there is no volume compensation. This means that the shock absorber responds initially only from relatively strong forces and has a relatively high stiffness. In particular, forces of 2 relatively high frequency and relatively low amplitude, which are produced, for example, in the case of vertical wheel movements of motor vehicles, are damped only insufficiently. In addition to an undesired high return flow of energy, which adversely affects the ride comfort, the components of the shock absorber are subjected to increased wear because of the stronger forces acting on them.

It is also disadvantageous that the damping characteristic of the shock absorber depends on the direction of movement of the piston rod. The aim is for no damping to occur during the rebound and for the shock absorber to be active during the return stroke into the middle position. This is achieved by means of a spring-loaded pressure valve body which, depending on the direction of movement, releases a first overflow duct, which is free from damping, or a second overflow duct, which is affected by damping. However, this shock absorber, whose damping properties depend on the direction of movement, cannot be used universally. Moreover, it has a complicated design and its construction is susceptible to faults. Again, the problem of effectively damping forces of variable magnitude and frequency is not solved.

The present invention seeks to provide a gas pressure shock absorber such that both low-frequency and higher- frequency vibrations are effectively damped.

According to the present invention there is provided a shock absorber for motor vehicles, having a working piston movable in a working cylinder, and mounted on a piston rod which piston divides the working cylinder into two chambers which are filled with hydraulic fluid and are interconnected via at least one overflow duct of an overflow valve, a displaceable piston ring in a piston ring gap of the working piston, and an overflow bypass which communicates with the piston ring gap and has reduced flow resistance and is arranged between the two chambers of the working cylinder, wherein the overflow bypass extends radially and connects the piston ring gap to the overflow duct, it being possible for the piston ring to be displaced between a position releasing the overflow bypass and a position sealing the overflow bypass.

The use of a separate gas reservoir with an elastic wall enables the separating piston with the expensive seal to be omitted. Virtually no frictional forces are produced when pressure is applied in conjunction with a change in volume caused thereby, because only the shape of the elastic gas reservoir is changed, and the position 3 of the gas reservoir in the interior of -the cylinder essentially does not change. The elimination of the frictional forces enables the shock absorber to dampen even relatively low amplitudes of vibration more effectively. The response can be set to the load respectively to be expected by appropriately selecting the material of the gas reservoir wall and the gas reservoir internal pressure.

The elimination of the frictional forces in the region of the pressure chamber causes an overall reduction in the level of forces in the shock absorber. This avoids additional elastic deformations of the sealing elements, which have a stiffening effect on small excitations. In addition, wear is reduced and the service life of the shock absorber is increased.

It is expedient for the wall of the gas reservoir to be constructed as a rubber diaphragm in the form of a torus. The rubber diaphragm permits large elastic deformations in conjunction with a high degree of tightness. The toroidal gas reservoir can be inserted particularly effectively into the end-face section in the interior of the working cylinder, it being advantageous to arrange a clamping ring in the cylinder, in order to fix the position of the gas reservoir.

For the purpose of further improving the damping characteristic, it is possible to provide additional measures such as elastic axial mounting on the piston rod and overflow valves with a construction which is particularly low in friction.

According to the invention, it is also possible to improve the damping response by providing in addition to the overflow duct an overflow bypass which has a reduced flow resistance with respect to the overflow duct. At the start of a movement or in the case of small movements of the working piston, the hydraulic fluid initially flows only through the additional overflow bypass arranged. Because of the reduced flow resistance. there is also, moreover, only a slight damping of the movement. The overflow bypass communicates with the piston ring gap, which is constructed between the lateral surface of the working piston and the inner wall of the working cylinder. The piston ring, which is supported in the piston ring gap, can in this case be displaced between a position which releases the overflow bypass and a position which seals the overflow bypass. In the case of small movements, the piston ring is initially located in its releasing position, with the result that the hydraulic fluid can flow through the bypass with only a slight flow resistance. By contrast, because of the larger resistance 4 caused by valve elements to which force is applied and which seal the overflow duct, the main flow path through the overflow duct is initially not taken into account.

As soon as the movements of the working piston are intensified because of the larger forces acting on the piston rod, the resistance of the valve elements sealing the overflow duct is overcome, and the hydraulic fluid flows through the overflow duct. The piston ring is displaced into its closed position, in which the bypass is sealed and can no longer have hydraulic fluid flowing through it. The hydraulic fluid must now make its way through the overflow duct.

The result of these measures is that higher-frequency excitations are expediently damped without impairing the damping properties with reference to relatively large amplitudes of lower frequency. It is possible to represent shock absorbers which are frequency-selective in large tuning ranges and which, particularly in the case of motor vehicles, stabilize the vehicle body to the extent required for safety and at the same time subject the vertical wheel movements to the slightest stiffening in favour of rolling comfort.

Further advantages and expedient embodiments are to be gathered from the further claims, the description of the figure and the drawings, in which a gas pressure shock absorber according to the invention is represented in section.

The gas pressure shock absorber 1, which is to be used in motor vehicles in particular, comprises a working cylinder 2, in which a working piston 4, which is mounted on a piston rod 3, is supported in an axially displaceable fashion. The interior of the working cylinder 2 is subdivided by the working piston 4 into two chambers 5, 6, a rod chamber 5 on the rod side, and a piston chamber 6 on the piston side. The two chambers 5, 6 are filled with a hydraulic fluid, in particular with hydraulic oil. A piston rod movement caused by a force F acting on the piston rod 3 immediately entails an axial displacement of the working piston 4 in the cylinder 2. Constructed in the working piston 4 are rebound/compression overflow valves 15, 16 through which the hydraulic oil flows in the event of a movement of the working piston. The flow resistance of the hydraulic oil damps the movement of the working piston 2 and thus also of the piston rod 3.

A pressure chamber 10 is provided in the interior between the piston chamber 6 and the end face 21 of the working cylinder. The pressure chamber 10 is located in the direct vicinity of the piston chamber 6, and is constructed as a gas reservoir 20. The gas reservoir 20 has an elastic wall 11 which is designed, in particular, as a rubber diaphragm. The gas reservoir 20 has the shape of a torus and bears directly against the inner surface of the end face 21 of the working cylinder 2. A clamping ring 12 between the gas reservoir 20 and the working piston 4 fixes the gas reservoir in its position in the working cylinder 2.

A change in pressure in the piston chamber 6 effects a change in volume of the gas reservoir 20. Under the action of a modified pressure as a consequence of the change in gas volume, the elastic wall 11, which is pressure-tight and designed as a rubber diaphragm, of the gas reservoir 20 expands or contracts. The rubber diaphragm is resistant and hardwearing. In the event of a change in shape of the gas reservoir 20, caused by changes in pressure in the piston chamber 6, there are virtually no frictional forces which have to be overcome.

Instead of a toroidal shape, the gas reservoir can also be constructed as ellipsoid or otherwise in the shape of a gas-filled pressure cushion.

The overflow valves 15, 16 in the working piston 4 respectively comprise an overflow duct 7, 8 and a plurality of juxtaposed valve plates 17, 18, which form the valve closure elements. The overflow valves 15, 16 operate in a directionally selective fashion. In the event of a movement of the working piston 4 in the direction of the closed end face 21 of the working cylinder 2, the hydraulic fluid flows through the first overflow valve 15, whose overflow duct 7 is open at the end facing the piston chamber 6. Arranged at the opposite end of the overflow duct 7 are the valve plates 17, which are displaced axially in the direction of the open position by the pressure of the hydraulic fluid and release the path for the hydraulic fluid from the piston chamber 6 into the rod chamber 5. The overflow duct 8 of the second overflow valve 16 is sealed during this movement by the valve plates 18.

In the event of the inverse movement of the working piston 4 in the direction away from the end face 21, the hydraulic fluid flows from the rod chamber 5 into the piston chamber 6 through the overflow duct 8 of the second overflow valve 16, whose valve plates 18 are displaced into the open position. At the same time, the valve plates 17 of the first overflow valve 15, which are held on the side of the working piston 4 facing the rod chamber 5, are displaced into their closed position; the 6 path through the overflow duct 7 of the first overflow valve 15 is thereby blocked.

In order to reduce the opening resistance of the valve plates 17, 18, the individual disc-shaped valve plates are provided with a friction-reducing surface coating, preferably Teflon (RTM). Immediately.juxtaposed individual valve plates can thereby he displaced with a low outlay on force from their closed position into their open position which releases the opening of the overflow duct.

In addition to the friction-reducine surface coating or as an alternative to this, it is possible for spacer discs to he arranged between in each case two individual valve plates. The result of this is that the oil gap thickness in the closed position of the valve plates is increased, and the opening resistance is reduced. In accordance with a further embodiment, this can also be achieved by means of a convex cross- sectional profile of the valve plates.

The working piston 4 is mounted on the piston rod 3 via elastic axial bearings 13, 14. The elastic axial bearings 13, 14 are arranged at both end faces of the t) Z:1 working piston 4. Each axial bearing 13, 14 comprise two convex shaped and mutually facing, axially compliant bearing disc springs 1-21, 23.

The effect of the axial bearing springs is that relatively small excitafion amplitudes are primarily damped via relatively softer bearing deformations; it is avoided that the response to these relatively small excitation amplitudes is exclusively or chiefly via the harder fundamental damping response. It' appropriate, it is possible in addition to provide axial play between the working piston 4 and piston rod 3.

It is provided as a further measure that the piston rod seal 27, by which the rod chamber 5 is sealed from the outside has radially extended scaling lips, in order to reduce the effective axial stiffness in conjunction with an unchanged scaling effect. It is also possible for this purpose to provide additional axial compliant parts, for example corrugated tube contours.

An additional overflow bypass is assigned to each overflow duct 7, 8. The overflow pipe is constructed as a radially and axially widened piston ring gap 9 between the lateral surface of the working piston 4 and the inner wall 19 of the working W c> cylinder 2. Alternatively or additionally, the overflow bypass is constructed as an essentially radially extending, unidirectional non- return valve 25 or 26 between the overflow ducts 7, 8 and the piston ring gap 9. In both instances, the piston ring 14 7 in the piston ring gap 9 takes over the task of keeping open or sealing the overflow bypass as a function of the magnitude of the movement of the working piston 4. At the start of a movement of the working piston 4, in particular in the event of a reversal of the direction of movement of the piston rod 3, the flow resistance via the overflow bypass is conspicuously smaller than the resistance which opposes the flow through the valve plates 17, 18 of the overflow valves 15, 16. Consequently, the hydraulic fluid flows via the bypass, and the shock absorber forces are very small. The flow path via the bypass is not thereby impeded by the piston ring 24.

Larger movements require correspondingly stronger shock absorber forces. These are achieved by having the piston ring 24 occupy a closed position in which the bypass is sealed. The piston ring 24, which initially remained unchanged in its initial position with the start of the piston movement because of the wall friction, strikes the side wall of the piston ring gap 9 in the further course of the movement and is moved along by the side wall against the frictional resistance of the piston ring. In this position, the piston ring is located in its closed position, blocking the bypass; the hydraulic fluid then has to take the path having the higher flow resistance via the overflow valve 15, 16.

With regard to its dimensioning and its preferably rectangular crosssectional shape, the piston ring 24 is particularly suitable for sealing the overflow bypass. The axial width of the piston ring 24 is small by comparison with the axial width of the piston ring gap 9 in order to create efficient play for movement of the piston ring in the first phase of relatively small movements. The axial width or the radial thickness of the piston ring 24 is, on the other hand, large enough in order to be able to seal both the flow opening of the radially extending non-return valve 25, 26 and the axial flow opening of the piston ring gap 9.

The non-return valves 25, 26 forming the bypass are constructed in a unidirectional fashion, and only permit the hydraulic fluid to flow through via the respective duct 7, 8 to the piston ring gap 9. For this purpose, each of the non-return valves 25, 26 has a ball which is springloaded in the direction of the overflow ducts 7, 8, is jammed in a tapered part of the duct belonging to the respective non-return valve 25, 26 and can be displaced only in the direction of the piston ring gap 9 to release the relevant duct.

8

Claims

1. A gas pressure shock absorber for motor vehicles, having a working piston movable in a working cylinder, and mounted on a piston rod which piston divides the working cylinder into two chambers which are filled with hydraulic fluid and are interconnected via at least one overflow duct of an overflow valve, a displaceable piston ring in a piston ring gap of the working piston, and an overflow bypass which communicates with the piston ring gap and has reduced flow resistance and is arranged between the two chambers of the working cylinder, wherein the overflow bypass extends radially and connects the piston ring gap to the overflow duct, it being possible for the piston ring to be displaced between a position releasing the overflow bypass and a position sealing the overflow bypass.

2. A shock absorber according to Claim 1, wherein the overflow bypass is constructed as a radial, unidirectional non-retum valve in the working piston between the overflow duct and the piston ring gap.

3.

A shock absorber according to Claim 2, wherein the non-return valve opens in the direction of the piston ring gap.

4. A shock absorber according to any one of Claims 1 to 3, wherein the piston ring gap forms an additional overflow bypass and is of radially and axially widened construction.

5. A shock absorber according to any one of Claims 1 to 4, wherein a gas filled pressure chamber comprising a separate component is provided between the working piston and one of the end faces of the working cylinder and is designed as a pressure-tight gas reservoir with an elastic wall.

6. A shock absorber according to Claim 5, wherein the wall of the gas reservoir comprises a rubber diaphragm.

9

7. A shock absorber according to Claim 5 or 6, wherein the gas reservoir is of toroidal construction.

8. A shock absorber according to any one of Claims 5 to 7, wherein a clamping ring is arranged between the gas reservoir and the working piston.

9. A shock absorber according to any one of Claims 5 to 8, wherein the working piston is mounted on the piston rod via an elastic axial bearing.

10. A shock absorber according to any one of Claims 5 to 9, wherein the overflow duct is part of a directionally selective rebound/compression overflow valve having a plurality of valve plates which seal the overflow duct and have a friction reducing surface coating.

C>

11. A shock absorber according to Claim 10, wherein the valve plates which seal the overflow duct have a surface coating which is a friction reducing coadng.

1) A shock absorber for motor vehicles, substantially as described herein with reference to and as illustrated in the accompanying drawing c>,

11. A shock absorber according to Claim 10, wherein the valve plates which seal the overflow duct have a surface coating of Teflon.

12. A shock absorber for motor vehicles, substantially as described herein with reference to and as illustrated in the accompanying drawing.

Amendments to the claims have been filed as follows Claims 1. A shock absorber for motor vehicles, having a working piston movable in a working cylinder, and mounted on a piston rod which piston divides the working cylinder into two chambers which are filled with hydraulic fluid and are interconnected via at least one overflow duct of an overflow valve, a displaceable piston ring in a piston ring gap of the working piston, and an overflow bypass which communicates with the piston ring gap and has reduced flow resistance and is arranged between the two chambers of the working cylinder, wherein the overflow bypass which is provided in addition to the overflow duct(s), extends radially and connects the piston ring gap to the overflow duct(s), it being possible for the piston ring to be displaced between a position releasing the overflow bypass and a position scaling the overflow bypass.

2. A shock absorber according to Claim 1, wherein the overflow bypass is constructed as a radial, unidirectional non-return valve in the working piston between the overflow duct and the piston ring gap.

3. A shock absorber according to Claim 2, wherein the non-return valve opens in the direction of the piston ring gap.

4. A shock absorber according to any one of Claims 1 to 3, wherein the piston ring gap forms an additional overflow bypass and is of radially and axially c widened construction.

5.

A shock absorber according to any one of Claims 1 to 4, wherein a gasfilled pressure chamber comprising a separate component is provided between the working piston and one of the end faces of the working cylinder and is designed as a pressure-tight gas reservoir with an elastic wall.

11 A shock absorber according to Claim 5 or 6, wherein the gas reservoir is of toroidal construction.

7.

A shock absorber according to any one of Claims 5 to 7, wherein a clamping ring is arranged between the gas reservoir and the workin. piston.

C> 9. A shock absorber according to any one of Claims 5 to 8, wherein the 0 working piston is mounted on the piston rod via an elastic axial bearing.