GB2151746A

GB2151746A - A hydropneumatic vehicle suspension with load-responsive damping

Info

Publication number: GB2151746A
Application number: GB08430443A
Authority: GB
Inventors: Hubert Beck; Heinz Knecht
Original assignee: Boge GmbH
Current assignee: ZF Boge GmbH
Priority date: 1983-12-23
Filing date: 1984-12-03
Publication date: 1985-07-24
Also published as: GB8430443D0; IT1179532B; DE3346660A1; DE3346660C2; FR2557036B1; GB2151746B; IT8424229A0; FR2557036A1

Abstract

A self-levelling hydropneumatic vehicle suspension is arranged to give an automatic increase in the damping of the vehicle on increase in the load and a reduction in the damping on reduction of the load on the vehicle. To achieve this a telescopic spring suspension strut 5 communicates with a pressure reservoir 4 via a path controlled by a throttle element comprising a piston 14 acted on by the fluid pressure medium in the system and by an independent pressure so that the effective throttling cross-sectional area 12 is regulated by axial displacement of the piston 14. The independent pressure may be atmospheric acting on the face 15, or a control pressure from a pump. The system pressure increases as the load on the vehicle increases. The levelling operation is conventional. <IMAGE>

Description

SPECIFICATION A hydropneumatic vehicle suspension with height control The invention relates to a hydropneumatic suspension with height control for a vehicle, in particular a motor vehicle, with at least two telescopic spring struts, including a damping device if necessary, arranged in the region of the vehicle wheels between the vehicle structure and the axle, the telescopic spring struts being connected by a fluid pipe on the one hand to a fluid pressure pump and a control element for controlling the desired vehicle height and on the other hand to a pressure resevoir, and a throttle element regulating a throttling cross-sectional area in the fluid pipe in accordance with the load on the vehicle.

Means for altering the damping force are known (e.g. DE-OS 3111 410), in which damping fluid is fed through the pipes into the spring cylinder by the pressure fluid pump so that the piston rod of the damper is extended by the pressure building up and accordingly the rear end of the vehicle is raised. This device allows the possibility of the vehicle reaching the desired height despite differing conditions of loading.

A drawback in this arrangement is that the control slide is made relatively long and is connected ahead of the pressure reservoir in such a way that a direct angular lead-off, as is often necessary on grounds of availability of space, is impossible. On the other hand with this design one must fear that the diaphragm which also gives travel in this case can take up unstable shapes, which adversely affects the necessary accuracy of control. Moreover the throttling cross-sectional area is controlled solely by the system pressure, and so the damping in the contraction and extension phases cannot be influenced independently. In addition the control plunger produces unwanted reaction forces on the dividing diaphragm when the damping force is generated.

Likewise spring strut assemblies are known with integrated damping valves operating in accordance with load (e.g. DE-PS 16 55 094), relating to a closed system. Through a spring-loaded valve slide this system influences the suspension of the vehicle. It does not influence the vehicle damping. A hydropneumatic suspension of this kind switches two or more gas pressure reservoirs of different pressures in a cascade manner according to vehicle load in order to achieve more favourable spring characteristics in this way.

Taking this as its starting point it is the aim of the invention to construct a hydropneumatic suspension in such a way that a simple and reliable control of the height is achieved, resulting in an improvement in the ride comfort, automatic increase in the vehicle damping with the increase in load, and a reduction in the damping when the load on the vehicle is reduced.

To solve this problem it is proposed according to the invention that the throttling element should comprise at least one axially displaceable piston in a closed bore, and a front face of the piston is exposed to a pressure independent of the pressure in the fluid pipe, the piston, under the action of the fluid pressure and the independent pressure, controlling the effective throttling cross-sectional area of the fluid pipe, and means being provided for balancing the pressure between the closed portion and the open portion of the bore.

In this arrangement it is of advantage that the vehicle damping is regulated automatically by means of the throttling element, and indeed on increase in the load and therefore also on increase in the pressure the throttling cross- sectional area is narrowed, whilst when the load is removed the throttling cross-sectional area increases, without having to react on the damping pressure modulation. By virtue of the increased damping on increase in the loading of the vehicle the large vehicle masses of driving operation can be more easily stabilised so that a significant improvement in the driving behaviour arises. Moreover for example excessive rolling movements of the vehicle structure are more rapidly reduced.

A simple and favourable integration of the throttling element is achieved according to a significant feature of the invention when the bore is provided in the telescopic spring strut. This has a particularly favourable effect on the axial iength of the structure if this bore is arranged transversely to the working chamber of the telescopic spring strut.

Such an arrangement also favours the connection of the fluid pressure pipes. The effective throttling cross-sectional area extends in a direction which is axial with respect to the telescopic spring strut and at right angies to the bore.

According to a further important feature it is provided that the piston comprises at least two cylindrical parts of different diameters. This results in advantages both in mounting and also in the axial guiding of the piston in its bore. Moreover the free space which arises in the region which is of smaller diameter simultaneously provides a path for the passage of the fluid under pressure from the fluid pipe to the throttling cross-sectional area.

A further important feature envisages that with the provision of two bores, each with a respective piston, the throttling cross-sectional area for the extension phase and the contraction phase can be controlled independently of one another. Such a construction is of particular advantage where the damping forces on the extension and contraction phases of the telescopic spring strut are to be damped and controlled in different ways.

To achieve a simple exposure of the piston to a pressure which is independent of the system pressure it is provided according to a further embodiment of the invention that the front face of the piston should be exposed to atmosphere as the independent pressure. A cylindrical portion of the piston is guided externally by a sealed bore and is exposed to atmosphere. In addition there is the further possibility, according to a further feature, of urging the piston in the direction of maximum throttling cross-sectional area by means of a compression spring. This results in the respective desired throttling cross-sectional area being set on a change of the system pressure.

An alternative feature envisages that the independent pressure which acts on the piston should be generated by a fluid pressure pump. A further pump can be provided as the fluid pressure pump or, where a corresponding control slide is used, the pump which is already present may be used. In such an arrangement it is of advantage that a further variation in the desired damping forces can be achieved in accordance with the load on the vehicle.

A simple form of the effective throttling crosssectional area envisages that the throttling crosssectional area should comprise at least one opening from the bore into the working chamber of the telescopic spring strut cylinder.

A further important feature envisages that the means for balancing the pressure should comprise at least one orifice opening into the fluid pressure pipe and/or into the working chamber of the telescopic spring strut cylinder. The orifices for balancing the pressure are so arranged, using two pistons, that the piston controlling the extension phase is received in a bore in the working chamber and the piston controlling the contraction phase is guided in a bore in the fluid pressure pipe. These bores for balancing the pressure are intended on the one hand to prevent compression on axial movement of the piston and on the other hand to have no negative effect on the independent pressure which acts on the piston.Furthermore these bores have the effect that when piston rod velocities become large the pressure differences which arise act on the respective exposed piston face and accordingly have the tendency towards an yen large ment of the throttling cross-sectional area.

In this variant (Figure 3) there is a two-fold influence of the throttling cross-sectional area because the piston reacts to the system pressure and to the damping pressure. This allows a greater variation in the damping characteristic which is sought.

Where a single piston is used, effective in the extension and contraction phases and where the telescopic spring strut provides the basic damping, it is provided according to an important feature that the means for balancing the pressure should comprise at least one recess or notch in the piston con necting the two parts of the bore. In this embodiment it is of advantage that the recess can be made in the form of at least one bore or chan nel or the like, so that a compression across the piston cannot result. This arrangement is free from any reactions which might be produced by the damping pressure.

Some preferred embodiments by way of exam ple are illustrated diagrammatically in the drawing, in which: Figure I is a diagrammatic illustration of a fully or partially supporting hydropneumatic suspension with height control; Figure 2 shows an upper portion of a telescopic spring strut with a throttling element; Figure 3 shows the upper part of a telescopic spring strut which in principle is like that in Figure 2 but which has two throttling elements acting independently for the extension and contraction phases; Figure 4 shows the upper part of a telescopic spring strut which in principle is like that of Figure 3 but with the difference that double pistons are provided and the pressure of the fluid medium is applied through a rotary valve; Figure 5 shows the rotary valve of Figure 4 alone and in three different possible positions; and Figure 6 is a diagrammatic illustration of a practical embodiment of Figure 2 by way of example.

The fully or partially supporting vehicle hydropneumatic suspension with height control, illustrated diagrammatically in Figure 1, comprises substantially a fluid pressure pump 1, pressure pipes 2, a control element 3, a pressure reservoir 4 and telescopic springs struts 5. Each strut 5 comprises a housing 6, an upper working chamber 7 and a lower chamber 8, and a piston rod 9 connected to a wheel guiding member (not shown); the housing 6 being connected to the vehicle structure. The upper working chamber 7 and the lower working chamber 8 are separated from one another by a damping piston 10 secured to the piston rod 9.

Figure 2 shows the upper portion of a telescopic spring strut 5 in which the upper working chamber 7 is separated from the lower working chamber 8 by the damping piston. Basic damping is achieved by the damping piston and associated damping valves 11. Damping in accordance with load is handled by an effective throttling cross-sectional area 12, the pressure from the pipe 2 acting on a piston 14 which is in a bore 13. The fluid pressure pipe 2 is fed from the pump 1 and the reservoir 4, and the reservoir pressure acts on the piston 14 as well as the atmospheric pressure which acts on the front face 15 of it. Pre-loading in a direction towards the end position of the piston 14 in its bore 13 is produced by a compression spring 16. The atmospheric pressure which acts on the face 15 of the piston 14 thus forms a pressure which is independent of system pressure.The piston 14 has a portion 17 of smaller diameter which extends outwards from the housing 6 through a bore 18. The O-ring 19 serves as a seal. To avoid compression arising in the closed part 20 of the bore 13, recesses 21 are provided in the piston 14. This embodiment envisages a single throttling crosssectional area 12 which is effective to an equal extent in the extension and contraction phases.

On an increase in the loading, more fluid under pressure must be fed into the pipe 2 from the pump 1, so as to pump the vehicle structure up to its desired height. There is then an increased pressure in the system. The piston 14 which is subjected to the pre-load of the compression spring 16 is acted on over its exposed diameter 17 sealed from the atmosphere and so the compression spring 16 is further loaded and the effective throttling cross-sectional area 12 is reduced. The reduction in the effective throttling cross-sectional area 12 increases the damping force so that there is an improvement in the behaviour of the vehicle when it is fully laden.

The pre-load of the compression spring 16 and the face 15 of the piston 14 are variable quantitites which can be matched to the respective type of vehicle and the conversion ratios (wheel travel to cylinder travel). When the maximum pressure is reached the compression spring 16 comes up against a stop and in this way limits the minimum opening of the effective throttling cross-sectional area 12. The individual nature of the damping valves 11 ensures a predetermined basic damping of the strut 5.

Figure 3 illustrates the upper part of a telescopic spring strut 5 of which the damping piston 10 operates without damping valves, the exchange of fluid from the upper chamber 7 to the lower chamber 8 being by means of a constant oil passage 23.

Separate bores 13a and 13b are provided for the damping in extension and compression respectively. The piston 14a serves for damping in compression and the piston 14b for damping on extension. The effective throttling cross-sectional areas 12a and 12b are arranged accordingly. Also in this embodiemnt the pistons 14a and 14b have end faces 15a and 15b exposed to atmosphere and corresponding compression springs 16a and 16b.

The closed portions 20a and 20b of the bores 13a and 13b each have a provision for pressure balance. The closed portion 20a is provided with an orifice 24 leading into the pipe 2 for pressure balance in the compression phase and the closed portion 20b is provided with an orifice 25 leading into the upper working chamber 7 for pressure balance in the extension phase.

The manner of operation of this embodiment corresponds to that shown in Figure 2, namely when the load is increased oil is pumped into the upper working chamber 7 from the pump 1 and the pipe 2 until the vehicle structure reaches the required height. With increased build-up of the pressure both springs 16a and 16b are compressed against the atmospheric pressure which acts on the faces 15a and 15b, and the effective throttling cross-sectional areas 12a and 12b are reduced. By choice of different dimensions for the springs 16a and 16b and also for the faces 15a and 15b different damping characteristics can be obtained in the extension and contraction phases. By virture of the orifices 24 and 25 the respective throttling crosssectional areas can react to damping pressures, so that an additional influence on the damping characteristic can be obtained.

Figure 4 shows a further embodiment in principle like that already illustrated in Figure 3 but with the difference that the faces 15a and 15b are correspondingly increased in size and are exposed to atmospheric pressure only through the openings 26a and 26b. In this embodiment this pistons 14a and 14b have three cylindrical portions. For controlling the fluid a rotary valve 27 is provided for predetermined additional functions. Again the pressure fluid pipe 2 is comparable in principle with that of the third embodiment. In addition there are pipes 28 arranged so that they act on the cylindrical regions 29a and 29b of the pistons 14a and 14b. The pipe 30 and the reservoir 31 serve to compensate for the volume of the fluid moved to and fro by the pistons 14a and 14b, symbolising in this case simultaneous exposure to atmospheric pressure.

In the position of the valve 27 illustrated in Figure 4 the hydropneumatic suspension operates as already described with reference to Figure 3.

Figure 5 shows three different positions for the valve 27. The position shown in Figure 5a corresponds to that already shown in Figure 4 and it corresponds likewise to the embodiment shown in Figure 3.

Figure 5b shows a position in which the pressure from the pressure reservoir passes through the pipe 2 to the pipes 28 and then to the cylindrical regions 29a and 29b of the pistons 14a and 14b, as they are shown in Figure 4. This achieves the result that the damping of the vehicle is increased by a predetermined amount, i.e. is taughtened whilst maintaining the load-dependent action of the damping. In this higher damping force mode, higher and lower damping forces are automatically regulated in accordance with the load on the vehicle.

Figure Sc shows the rotary valve 27 in a position in which an external control pressure can be applied from the pipe 32 to the cylindrical regions 29a and 29b of the pistons 14a and 14b so that likewise a load-dependent increase in the damping can be achieved up to the point of locking. The external control pressure in the pipe 32 is backed by a gas cushion 33 in order to allow that the pistons 14a and 14b to work in an axial direction. The locking of the pistons 14a and 14b and therefore the blocking of the telescopic spring strut from the pressure reservoir 4 makes it possible for the struts to serve as a rigid support in, for example, utility vehicles with crane structures so that the tilting moment cannot be exposed to any unfavourable influence by contraction of the springs.

In Figure 6 there is illustrated diagrammatically an embodiment of the version shown in Figure 2.

In detail it comprises a telescopic suspension strut cylinder 5 of which the interior is divided by the damping piston 10 into an upper working chamber 7 and lower working chamber 8. The fluid pressure pipe 2 leads through its connection 34 to the pressure reservoir 4 and through the connection 35 to the fluid reservoir 31 or pump 1. The piston 14 is acted on by the compression spring 16 and thereby influences the effective throttling crosssectional area 12. Compression in the closed part 20 of the bore 13 is prevented by the recess 21.

The supply of fluid to and from the upper working chamber 7 through the pipe 2 is achieved through an annular passage 36 together with a transverse bore 37.

The manner of operation of the embodiment of Figure 6 follows the principle illustrated already in Figure 2.

Claims

1. A hydropneumatic vehicle suspension with height control comprising at least two telescopic spring struts arranged in the region of the vehicle wheels between the vehicle structure and the axle, each telescopic spring strut being connected by a fluid pipe on the one hand to a fluid pressure pump and a control element for controlling the desired vehicle height and on the other hand to a pressure reservoir, and a throttle element regulating a throttling cross-sectional area in each fluid pipe in accordance with the load on the vehicle, the throttle element comprising at least one axially displaceable piston in a closed bore, a front face of the piston being exposed to a pressure which is independent of the pressure in the fluid pipe, the piston, under the action of the fluid pressure and the independent pressure, controlling the effective throttling cross-sectional area of the fluid pipe, and means being provided for balancing the pressure between the closed portion and the open portion of the bore.

2. A hydropneumatic vehicle suspension according to claim 1, in which there are two bores each with a respective piston, and the throttling cross-sectional areas of the extension phase and the contraction phase are controlled independently of one another.

3. A hydropneumatic vehicle suspension according to claim 1 or claim 2, in which the or each bore is provided in the telescopic spring strut.

4. A hydropneumatic vehicle suspension according to any preceding claim, in which the or each piston has at least two cylindrical portions of different diameters.

5. A hydropneumatic vehicle suspension according to any preceding claim, in which the or each piston is urged by a spring in the direction of maximum throttling cross-sectional area.

6. A hydropneumatic vehicle suspension according to any preceding claim, in which the independent pressure which acts on the front face of the or each piston is atmospheric pressure.

7. A hydropneumatic vehicle suspension according to any of claims 1 to 5, in which the independent pressure which acts on the front face of the or each piston is generated by a fluid pressure pump.

8. A hydropneumatic vehicle suspension according to claim 1, in which the throttling crosssectional area comprises at least one opening from the bore into the upper working chamber of the te lescopic suspension strut cylinder.

9. A hydropneumatic vehicle suspension according to any preceding claim, in which the means for balancing the pressure comprises at least one orifice opening from the closed portion of the bore into the fluid pipe and/or into the work ing chamber of the telescopic spring strut cylinder.

10. A hydropneumatic vehicle suspension according to any of claims 1 to 8, in which the means for balancing the pressure comprises at least one recess in the piston connecting the two parts of the bore.

11. A hydropneumatic vehicle suspension with height control substantially as described herein with reference to Figures 1 and 2 of the accompanying drawings.

12. A hydropneumatic vehicle suspension with height control substantially as described herein with reference to Figures 1 and 3 of the accompanying drawings.

13. A hydropneumatic vehicle suspension with height control substantially as described herein with reference to Figures 1, 4 and 5 of the accompanying drawings.

14. A hydropneumatic vehicle suspension unit with height control substantially as described herein with reference to Figures 1 and 2 and as illustrated by Figure 6 of the accompanying drawings.