GB2282864A

GB2282864A - Hydraulic shock absorber

Info

Publication number: GB2282864A
Application number: GB9420719A
Authority: GB
Inventors: Hassan Asadi; Michael Hurrlein; Andreas Foerster
Original assignee: Fichtel and Sachs AG
Current assignee: ZF Friedrichshafen AG
Priority date: 1993-10-15
Filing date: 1994-10-14
Publication date: 1995-04-19
Anticipated expiration: 2014-10-14
Also published as: DE4424991A1; FR2711201A1; GB9420719D0; GB2282864B; FR2711201B1

Description

2282864 1 HYDRAULIC SHOCK ABSORBER The invention relates to an hydraulic

shock absorber for a vehicle suspension system of the kind comprising a piston working in a cylinder and including damping means for damping the flow of fluid on extension and contraction of the piston, a damping force of the damping means being adapted to be varied in accordance with the hydraulic pressure in the suspension system dependent on the loading of the vehicle.

Such a shock absorber is known for example from DE 34 06 032. For load-dependent alteration of the damping force of a damping valve there is a control piston which is exposed on one side to the internal pressure of the suspension and on the other side to atmospheric pressure. Movement of the control piston alters the pre-load of a valve spring and increases or reduces a bypass connected in parallel with a damping piston. This enables the pre-load of only one damping valve to be altered. The adjustment of the bypass cross-section which bridges the damping valve presents real problems as such a bypass has only a small flow cross-section, which needs to be adjusted over a large pressure range, e.g. between 30 and 90 bar, with sufficient accuracy. Accordingly, a sensitive pre s sure -dependent alteration of the damping for may not be achieved with such an arrangement or only at great expense, as the components necessary for controlling the cross-section of the bypass need to be manufactured with extreme accuracy.

DE-36 12 006 also shows a load-dependent shock absorber having a damper piston which is movable axially between sets of springs. The tuning or setting 2 of the damping valve is relatively difficult. Moreover the spread of load- dependent damping is limited.

DE-OS 36 01 445 describes a load-dependent damping means for a hydropneumatic suspension strut in which the flow cross-section is altered in accordance with pressure, but the alteration of the damping force is of little effect, because the damping disc v alve is not variable. The pressure-dependent flow cross-section matches a load condition. The main damping force which is provided by the disc valve, however, remains invariable and accordingly independent of load.

It is an aim of the present invention to overcome with simple means the drawbacks known from the state of the art, and in particular to provide as large as possible a load-dependent increase in the damping force.

According to the present invention in an hydraulic shock absorber of the kind set forth the damping means includes a spring-loaded disc valve for damping in extension and contraction and passage means with a variable cross-section for fluid flow, in which spring-loading of the valve and the cross-section of the passage means are varied in accordance with the hydraulic pressure, the arrangement being such that for a given pressure the damping force characteristic has a first rising portion where fluid flows only through the passage means, and a second substantially constant portion where' fluid flows also through the disc valve, with a knee point between the first and second portions, the knee points lying on a line determined by the characteristic of the disc valve spring and the geometry of the cross-section of the passage means.

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3 This achieves damping force characteristics suitable for a shock absorber, matched to the load over its entire extent, as large flow cross-sections are available for the damping fluid.

Preferably, fluid flows through the damping means in one direction only. The construction of the damping valve is decisively simplified by the single direction of flow. In addition it is possible to estimate accurately what volume flows from the damping means need to be controlled as there is no bypass within the damping valve.

Preferably the variable cross-section of the passage means is formed by the cross-section of an axially movable control piston in conjunction with a passage. The desired characteristics can be achieved by altering the cross-section. The control piston is displaced axially in response to the system pressure. As the loading on the vehicle increases, the crosssection for flow is reduced with the consequence that the effect of the cross-section for flow and thereby the damping action is increased. In this respect there are no practical limitations to the design.

For example the control piston has over a defined length a constriction which passes through the passage in the form of a control bore in the damping means. This achieves a coaxial arrangement of the valve components which produce the damping force characteristic, so that a compact damping device is obtained.

In one embodiment the damping means includes a of non-return valves which control the 4 direction of flow within connecting passages between an accumulator for the suspension system and the damping valve and between the shock absorber cylinder and the damping valve. The shock absorber may have a base valve including at least one of the non-return valves. The damping valve may be connected to the accumulator through a bypass including at least one of the non-return valves.

Alternatively, in a simpler construction, the damping valve is connected to the shock absorber by a bypass. In such an arrangement the bypass is formed simply between the housing of the shock absorber and a concentrically arranged outer tube, with a bypass bore forming a connection between the cylinder of the shock absorber and the bypass.

So that there is no need to provide pipes, which would be exposed to impact f rom stones, in the wing of the vehicle and which again increase the space taken up by the connections, the damping means is arranged in a housing connected rigidly to the shock absorber.

Various embodiments of the invention are shown, by way of example, in the accompanying drawings, in which:- Figure 1 is a longitudinal section through a hydropneumatic suspension strut; Figure 2 shows the load-dependent damping means of Figure 1; Figure 3 shows a load-dependent damping means with non-return valves; Figure 4 is a block circuit diagram of the hydropneumatic suspension of Figure 3; and Figure 5 is a graph showing the damping force characteristic of the damping means.

Figure 1 shows a shock absorber in the form of an hydraulic cylinder 1 of a hydropneumatic suspension strut connected through a pressure pipe 3 to an accumulator 5 which is under the pre-load of a gas cushion 7. The cylinder 1 has a housing 9 which is enclosed concentrically in an outer tube 11. A base component 13 and a piston rod guide 15 having seals close the cylinder 1. Within the housing 9 there is arranged an axially movable piston rod 17 carrying a piston 19 which divides the housing 9 into upper and lower working chambers 21, 23. The piston 19 has damping valves 25, 27 for the extension and contraction directions. In additional a base valve 29 is installed within the housing 9.

The base valve 29 is connected by a passage 31 to a load-dependent damping means in the form of a valve 33 which is arranged within the base component 13. A bypass 35, formed by an annular space between the housing 9 and the outer tube 11, communicates with the upper working chamber 21 through a bypass bore 37.

Figure 2 shows the damping valve 33 in more detail. The damping valve 33 has a damping valve body 39 located between a supporting face 41 on the base component 13 and an attachment sleeve 43. The damping valve body 39 has a number of flow passages 45, closed-off by one or more discs 47 engaged by a spring 49 which in its turn is connected through a plate 51 to a control piston 53. The control piston 53 passes 6 through a control bore 55 in the damping valve body 39 and an outlet oper. --ng 57 in the base component 13 so that its upper face 59 is exposed through this opening to atmospheric pressure. An adjusting spring 61 acts on the control piston 53. Its lower face 63, in the region of the spring plate 51, is exposed to the suspension system pressure of the accumulator 5, acting on the hydropneumatic suspension strut. The control piston 53 is therefore axially movable in response to the system pressure which alters with the load of the vehicle. Movement of the piston 53 also alters the loading in the springs 49 and 61 to alter the point at which the valve discs 47 open.

The control piston 53 has a constriction which, together with the control bore 55 defines a passage means 67 with a variable flow cross-section. As the axially movable control piston 53 is displaced under increasing loaddependent system pressure.. the cross-section of the passage means 67 is reduced so that a damping force characteristic with a steep rise in accordance with requirements can always be set. Increasing system pressure increases the load in the springs 49 and 61 to increase the resistance to flow. The attachment sleeve 43 has a number of openings 69 which allow the damping fluid to flow into the passage means 67.

On a suspension movement in the extension direction the whole compressed volume of working chamber 21 passes through the bypass bore 37, into the bypass 35 and then into the base component 13. In accordance with the suspension movement the damping fluid flows through the passage means 67 and through the flow passages 45 when the valve disc 47 opens at the appropriate point for the system pressure, to i- 7 provide the required damping force characteristic as will be explained below.

In the contraction direction of suspension movement, a part of the displaced fluid volume is forced through the bypass bore 37 while the other part flows through the base valve. The flow path through the loaddependent damping valve 33 is identical with the flow path in extension, and the direction of flow is the same as is the operation of the valve to provide the appropriate damping forces. For this purpose the disc 47 blocks any flow from the connecting pipe 3 and the passage 31 in the region of the base valve 29 into the bypass 35.

Figure 3 shows a modification of Figures 1 and 2, and corresponding reference numerals have been applied to corresponding parts. In the embodiment of Figure 3 the shock absorber has only one housing 9. As in Figures 1 and 2, the base component 13 has a damping valve 33. However, in Figure 3 the damping valve 33 has a series of non-return valves (R 1 - R 4) arranged in various connecting passages, which ensure that only one direction of flow is possible through the valve 33. For this purpose two oppositely-acting non-return valves (R,,, R 2) are integrated into the base valve, and the valve R 1 also forms a damping valve. Two further nonreturn valves (R 3 ' R 4) are provided. R 4 opens a bypass 71 from the accumulator 5 to the load-dependent damping valve 33 but closes off the path from the load-dependent damping valve 33 through the bypass 71 to the accumulator 5, and R 3 opens the direct path from the load-dependent damping valve 33 to the accumulator 5. The valve 33 is connected to the base valve through a passage 73 and an annular groove 75 in the body of the base valve 8 connects the passage 73 to the valve R 2 The construction of the valve 33, with the control piston 53, control bore 55 and spring-loaded valve disc 47 is the same as that of Figures 1 and 2.

On inward movement (contraction) of the piston rod the volume of the piston rod is forced through the base valve 29 into the base component 13. In this event the damping fluid flows through the non-return valve R 1 and via the flow connection 31 into the base component 13. The disc 47 then lifts away from the damping valve body 39 so that subsequently the damping fluid opens the non-return valve R 3 and allows escape into the accumulator 5.

On movement of the piston rod in an outward direction (extension) the volume of the upper working chamber 21 is made up through the bypass 71 and the non-return valve R 4 In this case this volume flows again through the load-dependent valve 33 to the passage 73, whereby the non-return valve R 1 closes and the non-return valve R 2 opens.

The manner of operation of the load-dependent damping valve 33 is therefore identical with the embodiment of Figures 1 and 2. The bypass 35 is simply displaced from the cylinder of Figure 1 into the base component 13. Figure 4 shows the hydraulic block circuit diagram of Figure 3. In both embodiments the load-dependent damping valve 33 is arranged within the base component so that a complete unit can be fitted.

Figure 5 shows the damping force characteristic of the load-dependent shock absorber. In this arrangement the entire range included between the two outer characteristics can be covered without steps.

9 Each line on the graph represents a given system pressure, dependent on vehicle load. For example, the top line represents a high vehicle load, for which the system pressure is high, and the passage means 67 has a small cross-section for flow which determines the initial relatively steep rise. When a defined pressure is exceeded at a velocity V the valve disc 47 lifts off and the substantially flat branch of the characteristic is introduced. This part of the characteristic is also dependent on the loading of the vehicle and on the pressure difference between the accumulator and atmosphere. Between the two portions of the characteristic, at the point of opening of the valve disc 47 there is a knee point, which can be determined by tuning the cross-section of the passage means and the disc valve. The line joining the knee points for each system pressure can be varied as required, to form a straight line or a curve.

Claims

1. An hydraulic shock absorber of the kind set forth, in which the damping means includes a spring-loaded disc valve for damping in extension and contraction and passage means with a variable cross-section for fluid flow, in which spring-loading of the valve and the cross- section of the passage means are varied in accordance with the hydraulic pressure, the arrangement being such that for a given pressure the damping force characteristic has a first rising portion where fluid flows only through the passage means, and a second substantially constant portion where fluid flows also through the disc valve, with a knee point between the first and second portions, the knee points lying on a line determined by the characteristic of the disc valve spring and the geometry of the cross-section of the passage means.

2. An hydraulic shock absorber as claimed in claim 1, in which fluid flows through the damping means in one direction only.

3. An hydraulic shock absorber as claimed in claim 1 or claim 2, in which the variable cross-section of the passage means is formed by the crosssection of an axially movable control piston in conjunction with a passage.

4. An hydraulic shock absorber as claimed in claim 2, in which the control piston has a constriction over a defined length, the constriction passing through the passage in the form of a control bore in the damping means.

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5. An hydraulic shock absorber as claimed in any preceding claim, in which the damping means includes a series 0- non-return valves which control the direction of flow within connecting passages between an accumulator for the suspension system and the damping valve, and between the shock absorber cylinder and the damping valve.

6. An hydraulic shock absorber as claimed in claim 5, in which the shock absorber has a base valve including at least one of the non-return valves.

7. An hydraulic shock absorber as claimed in claim 5 or claim 6, in which the damping valve is connected to the accumulator through a bypass including at least one of the non-return valves.

8. An hydraulic shock absorber as claimed in any of claims 1 to 4, in which the damping means is connected to the shock absorber cylinder through a bypass.

9. An hydraulic shock absorber as claimed in claim 8, in which the bypass is formed between a housing of the shock absorber and a concentrically arranged outer tube, with a bypass bore forming a connection between the cylinder of the shock absorber and the bypass.

10. An hydraulic shock absorber as claimed in any preceding claim, in which the damping means is arranged in a housing connected rigidly to the shock absorber.

11. An hydraulic shock absorber of the kind set forth substantially as described herein with reference to and as illustrated in Figures 1, 2 and 5 of the accompanying drawings.

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12. An hydraulic shock absorber of the kind set forth substantially as described herein with reference to and as illustrated in Figures 3, 4 and 5 of the accompanying drawings.

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