GB2115756A

GB2115756A - Improvements in vehicle hydraulic suspension systems

Info

Publication number: GB2115756A
Application number: GB08236016A
Authority: GB
Inventors: Anthony William Harrison
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1982-01-13
Filing date: 1982-12-17
Publication date: 1983-09-14
Also published as: GB2115755A; FR2519781B1; FR2519781A1; GB2115756B; FR2519720A1; GB2115755B

Abstract

In a vehicle hydraulic suspension system in which a vehicle body is supported by at least two hydraulic suspension struts (26, 27, 28, 29) including suspension springs and operated from a common supply (30) of hydraulic fluid, the length of the struts (26, 27, 28, 29) being adjustable, by displacement of hydraulic fluid into and out of the struts (26, 27, 28, 29), to raise and lower the vehicle body, each strut (26, 27, 28, 29) has an associated control member (31, 32, 33, 34), located between the supply (30) and the strut (26, 27, 28, 29), and movable through a limited distance in order to control displacement of fluid into and out of the associated strut (26, 27, 28, 29). The system is further characterised in that movement of each control member (31, 32, 33, 34) is responsive to the pressure differential across the control member (31, 32, 33, 34), and the system incorporates synchronising means (37) for synchronising operation of the control members (31, 32, 33, 34) to provide an equal rate of adjustment of the length of all the struts (26, 27, 28, 29). <IMAGE>

Description

1

GB 2 115 756 A 1

SPECIFICATION

Improvements in vehicle hydraulic suspension systems

This invention relates to vehicle hydraulic 5 suspension systems of the kind in which a vehicle body is supported by at least two hydraulic suspension struts including suspension springs and operated from a common supply of hydraulic fluid, the length of the struts being adjustable, by 10 displacement of hydraulic fluid into and out of the struts, to raise and lower the vehicle body.

Suspension systems of the kind set forth may be used on buses to lower the body when the bus is stationary to facilitate the entry and exit of 15 passengers, and then to raise the body before the bus moves off. It is important to ensure that the attitude of the body is stable when it is being lowered or raised, which means that the rate of adjustment of the length of all the struts must be 20 controlled, and must be independent of the loading on the struts. If the rate is not controlled, fluid tends to displace more quickly into a lightly loaded strut than into a heavily loaded one, when the body is being raised, and more quickly out of a 25 heavily loaded strut than out of a lightly loaded one during a lowering operation. Clearly, this would alter the attitude of the body during adjustment.

According to our invention, in a vehicle 30 hydraulic suspension system of the kind set forth each strut has an associated control member, located between the supply and the strut, and movable through a limited distance in order to control displacement of fluid into and out of the 35 associated strut, movement of each control member being responsive to the pressure differential across the control member, and the system incorporates synchronising means for synchronising operation of the control members to 40 provide an equal rate of adjustment of the length of all the struts.

Thus the provision of the synchronising means ensures that the attitude of the body is stable during adjustment of the length of the struts. 45 Conveniently the pressure of the fluid in the supply is increased in order to raise the body, and is decreased in order to lower the body. The supply is normally at a relatively high pressure to keep the body in its raised position. 50 Preferably each control member comprises a displacement piston which is able to move freely between opposite ends of an associated bore, and is exposed on one side to the pressure of the supply, and on the other side to the fluid in the 55 associated strut.

The synchronising means may comprise two-way fluid flow regulators, with one regulator being located between each control member and its associated strut.

60 Conveniently, the regulator comprises a fluid flow regulator having a constant restriction connected in series with a variable restriction defined between two relatively movable members. Fluid flow through the regulator produces a pressure differential across the constant restriction which controls relative movement of the members to determine the size of the variable restriction such that an increase in the pressure differential causes a reduction in the size of the variable restriction. Resilient means is operative to bias a movable one of the relatively movable members against the action of the pressure differential across the constant restriction for each direction of fluid flow through the regulator, so that for each direction of fluid flow, relative movement of the relatively movable members is controlled such that an increase in the pressure differential causes a reduction in the size of the variable restriction.

With such a regulator when the body is to be lowered, for example, the strut with the most load has the greatest pressure differential across its control member and across the constant restriction of its regulator, so that the fluid flow from the strut through the regulator is the more restricted, thus ensuring that the rate of adjustment of the struts is equal.

Alternatively, the synchronising means may comprise a mechanical connection between the control members, which ensures that they move at the same rate to control the rate of adjustment.

One embodiment of our invention and some modifications are shown in the accompanying drawings, in which:—

Figure 1 shows a schematic representation of a vehicle hydraulic suspension system incorporating synchronising means comprising two-way fluid flow regulators;

Figure 2 is a longitudinal section through a fluid flow regulator which may be incorporated in the vehicle hydraulic suspension system of Figure 1 ;

Figure 3 shows the regulator of Figure 2 in a different mode of operation; and

Figure 4 shows a further fluid flow regulator which may be incorporated in the suspension system of Figure 1.

The suspension system of Figure 1 is adapted to support a vehicle body (not shown), and has four hydraulic suspension struts 26,27,28,29, each of which includes a gas or steel spring (not shown) for use in normal suspension operation. The length of each strut is adjustable, by displacing hydraulic fluid into and out of the struts, to raise and lower the vehicle body. The length of the struts 26,27,28,29 is adjusted from a common supply of fluid 30. Each strut is connected to the supply 30 through an associated control member 31, 32, 33, 34, each of which comprises a displacement piston 35 which has limited movement in a bore 39 in a cylinder 36, the movement being controlled by the pressure differential across it. A synchronising means 37 is provided, in the form of a two-way fluid flow regulator 38 located between each strut and its associated control member. The synchronising means 37 ensures that the rate of adjustment is the same for each strut, independent of the loads on the struts. The synchronising means 37 thus equalises the rate of adjustment of the struts, in

65

70

75

80

85

90

95

100

105

110

115

120

125

2

GB 2 115 756 A 2

order to maintain the body in a stable attitude during the raising and lowering operations. If the rates were not equalised, fluid would tend to displace more quickly into a lightly loaded strut 5 than into a more heavily loaded one, but less quickly out of the lightly loaded strut than out of the more heavily loaded one. Clearly, this would alter the attitude of the body during adjustment.

A system such as this is suitable for use on a 10 bus. When the bus is moving the body is in a raised position, but it can be lowered when the bus is stationary to facilitate the entry and exit of the passengers.

Thus in operation, the pressure of the fluid in 15 the supply 30 is higher than that in the struts 26, 27,28, 29, so that each displacement piston 35 abuts the right hand end of its cylinder 36.

When the body is to be lowered, requiring fluid to be displaced from the struts, the pressure of the 20 fluid in the supply 30 is reduced rapidly, to a value below the pressure of the fluid in the struts. This reverses the pressure differential across the displacement pistons 35 so that they move leftwardly, allowing fluid to be displaced from the 25 struts, through the regulators 38. The regulators 38 act to ensure that the rate of adjustment is the same for all the struts. If one strut, say 26, is more heavily loaded than the others, the pressure differential between the supply 30 and that strut 30 26 will be greater than that for the other struts. Fluid tends therefore, to be displaced more quickly from the strut 26 than from the others, but the regulator restricts the fluid flow, ensuring that the rate of adjustment will be the same as for the 35 other struts, irrespective of their individual loading.

Similarly, when the body is to be raised, the pressure of the supply 30 is increased rapidly again, which operates the displacement pistons 35 to displace fluid into the struts through the 40 regulators 38, which act to synchronise the rate of adjustment of the length of the struts.

Using the two-way fluid flow regulators 38 means that the construction of the system is relatively simple.

45 Preferably each fluid flow regulator comprises that shown in Figures 2 and 3.

The regulator shown in Figures 2 and 3 comprises a housing 1 provided with a stepped bore 2, in which a piston 3 works. The piston 3 50 and the wall of the bore 2 comprise the relatively movable members of the regulator. The piston 3 has a bore 4, which forms a constant restriction 5 at one end. When fluid flows through the bore 4 and through the constant restriction 5 a pressure 55 drop occurs across the constant restriction 5, and the pressure differential acting on the piston 3 controls the movement of the piston 3 in the bore 2. Radial passages 6 lead from the piston bore 4 to an annular recess 7 on the piston 3. The annular 60 recess 7, together with a passage 8 in the housing 1 leading to the bore 2 defines a variable restriction 9 between one edge of the recess 7 and a corresponding oppositely facing wall of the passage 8. A further passage 10 in the housing 1 65 leads from the bore 2, so that fluid flow into and out of the regulator is by the passages 8 and 10.

The piston 3 is biassed against the action of the pressure differential for each direction of fluid flow by resilient means comprising first and second 70 compression springs 11,12 respectively. Each spring 11, 12 is caged, and abuts at its outer end against the housing 1, and at its inner end against an annular abutment ring 13, 14. Each spring is able to act on the piston 3 through a respective 75 abutment ring 13,14, against the action of the pressure differential on the piston 3 for one direction of fluid flow. Thus each spring 11, 12 is active for one direction of fluid flow only, as can be seen in Figures 2 and 3, spring 11 being active in 80 Figure 2, and spring 12 in Figure 3. For the other direction of fluid flow the spring is inactive, and does not affect the piston 3 as the annular ring 13 or 14 abuts a step 15 or 16 in the bore 2.

The regulator is shown in two of its operative 85 positions in Figures 2 and 3. In Figure 2 fluid flows into the regulator through the passage 8, through the variable restriction 9, then through the constant restriction 5 and flows out through passage 10. Fluid flow through the constant 90 restriction 5 creates a pressure drop across the restriction, with the pressure on the upstream side being greater than that on the downstream side of the restriction 5. The pressure differential acts on the piston 3 to move it against the action of the 95 active spring 11, thus varying the size of the variable restriction 9. Clearly, the greater the pressure differential across the constant restriction 5, the smaller the size of the variable restriction 9 becomes. The spring 12 is inactive, as 100 the annular ring 14 is in abutment with the step 16 in the bore 2.

In Figure 3 the fluid flow is in the opposite direction, and the operation of the regulator is similar to that of Figure 2, with the spring 12 105 being active to bias the piston 3 against the action of the pressure differential to control the size of the variable restriction 9, and the spring 11 being inactive.

The regulator described above uses caged 110 springs but in a modification, any other type of resilient means could be used, such as uncaged springs. However, two requirements affecting the resilient means must be taken into account, and these requirements mean that the use of caged 115 springs has some advantages.

Firstly, it is desirable to minimise the lost-motion in the travel of the piston which occurs when the direction of fluid flow is reversed, that is the distance the piston travels between a position 120 for one direction of fluid flow, and a similar position for the other direction of fluid flow. Secondly, it is desirable to maintain a uniform flow of fluid through the regulator. If uncaged springs are used, a high spring rate is necessary to 125 minimise the lost-motion in the piston travel, but a low spring rate is necessary to maintain uniform flow. Thus if uncaged springs are used the spring rate has to be a compromise with respect to these two requirements. However, if caged springs are 130 used the first requirement can be met by the

3

GB 2 115 756 A 3

provision of the abutments for the springs (such as 15,16 in Figures 2 and 3), which ensures that the lost-motion is minimised, while the spring rates can be chosen to ensure uniform fluid flow.

5 Thus the use of caged springs, as in the regulator of Figures 2 and 3, has the advantage that the regulator operates efficiently, and also gives freedom from the effect of manufacturing tolerances.

10 The regulator shown in Figure 4 is a modification of that shown in Figures 2 and 3, and corresponding reference numerals have been applied to corresponding parts.

Thus the regulator of Figure 4 has a housing 1, 15 provided with a bore 2 in which a piston 3 works. The passages 8 and 10 are similar to those of Figure 2, as are the constant restriction 5 and the variable restriction 9.

The resilient means which biasses the piston 20 against the action of the pressure differential for each direction of fluid flow comprises a single spring 17, which acts both in compression and tension. The ends 18, 19 of the spring 17 are of reduced diameter and pitch, and each end has a 25 projecting part 20,21 respectively. The end 18 is mounted on a threaded extension 22 of the piston 3, and the end 19 is mounted on a threaded extension 23 of a plug member 24 which is screwed into one end of the bore 2. Normally the 30 ends 18,19 of the spring 17 grip their respective extensions 22, 23, in order that forces can be transmitted between the piston 3 and the spring 17 in both directions in operation. However, the length of the spring 17 can be adjusted by rotating 35 the extensions 22, 23. The piston 3 is provided with slots 25 on the end opposite the extension 22 which can be used to assist in the adjusting operation.

The operation of the regulator of Figure 4 is 40 similar to that of Figures 2 and 3. The length of the spring 17 is adjusted initially to ensure that the inoperative position of the piston 3 is correct, as shown in Figure 4.

In operation, when fluid flows into the regulator 45 through the passage 8, the pressure differential across the constant restriction 5 acts to move the piston 3 towards the passage 10. The spring 17 acts in tension against the action of the pressure differential to regulate the size of the variable 50 restriction 9. With fluid flow in the opposite direction the spring 17 acts in compression on the piston 3 against the action of the pressure differential.

In a modification of the suspension system of 55 Figure 1 (not shown) the synchronising means comprises a mechanical interconnection between the displacement pistons 35 to ensure that they move at the same rate, which thus means that the rate of adjustment of all the struts is the same. 60 The system shown in the drawing has four struts, but clearly any suitable number of struts, above the minimum of two, could be operated from the fluid supply 30. The struts may have equal effective areas, or they may have different 65 effective areas corresponding to the weight distribution of the vehicle. In the latter case the areas or the strokes of the displacement pistons may be altered to accommodate the different volumes of fluid which are displaced during adjustment. The suspension struts may also be of the self-levelling type, which include means for automatically maintaining the attitude of the body, regardless of changes in load on the body.

Claims

1. A vehicle hydraulic suspension system of the kind set forth, in which each strut has an associated control member, located between the supply and the strut, and movable through a limited distance in order to control displacement of fluid into and out of the associated strut, movement of each control member being responsive to the pressure differential across the control member, and the system incorporates synchronising means for synchronising operation of the control members to provide an equal rate of adjustment of the length of all the struts.

2. A vehicle hydraulic suspension system as claimed in claim 1, in which the pressure of the fluid in the supply is increased in order to raise the body, and is decreased in order to lower the body, with the supply normally at a relatively high pressure to keep the body in its raised position.

3. A vehicle hydraulic suspension system as claimed in claim 1 or claim 2, in which each control member comprises a displacement piston which is able to move freely between opposite ends of an associated bore, and is exposed on one side to the pressure of the supply, and on the other side to the fluid in the associated strut.

4. A vehicle hydraulic suspension as claimed in any preceding claim, in which the synchronising means comprises two-way fluid flow regulators, with one regulator being located between each control member and its associated strut.

5. A vehicle hydraulic suspension system as claimed in claim 4, in which each regulator has a constant restriction connected in series with a variable restriction defined between two relatively movable members, fluid flow through the regulator producing a pressure differential across the constant restriction which controls relative movement of the members to determine the size of the variable restriction such that an increase in the pressure differential causes a reduction in the size of the variable restriction, and the regulator incorporates resilient means operative to bias a movable one of the relatively movable members against the action of the pressure differential across the constant restriction for each direction of fluid flow through the regulator, so that for each direction of fluid flow, relative movement of the relatively movable members is controlled such • that an increase in the pressure differential causes a reduction in the size of the variable restriction.

6. A vehicle hydraulic suspension system as claimed in any of claims 1 to 3, in which the synchronising means comprises a mechanical connection between the control members, which

70

75

80

85

90

95

100

105

110

115

120

125

4

GB 2 115 756 A 4

ensures that they move at the same rate to control substantially as herein described with reference to the rate of adjustment. 5 and as illustrated in Figure 1 of the accompanying

7. A vehicle hydraulic suspension system drawings.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1983. Published by the Patent Office 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.