GB2073366A

GB2073366A - Hydraulic shock absorbers

Info

Publication number: GB2073366A
Application number: GB8011530A
Authority: GB
Original assignee: Armstrong Patents Co Ltd
Current assignee: Armstrong Patents Co Ltd
Priority date: 1980-04-08
Filing date: 1980-04-08
Publication date: 1981-10-14

Abstract

The present specification discloses a twin-tube, telescopic hydraulic piston and cylinder apparatus for use as a vehicle shock absorber. In such shock absorbers hydraulic lock can occur in the event of high velocity large impacts being applied. This is clearly undesirable as it results in a hard ride. The present invention seeks to overcome the above problem by applying the hydraulic pressure prevailing in the compression chamber 11 to a displaceable wall 42 which is supported by resilient means 44 acting in series opposition to said hydraulic pressure. Thus the wall 42 can yield to create a temporary increase in volume of said compression chamber 11 whenever the hydraulic pressure in the compression chamber 11 exceeds a value predetermined by said resilient means 44. Thus the possibility of hydraulic lock and a hard ride is reduced. The wall may instead slide in the inner tube 10 and carry the foot valves 32, 34. <IMAGE>

Description

SPECIFICATION Improvements in and relating to hydraulic piston and cylinder apparatus This invention concerns hydraulic piston and cylinder apparatus and relates more particularly to vehicle wheel suspension units such as telescopic hydraulic shock absorbers and wheel suspension struts.

One widely used construction of telescopic hydraulic shock absorber, which is also often embodied in a suspension strut, comprises a pair of concentrically arranged cylinders, of which the inner cylinder has a hydraulic piston displaceable therein by a piston rod extending out of one end of that cylinder, the cylinder end being closed and also being located relative to the outer cylinder by a sealing gland assembly which also incoporates a piston rod bearing. The opposite end of the inner cylinder is closed and the two cylinders are located relative to one another at that end by a so-called foot valve assembly which enables the compression chamber constituting the space beneath the piston in the inner cylinder to communicate with the annular space between the two cylinders, acting as a fluid reservoir.

Many constructions of so-called twin tube shock absorbers have been widely and satisfactorily employed for many years in the vehicle suspension art and, apart from their higher initial cost than most shock absorbers using only a single tube, have many advantages to offer in controlling vehicle ride characteristics.

However, in common with virtually all other shock absorbers of any type, that have only a limited ability to deal with high speed transient shock loads and thus under some conditions of ride surface and of vehicle loading, they may still permit the vehicle to experience a hard ride.

It is an object of the invention to provide a shock absorber which is better able to accommodate the effects of transient high shock loads whilst otherwise maintaining ride characteristics which are conventional under normal or less extreme riding conditions.

According to the broadest aspect of the present invention, the hydraulic pressure prevailing in the compression chamber of a twin tube, telescopic hydraulic piston and cylinder apparatus is applied to a displaceable wall which is supported by a resilient force acting in series opposition to said hydraulic pressure and which permits said wall to yield to create a temporary increase in the volume of said compression chamber whenever the hydraulic pressure in the latter exceeds a value predetermined by said force.

For this purpose, the compression chamber may be connected to means such as a pressure accumulator situated either internally or externally of said apparatus and provided with the said displaceable wall. Where an external pressure accumulator is used, several individual piston and cylinder apparatus may, if desired, be connected to the same accumulator. A preferred construction, however, lies in an apparatus in which one wall, or at least a part of one wall, of the compression chamber is resiliently dispiaceable responsive to transient pressure peaks in said chamber when said apparatus is subjected to abrupt compressive load increases, the corresponding transient increases in the volume of the compression chamber and the resilient reaction of said displaceable wall acting to flatten said pressure peaks and to cushion the shock effect of said loads.

The displaceable wall may take the form of a piston member or separator resiliently sealing the lower end of the pressure cylinder with respect to the hydraulic fluid in the compression chamber and such separator may conveniently be arranged to include the usual foot valve assembly by which the compression chamber communicates with the hydraulic reservoir. Alternatively, the shock absorber piston rod may be hollow and may have its internal end bounding the compression chamber sealed by a floating piston or separator which is resiliently supported within the piston rod of the shock absorber itself. In either case, although the resilient support may be a spring, it may alternatively take the form of a charge of compressed gas within the space situated on the opposite side of the separator from the hydraulic fluid in the compression chamber.

The invention will be described further, by way of example, with reference to the accompanying, generally diagrammatic drawings, in which: Figure 1 is an axial section through a twin tube telescopic hydraulic shock absorber embodying the invention, Figure 2 is a similar section through a secorid embodiment, and Figure 3 is a fragmentary section of a modification of the shock absorber shown in Fig. 1.

The shock absorber shown in Fig. 1 will be seen to include a pair of coaxial cylinders 10 and 1 2 arranged one within the other and defining an annular space 14 between them.

Within the inner or pressure cylinder 10, there is displaceable a piston 1 6 which is fitted with conventional fluid transfer valves 1 8 and 20 and which is carried on a piston rod 22 extending out of the upper end of the cylinder 10 as viewed in the drawing. The upper end of the cylinder 10 and the same end of the cylinder 1 2 are closed by a rod guide and bearing 24 through which the piston rod 22 passes and which also acts to locate the two cylinder ends relative to one another. Fluid closure is provided by a buttress seal 26 or the like which is spring mounted within an end cover 28 secured to the reservoir tube 12.The opposite ends of the cylinders 10 and 1 2 are closed and located relative to one another by a foot valve assembly 30 fitted with conventional damping and recuperation valves 34 and 32 and the foot valve assembly is protected by an end cover 36 carried at the bottom end of the reservoir tube 1 2. The end cover 36 is provided with a fitting 40 and the free end of the piston rod with a fitting 38 by which the shock absorber may be mounted in a vehicle suspension.

Operation of the shock absorber described above is conventional, in that, on compression strokes of the piston 16, there is transfer of hydraulic fluid from the compression chamber 11, that is to say, from the space in cylinder 10 in front of the piston, to the hydraulic reservoir 14 through the foot valve assembly 30 and at a rate determined by the velocity of the closing stroke and the piston rod crosssectional area. During extension strokes of the piston the flow of hydraulic fluid is reversed and is under the control of the piston valve 20 and the foot valve 32, the piston valves serving to control the flow of fluid into and out of the annular space between the cylinder to and piston rod 22.

As will be appreciated, if the shock absorber is subjected to an abrupt compressive load, or to a sequence of such loads, as when a vehicle is travelling over an unusually rough surface, the piston and foot valves 1 8 and 34 are unable to handle the transient flow demands suddenly made on them and to all intents and purposes, the hydraulic fluid in the compression chamber 11 may, for brief instants of time, become solid. Even at compressive loads not normally sufficiently great to give rise to this effect, it may still occur if such lesser loads are rhythmically applied to the shock absorber and in consequence set up standing waves of compression in the fluid in chamber 11-.

With a view to alleviating this problem, the invention provided a piston rod 22 which is hollow but which has its inner end closed in fluid sealing manner by a floating piston or separator 42, the interior of the piston rod containing a charge 44 of gas under pressure.

The gas charge 44 is predetermined in relation to the internal volume of the piston rod 22 to exert a resilient pressure on the separator 42 counterbalancing the fluid pressure in the compression chamber 11 and maintaining the separator 42 seated as on a radially inwardly directed end flange 23 of the piston rod 22. This condition is maintained for such a time as the velocity of movement of the piston 1 6 in the cylinder 10 generates only such fluid pressures and/or requires only the displacement of such fluid quantities as lie within the capabilities of the piston valve 1 8 and the foot valve 34.At higher piston velocities, however, above some threshold value determined by the pressure of the gas charge 44, the fluid pressure in the compression chamber 11 will exceed the gas pressure and the separator 42 will be moved inwardly into the piston rod interior to temporarily increase the effective volume of the compression chamber 11 and permit a reduction of the fluid pressure therein. The probability of a hydraulic lock condition, or of conditions closely approaching a hydraulic lock in the compression chamber 11 is thus greatly reduced and in a vehicle suspension employing a shock absorber as described, the incidence of harsh ride characteristics, even on sus-' tained rough terrain, is greatly lowered without sacrificing damper parameters which it is desirable to retain for more normal ride surfaces.

In a modification of the invention, a coil spring (not shown) may be substituted for or used to augment the gas charge 44 to permit the variation in the effective volume of the compression chamber 11 and to provide the resilient reaction to the transient compressive loading of the shock absorber.

In another modification of the invention, the separator member, instead of being arranged in a hollow piston rod, is arranged in the pressure cylinder 10, between the piston 1 6 and the end cover 36, as shown at 46 in Fig.

2. The separator 46 sealingly co-operates with the piston 1 6 to define the compression chamber 11 whilst a space for a compressed gas charge 44 is defined between the end cover 36 and the separator 46. For convenience, the separator 46 also constitutes the foot valve assembly of the shock absorber and carries valves 48 and 50 which correspond respectively to the foot valves 32 and 34 of Fig. 1. In order to allow communication of the foot valve assembly with the reservoir 14, the separator 46 is made hollow and has ports 52 in its side wall permitting fluid flow to cooperating ports 54 in the cylinder 10. The cylinder 10 also carries a radially inwardly directed stop 56 to establish the end position normally adopted by the separator in the cylinder 10 under the influence of the gas pressure. In all other aspects, the construction and operation of this embodiment of the invention is identical to that described with reference to Fig 1.

In the modifications shown in Fig. 3, a resilient reaction or cushion effect is made effective on both compression and and rebound strokes of the shock absorber piston 1 6 by closing the inner end of the piston rod as indicated at 23 and by providing a port 25 in the piston rod between the closed end 23 and the separator piston 42. The space within the piston rod 22 between the separator piston 42 and the closed rod end 23 thus communicates with the annular space between the rod 22 and the pressure cylinder 10, the latter space being pressurized on both compression and rebound strokes, since on compression strokes, the piston valve 18 is open.

Claims

1. A twin-tube telescope hydraulic piston and cylinder apparatus comprising a compression chamber, the hydraulic pressure prevailing therein being applied to a displaceable wall which is supported by resilient means acting in series opposition to said hydraulic pressure and which can yield to create a temporary increase in volume of said compression chamber whenever the hydraulic pressure in the compression chamber exceeds a value predetermined by said resilient means.

2. Apparatus as claimed in claim 1, in which said compression chamber is connected to a hydraulic accumulator located externally of the twin-tubes of the piston and cylinder apparatus, which accumulator incorporates said displaceable wall.

3. Apparatus as claimed in claim 2, in which additional twin-tube telescopic piston and cylinder apparatus are connected to said hydraulic accumulator.

4. Apparatus as claimed in claim 1, in which at least a part of one wall of the compression chamber forms said displaceable wall.

5. Apparatus as claimed in claim 4, in which the piston is displaceable in a pressure cylinder and is connected to a hollow piston rod which houses a separator member which is axially movable within said piston rod and which forms said displaceable wall, hydraulic pressure prevailing in said pressure cylinder acting on one side of the separator and said resilient means acting on the other side of said separator.

6. Apparatus as claimed in claim 5, in which said hollow piston rod opens into the compression chamber.

7. Apparatus as claimed in claim 5, in which said hollow piston rod opens into the annular space between the piston rod and the cylinder.

8. Apparatus as claimed in claim 4, in which said piston is displaceable in a pressure cylinder and said displaceable wall is formed by a piston member which closes one end of the pressure cylinder with respect to the hydraulic pressure in the compression chamber.

9. Apparatus as claimed in claim 8, which said piston member incorporates a foot valve which controls hydraulic fluid flow between the compression chamber and a hydraulic fluid reservoir formed between said pressure cylinder and an outer cylinder within which said pressure cylinder is housed.

10. Apparatus as claimed in any one of the preceding claims, in which said resilient means is in the form of a spring.

11. Apparatus as claimed in any one of claims 1 to 9, in which said resilient means is in the form of a charge of compressed gas.

1 2. Apparatus as claimed in any one of claims 1 to 9, in which said resilient means is in the form of a charge of compressed gas aided by a spring.

1 3. A twin-tube, telescopic hydraulic piston and cylinder apparatus constructed substantially as hereinbefore described with reference to and as illustrated in Fig. 1, Fig. 2 or Fig. 3 of the accompanying drawings.