GB2346665A - Spring and damper unit - Google Patents

Abstract

A spring and damper unit (10) comprises two telescopically movable housings (11, 12) bounding an interior space formed by a cavity (16) in one of the housings and a cavity (23) in the other housing, and a floating piston (24) disposed in one of the cavities (23) and dividing the interior space into a spring chamber (27) containing a gaseous medium such as air and a damping chamber (26) containing a damping fluid such as hydraulic oil, the housings being movable together in a compression stroke thereof to compress the gaseous medium through reduction in the volume of the spring chamber (27) and the piston (24) being movable by the compressed gaseous medium to cause a reduction in the volume of the damping chamber (26) for displacement of fluid therefrom. The spring thus has a dual function in the unit. The piston (24) is preferably arranged to produce an increase in the volume of the spring chamber (27) concurrently with the reduction in the volume of the damping chamber (27), the increase in the spring chamber volume being less than the mentioned reduction in that volume. As a result, a more linear rising spring rate is achieved.

Description

SPRING AND DAMPER UNIT The present invention relates to a spring and damper unit and has particular reference to springing and damping in a compression phase of the unit.

Spring and damper units of coaxial mode of construction conventionally consist of a coil spring effective in compression and surrounding a double-acting hydraulic, pneumatic or gas telescopic damper. The damper normally has a rod carrying a piston which slides in a cylinder filled with fluid and the movement of which within the cylinder is resisted by throttled flow of the fluid through an opening or openings in the piston during passage from one side of the piston to the other. The damped movement of the piston is superimposed, in the mounted state of the unit, on the spring travel. The opening or openings in the piston can be influenced to provide different degrees of throttling between the compression phase and the rebound phase of the damper and also variable throttling in dependence on the rate of compression and/or rate of rebound. Such a unit is relatively bulky and heavy and allows limited, if any, access to the damper for adjustment purposes.

Also known are units consisting of a rubber spring combined with a diaphragm-actuated damper in which fluid is displaced through flow-throttling openings in a dividing wall.

These units are again bulky and heavy, since the mass of rubber provides a volume and weight penalty similar to a coil spring.

Some spring and damper units utilise gaseous and hydraulic media, with the gaseous medium, for example air, nitrogen or freon, separated from the hydraulic medium by a separator piston. Damping is performed, as in a conventional telescopic damper, by a damping piston carried by a rod and moving in cylinder filled with hydraulic fluid which is displaced through openings in the piston. The gaseous medium is compressible under movement of the separator piston and functions as a spring as well as an anti-cavitation measure in the case of higher rates of movement of the damping piston. Such a unit, in which cylinder chambers for the hydraulic medium and a chamber for the gaseous medium are in series, is long in relation to available stroke and has a large rising spring rate.

It is accordingly the object of the invention to provide a spring and damper unit which can be of relatively light and compact construction and can provide effective springing and damping respectively by a gaseous medium, such as air, and a damping fluid, such as hydraulic oil. A further object is the provision of a unit in which the spring exercises the additional function of actuation of the damper in the compression phase, so as to save components and consequently weight. Yet another object is the provision of a unit with a more linear rising rate of the spring and thus a more progressive spring characteristic of the unit. Other objects and advantages of the invention will be apparent from the following description.

According to a first aspect of the present invention there is provided a spring and damper unit comprising two telescopically movable housings containing a spring chamber for a gaseous medium which is compressible on movement of the housings together in a compression stroke thereof, a damping chamber for a damping fluid which is displaceable therefrom during the compression stroke, and a piston movable by such compressed gaseous medium to reduce the volume of the damping chamber thereby to cause said displacement.

In this unit, actuation of the piston responsible for displacing damping fluid for damping purposes is undertaken by the spring, in particular by the air or gas undergoing compression in the spring chamber during the compression stroke of the housings. The spring thus exerts a dual function and an exclusively mechanical actuation of the piston, such as by a rigidly attached piston rod as in a conventional telescopic damper, is not required.

Preferably, the piston is arranged to produce an increase in the volume of the spring chamber concurrently with the reduction in the volume of the damping chamber, the increase in the spring chamber volume being less than a reduction in that volume produced by the movement of the housings together. Thus, an interchange between the volume of the spring chamber and the volume of the damping chamber takes place during operation of the unit, in particular the decrease in the volume of the damping chamber to bring about the damping action is accompanied by an increase in the available volume of the spring chamber to partially offset the decrease produced by movement of the housings together. For a given set of parameters the total reduction in the spring chamber volume is consequently not as great as in a conventional air or gas spring and a more linear rising spring rate is obtained. In a particularly favourable constructional configuration the spring chamber is formed by part of a cavity in one of the housings and part of a cavity in the other housing and the damping chamber is formed by another part of the latter cavity. In that case, the piston is preferably disposed in the latter cavity and sealing means can be present to provide a seal between the piston and a surrounding circumferential wall. The piston is preferably arranged to separate the spring chamber from the damping chamber. Such a construction allows the housing to be of simple form, for example telescopically interengaging tubes with a piston located exclusively in one of he housings and serving, by itself, to separate the two fluid media. The piston is preferably floating and thus mechanically unrestrained, the sole restraint to movement being provided by the static and dynamic fluid forces prevailing in the chambers and by any friction generated by the sealing means.

For preference, however, the unit also comprises drive means effective on movement of the housings together at a rate exceeding a predetermined rate to mechanically positively drive the piston. In practice, a high speed of movement of the housings in a compression stroke may exceed the speed of damped movement of the piston, thus producing a lag in piston travel. To overcome this, mechanically positive driving of the piston can come into effect to take over from or assist the fluid drive by the compressed gaseous medium.

Such an auxiliary drive can be conveniently realised by a drive member drivingly engaging the piston after overcoming a spacing therefrom. If the rate of compression of the gaseous medium sufficiently exceeds the rate of movement of the piston, the spacing is overcome and the drive member, effectively a fixed abutment, acts to mechanically drive the piston.

The predetermined rate beyond which the drive means takes over or supplements the driving may be variable inter alia by variation of the spacing.

The unit may also include cushioning means for cushioning at least one of an end stage of movement of the housings together in the compression stroke and an end stage of movement of the housings apart in a rebound stroke. Thus, the housings can be progressively braked to a stop in either or both senses of their relative movement.

According to a second aspect of the invention there is provided a spring and damper unit comprising two telescopically movable housings bounding an interior space formed by a cavity in one of the housings and a cavity in the other housing, and a floating piston disposed in one of the cavities and dividing the interior space into a spring chamber containing a gaseous medium and a damping chamber containing a damping fluid, the housings being movable together in a compression stroke thereof to compress the gaseous medium through reduction in the volume of the spring chamber and the piston being movable by the compressed gaseous medium to cause a reduction in the volume of the damping chamber for displacement of damping fluid therefrom.

Such a unit, based on the functional principes of the first aspect of the invention, provides a particuiariy simple constructional realisation of the invention through use of housings jointly bounding an interior space formed by two housing cavities, the piston serving to divide the interior space appropriately into the spring chamber and the damping chamber respectively filled with the gaseous medium and the damping fluid. In that case, the spring chamber is preferably formed by a part of the other cavity and part of the cavity with the piston, and the damping chamber is formed by another part of the latter cavity.

Similarly, to eliminate out-of-phase travel of the piston in more extreme cases of compression of the unit, the housing with the other cavity can be provided with a drive member which is disposed at a spacing from the piston in an unloaded state of the unit and arranged to drivingly engage the piston on movement of the housings together at a rate exceeding a predetermined rate. The drive member thus represents an abutment fixed relative to that housing and can conveniently be in the form of a rod extending along a common axis of the cavities and the piston. To ensure the rod does not, due to its crosssectional area and the limited space within the housings, hinder normal driving action compressed gaseous medium on the piston, the rod can be provided with a duct for conducting the gaseous medium to and from the region of the piston.

For preference, resilient means are present to cushion an end stage of the movement of the housings together and/or an end stage of the movement apart. The resilient means for cushioning movement apart can be conveniently provided by an elastomeric element mounted on the rod and co-operabie with an abutment at the housing with the cavity containing the piston. Location of the resilient means in this position, as distinct from at an end of the unit, enhances the compact construction of the unit.

Preferably, the housing with the cavity containing the piston additionally has a piston portion movable in the other cavity during the compression stroke to produce the reduction in the volume of the spring chamber. Such a piston portion, for example a partially closed head of the respective housing, can provide an effective reduction in the volume of the spring chamber without the need for a separate piston for that purpose.

An embodiment of the present invention will now be more particularly described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a schematic axial section of a spring and damper unit embodying the invention and shown in a state between a full compression setting and a full rebound setting; Fig. 2 is a view similar to Fig. 1, but showing the full compression setting ; and Fig. 3 is a view similar to Fig. 1, but showing the full rebound setting.

Referring now to the drawings, there is shown a coaxial pneumatic spring and hydraulic damper unit 10 comprising an outer housing 11 and an inner housing 12, the two housings being telescopically interengaged for movement together to execute a compression stroke and movement apart to execute a rebound stroke. In an installed state, the spring behaviour of the unit is effective primarily in the compression stroke to soften shock forces transmitted between two components, for example a vehicle wheel and vehicle body, intercoupled by the unit and the damping behaviour primarily in the rebound stroke to damp oscillations arising from spring restoration. Damping behaviour is also present in the compression stroke, but generally to a lesser degree.

The outer housing 11 comprises an end closure member 13, which incorporates an attachment eye 14, and a tube 15 threadedly connected to the closure member 13 so as to define a cylindrical cavity or bore 16 closed at one end by the member 13 and open at the other end. The inner housing 12 comprises a similar end closure member 17, which also incorporates an attachment eye 18, an inner tube 19 and outer tube 20, which are concentric with and radially spaced from each other and also concentric with and radially spaced from the tube 15 of the outer housing 13, and an annular piston member 21. The outer tube 20 is threadedly connected to both the closure member 17 and piston member 21 and the inner tube 19 is clamped between these two members by the process of tightening the members towards each other, a resilient sealing ring 22 being disposed between the inner tube 19 and piston member 21. The inner tube 19 defines a cylindrical cavity or bore 23 which is closed at one end by the closure member 17 and open at the other end via the opening in the piston member 21, the diameter of this opening being reduced relative to the bore diameter by an inwardly protruding lip of the piston member.

Slidably arranged in the bore 23 of the inner tube 19 of the inner housing 12 is a piston 24 provided with an annular seal 25 sealingly bearing against the inner wall surface of the tube 19. The piston 24 inclusive of seal 25 separates an inner damping chamber 26, on the right, filled with hydraulic oil from a spring chamber 27, on the left, filled with air under pressure. Since the bore 23 of the inner housing 12 communicates with the bore 16 of the outer housing 13 via the opening in the piston member 21, the spring chamber 27 is formed by parts of both bores. The inner damping chamber 26 is formed solely by part of the bore 23. The respective volumes of the two chambers 26 and 27 vary in dependence on the position of the piston 24.

The tube 15 of the outer housing 11 is provided at its free end with a radially inwardly directed projection in the form of two axially spaced beads or flanges 28 bounding a groove which receives an annular seal 29 in sealing contact with the outer wall surface of the outer tube 20 of the inner housing 12. An annular wiper seal 30 is present at the termination of the tube 15 to prevent ingress of foreign matter. The annular piston member 21 is analogously provided at its outer Gircumference with a radially outwardly directed projection in the form of two axially spaced beads or flanges 31 bounding a groove which receives a seal 32 in sealing contact with the inner wall surface of the tube 15 of the outer housing 11. The two projections and variable lengths of the concentric wall surfaces, which are disposed therebetween, of the tubes 15 and 20 bound an outer damping chamber 33 similarly filled with the hydraulic oil. The projection at the annular piston member 21 separates this chamber from the spring chamber 27. The outer damping chamber 33 is thus formed from a further part of the bore 23. The diameters of the bores 16 and 23 are so selected that the inner and outer damping chambers 26 and 33 have at least substantially the same area in cross-section.

The annular seals 25,29 and 32 preferably each consist of a compound body composed of two axially spaced polytetrafluoroethylene rings and intermediate rubber flange of a radially outer rubber seat for the rings. Each seal is at some stage in execution of the compression and rebound strokes caused to wipe a surface with residual hydraulic oil and thus is subject to oil lubrication. The closure member 13 of the outer housing 11 has an inlet (not shown) and a non-retum valve 34 for feed of pressurised air into the spring chamber 27. The closure member 17 of the inner housing 12 correspondingly has inlets 35 (only one shown) usable for feed of hydraulic oil into the damping chambers 26 and 33. The damping chambers are interconnected in terms of flow by a duct system formed by a space 36 left between the inner and outer tubes 19 and 20 of the inner housing 12, an opening 37 provided in the tube 20 and connecting the space 36 with the outer damping chamber 33, and two channels 38 (only one shown) provided in the closure member 17 and connecting the space 36 with the inner damping chamber 26. Removably mounted in the inlets 35 are ajustable flow control valves 39 (only one shown) each with a valve member 40 controlling flow through the respective channel 38 between the damping chambers 23 to 33.

The unit 10 incorporates an auxiliary drive for the piston 24, the drive being in the form of a drive rod 41 which extends along a common axis of the bores 16 and 23 and piston 24 and which is threadedly secured at one end in the closure member 13 of the outer housing 11 and provided at the other end with a radially enlarged head. In the unloaded state of the unit, the head is normally disposed at a spacing's'from the piston 21. The rod 41 has drillings 42 for feed of air to and from the region of the piston, which is mechanically separate from the rod 41 and other components of the unit. The piston thus floats in the bore 23, subject only to constraints on movement provided by friction and fluid loadings.

Finally, to cushion the end stages of movement of the housings 11 and 12 together in the compression phase and apart in the rebound phase the unit includes appropriate resilient cushioning elements. The cushioning element associated with the compression phase comprises an annular rubber body 43 seated in an intemal recess in the closure member 13 of the outer housing 11 and co-operable with the piston member 31 of the inner housing 12, the body 43 and member 31 having mutually facing abutment surfaces of generally complementary shape. The cushioning element associated with the rebound phase comprises a compressible elastomer cylinder 44 located between two end washers and slidably mounted on the drive rod 41 between the head thereof and a stop collar 45, for example a spring ring, secured on the rod at a spacing from the head. The elastomer cylinder 44 is co-operable, via the lefthand washer, with the inwardly protruding lip of the piston member 21 and can, under compression by the lip, deform by sliding movement of its constituent material along the rod.

In use of the unit 10, with the chambers 26,27 and 33 appropriately filled with air under pressure and hydraulic oil the two attachment eyes 14 and 18 of the housings 11 and 12 are respectively coupled to two components intended to be relatively movable by way of a damped spring coupling. The unit can, for example, be incorporated in a two-wheel vehicle between a wheel axle and a frame or chassis. Normally, two or more units would be associated with an individual axle.

In the case of a compression stroke of the housings 11 and 12, thus telescopic movement of the housings together (whether starting from a fully expanded setting as in Fig. 3 or an intermediate position such as in Fig. 1 if the weight of a supported one of the two coupled components induces partial compression of the unit 10 in the normal state), the piston member 21 moves in the bore 16 towards the closure member 13 and reduces the volume of the spring chamber 27, which causes progressive compression of the already pressurised air in the chamber. The compressing air acts on the piston 24 to move it in the bore 23 in opposite sense to the movement of the piston member 31, i. e. towards the other closure member 17, and thus causes the piston 24 to reduce the volume of the inner damping chamber 26. The movement of the piston member 21 along the bore 16 has the simultaneous effect of increasing the volume of the outer damping chamber 33. The reduction in volume of the inner damping chamber 26 and increase in volume of the outer damping chamber 33 constrains a displacement of hydraulic oil from the former to the latter via the duct system formed by the channels 38, space 36 and opening 37.

Preferably, one of the channels 38 is assigned to the flow in direction from the chamber 26 to the chamber 33 and the other chamber to the flow in reverse direction. The associated valves 39 are then effective in opposite sense to block flow in one direction and allow throttled flow in the other direction. The degree of throttling of the permitted flow determines the degree of damping, which is preferably relatively light in the compression phase of the unit. In this phase, the unit thus allows movement of the two coupled components towards one another against the resistance presented by the compressing air functioning as a pneumatic spring and damps that movement to a selected degree by the throttled displacement of the hydraulic oil. Due to the equality or approximate equality of the cross-sectional areas of the two damping chambers 26 and 33 the fluid displacement of the chambers is the same or substantially the same during operation of the unit and the piston 24 consequently remains in the same position relative to the outer housing 11 in both the compression phase and the rebound phase, as is evident from comparison of Figs. 1,2 and 3. This position of the piston 24 is selected to be close to the free end of the tube 15 of the outer housing 11, so that in the fully compressed state of the unit as shown in Fig. 2 both the piston and the free end of the tube 15 are closely adjacent to the closure member 17 of the inner tube 12. A maximum spring stroke and maximum damping stroke are thus derived from the maximum amount of intended available relative travel of the two housings 11 and 12 in the compression stroke. In the fully compressed state of the unit, as evident from Fig. 2, the piston member 21 abuts and deforms the rubber body 43, which thus cushions the end stage of movement of the housings 11 and 12 in the compression stroke.

The movement of the piston 24 to reduce the volume of the inner damping chamber 26 simultaneously causes an increase in the volume of the spring chamber 27, in effect a volume interchange between the two chambers. Because of the cross-section relationship of the bores 16 and 23, this increase is necessarily less than the decrease in volume of the spring chamber 27 produced at the same time by the moving piston member 21.

Compression of the air continues to take place, but with a controlled attenuation and consequently a reduction in the rising rate of the air spring by comparison with the rate that would result from constant reduction in the volume of the air chamber without a compensating, proportional increase in that volume.

In the event of a very high rate of compression of the unit 10, the air in the spring chamber 27 may compress so rapidly that the piston 24 cannot move, due to the pressure created by throttling of the outflow of hydraulic oil from the inner damping chamber 26, at the same rate as that of the relative movement of the two housings 11 and 12. If the two movements become out-of-phase in this manner, the piston 24 will move relative to the outer housing 11 in direction towards the closure member 13 of that housing. Should the piston travel towards the closure member 13 be sufficient to overcome the spacing's', the piston will then come into contact with the head of the drive rod 41 and thereafter be mechanically positively driven by the rod. The mechanical drive will persist for such time as the rate of compression of the unit, i. e. rate of relative movement of the housings 11 and 12 towards one another, exceeds a predetermined threshold rate. The threshold rate is determined primarily by the rate of fluid outflow permitted by the respective valve 39 and the dimension of the spacing's'. The fluid drive provided by the compressed air continues to be applied throughout and will take over from the mechanical drive if the rate of compression reduces below the threshold rate and a clearance is reinstated between the head of the rod 41 and the adjacent face of the piston 24. The drillings 42 in the rod 41 inclusive of the head thereof ensure that the cross-sectional obstruction of part of the air chamber 27 by the rod 41 and elastomer cylinder 44 does not diminish the force of compressed air acting on the piston 24.

In the case of the rebound stroke, on relief of the loading of the coupled components that induced the compression stroke, the force generated by the compressed air in the spring chamber 27 urges the two housings 11 and 12 apart, whereby the piston member 21 enlarges the volume of the spring chamber 27 and simultaneously reduces the volume of the outer damping chamber 33, thus causes an interchange of volume of the two chambers. The reducing volume of the damping chamber 33 places the hydraulic oil in that chamber under a positive pressure and displaces oil from the chamber 33 to the inner damping chamber 26 through the duct system provided by the opening 37, space 36 and associated one of the channels 38. In that case the valve 39 permitting flow through that channel exerts a relatively strong throttling effect on the flow to produce an appreciable retardation or damping of the relative movement of housings 11 and 12 apart. The positive pressure acting on the oil in the outer damping chamber 33 resists any tendency of the oil to cavitate in that chamber. Similarly, the compressed air continuing to act on the piston 24 in the sense of urging it towards the closure member 17 ensures that the volume enlargement of the inner damping chamber 26 is produced solely by inflowing oil, so that cavitation cannot occur in that chamber. The piston 24 moving in the bore 23 of the inner housing 12 again remains in the same position relative to the outer housing 11.

If the housings 11 and 12 in the rebound stroke attain the maximum extended position, the inwardly protruding lip of the piston member 21 contacts the elastomer cylinder 44, in particular the adjacent one of the end washers at the cylinder, towards the end of the relative movement of the housings and compresses the cylinder to cushion the final movement. Compression of the cylinder is accommodated by departure from abutment with the stop collar 45 and partial displacement along the rod 41. The end setting is shown in Fig. 3. The location of this rebound cushioning element within the stroke length of the unit 10 contributes to the compact form of the unit.

All three seals 25,29 and 32 in the unit are subject to lubrication by the hydraulic oil during use, the seal 25 primarily during the compression stroke and the seals 29 and 32 primarily during the rebound stroke. The lubrication increases sealing integrity and extends service life.

The valves 39 can be constructed to be individually ajustable and are readily accessible for adjustment purposes. In principle, a single valve and single channel can suffice to control flow between the damping chambers and different forms of duct system are possible. In the case of the described twin-tube construction of the inner housing, utilisation of a space between the tubes for fluid transfer is convenient and has the advantage of avoiding the need for long drillings in solid material, but a bore in a single solid circumferential wall, or another arrangement, is equally possible. Apart from constructional aspects, there is also scope for variation of chamber shapes, utilisation of additional pistons or different forms of pistons, and provision of assist or restorative springs or spring bodies. The spring chamber can employ a gas other than air and the damping fluid itself can be a gas, rather than a liquid, or even a gas and liquid mixture (emulsion).

Claims (24)

1. CLAIMS 1. A spring and damper unit comprising two telescopically movable housings containing a spring chamber for a gaseous medium which is compressible on movement of the housings together in a compression stroke thereof, a damping chamber for a damping fluid which is displaceable therefrom during the compression stroke, and a piston movable by such compressed gaseous medium to reduce the volume of the damping chamber thereby to cause said displacement.
2. 2. A unit as claimed in claim 1, wherein the piston is arranged to produce an increase in the volume of the spring chamber concurrently with the reduction in the volume of the damping chamber, the increase in the spring chamber volume being less than a reduction in that volume produced by the movement of the housings together.
3. 3. A unit as claimed in daim 1 or claim 2, wherein the spring chamber is formed by part of a cavity in one of the housings and part of a cavity in the other housing and the damping chamber is formed by another part of the latter cavity.
4. 4. A unit as claimed in claim 3, wherein the piston is disposed in the cavity in the other housing.
5. 5. A unit as claimed in claim 4, comprising sealing means providing a seal between the piston and a circumferential wall of the cavity in which it is disposed.
6. 6. A unit as claimed in any one of the preceding claims, wherein the piston is arranged to separate the spring chamber from the damping chamber.
7. 7. A unit as claimed in any one of the preceding claims, wherein the piston is a floating piston.
8. 8. A unit as claimed in any one of the preceding claims, comprising drive means effective on movement of the housings together at a rate exceeding a predetermined rate to mechanically positively drive the piston.
9. 9. A unit as claimed in claim 8, wherein the drive means comprises a drive member drivingly engaging the piston after overcoming a spacing therefrom.
10. 10. A unit as claimed in any one of the preceding claims, comprising cushioning means for cushioning at least one of an end stage of movement of the housings together in the compression stroke thereof and an end stage of movement of the housings apart in a rebound stroke thereof.
11. 11. A spring and damper unit comprising two telescopically movable housings bounding an interior space formed by a cavity in one of the housings and a cavity in the other housing, and a floating piston disposed in one of the cavities and dividing the interior space into a spring chamber containing a gaseous medium and a damping chamber containing a damping fluid, the housings being movable together in a compression stroke thereof to compress the gaseous medium through reduction in the volume of the spring chamber and the piston being movable by the compressed gaseous medium to cause a reduction in the volume of the damping chamber for displacement of damping fluid therefrom.
12. 12. A unit as claimed in claim 11, wherein the piston is arranged to produce an increase in the volume of the spring chamber concurrently with the reduction in the volume of the damping chamber, the increase in the spring chamber volume being less than said reduction in that volume.
13. 13. A unit as claimed in claim 11 or claim 12, wherein the spring chamber is formed by a part of the other cavity and part of said one cavity and the damping chamber is formed by another part of the latter cavity.
14. 14. A unit as claimed in any one of claims 11 to 13, wherein the housing with the other cavity is provided with a drive member which is disposed at a spacing from the piston in an unloaded state of the unit and arranged to drivingly engage the piston on movement of the housings together at a rate exceeding a predetermined rate.
15. 15. A unit as claimed in claim 14, wherein the drive member comprises a rod extending along a common axis of the cavities and the piston.
16. 16. A unit as claimed in claim 15, wherein the rod is provided with a duct for conducting the gaseous medium to and from the region of the piston.
17. 17. A unit as claimed in any one of claims 11 to 16, comprising resilient means for cushioning an end stage of movement of the housings together in the compression stroke thereof.
18. 18. A unit as claimed in any one of claims 11 to 17, comprising resilient means for cushioning an end stage of movement of the housings apart in a rebound stroke thereof.
19. 19. A unit as claimed in claim 18 when appended to claim 15 or claim 16, wherein the resilient means for cushioning movement apart comprises an elastomeric element mounted on the rod and co-operable with an abutment at the housing with said one cavity.
20. 20. A unit as claimed in any one of claims 11 to 19, wherein the housing with said one cavity has a piston portion movable in the other cavity during the compression stroke to produce the reduction in the volume of the spring chamber.
21. 21. A unit as claimed in any one of the preceding claims, wherein the housings are respectively provided with attachment eyes at mutually remote ends thereof.
22. 22. A unit as claimed in any one of the preceding claims, wherein the gaseous medium is air.
23. 23. A unit as claimed in any one of the preceding claims, wherein the damping fluid is hydraulic oil
24. 24. A spring and damper unit substantially has hereinbefore described with reference to the accompanying drawings.