GB2348473A - Spring and damper unit - Google Patents

Abstract

A spring and damper unit (10) comprises two telescopically movable housings (11, 12) containing a spring chamber (28) for a gaseous medium such as air and a damping chamber (27) for a damping fluid such as hydraulic oil, a first piston (21) for causing compression of the gaseous medium on movement of the housings together in a compression stroke and a second piston (22) for causing displacement of damping fluid from the damping chamber (27) during the compression stroke. Part of the spring chamber (28) is disposed between the two pistons (21, 22) so as to be reduced in volume by movement of the pistons towards one another on movement of the housings apart in a rebound stroke. A valve (24, 26) is provided to control communication of that part of the spring chamber with the rest of the spring chamber and is operable in an end stage of the rebound stroke to interrupt the communication and cause the movement of the housings apart in that end stage to be braked by compression of gaseous medium isolated in the part of the spring chamber between the pistons. The valve can have the form of a rod (24) extending through an opening in the first piston (21), the first piston being movable along the rod during the rebound stroke and the rod and opening being co-operable during that stroke to define a communicating passage between the two parts of the spring chamber until attainment of the end stage of the stroke and to cause the passage to be blocked during that end stage.

Description

SPRING AND DAMPER UNIT The present invention relates to a spring and damper unit and has particular reference to cushioning of an end stage of actuation of the unit in a rebound phase.

Dampers, including combined spring and damper units, of coaxial mode of construction conventionally contain cushioning elements to cushion the end stages of retraction in the compression phase and extension in the rebound phase. Cushioning at the end of the rebound phase, termed top-out, is commonly provided by a buffer element of rubber or other elastomeric material. Contact with such an element inevitably generates noise and causes an abrupt change in the rate of relative movement of the damper parts travelling apart. At higher frequencies of oscillation of the damper, as in the case of repeated shock loading, the noise may be evident as a series of knocks accompanied by jolts transmitted to the components damped by the damper. In addition to these undesirable operating characteristics, the buffer element contributes weight to the unit as a whole, which generally should be as light as possible, and increases the inertia of the damper part to which it is fixed.

It is therefore one object of the invention to provide a damper, in particular a combined spring and damper unit, which may provide a smoother transition to braking of the end stage of the rebound stroke and which avoids or substantially avoids generation of noise when the transition occurs. A further object of the invention is to eliminate the weight contribution made by a top-out cushioning element of solid material. Other objects and advantages of the invention will be apparent from the following description.

According to a first aspect of the invention there is provided a spring and damper unit comprising two telescopically movable housings containing a spring chamber for a gaseous medium and a damping chamber for a damping fluid, a first piston for causing compression of the gaseous medium on movement of the housings together in a compression stroke thereof, a second piston for causing displacement of damping fluid from the damping chamber during the compression stroke, part of the spring chamber being disposed between the pistons so as to be reduced in volume by movement of the pistons towards one another on movement of the housings apart in a rebound stroke thereof, and valve means controlling communication of said part of the spring chamber with the rest of the spring chamber and operable in an end stage of the rebound stroke to interrupt the communication and cause movement of the housings apart in that end stage to be braked by compression of gaseous medium isolated in said part of the spring chamber.

In the case of such a unit, the gaseous medium, for example air, employed for the springing is also utilise for top-out cushioning or braking of the housings in the end stage of the rebound phase. The initiation of braking is determined by the point, in the stroke length of the housings, at which the valve means operates to interrupt communication of the two parts of the spring chamber and this point can be selected to ensure that the housings will be normally brought to a complete stop in advance of attainment of the maximum possible pressure of the gaseous medium compressed between the pistons.

The transition to braking of the housings, thus the response of the valve means, takes place without the physical shock of impact against a solid cushioning body and the compression of the isolated gaseous medium exerts an effective, but progressive and therefore smooth, braking action. The transition to braking can be completely silent even when the housings are braked at high rates of relative movement and the omission of a solid cushioning body achieves a saving in weight.

Preferably, the part of the spring chamber between the pistons is formed by a part of a cavity in an inner one of the housings and, for preference, the first piston is arranged at an end of that housing. The rest of the spring chamber can then be formed by part of a cavity in an outer one of the housings.

In one advantageous form of the unit the valve means comprises a rod extending through an opening in the first piston, the first piston being movable along the rod during the rebound stroke and the rod and opening being co-operable during that stroke to define a communicating passage between said part of the spring chamber and the rest of the spring chamber until attainment of the end stage of the stroke and to cause the passage to be blocked during that end stage. In that case, the diameter of the rod can be smaller than that of the opening over a length portion of the rod traversed by the first piston up to attainment of the end stage of the rebound stroke and substantially equal to that of the opening over a length portion of the rod traversed by the first piston during that end stage.

The flow cross-section of the communication passage defined in the first piston determines the rate of flow of gaseous medium between the two parts of the spring chamber and this rate is preferably such that during the effective spring action, i. e. the compression phase, there is little or no lag in pressure equalisation between the two parts of the chamber. The point along the length of the rod where the enlargement in diameter is provided determines the onset of top-out braking in the rebound phase and this point can be selected in accordance with the constructional and operating characteristics of the spring and damper unit and also of the components which, in use, the unit serves to spring and damp. When extension of the housings in the rebound phase takes place to such a degree that the topout braking comes into effect and causes compression of the isolated gaseous medium, the unit may subsequently seek an equilibrium state in which the housings partially retract to reduce the elevated pressure of the isolated gaseous medium. Subject to consideration of the load applied by the sprung/damped components, the point along the length of the rod at which its diameter changes also influences the equilibrium state of the unit, i. e. extent of retraction of the housings, and can be positioned accordingly. It can be the case that the end stage of the rebound stroke represents a significant part of the total stroke length.

For preference, the opening is bounded by a seal sealingly engageable with the rod around the circumference thereof during the end stage of the rebound stroke. Such a seal can be conveniently located in recess in a bore in the first piston. The use of a seal reinforces the separation of the part of the spring chamber between the pistons from the rest of that chamber when the end stage of the rebound stroke is reached.

Expediently, the rod is connected to the second piston, in which case the rod can travel with that piston. Alternatively, the rod can be connected to, for example, the outer housing.

In a preferred embodiment of the unit, the second piston is movable by the compressed gaseous medium during the compression stroke to reduce the volume of the damping chamber thereby to cause the displacement of the damping fluid and the rod is effective on movement of the housings together at a rate exceeding a predetermined rate to mechanically positively drive this piston. The gaseous medium thus provides the normal driving force for the second piston and consequently the initiating force for the displacement of the damping fluid, for example hydraulic oil, and the rod has the purpose, in addition to forming part of the valve means, of providing an auxiliary mechanical drive for that piston. For preference, the rod is disposed at a spacing from a drive surface in an unloaded state of the unit and is arranged to be drivingly engaged by that surface on movement of the housings together at a rate exceeding the predetermined rate. If the rod is connected to the second piston, the drive surface can be located at the outer one of the housings.

The second piston can be arranged to separate the spring chamber from the damping chamber, so that gaseous medium compressed in the spring chamber in the compression phase for the spring effect or in the rebound phase for the top-out braking effect acts directly on the second piston. The housings can addition contain a further damping chamber for receiving displaced fluid from and retuming the fluid to the first-mentioned damping chamber, the further damping chamber being disposed between and bounded by the two housings. Such an arrangement of the further damping chamber, for example concentrically around the first-mentioned damping chamber, results in positive displacement of damping fluid into the further chamber during the rebound phase.

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 has an end portion 13, which incorporates a coupling eye 14, and a tubular body portion 15 defining a cavity in the form of a cylindrical blind bore 16. The inner housing 12 similarly comprises an end portion 17, which also has a coupling eye 18, and a tubular body portion 19 which is concentric with and radially spaced from the body portion 15 of the outer housing 11 and which defines a cavity in the form of a cylindrical blind bore 20. An annular, first piston member 21 is threadedly connected to the tubular body portion 19 at the end thereof remote from the end portion 17.

Slidably arranged in the bore 20 of the inner housing 12 is a second piston 22 provided with an annular seal 23, which sealingly bears against the inner wall surface of the bore 20, and with a piston rod 23, which extends along the common axis of the bores 16 and 20 via a bore in the first piston 21 and which, in the unloaded state of the unit 10, terminates at a spacing from the base of the bore 16. The piston 22 thus floats in the bore 20. The piston rod 24, which as later explained functions on the one hand as an auxiliary drive and on the other hand as part of a valve, has a step in diameter intermediate its ends. The step is provided by a chamfer 25 and is such that a length portion 24a of the rod adjoining the piston 22 has substantially the same diameter as the bore in the piston 21, whilst the remaining length portion 24b of the rod has a diameter reduced relative to that of the bore.

Arranged in a recess in the bore of the piston 21 is a seal 26 which is sealingly engageable with the length portion 24a, but not with the length portion 24b, and which completes the valve. Thus, when the length portion 24b extends through the bore in the piston 21, the seal 26 and the rod 24 co-operate to define a communicating passage between the bore 16 and the bore 20. Conversely, when the length portion 24a extends through the bore in the piston 21, the seal 26 and rod 24 co-operate to block communication of the bores 16 and 20.

The piston 22 inclusive of seal 23 separates an inner damping chamber 27 filled with hydraulic oil from a spring chamber 28 filled with air outer pressure. The air-fille spring chamber 28 is formed by parts of both blind bores 16 and 20 of the outer housing 11 and inner housing 12, respectively, and the inner damping chamber 27 is formed solely by part of the bore 20. The respective volumes of the two chambers 27 and 28 vary in dependence on the position of the piston 22 relative to the inner housing 12. When the bores 16 and 20 are in communication by way of the communicating passage as described above, the two parts of the spring chamber are connected in term of flow and all the air therein contributes to the springing provided by the unit 10. When the communicating passage is blocked, the air in the part of the spring chamber between the pistons changes its function to that of top-out cushioning or braking of the housings 11 and 12 in an end stage of movement of the housings apart, thus the rebound phase. Cushioning or braking of an end stage of movement of the housings together, thus the compression phase, is provided by an annular rubber body 29 located at the base of the bore 16 and co-operable with the piston 21.

The body portion 15 of the outer housing 11 is provided at its free end with a radially inwardly directed projection in which is seated an annular seal 30 in sealing contact with the outer wall surface of the body portion 19 of the inner housing 12. The body portion 19 is analogously provided at its free end with a radially outwardly directed projection in which is seated a seal 31 in sealing contact with the inner wall surface of the body portion 15 of the outer housing 11. The two projections and variable lengths of the concentric body portions 15 and 19, which are disposed therebetween, of the two housings bound an outer damping chamber 32 similarly filled with the hydraulic oil. The projection with the seal 30 separates this chamber from the spring chamber 28. The outer damping chamber 32 is thus formed by a further part of the bore 16.

The end portion 13 of the outer housing 11 has an inlet (not shown) and a non-return valve 33 for feed of pressurised air into the spring chamber 28. The end portion 17 of the inner housing 12 correspondingly has inlets 34 (only one shown) usable for feed of hydraulic oil into the damping chambers 27 and 32. The two damping chambers are interconnected in terms of flow by two ducts 35 (only one shown) each having a section extending in the end portion 17 of the inner housing 12 and opening at the end wall of the bore 20 and a section extending in the body portion 19 of the same housing and opening at the outer circumference of that body portion. The sections of the two ducts in the body portion 19 can, in fact, be provided by a common cavity therein and a common inlet and outlet port of the outer damping chamber 32. Removably mounted in the inlets 34 are ajustable flow control valves 36 (only one shown) each with a valve member 37 controlling flow through the respective duct 35 between the damping chambers 27 and 32. One duct 35 is allocated to serve as a flow path for oil from the inner damping chamber 27 to the outer damping chamber 32 and the associated valve 36 serves to regulate oil flow in that direction and prevent flow in the opposite direction. Conversely, the other duct 35 is allocated to serve as a flow path for oil from the outer damping chamber 32 to the inner damping chamber 27 and the associated valve 36 correspondingly regulates flow in that direction and blocks flow in the opposite direction.

In use of the unit 10, with the chambers 27,28 and 32 appropriately filled with air under pressure and hydraulic oil, the two coupling eyes 14 and 18 of the housings 11 and 12 are respectively coupled to two components intended to be relative 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 ormore usually-an intermediate position due to the weight of a supported one of the two coupled components inducing partial compression of the unit 10 in a rest state), the piston 21 moves in the bore 16 towards the end portion 13 of the outer housing 11 and reduces the volume of the part of the spring chamber 28 in that bore, which causes progressive compression of the already pressurised air in that chamber part. As soon as the seal 26 in the piston 21 travels over the chamfer 25 of the piston rod 24 so that the smaller diameter length portion 24b of the rod extends through the seal and defines therewith the communicating passage between the two parts of the spring chamber, the compressing air acts on the piston 22 to move it in the bore 20 in opposite sense to the movement of the piston member 21, i. e. towards the end portion 17 of the inner housing 12, and thus causes the piston 22 to reduce the volume of the inner damping chamber 27. The movement of the piston 21 along the bore 16 has the simultaneous effect of increasing the volume of the outer damping chamber 32. the reduction in volume of the inner damping chamber 27 and increase in volume of the outer damping chamber 32 constrain a displacement of hydraulic oil from the former to the latter via the allocated one of the ducts 35 and the associated one of the valves 36, the valve allowing a throttled flow through that duct. The degree of throttling determines the degree of damping provided by the unit, which is preferably relative light in the compression phase. In this phase, movement of the two coupled components towards one another takes place against the resistance presented by the compressing air functioning as a pneumatic spring and the movement is damped to a selected degree by the throttled displacement of the hydraulic oil.

Due to an equality or approximate equality of the cross-sectional areas of the two damping chambers 27 and 32, the fluid displacement of the chambers is the same or substantially the same during operation of the unit and the piston 22 consequently remains in essentially 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. In the fully compressed state of the unit, as shown in Fig. 2, the piston 21 contacts the rubber body 29, which thus cushions the end stage of movement of the housings 11 and 12 in the compression stroke.

The movement of the piston 22 to reduce the volume of the inner damping chamber 27 simultaneously causes an increase in the volume of the spring chamber 28, in effect a volume interchange between the two chambers. Because of the cross-sectional relationship of the bores 16 and 20, this increase is necessarily less than the decrease in volume of the spring chamber 28 produced by the moving piston 21. Compression of the air thus takes place with a controlled attenuation due to the movement of the piston 22.

In the event of a very high rate of compression of the unit 10, the air in the spring chamber 28 may compress so rapidly that the piston 22 cannot move, due to the pressure created by throttling of the outflow of hydraulic oil from the inner damping chamber 27, 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 22 together with the piston rod 24 will move relative to the outer housing 11 in direction towards the end portion 13 of that housing. Should the movement be sufficient to overcome the spacing of the free end of the rod 24 from the base of the bore 16, that end will then come into contact with the surface defining the base and the piston 22 with 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 towards one another, exceeds a predetermined threshold rate. 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 of the unit reduces below the threshold rate and a clearance is reinstated between the free end of the rod 24 and the base surface of the bore 16.

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 28 urges the housings 11 and 12 apart, whereby the piston 21 enlarges the overall volume of the spring chamber 28 and simultaneously reduces the volume of the outer damping chamber 32, thus causes an interchange of volume of those chambers.

The reducing volume of the damping chamber 32 places the hydraulic oil in that chamber under a positive pressure and displaces oil therefrom to the inner damping chamber 27 through the other one of the ducts 35. In that case, the valve 36 permitting flow through that duct exerts a relatively strong throttling effect on the flow to produce an appreciable damping of the relative movement of housings 11 and 12 apart. For as long as the piston 21 is travelling along the smaller diameter length portion 24b of the rod 24, the two parts of the spring chamber are in flow communication and the pressures therein tend to equalise via the communicating passage formed through the piston 21. When the piston 21 reaches the chamfer 25, the seal 26 and larger diameter length portion 24a sealingly interengage and block the communicating passage, thus cutting off the part of the spring chamber between the pistons 21 and 22 from the rest of the spring chamber. The transition at this point represents the start of the end stage of the rebound stroke, in which the movement of the housings apart is to be progressively braked to a stop. The braking is effected by compression of the air isolated in the cut-off part of the spring chamber between the pistons, which move towards one another as a consequence of the continuing increase in the volume of the rest of the spring chamber and concomitant decrease in the volume of the outer damping chamber 32, the latter change resulting in oil being displaced under pressure into the inner damping chamber 27. The cut-off part of the spring chamber thus now functions to provide top-cut cushioning. The rising pressure of air compressing between the two pistons, which is accompanied by a rising pressure of oil in both damping chambers 27 and 32, gradually brings the housings to a stop, in particular at a point at which the oil pressure in the outer chamber 32 comes into balance with the air pressure in the main part of the spring chamber, which is still increasing in volume. In order for the rebound stroke to continue beyond this point, the pressure of the oil in the outer damping chamber 32 and thus in the inner damping chamber 27 would have to rise sufficiently to cause further compression of the now highly-compressed air in the part of the spring chamber between the pistons. The volume relationships of the chambers and the proportion of the total rebound stroke constituting the end stage during which top-out braking is effective can be selected so that the anticipated level of forces externally acting on the housings in normal use of the unit will not be sufficient to cause compression of the isolated air in the cut-off part of the spring chamber beyond a predetermined maximum extended state of the unit.

After elimination of the forces inducing the rebound stroke, the housings 11 and 12, assisted by the static load acting thereon by way of the components intercoupled by the unit, will seek a rest state in which the air in the part of the spring chamber between the pistons undergoes partial expansion. The rest state is preferably a state in which the seal 26 is still in engagement with the larger diameter length portion 24a of the rod 24a, but close to the chamfer 25. In the event of a subsequent compression stroke as previously described, the residual excess pressure in the part of the spring chamber between the pistons acts to assist movement of the piston 22 for initial displacement of oil out of the inner damping chamber 27. As soon as the seal 26 passes the chamfer 25 and restores flow communication of the two parts of the spring chamber, the fluid drive of the piston 22 by the air in the entire spring chamber becomes effective.

The use of compressed air to provide top-out cushioning in the rebound stroke provides a smooth braking of the housings to a stop and saves weight through deletion of a solid body buffer element. Additional saving in weight and/or reduction in the number of components of the unit is achieved by assigning dual functions to both the pneumatic spring chamber and the auxiliary mechanical drive for the airloil separator piston.

Claims (15)

1. CLAIMS 1. A spring and damper unit comprising two telescopically movable housings containing a spring chamber for a gaseous medium and a damping chamber for a damping fluid, a first piston for causing compression of the gaseous medium on movement of the housings together in a compression stroke thereof, a second piston for causing displacement of damping fluid from the damping chamber during the compression stroke, part of the spring chamber being disposed between the pistons so as to be reduced in volume by movement of the pistons towards one another on movement of the housings apart in a rebound stroke thereof, and valve means controlling communication of said part of the spring chamber with the rest of the spring chamber and operable in an end stage of the rebound stroke to interrupt the communication and cause movement of the housings apart in that end stage to be braked by compression of gaseous medium isolated in said part of the spring chamber.
2. 2. A unit as claimed in claim 1, wherein said part of the spring chamber is formed by part of a cavity in an inner one of the housings.
3. 3. A unit as claimed in claim 2, wherein the first piston is arranged at an end of the inner one of the housings.
4. 4. A unit as claimed in any one of the preceding claims, wherein the valve means comprises a rod extending through an opening in the first piston, the first piston being movable along the rod during the rebound stroke and the rod and opening being cooperable during that stroke to define a communicating passage between said part of the spring chamber and the rest of the spring chamber until attainment of the end stage of the stroke and to cause the passage to be blocked during that end stage.
5. 5. A unit as claimed in claim 4, wherein the diameter of the rod is smaller than that of the opening over a length portion of the rod traversed by the first piston up to attainment of the end stage of the rebound stroke and substantially equal to that of the opening over a length portion of the rod traversed by the first piston during that end stage.
6. 6. A unit as claimed in claim 4 or claim 5, wherein the opening is bounded by a seal sealingly engageable with the rod around the circumference thereof during the end stage of the rebound stroke.
7. 7. A unit as claimed in claim 6, wherein the seal comprises a resilient sealing ring located in recess in a bore in the first piston.
8. 8. A unit as claimed in any one of claims 4 to 7, wherein the rod is connected to the second piston.
9. 9. A unit as claimed in any one of claims 4 to 8, wherein the second piston is movable by the compressed gaseous medium during the compression stroke to reduce the volume of the damping chamber thereby to cause the displacement of the damping fluid and the rod is effective on movement of the housings together at a rate exceeding a predetermined rate to mechanically positively drive this piston.
10. 10. A unit as claimed in claim 9, wherein the rod is disposed at a spacing from a drive surface in an unloaded state of the unit and arranged to be drivingly engaged by that surface on movement of the housings together at a rate exceeding the predetermined rate.
11. 11. A unit as claimed in any one of the preceding claims, wherein the second piston is arranged to separate the spring chamber from the damping chamber.
12. 12. A unit as claimed in any one of the preceding claims, wherein the housings addition contain a further damping chamber for receiving displaced fluid from and returning the fluid to the first-mentioned damping chamber, the further damping chamber being disposed between and bounded by the two housings.
13. 13. A unit as claimed in any one of the preceding claims, wherein the gaseous medium is air.
14. 14. A unit as claimed in any one of the preceding claims, wherein the damping fluid is hydraulic oil.
15. 15. A spring and damper unit substantially as hereinbefore described with reference to the accompanying drawings.