WO2009078791A1

WO2009078791A1 - Shock absorber with increasing damping force

Info

Publication number: WO2009078791A1
Application number: PCT/SE2008/051431
Authority: WO
Inventors: Nils Göran NYGREN; Magnus Danek; Johan Jarl
Original assignee: öHLINS RACING AB
Priority date: 2007-12-14
Filing date: 2008-12-10
Publication date: 2009-06-25
Also published as: US20110017558A1; SE0702798L; SE531501C2; JP6084643B2; CN101896732A; EP2232095A1; CN101896732B; EP2232095B1; EP2232095A4; JP2015121326A; JP2011506874A; US8955654B2

Abstract

The invention relates to an arrangement in a vehicle shock absorber (1) for generating an additional counter-force (F2) acting counter to the direction of movement in the limit position (s) (R1) of a movement /stroke between the parts (4, 2) of the shock absorber moving relative to one another, and to a shock absorber (1) comprising such an arrangement. The shock absorber in both cases substantially comprises a damping medium-filled damping cylinder (2), divided into two damping chambers (Cl, C2 ), by a main piston (5), axially moveable relative to the damping cylinder (2) and attached to a piston rod (4). Also arranged around the piston rod (4) is a second piston/damping piston (5), which contains one or more first and second through-ducts (9, 10), which are defined by first and second flow-limiting devices (11, 12), preferably consisting of multiple thin first washers. The damping piston (5) is intended to slide in a restricting space (6) located in the damping cylinder (2), and having an inside diameter smaller than the inside diameter of the damping cylinder. The additional counter- force counter (F2) is substantially constant throughout the entire stroke in order to create gentle braking of the damping movement.

Description

SHOCK ABSORBER WITH INCREASING DAMPING FORCE

Technical field The invention relates to a vehicle shock absorber in the main comprising a damping medium-filled damping cylinder divided into two damping chambers, a compression chamber and a return chamber, by a main piston attached to a piston rod. The main piston moves axially in the damping cylinder in a compression and return movement. When the shock absorber reaches a certain compressed state, the movement is braked by a function which increases the damping.

Background of the invention

This type of shock absorber is disclosed, for example, by EP0565832, GB998742 or the applicant's patent EP1006292B1, in which the compression movement is braked by a second piston, here referred to as the damping piston, which at a certain stroke length reaches a cup having a smaller diameter than the damping cylinder.

It has proved a problem, in a shock absorber with this type of progressive damping, to create a gentle braking of the damping movement in the limit position. When the second piston reaches the cup of smaller diameter, the piston has a high velocity, which generates a large braking force around the point of maximum engagement. The counter-force then diminishes as the velocity decreases .

A shock absorber configured according to EP1006292, for example, has a second piston with a number of thin washers, a so-called shim stack, which opens under a certain force. The opening force is generated when the pressure in the cup acts on the opening area of the shim stack. The opening characteristic of a shim stack can be varied by adjusting the size, thickness, number and reciprocal arrangement of the washers (the shims) . A relatively progressive damping characteristic is most often created, which gives a force increasing with the velocity, which leads to large increases in the force per unit time when the second piston comes into engagement at high velocity. This behavior can be a problem for a driver of the vehicle, since the braking may then be perceived as jerky. Rapid changes in the force between the wheel and the road surface run the risk of leading to rapid changes in the distribution of the adhesion between the road surface and the different wheels. This makes handling of the vehicle more difficult, since it becomes less predictable. It is therefore desirable, instead of having a counter-force which increases with increasing velocity, to have a counteracting damping force that is as constant as possible throughout braking of the movement.

EP0565832 also illustrates a third problem of the known technology, that is to say the problem of creating a function which will afford a fully controllable braking of the damping movement without the second piston at the same time contributing to a geometrically over- defined system, which may become locked or require extremely precise tolerances. In EP0565832 this problem is resolved in that the second piston has an outside diameter which is significantly smaller than the inside diameter of the cup, and that any movement in a radial direction is absorbed by a flexible washer. EP1006292 also resolves this problem in that the second piston has an outside diameter which is smaller than the inside diameter of the cup. In this case, however, the leakage flow of damping medium that passes on the outside of the piston is deducted from the total braking function, that is to say the braking characteristic is not solely determined by the opening characteristic of the shim stack. At low piston velocities the braking characteristic is in principle determined solely by the magnitude of the leakage flow. In both cases this leads to a very progressive damping characteristic, that is to say the damping force increases sharply with the velocity, with all the implicit disadvantages described above.

Object of the invention

The present invention is intended to solve the problem of creating a gentle braking of the movement in the limit position of a shock absorber movement, which will prevent bottoming in the shock absorbing device, the gentle braking creating the prerequisites for a comfortable vehicle predictable for the driver.

The invention also addresses the problem of ensuring that the force serving to brake the damping movement increases manifestly with the velocity.

A further problem addressed by the invention is that of allowing a fully controllable braking of the damping movement without the second piston contributing to a geometrically over-defined system that is capable of locking.

The invention is furthermore intended to solve these problems in a simple way that does not require extremely precise tolerances.

Summary of the invention

The invention relates to an arrangement in a vehicle shock absorber intended for creating an additional counter-force acting counter to the direction of movement in the limit position (s) of a movement/stroke between the parts of the shock absorber moving relative to one another, and to a shock absorber comprising such an arrangement .

The shock absorber in both cases basically comprises a hydraulic damping medium- filled damping cylinder divided into two damping chambers, for example a compression chamber and a return chamber. The two damping chambers are divided by a main piston, axialIy moveable relative to the damping cylinder and attached to a damping piston holder part, which is fastened to or which forms a part of a piston rod. Also arranged round the piston rod is at least a second piston/damping piston, the first side of which faces towards the main piston and contains one or more first and second through-ducts . The ducts are defined in a direction of flow by flow limiters, which produce a restriction of the damping medium flow, the flow limiters preferably being embodied as one or more thin first washers or shims. The first of these ducts, at least, are defined by a first flow limiter on the first side of the damping piston. The damping piston has a piston outside diameter smaller than the outer diameter of the main piston and is intended to slide in a restricting space located in and hydraulically coupled to the damping cylinder. The restricting space, which may take the form of a cup, for example, has an inside diameter smaller than the inside diameter of the damping cylinder.

A braking of the damping movement commences at a certain stroke length which corresponds to the distance as the dampincj piston reaches the restricting space. The counter-force from the shock absorber acting counter to the direction of movement derives partly from the pressure drop over the main piston and partly from the pressure drop over the damping piston when this is engaged in the restricting space. The force from the damping piston is substantially constant largely throughout the entire stroke as the damping piston moves towards its limit position and is situated in the parts of the restricting pace where full engagement occurs. This is due to the design of the damping piston, which generates a damping force that varies little as a function of the velocity. This has been achieved in that the damping medium flow acting on the first flow limiter is designed to act in a space located between the first flow-limiting device and the side of the damping piston remote from the direction of movement. This space is of such a large outer radial extent that when pressure differentials over the damping piston open the flow path through the first ducts, the restriction of the damping medium flow varies minimally with increased flow, that is to say it produces low lifting heights of the first flow limiter even in the case of relatively large flows. This results in small variations in the pressure drop over the first flow limiters as the flow varies.

Consequently when the flow path through the first ducts opens, the first flow limiter rises a certain lifting height from the damping piston, so that an opening area is created between the first flow limiter and the side of the damping piston remote from the direction of movement. This opening area determines the restriction of the damping medium flow.

The leakage flow past the piston has moreover been minimized, that is to say the flow between the damping piston and the inner surface of the restricting space varies less as a function of the pressure. A minimal leakage flow is made possible in that the damping piston is designed to move in a radial direction in relation to the damping piston holder. A modulation of the restricting space is then possible without the need for a large difference between the outside diameter of the damping piston and the inner surface of the restricting space.

In order to allow a certain radial movement of the separating piston it is moreover not fully locked in an axial direction. This means that the tensioning, which varies as a function of the position of the damping piston relative to the device holding it, is influenced by any axial movement. Tensioning is made possible by the geometric design of the first flow limiter. The flow limiter is held between an outer and an inner damping piston surface, which are axially offset in relation to one another. The maximum counter- force for opening the first flow limiter is required when the flow limiter is under maximum tension. Maximum tensioning is obtained when the axial movement of the damping piston towards the main piston reaches its maximum. The flow limiter then bears against both the inner and the outer damping piston surface. The lower the velocity of the piston rod, the lower the impinging flow through the damping piston and hence also the lower the pressure drop over the damping piston, that is to say the axial movement of the damping piston towards the main piston is reduced.

The design with a large surface for the pressure to act upon in order to create the tensioning means that the tensioning can be achieved even in the event of small pressure drops over the damping piston, that is to say the tensioning is kept constant down to very low velocities. Since the velocity is usually greatest just when the damping piston has reached full engagement in the cup-shaped restricting space, the tensioning and the flow through the first flow limiter also reach a maximum at this point, that is to say the force acting on the first flow limiter is then greatest. Because the design of the flow limiter contributes to a damping piston that gives a slight increase in the pressure drop as the flow increases, the counter-force is reliably maintained even as the velocity and hence also the flow diminish. When the velocity then falls as the damping piston approaches its limit position, the counter-force also diminishes.

In a further embodiment of the arrangement an additional counter-force is generated in the limit position of both stroke directions in that a damping piston is located in each damping chamber. Both damping pistons are then located with their first side facing towards the main piston. This design means that the damping piston is moveable in an axial and in a radial direction in relation to the piston rod. A modulation of the restricting space is therefore possible without the damping piston catching or without the need for a large amount of play between the damping piston and the inside diameter of the restricting space.

The shock absorber is characterized in that the/ the first of the damping piston through-ducts opens into a space between an inner and an outer radial extent located between the first side of the damping piston and the first flow-limiting device. The space has a compression area that is greater than the area of the through-duct/the first of the through-ducts or the total area. The space also has a certain depth, so that damping medium can move therein. The damping piston comprises at least two first through-ducts and the space links at least two first through-ducts together. The outer radial extent of the space creates a large opening area even when the flow limiter lifts to a small lifting height from the damping piston. This means that when the first flow limiter has opened, the damping force only varies slightly in response to a change in the flow.

The damping piston is also designed with a radial play around the damping piston holder part but with a minimal play between the damping piston and the inside diameter of the restricting space. The damping piston is therefore able to move radially in relation to the damping piston holder part. The damping piston can also move axially in relation to the damping piston holder part, a movement which is limited by two position- limiting parts arranged in or on the damping piston holder part. In a first embodiment of the invention the damping piston moves in opposition to a resistance in an axial direction. This resistance may be created by a resilient device arranged between the damping piston and the position-limiting parts.

In a further embodiment the damping piston moves in opposition to a resistance created by a tensioning of the first flow limiter. This tensioning is created in that the first flow limiter is designed in a closed position to bear against a first and a second damping piston surface, which are offset in relation to one another. There the first damping piston surface is arranged on the first side of the damping piston inside the inner radial extent of the space and the second damping piston is arranged outside the outer radial extent of the space. The damping piston can naturally also be arranged so that it moves in opposition to a combined resistance from both the resilient device and the tensioning force.

In a third embodiment of the shock absorber two damping pistons are arranged on the piston rod, one on a first side of the main piston and the other on a second side of the main piston. Both of the damping pistons have their first side facing towards the main piston.

The invention will now be described in more detail below, with references to the drawings attached.

Description of the drawings

Fig. 1 shows a shock absorber according to the invention

Fig. 2a shows a force-velocity diagram for the main piston, according to the invention and according to the state of the art

Fig. 2b shows a force-velocity diagram for the damping piston, according to the invention and according to the state of the art Fig. 2c shows a force-velocity diagram for the main piston with superimposed damping piston, that is to say the entire shock absorber, according to the invention and according to the state of the art Fig. 2d shows a diagram of the acceleration absorbed in the shock absorber over time, according to the invention and according to the state of the art Fig. 3a shows an enlarged view of the second piston/the damping piston according to the invention Fig. 3b shows a further enlarged view of the damping piston with a closed first flow limiter without tensioning

Fig. 3b shows a further enlarged view of the damping piston with an open first flow limiter and with tensioning

Figs. 4a-4c show the damping piston with the dimensions thereof .

Detailed description of the invention Fig. 1 shows a shock absorber 1 according to the invention. The shock absorber 1 in this embodiment comprises a damping cylinder 2 filled with damping medium Ml and divided into two damping chambers by a main piston 3 attached to a piston rod 4. The damping medium Ml is preferably hydraulic oil, which may contain associated additives of a type known in the art. Alternatively glycol and/or water may be used as fluid. The two damping chambers may be referred to as the compression chamber and the return chamber Cl and C2 respectively, since the main piston 3 moves axially in the damping cylinder 2 in a compression and a return movement in proportion to the movement of the vehicle wheel and chassis towards or away from one another in the directions Rl, R2. The one second piston or damping piston 5 of an outside diameter ds_y is seated on a damping piston holder part 13, located at an upper piston rod end 4a on the compression side of the main piston 3. The damping piston 5 is designed to move axially in a restricting space β, defined by a substantially cup-shaped part 6a. The restricting space 6 has an inside diameter d₆i which is smaller than the inside diameter d_2l of the damping cylinder 2. The lower piston rod end 4b has a first fixing member II, for example an end eye, which is intended for attaching the damper to a part of a vehicle in question that moves with the underlying road surface, preferably a wheel or runner. At the upper end 2a of the damping cylinder 2 the damper comprises a second fixing member 12, such as a cylinder head with associated top eye, which can be attached to a chassis or frame part of a vehicle in question. It is naturally also possible to reverse the direction of assembly.

In the exemplary embodiment the shock absorber is a pressurized damper wherein the damping medium in the damping cylinder, via the compression chamber, is pressurized by a pressurization reservoir 7, which is located externally or internally inside the damping cylinder and inside which a pressurizing member 7a in the form of a piston, a rubber bladder or the like is arranged. The pressurization reservoir in a first space 7b encloses damping medium, preferably the aforementioned hydraulic oil with any additives or other fluid (see above) . The pressurization reservoir is defined by the pressurizing member 7a also in a second space 7c, which contains a second medium more compressible than the damping medium. The compressible medium may consist of a gas, such as air, nitrogen gas or other gas with additives, for example. Filling the second space with compressible medium creates a basic pressure which pressurizes the damping medium. The compressible medium may also be replaced by a mechanical member such as a spring or the like.

The pressurization of the damper and also the damping characteristic of the damper are adjusted by one or more valves 8 according to the state of the art, arranged between the pressurization reservoir and the damping cylinder.

When the damper is working, the piston rod 4 moves at a certain velocity in relation to the damping cylinder 2 and a certain velocity gives a certain damping medium flow through the main piston 3, and in the limit position also through the damping piston 5. During a damping stroke the main piston 3 usually has the highest velocity when it is situated axially approximately in the middle of the damping cylinder 2. When the main piston begins to approach its limit position, the velocity diminishes before reaching zero in the reversal position and then increases again when the piston rod reverses its direction of movement. As long as the second piston/the damping piston 5 is free in the damping chambers, it is only the main piston 3 that contributes to the damping characteristic of the damper. When the piston rod 4 and the damping piston 5 move in the direction Rl towards the limit position, see Fig. Ia, and reach the cup βa, it begins to act in opposition to the volume of damping medium that is enclosed in the restricting space 6. An additional force F2 counteracting the movement Rl is then generated, which is superimposed on the damping force Fl, which is generated by the main piston 3 and which brakes the movement in the limit position and counteracts the bottoming of the damper. The magnitude of the counter-force F2 is determined by the pressure differential over the damping piston 5 and the effective compression area on which the pressure acts. The damping medium flow is determined by the pressure differential over the piston and the design of the restriction.

See Figs 2a-2c which show force-velocity diagrams for the main piston (Fig. 2a) and the damping piston (Fig. 2b) respectively and the main piston with damping piston superimposed (Fig. 2c) . Fig. 2d shows a diagram of the acceleration absorbed in the shock absorber over time .

Fig. 2a shows an example of the increase in force generated by the pressure differentials over the main piston 3 of the shock absorber. The appearance of the curve may vary considerably but always increases with the velocity. The velocity can be said to vary from V₀-

V₂ when the additional damping piston 5 comes into engagement at the velocity v₂.

In Fig. 2b the dashed line shows an example of the pressure build-up for a damping piston 5 according to the state of the art, wherein the force manifestly increases with the velocity v. The progressive increase in force derives from the relatively large leakage flow which in the state of the art passes between the damping piston and the damping cup that defines the restricting space. The solid line shows the pressure build-up for the damping piston 5 according to the invention, where the force remains substantially constant as the velocity increases. A slight shift in the force increase is shown even here, since a minimal leakage flow must be allowed to pass between the damping piston and the damping cup.

Fig. 2c shows the force absorbed for the main piston with superimposed damping piston, that is to say the force absorption for the complete shock absorber. The solid line represents the shock absorber according to the invention and the dashed line is the state of the art. Let it be assumed in this case that the compression of the damper occurs at the velocity v₂ when the damping piston engages in the damping cup. This means that the force increases from Fl₂ to F1₂+F2₂ when a force limiter according to the invention is used. At the same time the counter-force persists further down the velocity range and the damping energy absorbed (which can be said to correspond to the area below the graph) can be increased without any increase in the jerkiness. In a design construction according to the state of the art the damping force increases from Fl₂ to F1₂+F3₂ when the damping piston goes into engagement. This damping force F1₂+F3₂ is greater than Fl2+F2₂ but at lower velocity vi the combined damping force Fli+F3i is less than for the design construction according to the invention Fli+F2i. The combined damping energy which the design construction according to the state of the art can absorb is therefore less than the design construction according to the invention, and the risk of the shock absorber bottoming, that is to say reaching maximum stroke length, is then greater.

Fig. 2d shows how the retardation occurs over time with a shock absorber having an additional force limiter according to the state of the art (dashed line) and a shock absorber having an additional force limiter according to the invention (solid line) . A steep gradient of this curve corresponds to a heavy jerk, that is to say the driver experiences a large force from the damper. At time t₂ the additional damping piston 5 encounters the restricting space 6 and the velocity is braked heavily by an acceleration, which in the state of the art has the value a₂ and according to the invention a.χ. As can be seen, ai is much less than &2, which implies that the movement is braked with a much softer movement.

When the piston rod is working in the direction R2 from the limit position, see Fig. Ib, only a low pressure drop is generally generated over the damping piston 5 and only a negligible additional force occurs acting counter to the direction of movement. The force is generated by a second flow limiter 12, which acts as a non-return valve and opens even in the event of small pressure differentials over the damping piston. The damping characteristics in this direction R2 are in the main determined by the pressure drop that is generated over the main piston 3.

Fig. 3a shows an enlarged view of the second piston/the damping piston 5. The damping piston 5 has first and second through-ducts 9, 10, which are defined in a direction of flow by a first and a second flow limiter 11, 12 in the form of one or more thin washers. A third duct (not shown) may also be arranged running through the piston, which has a fixed restriction, through which the magnitude of the flow does not vary substantially as a function of the pressure. The number of ducts is limited by the total volume of the damping piston, since a certain quantity of piston material must be retained in order to maintain the stability and strength of the damping piston, but the total number of ducts is preferably 6-12. Of the ducts, the first ducts 9 conduct the flow in the event of a stroke in the direction of movement towards the limit position (compression) and the second ducts 10 in the event of a stroke away from the limit position (return) . The first flow limiter 11 is located on a first side 5a of the damping piston 5 and is therefore intended to restrict the flow of damping medium from the restricting space 6 to the ordinary damping chambers C1/C2. The second flow limiter 12 is located on a second side 5b of the damping piston 5 and limits the flow through the second ducts 10 when the damping piston 5 is carried into the restricting space 6, that is to say in the direction Rl. The second flow limiter 12 opens very slightly when the damping piston 5 moves in the direction away from the limit position, that is to say in the direction R2. A negligible counter-force is consequently created in the event of a stroke in the direction away from the limit position, but an open flow through the second ducts 10 is prevented in the case of a stroke in the direction towards the limit position. In order that the second flow limiter 12 will open slightly, it is embodied as a very thin washer, which flexes slightly and allows damping medium to pass through. It also has at least three support points 12a, 12b, 12c in order to reduce the flexure of the washer when the damping piston moves in the direction towards the limit position, that is to say in the direction Rl.

The damping piston holder 13 part has a certain first diameter di₃. The damping piston holder part 13 can also be said to comprise the piston rod 4, if the piston rod 4 is designed so that, together with a threaded spacing sleeve, for example, for squeezing together the main piston flow limiter, it replaces the damping piston holder part 13. The upper part 4a of the piston rod 4 can then be said to have the first diameter

The diameter di₃ of the damping piston holder part 13 is smaller than the inside diameter d_5l of the damping piston 5, so that a radial play x results between the damping piston 5 and the damping piston holder part 13. The damping piston 5 is able to move in a radial direction in relation to the damping piston holder part 13.

The location of the damping piston 5 on the damping piston holder 13 is determined by two position limiting parts 14, 15, which are arranged in the damping piston holder 13 part. In the embodiment according to Fig. 2a the first position limiting part is a locking ring 14 arranged at the first piston rod end 4a and the second position limiting part is a stop in the form of a heel

15 created by a variation in the area of the damping piston holder part 13. The distance yi between the locking ring 14 and the heel 15 is somewhat greater than the total height h_tot of the piston 5, including that of the flow limiters 9, 10, so that the piston can move axially by a distance y₂ in relation to the piston rod. This selected distance y₂ is preferably very short but is required so that the damping piston will be able to move freely in a radial direction. The axial degree of movement y₂ can also be adjusted by stop washers 16a, 16b, which have been placed between the locking ring 14 and the heel 15 and the flow limiters 9, 10, which bear against the first and second side 5a, 5b of the piston. The axial movement of the piston can be adjusted by resilient devices 18, preferably in the form of a corrugated washer or a resilient 0-ring, arranged between the damping piston 5 and the position- limiting parts 14, 15.

Fig. 3b and also Figs 4a-4c show an enlarged view of the damping piston wherein the first of the damping piston through-ducts 9 open into a space 17, which extends radially between an inside diameter d_i7l and an outside diameter di_7γ arranged between the first side 5a of the damping piston and the first flow-limiting device 11. The space 17 has a certain compression area 17' and a certain depth in relation to the first piston surface 5a. The inside diameter di_7i of the space 17 is greater than the diameter d₄ of the piston rod/the piston rod extender.

The surface of the first side 5a of the damping piston which results between the inside diameter dχ_7l of the space and the piston rod may be termed a first damping piston surface 5a'. The area of the first damping piston surface 5a' outside the outside diameter di_7γ of the space may be termed the second damping piston surface 5a' '.

Depending on the design of the damping piston 5, the compression area 17' may also extend all the way out to the outside diameter of the second damping piston surface 5a' '. The outside diameter di_7γ of the space is therefore shifted outwards and the first flow limiter 11 in the closed position rests on a minimal external surface of the second damping piston surface 5a''. When the flow limiter opens, the adhering characteristics of the damping medium against the damping piston surface 5a'' are minimized.

The first damping piston surface 5a is offset in relation to the second damping piston surface 5a'' so that the depth zl of the space in relation to the first damping piston surface 5a' is shallower than the depth z2 of the space in relation to the second damping piston surface 5a' ' .

The space 17 encloses the piston rod 4 and the first ducts 9 open out into this space. The damping medium that flows through these first ducts 9 can therefore be dispersed into the space 17 where it contributes to an increase in the force, which at a certain value generates such a large force on the first flow limiter 11 that it begins to open. The additional counter-force F2 is substantially constant basically throughout the stroke as the damping piston 5 moves towards its limit position and is situated in the restricting space 6. This is because the restriction between the damping piston 5 and the first flow limiter 11 diminishes rapidly with the lifting height s.

The outside diameter di_7y of the space 17 is so large that only a slight increase in the lifting height s of the first flow limiter 11 allows a greater quantity of damping medium to flow through the damping piston 5, see Fig. 3c. The damping medium flow is therefore determined by the opening area extending in an axial direction. The opening area is the enveloping area of the opening and therefore varies as a function of the outside diameter di_7γ of the space and the lifting height s .

Fig. 3c also shows how the first flow limiter 11 is transformed when it is exposed to a sufficient counter- force F2. The full opening force F2 occurs when the piston rod 4 moves at least at a certain minimum velocity in relation to the damping cylinder in the direction Rl towards the limit position and the damping piston 5 reaches the restricting space 6. Since no flow passes through the damping piston 5, the inner part 11a of the first flow limiter bears against the first damping piston surface 5a' and the outer part lib of the first flow limiter bears against the second damping piston surface 5a' '. When the first flow limiter opens, its outer part lib is therefore relieved from the second piston damping surface 5a' ' by a certain distance s, but the inner part 11a of the flow limiter continues to bear against the first damping piston surface 5a' .

Figs. 4a-4c show only the damping piston 5 with its duct orifices and flow spaces. The damping piston 5 has a chamfer at its outside diameter d_5γ in the direction towards its second surface 5b, thereby facilitating the centering of the damping piston 5 in the restricting space 6. Fig. 4a shows a view of the second surface 5b of the damping piston, with the support points 12a, 12b and 12c of the second flow limiter 12. Fig. 4c shows a view of the first surface 5a of the damping piston showing the outside diameter di_7y and inside diameter di_7l of the space 17. The first damping piston area 5a' extends between the inside diameter ds_a. of the damping piston and the inside diameter di_7l of the space, and the second damping piston area 5a' ' extends between the outside diameter di_7γ of the space and the outside diameter d_5γ of the damping piston.

In a further variant of the shock absorber two damping pistons 5 are used, see Fig. 5, one on either side of the main piston 3 in order to generate an additional counter-force in the limit position of both stroke directions Rl, R2. The damping pistons 5 are designed in the same way as described above. The invention is not limited to the embodiment shown, a shock absorber in which both sides of the main piston are pressurized also being feasible, for example by pressurizing one space common to both of the damping chambers adjoining the main piston or directly adjoining the pressurization reservoir. The flow limiters can furthermore be any form of cone valve and therefore need not be shimmed valves. The invention can also be modified without departing from the scope of the following patent claims or the idea of the invention.

Claims

Patent claims

1. An arrangement in a vehicle shock absorber (1) for generating an additional counter- force acting counter to the direction of movement in the limit position (s) of a movement/stroke between the parts (4, 2) of the shock absorber moving relative to one another, wherein the shock absorber substantially comprises a hydraulic damping medium-filled damping cylinder (2), divided into two damping chambers (Cl, C2), for example a compression chamber and a return chamber, by a main piston (3), axially moveable relative to the damping cylinder (2) and attached to a piston rod (4), and wherein at the first end (4a) of the piston rod (4) , centered around a damping piston holder part (13), at least a second piston/damping piston (5) is also arranged, the first side (5a) of which faces towards the main piston (3) and contains one or more first through-ducts (9) and second through- ducts (10) , wherein at least the first of these ducts

(9) are defined in a direction of flow on the first side (5a) of the damping piston by a first flow limiter

(11) , which produces a restriction of the damping medium flow through the fist flow ducts (9) , and wherein the damping piston (5) has an outside piston diameter (ds_y) smaller than the outside diameter of the main piston (3) and is intended to slide in a restricting space (6) located in and hydraulically coupled to the damping cylinder (2) , but with an inside diameter (d_6l) smaller than the inside diameter (d₂j of the damping cylinder, so that a braking of the damping movement occurs at a stroke of a certain magnitude, characterized in that the counter- force acting counter to the direction of movement is substantially constant throughout the entire stroke as the damping piston (5) moves towards its limit position and is situated in the restricting space (6), and in that a minimal damping medium flow passes between the damping piston (5) and the inside diameter (d_6l) of the restricting space (6) , whilst the damping medium flow which acts on the first flow limiter (11) acts in a space (17) arranged between the first flow limiter (11) and the side (5a) of the damping piston (5) remote from the direction of movement, which is of such a large outer radial extent

(di_7y) that when pressure differentials over the damping piston (5) open the flow path through the first ducts

(9), the restriction of the damping medium flow varies minimally with increased flow.

2. The arrangement as claimed in claim 1, characterized in that when the flow path through the first ducts (9) opens, the first flow limiter (11) rises a certain lifting height (s) from the damping piston (5), so that an opening area is created between the first flow limiter (11) and the side (5a) of the damping piston (5) remote from the direction of movement, which determines the restriction of the damping medium flow.

3. The arrangement as claimed in claim 2, characterized in that the damping piston (5) is designed to move in an axial and a radial direction in relation to the piston rod (4) , so that a modulation of the restricting space is possible without a large difference occurring between the outside diameter (ds_y) of the damping piston and the inner surface of the restricting space (6) of diameter (d₆i) .

4. The arrangement as claimed in one of claims 1 to 3, characterized in that the first flow limiter (11) is tensioned between an outer damping piston surface (5a'') and an inner damping piston surface (5a'), which are offset in relation to one another, so that a large counter-force (F2) is generated acting counter to the movement .

5. The arrangement as claimed in claim 4, characterized in that the fully developed counter- force (F2) for opening the first flow- limiting device (11) is required when the pressure against the damping piston (5) becomes so great that the first flow-limiting device (11) is under maximum tension and is bearing against both the inner and the outer damping piston surface (5a' , 5a' ' ) .

6. The arrangement as claimed in one of the preceding claims, characterized in that it is designed to generate an additional counter-force in the limit position of both stroke directions (Rl, R2 ) , and in that a damping piston (5) is located in each damping chamber (Cl, C2 ) , both with the first side (5a) thereof facing towards the main piston (3).

7. A vehicle shock absorber (1) which substantially comprises a hydraulic medium-filled damping cylinder (2) , divided into two damping chambers (Cl, C2 ) , for example a compression chamber and a return chamber, by a main piston (3), axially moveable relative to the damping cylinder (2) and attached to a piston rod (4), wherein at the first end (4a) of the piston rod (4) , centered around a damping piston holder part (13) of the piston rod (4) , at least a second piston/damping piston (5) is also arranged, the first side (5a) of which faces towards the main piston (3) and contains one or more first through-ducts (9) and second through- ducts (10), wherein at least the first of these ducts (9) are defined in a direction of flow on the first side (5a) of the damping piston by a first flow limiter (11), and wherein the damping piston (5) has an outside piston diameter (d_5y) smaller than the outside diameter of the main piston (3) and is intended to slide in a restricting space (6) located in and hydraulically coupled to the damping cylinder (2), but with an inside diameter (d_6l) smaller than the inside diameter (d_2i) of the damping cylinder, characterized in that the/the first of the damping piston through-ducts (9) opens into a space (17) between the first side (5a) of the damping piston and the first flow limiter (11) between an inner radial extent (di₇i) and an outer radial extent (divy) arranged where the space (17) has a compression area (17') which is greater than the area of the/the first through-ducts (9) or the total area.

8. The shock absorber (1) as claimed in claim 7, characterized in that the pressure acting on the compression area (17') lifts the first flow limiter a distance (s) which basically varies as a function of the outer radial extent (di_7y) of the space (17) .

9. The shock absorber (1) as claimed in claim 7 or 8, characterized in that the damping piston (5) is designed with a radial play (x) around the damping piston holder part (13) but with a minimal play between the damping piston (5) and the inside diameter (d_δi) of the restricting space (6) .

10. The shock absorber (1) as claimed in one of claims 7 to 9 , characterized in that the damping piston (5) is capable of moving axially in relation to the damping piston holder part (13) in that an axial play is created and limited by two position-limiting parts (14, 15) arranged in or on the damping piston holder part (13), one on either side of the damping piston (5) .

11. The shock absorber (1) as claimed in claim 10, characterized in that the movement of the damping piston (5) in an axial direction occurs in opposition to a resistance.

12. The shock absorber (1) as claimed in claim 11, characterized in that the movement resistance of the damping piston (5) in an axial direction is created by a resilient device (18) arranged between the damping piston (5) and either of the position limiting parts (14, 15) .

13. The shock absorber (1) as claimed in claim 11 and/or 12, characterized in that the movement resistance to the movement of the damping piston (5) in an axial direction is created by a tensioning of the first flow limiter (11) ) .

14. The shock absorber (1) as claimed in claim 13, characterized in that the tensioning of the first flow limiter (11) is created in that the first flow limiter in the closed position bears against a first damping piston surface (5a') and a second damping piston surface (5a'') on the first side (5a) of the damping piston, which are offset in relation to one another.

15. The shock absorber (1) as claimed in claim 14, characterized in that the first damping piston surface

(5a') is arranged inside the inner radial extent (dχ₇i) of the space and the second damping piston (5a'') is arranged outside the outer radial extent (di7_Y) of the space.

16. The shock absorber (1) as claimed in one of claims 7 to 15, characterized by two damping pistons (5) arranged on the piston rod (4) , one on a first side of the main piston (3) and one on a second side of the main piston (3), both having their first side (5a) facing towards the main piston (3) .