Damping force generating mechanism for shock absorber

A damping valve (10) resists a flow of fluid from a first fluid chamber (41) to a second fluid chamber (42) which are separated by a piston (1). A partitioning member (24, 27-29, 52) partitions an inflow space (A, B) of a passage (2) in the first fluid chamber (41). A spool (17, 31, 51) decreases a flow cross-sectional area of a flow path from the first fluid chamber (41) into the inflow space (A, B) according to a differential pressure between the fluid chambers. By ensuring a gap between the outer circumference of the valve disk (1) and the partitioning member (24, 27-29, 52) which permanently allows fluid to flow from the first chamber (41) into the inflow space (A, B), the damping force characteristic in a high speed region of piston displacement can be set independently of the damping force characteristic in other regions.

FIELD OF THE INVENTION

This invention relates to a damping force generating mechanism provided in a shock absorber.

BACKGROUND OF THE INVENTION

A hydraulic shock absorber for a vehicle comprises, for example, two fluid chambers separated in the interior of a cylinder by a piston, and a passage provided through the piston to connect these fluid chambers. A damping valve in the form of a leaf valve is provided at an outlet of the passage to generate a damping force relative to displacement of the piston. The leaf valve generally comprises a plurality of stacked leaves having a fixed inner circumferential part, and lifts an outer circumferential part to open the passage according to the differential pressure between the upstream and downstream sides of the leaves. With this construction, the damping force generated by the leaf valve tends to be excessive when in a middle to high speed region of piston displacement.

In order to improve the damping force characteristic of a leaf valve, JPH09-291961A, published by the Japan Patent Office in 1997, proposes a leaf valve in which the inner circumferential part is not fixed but supported resiliently by a coil spring.

Referring toFIG. 10, in a shock absorber in which this leaf valve is installed, a cylindrical piston nut N is secured onto a tip of a piston rod R penetrating the piston P. The leaf valve L closing an outlet of a passage Po which passes through the piston P is fitted on the outer circumference of the piston nut N such that it can displace in an axial direction. A coil spring S an end of which is supported by the piston nut N, resiliently supports the inner circumferential part of the leaf valve L via a push member M.

When the piston P moves upward in the figure, working oil in an oil chamber above the piston P flows into an oil chamber below the piston P via the passage Po and a damping force is generated due to a flow resistance of the leaf valve L at the outlet of the passage Po. When the piston displacement speed is in a low speed region, the leaf valve L bends the outer circumferential part downward in the figure from the inner circumferential part supported by the push member M. As the piston displacement speed reaches a middle to high speed region, the pressure in the passage Po becomes greater than the resilient force of the coil spring S such that the leaf valve L retreats from the piston P downward in an axial direction together with the push member M. As a result, the opening area of the leaf valve L becomes large so that the damping force is prevented from becoming excessive. As shown inFIG. 11, the damping force increase is gradual with respect to an increase in the piston displacement speed even in the middle to high speed region.

SUMMARY OF THE INVENTION

This valve structure is effective in suppressing an excessive increase in the damping force generated in the middle to high speed region of piston displacement. Since the leaf valve L is kept in a retreated position once the piston displacement speed has reached the middle to high speed region of piston displacement, the damping force characteristic does not vary as long as the piston displacement speed varies in this region. When a spring load is set to obtain a preferable damping force in the middle speed region, therefore, the damping force generated in the high speed region may become insufficient.

It is therefore an object of this invention to provide a damping force generating mechanism which realizes different damping force characteristics in a middle speed region and a high speed region of piston displacement such that a preferable damping force is obtained in the respective speed regions.

In order to achieve the above object, this invention provides a damping force generating mechanism for a shock absorber which comprises a first fluid chamber, a second fluid chamber, a valve disk which separates the first fluid chamber and the second fluid chambers, and a passage formed through the valve disk to connect the first fluid chamber and the second fluid chambers.

The mechanism comprises a damping valve which exerts a resistance on a flow of fluid in the passage from the first fluid chamber to the second fluid chamber, a partitioning member which covers the valve disk and partitions an inflow space into the passage in the first fluid chamber, a first flow path connecting the first fluid chamber and the inflow space, a spool which decreases a flow cross-sectional area of the first flow path when a fluid pressure in the first fluid chamber increases beyond a fluid pressure in the second fluid chamber by more than a predetermined pressure, and a second flow path formed by the partitioning member and facing the outer circumference of the valve disk so as to allow fluid to flow from the first fluid chamber to the inflow space.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 1of the drawings, a hydraulic shock absorber for a vehicle comprises a cylinder40, a piston1enclosed in the cylinder40, as a valve disk, so as to be free to slide in an axial direction, and a piston rod5connected to the piston1and projecting axially from the cylinder40.

The piston rod5has a small diameter part5aformed at its lower end via a step5b. The small diameter part5apenetrates a through-hole1bformed in the piston1. A cylindrical part4aof a piston nut4is screwed onto a male screw part formed at a lower end of the small diameter part5a. A outer circumferential part1fof the piston1slides on the inner circumference of the cylinder40. The piston1has a closed-end cylindrical form and is fixed to the small diameter part5ain a bottom-up state by a piston nut4.

The interior of the cylinder40is separated by the piston1into a first oil chamber41located above the piston1and a second oil chamber42located below the piston1. Working oil is enclosed in the first oil chamber41and the second oil chamber42. The first oil chamber41and the second oil chamber42communicate with each other via passages2and passages1dformed respectively as through-holes in the piston1. Although not shown in the drawings, a reservoir or an air chamber is provided inside or outside the cylinder40as a known component of a hydraulic shock absorber to compensate for capacity variation in the cylinder40due to elongation and contraction of the piston rod5with respect to the cylinder40.

A damping valve100is provided at openings of the passages1dformed on an upper end face of the piston1facing the first oil chamber41. The damping valve100generates a damping force during contraction of the shock absorber, in which the piston1displaces downward in the figure, by resisting a flow of working oil from the shrinking second oil chamber42to the expanding first oil chamber41through the passages1d. The damping valve100is constituted by a leaf valve and also functions as a check valve that prevents a reverse flow in the passages1d. Holes100aare formed so that the damping valve100does not prevent the working oil from flowing through the passages2.

A damping valve10is provided at openings3of the passages2formed on a lower end face1aof the piston1facing the second oil chamber42. The piston1has a skirt portion at its lower end and a space surrounded by the skirt portion is used to accommodate the damping valve10. Owing to this construction, the entire length of a piston part of the shock absorber from the upper end face of the piston1to the lower end of the piston nut4can be shortened while ensuring the length of the sliding surface of the piston1.

The damping valve10generates a damping force during elongation of the shock absorber in which the piston1displaces upward in the figure, by resisting the flow of working oil from the shrinking first oil chamber41to the expanding second oil chamber42through the passages2. The damping valve10also functions as a check valve which prevents a reverse flow in the passages2.

The damping valve10is constituted by a leaf valve that has a plurality of stacked leaves covering the openings of the passages2. More specifically, the damping valve10comprises a washer7, the leaves10a, a washer8, a push member11, and a coil spring15.

The washer7, the plural leaves10a, the washer8, and the push member11are fitted on the outer circumference of a small diameter part4cof the piston nut4which is formed continuously with the cylindrical part4aand projects upward therefrom. The washer7is in contact with the lower end face1aof the piston1. The leaves10aare gripped between the washers7and8. The push member11applies a resilient force of the coil spring15upward to the washer8.

The push member11comprises a cylindrical part11bwhich slides on the outer circumference of the small diameter part4cand a disk part11awhich extends in a radial direction from the top end of the cylindrical part11b. The coil spring15is interposed between the disk part11aand a flange4bwhich is formed at the bottom of the cylindrical part4aof the piston nut4.

The cylindrical part11bhas a function of centering the coil spring15and thereby ensuring the resilient force of the coil spring15to be applied evenly to the push member11. It should be noted however that the cylindrical part11bcan be omitted.

A circular valve seat1cprojecting downward is formed on the lower end face1aof the piston1to surround the openings of the passages2and face the outer circumference of the leaves10a. The damping valve10closes the openings3of the passages2by causing the leaves10ato be seated on the valve seat1c. Further, although not shown in the figure, the leaves10ahave a minute notch or minute notches on the outer circumference that connects the passages2and the second oil chamber42even when the openings3are closed by the leaves10a. It is also possible to form a minute orifice or minute orifices on the valve seat1cby stamping instead of providing a notch or notches on the leaves10a. Providing such a passage or passages having a minute flow sectional area in the damping valve is known in the art.

The number of the leaves10adepends on the required damping force characteristic, or in other words the required relationship between the piston displacement speed and the generated damping force. A single leaf may be used depending on the required damping force characteristic. Further, it is possible to stack plural leaves10ahaving different diameters depending on the required damping force characteristic.

According to the construction of the damping valve10as described above, the inner circumferential part of the leaves10ais pressed against the lower end face1aof the piston1by the push member11that applies the resilient force of the coil spring15. Herein, the thickness of the washer7is set to be smaller than the distance from the lower end face1aof the piston1to the crest of the valve seat1cin the axial direction, thereby providing an initial bend to the leaves10a.

By regulating the amount of the initial bend, the opening pressure with which the leaves10aare lifted off the valve seat1cto open the passages2can be regulated. The amount of the initial bend can be regulated by altering the thickness of the washer7or stacking a plurality of the washers7. The amount of the initial bend should therefore be set such that the best damping force characteristic is obtained for the vehicle that uses the shock absorber. The washer(s)7may be omitted depending on the distance in the axial direction from the lower end face1ato the crest of the valve seat1c.

A disc spring, leaf spring, or a resilient material such as rubber may be used instead of the coil spring15to apply a resilient force to the leaves10a.

The shock absorber further comprises a pressure responsive throttle16provided with a partitioning member24and a spool17so as to provide a different damping force characteristic in the high speed region of piston displacement to the middle speed region thereof.

A washer101, the partitioning member24, and a pressure chamber partitioning member22are disposed above the piston1. These members are fitted on the outer circumference of the small diameter part5aof the piston rod5in this order from above such that the pressure chamber partitioning member22is in contact with the step5band gripped between the step5band the piston nut4together with the piston1. The piston1has a recess1eon the lower end face1ato accommodate the tip of the small diameter part4of the piston nut4.

The partitioning member24is formed into a cylindrical shape which covers an upper end1gof the piston1. A cylindrical lower end24eof the partitioning member24is located nearby the outer circumference of the upper end1gof the piston1. A circular recess is formed on an upper end face24aof the partitioning member24. According to this construction, the partitioning member24delimits an inflow space A of working oil into the passages2in the first chamber41above the piston1.

The inflow space A communicates with the passages2permanently via the holes100ain the damping valve100. The inflow space A also communicates with the first oil chamber41via communicating holes24bserving as a first flow path, which are formed through the partitioning member24in the vicinity of the outer circumference of the recess. Further, the inflow space A communicates with the first oil chamber41permanently via a minute annular gap serving as a second flow path, which is located between the lower end24eand the outer circumference of the upper end1gof the piston1.

The pressure chamber partitioning member22is formed into a closed-end cylindrical shape. The small diameter part5aof the piston rod5passes through a through-hole22cformed in the center of a bottom portion22aof the pressure chamber partitioning member22. The bottom portion22ais gripped between the recess of the partitioning member24and the step5b. The diameter of the bottom portion22awhich comes into contact with the recess of the partitioning member24is made smaller than that of the other part of the pressure chamber partitioning member22so as not to block up the communicating holes24b. The pressure chamber partitioning member22comprises a cylindrical portion22bopening upward and a flange portion22dextending radially from the bottom portion22a.

The spool17is fitted onto the outer circumference of the piston rod5so as to be free to slide in the axial direction. The spool17comprises a bottom17awhich the piston rod5penetrates and a cylindrical part17bextending downward from the outer circumference of the bottom17a. The cylindrical part17bis fitted onto the outer circumference of the cylindrical portion22bof the pressure chamber partitioning member22. An enlarged inner diameter part17cis formed on the inside of a lower end of the cylindrical part17b. The enlarged inner diameter part17cis fitted onto the outer circumference of the flange part22dof the pressure chamber partitioning member22and has a tip facing an annular valve seat24cformed in the vicinity of a slanted wall face24dwhich forms the recess on the partitioning member24. The slanted wall face24dhas a conical shape which decreases in diameter downward and causes an annular gap formed between the tip of the enlarged inner diameter part17cand the slanted wall face24dto decrease gradually as the tip of the enlarged inner diameter part17capproaches the annular valve seat24c.

According to the above construction, a pressure chamber26having a ring-shaped horizontal cross-section is formed between the enlarged inner diameter part17cof the spool17and the cylindrical portion22bof the pressure chamber partitioning member22

In order to introduce fluid pressure from the second oil chamber42into the pressure chamber26, a pilot passage5dis formed through the small diameter part5aof the piston rod5. Further, a port22econnecting the pilot passage5dto the pressure chamber26is formed in the pressure chamber partitioning member22in a radial direction. An orifice23ais provided in the port22e. The orifice23ais formed in a plug23screwed into the inner circumference of the port22e.

A coil spring25is interposed between the spool17and the pressure chamber partitioning member22in a position surrounding the piston rod5. An upper end of the coil spring25is supported by the bottom17aof the spool17. A lower end of the coil spring25is supported by the bottom portion22aof the pressure chamber partitioning member22.

The coil spring25applies a resilient force to the spool17in a direction to cause the spool17to retreat from the partitioning member24, or in other words a direction for supplementing the action of the pressure in the pressure chamber26. Displacement of the spool17in this direction is limited by a stop ring18fitted onto the outer circumference of the piston rod5. This position of the spool17is expressed as a retreated position.

A space delimited by the spool17, the piston rod5and the pressure chamber partitioning member22is used to accommodate the coil spring15and is permanently connected to the first oil chamber41via a communicating hole17dpenetrating the bottom17aof the spool17, thereby preventing the working oil enclosed in this space from locking the axial displacement of the spool17.

The pressure in the first oil chamber41pushes the spool17downward due to a difference in the upward-facing pressure receiving area and the downward-facing pressure receiving area of the spool17. On the other hand, the pressure in the pressure chamber26and the resilient force of the coil spring25act upward on the spool17. The spool17, when displacing downward, causes the tip of the enlarged inner diameter part17cto approach the partitioning member24such that the annular gap formed between the tip of the enlarged inner diameter part17cand the inclined wall face24ddecreases gradually. As a result, the flow resistance to the working oil flowing from the first oil chamber41to the second oil chamber42via the annular gap increases. The downward displacing spool17finally causes the tip of the enlarged inner diameter part17cto be seated on the annular valve seat24con the partitioning member24such that the flow of working oil from the first oil chamber41into the second oil chamber42via the annular gap is shut off.

The damping valve10and the pressure-responsive throttle16function as described below.

When the piston1displaces upward in the cylinder40, or in other words when the shock absorber elongates, the first oil chamber41shrinks and the second oil chamber42expands. According to this action, the working oil in the first oil chamber41flows into the second oil chamber42via the inflow space A, the passages2, and the damping valve10.

When the piston displacement speed is very low, the opening pressure acting on the damping valve10is too low to cause the leaves10a, which are under the initial bend, to open the passages2. The working oil in the passages2flows into the second oil chamber42via the notch(es) formed in the leaves10aor the orifice(s) formed in the valve seat1cas described above. Since the flow rate of the working oil flowing into the second oil chamber42is very small in this state, the damping force generated by the damping valve10is also very small.

As the piston displacement speed increases, the leaves10abend downward from the outer rim of the washer8and the flow cross-sectional area of the working oil flowing out from the passages2into the second oil chamber42increases. The damping force generated by the damping valve10in this state depends on the elastic deformation of the leaves10a, and increases sharply with respect to an increase in the piston displacement speed, as shown in the low speed region inFIG. 2.

When the piston displacement speed reaches the middle speed region, the differential pressure between the first oil chamber41and the second oil chamber42increases further, and the leaves10amove downward inFIG. 1against the resilient force of the coil spring15. As the leaves10amove downward, the flow cross-sectional area of the opening3of the passages2increases greatly. The distance between the leaves1aand the opening3of the passages2increases as the piston displacement speed increases, and hence an increase in the damping force generated by the damping valve10in the middle speed region of piston displacement is much gentler than in the low speed region, as shown inFIG. 2.

When the piston displacement speed reaches the high speed region, the pressure-responsive throttle16displaces the spool17downward inFIG. 1from the retreated position against the resilient force of the coil spring25and the pressure in the pressure chamber26, causing the annular gap between the tip of the enlarged inner diameter part17cand the slanted wall face24dto narrow gradually. When the tip of the enlarged inner diameter part17cis seated on the annular valve seat24c, the flow of working oil from the first oil chamber41to the second oil chamber42via the annular gap is shut off.

According to the above action of the pressure responsive throttle16, the damping force generated during the elongation stroke of the hydraulic shock absorber increases greatly immediately after the piston displacement speed reaches the high speed region. This rapid increase characteristic of the damping force can be set differently by selectively setting the gradient of the slanted wall face24dof the partitioning member24in advance.

It should be noted that the spring load characteristic of the coil spring25is set in advance such that the enlarged inner diameter part17cis seated on the annular valve seat24cimmediately after the piston displacement speed reaches the high speed region. According to this setting of the spring load of the coil spring25, the pressure-responsive throttle16does not operate as long as the piston displacement speed stays in the middle speed region or low speed region. The damping force accompanying the flow of working oil from the first oil chamber41to the second oil chamber42in these speed regions is generated exclusively in the damping valve10.

After the tip of the enlarged inner diameter part17cof the spool17is seated on the annular valve seat24cin the high speed region, the working oil in the first oil chamber41flows into the passages2only via the minute annular gap between the lower end24eof the partitioning member24and the outer circumference of the upper end1gof the piston1. As a result, the pressure loss in the working oil flowing from the first oil chamber41to the second oil chamber42increases greatly with respect to an increase in the piston displacement speed.

According to this damping force generating mechanism, therefore, different damping force characteristics are obtained in the low speed region, the middle speed region, and the high speed region.

Further, by selectively setting the gradient of the slanted wall face24dof the partitioning member24in advance, the rapid increase characteristic of the damping force immediately after the piston displacement speed reaches the high speed region can be set according to circumstances. This ability to set the damping force characteristics according to circumstances is favorable in preventing the driver or passengers of the vehicle from feeling discomfort or a shock.

Further, since the pressure chamber26is connected to the second oil chamber42via the orifice23a, the pressure variation in the pressure chamber26always has a delay. Due to this delay, some time is required from the point at which the spool17starts to move until the tip of the enlarged inner diameter part17cbecomes seated on the annular valve seat24c. This means that the increase in the damping force is gentler than in a case where the port22eis not provided with the orifice23a, and hence the orifice23aalso helps in protecting the driver or passengers of the vehicle from feeling discomfort or a shock due to a rapid change in the damping force.

The rapid increase characteristic of the damping force generated immediately after the piston displacement speed has reached the high speed region will now be described in detail.

Assuming that the slanted wall face24ddoes not exist in the vicinity of the tip of the enlarged inner diameter part17c, the annular gap which generates resistance against the flow of working oil is always formed between the tip of the enlarged inner diameter part17cof the spool17and the annular valve seat24c. When on the other hand the slanted wall face24dexists in the vicinity of the tip of the enlarged inner diameter part17cas shown inFIG. 1, the distance between the tip of the enlarged inner diameter part17cof the spool17and the slanted wall face24dbecomes shorter than the distance between the tip of the enlarged inner diameter part17cof the spool17and the annular valve seat24c, and hence the annular gap formed between the tip of the enlarged inner diameter part17cand the slanted wall face24dis dominant in determining the magnitude of the generated damping force. In the former case, the damping force increases in a step like manner at the instant when the tip of the enlarged inner diameter part17cbecomes seated on the annular valve seat24c. In the latter case, since the annular gap is smaller than in the former case at the same stroke position of the spool17, a greater damping force is generated than in the former case. As a result, an increase in the generated damping force with respect to the displacement amount of the spool17in the latter case is gentler than in the former case.

FIG. 3shows a relation between the damping force generated by the hydraulic shock absorber and the stroke position of the piston1in an operating situation. In the hydraulic shock absorber, the piston displacement speed reaches a maximum when it passes a neutral position, irrespective of the piston stroke direction, i.e., the elongation stroke or the contraction stroke. The maximum damping force is therefore generated in the neutral position. In contrast, in the most elongated position and the most contracted position of the shock absorber, the piston displacement speed becomes zero and the generated damping force also becomes zero. The dotted line in the figure denotes a damping force generated when the slanted wall face24ddoes not exist in the vicinity of the tip of the enlarged inner diameter part17c, and the solid line in the figure denotes a damping force generated when the slanted wall face24dexists in the vicinity of the tip of the enlarged inner diameter part17c.

As can be understood from the figure, by providing the slanted wall face24din the vicinity of the tip of the enlarged inner diameter part17c, the rapid increase characteristic if the damping force immediately after the piston displacement speed reaches the high speed region can be made gentler. This action of the slanted wall face24d, in association with an effect brought about by a time delay in pressure variation in the pressure chamber26due to the orifice23a, brings a particularly favorable effect in terms of preventing noise and discomfort to the driver or passengers due to a rapid change in the damping force of the shock absorber.

On the other hand, when the piston1displaces downward inFIG. 1in the cylinder40, or in other words when the shock absorber contracts, the second oil chamber42shrinks and the first oil chamber41expands. According to this action, the working oil in the second oil chamber42flows into the first oil chamber41via the passages1d, and the damping valve100generates a damping force by applying a flow resistance to the flow of working oil through the passages1d.

Various variations are possible with respect to this embodiment.

According to this embodiment, the lower end24eof the partitioning member24is located in the vicinity of the outer circumference of the upper end1gof the piston1such that a minute annular gap left therebetween is used as the second flow path of the working oil that flows from the first oil chamber41into the inflow space A after the spool17is seated on the annular valve seat24c. However, the lower end24of the partitioning member24may be disposed in the vicinity of the inner circumference of the cylinder40such that a minute gap formed between the lower end24of the partitioning member24and the inner circumference of the cylinder40is used as the second flow path of the working oil that flows from the first oil chamber41into the inflow space A.

Referring toFIG. 4, the partitioning member24may be replaced by a partitioning member27which has a slanted wall face27don the inside of an annular valve seat27c. The partitioning member27further comprises a main body27a, communicating holes27b, and a lower end27eextending downward from the outer circumference of the main body27a. The slanted wall face27dis formed into a conical shape and disposed on the inside of the annular valve seat27cand on the outside of the communicating holes27b. The lower end27ereaches the vicinity of the outer circumference of the upper end1gof the piston1.

Referring toFIG. 5, the partitioning member24may be replaced by a partitioning member28in which the annular valve seat is omitted and the tip of the enlarged inner diameter part17cof the spool17is seated directly on a slanted wall face28d. A circular recess formed on the upper end face28aand communicating holes28bformed in the vicinity of the slanted wall face28dare equivalent respectively to the recess formed on the upper end face24aand the communicating holes24bof the partitioning member24. A lower end28cof the partitioning member28has a shape and function equivalent to those of the lower end24eof the partitioning member24.

It is also possible to omit the annular valve seat27cfrom the partitioning member27shown inFIG. 5such that the tip of the enlarged inner diameter part17cof the spool17is seated directly on the slanted wall face27d.

Referring toFIG. 6, instead of the combination of the spool17and the partitioning member24, a spool30having a tapered face30aon the outer circumference of a lower end of an enlarged inner diameter part30cmay be used together with a partitioning member29which has a recess surrounded by an upright wall face29a. The shape and function of the communicating holes29bformed in the vicinity of the upright wall face29aand a lower end29eof the partitioning member29are equivalent to those of the communicating holes24band the lower end24eof the partitioning member24.

According to the combination of the spool30and the partitioning member29, an effect of smoothing out the increase characteristic of the generated damping force with respect to an increase in the displacement amount of the spool17is obtained by causing the tapered face30ato approach the upright wall surface20dsteadily as the pool30displaces, as in the case where the tip of the enlarged inner diameter part17cof the spool17approaches the slanted wall face24dof the partitioning member24shown inFIG. 1.

All the partitioning members24and27-29are arranged to form a minute annular gap between the lower end thereof and the outer circumference of the upper end1gof the piston1, thereby forming a the second flow path to ensure a flow of working oil from the first oil chamber41into the inflow space A after the spool17(30) is seated on the partitioning member24(27-29). A similar function may be derived from a notch or notches formed on the tip of the enlarged inner diameter part17a,30aof the spool17,30. The notch(es) enables a small amount of working oil to flow from the first oil chamber41into the inflow space A even when the spool17(30) is seated on the partitioning member24(27-29), thereby allowing the partitioning member24(27-29) to be fitted tightly onto the outer circumference of the upper end1gof the piston1without clearance.

Next, referring toFIG. 7, a second embodiment of this invention will be described.

A damping force generating mechanism according to this embodiment comprises a pressure responsive throttle37comprising a pressure chamber partitioning member32and a spool31, instead of the pressure responsive throttle16.

The pressure chamber partitioning member32is gripped between the washer101fitted onto the small diameter part5aof the piston rod5and the step5bformed on the piston rod5. The pressure chamber partitioning member32is formed into a cylindrical shape having a bottom portion32awhich the small diameter part5apenetrates. The pressure chamber partitioning member32further comprises a cylindrical portion32bprojecting upward from the bottom portion32aand a flange portion32dprojecting radially from the bottom portion32a.

The spool31is fitted onto the outer circumference of the piston rod5so as to be free to slide in the axial direction. The spool31comprises a bottom31a, the center of which is penetrated by the piston rod5, and a cylindrical part31bprojecting downward from the outer circumference of the bottom31a. The cylindrical part31bis fitted onto the outer circumference of the cylindrical portion32bof the pressure chamber partitioning member32. An enlarged inner diameter part31cis formed on the inside of the lower end of the cylindrical part31b. The enlarged inner diameter part31cis fitted onto the outer circumference of the flange portion32dof the pressure chamber partitioning member32such that the tip of the enlarged inner diameter part31coverlaps the outer circumference of the upper end1gof the piston1as the spool31displaces downward. Slits31eare formed on the tip of the enlarged inner diameter part31c.

According to the above construction, a pressure chamber34having a ring-shaped horizontal cross-section is formed between the enlarged inner diameter part31cof the spool31and the cylindrical portion32bof the pressure chamber partitioning member32. The pressure chamber partitioning member32also functions as a partitioning member which delimits the inflow space A to the passages2from the first oil chamber41. As a result, the partitioning member24of the first embodiment is herein omitted.

To introduce the pressure of the second oil chamber42into the pressure chamber34, a pilot passage5dis formed to penetrate the small diameter part5aof the piston rod5. In the pressure chamber partitioning member32, a port32cis formed in the radial direction to connect the pilot passage5dto the pressure chamber34. An orifice35ais provided in the port32c. The orifice35ais formed in a plug35screwed into the inner circumference of the port32c.

A coil spring33is interposed between the spool31and the pressure chamber partitioning member32around the piston rod5. An upper end of the coil spring33is supported by the bottom31aof the spool31. A lower end of the coil spring33is supported by the bottom portion32aof the pressure chamber partitioning member32.

The coil spring33applies a resilient force to the spool31in a direction that causes the spool31to retreat from the piston1, or in other words a direction for supplementing the action of the pressure in the pressure chamber34. Displacement of the spool31in this direction is limited by a stop ring36fitted onto the outer circumference of the piston rod5.

A space accommodating the coil spring33is surrounded by the spool31, the piston rod5, and the pressure chamber partitioning member32. This space is connected permanently to the first oil chamber41via a communication hole31dpenetrating the bottom31aof the spool31, thereby preventing the working oil in the space from locking the axial displacement of the spool21.

The pressure in the first oil chamber41acts downward on the pressure receiving area of the spool31exposed upward in the first oil chamber41. In contrast, the pressure in the pressure chamber34and the resilient force of the oil spring33act upward on the spool31. The spool displaces in a direction to cause the tip of the enlarged inner diameter part31cto approach the outer circumference of the upper end1gof the piston1as the piston displacement speed in the elongation direction increases, thereby reducing the annular gap between the tip of the enlarged inner diameter part31cand the outer circumference of the upper end1gof the piston1. As a result, the flow resistance of the working oil flowing into the inflow space A from the first oil chamber41via the annular gap increases. After the tip of the enlarged inner diameter part31coverlaps the outer circumference of the upper end1gof the piston1, the oil can flow from the first oil chamber41into the inflow space A only through the slits31eformed in the tip of the enlarged inner diameter part31c, and hence the working oil flowing from the first oil chamber41to the second oil chamber42is subjected to large resistance. As the spool31displaces further downward, the flow cross-sectional area of the slits31edecreases further. The flow resistance reaches a maximum when the tip of the enlarged inner diameter part31cbecomes seated on a step1hformed on the upper end1gof the piston1. It should be noted that the vertical length of the slits31eis set in advance such that the slits31eare not blocked entirely by the outer circumference of the upper end1gof the piston1even when the tip of the enlarged inner diameter part31cis seated on the step1h.

Also according to this damping force generating mechanism, different damping force characteristics are obtained when the piston displacement speed is in the low speed region, the middle speed region, and the high speed region, respectively. Further, the damping force characteristic in the high speed region of piston displacement can be altered by altering the shape, the size, or the number of the slits31e. According to this embodiment, therefore, setting the damping force characteristic in the high speed region is easier than in the first embodiment in which the damping force characteristic in the same high speed region is dependent on the minute annular gap.

Referring toFIG. 8, a third embodiment of this invention will be described. This invention corresponds to the damping force generating mechanism of the second embodiment in which the pressure responsive throttle37is replaced by a pressure responsive throttle57.

The pressure responsive throttle37increases the generated damping force in the high speed region of piston displacement by causing the tip of the enlarged inner diameter part31cof the spool31to overlap the outer circumference of the upper end1gof the piston1. In contrast, the pressure responsive throttle57according to this embodiment obtains the same result by causing a tip of an enlarged inner diameter part51bof a spool51to close communicating holes52dformed though a pressure chamber partitioning member52.

An annular weir60projecting upward is formed on the outer circumference of the upper end1gof the piston1. The pressure chamber partitioning member52comprises a flange52cwhich is located just above the annular weir60such that an inflow space B into the passages2is formed above the damping valve100. The communicating holes52dpenetrate the bottom52aof the pressure chamber partitioning member52diagonally to connect the first oil chamber41to the inflow space B. When the piston displacement speed reaches the high speed region, the spool51displaces downward and the tip of the enlarged inner diameter part51bstarts to close the communicating holes52d. A minute annular gap is provided between the flange52cand the annular weir60in advance such that, after the communicating holes52dare closed in the high speed region of piston displacement speed, working oil flows from the first oil chamber41into the inflow space B only through this minute annular gap.

The other components of this embodiment are equivalent to the corresponding components of the second embodiment. Specifically, a bottom51aand a communication hole51cof the spool51are equivalent to the bottom31aand the communication hole31dof the spool31. A cylindrical portion52band a port52eof the pressure chamber partitioning member52correspond to the cylindrical portion32band the port32e, respectively. A coil spring53corresponds to the coil spring33. A pressure chamber54corresponds to the pressure chamber34. A plug55and an orifice55acorrespond to the plug35and the orifice35a, respectively. A stop ring56corresponds to the stop ring36

With this pressure responsive throttle57, when the piston displacement speed reaches the high speed region, the spool52gradually reduces the opening area of the ports52das the piston displacement speed increases, and after the spool52closes the ports52, only the minute annular gap between the flange52cand the annular weir60allows working oil to flow from first oil chamber41into the inflow space B.

Also according to this embodiment, damping forces of different characteristics are generated according to the speed regions of piston displacement, i.e., the low speed region, the middle speed region, and the high speed region, as in the case of the second embodiment.

Further, according to this embodiment, a decreasing gain of the flow cross-sectional area of the communicating holes52dwith respect to the displacement amount of the spool51is smaller than in the case of the pressure responsive throttle37which is designed to decrease the annular gap. In other words, a decrease in the flow cross-sectional area with respect to an increase in the differential pressure between the first oil chamber41and the second oil chamber42can be made gentler than in the second embodiment. It should be noted that instead of having a plurality of communicating holes52, the pressure chamber partitioning member52may have a single communicating hole52.

Referring toFIG. 9, a fourth embodiment of this invention will be described.

A damping force generating mechanism according to this embodiment is provided with a pressure responsive throttle64. The pressure responsive throttle64corresponds to the pressure responsive throttle57according to the third embodiment, to which an annular groove61and a tapered face63are further provided. Communicating holes62correspond to the communicating holes52of the pressure responsive throttle57, but communicate with the first oil chamber41via the annular groove61which is formed on the outer circumference of the bottom52aof the pressure chamber partitioning member52. The tapered face63is formed on the tip of the enlarged inner diameter part51cof the spool51, and enlarges a diameter of the enlarged inner diameter part51cdownward. The other components of the damping force generating mechanism are equivalent to those of the damping force generating mechanism according to the third embodiment.

The pressure responsive throttle64provided with the annular groove61and the tapered face63can further reduce the decreasing gain of the flow cross-sectional area with respect to the stroke position of the spool51in comparison with the pressure responsive throttle57according to the third embodiment.

As described above, the damping force generating mechanism according to this invention generates damping forces of different characteristics depending on the piston displacement speed region, e.g., the low speed region, the middle speed region, and the high speed region. Further, the damping force generating mechanism according to this invention can vary a decrease gain of the flow sectional area when the piston displacement speed reaches the high speed region, thereby enabling arbitrary setting of the rapid increase characteristic of the damping force when the piston displacement speed reaches the high speed region.

The contents of Tokugan 2006-348838, with a filing date of Dec. 26, 2006 in Japan are hereby incorporated by reference.

Although the invention has been described above with reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, within the scope of the claims.

For example, this invention does not depend on the construction of the damping valve. The damping force generating mechanism according to this invention can be applied to a shock absorber having a damping valve which varies a generated damping force depending only on the elastic deformation of a leaf valve, the inner circumference of which is fixed. In this case also, the damping force characteristic in the high speed region of piston displacement can be varied independently from the other speed regions.

In the embodiments described above, the damping force generating mechanism is applied for varying the damping characteristic during the elongation stroke of a shock absorber, but the damping force generating mechanism according to this invention may also be applied to vary the damping force characteristic during the contraction stroke of a shock absorber.

In the embodiments described above, the valve disk is constituted by the piston1, but it is possible to constitute the valve disk by a base valve which is fixed to the bottom of the cylinder40to separate the second oil chamber42from a reservoir provided outside the cylinder40. In this case the second oil chamber42and the reservoir correspond to the first fluid chamber and the second fluid chamber, respectively.

The passages2may be replaced by a single passage. Similarly, the communicating holes24b,27b,28d,29bmay be replaced by a single communicating hole.

The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows: