Shock absorber with variable damping characteristics depending upon stroke speed

Abstract: A shock absorber is provided a piston stroke dependent variable shock absorbing characteristics by means of a first and second flow restriction valves associated with a piston (100). The first and second flow restriction valves (130, 140) are arranged in a fluid path for communicating first and second working chambers defined within a shock absorber cylindrical housing in series. The first flow restriction valve is associated with an orifice forming a part of the fluid path and has variable flow restriction rate in response to a pressure difference between the first and second chambers greater. The second flow restriction means is disposed in tandem fashion with the first flow restriction valve for providing a predetermined constant flow restriction rate in response to the pressure difference smaller than a predetermined criterion and providing a variable flow restriction as a function of the piston stroke speed in response to the pressure difference greater than the predetermined criterion.

Description: The present invention relates generally to a hydraulic shock absorber, such as for automotive suspension system. More specifically, the invention relates to a valve construction to be employed in the hydraulic shock absorber for achieving piston stroke speed dependent variable damping characteristics. It should be appreciated that, in an automotive suspension system, shock absorbers are required to produce two contradictory effects. Namely, the shock absorber has to absorb vibration energy that would cause vibration in the vehicle body, in order to provide riding comfort in the vehicle. On the other hand, the shock absorber has to damp vibration for suppressing relative displacement between the vehicle body and a suspension member so as to suppress attitude changes, for driving stability. It should also be appreciated that vibration energy generated in a road wheel due to unevenness generally influences riding comfort and has relatively small magnitude and high frequency, causing small magnitude, high speed piston movements in the shock absorber. On the other hand, vibration energy induced in the vehicle body generally influences driving stability, by causing attitude change, such as pitching, rolling and so forth, and has relatively great magnitude and low frequency, thus causing high magnitude, low speed piston movements. In order to provide both riding comfort and driving stability, it is therefore required to absorb high frequency vibration and to damp low frequency vibration. In the prior art, there have been proposed various shock absorbers which attempted to provide piston stroke dependent damping characteristics for accomplish both of the aforementioned tasks. For example, JP-A-Showa 61-47134 (on which the preamble of claim 1 is based) discloses a shock absorber with a multi-stage valve assembly employed in a shock absorber piston. In that valve assembly, first stage and second stage disc valves are arranged in series or in tandem fashion with respect to a fluid path for fluid communication between upper and lower working chambers defined in a shock absorber cylinder. The first stage valve is designed to respond to a pressure difference across it to open for fluid communication therethrough. On the other hand, the second stage valve is designed to respond to a higher pressure difference than the first stage valve to open for fluid communication therethrough. The second stage valve also defines a constant orifice or orifices having a predetermined fixed fluid path area to permit a limited fluid flow therethrough even when the second stage valve is closed. With that previously proposed construction, a low speed piston stroke creates a small pressure difference between the upper and lower working chambers and thus exerts a small pressure on the first stage and second stage valves. The first stage valve is responsive to this small pressure to open to establish fluid communication between the upper and lower fluid chambers. Therefore, the working fluid flows through the gap formed in the first stage valve and through the constant orifice. In such a case, since the path area is limited, a substantial flow restriction for the working fluid is provided, to generate a great damping force to damp the vibration induced in the vehicle body. On the other hand, in response to a high frequency piston stroke which creates a greater pressure difference between the upper and lower working chambers, both the first stage and the second stage valves are open to provide increased fluid path area and a smaller damping force. Therefore, the vibration energy input from the road wheel can be absorbed for avoiding a rough ride. In such a valve construction, the first stage valve is driven to deform substantially more often than the second stage valve. Therefore, the first stage valve may have a shorter life than the second stage valve. Particularly, when the first stage valve has constant resilient characteristics, it may be subject to high stresses, further shortening its life. On the other hand, at a pressure difference greater than the pressure relief point of the second stage valve, the damping characteristic at a flow restriction orifice extending through the shock absorber piston is proportional to the square of the piston stroke speed. On the other hand, at the same time, the damping characteristic at the first and second stage valves is proportional to the two-thirds power of the piston stroke speed. As a result, the variation characteristics of the overall damping characteristics of the shock absorber cannot be linear, which causes difficulty in setting desired variation characteristics. Therefore, it is an object of the present invention to provide a hydraulic shock absorber which has linear damping characteristics in relation to a piston stroke speed. Another object of the invention is to provide a hydraulic shock absorber which can avoid fatigue of a valve member for generating damping force, and thus extend the life of the shock absorber. The invention provides a flow restriction valve unit for a hydraulic shock absorber, which valve unit is provided in one of a piston assembly or a bottom valve assembly and associated with a fluid path for communication between first and second fluid chambers, comprising: a flow restricting orifice forming part of the fluid path for permitting fluid flow therethrough at a first, limited flow rate; a first valve associated with the flow restricting orifice and normally closing one end of the orifice for blocking fluid communication between the first and second fluid chambers, and being responsive to a fluid pressure difference between the first and second fluid chambers greater than a predetermined magnitude to open the said one end of the orifice to establish fluid communication permitting fluid flow from the first fluid chamber to the second fluid chamber; and a second valve associated with, and arranged downstream of, the first valve in series with the latter with respect to the fluid flow from the first fluid chamber to the second fluid chamber, the second, downstream valve having a flow path of predetermined constant path area for communication between the downstream side of the first, upstream valve and the second fluid chamber, and being responsive to a fluid pressure difference between the downstream side of the first valve and the second fluid chamber greater than a second predetermined magnitude to increase its path area; characterised in that the first valve includes means for modifying the opening characteristics of the first valve so as to restrict further opening of the first valve when subject to a pressure difference across it substantially corresponding to the presence of a pressure difference of the said second predetermined magnitude between the downstream side of the first valve and the second fluid chamber; and that the overall combined damping characteristic of the first and second valves (138, 136) varies substantially linearly with the pressure difference between the first and second fluid chambers (12, 14). The invention also provides a hydraulic shock absorber arranged to be disposed between relatively movable first and second objects for absorbing vibration energy which causes relative movement between the first and second objects, comprising: a cylinder tube defining an internal space and arranged to be connected to the first object for movement therewith; a piston assembly disposed within the internal space of the cylinder tube for defining first and second fluid chambers therein, the piston being arranged to be connected to the second object by means of a piston rod for movement therewith; and a flow restriction valve unit according to the invention provided in the piston assembly associated with a fluid path for communication between the said first and second fluid chambers. Preferably, the first, upstream valve has a valve member capable of shifting in accordance with increasing fluid pressure in the first fluid chamber, the valve member cooperating with a stopper means which limits the shifting range of the valve member. The valve member may comprise a resiliently deformable disc, and the stopper means may then comprise a disc shaped member having substantially the same diameter as the valve member so that at least the circumferential edge portion of the valve member seats thereon at a predetermined magnitude of resilient deformation, which stopper means defines a clearance to permit fluid flow from the first, upstream valve to the second, downstream valve through the clearance. The disc shaped member may comprise a first, smaller diameter disc and a second, larger diameter disc which has essentially the same diameter as the valve member, the second disc being positioned to limit deformation of the valve member by making contact with the circumferential edge of the valve member and the first disc being positioned to limit deformation at a portion of the valve member of intermediate radius. Alternatively, the disc shaped member may be resiliently deformable. In a further alternative embodiment, the disc shaped member may comprise a plurality of resiliently deformable discs laminated to each other. The hydraulic shock absorber may comprise a plate-like member disposed between the first, upstream valve and the discharge outlet of the orifice for receiving working fluid flow and distributing the uniform fluid pressure over the whole effective area of the first upstream valve. In an alternative embodiment, the pressure difference between the first and second fluid chamber is created by a stroke of the piston assembly. In a preferred construction, the orifice provides damping characteristics proportional to the square of the stroke speed of the piston assembly; the first, upstream valve provides damping characteristics proportional to the two-thirds power of the stroke speed of the piston assembly; and the second, downstream valve provides damping characteristics in response to stroke speeds of the piston assembly lower than or equal to a predetermined piston speed criterion, proportional to the square of the stroke speed of the piston assembly, and in response to stroke speeds of the piston assembly higher than the piston stroke criterion, proportional to the two-thirds power of the stroke speed of the piston assembly. The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiments of the present invention, which, however, should not be taken to limit the invention to the specific embodiment or embodiments, but are for explanation and understanding only. In the drawings: Fig. 1 is a section of a major part, including a piston assembly, of the first embodiment of a hydraulic shock absorber according to the present invention; Fig. 2 is an enlarged section of a flow restricting valve unit employed in the first embodiment of the hydraulic shock absorber according to the invention; Figs. 3 and 6 show examples of stopper plates to be employed in the first embodiment of the hydraulic shock absorber of Fig. 1 ; Fig. 7 is a section of a piston assembly in the second embodiment of a hydraulic shock absorber according to the present invention; Figs. 8 and 9 are enlarged sections of valve units employed in the second embodiment of the hydraulic shock absorber in different mode positions; Fig. 10 is an enlarged section of a modification of valve unit to be employed in the second embodiment of the hydraulic shock absorber of Fig. 7 ; Fig. 11 is a section of a piston assembly in the third embodiment of a hydraulic shock absorber according to the present invention; Figs. 12 and 13 are enlarged sections of valve units employed in the third embodiment of the hydraulic shock absorber in different mode positions; Fig. 14 is a graph showing variation of damping force to be generated at various piston stroke speed; Fig. 15 is a section of a piston assembly in the fourth embodiment of a hydraulic shock absorber according to the present invention; Fig. 16 is an enlarged section of a valve unit employed in the piston assembly of the fourth embodiment of the hydraulic shock absorber of Fig. 15 ; and Figs. 17 to 19 are enlarged sections showing variation of the valve unit to be employed in the fourth embodiment of the hydraulic shock absorber of Fig. 16 . Referring now to the drawings, particularly to Fig. 1 , the first embodiment of a hydraulic shock absorber according to the present invention, employs a piston assembly 100 disposed within a cylinder tube 10 which forms a shock absorber housing filled with a hydraulic working fluid. The piston assembly 100 is disposed within the internal space of the cylinder tube 10 in order to divide the internal space into an upper and a lower fluid chamber 12 and 14 . The piston assembly 100 is mounted at the lower end of a piston rod 20 which has an upper end extending through the upper end of the cylinder tube 10 . In practical use, the shock absorber is disposed between upper and lower objects which are movable relative to each other, so as to absorb vibration energy causing relative movement of the upper and lower objects. Though the discussion herein is given in terms of vertical installation of the shock absorber, the shock absorber is not limited to installation in a vertical orientation. The shock absorber can be installed horizontally so as to absorb vibration energy between horizontally arranged objects. When the shock absorber is vertically installed as hereinbelow discussed, the piston rod 20 is connected to the upper object for moving the piston assembly 100 , and the lower end of the cylinder tube 10 is connected to the lower object. If the shock absorber is employed in an automotive suspension system, the piston rod 20 is connected to a vehicle body at the top end and the cylinder tube 10 is connected to a suspension member which rotatably supports a road wheel. Though the shock absorber is hereinafter discussed as being positioned with the piston rod upwards, it is possible instead to position the shock absorber with the piston rod downwards. As seen from Fig. 1 , the piston rod 20 has a greater diameter major section 22 and a smaller diameter lower end section 24 . The lower end section 24 has a threaded lower end 26 . The lower end section 24 with the threaded lower end 26 forms a piston assembly receptacle 28 . The piston assembly 100 has a piston body 102 as principle component of the piston assembly. The piston body 102 has a center hole 104 , through which the lower end section 24 of the piston rod 20 extends. The piston body 102 has an upper surface facing the upper working chamber 12 and a lower surface facing the lower working chamber 14 . Coaxially arranged inner and outer annular grooves 106 and 108 are formed on the upper surface of the piston body 102 . On the other hand, a circular groove 110 is formed on the lower surface of the piston body 102 . Coaxially arranged inner and outer annular grooves 112 and 114 are formed on the surface facing the groove 110 . The inner annular grooves 106 and 112 are vertically aligned with each other. One or more fluid flow orifices 116 are formed through the piston body parallel to the axis of the piston rod 20 for communication between the inner grooves 106 and 112 . Similarly, the outer grooves 108 and 114 are arranged in vertical alignment and are connected along one or more communication paths 118 . Along the inner and outer grooves 106 and 108 on the upper surface of the piston body 102 , annular lands 120 and 122 are formed. The lands 120 and 122 have upper planar surfaces which serve as valve seats for seating an annular disc shaped check valve member 124 . The check valve member 124 is biased toward the valve seat surfaces of the lands 120 and 122 by means of a coil form check spring 126 which is disposed between the check valve member 124 and a retainer disk 128 . With this construction, the check valve member 124 closes the annular grooves 106 and 108 . A plurality of through openings 130 are formed in circumferential alignment at a position corresponding to the radial position of the inner groove 106 . Therefore, the inner groove 106 is normally in communication with the upper working chamber 12 via these through openings 130 . On the other hand, the check valve member 124 has a solid construction at the position corresponding to the outer annular groove 108 . Therefore, the check valve member 124 normally blocks fluid communication, but if the fluid pressure in the lower working chamber 14 exceeds that in the upper working chamber by more than a predetermined relief pressure, the check valve is lifted, overcoming the spring force of the check spring 126 , to establish fluid communication between the lower and upper working chambers 14 and 12 . On the other hand, the check valve member 124 constantly blocks fluid flow via the communication paths 118 from the upper working chamber 12 to the lower working chamber 14 . Lands 134 and 136 are formed along the circumferential edges of the inner and outer annular grooves 112 and 114 . The lands 134 and 136 have lower end planar surfaces. A piston stroke speed dependent variable damping characteristics valve unit 137 is provided in opposition to the lower surfaces of the lands 132 and 134 . As shown in Figs. 1 and 2 , the valve unit 137 comprises an upper disc valve member 138 and lower disc valve members 140 . An annular washer 142 and a stopper plate 144 are disposed between the upper and lower disc valve members 138 and 140 . These upper and lower disc valve members 138 and 140 , the washer 142 and the stopper plate 144 form the flow restriction valve unit 136 and are supported by an annular cylindrical collar 146 via disc shaped washer 148 on a retainer nut 150 which engages with the threaded lower end 26 of the piston rod 20 . The retainer nut 150 has a laterally extending flange 152 , on which a lower end of an assist coil spring 154 is seated. The upper end of the assist coil spring 154 is seated on a spring seat member 156 . The spring force of the assist spring 154 is thus exerted on the outer circumferential edge portion of the lower disc valve member 140 via the spring seat member 156 so that the outer circumferential edge of the lower disc valve member 140 is normally urged toward the lower planar surface of the outer annular land 136 . The upper and lower disc valve members 138 and 140 are formed of resiliently deformable material and provide spring characteristics for self-biasing their respective circumferential edge portions toward the opposing planar lower surfaces of the lands 134 and 136 . The upper disc valve member 138 thus normally seats its outer circumferential edge portion onto the lower planar surface of the land 134 , closing the inner annular groove 112 . Therefore, when the working fluid pressure in the upper working chamber 12 , which is introduced into the inner annular groove 112 via the orifices 116 , does not exceed the working fluid pressure in the lower working chamber 14 by a predetermined amount which is the set pressure of the upper disc valve member 138 and is determined by the resiliency thereof, the inner annular groove 112 is held closed. Therefore, fluid communication between the upper and lower working chambers 12 and 14 via the orifices 116 is blocked. The upper disc valve member 138 is overcome when the working fluid pressure in the upper working fluid chamber 12 exceeds that in the lower working chamber 14 by more than the set pressure to form a gap between the upper disc valve member 138 and the lower end planar surface of the land 134 to permit fluid flow therethrough. The stopper plate 144 is formed with one or more cut outs 158 on the outer circumference. On the other hand, the outer annular land 136 is formed with one or more cut outs 160 which cooperate with the lower disc valve member 140 , when the latter is seated on the seating surface of the land 136 , to form a constant path area orifice for fluid communication. Therefore, when the gap is formed between the upper disc valve member 138 and the mating surface of the land 134 , the working fluid flows through the cut outs 158 of the stopper plate 144 and the cut outs 160 of the land 136 . At this time, since the fluid flow path area at the gap between the upper disc valve member 138 and the land 134 is limited to restrict fluid flow, the gap serves as first stage orifice for generating damping force. On the other hand, the constant fluid flow path area orifices defined by the cut outs 160 of the land 136 provide another flow restriction for the fluid flowing from the upper working chamber 12 to the lower working fluid chamber and thus serve as a second stage flow restriction orifice. It should be noted that the resilience of the lower disc valve member 140 is cooperative with the spring force of the assist spring 154 to determine the set pressure of the lower disc valve member 140 . The set pressure of the lower disc valve member 140 is determined to be greater than that of the upper disc valve member 138 . In addition, as clearly seen from Fig. 2 , the effective area of the first disc valve member 138 , on which the working fluid pressure in the annular groove 112 is exerted, is set to be much greater than the effective area of the second disc valve member 140 . Therefore, the upper disc valve member 138 has a higher sensitivity to an excess of the working fluid pressure in the upper working chamber 12 over that in the lower working chamber 14 , than does the lower disc valve member 140 . As shown in Figs. 2 and 3 , the stopper plate 144 is formed of a rigid material and has a diameter slightly smaller than the internal diameter of the lower end of the annular land 136 to define a clearance therebetween. On the other hand, the cut outs 158 are formed in an essentially U-shaped configuration and arranged along the outer circumference at regular angular intervals. In practical operation, damping force for absorbing vibration energy during a piston stroke toward the lower working chamber, compressing the working fluid in the lower working chamber, is generated by flow restriction in the openings 118 which provide limited fluid flow path area and by the spring force of the check spring 126 . On the other hand, during a piston stroke toward the upper working chamber 12 , compressing the working fluid in the upper working chamber, the valve unit 136 operates in two different modes depending upon the piston stroke speed. When the piston strokes at relatively low speed, the rate of change of the working fluid pressure in the upper and lower fluid chambers 12 and 14 is relatively low. Therefore, the rate of change of pressure difference between the upper and lower working chambers 12 and 14 remains low. As a result, the working fluid force acting on the valve unit as a result of the difference between the working fluid pressures in the upper and lower working chambers 12 and 14 increases relatively slowly. When the fluid force has become greater than the set pressure of the upper disc valve member 138 , the upper disc valve member 138 is deformed to shift the circumferential edge portion away from the seating surface of the land 134 to form a gap therebetween to permit fluid flow therethrough. On the other hand, at this time, the working fluid pressure acting on the lower disc valve member 140 remains smaller than the set pressure thereof. Therefore, the working fluid flowing through the gap formed by deformation of the upper disc valve member 138 flows through the constant flow area orifices 160 formed through the land 136 . At this time, the damping characteristic at the first stage orifice formed by the deformation of the upper disc valve member 138 is proportional to the two-thirds power of the piston stroke speed. On the other hand, the damping characteristic at the second stage orifice defined by the constant fluid path area orifices 160 is proportional to the square of the piston stroke speed. Therefore, the damping force to be created in response to relatively low piston stroke speed becomes relatively greater. On the other hand, when the piston strokes at relatively high speed, the pressure difference produced in the working fluid in the upper and lower working chambers 12 and 14 varies at greater rate than in the low piston stroke speed case set forth above. When the piston stroke speed is higher than a certain speed, the working fluid pressure exerted on the lower disc valve 140 becomes greater than the set pressure of the lower disc valve member 140 and overcomes the combined spring force of the valve member and the assist spring 154 . Therefore, the lower disc valve member 140 is deformed to shift the circumferential edge portion away from the seating surface of the land 136 to form a gap therebetween. By this, the fluid flow area defined by the second stage orifice becomes greater than in the low speed piston stroke. At this time, the damping characteristic at the second stage orifice becomes proportional to the two-thirds power of the piston stroke speed. Therefore, the flow restriction for the working fluid at the second stage orifice becomes less to provide smaller damping force in comparision with that in the low piston stroke speed mode. It should be appreciated that the stopper plate 144 is made of rigid material so as not to deform in response to the fluid pressure acting thereon. As seen from Fig. 2 , the stopper plate 144 is placed away from the upper disc valve member 138 , leaving a clearance defined by the height of the washer 142 . This construction limits deformation of the upper disc valve member 138 to prevent excessive deformation of the latter. This extends the life of the upper disc valve member and thus increases the durability of the shock absorber. Figs 4 to 6 show modifications of configuration of the stopper member to be employed in the foregoing first embodiment. In Fig. 4 , the stopper plate 144 is formed into hexagonal configuration to form clearance 162 between the inner circumferential edge of the land 136 and the straight portion between the adjacent corners of the hexagon. On the other hand, In Fig. 5 , the stopper plate 144 is formed into a circular disc shaped configuration. The diameter of the circular disc shaped stopper plate 144 is set to be slightly smaller than the inner diameter of the inner circumferential edge of the land 136 in order to define an annular clearance 163 . A plurality of circular through openings 164 are formed in the vicinity of the circumferential edge. The pressurized fluid flowing through these openings 164 acts on the corresponding portion of the lower disc valve member 140 so as to prevent the second disc valve member from sticking on the stopper plate. In Fig. 6 , the stopper plate 144 is formed with a plurality of elongated arc shaped openings 166 . The construction shown in Fig. 6 produces substantially the same effect as the construction of Fig. 5 . Fig. 7 shows the second construction of the hydraulic shock absorber according to the invention. In the discussion given herebelow, the components common to the first and second embodiments will be represented by the same reference numerals as in the former embodiment and detailed discussion thereof will be omitted. In the shown embodiment, the construction of the piston assembly 100 is almost same as that of the former embodiment except for a stopper plate 200 which is modified from the stopper plate 144 in the first embodiment. Namely, in the second embodiment, the stopper plate 200 comprises a pair of thinner plates 202 and 204 . The plates 202 and 204 are formed in disc shaped configurations. The external diameter of the plate 204 is substantially equal to the diameter of the upper disc valve member 138 . The plate 202 has a smaller diameter than the plate 204 . The plate 202 is placed on the plate 204 . In the construction set forth above, the valve unit 136 operates to produce damping force in response to the piston stroke toward the upper working chamber 12 in a manner substantially as illustrated with respect to the foregoing first embodiment of the invention. In the second embodiment of Fig. 7 , when the piston stroke speed is relatively low, the magnitude of deformation of the upper disc valve member 138 is relatively small. In such a case, the circumferential edge portion of the upper disc valve member 138 comes into contact with the plate 204 to restrict further deformation as shown by phantom line in Fig. 8 . On the other hand, when the piston stroke speed is relatively high, whole body deformation is caused in the upper disc valve member 138 to cause deformation at the intermediate portion of the upper disc valve member. This causes expansion of the first stage orifice permitting greater flow of working fluid. By the whole body deformation, the intermediate portion of the upper disc valve member 138 comes into contact with the plate 202 to be restricted from further deformation. Such construction of valve unit may provide variable damping characteristics even in the low piston stroke speed mode of operation. Furthermore, limitation of the deformation of the upper disc valve member 138 will successfully prevent the upper disc valve member from suffering excessive deformation which might cause shortening of life of the upper disc valve member. Fig. 10 shows a modification of the aforementioned second embodiment of the valve unit to be employed in the second embodiment of the hydraulic shock absorber according to the invention. In this embodiment, the stopper plate 200 is replaced with a plate like member 210 with a slanted upper surface. As seen from Fig. 10 , the plate like member 210 is formed in a disc shaped configuration and has a relatively thin circumferential edge with thickness increasing toward the centre. With this construction, the deformation stroke of the upper disc valve member 138 is gradually and linearly increased toward the outer circumferential edge. With the shown construction, substantially the same effect for limiting deformation magnitude as that achieved by the embodiment of Fig. 7 can be achieved. Fig. 11 shows the third embodiment of the hydraulic shock absorber according to the invention. Similarly to the former embodiment of Fig. 7 , the shown embodiment has substantially the same structural components to the first embodiment of Fig. 1 . In order to avoid redundant discussion, components common to the shown embodiment and the first embodiment will be represented by the same reference numerals. The third embodiment of the shock absorber is differentiated from the former embodiments of Figs. 1 and 7 in the stopper plate 300 in the valve unit 136 . The stopper plate 300 employed in the shown embodiment comprises a plurality of thin resiliently deformable discs 302 as particularly shown in Figs. 12 and 13 . The thin discs 302 are laminated to each other to form an integral deformable disc. Also, the upper and lower disc valve members 304 and 306 respectively comprise a plurality of thin resiliently deformable discs 308 and 310 . In the shown construction, the upper and lower disc valve members 304 and 306 and the stopper plate 300 are formed by laminated pairs of thin discs 308 , 310 and 302 , respectively. The resilience coefficients of respective thin discs 308 , 310 and 302 are differentiated from each other so as to establish desired resilience coefficients in the laminated forms. In the preferred construction, the resilience coefficient of the upper disc valve member 304 is set to be smallest among three laminated discs. On the other hand, the resilience coefficient of the stopper plate 300 may be set to be the greatest among three laminated plates. As seen from Figs. 11 to 13 , the seating surfaces of the lands 134 and 136 are vertically separated by a distance h₁ in the vertical direction. In relation to the distance h₁ , the thicknesses of the thin discs 308 and 302 and the height of the washer 142 are so selected as to have the overall thickness h₂ as assembled smaller than the distance h₁ . Therefore, the lower disc valve member 140 , which has the circumferential edge portion seating on the seating surface of the land 136 , is slightly deformed at the normal position as clearly shown in Fig. 12 . This forms a clearance 312 between the lower disc valve member 306 and the stopper plate 300 . The clearance 312 formed between the stopper plate 300 and the lower disc valve member 306 cancels the fluid pressure exerted on the stopper plate when the upper disc valve member 304 is deformed to form the first stage orifice. Therefore, the first stage orifice may not become too great to provide the desired flow restriction for generating damping force. Similarly to the foregoing first embodiment, the valve unit 136 performs two different modes of flow restriction for generating damping force. When the piston strokes at relatively low speed in a piston stroke speed range as illustrated by the range b in Fig. 14 , the rate of change of the working fluid pressure in the upper and lower fluid chambers 12 and 14 remains relatively low. Therefore, the speed of variation of the pressure difference between the upper and lower working chambers 12 and 14 remains low. As a result, the working fluid force acting on the valve unit as a result of the difference between the working fluid pressures in the upper and lower working chambers 12 and 14 increases relatively slowly. When the fluid force has become greater than the set pressure of the upper disc valve member 304 , the upper disc valve member 304 is deformed to shift the circumferential edge portion away from the seating surface of the land 134 to form a gap therebetween to permit fluid flow therethrough. At this position, the damping force produced by the orifices is proportional to the square of the piston stroke speed, as illustrated by the broken line A of Fig. 14 . Also, the damping force produced at the first stage orifice is proportional to the two-thirds power of the piston stroke speed as illustrated by broken line B in Fig. 14 . On the other hand, at this time, the working fluid pressure acting on the lower disc valve member 140 remains smaller than the set pressure thereof. Therefore, the working fluid flowing through the gap formed by deformation of the upper disc valve member 138 flows through the constant flow area orifices 160 formed through the land 136 . The damping force produced at the second stage orifice is proportional to the square of the piston stroke speed, as illustrated by the broken line C of Fig. 14 . Since these orifices are arranged in tandem fashion or in series, the overall damping force produced in this mode becomes almost linear, as illustrated by the solid line D of Fig. 14 . In response to piston stroke speeds in the range of d of Fig. 14 , the upper disc valve member 304 contacts the stopper plate 300 which increases its effective stiffness. Therefore, at a point e in Fig. 14 , the stiffness of the upper disc valve member 304 is changed. Therefore, the damping force comprises a component which is proportional to the two-thirds power of the piston stroke speed, and an initial component which is the damping force at the point e . On the other hand, at a point f of Fig. 14 , the lower disc valve member 306 starts to deform, forming a gap between the seating surface of the land 136 and the circumferential edge portion of the lower disc valve member. After the deformation of the lower disc valve member 306 has started, the variation characteristic of the damping force becomes proportional to the two-thirds power of the piston stroke speed. Therefore, as shown by the solid line D in Fig. 14 , even at this high piston speed range, the damping force variation characteristic becomes linearly proportional to the piston stroke speed. Figs. 15 and 16 show the fourth embodiment of the shock absorber according to the present invention. The shown embodiment is illustrated in a form in which the stopper plates of the former embodiments are not provided. Though the shown embodiment does not have the stopper plate, it is of course possible to provide the stopper plate in this embodiment. The particular point of the shown embodiment resides in an annular disc plate 400 . The annular disc plate 400 is provided within the inner annular groove 112 above the upper disc valve member 138 . The annular disc plate 400 has a width to receive all of the working fluid discharged through the orifices 116 . On the other hand, the outer diameter of the annular disc plate 400 is smaller than the diameter of the external circumferential edge of the inner annular groove 112 so that the working fluid introduced into the groove can flow through the gap therebetween. This annular disc plate 400 prevents the working fluid from directly contacting the upper disc valve member 138 to cause concentration at the positions where the orifices 116 are formed. Since the concentration of the fluid pressure at the positions of the orifices will cause uneven deformation in the upper disc valve member and thus cause fluctuation of damping characteristics, it is desired to be avoided. A similar effect for avoiding concentration of the fluid pressure at the particular section of the upper disc valve member 138 can be achieved by various constructions of members. For example, in the construction of Fig. 17 , the annular disc valve member 402 is fixed onto the inner peripheral edge of the groove 112 so that the annular disc valve member 402 is placed away from the upper disc valve member 138 to leave a clearance 404 therebetween. On the other hand, in the example of Fig. 18 , a clearance 406 is provided by a washer 408 disposed between the annular disc 410 and the upper disc valve member 138 . Though it is preferable for the annular disc to be spaced apart from the upper disc valve member 138 , it is possible to provide the annular disc 412 together with the upper disc valve member 138 in engagement with the piston rod 24 , as shown in Fig. 19 .

Claims: A flow restriction valve unit for a hydraulic shock absorber, which valve unit is provided in one of a piston assembly (100) or a bottom valve assembly and associated with a fluid path for communication between first and second fluid chambers (12, 14), comprising: a flow restricting orifice (116) forming part of the said fluid path for permitting fluid flow therethrough at a first, limited flow rate; a first valve (138) associated with the said flow restricting orifice (116) and normally closing one end of the orifice (116) for blocking fluid communication between the first and second fluid chambers (12, 14), and being responsive to a fluid pressure difference between the first and second fluid chambers (12, 14) greater than a predetermined first magnitude to open the said one end of the orifice (116) to establish fluid communication permitting fluid flow from the first fluid chamber (12) to the second fluid chamber (14); and a second valve (136) associated with, and arranged downstream of, the said first valve (138) in series with the latter with respect to fluid flow from the first fluid chamber (12) to the second fluid chamber (14), the second valve (136) having a flow path of predetermined constant path area (160) for communication between the downstream side of the first valve (138) and the second fluid chamber (14), and being responsive to a fluid pressure difference between the downstream side of the first valve (138) and the second fluid chamber (14) greater than a second predetermined magnitude to increase its said path area; characterised in that the first valve (138) includes means for modifying the opening characteristics (142, 144) of the first valve (138) so as to restrict further opening of the first valve when subject to a pressure difference across it substantially corresponding to the presence of a pressure difference of the said second predetermined magnitude between the downstream side of the first valve (138) and the second fluid chamber (14); and that the overall combined damping characteristic of the first and second valves (138, 136) varies substantially linearly with the pressure difference between the first and second fluid chambers (12, 14). A valve unit as claimed in claim 1, wherein the first valve has a valve member (138) capable of shifting in accordance with increasing fluid pressure in the first fluid chamber, the said valve member cooperating with a stopper means (144) which limits the movement of the said valve member. A valve unit as claimed in claim 2, wherein the said valve member comprises a resiliently deformable disc (138), and the stopper means comprises a disc shaped member (144) having substantially the same diameter as the said valve member so that at least the circumferential edge portion of the said valve member seats thereon at a predetermined magnitude of resilient deformation, which stopper means defines a clearance (163) to permit fluid flow from the first valve to the second valve through the said clearance. A valve unit as claimed in claim 3, wherein the said disc shaped member comprises a first disc (202) and a second disc (204) which is of larger diameter than the first disc and has essentially the same diameter as the said valve member (138), the second disc being positioned to limit deformation of the valve member by contact with the circumferential edge of the valve member and the fist disc being positioned to limit deformation at a portion of the valve member of intermediate radius. A valve unit claimed in claim 3, wherein the said disc shaped member (300) is resiliently deformable. A valve unit as claimed in claim 3 or claim 5, wherein the said disc shaped member (300) comprises a plurality of resiliently deformable discs (302) laminated to each other. A valve unit as claimed in any one of claims 1 to 6, which comprises a plate like member (400, 402, 410, 412) disposed between the first valve (138) and the discharge outlet of the said orifice (116) for receiving working fluid flow and distributing the uniform fluid pressure over the whole effective area of the first valve. A valve unit as claimed in any one of claims 1 to 7, wherein the pressure difference between the first and second fluid chambers (12, 14) is created by stroke of the said piston assembly (100). A valve unit as claimed in claim 8, wherein the said orifice (116) provides damping characteristics proportional to the square of the stroke speed of the piston assembly (100); the first valve (138) provides damping characteristics proportional to the two-thirds power of the stroke speed of the piston assembly; and the second valve (136) provides damping characteristics at stroke speeds of said piston assembly lower than or equal to a predetermined piston speed criterion, proportional to the square of the stroke speed of said piston assembly, and in response to stroke speeds of the piston assembly higher than the said piston speed criterion, proportional to the two-thirds power of the stroke speed of said piston assembly. A hydraulic shock absorber arranged to be disposed between relatively movable first and second objects for absorbing vibration energy which causes relative movement between said first and second objects, comprising: a cylinder tube (10) defining an internal space and arranged to be connected to the first object for movement therewith; a piston assembly (100) disposed within the internal space of the cylinder tube for defining first and second fluid chambers (12, 14) therein, the piston being arranged to be connected to the second object by means of a piston rod (20) for movement therewith; and a flow restriction valve unit as claimed in any one of claims 1 to 9 provided in said piston assembly associated with a fluid path (116) for communication between the said first and second fluid chambers.

Inventors: SASAKI MITSUO, YAMAOKA FUMIYUKI, YAMAURA TAMOTSU, SASAKI, MITSUO, YAMAOKA, FUMIYUKI, YAMAURA, TAMOTSU
Applicants: ATSUGI UNISIA CORP, UNISIA JECS CORPORATION
Application year: 1989
Main classification: F16F9
IPC codes: F16F9