Abstract:
A shock absorber comprises a first passage ( 2   a,    2   b ) and a second passage ( 15 ) connecting a first fluid chamber ( 41 ) and a second fluid chamber ( 42 ). A throttle ( 12, 14 ) narrows an inflow of fluid to the first passage ( 2   a,    2   b ) according to an applied displacement pressure. The displacement pressure includes a fluid pressure in one of the two fluid chambers ( 41, 42 ) and a pressure that depends on a velocity of the fluid flow through the throttle ( 12, 14 ). A spring ( 25, 29 ) biases the throttle ( 12, 14 ) in the opposite direction to narrow the fluid flow. The second passage ( 15 ) comprises a pair of orifices ( 16   a,    17   a ). By exerting a pressure between the pair of orifices ( 16   a,    17   a ) on the throttle ( 12, 14 ) in the opposite direction to narrow the fluid flow, the spring load required of the spring ( 25, 29 ) is reduced.

Description:
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 the 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 a differential pressure between the upstream and downstream sides of the leaves. With this construction, however, the damping force generated by the leaf valve tends to be excessive in a medium-speed to a high-speed region of piston displacement. 
     To improve the damping force characteristic of a leaf valve for a damping 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 to  FIG. 3 , 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. A leaf valve L closing an outlet of a passage Po which passes through the piston P is fitted to 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 the 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 outer circumferential part of the leaf valve L is bent downward in the figure from the inner circumferential part supported by the push member M. As the piston displacement speed reaches the medium-speed to high-speed region, the pressure in the passage Po becomes greater than the resilient force of a 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, thereby preventing the damping force from becoming excessive. As shown in  FIG. 4 , the damping force increase is gradual with respect to an increase in the piston displacement speed, in the medium-speed to high-speed region. 
     The prior art therefore prevents a damping force from becoming excessively large in the medium-speed to high-speed region of the piston displacement, thereby increasing the riding comfort of the vehicle. 
     SUMMARY OF THE INVENTION 
     This valve structure is effective in suppressing an excessive increase in the damping force generated in the medium-speed 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 medium-speed 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 medium-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 the middle-speed region and the high-speed region of piston displacement such that a preferable damping force is obtained in each of the speed regions. 
     To achieve the above object, this invention provides a damping force generating mechanism for such a shock absorber that comprises a first fluid chamber, a second fluid chamber, and a first passage which allows a fluid flow between the first fluid chamber and the second fluid chamber. The damping force generating mechanism comprises a damping valve which generates a damping force against a fluid to flow through the first passage, a throttle which narrows an inflow to the first passage according to an applied displacement pressure, a biasing member which biases the throttle in the opposite direction to narrow the inflow to the first passage, a second passage which connects the first fluid chamber and the second fluid chamber via a pair of orifices, and a pressure chamber which exerts a pressure in the second passage between the pair of orifices on the throttle in the opposite direction to narrow the inflow to the first passage. 
     The displacement pressure includes a pressure in one of the fluid chambers which biases the throttle in a direction to narrow the inflow to the first passage and a pressure that acts on the throttle in an opposite direction to narrow the inflow to the first passage depending on a velocity of the fluid flow through the throttle. 
     The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal sectional view of essential parts of a hydraulic shock absorber showing a damping force generating mechanism according to this invention. 
         FIG. 2  is a diagram showing the characteristic of a damping force generated by the damping force generating mechanism. 
         FIG. 3  is a longitudinal sectional view of essential parts of a hydraulic shock absorber including a damping force generating mechanism according to the prior art. 
         FIG. 4  is a diagram showing the characteristic of a damping force generated by the damping force generating mechanism according to the prior art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1  of the drawings, a hydraulic shock absorber for a vehicle comprises a cylinder  40 , a piston  1  enclosed in the cylinder  40  so as to be free to slide in an axial direction, and a piston rod  5  connected to the piston  1  and projecting axially from the cylinder  40 . 
     The piston rod  5  has a small diameter part  5   a  formed at its lower end via a step  5   b . The small diameter part  5   a  penetrates the center of the piston  1  and a piston nut  30  is screwed onto a male screw  5   c  formed at a lower end of the small diameter part  5   a . The outer circumferential part of the piston  1  slides on the inner circumference of the cylinder  40 . 
     The interior of the cylinder  40  is separated by the piston  1  into a first oil chamber  41  located above the piston  1  and a second oil chamber  42  located below the piston  1 . Working oil is enclosed in the first oil chamber  41  and the second oil chamber  42 . As a known construction of a hydraulic shock absorber, a reservoir or an air chamber to compensate for capacity variation in the cylinder  40  accompanying elongation and contraction of the piston rod  5  with respect to the cylinder  40  is provided inside or outside the cylinder  40 . 
     First passages  2   a  and  2   b  which respectively penetrates the piston  1  obliquely, a ring groove  3   a  serving as an outlet of the first passage  2   a , and a ring groove  3   b  serving as an outlet of the first passage  2   b  are formed in the piston  1 . 
     An elongation damping valve  10   a  facing the ring groove  3   a  is provided under the piston  1 . A contraction damping valve  10   b  facing the ring groove  3   b  is provided above the piston  1 . A leaf valve comprising a stack of a plurality of leaves constitutes the elongation damping valve  10   a  and the contraction damping valve  10   b . The elongation damping valve  10   a  closes the ring groove  3   a  by seating its outer circumferential part on a valve seat  1   a  formed on the piston  1  along the outer circumference of the ring groove  3   a . The elongation damping valve  10   b  closes the ring groove  3   b  by seating its outer circumferential part on a valve seat  1   b  formed on the piston  1  along the outer circumference of the ring groove  3   b.    
     A partitioning member  22  having an inverted cylindrical shape covers an inlet of the first passage  2   a  and the contraction damping valve  10   b . The partitioning member  22  comprises a bottom part  22   a  and a tubular part  22   b  which extends axially downward from the outer circumference of the bottom part  22   a . The small diameter part  5   a  of the piston rod  5  penetrates a hole part  22   c  formed in a center of the bottom part  22   a . A tip of the tubular part  22   b  is fitted onto the outer circumference of the piston  1 . The partitioning member  22  thus constructed forms a chamber R 1  above the inlet of the first passage  2   a  and the contraction damping valve  10   b . The bottom part  22   a  has a plurality of first ports  22   e  connecting the first oil chamber  41  and the chamber R 1 . 
     The bottom part  22   a  has a ring-shaped projection projecting into the first oil chamber  41  on its outer circumference. A plurality of second ports  22   d  connecting the first oil chamber  41  and the chamber R 1  are formed through the ring-shaped projection. The ring-shaped projection is provided with a conically inclined wall face  22   g  on its inner circumference surrounding the first ports  22   e.    
     A throttle  12  is provided above the partitioning member  22  so as to face the plurality of first ports  22   e . The throttle  12  is fitted onto the outer circumference of a cylindrical holder  23  which is fixed onto the outer circumference of the small diameter part  5   a  of the piston rod  5   
     The throttle  12  is formed in the shape of a double tube having a bottom. The throttle  12  comprises a bottom part  12   c  surrounding a hole part  12   b  through which the small diameter part  5   a  of the piston rod  5  penetrates, an inner tube  12   a  extending downward from the inner circumference of the bottom part  12   c  in an axial direction along the outer circumference of the holder  23 , and an outer tube  12   d  extending downward from the outer circumference of the bottom part  12   c  in the axial direction. 
     The throttle  12  and the partitioning member  22  are biased by a coil spring  25  so as to be detached from each other. The coil spring  25  is interposed between the bottom part  12   c  of the throttle  12  and the bottom part  22   a  of the partitioning member  22  through a space that is formed between the inner tube  12   a  and the outer tube  12   d  so as to have a ring-shaped cross section. 
     A ring-shaped stopper  24  is gripped between the step  5   b  of the piston rod  5  and the holder  23 . The stopper  24 , by contacting the bottom part  12   c  of the throttle  12 , prevents the throttle  12  from displacing upward beyond a predetermined distance. In the surface of the bottom part  12   c  of the throttle  12  which contacts the stopper  24 , radial grooves  12   e  are formed so as to ensure that a hydraulic pressure in the first oil chamber  41  acts on the bottom part  12   c  contacting the stopper  24 . The bottom part  22   a  of the partitioning member  22  around the hole part  22   c  is gripped between the holder  23  and the inner circumferential part of the contraction damping valve  10   b  via a washer  21 . 
     A tip of the outer tube  12   d  of the throttle  12  faces a valve seat  22   f  which is formed between the first ports  22   e  of the bottom part  22   a  of the partitioning member  22  and the conically inclined wall face  22   g.    
     The throttle  12  displaces downward against the coil spring  25 , thereby narrowing a flow sectional area between the first oil chamber  41  and the plurality of first ports  22   e  or shutting of the flow therebetween. Specifically, in an elongation stroke of the piston  1  during which the throttle  12  approaches the partitioning member  22 , the tip of the outer tube  12   d  approaches the inclined wall face  22   g  so as to narrow the flow cross sectional area formed therebetween in relation to a flow of working oil from the first oil chamber  41  to the first passage  2   a  via the first ports  22   e . Further, when the tip of the outer tube  12   d  is seated on the valve seat  22   f , the flow of oil from the first oil chamber  41  to the first passage  2   a  via the first ports  22   e  is completely shut off, and the entire amount of working oil flowing from the first oil chamber  41  to the first passage  2   a  passes through the second ports  22   d . The throttle  12  thus has a function to narrow the flow path from the first oil chamber  41  to the first passage  2   a  when the piston  1  strokes in the elongation direction of the shock absorber at a high speed. 
     A partitioning member  27  having a cylindrical shape covers an inlet of the first passage  2   b  and the elongation damping valve  10   a . The partitioning member  27  comprises a bottom part  27   a  and a tubular part  27   b  which extends axially upward from the outer circumference of the bottom part  27   a . The small diameter part  5   a  of the piston rod  5  penetrates a hole part  27   c  formed in a center of the bottom part  27   a . A tip of the tubular part  27   b  is fitted onto the outer circumference of the piston  1 . The partitioning member  27  thus constructed forms a chamber R 2  under the inlet of the first passage  2   b  and the elongation damping valve  10   a . The bottom part  27   a  has a plurality of first ports  27   e  connecting the second oil chamber  41  and the chamber R 2 . 
     The bottom part  27   a  has a ring-shaped projection projecting into the second oil chamber  42  on its outer circumference. A plurality of second ports  27   d  connecting the second oil chamber  42  and the chamber R 2  are formed through the ring-shaped projection. The ring-shaped projection is provided with a conically inclined wall face  27   g  on its outer circumference surrounding the first ports  27   e.    
     A throttle  14  is provided under the partitioning member  22  so as to face the plurality of first ports  27   e . The throttle  14  is fitted onto the outer circumference of a cylindrical holder  28  which is fixed onto the outer circumference of the small diameter part  5   a  of the piston rod  5 . 
     The throttle  14  is formed in the shape of a double tube having a bottom. The throttle  14  comprises a bottom part  14   c  surrounding a hole part  14   b  through which the small diameter part  5   a  of the piston rod  5  penetrates, an inner tube  14   a  extending upward from the inner circumference of the bottom part  14   c  in an axial direction along the outer circumference of the holder  28 , and an outer tube  14   d  extending upward from the outer circumference of the bottom part  14   c  in the axial direction. 
     The throttle  14  and the partitioning member  27  are biased by a coil spring  29  so as to be detached from each other. The coil spring  29  is interposed between the bottom part  14   c  of the throttle  14  and the bottom part  27   a  of the partitioning member  27  through a space formed between the inner tube  14   a  and the outer tube  14   d  so as to have a ring-shaped cross section. 
     The bottom part  27   a  of the partitioning member  27  around the hole part  27   c  is gripped between the holder  28  and the inner circumferential part of the elongation damping valve  10   a  via a washer  26 . The piston nut  30 , when screwed onto the tip of the small diameter part  5   a  of the piston rod  5 , fixedly retains the holder  28 , partitioning member  27 , washer  26 , elongation damping valve  10   a , piston  1 , contraction damping valve  10   b , washer  21 , partitioning member  22 , holder  23 , and stopper  24  against the step  5   b  of the piston rod  5  on the outer circumference of the small diameter part  5   a.    
     The piston nut  30  also serves as a stopper which prevents the throttle  14  from displacing downward beyond a predetermined distance. In the surface of the bottom part  14   c  of the throttle  14  contacting the piston nut  30 , radial grooves  14   e  are formed to ensure that a hydraulic pressure in the second oil chamber  42  acts on the bottom part  14   c  contacting the piston nut  30 . 
     A tip of the outer tube  14   d  of the throttle  14  faces a valve seat  27   f  which is formed between the first ports  27   e  of the bottom part  27   a  of the partitioning member  27  and the conically inclined wall face  27   g.    
     The throttle  14  displaces upward against the coil spring  29 , thereby narrowing a flow sectional area between the second oil chamber  42  and the plurality of first ports  27   e  or shutting of the flow therebetween. Specifically, in a contraction stroke of the piston  1  where the throttle  14  approaches the partitioning member  27 , the tip of the outer tube  14   d  approaches the inclined wall face  27   g  so as to narrow the flow cross-sectional area formed therebetween in relation to a flow of working oil from the second oil chamber  42  to the first passage  2   b  via the first ports  27   e . Further, when the tip of the outer tube  14   d  is seated on the valve seat  27   f , the flow of oil from the second oil chamber  42  to the first passage  2   b  via the first ports  27   e  is completely shut off, and the entire amount of working oil flowing from the second oil chamber  42  to the first passage  2   b  passes through the second ports  27   d . The throttle  14  thus has a function to narrow the flow path from the second oil chamber  42  to the first passage  2   b  when the piston  1  strokes in the contraction direction of the shock absorber at a high speed. 
     A second passage  15  which connects the first oil chamber  41  and the second oil chamber  42  without passing through the damping valves  10   a  and  10   b  is formed through the small diameter part  5   a  of the piston rod  5 . An upper end of the second passage  15  is connected to the first oil chamber  41  via two orifices  16   a  disposed in series. A lower end of the second passage  15  is connected to the second oil chamber  42  via two orifices  17   a  disposed in series. 
     The orifice  16   a  is formed inside a plug  16  which is screwed into a lateral hole  15   b  connecting the upper end of the second passage  15  in the piston rod  5  to the first oil chamber  41 . The orifice  17   a  is formed inside a plug  17  which is screwed into the lower end of the second passage  15  opening onto the second oil chamber  42 . Although the two orifices  16   a  and the two orifices  17   a  are used in this embodiment, the number of the orifices  16   a ,  17   a  can be set differently. 
     The second passage  15  thus communicates with the first oil chamber  41  via the two orifices  16   a , and communicates with the second oil chamber  42  via the two orifices  17   a . The second passage  15  between the orifices  16   a  and the orifices  17   a  maintains a substantially constant pressure irrespective of the stroke direction or stroke speed of the piston  1 . 
     In this hydraulic shock absorber, the pressure in the second passage  15  stabilized as described above is caused to act on the throttles  12  and  14 , and therefore the coil springs  25  and  29  can be made compact and reduced in weight. 
     Specifically, with respect to the throttle  12 , a lateral hole  15   c  is formed in the small diameter part  5   a  of the piston rod  5  so as to connect the second passage  15  to a ring groove  23   c  formed on the inner circumference of the holder  23 . Further, a lateral hole  23   d  is formed in the holder  23  to connect the ring groove  23   c  to a pressure chamber  18  formed between the inner tube  12   a  of the throttle  12  and the holder  23 . 
     The pressure chamber  18  is formed in a space having a ring-shaped cross section between an enlarged diameter part  12   f  on the inner circumference of the inner tube  12   a  of the throttle  12  and a small diameter part  23   a  on the outer circumference of the holder  23 . An upper end of the pressure chamber  18  is delimited by the inner tube  12   a  of the throttle  12  and a lower end of the pressure chamber  18  is delimited by the holder  23 . The pressure introduced into the pressure chamber  18  from the second passage  15  therefore exerts an upward force permanently on the throttle  12 . 
     With respect to the throttle  14 , a lateral hole  15   d  is formed in the small diameter part  5   a  of the piston rod  5  so as to connect the second passage  15  to a ring groove  28   c  formed on the inner circumference of the holder  28 . Further, a lateral hole  28   d  is formed in the holder  28  to connect the ring groove  28   c  to a pressure chamber  19  formed between the inner tube  14   a  of the throttle  14  and the holder  28 . 
     The pressure chamber  19  is formed in a space having a ring-shaped cross section between an enlarged diameter part  14   f  on the inner circumference of the inner tube  14   a  of the throttle  14  and a small diameter part  28   a  on the outer circumference of the holder  28 . An upper part of the pressure chamber  19  is delimited by the holder  28  and a lower end of the pressure chamber  19  is delimited by the inner tube  14   a  of the throttle  14 . The pressure introduced into the pressure chamber  19  from the second passage  15  therefore exerts a downward force permanently on the throttle  14 . 
     In this hydraulic shock absorber, the opening pressure of the elongation damping valve  10   a  and the opening pressure of the contraction damping valve  10   b  are set in advance such that they are not reached as long as the stroke speed of the piston  1  is within a low-speed region irrespective of the stroke direction of the piston  1 . Further, the spring load of the coil spring  25  and the spring load of the coil spring  29  are set in advance such that the throttle  12  and the throttle  14  do not operate before the stroke speed reaches a high-speed region. When designing the oil springs  25 , their size may inevitably increase depending on the required spring loads. In this hydraulic shock absorber, the pressures in the pressure chambers  18  and  19  act on the throttles  12  and  14  in the same direction as the biasing forces of the coil springs  25 ,  29 , respectively. The pressure chambers  18  and  19  therefore help to reduce the spring loads of the oil springs  25 ,  29 , thereby enabling the coil springs  25  and  29  to be compact and light weight. 
     According to the above constructions, when the piston  1  performs an elongation stroke or a contraction stroke at a low speed, a flow of working oil is formed through the second passage  15  while the elongation damping valve  10   a  and the contraction damping valve  10   b  remain closed. Specifically, in the elongation stroke of the piston  1 , working oil flows from the first oil chamber  41  to the second oil chamber  42  through the second passage  15  and generates an elongation damping force in the orifices  16   a  and  17   a . In the contraction stroke of the piston  1 , working oil flows from the second oil chamber  42  to the first oil chamber  41  through the second passage  15  and generates a contraction damping force in the orifices  16   a  and  17   a.    
     When the piston  1  performs an elongation stroke or a contraction stroke at a medium speed, a differential pressure between the first oil chamber  41  and second oil chamber  42  exceeds the opening pressure of the elongation damping valve  10   a  or the opening pressure of the contraction damping valve  10   b  such that the elongation damping valve  10   a  or the contraction damping valve  10   b  opens depending on the stroke direction of the piston  1  while allowing a flow of working oil through the second passage  15 . 
     In other words, when the piston  1  performs an elongation stroke in the medium-speed region, the elongation damping valve  10   a  opens so as to cause working oil in the first oil chamber  41  to flow into the second oil chamber  42  via the chamber R 1 , the first passage  2   a , and the chamber R 2  while generating an elongation damping force based on the opening pressure of the elongation damping valve  10   a . When the piston  1  performs a contraction stroke in the medium-speed region, the contraction damping valve  10   b  opens so as to cause working oil in the second oil chamber  42  to flow into the first oil chamber  41  via the chamber R 2 , the first passage  2   b , and the chamber R 1  while generating a contraction damping force based on the opening pressure of the contraction damping valve  10   b.    
     When the piston  1  performs an elongation stroke, or in other words the piston  1  displace upward in  FIG. 1 , the pressure in the first oil chamber  41  acts on the throttle  12  both upward and downward. With respect to the pressure in the first oil chamber  41 , the upward pressure receiving area of the throttle  12  is smaller than the downward pressure receiving area of the same due to the pressure chamber  18 . Since the pressure in the pressure chamber  18  and the biasing force of the coil spring  25  act upward on the throttle  12 , the throttle  12  stays in a lifted position as shown in  FIG. 1  as long as the piston  1  displaces upward in a low-speed region or a middle-speed region. However, when the piston  1  displaces upward in a high-speed region, the pressure in the first oil chamber  41  greatly increases while the pressure in the pressure chamber  18  remains constant. As a result, the downward force acting on the throttle  12  relatively increases and drives the throttle  12  downward, thereby narrowing a gap between the tip of the outer tube  12   d  and the inclined wall face  22   g  of the partitioning member  22 . 
     As the flow path of the working oil between the first oil chamber  41  and the chamber R 1  is thus narrowed, an additional elongation damping force is generated in addition to the elongation damping force generated by the elongation damping valve  10   a.    
     The difference between the downward force and the upward force acting on the throttle  12  increases as the elongation stroke speed of the piston  1  becomes higher. Accordingly, the throttle  12  displaces further downward as the elongation stroke speed of the piston  1  becomes higher so as to narrow a gap between the tip of the outer tube  12   d  and the inclined wall face  22   g  of the partitioning member  22  further. As the flow path of the working oil between the first oil chamber  41  and the chamber R 1  is thus narrowed further, the additional elongation damping force increases-further. 
     When the piston  1  performs a contraction stroke, or in other words the piston  1  displace downward in  FIG. 1 , the pressure in the second oil chamber  42  acts on the throttle  14  both upward and downward. With respect to the pressure in the second oil chamber  42 , the downward pressure receiving area of the throttle  14  is smaller than the upward pressure receiving area of the same due to the pressure chamber  19 . Since the pressure in the pressure chamber  19  and the biasing force of the coil spring  29  act downward on the throttle  14 , the throttle  14  stays in a lifted position as shown in  FIG. 1  as long as the piston  1  displaces downward in a low-speed region or a middle-speed region. However, when the piston  1  displaces downward in a high-speed region, the pressure in the second oil chamber  42  greatly increases while the pressure in the pressure chamber  19  remains constant. As a result, the upward force acting on the throttle  12  relatively increases and drives the throttle  14  upward, thereby narrowing a gap between the tip of the outer tube  14   d  and the inclined wall face  27   g  of the partitioning member  27 . 
     As the flow path of the working oil between the first oil chamber  42  and the chamber R 2  is thus narrowed, an additional contraction damping force is generated in addition to the contraction damping force generated by the elongation damping valve  10   a.    
     The difference between the upward force and the downward force acting on the throttle  14  increases as the contraction stroke speed of the piston  1  becomes higher. Accordingly, the throttle  14  displaces further upward as the contraction stroke speed of the piston  1  becomes higher so as to narrow a gap between the tip of the outer tube  14   d  and the inclined wall face  27   g  of the partitioning member  27  further. As the flow path of the working oil between the second oil chamber  42  and the chamber R 2  is thus narrowed further, the additional contraction damping force increases further. 
     Referring to  FIG. 2 , through the actions described above, the hydraulic shock absorber generates a small damping force when the stroke speed of the piston  1  is in a low-speed region. When the stroke speed of the piston  1  reaches a medium-speed region, the damping force generated by the orifices  16   a  or  17   a  increases rapidly, and either the elongation damping valve  10   a  or the contraction damping valve  10   b  opens depending on the stroke direction of the piston  1 . When the stroke speed of the piston  1  varies within the medium-speed region, the opening of the elongation damping valve  10   a  or the contraction damping valve  10   b  varies depending on the stroke speed such that the generated damping force maintains a constant level. 
     When the stroke speed of the piston  1  reaches the high-speed region, the throttle  12  or  14  starts to move. As a result, in the elongation stroke, the gap between the tip of the outer tube  12   d  and the inclined wall face  22   g  of the partitioning member  22  narrows such that resistance to the flow of working oil from the first oil chamber  41  to the chamber R 1  increases. However, since the tip of the outer tube  12   d  is arranged to be seated on the valve seat  22   f  after narrowing the gap with respect to the inclined wall face  22   g , resistance to the flow of working oil increases gradually from the beginning of displacement of the throttle  12  in contrast to a case where the inclined wall face  22   g  is not provided. Accordingly, as shown in the figure, the elongation damping force in the high-speed region increases at a higher rate than in the medium-speed region with respect to an increase in the stroke speed. 
     In the contraction stroke, the gap between the tip of the outer tube  14   d  and the inclined wall face  27   g  of the partitioning member  27  narrows such that resistance to the flow of working oil from the second oil chamber  42  to the chamber R 2  increases. However, since the tip of the outer tube  14   d  is arranged to be seated on the valve seat  27   f  after narrowing the gap with respect to the inclined wall face  27   g , resistance to the flow of working oil increases gradually from the beginning of the displacement of the throttle  1   r  in contrast to a case where the inclined wall face  27   g  is not provided. Accordingly, as shown in the figure, the contraction damping force in the high-speed region increases at a higher rate than in the medium-speed region with respect to an increase in the stroke speed, as in the case of the elongation stroke. 
     As described above, in the hydraulic shock absorber according to this invention, the damping force does not become insufficient and a high shock absorbing performance is realized even when the piston  1  strokes at high speed. 
     Further, in this hydraulic shock absorber, throttles are provided to reduce the flow cross-sectional area of the working oil when the piston  1  strokes at high speed, and hence different damping force characteristics can be obtained in the medium-speed region and the high-speed region. Further, not only different damping force characteristics, but also different damping force increase rates corresponding to the stroke speed can be obtained in the medium-speed region and the high-speed region. Therefore, detailed setting of the damping force characteristics can be realized in this hydraulic shock absorber. 
     In the embodiment as described above, the piston stroke speed is divided into the low-speed region, the medium-speed region and the high-speed region. The stroke speed which corresponds to a boundary between the regions can be set arbitrarily. Further, it is also possible to construct a shock absorber in which the damping force characteristics vary only when the stroke speed shifts between the medium-speed region and the high-speed region while maintaining an identical damping force characteristics when the stroke speed shifts between the low-speed region and the medium-speed region. 
     Instead of using the coil springs  25  and  29 , it is possible to bias the throttles  12  and  14  using disc springs or leaf springs. It is also possible to use biasing means other than a spring to bias the throttles  12  and  14 . 
     The throttles  12  and  14  vary only the flow cross-sectional area to the first ports  22   e  and  27   e  and do not vary the cross-sectional area of the ports  22   e  and  27   e . Accordingly, high operation stability is realized with respect to a case where the total flow cross-sectional area between the first oil chamber  41  and the chamber R 1  is varied or a case where the total flow cross-sectional area between the second oil chamber  42  and the chamber R 2  is varied. 
     The contents of Tokugan 2007-231470, with a filing date of Sep. 6, 2008 in Japan, are hereby incorporated by reference. 
     Although the invention has been described above with reference to certain embodiments, 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, in the embodiment described above, the damping forces generated in the elongation stroke and the contraction stroke of the hydraulic shock absorber are identical, but setting may be performed such that different damping forces are generated in the elongation stroke and the contraction stroke. 
     The valve disc is not limited to the piston  1 . This invention can be applied to a base valve which is installed in the bottom of the cylinder  40 .