shock absorbers

Abstract

Shock absorber comprising: a cylinder (1) sealed and containing hydraulic fluid; a piston (2; 71) movably fitted into the cylinder to define two chambers (A, B) in the cylinder; a piston rod (3; 72; 81) secured at one end to the piston, the other end of the piston rod protruding from the cylinder; a main passage (2A, 2B, 71A, 71B) through which the hydraulic fluid flows between the two chambers in the cylinder in response to movement of the piston; a first passage and a second passage provided parallel to the main passage, through which the hydraulic fluid flows from one of the two chambers in the cylinder to the other in response to movement of the piston; a main damping valve (7, 16, 73, 74) provided in the main passage to control the flow of hydraulic fluid induced by the movement of the piston. to regulate in order to generate a damping force;a damping force generation mechanism (4, 13; 78C, 78D, 78E) provided in the first passage to regulate the flow of hydraulic fluid induced by the movement of the piston in order to generate a damping force; a free piston (29) provided in the second passage to divide the second passage into an upstream section and a downstream section; and a passage range changing mechanism (32; 86) provided midway through the first and second passages, wherein the passage range changing mechanism, capable of adjusting passage ranges of the first and second passages accordingly, is actuated by a single control rod (34).

Description

BACKGROUND OF THE INVENTIONThe present invention relates to shock absorbers suitably used for, for example, damping vibration of vehicles.In vehicles such as two-wheel vehicles or four-wheel vehicles, a hydraulic shock absorber is generally provided between a wheel-side member and a vehicle-body-side member to attenuate vertical vibrations, etc., generated during traveling of the vehicle. This hydraulic shock absorber uses an actuator to change the opening area through which hydraulic fluid passes, thereby making it possible to appropriately adjust the generated damping force over a range from a low damping force to a high damping force. Further, the hydraulic shock absorber is additionally provided with a frequency dependent mechanism (i.e., a mechanism by which the damping force decreases according to the input vibration frequency). The combination of the frequency dependent mechanism and the damping force control mechanism enables improvement of ride quality (see, for example, Japanese Patent Application Publication No. JP H06-94 065 A).The frequency dependent mechanism makes it possible to provide a decreased damping force for a high frequency vibration input even when the damping force characteristics are set to "hard" as shown in, for example, FIG. 2 of Japanese Patent Application Publication No. JP H06-94 065A.Furthermore, document US 2003 / 0 132 073 A1 is known, which describes a shock absorber.In DE 41 38 117 A1 a hydraulic damper for a vehicle chassis suspension system is shown.SUMMARY OF THE INVENTIONThe above-described prior art shock absorber has a fixed opening area for determining a variable width of the damping force generated by the frequency dependent mechanism. Consequently, when the damping force generated from the damping force control hydraulic shock absorber is changed to the damping force characteristic "hard", "medium", or "soft", the variable width of the damping force of the frequency dependent mechanism cannot be freely changed according to each of the selected damping force characteristics ("hard", "medium", or "soft").For example, in a state where "hard" damping force characteristics are set to avoid rolling during cornering, when the variable width of the damping force is increased to reduce high-frequency vibrations from the road surface and then the "soft" damping force characteristics are set to improve the ride quality during good road travel, it is impossible to obtain such characteristics that reduce the variable width of the damping force to suppress vibrations at the unsprung resonance frequency.The present invention has been made in view of the above-described problem of the related art. Accordingly, it is an object of the present invention to provide a shock absorber configured to enable the variable width of the damping force of a frequency dependent mechanism to be freely changed according to each selected damping force characteristic when it is switched from one to another.In order to solve the above-described problem, the present invention provides a shock absorber having the features according to claim 1.According to the present invention, the area of the first passage and the area of the second passage can be set independently of each other by the passage area changing mechanism. Consequently, regardless of which damping force characteristic, for example, a "hard", "medium", or "soft" characteristic, the damping force to be generated by the shock absorber is changed, the variable width of the damping force of a frequency dependent mechanism including the free piston can be changed freely according to each of the selected damping force characteristics.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a vertical sectional view of a hydraulic shock absorber according to a first embodiment of the present invention. FIG. 2 is an enlarged vertical sectional view showing a main valve and a disk valve constituting a compression control valve of FIG. 1. FIG. 3 is a bottom plan view showing the main valve of FIG. 2 as a separate component. FIG. 4 is a bottom plan view showing the disk valve of FIG. 2 as a separate component. FIG. 5 is a fragmentary enlarged sectional view showing a piston rod and a fastener of FIG. 1. FIG. 6 is a horizontal sectional view of the shutter as viewed from the direction of arrow VI-VI in FIG. 5. FIG. 7 is a horizontal sectional view of the shutter as viewed from the direction of the arrow VII-VII in FIG. 5. FIG. 8 is a characteristic diagram showing the relationship between the shutter position and the opening area in the first embodiment. FIG. 9 is a characteristic diagram showing the relationship between the frequency and the damping force in the first embodiment. FIG. 10 is a vertical sectional view of a hydraulic shock absorber of a second embodiment of the present invention. FIG. 11 is a vertical sectional view of a hydraulic shock absorber of a third embodiment of the present invention. FIG. 12 is a vertical sectional view showing the overall structure of the hydraulic shock absorber according to the first and second embodiments.DETAILED DESCRIPTION OF THE INVENTIONThe present embodiments exhibit not only the above-described technical problems and advantageous effects of the present invention, but also various other solutions to problems and advantages. Main problems which solve the following embodiments, including those set forth above, are enumerated below.[Characteristic Improvement]For example, when damping force characteristics (damping force with respect to the piston speed) are changed from one to another according to vibration conditions, it is desirable that the damping force characteristics are changed as smoothly as possible. The reason for this is as follows. When a dip is switched between damping force characteristics generating a small damping force and damping force characteristics generating a large damping force, the damping forces actually generated are also switched dip, thereby degrading ride quality. When switching of damping forces occurs during steering of the vehicle, the vehicle behavior becomes unstable, so that the driver feels uncomfortable in the steering operation. Under these circumstances, studies have been made to enable the damping force characteristics to be changed as smoothly as possible as disclosed in the above-mentioned Japanese Patent Application Publication No. Hei 06-94065. In this connection, however, further improvement of characteristics has been demanded.[Avoidance of Structural Enlargement)A frequency dependent mechanism requires an area in which the piston is free to move up and down. Consequently, as the range for the free piston increases, the axial length of the cylinder device increases. By enlarging the cylinder device in size, the degree of freedom with which the cylinder device can be attached to the vehicle body is reduced. Consequently, increasing the axial length of the cylinder device is a significant problem. When the cylinder device is provided with a mechanism for controlling the damping force from the outside, the size of the device inevitably increases by an amount corresponding to the size of the mechanism. Consequently, a structural size reduction or "downsizing" of the frequency-dependent mechanism is strongly demanded.[Reduction in Number of Components]The frequency dependent mechanism requires components such as a housing and a free piston in addition to a piston. Consequently, the number of parts increases. An increase in the number of parts results in adverse effects on productivity, durability, reliability, etc. Accordingly, it is desirable to reduce the number of parts while obtaining the desired characteristics, i.e., the damping force characteristics corresponding to a wide range of vibration frequencies.Hereinafter, shock absorbers according to embodiments of the present invention will be described in detail with reference to the accompanying drawings by way of an example in which the present invention is applied to a hydraulic shock absorber for a vehicle.Figs. 1 to 9 and 12 show a first embodiment of the present invention. Fig. 12 shows the overall structure of the first embodiment. Referring to FIG. 12, an outer tube 100 is provided around the outer periphery of an inner tube 1. Between the inner tube 1 and the outer tube 100, a reservoir chamber R including hydraulic oil and a gas sealed therein is provided. The reservoir chamber R and the bottom-side oil chamber B are connected via a bottom valve mechanism 101. The bottom valve mechanism 101 includes a shut-off valve 102 that allows the hydraulic oil to flow substantially resistance-free from the reservoir chamber R to the bottom-side oil chamber B. The bottom valve mechanism 101 further includes a damping valve 103 that generates a relatively large damping force.A piston rod 3 connected to a piston 2 (described later) extends through a rod guide 104 and a packing 105 and protrudes from the outer tube 100. A control rod 34 (described later) protrudes from the protruding end of the piston rod 3. The distal end of the piston rod 3 is secured to the vehicle via a mounting member (not shown). To the piston rod 3 when mounted on the vehicle, a direct acting actuator 106 is mounted from the engine room or trunk of the vehicle. Reference numeral 107 denotes an eyelet for attaching the hydraulic shock absorber to a wheel-side member.In FIG. 1, the inner pipe 1 has hydraulic oil sealingly contained therein as hydraulic fluid. An annular reservoir chamber R is formed between the outer tube 1 and the inner tube 1.Reference numeral 2 denotes a piston slidably fitted in the inner tube 1. The piston 2 divides the inner region of the inner tube 1 into two chambers, i.e., a rod-side oil chamber A and a bottom-side oil chamber B. The piston 6 has a plurality of circumferentially spaced oil paths 2A and a plurality of circumferentially spaced oil paths 2B that allow communication between the rod-side oil chamber A and the bottom-side oil chamber B. The oil paths 2A and 2B are oil ports inclined with respect to the axis of the piston 2. The oil paths 2A and 2B form a main passage through which the hydraulic oil flows between the rod-side oil chamber A and the bottom-side oil chamber B.The piston 2 has a lower end surface defined as one end thereof. The lower end surface of the piston 2 is provided with an annular recess 2C surrounding the opening at one end of each oil path 2A, and is further provided with an annular valve seat 2D positioned radially outward with respect to the annular recess 2C. The main disc 16 (described later) selectively seats and disengages from the annular valve seat 2D. The upper end surface, i.e., the other end, of the piston 2 is provided with an annular recess 2E surrounding the openings at the other ends of the oil paths 2B, and is further provided with an annular valve seat 2F provided radially outward with respect to the annular recess 2E. A main disc 7 (described later) selectively seats and disengages from the annular valve seat 2F.Reference numeral 3 denotes a piston rod which extends axially into the inner tube 1. The piston rod 3 has a lower end defined as one end thereof. The lower end of the piston rod 3 is inserted into the inner tube 1 and secured to the piston 2 by a cap nut 27 of a housing 26 (described later), etc. The upper end, as the other end, of the piston rod 3 projects beyond the outer and inner tubes 100 and 1 via the rod guide 104, etc. The piston rod 3 is provided with a lock fitting hole 3A and a reduced diameter rod insertion hole 3B axially extending through the inner diameter thereof. The shutter fitting hole 3A opens at the lower end of the piston rod 3 and has a shutter 33 (described later) rotatably fitted thereto. The rod insertion hole 3B extends upward from the upper end of the shutter fitting hole 3A.The piston rod 3 is provided with axially spaced groups of a plurality of circumferentially spaced oil holes 3C, 3D, 3E, 3F and 3G extending radially outward from the shutter fitting hole 3A. Among the oil holes 3C to 3G, the oil holes 3D to 3E are arranged in the rod-side oil chamber A defined in the inner pipe 1 by the piston 2, and the remaining oil holes 3F and 3G are arranged in the bottom-side oil chamber B in the inner pipe 1.Among the oil holes 3C to 3G, the uppermost oil holes 3C are selectively brought into communication with an inner hole 33A of the shutter 33 (described later) via radial holes 33B, and the communication is selectively canceled. The oil ports 3D and 3E are selectively communicated and separated with each other via oil grooves 33C and 33D of the shutter 33 (described later). The oil ports 3F and 3G are selectively communicated and separated with each other via oil grooves 33E and 33F of the shutter 33 (described later). Further, the piston rod 3 has an annular stepped portion 3H formed on the outer periphery thereof. A terminal member 22 (described later) is axially positioned at the stepped portion 3H.Reference numeral 4 denotes a compression damping force generation mechanism (hereinafter referred to as "compression damping mechanism 4") used in this embodiment. The compression damping mechanism 4 is, as shown in FIG. 1, provided in the rod-side oil chamber A of the inner pipe 1 and secured to the upper side of the piston 2. When the piston 2 is slidably displaced downward in the inner tube 1 during the compression stroke of the piston rod 3, the compression damping mechanism 4 resists the hydraulic oil flowing from the bottom side oil chamber B to the rod side oil chamber A through the oil paths 2B of the piston 2, a pressure chamber C (described later), the oil ports 3E and 3D of the piston rod 3, the oil grooves 33C and 33D of the shutter 33 (described later), etc., thereby generating a certain damping force.The compression damping mechanism 4 includes an upper case member 5 in the shape of a cylinder, an upper end of which is closed. The upper case member 5 is secured to the outer periphery of the piston rod 3 between a connector 23 (described later) and the piston 2. The compression damping mechanism 4 further includes a main valve 6 having an elastic sealing member 8 (described later) press-fitted to the lower end surface of the upper case member 5 to form an annular pressure chamber C between the main valve 6 and the upper case member 5. Further, the compression damping mechanism 4 includes an outer check valve 9, an inner check valve 10, and a disk valve 11 which will be described later.The upper case member 5 of the compression damping mechanism 4 is provided with an annular valve 5A formed on an upper end surface thereof. The outer disk valve 5 selectively seats on and is selectively disengaged from the annular valve seat 5A. The upper case member 5 is further provided with openings 5B radially provided in the annular valve seat 5A to serve as axial oil paths that allow the pressure chamber C to communicate with a region inside the annular valve seat 5A. Further, the upper case member 5 is provided with oil grooves 5C serving as radial oil paths that allow the pressure chamber C to constantly communicate with the oil ports 3E of the piston rod 3. The outer check valve 9 constitutes a relief valve which opens when the pressure in the pressure chamber C increases to a certain fixed positive pressure. At events different from the above, the outer shut-off valve 9 is seated on the annular valve seat 5A and thus kept closed.The main valve 6 includes a main disk 7 selectively seated on and detached from the annular valve seat 2F of the piston 2, and an annular elastic sealing member 8 secured to the outer periphery of the upper side of the main disk 7 by vulcanization, firing, or a similar method. The elastic seal member 8 is formed in the shape of a thick-walled ring using an elastic material such as rubber to seal the pressure chamber C from liquid contained in the elastic seal member 8 with respect to the rod-side oil chamber A contained outside with respect to the elastic seal member 8.The main disk 7 of the main valve 6 is configured to serve as a main damping valve which is a constituent part of the present invention. The main valve 6 functions as follows. When a pressure difference between the oil chambers A and B increases to a certain predetermined value during the compression stroke of the piston rod 3, the main disc 7 disengages from the annular valve seat 2F to generate a certain compression damping force. When the main valve 6 (main disk 7) opens, the oil chambers A and B communicate with each other via the oil paths 2B of the piston 2. Thus, a main passage forming a part of the present invention is formed.The main disc 7 of the main valve 6, as shown in FIGS. 2 and 3, is provided with a circular opening 7A and a plurality of circumferentially spaced arc openings 7B disposed radially outward with respect to the circular opening 7A. The main disk 7 is attached to the outer periphery of the piston rod 3 via the circular opening 7A. The arc holes 7B constantly communicate between the annular recess 2E of the piston 2 and the pressure chamber C through cut-out portions 11B of the disk valve 11 (described later).Reference numeral 10 denotes an internal check valve provided in the annular recess 2E of the piston 2. The inner check valve 10 selectively disengages and seats on the bottom of the annular recess 2E to bring the oil paths 2B of the piston 2 into communication with the annular recess 2E and cancel the communication. The inner check valve 10 allows the hydraulic oil in the bottom-side oil chamber B to flow from the oil paths 2B to the piston 2 to the annular recess 2E, and blocks the reverse flow of the hydraulic oil from the annular recess 2E to the oil paths 2B.Reference numeral 11 denotes a disk valve provided over the inner check valve 10 with a spacer or the like provided therebetween. The disk valve 11 is, as shown in FIGS. 2 and 4, provided with a circular opening 11A and a plurality of circumferentially spaced cut-out openings 11B positioned outward with respect to the circular opening 11A. The disk valve 11 is attached to the outer periphery of the piston rod 3 via the circular opening 11A. Each cut-out hole 11B is formed as an oil hole of substantially a T-shape. The cut-out holes 11B constantly communicate between the annular recess 2E of the piston 2 and the pressure chamber C via the arc holes 7B of the main disk 7. in this case, the cut-out holes 11B have a substantially smaller passage area than the arc holes 7B of the main disk 7 to apply a restricting action to the hydraulic oil flowing therethrough, thereby generating the damping force.Reference numeral 12 denotes a retainer disc sandwiching the disc valve 11 between itself and the main disc 7. The retainer 12 is attached to the outer periphery of the piston rod 3 via a circular opening 12A. The retainer 12 reinforces the disk valve 11 and the main disk 7 by abutting them from below on the upper end surface of the retainer 12, and the retainer 12 is disposed in the annular recess 2E of the plunger 2 to serve as a retainer that regulates the maximum opening degree of the inner check valve 10.Reference numeral 13 denotes an extension damping force generation mechanism (hereinafter referred to as "extension damping mechanism 13") used in this embodiment. The extension damping mechanism 13 is, as shown in FIG. 1, disposed in the bottom-side oil chamber B of the inner pipe 1 and secured to the lower side of the piston 2. When the piston 2 is displaced upward in the inner tube 1 during the extension stroke of the piston rod 3, the extension damping mechanism 13 provides resistance against the hydraulic oil flowing from the rod-side oil chamber A to the bottom-side oil chamber B through the oil paths 2A of the piston 2, a pressure chamber D in the lower housing member 14, the oil ports 3F and 3G of the piston rod 3, the oil grooves 33E and 33F of the shutter 33 (described later), etc., thereby generating a certain damping force.The extension damping mechanism includes a lower case member 14 in the form of a cylinder, a lower end of which is closed. The lower case 14 is secured to the outer periphery of the piston rod 3 between a connector 24 (described later) and the piston 2. The extension damping mechanism 13 further includes a main valve 15 having an elastic sealing member 17 (described later) press-fitted to the upper end surface of the lower case member 14 to form an annular pressure chamber D between the main valve 15 and the lower case member 14. Further, the extension damping mechanism 13 includes an outer check valve 18, an inner check valve 19, and a disk valve 20 which will be described later.The lower case member 14 of the extension damping mechanism 13 has substantially the same structure as the upper case member 5 of the compression damping mechanism 4. The lower case member 14 has an annular valve seat 14a, oil ports 14b as axial oil paths, and oil grooves 14C as radial oil paths. The outer check valve 18 constitutes a relief valve which opens when the pressure in the pressure chamber D increases to a certain fixed positive pressure. At events different from the above, the outer check valve 18 is seated on the annular valve seat 14a and thus kept closed.The main valve 15 has substantially the same structure as the main valve 6 of the compression damping mechanism 4. That is, the main valve 15 includes a main disk 16 selectively seated on and detached from the annular valve seat 2D of the piston 2, and an annular elastic sealing member 17 secured to the outer periphery of the lower side of the main disk 16. The main valve 15 is configured to serve as a main damping valve which is a constituent part of the present invention.The main valve 15 functions as follows. When a pressure difference between the oil chambers A and B increases to a certain predetermined value during the extension stroke of the piston rod 3, the main disc 16 disengages from the annular valve seat 2D to generate a certain extension damping force. When the main valve 15 (main disk 16) opens, the oil chambers A and B communicate with each other via the oil paths 2A of the piston 2, thus forming a main passage which is a constituent part of the present invention. The main disk 16 of the main valve 15 is also provided with a circular opening 16A and a plurality of arc openings 16B as shown in FIGS. 2 and 3.Reference numeral 19 denotes an internal check valve provided in the annular recess 2C of the piston 2. The inner check valve 19 selectively disengages from and seats on the bottom of the annular recess 2C to bring the oil paths 2A of the piston 2 into communication with and cancel the annular recess 2C. The inner check valve 19 allows the hydraulic oil in the rod-side oil chamber A to flow from the oil paths 2A of the piston 2 to the annular recess 2C and blocks the reverse flow of the hydraulic oil from the annular recess 2C to the oil paths 2A.Reference numeral 20 denotes a disk valve provided under the inner check valve 19 with a spacer or the like provided therebetween. The disk valve 20 has the same structure as the disk valve 11 of the compression damping mechanism 4. That is, as shown in FIGS. 2 and 4, the disk valve 20 has a circular opening 20A and a plurality of cut-out openings 20B. The cut-out ports 20B of the disk valve 20 constantly communicate between the annular recess 2C of the piston 2 and the pressure chamber D via the arc ports 16A of the main disk 16.Reference numeral 21 denotes a retainer disc which sandwiches the disc valve 20 between itself and the main disc 16. The retainer 21 is attached to the outer periphery of the piston rod 3 via a circular opening 21A. The retainer 21 reinforces the disk valve 20 and the main disk 16 by abutting against them from above on the lower end surface of the retainer 21, The retainer 12 is disposed in the annular recess 2C of the piston 2 to function as a retainer that regulates the maximum opening degree of the inner check valve 19.Reference numerals 22 and 23 denote terminal members provided between the stepped portion 3H of the piston rod 3 and the upper case member 5. The connecting elements 22 and 23 are rings or the like which are adapted around the outer circumference of the piston rod 3. The port member 22 allows the hydraulic oil to flow between the rod-side oil chamber A and the oil ports 3C of the piston rod 3 clean and out. The port member 23 allows the hydraulic oil to flow between the rod-side oil chamber A and the oil ports 2D of the piston rod 3 clean and out.Reference numeral 24 denotes another terminal member provided between the lower case member 14 of the extension damping mechanism 13 and the capsule nut 27. The connecting element 24 is also a ring or the like which is adapted around the outer circumference of the piston rod 3. The connector 24 allows the hydraulic oil to flow clean and out between the bottom-side oil chamber B and the oil holes 3G of the piston rod 3.Reference numeral 25 denotes a frequency dependent mechanism provided at the lower end of the piston rod 3. The frequency dependent mechanism 25 includes, as shown in FIG. 1, a tubular housing 26 slidable in the inner tube 1 together with the piston rod 3 as a unit, and a free piston 29 (described later) slidable in the housing 26 and relative thereto. The frequency dependent mechanism 25 further includes O-rings 30 and 31. The housing 26 has a cap nut 27 serving as a cap member screwed on the lower end of the piston 3, and a bottom-closed tubular member 28.The capsule nut 27 has an inner nut portion 27A screwed on the outer periphery of the lower end of the piston rod 3, an annular lid portion 27B extending radially outward from the upper end of the inner nut portion 27A, and an outer tubular downwardly extending portion 27C extending downward from the outer periphery of the annular lid portion 27B and having an inner peripheral surface serving as a guide surface for the free piston 29. The lower end surface of the tubular downward extending portion 27C forms a housing contact surface that is in contact with an O-ring 30 (described later).The bottom-closed tubular member 28 has a tubular portion 28A having an upper end secured to the annular lid portion 27B of the capsule nut 27 from the outside by caulking or similar method. The tubular portion 28A extends downward in the inner tube 1. The bottom-closed tubular member 28 further includes an annular bottom portion 28B that closes the lower end of the tubular portion 28A. The lower portion 28B has a communication hole 28C formed at the center thereof. The communication port 28C communicates between a lower chamber 11 (described later) and the bottom-side oil chamber B.The tubular portion 28A has an inclined bent surface 28D formed on the inner periphery thereof. The inclined bent surface 28D serves as a housing contact surface that is in contact with an O-ring 31 (described later). The inclined curved surface 28D is inclined to the moving direction (i.e., the axial direction) of a free piston 29 (described later) and has a curved surface. The inclined bent surface 28D elastically and compressibly deforms the O-ring 31 between itself and an annular protrusion 29A (described later) when the free piston 29 is displaced downward. Thus, the inclined curved surface 28D has the function of suppressing the displacement of the free piston 29 toward the stroke end thereof by means of a resistance provided from the O-ring 31.Reference numeral 29 denotes a free piston slidably provided in the housing 26. As shown in FIG. 1, the free piston 29 is formed as a tubular piston with a lower end thereof closed. The free piston 29 has an annular protrusion 29A protruding radially outward from an axially intermediate position on the outer periphery thereof. The free piston 29 has a lower end which is an axial end which is movably fitted in the piston-shaped portion 28A of the bottom-closed tubular member 28. The upper end of the free piston 29, which is the other axial end, is slidably fitted into the tubular downwardly extending portion 27C of the capsule nut 27.The free piston 29, which is axially displaceable in and relative to the housing 26, is abuttable against the annular cap portion 27B of the capsule nut 27 and the bottom portion 28B of the bottom-closed tubular member 28. The upper and lower stroke ends of the free piston 29 are defined by abutment of the free piston 29 against the annular cap portion 27B and the bottom portion 28B, respectively. The free piston 29 divides the interior of the housing 26 (i.e., the second passage) into two chambers, i.e., an upper chamber E which is an upstream chamber and a lower chamber F which is a downstream chamber. The second passage is defined by the free piston 29, and no displacement flow of hydraulic oil occurs between the rod-side oil chamber A and the bottom-side oil chamber B. However, as the free piston 29 moves relative to the housing 26, the hydraulic oil in the rod-side oil chamber A flows into the upper chamber E, and the same amount of hydraulic oil is pushed out from the lower chamber F to the bottom-side oil chamber B. Consequently, a flow of hydraulic oil practically occurs between the rod-side chamber A and the bottom-side chamber B.The annular projection 29A provided on the outer periphery of the free piston 29 has inclined arcuate surfaces 29B and 29C formed respectively by the upper and lower surfaces thereof. The inclined surfaces 29B and 29C are free piston contact surfaces which come into contact with O-rings 30 and 31 (described later), respectively. The inclined arcuate surfaces 29B and 29C respectively form surfaces having curved surfaces inclined to the axial direction of the free piston 29. The inclined arcuate surface 29B of the free piston 29 axially faces the lower end surface of the tubular downwardly extending portion 27C via the O-ring 30. the inclined arcuate surface 29C axially faces the inclined arcuate surface 28D of the bottom-closed tubular member 28 via the O-ring 31.The O-rings 30 and 31 are elastic members, which respectively constitute resistance elements of the frequency dependent mechanism 25. The O-rings 30 and 31 are disposed between the tubular portion 28A of the housing 26 and the outer circumferential surface of the free piston 29 to liquid-tightly seal therebetween. The upper and lower chambers E and F in the housing 26 are kept sealed from each other by the O-rings 30 and 31.When the free piston 29 is displaced upward in the housing 26, the O-ring 30 is elastically and compressively deformed between the lower end surface of the tubular downward extending portion 27C and the annular portion 29A (inclined arcuate surface 29B) of the free piston 29. At the time, the O-ring 30 generates a resistance to the upward displacement of the free piston 29 toward the stroke end. When the free piston 29 is displaced downward in the housing 26, the O-ring 31 is elastically and compressively deformed between the inclined arcuate surface 28D of the tubular portion 28A and the annular protrusion 29A (inclined arcuate surface 29C) of the free piston 29. At the time, the O-ring 31 generates a resistance against the downward displacement of the free piston 29 toward the stroke end.Reference numeral 32 denotes a passage area moving mechanism 32 and a passage area changing mechanism 32 used in this embodiment, respectively. The displacement mechanism of the passage portion 32 includes a shutter 33 (described later), a control rod 34, and an actuator (not shown), for example, a stepping motor. The actuator of the displacement mechanism of the passage portion 32 is provided at the protruding end of the piston rod 3, for example, to rotate the shutter 33 by means of the control rod 34.The shutter 33 is provided in the shutter fitting hole 3A of the piston rod 3. The shutter 33 constitutes an opening portion adjusting member of the passage portion changing mechanism 32. the shutter 33 is provided at the lower end of the control rod 34 so as to rotate together with the control rod 34 as a unit and rotate together with the control rod 34 in the shutter fitting hole 3A of the piston rod 3. The control rod 34 is provided in the rod insertion hole 3B of the piston rod 3. The upper end of the control rod 34 is connected to an output shaft (not shown) of the actuator.The shutter 33 has an inner opening 33A axially extending through the inner periphery thereof. The lower end of the inner opening 33A always communicates with the upper chamber E in the housing 26. the shutter 33 further includes openings 33B as openings extending radially outward from the inner opening 33A, oil grooves 33C and 33D formed in the outer circumferential surface of the shutter 33 spaced apart from the openings 33B in the axial direction of the shutter 33, and other oil grooves 33E and 33F formed in the outer circumferential surface of the shutter 33 spaced apart axially from the oil grooves 33C and 33D.The openings 33B are, as shown in FIG. 5, disposed at respective positions where the openings 33B may radially face the oil openings 3C of the piston rod 3. The openings 33B selectively bring the oil openings 3C into communication with the inner opening 33A and terminate communication according to the rotational position of the shutter 33. The second passage, which is a constituent part of the present invention, includes a connector 22 that communicates with the rod-side oil chamber A, the oil openings 3C of the piston rod 3, the openings 33B and inner opening 33A of the shutter 33, and the housing 26. The second passage is parallel to the main passage. The inner portion of the housing 26 forming part of the second passage is divided into an upper chamber E and a lower chamber F by means of the free piston 29.The oil grooves 33C and 33D of the shutter 33 bring the oil holes 3D and 3E of the piston rod 3 into communication with each other according to the rotational position of the shutter 33, and end the communication. As shown in FIG. 5, the oil grooves 33C extend over a distance α in the axial direction of the shutter 33, and the oil grooves 33D are formed over a shorter distance β (β<α). The oil grooves 33C are, as shown in FIG. 6, arc-shaped grooves formed on the outer circumferential surface of the shutter 33. The groove width of the oil grooves 33C is substantially uniform in the circumferential direction of the shutter 33. the oil grooves 33D are formed by cutting the outer circumferential surface of the shutter 33 in an arc shape as shown in FIG. 7. The groove width of the oil grooves 33 gradually decreases in the circumferential direction of the shutter 33.The oil grooves 33C and 33D of the shutter 33 form a first compression passage, which is a constituent part of the present invention, together with the oil ports 3D and 3E of the piston rod 3, the pressure chamber C of the compression damping mechanism 4, the arc ports 7B of the main disk 7, the cut ports 11B (see FIGS. 2 to 4 ) of the disk valve 11, the oil paths 2B of the piston 2, etc. The first passage is parallel to the main passage.The oil grooves 33E and 33F of the shutter 33 bring the oil holes 3F and 3G of the piston rod 3 into communication with each other according to the rotational position of the shutter 33, and end the communication. The oil grooves 33E are formed to extend a distance β in the axial direction of the shutter 33, and the groove width of the oil grooves 33E gradually increases in the circumferential direction of the shutter 33, as shown in FIG. 7, in substantially the same manner as the oil grooves 33D described above. The oil grooves 33F are formed to extend a distance α in the axial direction of the shutter 33, and the groove width of the oil grooves 33F is substantially uniform in the circumferential direction of the shutter 33, as shown in FIG. 6, in substantially the same manner as the oil grooves 33C described above.The oil grooves 33E and 33F of the shutter 33 form a first extension passage, which is a constituent part of the present invention, together with the oil ports 3F and 3G of the piston rod 3, the pressure chamber D of the extension damping mechanism 13, the arc ports 16B of the main disk 16, the cut ports 20B (see FIGS. 2 to 4 ) of the disk valve 20, the oil paths 2A of the piston 2, etc. The first passage is parallel to the main passage.In the shutter fitting hole 3A of the piston rod 3, a tubular member 35 is provided at the lower end (one axial end) of the shutter 33, and a tubular guide member 36 and a seal member 37 are provided at the upper end (the other axial end) of the shutter 33. The seal member 37 seals between the shutter fitting hole 3A and the control rod 34 to block the hydraulic oil from leaking from the outside. The tubular member 35 constitutes a falling-down prevention member for preventing the shutter 33 from falling down from the shutter fitting hole 3A. The tubular member 35 has an inner opening 35A extending through the inner periphery thereof. The inner opening 35A also forms a part of the second passage described above.Next, an explanation will be given of the operation of the hydraulic shock absorber according to the first embodiment constructed as above.For attaching the hydraulic shock absorber to a vehicle, the upper end of the piston rod 3 is secured to a vehicle body side member, and the bottom of the outer tube is secured to a wheel side member. During the running of the vehicle, for example, when vertical vibrations are generated by unevenness on the road surface, the piston rod 3 moves to extend from and retract into the inner tube 1. Consequently, damping forces are generated by the compression damping mechanism 4, the extension damping mechanism 13, and so on. Thus, the vibrations of the vehicle can be attenuated.More specifically, during the compression stroke of the piston rod 3, the piston rod 3 enters the inner tube 1. As a result, the pressure in the bottom side oil chamber B becomes higher than that in the rod side oil chamber A. Consequently, the hydraulic oil in the bottom side oil chamber B flows into the annular recess 2E, flows from the oil paths 2B of the piston 2 through the internal check valve 10, and flows through the cut-out openings 11B of the disk valve 11 and the arc openings 7B (see FIGS. 2 to 4 ) of the main disk 7 into the pressure chamber C of the compression damping mechanism 4.The hydraulic oil in the pressure chamber C flows from the oil grooves 5C of the upper housing member 5 into the rod-side oil chamber A through the oil holes 3E of the piston rod 3, the oil grooves 33D and 33C of the shutter 33, the oil grooves 3D of the piston rod 3, and the port member 23 A. Thus, a compression damping force can be generated, for example, through the cut holes 11B of the disk valve 11, the oil grooves 33D and 33C of the shutter 33, and so on.In this state, when the contraction speed of the piston rod 3 increases and the pressure difference between the oil chambers A and B exceeds the above-mentioned set value, the main disc 7 of the main valve 6 constituting the main damping valve disengages from the annular valve seat 2F to open. Thus, a certain compression damping force can be generated. It should be noted that the external check valve 9 of the compression damping mechanism 4 opens when the pressure in the pressure chamber C increases to the set relief pressure. Thus, the outer check valve 9 functions as a relief valve that suppresses the increase in the pressure in the pressure chamber C.On the other hand, during the extension stroke of the piston rod 3, the pressure in the rod-side oil chamber A becomes larger than that in the bottom-side oil chamber B. Consequently, the hydraulic oil in the rod-side oil chamber A flows into the annular recess 2C, from the oil paths 2A of the piston 2 through the check valve 19, and flows into the pressure chamber D of the extension damping mechanism 13, through the cut-out openings 20B of the disk valve 20 and the arc openings of the main disk 16.The hydraulic oil in the pressure chamber D flows from the oil grooves 14C of the lower case member 14 into the bottom-side oil chamber B through the oil grooves 3F of the piston rod 3, the oil grooves 33E and 33F of the shutter 33, the oil grooves 3G of the piston rod 3, and the port member 24 Thus, an extension damping force can be generated by, for example, the cut openings 20B of the disk valve 20, the oil grooves 33E and 33F of the shutter 33, and so on.In this state, when the extension speed of the piston rod 3 increases and the pressure difference between the oil chambers A and B exceeds the above-mentioned set value, the main disc 16 of the main valve 15 constituting the main damping valve disengages from the annular valve seat 2D to open. Thus, a certain extension damping force can be generated. The outer check valve 18 of the extension damping mechanism 13 opens when the pressure in the pressure chamber D increases to the set relief pressure. Thus, the outer check valve 18 functions as a relief valve that suppresses the increase in the pressure in the pressure chamber D.In the following, an explanation will be given of the manner in which a damping force is variably adjusted by adjusting the passage area of each of the first and second passages with the shutter 33 of the passage area changing mechanism 32.In the first embodiment, as shown in FIG. 8 with a slanting line portion, when the shutter 33 is set at a position A, the oil grooves 33C and 33D of the shutter 33 directly face the oil grooves 3D and 3E of the piston rod 3, and at the time, the opening area between the respectively opposite oil grooves and openings (i.e., the passage area of the first passage, which is the damping force control mechanism side passage) reaches the maximum opening area. Also, when the shutter 33 is rotated from the position a to the position b, the opening area is kept maximum. However, when the shutter 33 is rotated from the position b to the position c and then to the position d, the opening area gradually decreases, for example, by the oil grooves 33D. At the position d, the opening area is zero. The oil holes 3D and 3E are closed by the outer circumferential surface of the shutter 33, and communication thereof is ended. Also, when the shutter 33 is rotated from the position d to the position e, the opening area is kept equal to zero.On the other hand, the opening area between the oil ports 3C of the piston rod 3 and the ports 33B (i.e., the passage area of the second passage, which is a passage on the frequency dependent mechanism side) changes as follows. As shown with the obliquely ruled portion 39 in FIG. 8, when the shutter 33 is set at the position a, the opening area is zero. However, when the shutter 33 is rotated from the position a to the position b, the openings 33B directly face the oil openings 3C, and the opening area becomes maximum. When the shutter 33 is rotated from the position b to the position d, the opening area is kept maximum. When the shutter 33 is rotated to the position e, the opening area becomes zero.Thus, in the first embodiment, when the shutter position is changed from a to b, the opening area of the damping force control mechanism-side passage is constant (maximum opening area), whereas the opening area of the frequency dependent mechanism-side passage is changed from zero to maximum. When the shutter position is changed from b to d, the opening area of the damping force control mechanism gradually decreases to zero, whereas the opening area of the frequency dependent mechanism is constant (maximum opening area). When the shutter position is changed from b to d, the opening area of the damping force control mechanism is zero, whereas the opening area of the frequency dependent mechanism gradually decreases and becomes zero at the position e.In this connection, assuming that the contraction speed (or extension speed) of the piston rod 3 is constant, the characteristics of the damping force with respect to the extension / contraction frequency of the piston rod 3 can be represented by the characteristic curves 40 to 43 shown in FIG. 9. That is, when the rotational position of the shutter 33 is at a, the opening area of the damping force control mechanism is large, whereas the opening area of the frequency dependent mechanism is zero. Consequently, damping force characteristics can be set at the position a as shown by the characteristic curve 40. That is, the damping force is "soft" and fixed in frequency.When the rotational position of the shutter 33 is switched from the position a to the position b, the opening area of the damping force control mechanism is kept constant, whereas the opening area of the frequency dependent mechanism is increased to the maximum. Consequently, at the position b, damping force characteristics can be set as shown by the characteristic curve 41. That is, the damping force is "soft" in the low frequency range and still "soft" in the high frequency range.When the rotational position of the shutter 33 is switched from the position b to the position d, the opening area of the damping force control mechanism gradually decreases, whereas the opening area of the frequency dependent mechanism is kept maximum. Consequently, at the position d, the damping force characteristics can be set as shown by the characteristic curve 42 shown in FIG. 9. That is, the damping force is "hard" in the low frequency range and "soft" in the high frequency range.When the rotational position of the shutter 33 is switched from the position d to the position e, the opening area of the damping force control mechanism is maintained at zero, whereas the opening area of the frequency dependent mechanism is gradually decreased to zero. Consequently, at the position e, damping force characteristics can be set as shown by the characteristic curve 43 shown in FIG. 9. That is, the damping force is "hard" in the low frequency range and is kept "hard" even in the high frequency range, i.e., it is invariable in frequency.Thus, according to the first embodiment, it is possible to set the variable width of the damping force (reduction rate) in the high frequency range at each rotational position of the shutter 33 in addition to the damping force in the low frequency range, as shown by the characteristic curves 40 to 43 in FIG. 9. Consequently, when the rotational position of the shutter 33 is set to the position b, for example, the damping force can be controlled in a comfort mode in which the vehicle running quality is considered, as shown by the characteristic curve 41. That is, during low-speed running, the damping force is controlled to be "soft" in the low frequency range, and is made still "soft" in the high frequency range by utilizing the frequency-dependent control function.When the shutter 33 is switched to the position a to set the damping force characteristics as shown by the characteristic curve 40, the frequency-dependent control function is off, so that the damping force can be kept high even in the vicinity of the unsprung resonance frequency. Consequently, it is possible to suppress unsprung vibrations after the vehicle runs over a bump, for example. During high-speed driving, the damping force may be controlled according to a sport mode in which the longitudinal stability is taken into account. When the shutter 33 is switched to the position d to set the damping force characteristics as shown by the characteristic curve 42, it is possible to obtain a "hard" damping force for use in ride quality control such as sprung vibration control, and it is also possible to suppress high-frequency vibration by utilizing the frequency-dependent control function. Consequently, compared with shock absorbers having no frequency-dependent control function, the shock absorber of this embodiment can execute higher-order ride quality control or execute equivalent-level ride quality control by a simpler control operation. When the shutter 33 is switched to the position e to set the damping force characteristics as shown by the characteristic curve 43, it is possible to obtain a "hard" damping force for use in suppressing rolling when steering, for example, and further possible to set the longitudinal stability control performance at maximum because the frequency dependent control function is off (off).Thus, according to this embodiment, the first compression passage can be formed by means of the oil ports 3D and 3E of the piston rod 3, the oil grooves 33C and 33D of the shutter 33, the pressure chamber C of the compression damping mechanism 4, the arc ports 7B of the main disk 7, the cut-out ports 11B of the disk valve 11, the oil paths 2B of the piston 2, and so on. Further, the first extension passage may be formed by the oil holes 3F and 3G of the piston rod 3, the oil grooves 33E and 33F of the shutter 33, the pressure chamber D of the extension damping mechanism 13, the arc holes 16B of the main disk 16, the cut holes 20B of the disk valve 20, the oil paths 2A of the piston 2, and so forth. Further, the second passage may be formed through the connector 22, the oil holes 3C of the piston rod 3, the holes 33B and the inner hole 33A of the shutter 33, the housing 26, and so on.The respective areas of the first and second passages formed as set forth above can be adjusted separately from each other by means of the shutter 33 of the passage area changing mechanism 32. Consequently, regardless of which damping force characteristics, i.e., characteristic "hard", "medium", or "soft", the damping force to be generated by the shock absorber is changed, the variable width of the damping force of the frequency dependent mechanism 25 having the free piston 29, etc., can be changed freely according to each of the selected damping force characteristics. Consequently, it is possible to realize an appropriate damping force control that takes into account the vehicle running quality and a damping force control that takes into account the steering stability.In this case, the opening area of each compression and extension damping mechanism 4 and 13 and the opening area of the frequency dependent mechanism 25 that determines a variable width of the damping force can be independently adjusted according to the characteristics inherent in the subject vehicle and the target of the damping force control (i.e., for example, the damping force control that takes into account the vehicle running quality or the damping force control that takes into account the steering stability). Further, the damping force can be reduced even in the high-frequency region in which the damping force is difficult to control by the frequency dependent mechanism 25, and the damping force can be reduced even in the high-frequency region. Consequently, complicated control is not required. Thus, a CPU with simplified specification results in a less expensive structure. In addition, since the control frequency is low, this embodiment is also advantageous in terms of durability.Further, O-rings 30 and 31 constituting resistance elements are provided between the housing 26 and the free piston 29 of the frequency dependent mechanism 25. The O-rings 30 and 31 can provide resistance to the free piston 29 when it is axially displaced within the housing 26. Consequently, the damping force generated by the frequency dependent mechanism can be changed smoothly.Further, the free piston contact surfaces at which the O-rings 30 and 31 are in contact with the free piston 29 are defined by inclined bent surfaces 29B and 29C formed on upper and lower sides corresponding to the annular protrusion 29A. The housing contact surface at which the O-ring 31 contacts the housing 26 is defined by the inclined arc-shaped surface 28D formed on the inner circumferential surface of the tubular portion 28A. The inclined bent surfaces 28D, 29B, and 29C are configured to face each other in the moving direction of the free piston 29. Further, the inclined curved surfaces 28D, 29B and 29C are inclined with respect to the moving direction of the free piston 29 and have respective curved surfaces.Consequently, when the O-ring 31 is elastically deformed between the inclined bent surfaces 28D and 29C, for example, it is possible to prevent the O-ring 31 from rapidly deforming, and to allow the O-ring 31 to be smoothly deformed. Further, the O-rings 30 and 31 can be smoothly deformed according to the displacement of the free piston 29 by setting the curvature of the inclined curved surfaces 28D, 29B and 29C larger than the curvature of the O-rings 30, 31 before the elastic deformation. As a result, the damping force can be controlled smoothly.Fig. 10 shows a second embodiment of the present invention. The feature of the second embodiment is that the first compression passage and the first extension passage are formed as a single passage common to the compression and extension strokes. It should be noted that, in the second embodiment, the same constituent elements as those in the aforementioned first embodiment are denoted by the same reference numerals as used in the first embodiment, and a description thereof is omitted.In FIG. 10, reference numeral 21 denotes a piston used in the second embodiment. The piston 71 has substantially the same structure as the piston 2 described in the first embodiment. That is, the piston 71 divides the inner region of the inner tube 1 into two chambers, i.e., a rod-side oil chamber A and a bottom-side oil chamber B. The piston 71 has a plurality of circumferentially spaced oil paths 71A and a plurality of circumferentially spaced oil paths 71B that allow communication between the rod-side oil chamber A and the bottom-side oil chamber B. The oil paths 71A and 71B form a main passage through which hydraulic oil flows between the rod-side oil chamber A and the bottom-side oil chamber B.The plunger 71 has a lower end surface defined as one end thereof. The lower end surface of the piston 71 is provided with an annular recess 71C surrounding the opening at one end of each oil path 71A, and is further provided with an annular valve seat 71D provided radially outward with respect to the annular recess 71C. An extension disk valve 73 (described later) selectively seats on and disengages from the annular valve seat 71D. The upper end surface, i.e., the other end of the piston 71 is provided with an annular recess 71E surrounding the openings at the other ends of the oil paths 71B, and is further provided with an annular valve seat 71F positioned radially outward with respect to the annular recess 71E. A compression disk valve 74 (described later) seats on and selectively disengages from the annular valve seat 71F.Reference numeral 72 denotes a piston rod used in the second embodiment. The piston rod 72 has substantially the same structure as the piston rod 3 described in the first embodiment. The piston rod 72 has a lower end defined as one end thereof. The lower end of the piston rod 72 is secured to the piston 72 by a cap nut 27 of a housing 26, etc. The piston rod 72 is provided with a lock fitting hole 72A and a reduced diameter rod insertion hole 72B axially extending through the inner periphery thereof. The shutter fitting hole 72A opens at the lower end of the piston rod 72 and has a shutter 78 (described later) fitted therein. The rod insertion hole 72B extends upward from the upper end of the shutter fitting hole 72A.The piston rod 72 is provided with axially spaced groups of a plurality of circumferentially spaced oil ports 72C, 72D, and 72E extending radially outward from the closure fitting port 72A. Among the oil holes 72C to 72E, the oil holes 72C and 72D are provided to open into the rod-side oil chamber A, and the remaining oil holes 72E are provided to open into the bottom-side oil chamber B in the inner pipe 1.Among the oil holes 72C to 72E, the uppermost oil holes 72C are selectively communicated and separated with an inner hole 78A of the shutter 78 (described later) via radial holes 78B. The oil ports 72D and 72E are selectively communicated and separated via oil grooves 78C and 78D of the shutter 78 (described later). Further, the piston rod 72 has an annular stepped portion 72F formed on the outer periphery thereof. A terminal member 75 (described later) is axially positioned at the stepped portion 72F.Reference numerals 73 to 74 denote disc valves which are used as main damping valves, respectively, in this embodiment. Of the two disk valves 73 and 74, the disk valve 73 for the extension stroke is provided on the lower end surface of the spool 71, which is an end surface thereof. When the piston 71 is displaced upward during the extension stroke of the piston rod 72, the extension disk valve 73 resists the hydraulic oil flowing through the oil paths 71A to generate a certain damping force. The disk valve 74 for the compression stroke is provided on the upper end surface of the piston 71 which is the other end surface thereof. When the piston 71 is displaced downward during the compression stroke of the piston rod 72, the compression disk valve 74 resists the hydraulic oil flowing through the oil paths 71B to generate a certain damping force.Reference numerals 75 and 76 denote terminal members provided between the stepped portion 72F of the piston rod 72 and the piston 71. The connecting members 75 and 76 are annular rings or the like fitted around the outer periphery of the piston rod 72. The port member 75 allows the hydraulic oil to flow in and out between the rod-side oil chamber and the oil holes 72C of the piston rod 72. The port member 75 allows the hydraulic oil to flow in and out between the rod-side oil chamber A and the oil holes 72D of the piston rod 72.Reference numeral 77 denotes another connecting member provided between the piston 71 and the capsule nut 27. The connecting member 77 is also an annular ring or the like fitted around the outer periphery of the piston rod 72. The port member 77 allows the hydraulic oil to flow in and out between the bottom-side oil chamber B and the oil holes 72E of the piston rod 72.Reference numeral 78 denotes a shutter used in this embodiment. The shutter 78 constitutes an opening area adjusting member of the passage area changing mechanism 32 in substantially the same manner as the shutter 3 described in the first embodiment. That is, the shutter 78 has an axially extending inner opening 78A, openings 78B provided as openings extending radially outward from the inner opening 78A, and oil grooves 78C, 78D, and 78E formed on the outer circumferential surface of the shutter 78 that are spaced apart from the openings 78B in the axial direction of the shutter 78. The oil grooves 78C, 78D, and 78E constitute a damping force generating mechanism, together with the oil grooves 72D and 72E of the piston rod 72.The oil grooves 78C, 78D, and 78E of the shutter 78 are formed of communicated grooves to selectively communicate and cancel the communication with each other of the oil holes 72D and 72E of the piston rod 72, according to the rotational position of the shutter 78. The oil grooves 78D have a structure similar to that of the oil grooves 33C (see FIG. 6 ) described in the first embodiment. The oil grooves 78C to 78E of the shutter 78 form a first passage, which is a constituent part of the present invention, together with the oil holes 72D and 72E of the piston rod 72, the connectors 76 and 77, etc. The first passage is parallel to the main passage described above.The openings 78B are disposed at corresponding positions where the openings 78B may radially face the oil openings 72C of the piston rod 72. The openings 78B selectively bring the oil openings 72C into communication with the inner opening 78A and cancel the communication according to the rotational position of the shutter 78 The second passage, which is a constituent part of the present invention, includes the port member 75 that communicates with the rod-side oil chamber A, the oil openings 72C of the piston rod 72, the openings 78B and the inner opening 78A of the shutter 78, and the housing 26. The second passage is parallel to the main passage.Thus, the second embodiment constructed as described above provides substantially the same advantageous effects as the aforementioned first embodiment. In the second embodiment, in particular, the first passage including the oil holes 72D and 72E of the piston rod 72, the oil grooves 78C to 78E of the shutter 78, etc. may be formed as a single passage common to the extension and compression strokes. Consequently, the overall structure can be simplified. Further, substantially the same advantageous effects can be implemented in the above-described first embodiment and the first and second modifications at a further reduced cost.Fig. 11 shows a third embodiment of the present invention. The feature of the third embodiment resides in that the actuator of the passage area changing mechanism includes an electromagnetic proportional solenoid, and a passage area adjusting member is offset in the axial direction of the piston rod. It should be noted that, in the third embodiment, the same components as those in the aforementioned first embodiment are denoted by the same reference numerals as used in the first embodiment, and a description thereof is omitted.In Fig. 11, reference numeral 81 denotes a piston rod used in the third embodiment. The piston rod 81, as shown in FIG. 11, includes a tubular rod 82 axially extending in the inner tube 1, a tubular housing 84 connected to a (lower) end of the tubular rod 82 via a connecting member 83 and accommodating therein a proportional solenoid 87 (described later), and a stepped rod 85 detachably secured to the lower end of the tubular housing 84. The other (upper) end of the tubular rod 82 protrudes beyond the inner tube 1 to form a protruding end.The stepped rod 85 has substantially the same structure as the lower portion of the piston rod 3 described in the first embodiment, and is secured to the piston 2 by the cap nut 27 of the housing 26, etc. The upper end of the stepped rod 85 forms an enlarged diameter attachment boss 85A that fits the lower end of the tubular housing 84. The stepped rod 85 is configured in substantially the same manner as the lower portion of the piston rod 3 (see FIG. 1 ) in the first embodiment except for the attachment boss 85A.That is, the inner periphery of the stepped rod 85 forms an axially extending slide displacement hole 85B which opens at the lower end of the stepped rod 85. The slider displacement hole 85B is slidably fitted with a slider 88 (described later). The stepped rod 85 is provided with axially spaced groups of a plurality of circumferentially spaced oil ports 85C, 85D, 85E, 85F, and 85G extending radially outward from the spool displacement port 85B. Among the oil holes 85C to 85G, the oil holes 85C to 85E are disposed in the rod-side oil chamber A defined in the inner pipe 1 by the piston 2, and the remaining oil holes 85F and 85G are disposed in the bottom-side oil chamber B in the inner pipe 1.Among the oil ports 85C to 85G, the uppermost oil ports 85C are selectively communicated and separated with an inner port 88A of the spool 88 (described later) via radial ports 88B. The oil ports 85D and 85E are selectively communicated and separated via oil ports 88C of the spool 88 (described later). The oil ports 85F and 85G are selectively communicated and separated via oil grooves 88D of the spool 88 (described later). Further, the stepped rod 85 has an annular stepped portion 85H formed on the mounting boss 85A. The terminal member 22 is axially positioned with respect to the stepped portion 85H.Reference numeral 86 denotes a passage area changing mechanism used in this embodiment. Reference numeral 87 denotes an electromagnetic proportional solenoid (hereinafter referred to as "proportional solenoid 87") constituting an actuator of the passage area changing mechanism 86. The proportional solenoid 87 includes a tubular coil part 87A accommodated in the tubular housing 84 of the plunger rod 81, a fixed holder 87B fixedly provided on the inner periphery of the coil part 82A, a movable core 87C movably provided on the inner periphery of the coil part 87A in an axially opposed relation to the fixed holder 87B, an output rod 87D fixedly provided at the center of the movable core 87C, and a spring 87E interposed between the fixed holder 87B and the movable core 87C for urging the output rod 87D axially upward together with the movable core 87C.The proportional solenoid 87 functions as follows. When an electric current is externally supplied to the coil part 87A via a lead wire 87F, etc., the movable core 87C and the output rod 87D are axially displaced together with the spring 87E. At this time, the amount of axial displacement of the output rod 87D is controlled in proportion to the value of the electric current flowing through the coil part 87A. Thus, a slider 88 (described later) is axially slidably displaced in the slider displacement hole 85B of the stepped rod 85, as shown in FIG. 8 illustrating the first embodiment.Reference numeral 88 denotes a slider used in this embodiment as an opening portion adjusting member. The slider 88 is slidably provided in the slider displacement hole 85B of the piston rod 81 (stepped rod 85) instead of the shutter 33 described in the first embodiment. The spool 88 is axially linearly moved in the spool displacement hole 85B by means of the output rod 87D of the proportional solenoid 87. The spool 88 is provided with an axially extending inner opening 88A, openings 88B bored in the spool 88 radially outward from the inner opening 88A to serve as openings, and annular oil grooves 88C and 88D formed on the outer circumferential surface of the spool 88. The oil grooves 88C and 88D are spaced from the openings 88B in the axial direction of the spool 88.The openings 88B are, as shown in FIG. 11, disposed at corresponding positions where the openings 88B may radially face the oil openings 85C of the stepped rod 85. The openings 88B selectively bring and cancel the oil openings 85C into communication with the inner opening 88A according to the displacement position of the spool 88, The openings 88B and the inner opening 88A of the spool 88 form a second passage, which is a constituent part of the present invention, together with the connector 22 that communicates with the rod-side oil chamber A, the oil openings 85C of the stepped rod 85, the housing 26, and so forth.The oil grooves 88C of the spool 88 selectively bring the oil ports 85D and 85E of the stepped rod 85 into communication with and cancel each other according to the displacement position of the spool 88, The oil grooves 88C form a first compression passage, which is a constituent element of the present invention, together with the oil ports 85D and 85E of the stepped rod 85, the pressure chamber C of the compression damping mechanism 4, the arc ports 7B of the main disk 7, the cut ports 11B (see FIGS. 2 to 4 ) of the disk valve 11, the oil paths 2B of the piston 2, and so on.The oil grooves 88D of the spool 88 selectively bring the oil ports 85F and 85G of the stepped rod 85 into communication with and lift them, according to the displacement position of the spool 88, The oil grooves 88D form a first extension passage, which is a constituent element of the present invention, together with the oil grooves 85F and 85G of the stepped rod 85, the pressure chamber D of the extension damping mechanism 13, the arc ports 16B of the main disk 16, the cut ports 20B of the disk valve 20, the oil paths 2A of the piston 2, etc.The spool displacement hole 85B of the stepped rod 85 is provided therein with a tubular member 89 at the lower (an axial) end of the spool 88. The inner periphery of the tubular member 89 defines an inner opening 89A which is a part of the second passage described above. A return spring 90 is provided between the lower end of the spool 88 and the tubular member 89. The return spring 90 constantly urges the spool 88 toward the output rod 87D of the proportional solenoid 87.Thus, the third embodiment provided as set forth above also provides substantially the same advantageous effects as the aforementioned first embodiment. That is, the first passage (the opening area of the damping force control mechanism) and the second passage (the opening area of the frequency dependent mechanism) can be individually adjusted from each other, as shown with the slant ruled portions 38 and 39 in FIG. 8 of the first embodiment, by displacing (linearly moving) the spool 88 with the output rod 87D of the proportional solenoid 87.According to the third embodiment, in particular, the proportional solenoid 87 is used as an actuator of the passage area changing mechanism 86. Consequently, the adjustment of the damping force can be continuously performed, and thus the damping force can be adjusted with higher accuracy. Consequently, a high control effect can be obtained. Further, since the proportional solenoid 87 is installed as an actuator in the inner tube 1 (i.e., in the tubular housing 84 of the piston rod 81), it is possible to improve the installability of the shock absorber on the vehicle. That is, the shock absorber can be installed without the actuator protruding into the machine room. Thus, it is possible to realize space saving and secure installation of the shock absorber.It should be noted that, in the aforementioned first and third embodiments, the first compression passage and the first extension passage have the same configuration by way of example. However, the present invention is not limited to this structure. For example, the configurations of the compression oil grooves 33C and 33D (oil grooves 88C) and the extension oil grooves 33E and 33F (oil grooves 88D) may be different from each other to obtain different damping force characteristics for the compression and the extension strokes. For example, when "hard" damping force characteristics are set for the extension stroke, "soft" damping force characteristics are set for the compression stroke, or when "soft" damping force characteristics are set for the extension stroke, "hard" damping force characteristics are set for the compression stroke.In the aforementioned embodiment, the present invention has been described by way of an example in which the respective passage areas of the first and second passages are variably set using a single shutter 33. However, the present invention is not limited thereto. For example, the respective passage areas of the first and second passages may be individually adjusted from each other using respective shutters. The closures can be rotated by means of corresponding actuators. The shutter of the passage area changing mechanism can also be rotated manually without an actuator. The same applies to the second and third embodiments.In the aforementioned first embodiment, the O-rings 30 and 31 are provided as resistance elements between the housing 26 and the free piston 29, for example. However, the present invention is not limited thereto. For example, a spring such as a coil spring, a leaf spring, etc. may be used as the resistance element as in the shock absorbers disclosed in Japanese Patent Application Publication No. Hei 06-94065 (mentioned above) and Japanese Utility Model Application Publication No. Hei 07-19642. An elastic member used as the resistance member is not limited to an O-ring, but may be an elastic ring having a rectangular or non-circular cross-sectional configuration. The same applies to the second and third embodiments.In the foregoing embodiments, the invention has been explained using a hydraulic shock absorber as an example of a shock absorber provided in a vehicle such as an automobile. However, the present invention is not limited thereto, but may also be applied to, for example, damping force control shock absorbers for use in various machines, architectural structures, etc. constituting vibration sources.As stated in the aforementioned embodiments, the damping force can be reduced by the action of the frequency dependent mechanism even in the high frequency region where the damping force is difficult to control. Consequently, complicated control is not required. Thus, simplification of the specification of the control CPU results in less expensive structure. Since the control frequency is low, the present invention is also advantageous in terms of durability. Further, the actuator may be provided in the cylinder, and thus it is not necessary to install the actuator in the trunk room. Thus, the present invention is advantageous in terms of vehicle stability and safety.According to the present invention, the free piston is movably provided in the housing in which a flow path forming at least a part of the second passage is formed, and a resistance member is provided between the housing and the free piston to generate resistance against the displacement of the free piston. With this structure, resistance can be generated when the free piston is axially displaced in the housing. Consequently, the damping force generated by the frequency dependent mechanism can be changed smoothly.According to the present invention, the resistance element is a spring. In this case, resistance against the displacement of the free piston can be controlled by a spring (resistance element) such as a leaf spring as in the shock absorber disclosed in the above-mentioned Japanese Patent Application Publication No. Hei 06-94065.According to the present invention, at least one elastic member is provided between the free piston and the housing. The free piston has a free piston contact surface at which the elastic member is in contact with the free piston. The housing has a housing contact surface at which the elastic member is in contact with the housing. At least one of the contact surface of the free piston and the housing contact surface has an inclined surface inclined to the moving direction of the free piston. The inclined surface is a curved surface. Thus, when the elastic member (for example, the O-ring) is deformed by the inclined curved surface, it is possible to prevent the elastic member from being rapidly deformed, and to allow the deformation to be smoothly performed. As a result, the damping force generated by the frequency dependent mechanism can be changed smoothly.According to the present invention, a plurality of elastic members are provided between the free piston and the housing. The free piston has a free piston contact surface at which the elastic members contact the free piston. The housing has housing contact surfaces at which the elastic members are in contact with the housing. The free piston contact surfaces and the housing contact surfaces have respective portions facing each other in the direction of movement of the free piston. Thus, when the elastic members (for example, O-rings) are elastically deformed between the contact surfaces, it is possible to prevent the elastic members from being rapidly deformed, and to allow the deformation to take place smoothly.Further, according to the present invention, the elastic members include an elastic member which is compressively deformed when the free piston moves in one direction and another elastic member which is compressively deformed when the free piston moves in the other direction. With this configuration, the damping force generated by the frequency dependent mechanism can be changed smoothly.Further, according to the present invention, the damping force generating mechanism includes an extension damping force generating mechanism and a compression damping force generating mechanism. The first passage includes a first extension passage passing through the extension damping force generation mechanism and a first compression passage passing through the compression damping force generation mechanism. The passage area changing mechanism is capable of setting the respective passage areas of the first extension passage and the first compression passage. With this configuration, the passage area changing mechanism can adjust the passage area of the first extension passage and can further adjust the passage area of the first compression passage.Although only some exemplary embodiments of these inventions have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.

Claims

A shock absorber comprising: a cylinder (1) sealingly having hydraulic fluid therein; a piston (2; 71) movably fitted into the cylinder to define two chambers (A, B) in the cylinder; a piston rod (3; 72; 81) secured to the piston at one end, the other end of the piston rod protruding from the cylinder; a main passage (2A, 2B, 71A, 71B) through which the hydraulic fluid flows in response to movement of the piston between the two chambers in the cylinder; a first passage and a second passage provided parallel to the main passage, and through which the hydraulic fluid flows from one of the two chambers in the cylinder into the other of the two chambers in response to movement of the piston; a main damping valve (7, 16, 73, 74) provided in the main passage for regulating a flow of the hydraulic fluid induced by the movement of the piston to generate a damping force; a damping force generating mechanism (4, 13; 78C, 78D, 78E) provided in the first passage for regulating a flow of the hydraulic fluid induced by the movement of the piston to generate a damping force; a free piston (29) provided in the second passage for dividing the second passage into an upstream portion and a downstream portion; and a passage area changing mechanism (32; 86) provided halfway in the first passage and the second passage, wherein the passage area changing mechanism capable of adjusting passage areas of the first passage and the second passage is operable with a single control rod (34).The shock absorber according to claim 1, wherein the passage area changing mechanism has a fixed area where the passage area of the second passage is fixed in a variable area where the passage area of the first passage decreases.The shock absorber according to any one of claims 1 to 2, wherein the second passage is formed in the piston rod, the passage area changing mechanism comprising: an opening area changing member (33; 78; 88) provided with an opening (33B; 38B; 88B) disposed in the piston rod so as to be in communication with an oil opening of the piston rod; and an actuator that causes either rotation or rectilinear movement of the opening area changing member.The shock absorber according to any one of claims 1 to 3, wherein the free piston is movably provided in a housing (26) in which a flow path forming at least a part of the second passage is formed; the shock absorber further comprising: a resistance member disposed between the housing and the free piston to generate resistance against the displacement of the free piston.The shock absorber of claim 4, wherein the resistance element is a spring.The shock absorber according to claim 4, wherein at least one elastic member (30, 31) is provided between the free piston and the housing; the free piston has a free piston contact surface (29B, 29C) at which the at least one elastic member is in contact with the free piston; the housing has a housing contact surface (28D) at which the at least one elastic member is in contact with the housing; at least one of the free piston contact surface and the housing contact surface has an inclined surface inclined to the moving direction of the free piston; the inclined surface is a curved surface.The shock absorber according to claim 4, wherein a plurality of elastic members (30, 31) are provided between the free piston and the housing; the free piston has free piston contact surfaces (29B, 29C) at which the elastic members are in contact with the free piston; the housing has housing contact surfaces at which the elastic members are in contact with the housing; the free piston contact surfaces and the housing contact surfaces have respective portions facing each other in the direction of movement of the free piston.A shock absorber according to claim 6 or 7, wherein the elastic members include an elastic member which is compressively deformed when the free piston moves in one direction and another elastic member which is compressively deformed when the free piston moves in the other direction.The shock absorber according to any one of claims 1 to 7, wherein the damping force generating mechanism includes an extension damping force generating mechanism (13) and a compression damping force generating mechanism (4); wherein the first passage includes: a first extension passage (33E, 33F, 88D) passing through the extension damping force generating mechanism; and a first compression passage (33C, 33D; 88C) passing through the compression damping force generating mechanism; wherein the passage area changing mechanism is capable of adjusting respective passage areas of the first extension passage and the first compression passage.

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