JP2001099219A - Hydraulic damping device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively prevent collision of a piston rod on a stopper rubber member at a stroke end. SOLUTION: A piston 19 having a piston rod 20 connected thereto is fitted into a cylinder 14, and a pump rod 45 is inserted into the rod 20, thereby forming a pump chamber 46. Through extension/contraction of the rod 20, the chamber 46 is expanded/contracted to supply oil from a reserver 15 to a pressure accumulation chamber 16 for pressure accumulation. When the rod 20 comes to a stroke end, a groove 51 or 52 of the rod 45 is communicated with a port 38 or 39, to thereby allowing supply of high pressure oil inside the chamber 16 to a cylinder chamber 13a or 13b for forcibly returning the piston 19 to the side of neutral position, resulting in effective prevention of collision of the rod to a stopper rubber member. Accordingly, riding comfortatableness on a vehicle and vehicle running stability are enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydraulic shock absorber used for a vibration damping device and a suspension device of a vehicle such as a railway vehicle.

[0002]

2. Description of the Related Art For example, in a railway vehicle, a vehicle body is elastically supported in a left-right direction by a spring means with respect to a vehicle having wheels mounted thereon, and a hydraulic shock absorber is mounted between the vehicle and the vehicle body. There is a vehicle equipped with a lateral vibration damping device that suppresses lateral vibration of a vehicle body and improves ride comfort by absorbing a vibration of a bogie following a track vibration by a spring means and a hydraulic shock absorber. .

An example of a hydraulic shock absorber mounted on this type of lateral vibration damping device will be described with reference to FIG. FIG.
As shown in FIG. 1, in the hydraulic shock absorber 1, a piston 4 to which a piston rod 3 is connected is slidably fitted in a cylinder 2 in which an oil liquid is sealed, and the cylinder inside the cylinder 2a is moved by the piston 4. , 2b. A reservoir 5 in which oil and gas are sealed is provided on the outer periphery of the cylinder 2. The cylinder chambers 2a and 2b are connected to the reservoir 5 via check valves 6 and 7, respectively. The check valves 6 and 7 are respectively connected to the cylinder chambers 2a and 2a from the reservoir 5 side.
Only the flow of the oil liquid to the 2b side is allowed.

The piston 4 has a relief valve 8 for permitting the flow of oil from the cylinder chamber 2a to the cylinder chamber 2b, and a relief valve for permitting only the flow of oil from the cylinder chamber 2b to the cylinder chamber 2a. A valve 9 is provided. An orifice passage 10 is provided in the side wall of the cylinder 2 near the end of the cylinder 2 on the side of the piston rod 3 to communicate the cylinder chamber 2a with the reservoir 5, and a cylinder chamber 2b is provided near the bottom of the cylinder 2 in the reservoir. An orifice passage 11 is provided for communication with the orifice 5.

[0005] With this configuration, during the extension stroke of the piston rod 3, the oil liquid in the cylinder chamber 2 a is pressurized and flows to the reservoir 5 through the orifice oil passage 10 in a low piston speed range. In the high-speed region of the piston speed, damping force is generated by flowing into the cylinder chamber 2b through the relief valve 8. At the same time, the oil liquid in the reservoir 5 opens the check valve 7 and flows into the cylinder chamber 2b. At this time, when the piston rod 3 extends to the vicinity of the stroke end on the extension side, the piston 4 closes the orifice passage 10 to increase the damping force, and a stopper rubber (see FIG. (Not shown).

During the compression stroke of the piston rod 3, the hydraulic fluid in the cylinder chamber 2b is pressurized, and passes through the orifice oil passage 11 through the orifice oil passage 11 in a low piston speed range.
To the relief valve 9 at high piston speeds.
The damping force is generated by flowing through the cylinder chamber 2a through the cylinder chamber 2a. At the same time, the oil liquid in the reservoir 5 opens the check valve 6 and flows into the cylinder chamber 2a. At this time, the piston rod 3
When is shortened to the vicinity of the stroke end on the contraction side, the piston 4 closes the orifice passage 11 to increase the damping force, thereby preventing and cushioning the collision with the stopper rubber (not shown) in the same manner as described above.

[0007] In this manner, by applying a damping force to the bogie's run-out, the vehicle body can be restrained from swaying, and by increasing the damping force near the stroke end of the piston rod 3. Thus, collision with the stopper rubber can be prevented and cushioned, and the riding comfort can be improved.

[0008]

However, in the above-described conventional hydraulic shock absorber 1, since a reaction force is generated only by the damping force against the vibration of the vehicle body, for example, a railway vehicle passes on a long curved track. In this case, the centrifugal force acts on the vehicle body in the same direction for a long time, so that the vehicle body gradually approaches the stopper rubber, and there is a problem that the vehicle easily hits the stopper rubber.

The present invention has been made in view of the above points, and has as its object to provide a hydraulic shock absorber that can effectively prevent collision with a stopper rubber.

[0010]

In order to solve the above-mentioned problems, the invention according to claim 1 comprises a cylinder filled with an oil liquid, a piston slidably fitted in the cylinder,
A piston rod connected to the piston and extending to the outside of the cylinder, wherein the hydraulic shock absorber generates a damping force by controlling a flow of an oil liquid generated in an oil passage by a stroke of the piston in the cylinder. In the above, the piston generates a low damping force when near the neutral position,
The damping force increases as the distance from the neutral position increases, and when reaching a predetermined position near the stroke end, a force urging the piston toward the neutral position acts on the piston.

With such a configuration, the damping force increases as the piston is displaced from the neutral position to the stroke end, and is returned to the neutral position when the piston reaches a predetermined position near the stroke end.

According to a second aspect of the present invention, in the hydraulic shock absorber according to the first aspect, the hydraulic fluid is accumulated in the accumulator by the pump, and when the piston strokes to a predetermined position, the hydraulic fluid is transferred from the accumulator into the cylinder. An oil liquid is supplied to urge the piston.

With this configuration, the piston is returned to the neutral position by the pressure of the oil liquid accumulated by the pump means.

According to a third aspect of the present invention, in the hydraulic shock absorber according to the second aspect, the pump means accumulates oil in the accumulator by expanding and contracting the piston rod.

[0015] With this configuration, the pump means is operated by the expansion and contraction of the piston rod, and the oil is accumulated in the accumulator.

[0016]

Embodiments of the present invention will be described below in detail with reference to the drawings. A first embodiment of the present invention will be described with reference to FIGS. As shown in FIG. 1, the hydraulic shock absorber 12 of the first embodiment has a double cylinder structure in which a bottomed cylindrical outer cylinder 14 is provided outside a cylinder 13. An annular reservoir 15 is formed therebetween, and a pressure accumulating chamber 16 (pressure accumulating means) is formed at the bottom of the outer cylinder 14. On the bottom side of the outer cylinder 14, a base valve 17 for partitioning the inside of the cylinder 13, the reservoir 15 and the pressure accumulating chamber 16 is provided. 13a, 13b and a reservoir 15 are formed.

In the cylinder 13, a piston 19 is slidably fitted.
The interior is defined by two chambers, a cylinder 13a and a cylinder 13b. A hollow piston rod 20 is inserted into and connected to the piston 19, and one end of the piston rod 20 is inserted into the guide seal 18 and extends to the outside, and the other end is formed in the guide bore 21 of the base valve 17. And is slidably guided. Thereby, the cylinder chamber 13 of the piston 19
The pressure receiving areas for a and 13b are equal. Accumulator
A free piston 22 is fitted in 16 and the inside thereof is defined by an oil chamber 16a and a gas chamber 16b. An oil liquid is sealed in the cylinder 13 and the oil chamber 16a, an oil liquid and a low-pressure gas are sealed in the reservoir 15, and a high-pressure gas is sealed in the gas chamber 16b.

The base valve 17 has oil passages 23, 24 for communicating the cylinder chamber 13b with the reservoir 15, an oil passage 25 for communicating the reservoir 15 with the oil chamber 16a, and 26 for communicating the reservoir 15 with the guide bore 21. Is provided. The oil passage 23 is provided with a check valve 27 that allows only the flow of the oil liquid from the reservoir 15 side to the cylinder chamber 13b side, and the oil passage 24 is configured so that the pressure of the oil liquid in the cylinder chamber 13b side becomes a predetermined pressure. A relief valve 28 is provided to relieve the pressure to the reservoir 15 side when the pressure reaches the oil reservoir 16 .When the pressure of the oil liquid in the oil chamber 16a reaches a predetermined pressure, the relief valve 28 A relief valve 29 for relief is provided.

The guide seal 18 is provided with oil passages 30, 31 for communicating with the cylinder chamber 13a and the reservoir 15. The oil passage 30 is provided with a check valve 32 that allows only the flow of the oil liquid from the reservoir 15 side to the cylinder chamber 13a side. Oil passage
When the pressure of the oil liquid in the cylinder chamber 13a reaches a predetermined pressure, the relief valve 31 releases the pressure to the reservoir 15 side.
33 are provided.

The piston 19 is provided with oil passages 34 and 35 for communicating between the cylinder chambers 13a and 13b. The oil passage 34 is provided with a pilot-type relief valve 36 for relieving the pressure of the oil liquid in the cylinder chamber 13a to the cylinder chamber 13b when the pressure reaches a predetermined pressure.
When the pressure of the oil liquid on the 13b side reaches a predetermined pressure, the pilot-type relief valve releases this to the cylinder chamber 13a side.
37 are provided. The piston 19 has ports 38 and 39 provided on the side wall of the piston rod 20 and cylinder chambers.
Oil passages 40 and 41 are provided to communicate the passages 13a and 13b, respectively. Oil passages 40 and 41 are each provided with a relief valve.
Connected to pilot passages 36a, 37a of 36, 37,
The relief pressure of the relief valves 36 and 37 increases as the pilot pressure increases.

On the side wall of the cylinder 13, a plurality (three in the illustrated example) of orifice passages 43 for communicating the cylinder chamber 13a with the reservoir 15 at a position on the guide seal 18 side are arranged at predetermined intervals along the axial direction. A plurality of (three in the drawing) orifice passages 44 for connecting the cylinder chamber 13b and the reservoir 15 to a portion on the base valve 17 side.
Are arranged at predetermined intervals along the axial direction. And
When the piston 19 is displaced from the neutral position in the center of the cylinder 13 to a predetermined position near the stroke end on the guide seal 18 side, the orifice passage 43 is sequentially closed by the outer peripheral surface of the piston 19, and also near the stroke end on the base valve side. , The orifice passages 44 are sequentially closed.

A distal end side of a tubular pump rod 45 whose base end is connected to the base valve 17 is slidably fitted in the hollow piston rod 20, and a pump chamber 46 (pump) is inserted in the piston rod 20. Means) are formed. Pump room 46
Is connected to the oil chamber 16a by an oil passage 47 in the pump rod 45, and between the pump chamber 46 and the oil chamber 16a, only the flow of the oil liquid from the pump chamber 46 side to the oil chamber 16a side. An allowable check valve 48 is provided. Further, the pump chamber 46 is connected to the reservoir 15 by a flexible conduit 49 connecting the piston rod 20 and the outer cylinder 14, and the reservoir 15 is provided between the pump chamber 46 and the reservoir 15. A check valve 50 that allows only the flow of the oil liquid from the side to the pump chamber 46 side is provided.

On the outer periphery of the pump rod 45, grooves 51, 52 extending along the axial direction thereof are formed so as to face the ports 38, 39 of the piston rod 20, respectively.
The oil passage 52 communicates with an oil passage 47 in the pump rod 45 by oil passages 53 and 54. When the piston 19 is displaced from the neutral position toward the guide seal 18 to close the orifice passage 43 and is further displaced, the ports 51 and 52 of the piston rod 20
Communicates with the groove 51, and is displaced toward the base valve 17 side,
When the orifice passage 44 is closed and further displaced, the port
39 is arranged so as to communicate with the groove 52.

The operation of the embodiment constructed as described above will now be described. The hydraulic shock absorber 12 is mounted between a vehicle and a vehicle body, which is elastically supported in a lateral direction, such as a railway vehicle, and generates a damping force against a lateral shake therebetween.

When the piston rod 20 expands and contracts, the pump chamber
46 is expanded and contracted.
Through, the check valve 50 is opened and flows into the pump chamber 46, and when contracted, the oil liquid in the pump chamber 46 is pressurized and the check valve 48 is opened,
The gas discharged through the oil passage 47 to the oil chamber 16a of the accumulator 16 is compressed to accumulate the gas in the gas chamber 16b. In this way, the expansion and contraction of the piston rod 20 constantly accumulates high-pressure oil liquid in the oil chamber 16a of the pressure accumulation chamber 16. Oil chamber 16a
When the pressure reaches a predetermined pressure, the relief valve 29 is opened, and the oil liquid in the oil chamber 16a is relieved to the reservoir 15 through the oil passage 25.

In the extension stroke of the piston rod 20, when the piston 19 is near the neutral position as shown in FIG. 2A, the check valve 32 of the guide seal 18 is closed and the cylinder chamber 13 is closed.
a flows into the reservoir 15 through the orifice passage 43 and flows into the cylinder chamber 13b through the relief valve 36, so that a small damping force is generated by the orifice passage 43 and the relief valve 36, thereby causing the vehicle to sway. (See FIGS. 6 and 7). At this time, ports 38 and 39
Is closed by the pump rod 45, and since the pressure in the cylinder chamber 13a is low, the pilot pressure of the relief valve 36 does not increase, and a low damping force is generated. The oil liquid in the reservoir 15 opens the check valve 27 and flows into the cylinder chamber 13b.

Further, the piston 19 is displaced, and FIG.
When the orifice passages 43 are sequentially closed as shown in FIG. 5, the flow area of the oil liquid decreases by that amount, and the damping force sequentially increases,
The vehicle body is restrained from swaying to prevent collision with the stopper rubber (see FIGS. 6, 7 and 7).

Further, as shown in FIG.
When 19 is displaced near the stroke end, the piston rod
The port 38 of 20 communicates with the groove 51 of the pump rod 45,
The high-pressure oil liquid accumulated in 16 flows into cylinder chamber 13a through oil passage 47, oil passage 53, groove 51, port 38 and oil passage 40. At this time, the orifice passage 43 is closed by the piston 19, and a high-pressure oil from the accumulator 16 is introduced into the pilot passage 36a, so that the relief pressure of the relief valve 36 is increased. Rise,
Push the piston 19 back to the neutral position (FIGS. 6 and 7).
reference). The oil liquid in the cylinder chamber 13b flows into the orifice passage 44
Through the reservoir 15.

When the piston 19 moves to the neutral position and the port 38 is cut off from the groove 51,
The supply of oil liquid to 13a is stopped. Also, the movement of the piston 19 opens the orifice passage 43, and the cylinder chamber 13a
Is communicated with the reservoir 15, and the pressure in the cylinder chamber 13a decreases to the pressure in the reservoir 15. Thereby, the piston 19 can be returned to the neutral position side, and the collision with the stopper rubber can be effectively prevented. When the pressure in the cylinder chamber 13a rises excessively, the relief valve 33 of the guide seal 18 is opened, and the oil liquid is relieved to the reservoir 15. The reaction force when the piston 16 is pushed back to the neutral position side by the pressure of the oil liquid supplied from the pressure accumulating chamber 16 to the cylinder chamber 13a is as shown in FIG.

On the other hand, when the piston 19 is in the vicinity of the neutral position during the contraction stroke of the piston rod 20, the check valve 27 of the base valve 17 closes, and the oil in the cylinder chamber 13b flows through the orifice passage 44 and the reservoir 15 To the cylinder chamber 13a via the relief valve 37, a small damping force is generated by the orifice passage 44 and the relief valve 37, thereby absorbing the vehicle body sway (FIGS. 6 and 7).
reference). At this time, the ports 38 and 39 are connected to the pump rod 45
Further, since the pressure in the cylinder chamber 13b is low, the pilot pressure of the relief valve 37 does not increase, and a low damping force is generated. The oil liquid in the reservoir 15 opens the check valve 32 and flows into the cylinder chamber 13a.

Further, when the piston 19 is displaced and the orifice passage 44 is sequentially closed, the flow path area of the oil liquid is reduced by that amount, the damping force is sequentially increased, and the lateral vibration of the vehicle body is suppressed to prevent the stopper rubber. (See FIGS. 6, 7 and 7).

Further, when the piston 19 is displaced to near the stroke end, the port 39 of the piston rod 20 communicates with the groove 52 of the pump rod 45, and the high-pressure oil stored in the pressure accumulating chamber 16 is supplied to the oil passage 47, the oil passage 47. The fluid flows into the cylinder chamber 13b through the groove 54, the groove 52, the port 39, and the oil passage 41. At this time, the orifice passage 44 is closed by the piston 19, and high-pressure oil liquid from the accumulator 16 is introduced into the pilot passage 37a,
Since the relief pressure of the relief valve 37 is increased, the pressure in the cylinder chamber 13b rises and pushes the piston 19 back to the neutral position (see FIGS. 6 and 7). Cylinder chamber 13a
Flows into the reservoir 15 through the orifice passage 43.

When the piston 19 moves to the neutral position and the port 39 is shut off from the groove 52, the pressure accumulating chamber 16
The supply of oil to 13b is stopped. Also, the movement of the piston 19 opens the orifice passage 44, and the cylinder chamber 13b
Is communicated with the reservoir 15, and the pressure in the cylinder chamber 13b decreases to the pressure in the reservoir 15. Thus, similarly to the extension stroke, the piston 19 can be returned to the neutral position side, and the collision with the stopper rubber can be effectively prevented. If the pressure in the cylinder chamber 13b rises excessively, the relief valve 28 of the base valve 17 opens, and the oil liquid is relieved to the reservoir 15. The piston 16 is driven by the pressure of the oil liquid supplied from the pressure accumulating chamber 16 to the cylinder chamber 13b.
The reaction force at the time of pushing back to the neutral position is as shown in FIG.

As described above, when the piston 19 makes a stroke near the neutral position, a small damping force is generated to absorb the lateral sway of the vehicle body, and when the piston 19 is displaced near the stroke end, the damping force increases stepwise. Collision with the stopper rubber can be effectively prevented by preventing collision with the stopper rubber and applying hydraulic pressure to push back to the neutral position when a stroke is made. This prevents the collision with the stopper rubber by forcibly pushing the piston 19 back to the neutral position side even if centrifugal force acts on the vehicle body for a long time in the same direction when passing on a long curved track. The driving comfort and running stability can be improved.

The cylinder chambers 13a, 13b of the piston 19
, The same damping force and reaction force characteristics can be obtained on the extension side and the contraction side. Since the hydraulic mechanism for pushing back the piston 19 is integrated into the hydraulic shock absorber, the conventional hydraulic shock absorber can be easily replaced. The guide seal 18 and the relief valves 33 and 28 of the base valve 17 may be omitted by appropriately setting the maximum relief pressure of the relief valves 36 and 37 of the piston 19.

Next, a modified example of the above embodiment will be described with reference to FIGS. In the following description, the same parts as those of the above-described embodiment are denoted by the same reference numerals, and only different parts will be described in detail.

In the first modification shown in FIG. 3, the grooves 51a, 52a of the pump rod 45 are different from those of the embodiment shown in FIG.
At the end of the stroke, the grooves 51a,
52a is cut off from ports 38 and 39.
The oil passages 40 and 41 of the piston 19 have cylinder chambers 13a and 13a, respectively.
Check valves 55 and 56 are provided to prevent the pressure of 13b from acting on the pilot passages 36a and 37a. In addition,
In this modification, the guide seal 18 and the relief valves 33 and 28 of the base valve 17 are omitted.

With this structure, when the stopper rubber collides with the stopper rubber at the stroke end, the groove 51a
, 51b and the ports 38, 39 are shut off, the supply of the oil liquid from the pressure accumulating chamber 16 to the cylinder chambers 13a, 13b is stopped, and the damping force (reaction force) is reduced appropriately (arrow A in FIG. 9). FIG.
, FIG. 11), the impact force at the time of collision (the sum of the damping force of the hydraulic shock absorber and the repulsive force of the stopper rubber) is reduced, and the riding comfort and running stability can be improved. FIGS. 9 to 11 show hydraulic damper damping force characteristics of the first modified example.

In the second modification shown in FIG. 4, the pump means by the pump rod 45 is omitted from the modification shown in FIG. 3, and a pump mechanism 57 is provided outside the outer cylinder 14 instead. I have. The suction side of the pump mechanism 57 is connected to the reservoir 15 by a pipe 58, and the discharge side is connected to the oil chamber 16a of the accumulator 16 via a check valve 60 by a pipe 59. As the pump mechanism 57, any pump means such as a mechanical mechanism such as a piston-cylinder mechanism that operates in accordance with the stroke of the piston rod 20, or an electrical mechanism such as an electric pump can be applied.

With this configuration, high-pressure oil can be accumulated in the oil chamber 16a of the pressure accumulating chamber 16 by the pump mechanism. As in the modification shown in FIG. From the cylinder chambers 13a, 13
b can be supplied with oil liquid. It should be noted that a pressure sensor or the like may be provided in the pressure accumulating chamber 16 and a pump mechanism that operates when the pressure in the pressure accumulating chamber 16 decreases may be used. In this case, the pressure can be accumulated in the pressure accumulating chamber in advance, and repeated reaction force generation Can respond quickly.

In the third modification shown in FIG. 5, the guide bore 21 of the base valve 17 is different from that of the embodiment shown in FIG.
The inside is a pump chamber 21a, and a check valve 61 that allows only the flow of the oil liquid from the reservoir 15 side to the pump chamber 21a side is provided in an oil passage 26 that communicates the pump chamber 21a and the reservoir 15. An oil passage 62 that connects the pump chamber 21a to the oil chamber 16a and a check valve 63 that allows only the flow of the oil liquid from the pump chamber 21a side to the oil chamber 16a side of the oil passage 62 are provided.

With this configuration, the oil is supplied from the reservoir 15 to the oil chamber 16a through the check valves 61 and 63 to accumulate the pressure by expanding and contracting the pump chamber 21a in addition to the pump chamber 46 by expansion and contraction of the piston rod 20. And the pump efficiency can be increased.

[0043]

As described above in detail, according to the hydraulic shock absorber of the first aspect of the present invention, the damping force increases as the piston is displaced from the neutral position toward the stroke end.
By absorbing vibration near the neutral position, it is possible to suppress collision with the stopper rubber near the stroke end, and when the piston reaches a predetermined position near the stroke end, the piston is forcibly pushed back to the neutral position side. In addition, the collision with the stopper rubber can be effectively prevented, and the riding comfort and running stability can be improved.

According to the hydraulic shock absorber of the second aspect of the present invention, the piston can be returned to the neutral position by the pressure of the oil liquid accumulated by the pump means.

According to the hydraulic shock absorber of the third aspect of the present invention, the pump can be operated by the expansion and contraction of the piston rod to accumulate the oil in the accumulator.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a hydraulic shock absorber according to one embodiment of the present invention.

FIG. 2 is a diagram showing an operation state of an extension side of the hydraulic shock absorber of FIG. 1;

FIG. 3 is a longitudinal sectional view of a hydraulic shock absorber according to a first modification of the embodiment of FIG.

FIG. 4 is a longitudinal sectional view of a hydraulic shock absorber according to a second modification of the embodiment of FIG.

FIG. 5 is a longitudinal sectional view of a hydraulic shock absorber according to a third modification of the embodiment of FIG.

FIG. 6 is a diagram showing damping force characteristics (relationship between piston speed and damping force) of the device of FIG. 1;

FIG. 7 is a diagram illustrating a relationship between a stroke and a damping force when the device in FIG. 1 performs a stroke toward a stroke end.

FIG. 8 is a diagram showing a relationship between a stroke and a damping force when the device of FIG. 1 makes a stroke toward the neutral position.

FIG. 9 is a diagram showing the device damping force characteristic (relation between piston speed and damping force) of FIG. 3;

FIG. 10 is a diagram illustrating a relationship between a stroke and a damping force when the device in FIG. 3 performs a stroke toward a stroke end.

FIG. 11 is a diagram showing a relationship between a stroke and a damping force when the device of FIG. 3 makes a stroke toward the neutral position.

FIG. 12 is a schematic view showing the structure of a conventional hydraulic shock absorber.

[Explanation of symbols]

 12 Hydraulic shock absorber 13 Cylinder 16 Accumulation chamber (accumulation means) 19 Piston 20 Piston rod 46 Pump chamber (Pump means)

Claims (3)

[Claims] A cylinder filled with an oil, a piston slidably fitted in the cylinder, and a piston rod connected to the piston and extending out of the cylinder; In a hydraulic shock absorber that generates a damping force by controlling a flow of an oil liquid generated in an oil passage by a stroke of the piston in the cylinder, the piston generates a low damping force when the piston is near a neutral position, and a neutral position. The hydraulic shock absorber is characterized in that the damping force increases as the distance from the stroke increases, and when the piston reaches a predetermined position near the stroke end, a force for urging the piston toward the neutral position acts on the piston. 2. The method according to claim 1, wherein the oil pressure is accumulated in the pressure accumulating means by a pump means, and when the piston strokes to a predetermined position, the oil liquid is supplied from the pressure accumulating chamber into the cylinder to urge the piston. The hydraulic shock absorber according to claim 1, wherein 3. The hydraulic shock absorber according to claim 2, wherein said pump means accumulates oil in said pressure accumulating means by expansion and contraction of said piston rod.