JPH02293265A - Rear-wheel steering gear for four-wheel steering vehicle - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is suitable for use in obtaining tight turning ability at low speeds by steering the rear wheels in the opposite direction in conjunction with the front wheels by steering operation. This paper relates to improvements to rear wheel steering devices for four-wheel steering vehicles. [Conventional technology] In recent years, turning the rear wheels in the opposite direction or the same direction according to the amount of steering (steering amount) of the front wheels has been developed to improve the ability to turn around at low speeds, and to improve turning ability during medium and high speed driving. Four-wheel steering vehicles that can improve driving stability are attracting attention. For example, when driving at low speeds, the front wheels are steered with a large steering angle. At this time, by setting the steering directions of the front and rear wheels in opposite phases (reverse direction steering), the vehicle's turning radius is minimized and the turning (small turning) performance is improved. It is particularly suitable for use in long vehicles such as trucks. As a rear wheel steering device for this type of four-wheel steering vehicle, for example, Japanese Patent Laid-Open No. 59
- Publication No. 128054, JP-A-59-14371
Publication No. 39, Japanese Utility Model Publication No. 81-53271, Japanese Patent Application Publication No. 1987
Various types of hydraulic devices have been proposed in the past, including the hydraulic device shown in Publication No. 31-87885. In particular, in hydraulic devices, hydraulic pressure controlled to a predetermined pressure is selectively supplied to the left and right chambers of the hydraulic cylinders for steering the rear wheels using servo valves, etc., to steer the rear wheels in the desired direction. A device that can be steered and has the advantage of having a greater degree of freedom in terms of movement and structure than a mechanical device that mechanically connects the front and rear wheels with a connecting shaft, etc. Met. [Problems to be Solved by the Invention] However, the above-mentioned conventional device still has advantages and disadvantages in terms of both structure and operational performance, and there is still room for improvement in putting it into practical use. For example, this type of rear wheel steering device requires the minimum number of component parts, unitizes each part to improve workability and assembly, and reduces manufacturing costs, as well as hydraulic piping, electrical wiring, etc. It is desired that the motors can be arranged efficiently and in line, that various drive controls can be performed easily and appropriately, and that there are sufficient measures against failures. Among these demands, what is most needed is a control system that can appropriately steer the rear wheels in conjunction with the steering operation of the front wheels that accompanies steering operations. ．． In other words, when the rear wheels are steered in conjunction with the steering operation of the front wheels, the start point of the rear wheel steering operation and its maximum steering angle are adjusted and controlled depending on the vehicle speed, and the steering operation is controlled from the front wheel side to the rear wheel side. It is desirable to have a structure in which the steering request is transmitted with some play, and
In this case, what is needed is a configuration that does not cause a time delay when it is necessary to steer the rear wheels and has excellent responsiveness. It is desirable that some kind of measure be taken to satisfy these demands. [Means for Solving the Problems] In order to meet such demands, the rear wheel steering device for a four-wheel steering vehicle according to the present invention employs electromagnetic proportional pressure control that variably controls the hydraulic passage from the hydraulic source according to the vehicle speed. A valve and a piston supported movably within a cylinder to which respective cylinder chambers are connected above and downstream of the pressure control valve, and the amount of steering displacement on the front wheel side is required according to the piston movement position. A displacement transmission control mechanism for rear wheel steering that transmits data to the rear wheels with a variable play stroke of It consists of a rear wheel steering control valve that operates a power cylinder for the rear wheels. [Operation] According to the present invention, the movement of the piston in the cylinder that constitutes the displacement transmission control mechanism for steering the rear wheels is controlled by the pressure difference generated when the hydraulic passage is constricted according to the vehicle speed, and the movement position of the piston is controlled. Transmission and control of steering displacement information from the front wheel side to the rear wheel side is performed using an idle stroke that is varied according to the amount of play, and the power cylinder for rear wheel steering is activated by switching the hydraulic passage of the control valve for rear wheel steering which is activated by this control valve. It is capable of driving and steering the rear wheels in the required state. [Embodiment] Figures 1 to 10 show an embodiment of the rear wheel steering device for a four-wheel steering vehicle according to the present invention. A steering link mechanism for wheel steering (only the rear wheel side is indicated by reference numeral 1, and the front wheel side is omitted from illustration) is connected to a power cylinder 2 which is a hydraulic actuator.
．． 3, and only the front wheel steering displacement information is transmitted as a hydraulic signal to the rear wheel steering drive system, and the rear wheel steering displacement (the amount of rod movement of the power cylinder 3) is transmitted to the tension cable. A rear wheel steering device 1O which is also equipped with a feedback signal system 5 which feeds back to the control system (rear wheel steering control valve 15 to be described later) by 4 will be explained.Here, 6 and 7 are front and rear wheel side power. An oil bomb 8 is an oil tank for supplying pressure oil to the cylinders 2.3, and the pressure oil from the front wheel side pump 6 is guided to the front wheel side power cylinder 2 through a hydraulic piping 6a, and is sent to the oil tank 8 through a return piping 6b. A well-known front wheel side power steering device is configured to recirculate the power. In addition, 9 is the car battery,
In addition, in FIG. 2, hydraulic piping is shown as a double line, and electrical wiring is shown as a single solid line.Furthermore, as the oil pumps 6 and 7, a two-barrel bomb simultaneously driven by an automobile engine (not shown) is shown as an example. However, any bomb that can be configured with independent hydraulic systems for the front and rear wheels will suffice. According to the invention, as shown in FIGS. 1 and 2, the rear wheel steering device 10 is configured such that the hydraulic passage 7a from the rear wheel side oil bomb 7 as a hydraulic source is controlled in proportion to the vehicle speed. An electromagnetic proportional pressure control valve l1 having a variable throttle lla for variable control, and an upper and downstream side (PH,
A piston l movably supported within a cylinder 12 to which each cylinder chamber 12a, 12b is connected to
3, and adjusts the amount of steering displacement on the front wheel side according to the movement position of the piston l3 with a required variable play stroke CQC)
The rear wheel steering displacement transmission control gI mechanism 14 transmits the transmission to the rear wheel side, and the hydraulic passage from the rear wheel side oil bomb 7 is controlled to switch according to the displacement obtained by this control mechanism 14, thereby controlling the rear wheel steering. The main feature of this vehicle is that it includes a rear wheel steering control valve l5 that operates the power cylinder 3. Here, the electromagnetic proportional pressure control valve 11 described above selectively controls drive only when the vehicle speed is 20 km/b or less, based on an output signal obtained by calculating a signal from the vehicle speed sensor 16 by the controller l7. The required differential pressure is applied to the cylinder side chambers 12a, 12b.
The structure is such that the piston 13 can be moved in a desired direction. In this embodiment, the rear wheel pump 7 is used as the hydraulic pressure source, but it is also possible to use the front wheel pump 6. In addition, the one shown at 18 in FIGS. 1 and 2 is provided in the middle of the hydraulic passage from the rear wheel side bomb 7, and is connected to the rear wheel steering control valve 15 side via the pressure control valve 1l and to the rear wheel neutral side, which will be described later. This is a flow divider that divides and supplies pressure oil to the position lock mechanism 60 side.

According to such a configuration, the pressure difference generated when the hydraulic passage 7a is throttled by the variable throttle lla according to the vehicle speed,
Cylinder 1 configuring displacement transmission control mechanism 14 for rear wheel steering
The movement of the piston 13 is controlled within 2, and the transmission of steering displacement information from the front wheel side to the rear wheel side is controlled using an idle stroke QC that is varied according to the movement position of the piston 13, and the rear wheel steering is operated thereby. By switching the hydraulic passage with the control valve 15, the rear wheel steering power cylinder 3 can be driven and the rear wheels can be steered in a desired state.To explain this in detail, the rear wheel displacement transmission control mechanism 4 is , FIGS. 4 and 5(a). The configuration is shown in (b). That is, reference numeral 20 denotes an input shaft to which steering displacement information on the front wheels is transmitted by being connected to the output shaft of the front wheel side power cylinder 2 (which may be part of the steering system by the steering handle 2a), etc.
Reference numeral 1 denotes an output shaft disposed coaxially with this input shaft 20, which constitutes a displacement extraction shaft. And input shaft 2
5(a), a piston 13 is rotatably connected to the piston 13 by a spline connection and supported so as to be movable only in the axial direction.
．． As shown in (b), the groove width gradually expands and opens toward one end in the axial direction, and the transmission play stroke QC
A notch groove 22 is formed that allows the angle to be varied. In addition,
1 in the figure to form the notch groove 22.
As shown in 3A and 13B, it is divided into two parts and integrated with screws. 23. A sleeve 23b is provided on the output shaft 21 so as to be inserted into the notch groove 22 described above, and a portion contacting the side edge of the notch groove 22 is fitted with a spherical bearing 23a. As is clear from FIG. 5(b), the idle stroke until the engaging means 23 comes into contact with the side edge of the groove 22 changes depending on the position of the piston 13. therefore,
The piston 13, which is constantly urged by a spring 24 toward the pump-side chamber 12a in the cylinder l2, is moved toward the variable throttle 11a# by the pressure control valve 11. When it moves in the axial direction due to the pressure difference that occurs later, the input shaft 20 on the front wheel side rotates as shown in FIG. 7(a), using the stroke defined by the movement position as play. As is clear from (b), the output shaft 21
As a result, the steering angle of the rear wheels relative to the steering angle of the front wheels changes as shown in the characteristic curve shown in Figure 6, depending on the vehicle speed. According to this configuration, the steering angle of the rear wheels can be controlled to a desired state according to the vehicle speed with a simple structure, and the vehicle can function as a four-wheel steered vehicle. Here, the schematic structure of the rear wheel steering device 10 of the four-wheel steering vehicle exemplified in this embodiment will be briefly explained using FIG. 2, FIG. 4, etc. 30 is the displacement transmission for rear wheel steering described above. A rear wheel steering control valve 15 for switching the hydraulic circuit to the rear wheel side power cylinder 3 is connected to a displacement takeoff shaft (20
．． 21) In the rear wheel steering control mechanism unit arranged in parallel above, the control valve l5 is connected to the output shaft 21 and a feedback shaft 32 which is rotatably connected to the output shaft 21 through a torsion bar 31. It consists of a sleeve part 21a provided at the shaft end of the rotor 33 and a rotor 33. The configuration and operation of this valve 15 are well known, and it switches the pressure oil passage between the pump 7 (P) and tank 8 (T) and the rear wheel cylinder chambers Cl and C2. Reference numeral 34 denotes a coil spring as a counter spring, which is wound around the shaft in a state in which engagement pins 34a and 34b protruding from the output shaft 21 and the valve body 35 are sandwiched in the left side chamber 12a of the cylinder. As is clear from FIG. 8, a centering spring mechanism 36 is provided which always urges the control valve 15 to return to the absolute position (neutral position). Reference numeral 37 denotes a cable connection for feeding back the actual position of the rear wheel, to which the other end of the tension cable 4 of the feedback signal system 5 is connected, one end of which is connected to the arm 5a provided on a part of the rear wheel link mechanism l. The lever is provided on a sleeve 37a rotatably mounted on a feedback shaft 32 disposed coaxially with the displacement take-out shaft (20.21) as shown in FIGS. 4 and 10. A well-known electromagnetic clutch 38 is provided between the sleeve 37a and the feedback shaft 32, and is configured to selectively connect these two members. This is a feedback signal transmission system that constantly adjusts the misalignment between the rear wheels and control valve l5 and absorbs suspension movement by separating the rear wheel steering control system from the feedback system at all times and connecting it when necessary. This is to obtain. Here, in this embodiment, contrary to a general rotary valve, the sleeve 21a is connected to the input shaft side and the rotor 33 is connected to the output side (feedback side). This is to connect the 37 side to the rotor 33, which has small inertia. Furthermore, in this embodiment, this mechanical unit 30
The piston l3, the centering spring mechanism 36, the rotary rear wheel steering control valve l5, and the electromagnetic clutch 38, which constitute the rear wheel steering displacement transmission control mechanism 14, are used together with the feedback shaft 32 of the feedback system 5 as a displacement extraction shaft. Person arranged on the same axis, output shaft 20.2
This unit configuration simplifies the overall configuration of the device and is also superior in terms of ease of assembly of the mechanism 22 and 30. , it has great practical advantages. Here, in this embodiment, the above-mentioned feed bar, shaft 32 and torsion bar are
The output shaft 21, the rotor 33, and the sleeve 21a, which are connected via the rotor 31, are repurposed parts used for a rotary flow path switching valve for a front wheel power steering device that is commonly used in the past. It is advantageous. Reference numeral 40 denotes a flange member which is provided on the input shaft 20 in the cylinder 12 and also serves as a spring holder for the coil spring 24 that biases the piston 13. As shown in FIG. First and second rear wheel turning start point detection switches 4 3 which mechanically detect the state by being selectively engaged and disengaged by the front wheel turning input to the input shaft 20 . 4 4 (SW-A, SW-B) are provided, and these detection switches 43 and 44 control the rear wheel steering control system to a state where the rear wheels can be steered. In other words, this type of rear wheel steering device 10 has a vehicle speed of 20 km.
Even when driving at low speeds below /h, the steering wheel 2a
It is best to activate it only when the steering wheel is operated.
It is desirable to leave everything else inactive. but,
In order to perform rear wheel steering using hydraulic control, it is desirable to place the hydraulic system and the electrical system that controls it in advance in a standby state in which the rear wheels can be steered, and for this reason, in this embodiment, they are operated at different timings. Two mechanical activation switches are installed in positions that allow the front wheels to be turned before the rear wheels are turned. To explain this in detail, see Figures 2 and 3.
As is clear from the diagram, the hydraulic circuit system that makes up the rear wheel steering device 1O has a problem in which the rear wheels are steered or the steered state is maintained when the hydraulic system or the electrical system fails. In order to prevent this, the supply side hydraulic piping and the return side piping of the rear wheel side pump 6 are normally connected through the bypass passage 50a, so that the hydraulic pressure to the rear wheel steering system is An electromagnetic emergency valve 50 is interposed to maintain a state in which hydraulic pressure cannot be supplied, and when this rear wheel steering system is energized, this communication section is cut off and hydraulic pressure can be supplied. ．． Further, a manual switch 51 is provided on this energization circuit to disable the rear wheel steering system in response to a driver's request to maintain the two-wheel steering system actively depending on driving conditions. ．． Further, in Fig. 3, 52 is an ignition key inch, 53 is an IOA fuse, which is connected in series to the above-mentioned manual switch 51, and 55 is a supply side and return link in the hydraulic circuit system from the rear wheel pump 7. This is a vehicle speed control bypass valve placed in parallel with the emergency valve 50 that selectively opens and closes the front and rear wheels, and is activated so that pressure oil is not supplied to the rear wheel steering system when the vehicle speed is 20 km/h or more. Note that 56 is a vehicle speed switch attached to the speedometer 54, etc., which is turned off when the vehicle speed is 20 km/h or more, and turned on when the vehicle speed is 20 km/h or less. Further, 57 is a cable breakage switch that is connected in series to the vehicle speed switch 2 53 and is used to disable the rear wheel steering system when a breakage occurs in the cable 4 constituting the feedback signal system 5. Further, an electromagnetic clutch 38 is connected in series with these and in parallel with the vehicle speed control bypass valve 55. Reference numeral 60 denotes a rear wheel neutral position locking mechanism provided in the steering link mechanism l on the rear wheel side to restrict movement on the power cylinder 3 side. For this reason, a rear wheel neutral position lock operation detection switch 61 is provided, and the hydraulic pressure for operating this port 7 mechanism 60 is introduced from the middle of the hydraulic piping from the rear wheel side pump 7 through a flow divider 18. It is written,
In addition, in the middle of this hydraulic piping, there is an electromagnetic rear wheel neutral position lock control valve 6 that opens and closes the flow of pressure oil using an electric signal.
2 are installed. Here, 63 is this control valve 6
An unloading valve 64 is provided on the upstream side of 2 to maintain a constant oil pressure to the leakage lock mechanism 60, and 64 is an accumulator that accumulates pressure oil on the downstream side thereof. In addition,
The above-mentioned rear wheel neutral position locking mechanism 60, although not shown in the drawings, normally operates by engaging the locking pin with the tie rod etc. that constitute the rear wheel side steering linkage mechanism 1 using the biasing force of a spring. At the same time, the lock pin is pulled out using pressurized oil to release the lock. Further, 65 is a check valve provided in the middle of the hydraulic pressure supply passage to the power cylinder 3 on the rear wheel side, and is used to recover the hydraulic pressure to the neutral position when the rear wheels are operating in the event of a hydraulic system failure. ．． In order to operate the rear wheel steering system using such various electromagnetic valves and electric switches, the rear wheel turning start point detection switch 43. 44 and a rear wheel neutral position lock operation detection switch 6l, thereby performing rear wheel steering control in a desired state. That is, the engine is started, and the pumps 6 and 7 send pressurized oil to the front and rear wheel steering systems. At this time, the flow divider l
The pressure oil diverted at 8 passes through the unloading valve 63 and is sequentially accumulated in the accumulator 64 between the neutral position lock control valves 62 which are in the OFF state, increasing the oil pressure in this part. In addition, since the vehicle speed control bypass valve 55 is in the off state, the remaining pressure oil passes through the tank 8.
It is being refluxed to the side. On the other hand, the pressure oil from the pump 7 passes through the pressure control valve 1l and flows into the valve section of the transmission control ml mechanism 14. In this state, when a steering operation is performed using the steering wheel 2a, the front wheels are steered and the input shaft 20
rotates and the switch 4 is turned at a predetermined rotation angle (QA in Figure 9).
3 (SW-A) is turned on, thereby operating the vehicle speed control bypass valve 55, cutting off the short circuit to the tank, guiding the discharged oil to the control valve l5, and turning on the electromagnetic clutch 38. The amount of handle rotation increases, Q
At B, the switch 4 4 (SW-8) is turned on, the neutral position lock control valve 62 is opened, and high pressure is guided to the neutral position lock mechanism 60, the lock bin is pulled out and the rear wheels can be steered. It will be on standby. Furthermore, when the handle is steered beyond the stroke QC defined by the notch groove 22 in the control mechanism 14, the output shaft 21, which is the input side of the control valve l5, is rotated, thereby operating the valve E5. Pressure oil is guided to one of the left and right chambers of the power cylinder 3 on the rear wheel side to steer the rear wheels, and the amount of steering is fed back from the feedback shaft 32 in the control valve 15 by the feedback signal system 5 via the cable 4. Here, the above-mentioned steering wheel steering angle QC is determined by changing the differential pressure obtained by the pressure control valve 11 according to the vehicle speed. Control valve 1 changes with the movement position and has the required play.
5 is rotationally displaced, and the rear wheels can be steered.
Further, when the vehicle speed reaches 20 Km/h or higher, the vehicle speed switch 56 is turned off, the vehicle speed control bypass valve 55 is turned off, a flow path is formed to the tank, and the electromagnetic clutch 38 and the like are also turned off. Furthermore, if some abnormal condition occurs while driving, the electrical system,
When the hydraulic system fails, it is recommended to use the emergency valve 50, manual switch 51, etc. to put the rear wheels in a non-steerable state. By connecting switches SW-A and SW-C in parallel, even if SW-A is turned off, the rear wheel steering system remains active as long as the rear wheels are not locked in the neutral position, and four-wheel steering is possible. This allows the vehicle to maintain its running condition. What is desired when configuring the above-mentioned rear wheel steering device is to ensure straight-line driving performance at high speeds and driving stability on low-friction roads such as snowy roads, and only when necessary. The wheel steering system is in a standby state and an active state, and when it is not needed, it is in a locked state to ensure two-wheel steering. Furthermore, one of the problems with installing the above-mentioned rear wheel steering device is the loss of driving force due to a failure in the rear wheel steering drive system and the failure in the command transmission system of the rear wheel steering control system. When this happens, it is necessary to keep the rear wheels locked in a neutral state and enter a two-wheel steering state, so it is better to adopt the configuration described above. In addition, a problem in configuring the rear wheel steering device Hto described above is that it is required to position and incorporate the neutral position on the device 10 side and the vehicle side, and in particular, the rear wheel steering start point detection switch 43 is required to be installed. ．． 44 and the input shaft 20 side should be correctly connected to the vehicle side. In this embodiment, in order to solve such a problem, as shown in FIG. A neutral positioning mechanism 48 is provided in which the input shaft 20 is pressed against the unit body by a spring 47, and the input shaft 20 is positioned with respect to the unit body, so that the input shaft 20 can be easily positioned with respect to the vehicle side. ing. Of course, this pole 46 is always movable by steering the steering wheel, but it does not hinder the movement of the input shaft 20. Note that the present invention is not limited to the structure of the above-described embodiment, and may vary depending on the shape of each part of the rear wheel steering device 10. You are free to modify or change the structure etc. as appropriate, and various modifications may be possible. [Effects of the Invention] As explained below, the rear wheel steering device for a four-wheel steering vehicle according to the present invention includes an electromagnetic proportional pressure control valve that variably controls the hydraulic passage from the hydraulic source according to the vehicle speed; The pressure control valve has a piston movably supported in a cylinder to which each cylinder chamber is connected on the upper and downstream sides, and has a variable play stroke to control the amount of steering displacement on the front wheel side according to the piston movement position. A displacement transmission control mechanism for rear wheel steering that selectively transmits power to the rear wheels, and a switching control of the hydraulic passage from the rear wheel hydraulic power source according to the amount of displacement obtained by this control mechanism to generate power for rear wheel steering. It consists of a rear wheel steering control valve that operates a cylinder, so despite its simple configuration, it can variably control the rear wheel steering angle relative to the front wheel steering angle depending on the vehicle speed, and from the front wheel side. It has various excellent effects such as appropriately transmitting steering displacement information to the rear wheels and improving maneuverability such as tight turning ability when the vehicle is running at low speed. Further, according to the present invention, the front and rear wheel side steering link mechanisms are independently driven, and the rear wheel steering device can be configured by simply adding a steering signal output section on the front wheel side. It is easy to assemble, and
It has a large degree of freedom in terms of installation space, and also has the advantage of being able to obtain appropriate rear wheel steering conditions.

[Brief explanation of the drawing]

The figure shows an embodiment of the rear wheel steering device for a four-wheel steering vehicle according to the present invention, and FIG. Displacement transmission control for rear wheel steering equipped with a magnetic proportional pressure control valve and variable play
2 is a schematic configuration diagram of the entire device, FIG. 3 is its electrical circuit diagram, and FIG. 4 is a cross-sectional view of the main parts of the rear wheel steering control mechanism unit. Figure 5 (a)
, (b) is a schematic perspective view and an enlarged view of the main parts explaining the configuration for varying the displacement transmission play stroke, and FIG. 6 is the front,
Characteristic diagram showing the relationship between rear wheel steering angles, FIG. 7(a). (b)
is a sectional view taken along the line ■-■ in Fig. 4 and an explanatory diagram showing its movement, and Figs. 8 and 9 are a cross-sectional view taken along the line ■-■ in Fig. 4,
The line sectional view, Figure 10, is a side view of a portion of the feedback cable connection lever. 1... Rear wheel side steering link mechanism, 2, 3... Front,
Power cylinder for rear wheel steering, 5... Feedback signal system, 6.7... Front and rear wheel side oil pumps, 8...
...Oil tank. 10... Rear wheel steering device, 11
...Electromagnetic proportional pressure control valve, lla...variable throttle, l2...cylinder, 13...piston,
14... Displacement transmission control mechanism for rear wheel steering. 15...
・Rear wheel steering control valve, 16...Vehicle speed sensor, 2
0.21...Input, output shaft, 22...Notch groove,
23... Engagement means, 30... Rear wheel steering control mechanism unit}, 31... Torsion bar, 32---
- Feedback shaft, 36...centering spring mechanism, 38...electromagnetic clutch. 40...flange part, 41.42...engaging part, 43.44...
... Rear wheel steering start point detection switch, 50 ... Emergency valve, 55 ... Vehicle speed control bypass valve, 56 ... Vehicle speed switch, 60 ... Rear wheel neutral position low 2 mechanism, 61... Rear wheel neutral position lock operation switch, 62... Rear wheel neutral position lock control valve. Figure 1 Patent Applicant Automotive Equipment Co., Ltd. Isuy Automobile Co., Ltd. Agent Masaki Yamakawa No. 5
2 (G) Figure 6 No. 82

Claims (1)

[Claims] An electromagnetic proportional pressure control valve that variably controls the hydraulic passage from the hydraulic power source according to vehicle speed, and a cylinder chamber connected to the upper and downstream sides of this electromagnetic proportional pressure control valve, which is movably supported within a cylinder. A displacement transmission control mechanism for rear wheel steering that includes a piston and selectively transmits the amount of steering displacement on the front wheels to the rear wheels with variable play according to the piston movement position; A rear wheel steering control valve controls switching of the hydraulic passage from the rear wheel hydraulic pressure source according to the amount of displacement, and the rear wheel steering link mechanism is steered by the hydraulic pressure obtained from this rear wheel steering control valve. 1. A rear wheel steering device for a four-wheel steering vehicle, comprising: a rear wheel steering power cylinder for controlling the rear wheels.