JPH11301503A - Steering device of running vehicle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reliably return the steering wheel to neutral after steering in a structure wherein the vertical steering cylinder is connected with a closed circuit. SOLUTION: In a wheel crane, straight recovery force received by the wheels 18, 18 is transmitted to a steering wheel 16 through static oil pressure of a closed circuit A including vertical steering cylinders 25 through 28, thus ensuring straight recovery. Then, a rod 37 of a neutral return cylinder 34 is connected to a link 24 of a steering unit 23 which is an assistor. After steering, thrust from the same cylinder 34 reliably returns the steering wheel 16 to the neutral position, resisting the sliding resistance of each of the cylinders 25 through 28.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a traveling vehicle, such as a wheel crane or a wheel shovel, in which an upper revolving unit is mounted on a lower traveling unit and a wheel on the lower traveling unit is operated from a cab of the upper revolving unit. The present invention relates to a steering device.

[0002]

2. Description of the Related Art A traveling vehicle provided with an upper revolving structure as described above (hereinafter referred to as an upper revolving type traveling vehicle) is equipped on a general traveling vehicle such as a truck due to the uniqueness that the upper revolving structure rotates. A well-known integral power steering unit, which is a booster for amplifying the steering force and transmitting it to the wheel, is connected to the front wheel via a mechanical link mechanism such as an arm or a drag link. The configuration cannot be adopted as it is.

For this reason, in a conventional upper turning type vehicle, as shown in FIG. 4, a steering unit 1 is disposed on an upper revolving unit, and steering cylinders 2 and 3 are disposed on a lower traveling unit. The front wheel is steered by supplying high-pressure hydraulic oil corresponding to the above to the steering cylinders 2 and 3 from the steering unit 1 through the rotary joint 5 arranged at the center of rotation of the upper revolving superstructure. Japanese Patent Publication No. Hei 2-8940).

In FIG. 4, reference numeral 6 denotes a steering hydraulic pump driven by an engine. Further, in FIG. 2, the internal mechanism of the steering unit 1 is schematically represented by hydraulic symbols of the servo valve 7 and the hydraulic motor 8.

[0005]

However, according to the above-mentioned prior art, since the rectilinear restoring force applied to the front wheel by the well-known wheel alignment mechanism is not transmitted to the steering wheel 4 during traveling, the neutral position (straight traveling position) ) Does not work automatically.

That is, the rectilinear restoring force received by the front wheel is applied to the steering cylinders 2 and 3, and the cylinders 2 and 3 try to discharge hydraulic oil toward the steering unit 1. The steering wheel automatically returns to the neutral position after the operation of the steering wheel. In this neutral position, the discharged hydraulic oil is blocked, so that the straight forward restoring force received by the front wheel is not transmitted to the steering wheel 4.

For this reason, the driver has to return the steering wheel 4 to the neutral position by measuring each time after turning the steering wheel 4 and turning the curve.

For this reason, the vehicle body may fluctuate particularly when changing lanes where quick steering operation is required, which not only reduces the traveling safety of the vehicle but also increases the size of the upper turning type traveling vehicle. In addition to this, the driver of the surrounding vehicle sometimes gives fear.

[0009] Under the circumstances described above, the conventional upper turning type traveling vehicle requires a long time to learn driving skills because the steering wheel operation is not easy, and requires special driving skills considerably different from those of ordinary traveling vehicles. I was

Therefore, the present inventor has disclosed in Japanese Patent Application No. 9-34527.
In the application of No. 5, the upper turning body of the upper turning type traveling vehicle is provided with an upper steering cylinder, and the lower traveling body is provided with a lower steering cylinder for changing a steering angle of a wheel, and the two cylinders are connected by an oil path and closed. A technique was proposed in which a circuit was configured to transmit the straight-line restoring force received by the wheel during traveling to the steering wheel via the hydraulic pressure of the closed circuit and the steering unit.

According to this steering apparatus, since the force is transmitted by the static pressure of the closed circuit, the steering wheel is automatically returned to the neutral position, similarly to a general traveling vehicle such as a truck.

[0012] For this reason, even though the vehicle is an upper turning type traveling vehicle, it is excellent in straight-line restorability, and the steering wheel operation becomes easy. In addition, there is no risk of the vehicle body fluctuating when changing lanes.

However, according to this structure, the sliding resistance of the upper steering cylinder in addition to the lower steering cylinder becomes a resistance for returning the steering wheel to the neutral position. Therefore, the neutral returning force of the wheel is lost before the steering wheel returns to the neutral position. As a result, a new problem arises in that the return of the steering wheel becomes poor and the straight traveling performance is reduced.

Another problem is that the total number of steering cylinders increases to four, so that the internal leakage of the cylinder increases, and the oil in the closed circuit decreases due to the leakage of oil from the rotating joint. A situation occurs where the neutral position deviates from the straight traveling position.

In this case, the straight running performance is reduced, the effective stroke of the cylinder is shortened, the steering angle of the wheel is reduced, and the original turning ability during running cannot be obtained.

Accordingly, the present invention provides a steering apparatus for a traveling vehicle which can surely return a steering wheel to a neutral position after steering, assuming a configuration in which upper and lower steering cylinders are connected by a closed circuit.

Another object of the present invention is to provide a steering apparatus for a traveling vehicle which can easily correct a neutral position of a steering wheel due to an internal leak of oil.

[0018]

According to a first aspect of the present invention, an upper revolving structure having a driver's seat having a steering handle is mounted on an upper portion of a lower traveling structure having wheels so as to be pivotable. A vehicle having a steering unit that amplifies the operating force of a steering wheel and transmits the steering force to the wheel as the steering force has the following configuration.

(I) The upper revolving structure is provided with an upper steering cylinder that operates in response to the operation of the handle.

(Ii) The lower traveling body is provided with a lower steering cylinder for changing a steering angle of the wheel.

(Iii) The upper and lower steering cylinders are connected to each other by an oil passage via a rotary joint disposed at the center of rotation of the upper revolving structure to form a closed circuit.

(Iv) The straight running restoring force received by the wheel during traveling can be transmitted to the steering wheel via the hydraulic pressure of the closed circuit and the steering unit.

(V) Neutral return means for applying a force in a direction for returning the handle to the neutral position is provided.

According to a second aspect of the present invention, in the first aspect, a neutral return cylinder including a hydraulic cylinder is used as the neutral return means.

According to a third aspect of the present invention, in the configuration of the first or second aspect, the neutral return cylinder is connected to both oil chambers connected to each other, a free piston slidably provided in the oil chambers on both sides, and And a rod for transmitting a force between the two free pistons.

According to a fourth aspect of the present invention, in the configuration of any one of the first to third aspects, the oil chambers of the upper and lower steering cylinders are switched between a position where the oil chamber communicates with the tank and a position where the oil chamber is shut off from the tank. An operating neutral return valve is provided.

According to a fifth aspect of the present invention, in the configuration of the fourth aspect, an electromagnetic switching valve is used as the neutral return valve.

According to a sixth aspect of the present invention, in the configuration of the fifth aspect, the electromagnetic switching valve as a neutral return valve is configured to be operated by operating a neutral return switch provided in an operator's cab.

According to the above configuration, in the upper turning type traveling vehicle, the rectilinear restoring force received by the wheel is transmitted to the steering wheel through the closed hydraulic pressure of the closed circuit, thereby ensuring the straight traveling performance.

By providing the neutral return means, the steering wheel after steering is reliably returned to the neutral position by the force of the neutral return means during traveling.

In this case, according to the configuration of the second aspect using a hydraulic cylinder (neutral return cylinder) as the neutral return means, the cylinder is incorporated in a steering hydraulic circuit to easily secure a power source (hydraulic source) thereof. be able to.
That is, there is no need to separately add a power source.

According to the third aspect of the present invention, the neutral return cylinder includes a cylinder having free pistons on both sides and a rod for transmitting a force between the two pistons. The cylinder equipment is simpler and smaller than when using a cylinder.

On the other hand, according to the configuration of the fourth to sixth aspects, when the neutral position of the steering wheel is displaced by an internal leak generated by each steering cylinder or the rotary joint, the operation of the neutral return valve causes the operation of each steering cylinder. By operating the neutral return means in a state where the oil chamber is communicated with the tank and the pressure is equal, the handle can be corrected to the neutral position.

As a result, the straight running performance can be maintained, and the effective stroke (wheel steering angle) of the cylinder can be prevented from decreasing, so that the original running turning ability can be secured.

In this case, according to the configuration of claim 5 in which an electromagnetic switching valve is used as the neutral return valve, the electromagnetic switching valve can be remotely operated by a switch, so that the driver can optionally operate the neutral correction function of the steering wheel. Can be.

In particular, according to the configuration of claim 6, when the shift of the neutral position of the steering wheel is detected, the correction function can be immediately and easily operated by operating the switch in the cab.

[0037]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIGS.

FIG. 1 shows an overall schematic configuration of a wheel crane which is an example of an upper turning traveling vehicle to which the present invention is applied.

In the figure, reference numeral 11 denotes a lower traveling body, on which an upper revolving body 12 is mounted via a revolving bearing 13 so as to be rotatable around a vertical axis X.

The upper revolving unit 12 has a telescopic boom 1
4 is provided so that it can be raised and lowered, and the cabin (operator's cab)
A steering wheel 16 is provided in the cabin 15.

On the other hand, the lower traveling body 11 is provided with a front wheel 18 which is a steering wheel in this embodiment at a front portion of a traveling body frame 17 and a rear wheel 19 at a rear portion on both right and left sides.

The traveling body frame 17 is provided at its rear end with an engine box 20 in which an engine (not shown) for driving and operating a crane is accommodated. Outriggers 21 are provided to lift and support the body during crane operation.

Further, a rotary joint 22 (shown in FIG. 2) is provided around a vertical axis X, which is the center of rotation of the upper revolving unit 12, and the hydraulic pipes on the lower traveling unit 11 side and the upper revolving unit 12 side are connected to each other. The connection is made via the joint 22.

Next, the hydraulic circuit configuration of the steering device will be described with reference to FIG.

A steering unit 23 is disposed on the upper revolving structure side above the rotary joint 22, and a handle shaft 16 a that rotates by operating the handle 16 is connected to an input side of the steering unit 23.

The steering unit 23 is a well-known integral type power steering unit, which operates according to the operation of the steering wheel, and is output as the operation of the link 24.

The steering unit 23 is not limited to the integral type, and another type of steering unit may be used. For example, a booster type (linkage type) may be used.

On the upper revolving unit side, upper left and right steering cylinders 25 and 26 are provided.

The cylinders 25 and 26 are double-acting single-rod cylinders having the same specifications.
R and 26R are connected to the link 24 in a state where they are connected to each other.

On the other hand, lower left and right steering cylinders (double-acting single rod hydraulic cylinders having the same capacity as the upper steering cylinders 25 and 26) 27 and 28 are provided on the lower traveling body side below the rotary joint 22. The rods of the two cylinders 27, 28 are connected to the front left and right wheels 18,
18 knuckle arms 18a, 18a. A tie rod 29 connects the left and right knuckle arms 18a, 18a to each other.

Each of these steering cylinders 25, 2
6, 27 and 28, the rod-side oil chamber 2 of the upper left steering cylinder 25
5b and the head side oil chamber 2 of the right steering cylinder 26
6a is connected by a cylinder cylinder oil passage 30, and the head side oil chamber 2 of the upper left steering cylinder 25
5a and the rod-side oil chamber 2 of the right steering cylinder 26
6b is connected by a cylinder oil passage 31, one cylinder oil passage 30 is connected to a rod-side oil chamber 27b of a lower left steering cylinder 27 and a head-side oil chamber 28a of a lower right steering cylinder 28, and the other cylinder oil passage 31 is Upper and lower connecting oil passages 32 and 33 passing through the rotary joint 22 are connected to a head side oil chamber 27a of the lower left steering cylinder 27 and a rod side oil chamber 28b of the lower right steering cylinder 28, respectively.

Each of these steering cylinders 25 to 28
A closed circuit A having a constant pressure is constituted by the oil passages 30, 31, 32, and 33. The static pressure of the closed circuit A transmits the steering wheel operating force to the wheels 18, 18, and The straight-line restoring force received by the steering wheel 18 is transmitted to the steering wheel 16.

The operation of this point will be described below.

When the handle 16 is turned clockwise as shown by the arrow in FIG. 2, the movement is transmitted to the link 24 via the steering unit 23, and the rods 25R, 26R of the upper left and right steering cylinders 25, 26 are respectively moved. Move right.

As a result, hydraulic oil is discharged from the rod-side oil chamber 25b of the upper left steering cylinder 25 and the head-side oil chamber 26a of the right steering cylinder 26, and passes through the oil passages 30 and 32 to the lower left steering cylinder 27. The oil is supplied to the rod-side oil chamber 27b and the head-side oil chamber 28a of the right steering cylinder 28.

As a result, the lower left and right steering cylinders 27 and 28 operate to operate the front left and right wheels 18 and 1.
8 is steered to the right.

At this time, the lower left steering cylinder 2
The hydraulic oil discharged from the head-side oil chamber 27a of the upper left steering cylinder 25 through the oil passages 33 and 31 passes through the oil passages 33 and 31, The oil is supplied to the rod-side oil chamber 26b of the steering cylinder 26, respectively.

On the other hand, when the steering wheel 16 is turned to the left, each part operates in the opposite direction to the above, and the wheels 18, 18 are steered to the left.

In the case of the first embodiment, since the cylinder capacity of each of the steering cylinders 25 to 28 is the same, the amount of hydraulic oil supplied and discharged between the upper and lower cylinders is substantially equal.

However, each of the steering cylinders 25 to 2
It is not necessary that the cylinder capacities of the cylinders 8 are completely the same, and they may be slightly different.

Further, a difference in pressure receiving area may be provided between the upper and lower steering cylinders. In this case, the handle 16
And the ratio of the turning angles of the wheels 18, 18 differ. That is, if the pressure receiving area of the cylinder above the lower part is made smaller, the cut of the wheels 18, 18 becomes smaller, and if the cylinder pressure receiving area of the lower part of the upper part is made smaller, the cut of the wheels 18, 18 becomes larger.

After the steering, the straight-line restoring force applied to the wheels 18, 18 is reduced by the lower steering cylinder 27,
From 28, the hydraulic pressure of the closed circuit A is transmitted to the upper steering cylinders 25, 26 and transmitted to the handle 16 via the link 24 and the steering unit 23.

Thus, at the same time when the wheels 18, 18 return to the straight-ahead position, the handle 16 automatically returns to the neutral position (the straight-ahead position).

In this case, each of the upper and lower steering cylinders 25 to 28 has a sliding resistance, and the sum of the cylinder sliding resistances is the resistance of the steering wheel neutral return. However, the steering wheel 16 may not completely return to the neutral position without being transmitted 100%.

Therefore, a configuration is adopted in which the handle 16 is reliably returned to the neutral position by the neutral return cylinder 34 as neutral return means.

The neutral return cylinder 34 is provided with two oil chambers 34a and 34b on the left and right sides as shown in an enlarged view in FIG.
5, 36 (hereinafter referred to as left and right oil chambers or free pistons according to the direction of the drawing).

Between these two free pistons 35 and 36,
A head 37a of a headed rod 37 for transmitting a force between the two free pistons 35 and 36 is introduced, and a tip end of the rod 37 is connected to an intermediate portion of the link 24 of the steering unit 23.

In FIG. 3, reference numeral 38 denotes both free pistons 35, 3
Stopper 6 for determining the stop position at the center of the cylinder, 3
Reference numeral 9 denotes an oil chamber formed between the free pistons 35 and 36, and the oil chamber 39 is connected to the tank T via a tank port 40.
And the oil is supplied to and discharged from the oil chamber 39 in accordance with the movement of the free pistons 35 and 36.

The left and right oil chambers 34a and 34b are connected to neutral cylinder oil passages 41 and 42, respectively.

The two neutral cylinder oil passages 41 and 42 join as shown in FIG. 2 and are connected to a neutral return cylinder line 43. The neutral return cylinder line 43 is connected to a neutral return cylinder hydraulic pump 44. I have.

That is, the same oil pressure is supplied from the pump 44 to both the oil chambers 34a, 34b (both free pistons 35, 36) of the neutral return cylinder 34.

It is also possible to connect an accumulator to the neutral return cylinder line 43 and supply the pressure of this accumulator to the neutral return cylinder 34.

It is desirable that equal pressure acts on both side free pistons 35 and 36.
5 has a smaller pressure receiving area than the right piston 36. If the difference in the pressure receiving areas becomes a problem, the diameter of the right cylinder of the neutral return cylinder 34 may be made smaller than the diameter of the left cylinder to achieve perfect equal pressure.

On the other hand, the upper steering cylinders 25 and 2
Tank lines 45 and 46 are connected to the two cylinder oil passages 30 and 31 connecting the six cylinders 6, respectively.

The two tank lines 45 and 46 are connected to a merging tank line 49 via neutral return valves 47 and 48, which are solenoid-operated directional control valves that switch between an open position A and a shut-off position B, respectively. The merging tank line 49 is connected to the tank T.

The neutral return valves 47 and 48 are operated by a neutral return switch 50 provided in the cabin 15 of FIG.

In FIG. 2, reference numeral 51 denotes a steering hydraulic pump as a hydraulic source of the steering unit 23;
2 is a flow divider provided in an oil passage connecting both the pump and tank ports of the steering unit 23 to the pump 51 and the tank T, 53 is a relief valve for setting the discharge pressure of the neutral return cylinder hydraulic pump 44, and 54 is This is a relief valve for setting the discharge pressure of the steering unit hydraulic pump 51.

The neutral return / neutral position correcting operation will be described.

I. Automatic Neutral Return Function After Steering The neutral return valves 47 and 48 are normally in the illustrated shut-off position B, and in this state, the closed circuit A is closed to the outside.

When the neutral return cylinder 34 is not steered, the oil chambers 34a, 34b on both sides have substantially the same pressure, and the rod head 37a is pushed by the free pistons 35, 35 on both sides to be located at the center of the cylinder.

When the steering is performed in this state, the cylinder 34 is pushed to the left or right in the drawing by the output of the link 24, and a pressure difference is generated between the oil chambers 34a and 34b on both sides of the cylinder.

At this time, the thrust of the neutral return cylinder 34 acts as a resistance to the output of the steering unit 23. However, there is no problem if the output of the steering unit is made sufficiently large.

After steering, as described above, the wheels 18, 1
8 is transmitted to the link 24 of the steering unit 23 at the same time as the neutral return cylinder 3
4 tries to return to the neutral position, and this cylinder return force is applied to the link 24 together with the rectilinear restoring force.

This cylinder return force is set to be larger than the sum of the sliding resistances of the steering cylinders 25 to 28, and the steering wheel 16 is reliably returned to the neutral position by the total return force.

Even when the thrust of the neutral return cylinder 34 is set to be smaller than the sum of the sliding resistances of the steering cylinders 25 to 28, this assists the return of the steering wheel 16 to the neutral position, and is more effective than when the cylinder 34 is not provided. Needless to say, the handle 16 returns well.

Thus, after steering, the steering wheel 16 is reliably returned to the neutral position, and the desired straight running performance can be secured.

On the other hand, in the closed circuit A, the amount of oil decreases due to internal leaks in the steering cylinders 25 to 28 and the rotary joint 22.
The neutral position is shifted from the straight position. In other words, the return position of the steering wheel 16 after steering is accurate to the neutral position (wheel 1
8, 18), and not only the straight running performance is impaired, but also the effective stroke of the steering cylinders 25 to 28 is shortened, and the inherent turning ability of the crane during running is reduced.

Therefore, if necessary, the handle 16 is neutralized as follows.

II. Neutral Position Correcting Operation The neutral return switch 47 is operated by operating the neutral return switch 50 with the wheels 18, 18 being visually adjusted to the straight traveling position, and switching from the shut-off position B to the open position A in the figure.

Then, the closed circuit A is communicated with the tank T, the oil chambers 25a, 25b, 26a, 26b of the left and right upper steering cylinders 25, 26 are adjusted to the tank pressure. The steering cylinders 25 and 26 and the handle 16 are returned to the neutral position.

The lower steering cylinders 27, 2
8 is neutral when the wheels 18, 18 are adjusted to the neutral position.

Thus, the deviation of the steering wheel 16 from the neutral position can be corrected, and the straight traveling performance and the original effective stroke of the steering cylinders 25 to 28 can be secured.

Further, according to the above-described position correcting method, the neutral position of the upper steering cylinders 25 and 26 can be left unclear, and sensors for detecting the neutral position of the cylinder are not required.

By the way, when the temperature of the oil in the closed circuit A rises during the crane operation and the internal pressure rises, it is desirable to perform so-called depressurization.

Therefore, as one method of depressurization,
A sensor detects that the outrigger has been extended (the crane operation that causes an increase in the oil temperature is started), and the neutral return valves 47 and 48 are operated based on the outrigger extended signal from the sensor. Is also good. in this case,
The neutral return cylinder 34 is locked to be inoperable.

In the above-described embodiment, the cylinder including the free pistons 35 and 36 and the rod 37 is used as the neutral return cylinder 34. Neutral return means can also be configured by the cylinder configuration.

Further, the steering wheel 16 may be mechanically returned to the neutral position by using a spring or the like in place of the hydraulic cylinder as the neutral return means.

Further, the neutral return means does not necessarily need to completely return the steering wheel 16 to the neutral state when the vehicle travels, and may return the neutral state to a range where the vehicle does not interfere with the travel (a state close to the complete neutral state). Good.

[0099]

As described above, according to the present invention, the straight running restoring force received by the wheel is transmitted to the steering wheel through the closed hydraulic pressure including the upper and lower steering cylinders to the steering wheel in the upper turning type traveling vehicle. Nature can be secured.

By providing the neutral return means, the steering wheel can be reliably returned to the neutral position against the sliding resistance of each steering cylinder.

In this case, according to the second aspect of the present invention in which a hydraulic cylinder (neutral return cylinder) is used as the neutral return means, the power source (hydraulic source) can be easily secured by incorporating the cylinder into the steering hydraulic circuit. There is no need to add a separate power source.

In particular, according to the third aspect of the present invention, a pair of cylinders for pushing and pulling is used as the neutral return cylinder, which is a cylinder consisting of free pistons on both sides and a main piston interposed therebetween. The cylinder equipment is simpler and smaller than that of

On the other hand, according to the invention of claims 4 to 6, when the neutral position of the steering wheel is displaced by an internal leak generated by each steering cylinder or the rotating joint, the operation of the neutral return valve causes the operation of each steering cylinder. By operating the neutral return means in a state where the oil chamber is communicated with the tank and the pressure is equal, the handle can be corrected to the neutral position.

As a result, it is possible to maintain the straight running performance and prevent the effective stroke (wheel steering angle) of the cylinder from decreasing, thereby ensuring the original turning ability during running.

In this case, according to the invention of claim 5, wherein the electromagnetic switching valve is used as the neutral return valve, the electromagnetic switching valve can be remotely operated by a switch. Can be.

In particular, according to the invention of claim 6, when the shift of the neutral position of the steering wheel is detected, the correction function can be immediately and easily operated by operating the switch in the cab.

[Brief description of the drawings]

FIG. 1 is a side view showing an overall schematic configuration of a wheel crane to which the present invention is applied.

FIG. 2 is a hydraulic circuit configuration diagram of the steering device according to the embodiment of the present invention.

FIG. 3 is an enlarged sectional view of a neutral return cylinder used in the device.

FIG. 4 is a hydraulic circuit configuration diagram of a conventional steering device.

[Explanation of symbols]

 Reference Signs List 16 Steering wheel 18, 18 Front wheel to be steered 23 Steering unit 25, 26 Upper steering cylinder 27, 28 Lower steering cylinder A Closed circuit 30 to 33 Oil path constituting closed circuit 22 Rotary joint 34 Neutral return as neutral return means Cylinder 35, 36 Free piston on both sides 37 Rod 37 Rod head 47, 48 Neutral return valve 50 Neutral return switch

Claims (6)

[Claims] 1. An upper part of a lower traveling body having wheels,
An upper revolving structure provided with a driver's seat having a steering wheel is rotatably mounted, and a vehicle having a steering unit that amplifies the operating force of the steering wheel and transmits the steering force to the wheel as a steering force includes the following configuration. A steering device for a traveling vehicle, comprising: (i) The upper revolving superstructure is provided with an upper steering cylinder that operates in response to the operation of the handle. (ii) The lower traveling body is provided with a lower steering cylinder that changes a steering angle of the wheel. (iii) The upper and lower steering cylinders are connected to each other by an oil passage via a rotary joint disposed at the center of rotation of the upper revolving structure to form a closed circuit. (iv) The straight-line restoring force received by the wheel during traveling can be transmitted to the steering wheel via the hydraulic pressure of the closed circuit and the steering unit. (v) A neutral return means for applying a force in a direction for returning the handle to the neutral position is provided. 2. The steering apparatus according to claim 1, wherein a neutral return cylinder including a hydraulic cylinder is used as the neutral return means. 3. A two-sided oil chamber connected to each other, a free piston slidably provided in the two-sided oil chamber, and a rod for transmitting force between the two free pistons. 3. The method according to claim 2, wherein
A steering device for a traveling vehicle according to claim 1. 4. A neutral return valve which is operated to switch between a position in which the oil chambers of the upper and lower steering cylinders communicate with the tank and a position in which the oil chambers are shut off from the tank. 4. The steering device for a traveling vehicle according to any one of claims 1 to 3. 5. The steering device for a traveling vehicle according to claim 4, wherein an electromagnetic switching valve is used as the neutral return valve. 6. The steering device for a traveling vehicle according to claim 5, wherein the electromagnetic switching valve as a neutral return valve is configured to be operated by operating a neutral return switch provided in a cab.