JPH11268656A - Forward and backward switching circuit for work vehicle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To switch to forward and backward by a second forward and backward switching operation means when a first forward and backward switching operation means is set neutral and a selection switch is turned ON, thereby forward and backward switching caused by wrong operations is prevented. SOLUTION: In this circuit, a starter motor is actuated only when a forward and backward switching lever is set at a neutral position N, which prevents a vehicle from suddenly going forward or backward. When a hand leaves an engine start switch SW-S and it is set at ON position, power is supplied either to the side of a forward and backward switching lever SW-D, or to the side of forward and backward switching switches GW-F, SW-R, and SW-N. When forward and backward switching is made with the forward and backward switching switches SW-F, SW-R and SW-N on the side of a multi-functional lever 27, a selection switch SW-J is pressed first.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a forward / backward switching circuit for a working vehicle, and more specifically, to enable forward / backward switching (forward selection or reverse selection) on either the handle side or the work lever side. And a forward / backward switching circuit for a working vehicle such as a wheel loader.

[0002]

2. Description of the Related Art FIG. 3 is a side view showing an example of a wheel loader. As shown in FIG. 1, the wheel loader 1 travels (forward or backward) by wheels 2, has a driver's cab 3 in the center of the vehicle body, and has an arm 4 and a front end of the arm 4 at the front of the vehicle body. And a work machine including the provided bucket 5. In the wheel loader 1, a forward or backward traveling of the vehicle is performed by a traveling operation operation of an operator in a driver's cab 3, and a lifting and lowering, a tilt and a dump of the bucket 5 are performed by the operation operation to load the truck. Performs loading of earth and sand.

FIG. 4 is a perspective view showing a configuration of a main part in a cab of the wheel loader. As shown in the figure, a steering (steering wheel) 8 provided on a steering post 15 and a forward / reverse switching lever provided on the left side of the steering wheel 8 are provided on a traveling driving operation unit 6 on the left side as viewed from the front of the vehicle. 10, an engine start switch (key switch) 11 provided on the right side of the steering wheel 8, and a steering wheel 1
0 and an accelerator pedal 9 provided below the right side of the vehicle.

The contact of the forward / reverse switching lever 10 constitutes a part of a forward / backward switching circuit (sequence circuit) (not shown). When the forward / backward switching lever 10 is tilted upward, forward is selected. The neutral position is selected when 10 is in the horizontal position (the state shown), and the reverse is selected when the forward / reverse switching lever 10 is tilted downward. A knob 12 is attached to the handle 8 so that an operator can grip the knob 12 with one hand to rotate the handle 8.

[0005] On the other hand, a work implement lever 13 is provided in the work operation operation unit 7 on the right side as viewed from the front of the vehicle. Then, the worker lowers or raises the bucket 5 by grasping the head 14 and tilting the work implement lever 13 forward or backward, and tilts the work implement lever 13 left or right to raise or lower the bucket 5. It can be rotated downward (tilt or dump).

Therefore, when loading a truck or the like with the wheel loader 1, the worker grasps the head 14 with his right hand and tilts the work machine lever 13 forward, backward, left and right to perform a work driving operation. Alternatively, the driving operation can be performed by grasping the knob 12 with the left hand and turning the handle 12.

[0007]

However, in the conventional wheel loader 1, when the operating lever 13 is operated by the right hand and the handle 12 is operated by the left hand at the same time, forward and backward switching is performed. In such a case, the user has to release the handle 8 once and operate the forward / reverse switching lever 10.

Therefore, recently, some work vehicles such as a wheel loader are provided with a forward / backward changeover switch on the work implement lever side in order to improve operability of forward / backward changeover (see FIG. 1). ).

However, if the forward / backward switching can be performed simply by operating the forward / backward switching switch on the side of the working machine lever (or simply by pressing the push button switch), the working machine lever will not work. Since the work implement is frequently operated, there is a risk that the forward / reverse selector switch may be accidentally operated during the operation of the work implement lever. That is, there is a possibility that the vehicle may be switched forward or backward by mistake and the vehicle may unexpectedly move forward or backward.

When the operation such as loading is completed and the operation is shifted from the work driving operation to the traveling driving operation, even if there is a forward / reverse changeover switch on the working machine lever side, the worker (especially a skilled worker) The driving operation can be performed more smoothly when the forward / backward switching is performed by the forward / backward switching lever on the handle side used by the driver.

When the forward / backward changeover switch provided on the working machine lever side is an individual switch such as a push button switch (see FIG. 1), the forward / backward changeover can be performed simply by pushing the forward switch or the reverse switch. In such a case, there is a possibility that an impact is generated with this switching.

Accordingly, the present invention has been made in view of the above prior art, and provides a forward / backward switching circuit for a working vehicle which can prevent forward / reverse switching from being performed due to erroneous operation of forward / backward switching means provided in a work driving operation unit. This is the first task.

Further, in the case where the forward / reverse selection by the forward / reverse switching means of the work driving operation unit has been performed first, the forward / backward switching by the forward / reverse switching means of the traveling driving operation unit can be prioritized. A second object is to provide a forward / reverse switching circuit.

It is a third object of the present invention to provide a forward / backward switching circuit for a working vehicle which can prevent the occurrence of an impact accompanying the forward / backward switching when the forward / backward switching means of the work driving operation unit is switched. .

[0015]

According to a first aspect of the present invention, there is provided a forward / backward switching circuit for a working vehicle, comprising: a first forward / backward switching operation means provided in a traveling driving operation section; A forward / backward switching circuit for a work vehicle having a second forward / backward switching operation means provided, wherein the first forward / backward switching operation means includes a selection switch for selecting the second forward / backward switching operation means. When the selection switch is turned ON, the second forward / reverse switching operation means can perform forward / reverse switching.

Further, the forward / backward switching circuit for a working vehicle according to a second invention for solving the second problem is a forward / backward switching circuit for a working vehicle according to the first invention. When the forward / reverse switching by the first forward / reverse switching operation means is performed after the selection of the first forward / reverse switching operation means, the forward / reverse selection by the second forward / reverse switching operation means is released and the first forward / reverse switching operation means is released. It is characterized in that it is configured to be able to switch back and forth.

In addition, the work vehicle forward / reverse switching circuit according to a third invention for solving the third problem is a work vehicle forward / reverse switching circuit according to the first or second invention, wherein the second forward / backward switching operation means is provided. It consists of an individual forward switch, reverse switch and neutral switch, and the forward / backward switching by the forward switch or the reverse switch is performed once.
The present invention is characterized in that the operation can be performed after the reverse selection by the reverse switch or the forward selection by the forward switch is canceled by operating the neutral switch.

[0018]

Embodiments of the present invention will be described below in detail with reference to the drawings. Since the present invention has been devised for the forward / reverse switching circuit of the work vehicle, this point will be described in detail below, and the overall structure of the work vehicle is assumed to be the same as the conventional one (see FIG. 3). Description and illustration are omitted.

[First Embodiment] FIG. 1 is a perspective view showing a main part in a cab of a wheel loader provided with a forward / reverse switching circuit according to an embodiment of the present invention, and FIG. 2 is a configuration of the forward / backward switching circuit. FIG.

<Structure> As shown in FIG. 1, a steering wheel (steering wheel) 23 provided on a steering post 26 and a steering wheel 23 provided on the left side of the steering wheel 23 are provided in a traveling operation operation unit 21 on the left side as viewed from the front of the vehicle. A forward / reverse switching lever SW-D serving as a first forward / reverse switching operation means, an engine start switch (key switch) SW-S provided on the right side of the handlebar 23, and a rightward lower portion of the handlebar 23. An accelerator pedal 25 is provided.

The contact of the forward / reverse switching lever SW-D constitutes a part of a forward / backward switching circuit (to be described in detail later). When the forward / backward switching lever SW-D is tilted upward, forward is selected, and forward / backward is selected. The neutral position is selected when the switching lever SW-D is in the horizontal position (the state shown), and the reverse is selected when the forward / reverse switching lever SW-D is tilted downward. Also, a knob 24 is attached to the handle 23, and the operator can grasp the knob 24 with one hand and rotate the handle 23.

On the other hand, a multi-function lever 27 is provided in the work driving operation section 22 on the right side as viewed from the front of the vehicle. This multifunction lever 27
Has a function as a conventional working machine lever (see FIG. 4),
It has both forward and backward switching functions.

That is, similarly to the conventional working machine lever, the operator grasps the head 28 and
Tilting the bucket forward or backward lowers or raises the bucket (see FIG. 3), and tilts the multifunction lever 27 left or right to rotate (tilt or dump) the bucket upward or downward. I can do it.

Further, on the head 28 of the multi-function lever 27, a forward / backward switch SW-F, a neutral switch SW-N, and a reverse switch SW-R are provided as forward / backward changeover switches as second forward / backward changeover operation means. Are provided. In the vicinity of the multifunction lever 27, the forward / reverse selector switch SW-F, S
Selection switch SW- for selecting W-N, SW-R
J is provided. These switches SW-F, SW
-N, SW-R, and SW-J also constitute part of the forward / reverse switching circuit. Note that these switches SW-F, SW-
N, SW-R, and SW-J are all push-button types, and the contact closes (turns on) when pressed with a finger, and returns with spring force to open the contact (off) when the finger is released.
).

Here, the configuration of the forward / reverse switching circuit (sequence circuit) will be described with reference to FIG.

As shown in FIG. 2, the terminal B of the engine start switch SW-S is connected to a 12 V power supply bus 31, and the terminals M and S are connected to the forward / reverse switching lever SW- via cables (harnesses) 33 and 34. D are connected to terminals B ₁ and B ₂ , respectively. When the engine start switch SW-S is turned to the ON position, the terminal B and the terminal M are connected. When the engine start switch SW-S is set to the START position, the terminal B is connected to the terminal B.
And the terminal M and the terminal S are connected. Also, the engine start switch SW-S is set to START
Release the hand after setting the position to return to the ON position.

The forward-reverse switching lever SW-D terminal F of traveling through the diode D ₄ the pump forward solenoid SOL1
And it is connected to one end of the relay R _10, the other end of the travel pump forward solenoid SOL1 and the relay R ₁₀ is connected to the ground bus 32. Further, the terminal F and the bus 32
It is also connected a relay R ₈ is between. Before the terminal R of the reverse switching lever SW-D is connected to one end of the travel pump reverse solenoid SOL2 and the relay R ₁₁ via the diode D _5, the travel pump reverse solenoid SOL
The other end of the 2 and the relay R ₁₁ is connected to the ground bus 32. A relay R ₉ is also connected between the terminal R and the bus 32. A contact R ₇ -b ₁ of a relay R ₇ and a starter motor M are connected in series between the terminal N of the forward / reverse switching lever SW-D and the bus 32.

Further, the forward-reverse switching lever when the SW-D forward selection position tilt the F is connected to the terminal B ₂ and the terminal F, terminal B when the neutral position N
₁ and is a terminal N connected, so that the and the terminal B ₂ and the terminal R is connected when the reverse selection position R tilted down. The forward / reverse switching lever SW-D is held at each position F, N or R until the operator manually switches to another position.

On the other hand, a selection switch SW-J and a contact R ₁ -a of the relay R ₁ are provided between the cable 34 and the cable 35.
₁ are connected in parallel, and the selection switch SW
Diode D ₁ is connected between the -J and the contact R ₁ -a _1.

[0030] In the cathode side of the diode D _1, between the cable 35 and the bus 32, the contact R ₉ -b ₁ and a series of relay R ₁ and the contact R ₈ -b ₁ and the relay R ₉ of the relay R ₈ It is connected to the.

Diode D₁On the anode side of the
Between the bull 35 and the bus 32,
Yellow selection lamp YL, series neutral switch S
W-N and relay R_Two, A series forward switch SW-F and
Leh R_ThreeAnd relay R₆Contact R₆-B₁And relay R
_TwoContact R of_Two-B₁, Series relay R_ThreeContact R of_Three-A
₁And relay R_Four, Series reverse switch SW-R and relay
R_FiveAnd relay R_FourContact R of_Four-B₁And relay R_TwoContacts
R_Two-B_Two, Series relay R_FiveContact R of_Five-A ₁And Lille
-R₆, And relay R₇Are connected in parallel, respectively.
ing.

The forward switch SW-F and the contact R _3-
is connected the diode D ₂ is between a _1, the backward switch SW-R and the contact R ₅ -a ₁ diode D ₃ between the
Is connected. Then, the contact R ₃ -a ₁ is also connected to the running pump forward solenoid SOL1 and the relay R ₁₀ via the diode D _6, contacts R ₅ -a ₁ and the travel pump reverse solenoid SOL2 through the diode D ₇ It is also connected to the relay R _11.

A parallel relay is provided between the buses 31 and 32.
-R_TenContact R of_Ten-A₁And relay R ₁₁Contact R of₁₁−
a₁, Parking drag prevention buzzer BZ and par
King switch SW-P is connected in series.

Further, among the above relay contacts, the contact R
₁ to code like _one such -a those denoted by a is the contact to close when the relay is energized, the code that denoted by b in the relay unexcited as such contacts R ₂ -b ₂ This is the contact that closes when.

<Operations / Effects> Accordingly, when loading a truck or the like by the above-mentioned wheel loader, the worker grips the head 28 with the right hand and tilts the multifunction lever 27 back and forth and right and left to perform a work operation. The operation can be performed, and at the same time, the driving operation can be performed by grasping the knob 24 with the left hand and turning the handle 23.

At the time of this operation, the forward / reverse selector switch SW- on the multifunction lever 27 side is operated.
Forward / reverse switching can be performed by F, SW-R, and SW-N, and forward / reverse switching can be performed by a forward / reverse switching lever SW-D on the steering wheel 23 side during driving operation. The switching priority and the like are defined by the forward / reverse switching circuit. That is, according to the above forward / reverse switching circuit, the following operation
The effect is obtained.

First, when the engine start switch SW-S is turned to the START position with the forward / reverse switching lever SW-D at the neutral position N, the cable 3
3, the power is supplied to the starter motor M via
The starter motor M operates to start an engine (not shown), and the engine drives a hydraulic pump (not shown). That is, unless the forward / reverse switching lever SW-D is in the neutral position N, the starter motor M does not operate, thereby preventing the vehicle from suddenly moving forward or backward. When a hydraulic pump (traveling pump) is driven, a hydraulic motor (not shown) is driven by the hydraulic pump to enable traveling.

The starter motor M has a contact R ₇ -b
_{Since 1} are connected in series, the starter motor M even when the relay R ₇ is energized the contact R ₇ -b ₁ open
Does not work. That is, the multi-function lever 27
The forward-reverse changeover switch SW-F, SW-R, even when the power is in the SW-N side, the starter motor M is prevented from operating by exciting the relay R _7.

Next, an engine start switch SW-S
Release the switch from the engine start switch SW-S
At the N position, the forward / reverse selector lever SW-D or the forward / reverse selector switch SW-F, SW is connected via the cable 34.
-R, the state where power is supplied to the SW-N side.

In this state, when the forward / reverse switching lever SW-D is operated to select the forward selection position F or the reverse selection position R,
When the traveling pump forward solenoid SOL1 or the traveling pump backward solenoid SOL2 is excited, the vehicle moves forward or backward. At the same time, the relay R ₈ or relay R ₉ is energized to open the contacts R ₈ -b ₁ or contacts R ₉ -b _1.

On the other hand, when the forward / reverse switching (forward selection or reverse selection) is performed by using the forward / backward changeover switches SW-F, SW-R, and SW-N on the multifunction lever 27 side, first, the selection switch SW-J is selected. push. As a result, if the contacts R ₈ -b ₁ and R ₉ -b ₁ are closed, the relay R ₁ is excited and the contact R ₁ -a ₁ is closed, so that power is also supplied to the cable 35 on the anode side of the diode D _1. Is supplied (hereinafter, this state is referred to as a state in which the master power supply is turned on). Therefore, the forward / reverse selector switch SW-
Forward / backward selection by F, SW-R becomes possible.

[0042] However, when the forward-reverse switching lever SW-D therefore forward or backward is selected, i.e., when the forward-reverse switching lever SW-D is in the forward selection position F or reverse selection position R the relay R ₈ or Since the relay R ₉ is excited and the contact R ₈ -b ₁ or the contact R ₉ -b ₁ is open,
Without relay R ₁ by pressing a selection switch SW-J is excited, the contact R ₁ -a ₁ remains open. Therefore, forward / reverse selection cannot be performed by the forward / backward changeover switches SW-F and SW-R.

That is, the forward / reverse selector lever SW-D is in the neutral position N, and the selection switch SW-J is
N, when the forward / reverse selector switch SW-F, SW-
The forward / reverse selection by the forward / reverse switching lever SW-D is made to be prioritized over the forward / reverse selection by the forward / backward changeover switches SW-F and SW-R.

Therefore, the forward / reverse selector switch SW-
The forward / reverse switching is not performed only by pressing the F / SW-R. Even if the forward / reverse switching switches SW-F and SW-R are erroneously pressed during the operation of the forward / reverse switching lever SW-D, this erroneous operation causes It is possible to prevent the forward / backward switching from being performed. For this reason, there is no possibility that the vehicle may unexpectedly move forward or backward.

When the master power is turned on, that is, when the contact R ₁ -a ₁ is closed and the selection lamp YL is turned on, the selection switch SW-J is released and the selection switch SW-J is turned off. also, because it is held by the contact R ₁ -a ₁ is closed, the master power-on state is maintained.

With the master power turned on, the forward switch is
When the switch SW-F or the reverse switch SW-R is pressed, the relay
R_ThreeOr relay R_FiveIs excited and the contact R_Three-A₁Or contact
Point R _Five-A₁Is closed because the traveling pump forward solenoid
SOL1 or traveling pump reverse solenoid SOL2 is excited
Then, the vehicle moves forward or backward. In addition, this
, The forward switch SW-F or the reverse switch SW
-Release the switch from R and move forward switch SW-F or reverse switch.
Even if the switch SW-R is turned off, the contact R_Three-A₁Or
Contact R_Five-A₁Was held by closing
Therefore, the forward selection state or the reverse selection state is maintained.

Then, as described above, the forward switch SW-F
Or after being advanced selected or reverse selection by pressing the reverse switch SW-R, when the forward-reverse switching lever SW-D to the operation forward selection position F or reverse selection position R, a relay R ₈
Or, the relay R ₉ is excited and the contact R ₈ -b ₁ or the contact R
₉ -b ₁ is for opening, contact relay R ₁ is unexcited R
₁ -a ₁ is open. For this reason, the forward selection or the reverse selection by the forward switch SW-F or the reverse switch SW-R is released, and the forward / reverse selection by the forward / backward switching lever SW-D can be performed.

Therefore, when the operation such as loading is completed and the operation is shifted from the work driving operation to the traveling driving operation, the worker immediately
The forward / reverse switching can be preferentially performed with the forward / backward switching lever SW-D on the handle 23 side to which the user is accustomed. For this reason,
The operator can perform the driving operation smoothly.

Also, the forward switch SW-F or the reverse switch
Switch SW-R to select forward or reverse
At the same time, the relay R_FourOr relay R₆Is excited and connected
Point R _Four-B₁Or contact R₆-B₁Is open
When the neutral switch SW-N is pressed,
Relay R_TwoIs excited and the contact R_Two-B₁And contact R_Two−
b_TwoIs designed to open.

For this reason, for example, first, the forward switch SW
Press -F to switch to reverse when forward is selected
If the reverse switch SW-R is simply operated,
Point R _Four-B₁Is open, relay R_FiveIs excited
No backward selection is made. On the other hand, the reverse switch SW-R
Before pressing, press the neutral switch SW-N once.
Then relay R_TwoIs excited and the contact R_Two-B₁Opened
The relay R_ThreeBecomes non-excited and the forward switch SW-F
Is canceled. Therefore,
If switch SW-R is pressed, contact R_Four-B₁
Is closed so you can select reverse. Of course, this
This means that the reverse is selected first by the reverse switch SW-R.
When switching to forward
Is the same.

That is, the forward / reverse switching by the forward switch SW-F or the reverse switch SW-R is performed by pressing the neutral switch SW-N once to select reverse by the reverse switch SW-R or forward / backward by the forward switch SW-F. This can be done after canceling the hex selection.

For this reason, simply pressing the forward switch SW-F or the reverse switch SW-R does not cause the forward / backward switching, but temporarily cancels the forward / backward selection by the neutral switch SW-N. Thus, it is possible to prevent the occurrence of an impact.

When the parking switch SW-P is operated, that is, when the parking brake is applied, the forward / reverse switching lever SW-D or the forward / backward switching switches SW-F, SW-R is used to move forward or backward. When you try to select, by the relay R ₁₀ or relay R ₁₁ is energized the contact R ₁₀ -a ₁ or contacts R ₁₁ -a ₁ closed,
The parking drag prevention buzzer BZ sounds to notify the worker that the parking brake is applied. For this reason, squeezing can be prevented.

The present invention is not limited to a wheel loader.
The present invention can be widely applied to various types of work vehicles that perform civil engineering work, construction work, and the like.

[0055]

As described above in detail with the embodiments of the present invention, the forward / reverse switching circuit of the work vehicle according to the first invention includes a first forward / reverse switching operation means provided in a traveling operation operation unit, A forward / reverse switching circuit for a working vehicle, comprising: a second forward / backward switching operation means provided in a work driving operation unit;
A selection switch for selecting the forward-reverse switching operation means, wherein the first forward-reverse switching operation means is neutral, and when the selection switch is turned on, the second forward-reverse switching operation means It is characterized in that it is configured to be able to perform forward switching.

Therefore, according to the forward / backward switching circuit of the working vehicle of the first invention, the forward / backward switching is not performed simply by operating the second forward / backward switching operating means. Even if the second forward / reverse switching operation means is erroneously operated during the driving operation, the erroneous operation can prevent the forward / reverse switching from being performed. For this reason, there is no possibility that the vehicle may unexpectedly move forward or backward.

Further, the forward / backward switching circuit for a work vehicle according to the second invention is the same as the forward / backward switching circuit for a work vehicle according to the first invention,
After forward or reverse is selected by the second forward / reverse switching operation means, when forward / reverse switching is performed by the first forward / reverse switching operation means, the forward / reverse selection by the second forward / reverse switching operation means is released. The first forward / reverse switching operation means can be used to perform forward / reverse switching.

Therefore, according to the forward / backward switching circuit of the work vehicle of the second invention, when the work such as loading is completed and the operation is shifted from the work driving operation to the traveling driving operation, the worker immediately becomes familiar with the traveling driving operation. The forward / reverse switching can be preferentially performed by the first forward / reverse switching operation means on the operation unit side. For this reason, the worker can perform the driving operation smoothly.

The forward / backward switching circuit for a work vehicle according to the third invention is a forward / backward switching circuit for a work vehicle according to the first or second invention, wherein the second forward / backward switching operation means is a separate forward switch and reverse switch. Forward and backward switching by the forward switch or the reverse switch, once after releasing the reverse selection by the reverse switch or the forward selection by the forward switch by operating the neutral switch. It is characterized in that it can be performed.

Therefore, according to the forward / backward switching circuit of the work vehicle of the third invention, forward / backward switching is not performed simply by pressing the forward switch or the reverse switch, but the forward / backward selection is once released by the neutral switch. In addition, it is possible to prevent an impact from being caused by the forward / backward switching.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a main part in a cab of a wheel loader provided with a forward / reverse switching circuit according to an embodiment of the present invention.

FIG. 2 is a configuration diagram of the forward / reverse switching circuit.

FIG. 3 is a side view showing an example of a wheel loader.

FIG. 4 is a perspective view showing a configuration of a main part in a cab of the wheel loader.

[Explanation of symbols]

31, 32 Bus bar 33, 34, 35 Cable SW-S Engine start switch SW-D Forward / reverse switching lever SW-J Selection switch SW-F Forward switch SW-N Neutral switch SW-R Reverse switch M Starter motor SOL1 Traveling pump forward Solenoid SOL2 Traveling pump reverse solenoid R _{1 to} R ₁₁ relay R ₁ -a _{1 to} R ₁₁ -a ₁ contact

Claims (3)

[Claims] 1. A forward / backward switching circuit for a working vehicle, comprising: a first forward / backward switching operation unit provided in a traveling driving operation unit; and a second forward / backward switching operation unit provided in a work driving operation unit. A selection switch for selecting a second forward / reverse switching operation means, wherein the first forward / reverse switching operation means is neutral, and when the selection switch is turned on, the second forward / reverse switching operation means is provided. A forward / backward switching circuit for a working vehicle, wherein the forward / backward switching circuit is configured to be able to perform forward / reverse switching. 2. The forward / reverse switching circuit according to claim 1, wherein the forward / reverse switching is performed by the first forward / reverse switching operation means after the forward / reverse operation is selected by the second forward / reverse switching operation means. Is performed, the forward / reverse selection by the second forward / reverse switching operation means is released, and the forward / backward switching by the first forward / reverse switching operation means can be performed. Forward / backward switching circuit. 3. The forward / reverse switching circuit for a work vehicle according to claim 1, wherein the second forward / reverse switching operation means includes an individual forward switch, a reverse switch, and a neutral switch. Forward / reverse switching by the switch or the reverse switch is configured to be performed once by operating the neutral switch to release the reverse selection by the reverse switch or the forward selection by the forward switch. Forward / backward switching circuit for work vehicle.