JPH0640348A - Travel transmitting structure for working vehicle - Google Patents

Abstract

PURPOSE:To change and adjust a turning radius optionally according to an operation lever operating position regardless of the magnitude of travel resistance of the ground surface in a working vehicle turning operating structure constituted so that the turning radius can be changed and adjusted optionally by changing operating oil pressure of a friction clutch for turning according to the operating position when a turning operating operation lever is operated to the right or left turning position side after a pair of left and right travel devices are equipped. CONSTITUTION:In an open position to allow a steep increase and in an orifice position to restrain the steep increase of operating oil pressure supplied to a hydraulic actuator 33 of a friction clutch RC for turning, a directional control valve 31 is provided so as to be switched freely. When the ground surface has large travel friction, while interlocking with operation to the right or left turn position R2 or L2 side of an operation lever 40, the directional control valve 31 is operated and held in the open position, and the operating oil pressure of the hydraulic actuator 33 is operated so as to be increased. When the ground surface has small travel resistance to the contrary, after the directional control valve 31 is operated and held in the orifice position, the operating oil pressure of the hydraulic actuator 33 is operated so as to be increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a traveling power transmission structure for a work vehicle provided with a pair of left and right traveling devices of four wheels on one side, such as a multi-wheel type or a crawler type.

[0002]

2. Description of the Related Art As an example of a work vehicle as described above, there is a combine equipped with a pair of left and right traveling devices of a crawler type.
As disclosed in Japanese Laid-Open Patent Publication No. 59577, one of the selected traveling devices is driven to rotate forward at a lower speed than the other so that a gentle turn can be performed, and one of the traveling devices is driven to rotate in the reverse direction. Some are configured so that they can turn. In the above-mentioned structure, as shown in the drawing of the publication, the left and right side gears (12 of the drawing of the publication) are attached to the first transmission gear (7 of the publication of the publication) to which the forward power for straight traveling is transmitted. When occluded (corresponding to the straight-ahead position), the forward rotation power of the same speed is transmitted from the left and right side gears to the left and right traveling devices, and the aircraft moves straight.

When the operating lever (40 in the drawing of the publication) for turning operation is operated to the left turning side from this straight traveling state, the control valve (35 in the drawing of the publication) is switched and the left hydraulic actuator. (3 of the drawings in the above publication
The hydraulic oil is supplied to 3), and the left side gear is operated to be separated from the first transmission gear to the left side of the drawing by the hydraulic actuator on the left side. Then, when the operating lever is further operated to the left turning side, the variable relief valve (37 in the drawing of the above-mentioned publication) is squeezed, the operating oil pressure of the left hydraulic actuator rises, and the left side gear moves. The friction clutch (RC in the drawing of the above publication) is pressed and operated (corresponding to the turning position). In this case, if low-speed forward rotation power is transmitted to the second transmission gear (9 in the drawing of the above publication),
The left traveling device is driven at a lower speed than the right traveling device,
The aircraft slowly turns to the left due to the speed difference between the left and right traveling devices. Then, if the reverse rotation power is transmitted to the second transmission gear, the left traveling device is reversely driven, and the airframe makes a super turning turn to the left.

[0004]

In the above-mentioned structure, the operating position of the operating lever to the right or left turning side causes
The operating pressure of the hydraulic actuator can be arbitrarily adjusted and changed by changing the throttle opening of the variable relief valve. Thereby, the pressing force of the friction clutch by the hydraulic actuator and the side gear is changed, and the transmission torque of the low speed forward rotation power or reverse rotation power to one traveling device is changed,
The turning radius of the gentle turning or the super turning turning can be changed.

As described above, the turning radius can be arbitrarily adjusted in accordance with the operating position of the operating lever when traveling in a work site having some running resistance, such as in a relatively soft field. . This is because if there is a certain amount of running resistance, the power of the running device on the side that is normally or reversely driven at a low speed is reliably transmitted to the ground even in the range of low transmission torque, and turning can be performed. Conversely, if the operating lever is operated to the right or left turning side on a paved road with a relatively low running resistance, the running device on the side driven in the forward or reverse rotation at a low speed slips, and the power for the turning is reduced. It may not be transmitted to the ground and may go straight. In such a state, the operator tries to turn the machine body by further operating the operation lever to the right or left turning side, so that the variable relief valve is suddenly throttled and the hydraulic pressure of the hydraulic actuator rises sharply. As a result, one traveling device is driven forward or reversely at a low speed due to a suddenly high transmission torque,
The state in which the machine body suddenly starts to turn is no longer a state in which the turning radius is arbitrarily adjusted according to the operation position of the operation lever. It is an object of the present invention to be configured so that the turning radius can be arbitrarily adjusted according to the operating position of the operating lever regardless of the state of the ground on which the vehicle travels.

[0006]

A feature of the present invention is that the traveling power transmission structure for a work vehicle as described above is configured as follows. That is, the first transmission gear to which the forward rotation power for straight traveling is transmitted and the low-speed forward rotation power or reverse rotation power are transmitted to each of the pair of left and right side gears that transmits power to each of the pair of left and right traveling devices. A second transmission gear that is transmitted is disposed, a friction clutch is disposed between the second transmission gear and the side gear, and each of the left and right side gears is set to the first side.
The left and right side gears are independently slid between the straight position where they are linked to the transmission gear and the turning position where the left and right side gears are pressed and operated to link the left and right side gears with the second transmission gear. A control valve that supplies a hydraulic oil for sliding the side gear to the turning position side and a hydraulic pressure that is supplied from the control valve to the hydraulic actuator Equipped with a variable relief valve that can be changed and adjusted, and a switching valve that can switch between a throttle position that suppresses a sudden rise in the operating hydraulic pressure supplied from the control valve to the hydraulic actuator and an open position that allows a sudden rise, and that is operated artificially. When the operating lever for operation is operated to the neutral position, operate the control valve to the neutral position to move the left and right side gears. When the operating lever is operated to the right or left turning position side while being moved to the forward position, the control valve is switched to supply hydraulic oil to the hydraulic actuator on the right or left turning side, and the operating lever moves to the right or left. The first control means for operating the variable relief valve so that the operating pressure of the hydraulic actuator becomes higher as it is operated to the left turning position side, and the operation lever is interlocked with the operation to the right or left turning position side. Then, after the control valve has been switched by the first control means, the switching valve is operated and held at the open position, and thereafter the first state in which the operation of the variable relief valve by the first control means is allowed, and the right or left rotation of the operating lever. In conjunction with the operation to the turning position side, after the control valve is switched by the first control means, the switching valve is operated and held at the throttle position, and then the second relief valve is allowed to be operated by the first control means. State and Switchable second
A control means is provided.

[0007]

With the configuration described above, when traveling on a work site having a certain amount of traveling resistance, such as a relatively soft field, the first state is selected as shown in FIGS. 1 and 3, for example. To do. In this first state, when the operation lever 40 is operated from the neutral position N to, for example, the left turning position L2, the variable relief valve 37 is operated to the throttle side in correspondence with the operation position of the operation lever 40, and the operation shown in FIG. As shown by the solid line, the hydraulic pressure of the left hydraulic actuator 33 rises, the friction clutch RC is pushed and operated, and the turning radius according to the operating position of the operating lever 40 can be set. In this case, since the switching valve 31 of FIG. 1 is operated and held in the open position, the operation lever 40 is slowly operated from the neutral position N to the right or left turning position R2, L2, or the operation lever 40 is moved to the neutral position N. The turning radius can be arbitrarily changed by operating and holding the position between the left and right turning positions R2 and L2.

On the other hand, the second state is selected on the pavement where the running resistance is relatively small. In this state,
Move the operation lever 40 from the neutral position N to, for example, the left turning position L2.
1, the switching valve 31 of FIG. 1 is switched to the throttle position, and then the variable relief valve 37 is operated to the throttle side in accordance with the operating position of the operating lever 40. As a result, due to the action of the throttle position of the switching valve 31, FIG.
As shown by the alternate long and short dash line, the operating oil pressure of the left hydraulic actuator 33 gradually rises corresponding to the operation position of the operation lever 40. In this way, by slowly transmitting the turning power to one of the traveling devices while suppressing the turning power, even on a paved road with a small traveling resistance, the one driving device supplies the turning power to the ground while suppressing slip. Can be transmitted to. As a result, the turning radius can be finely changed and adjusted by operating and holding the operation lever 40 at a position between the neutral position N and the right or left turning positions R2, L2. Then, the operation lever 40 is moved to the right or left turning position R2.
Even if L2 is rapidly operated, the increase in the operating oil pressure of the hydraulic actuator 33 is suppressed by the action of the throttle position of the switching valve 31, so that the aircraft does not start to turn rapidly.

[0009]

As described above, by providing the switching valve capable of switching between the state in which the hydraulic pressure of the hydraulic actuator for swing operation is suppressed and the state in which the hydraulic pressure is not suppressed, the rotary lever is used to swing regardless of the running resistance. The radius can be changed and adjusted arbitrarily, and the turning performance of the work vehicle can be improved.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 shows the structure inside the transmission case 2 of the traveling system of the combine, which is one of the work vehicles, in which the power from the engine (not shown) drives a belt transmission mechanism (not shown) equipped with a tension clutch. Through the input pulley 16 of the hydrostatic continuously variable transmission M. The power from the output shaft 1 of the hydrostatic continuously variable transmission M is supplied to the first transmission shaft 14
Is transmitted to the reaper (not shown) via the output pulley 15.

Power from the output shaft 1 is transmitted to the second transmission shaft 21 via the first gear 10. This second transmission shaft 21
The first high-speed gear 22 is fitted to the outside so as to be relatively rotatable, and the shift gear 3 is slidably attached by a spline structure. A low speed gear 25, a medium speed gear 8 and a second high speed gear 23 are attached to the third transmission shaft 24 in a spline structure, and the first and second high speed gears 22 and 23 are in mesh with each other. As a result, the shift gear 3 is slid to engage with the first high speed gear 22, the medium speed gear 8 and the low speed gear 25, whereby the power can be shifted to three stages of high, medium and low, and this power is transmitted to the medium speed gear 8. First transmission gear 7 that engages
Be transmitted to.

The right and left side gears 12 are fitted to the support shaft 6 supporting the first transmission gear 7 so as to be rotatable relative to each other,
The input gears 18 of the left and right axles 5 are constantly meshed with the left and right side gears 12. As a result, the articulating first clutch F is provided between the right or left side gear 12 and the first transmission gear 7.
C is configured, and the convex occlusion part 1 of the side gear 12 is formed.
By engaging 3 with the first transmission gear 7, the forward rotation power for straight traveling is transmitted to the left and right crawler type traveling devices 4. The spring 19 biases the side gear 12 toward the first transmission gear 7 side.

A pair of second transmission gears 9 are fitted on the left and right of the support shaft 6, and a multi-plate friction type second clutch RC (corresponding to a friction clutch) is provided between the second transmission gear 9 and the side gear 12. Has been. The second gear 11 is provided at both ends of the third transmission shaft 24.
Is fixed and the sleeve 38 is externally fitted to the bearing so that the third transmission shaft 24 can be slid to the left and right. In this case, since the low speed gear 25 and the middle speed gear 8 are positioned by the sleeve 26, the positions of the low speed gear 25 and the middle speed gear 8 do not change even if the third transmission shaft 24 is slid.

As shown in FIG. 2, a manipulating type switching lever 32 is provided in the control section of the fuselage for sliding operation of the third transmission shaft 24. A pair of fourth transmission shafts 39 are supported on the left and right of the transmission case 2, the third gear 27 and the fourth gear 28 are fixed to the fourth transmission shaft 39, and the left and right fourth gears 28 are the second. The transmission gears 9 are in mesh with each other.

With the above-described structure, when the switching lever 32 is operated to the superposition turning position and the third transmission shaft 24 is slid to the left side of the drawing, the second gears 11 on the left and right of the third transmission shaft 24 are operated.
Engages with the left and right third gears 27. As a result, the power of the third transmission shaft 24 passes through the third gear 27, the fourth transmission shaft 39, and the fourth gear 28, and is rotated in the reverse direction.
Be transmitted to. Therefore, for example, when the left side gear 12 is operated so as to be separated from the first transmission gear 7 and the second clutch RC is pressed and operated by the pressing portion 17 of the left side gear 12, the reverse rotation power of the left second transmission gear 9 is changed to the left. After being transmitted to the traveling device 4, the aircraft makes a super turning turn to the left.

Next, when the switching lever 32 is operated to the gentle turning position and the third transmission shaft 24 is slid rightward on the paper surface,
The left and right second gears 11 of the third transmission shaft 24 directly mesh with the left and right second transmission gears 9. As a result, the normal rotation power that is slower than the normal rotation power transmitted from the third transmission shaft 24 to the first transmission gear 7 is transmitted from the third transmission shaft 24 to the second transmission gear 9. Therefore, for example, the left side gear 12 is separated from the first transmission gear 7, and the second side is moved by the pressing portion 17 of the left side gear 12.
When the clutch RC is pressed and operated, the left traveling device 4 is driven to rotate in the forward direction at a lower speed than the right traveling device 4, and the vehicle body slowly turns to the left due to the speed difference between the left and right traveling devices 4.

Next, the sliding operation structure of the left and right side gears 12 will be described. As shown in FIGS. 1 and 2,
A pair of operating arms 20 for sliding operation of the left and right side gears 12, and a pair of hydraulic cylinders 3 for rocking the operating arms 20.
3 (corresponding to a hydraulic actuator), and the hydraulic cylinder 33 is internally provided with a spring (not shown) for urging the hydraulic cylinder 33 toward the contracting side. An electromagnetically-operated control valve 35 that selectively supplies the hydraulic oil from the pump 34 to one hydraulic cylinder 33 is provided.
Electromagnetically operated switching valve 31 that can be switched between two positions, a throttle position that suppresses a sudden rise in the operating hydraulic pressure and an open position that allows a sudden rise.
Is provided between the pump 34 and the control valve 35.
The side gear 12 is separated from the first transmission gear 7 (disengagement of the first clutch FC), and the second clutch RC
When the hydraulic cylinder 33 extends until it moves to a neutral stop position where the hydraulic cylinder 33 is not pressed and operated, a drain oil passage 36 for draining hydraulic oil from the hydraulic cylinder 33 and stopping the hydraulic cylinder 33 at the neutral stop position is provided. Oil passage 3
6, an electromagnetically operated variable relief valve 37 is provided.

Next, the control valve 35 and the variable relief valve 37.
The operation of will be described. As shown in FIG. 1, an operation lever 40 that is manually operated for turning operation is provided on the control section of the aircraft.
And a switching lever 41 for selecting between the soft turning state and the quick turning state, and the operating positions of the operating lever 40 and the switching lever 41 are input to the control device 42. The state shown in FIGS. 1 and 2 is a straight traveling state in which the left and right side gears 12 are engaged with the first transmission gear 7 by operating the operation lever 40 to the neutral position N (the left and right side gears 12).
Equivalent to the straight ahead position). Then, the switching lever 41 is in the state of being operated to the quick turning position (corresponding to the first state), and the switching valve 31 of FIG. 1 is in the state of being operated and held in the open position by the control device 42.

From the above state, as shown in FIGS. 1 and 3, for example, when the operating lever 40 is operated to the first left turning position L1, only the control valve 35 is switched by the control device 42, and the left hydraulic pressure is changed. Hydraulic oil is supplied to the cylinder 33, and the left side gear 12 slides to the left side of the drawing to move to the first side.
Separate from the transmission gear 7 (disengagement of the first clutch FC). In this case, the control device 42 controls the variable relief valve 3
Since 7 is operated in the fully open state, the left side gear 1
2 is the neutral stop position before pressing the second clutch RC,
The drain oil passage 36 is opened, hydraulic oil is drained from the variable relief valve 37, and the hydraulic pressure of the left hydraulic cylinder 33 is maintained at the pressure P1 corresponding to this neutral stop position, and the left side gear 12 is neutrally stopped. Stop at position. This is the state in which the transmission to the left traveling device 4 is cut off, and the aircraft gently turns to the left.

Next, when the operating lever 40 is operated from the first left turning position L1 to the second left turning position L2 (corresponding to the left turning position), control is performed in accordance with the operating position of the operating lever 40. The device 42 operates the variable relief valve 37 toward the throttle side. As a result, as the operating lever 40 is operated toward the left second turning position L2 side in accordance with the operating position of the operating lever 40 as shown by the solid line in FIG. 3, the operating oil pressure of the left hydraulic cylinder 33 rises. Then, the left side gear 12 presses and operates the second clutch RC (corresponding to the turning position of the side gear 12). Therefore, depending on the operating position of the operating lever 40, the pressing force of the left side gear 12 on the left second clutch RC is changed, and the transmission torque of the left second clutch RC is changed and adjusted.

In this case, as shown in FIG. 2, the switching lever 3
If 2 is operated to the super-spinning position and the left and right second gears 11 of the third transmission shaft 24 are engaged with the third gear 27, the left traveling device 4 is reversely driven and the super-spinning is performed. You can do it. On the contrary, if the switching lever 32 is operated to the gentle turning position and the second gears 11 on the left and right of the third transmission shaft 24 are directly engaged with the second transmission gear 9, the left traveling device 4 moves at a low speed. It can be driven to rotate gently. Here, as described above, the operating lever 40 is operated between the left first turning position L1 and the second turning position L2 to change the pressing force applied to the second clutch RC by the hydraulic cylinder 33 and the side gear 12. As a result, the turning radii of the super turning turn and the gentle turning can be arbitrarily changed. Then, at the left second turning position L2, the operating oil pressure of the hydraulic cylinder 33 becomes the pressure P2 at which the second clutch RC is completely engaged, and the turning radius of the super turning turn and the gentle turning becomes the minimum. . The above operation is similarly performed at the right first turning position R1 and the second turning position R2 (corresponding to the right turning position) (the above corresponds to the first control means).

In the state shown in FIG. 1, the switching lever 41 is operated to the quick turning position and the switching valve 31 is operated to the open position, and the operating lever 40 is moved to the first and second turning positions R1 and R1. Even if operated to R2, L1, L2, the switching valve 31
Is the first state in which the operation is held in the open position. The selection of the first state by the switching lever 41 is suitable when traveling on a work site having some traveling resistance, such as a relatively soft field. In this state, the operation lever 40 is slowly operated from the first turning position R1, L1 to the second turning position R2, L2 as described above, or the operation lever 40 is operated by the first turning position R1, L1 and the second turning position. By operating and holding in the position between R2 and L2,
The turning radius of super turning and gentle turning can be changed arbitrarily.
Then, if the operating lever 40 is rapidly operated from the first turning position R1, L1 to the second turning position R2, L2, the transmission to one traveling device 4 is discontinued (the first clutch FC is disengaged). It is possible to rapidly start the super turning and gentle turning (corresponding to the second control means).

On the other hand, on a paved road having a relatively low running resistance, the switching lever 41 is operated to the soft turning position (corresponding to the second state). In this state, the operation lever 40
Is operated to the neutral position N, the left and right side gears 12
Engages with the first transmission gear 7 and goes straight. And
For example, when the operating lever 40 is operated to the left first turning position L1, the left side gear 12 is operated to the left side of the drawing from the first transmission gear 7 by the left hydraulic cylinder 33 as described above, and the machine body is gently moved. Turn left to go. During this time, the switching valve 31 in FIG. 1 is operated and held in the open position.

Next, when the operation lever 40 is started to be operated from the first left turning position L1 to the second left turning position L2, the control device 42 first switches the switching valve 31 shown in FIG. After that, the variable relief valve 37 is operated toward the throttle side in accordance with the operation position of the operation lever 40 as described above. As a result, as the operating lever 40 is operated toward the left second turning position L2 side in accordance with the operating position of the operating lever 40 as shown by the one-dot chain line in FIG. 3, the operating hydraulic pressure of the left hydraulic cylinder 33 is increased. It will rise gradually.

In this case, as shown in FIG. 2, the switching lever 3
If 2 is operated to the super-spinning position and the left and right second gears 11 of the third transmission shaft 24 are engaged with the third gear 27, the left traveling device 4 is reversely driven and the super-spinning is performed. You can do it. On the contrary, if the switching lever 32 is operated to the gentle turning position and the second gears 11 on the left and right of the third transmission shaft 24 are directly engaged with the second transmission gear 9, the left traveling device 4 moves at a low speed. It can be driven to rotate gently. Then, as described above, the operation lever 40 is provided between the left first turning position L1 and the left turning position L2.
By operating, the turning radius of the super turning turn and the gentle turning can be arbitrarily changed. Then, the left second turning position L2
In, the turning radius of the super-spinning turn and the gentle turn is minimized. The above operation is similarly performed at the right first turning position R1 and the second turning position R2 (the above corresponds to the first control means).

As described above, in the state where the switching lever 41 is operated to the soft turning position (the state indicated by the alternate long and short dash line in FIG. 3),
Manipulated to the quick turn position (solid line in Fig. 3)
Than the operating pressure of the hydraulic cylinder 33 is higher than the switching valve 3
It is suppressed by the action of the diaphragm position of 1. As a result, the operating lever 40 is moved to the first turning position R1,
By slowly operating from L1 to the second turning position R2, L2, or by operating and holding the operating lever 40 at a position between the first turning position R1, L1 and the second turning position R2, L2, The turning radius of turning and gentle turning can be finely changed and adjusted. Then, the operation lever 40 is moved to the first turning position R
Even if the first turning position L1, L1 is rapidly changed to the second turning positions R2, L2, the increase in the working hydraulic pressure of the hydraulic cylinder 33 is suppressed by the action of the throttle position of the switching valve 31, so that the aircraft quickly moves to the supercritical position. There is no start of turning or gentle turning (the above corresponds to the second control means).

Next, a structure for temporarily holding the machine body by operating and holding the left and right side gears 12 at the neutral stop position will be described. As shown in Fig. 1, it is possible to switch between a closed position that gives sufficient resistance to the discharge of hydraulic oil (the throttle opening at this position can be adjusted) and a fully open position. An on-off valve 30 biased to the side is provided in an oil discharge passage 29 from the control valve 35 to both hydraulic cylinders 33. With the above structure, when the opening / closing valve 30 is operated to the closed position while the operation lever 40 is operated to the neutral position N (the control valve 35 is operated to the neutral position), the hydraulic oil from the pump 34 Left and right hydraulic cylinders 3
3 are simultaneously supplied to the left and right hydraulic cylinders 33, and the left and right side gears 12 are operated to be separated from the first transmission gear 7 left and right. In this case, since the operation lever 40 is operated to the neutral position N and the variable relief valve 37 is in the fully open state,
At the neutral stop position before the left and right side gears 12 press the second clutch RC, the drain oil passage 36 opens and hydraulic oil escapes from the variable relief valve 37, so that the left and right side gears 12 stop at this neutral stop position. As a result, the power transmission to the left and right traveling devices 4 is cut off, and the aircraft temporarily stops.

[Other Embodiment] In the embodiment of FIGS. 1 and 2,
The first and second states are selected by the switching lever 41. However, when the switching lever 41 is abolished and the switching lever 32 of FIG. The second state may be set by operating to the turning position. Further, in the case of such a configuration, the switching lever 32 may be operated to the super-spot turning position or to be switched from the second state to the first state.

In the embodiment shown in FIG. 2, a state in which a low speed forward rotation power is transmitted to the second transmission gear 9 (a gentle turning state),
It is configured to be switchable between two states, that is, a state in which reverse rotation power is transmitted (a super-spinning state).
On the other hand, it may be configured such that only the low speed forward rotation power is transmitted, or that only the reverse rotation power is transmitted to the second transmission gear 9. In the embodiment shown in FIG. 2, the first clutch FC is an occlusal type and the second clutch RC is a friction type.
Both the first and second clutches FC and RC may be friction clutch type.

It should be noted that although reference numerals are given in the claims for convenience of comparison with the drawings, the present invention is not limited to the configurations of the accompanying drawings by the entry.

[Brief description of drawings]

1] Hydraulic cylinders for left and right side gears, control valves,
Hydraulic circuit diagram of switching valve and variable relief valve

FIG. 2 is a schematic diagram showing a transmission structure in a mission case.

FIG. 3 is a diagram showing a relationship between an operation position of an operation lever and an operating oil pressure of a hydraulic cylinder.

[Explanation of symbols]

 4 Traveling device 7 1st transmission gear 9 2nd transmission gear 12 Side gear 31 Switching valve 33 Hydraulic actuator 35 Control valve 37 Variable relief valve 40 Operation lever N Neutral position of operation lever R2, L2 Right or left turning position of operation lever RC Friction clutch

Claims (1)

[Claims] 1. A first transmission gear (7) in which a forward rotation power for straight traveling is transmitted to each of a pair of left and right side gears (12) which transmits power to each of a pair of left and right traveling devices (4). ) And a second transmission gear (9) to which low-speed forward rotation power or reverse rotation power is transmitted, and a friction clutch (RC) is provided between the second transmission gear (9) and the side gear (12). ) Is arranged, and the left and right side gears (12) are interlockingly connected to the first transmission gear (7) in a straight-ahead position, and the left and right side gears (12) press and operate the friction clutch (RC). Then, a pair of hydraulic actuators (33) that can independently slide the left and right side gears (12) over the turning position where the left and right side gears (12) are interlockingly connected with the second transmission gear (9) are installed. Prepared and selected The square of the hydraulic actuator (33), the control valve supplies the working oil to the sliding operation side gears (12) in the pivoted position side (35), wherein the control valve (3
5) A variable relief valve (37) capable of changing and adjusting the working oil pressure supplied to the hydraulic actuator (33), and a throttle for suppressing a sudden increase in the working oil pressure supplied from the control valve (35) to the hydraulic actuator (33). And a switching valve (31) that is switchable to a position and an open position that allows a sudden rise, and the operating lever (40) for turning operation that is manually operated is operated to the neutral position (N). Control valve (3
5) to the neutral position to move the left and right side gears (1
2) is moved to the straight ahead position, and when the operation lever (40) is operated to the right or left turning position (R2), (L2) side, the control valve (35) is switched to turn right or left. Side hydraulic actuator (33) is supplied with hydraulic oil, and the operating lever (40) is at the right or left turning position (R).
2), the first control means for operating the variable relief valve (37) so that the operating pressure of the hydraulic actuator (33) increases as it is operated toward the (L2) side, and the operating lever (40). ) Right or left turning position (R2),
The switching valve (3) is operated after the control valve (35) is switched by the first control means in conjunction with the operation to the (L2) side.
1) is operated and held in the open position, and thereafter the first control means allows the variable relief valve (37) to be operated, and the right or left turning position (R) of the operation lever (40).
2), the switching valve (31) is operated and held at the throttle position after the control valve (35) is switched by the first control means in synchronism with the operation toward the (L2) side. 1
A traveling power transmission structure for a work vehicle, comprising a second control means capable of setting change to a second state in which the variable relief valve (37) can be operated by the control means.