JP3612419B2 - Work vehicle - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention includes a drive type left and right front wheel configured to be steerable and a drive type left and right rear wheel, and the peripheral speed of the front wheel is adjusted in conjunction with the steering operation of the front wheel. More specifically, the work vehicle having the control means for increasing the speed of the peripheral speed of the vehicle automatically switches the front wheels to the accelerated state and prevents the acceleration based on the traveling speed of the vehicle body during the steering operation. It relates to the technology to switch to.
[0002]
[Prior art]
As a work vehicle configured as described above, there is one shown in Japanese Patent Laid-Open No. 5-262151. In this conventional example, the traveling speed of the vehicle body is measured, and the vehicle body travels at a lower speed than the predetermined traveling speed. The control operation is set so as to allow the front wheel to speed up only when the steering operation is performed in a state where the vehicle is in the vehicle.
[0003]
[Problems to be solved by the invention]
Here, the direction of travel when traveling at an extremely low speed, such as immediately after starting the engine of the vehicle body with the front wheel being steered and immediately after starting from a stopped state at an intersection on the road, etc. Considering the case of changing, in the case where acceleration of the front wheels is allowed during low-speed traveling as in the conventional example, a sudden vehicle turn is started simultaneously with the start of traveling after the engine is started. Alternatively, when the steering handle operation is started at the time of starting traveling at this intersection or the like, the vehicle body turns at a turning radius corresponding to the steering operation angle at the beginning of the operation, but the steering angle of the front wheels exceeds a predetermined value. As a result, the speed of the front wheels is increased, resulting in a sudden turn of the vehicle body with a turning radius smaller than the steering wheel operation angle. There is room for improvement in terms of driving the vehicle in a direction against the will. In particular, as described in the conventional example, the above phenomenon appears more prominently when braking the rear wheel inside the turn in conjunction with the speed increase of the front wheel. There has been a demand for a work vehicle that can turn the vehicle body with a certain sense of operation.
[0004]
The purpose of the present invention is to rationalize a work vehicle that can turn at a small radius by increasing the speed of the front wheels during work and can prevent turning at a small radius against the will of the operator when traveling at a very low speed. It is in the point to be configured.
[0005]
[Means for Solving the Problems]
A first feature of the present invention (Claim 1) is, as described at the beginning, provided with a drive type left and right front wheels configured to be steerable and a drive type left and right rear wheels. In the working vehicle provided with the control means for increasing the peripheral speed of the front wheel in comparison with the peripheral speed of the rear wheel in conjunction with the steering operation of the vehicle, the vehicle has a vehicle speed determination means for determining the traveling speed of the vehicle body, and the vehicle speed determination means When the traveling speed discriminated in (1) is equal to or higher than the high speed side set value, and smaller than the high speed side set value and less than the low speed side set value set to the lower limit of the traveling speed during work. Is provided with check means for checking and preventing the front wheel from appearing in the accelerated state, and its operation and effect are as follows.
[0006]
A second feature of the present invention (Claim 2) is that in Claim 1, when the appearance of the acceleration state of the front wheel is inhibited by the restraining means, it is informed that it is in the restraining state. The function and the effect are as follows.
[0007]
According to a third aspect (Claim 3) of the present invention, in Claim 1, a braking device that brakes the left and right rear wheels independently is provided, and a traveling speed determined by the vehicle speed determining means is set to the high speed side setting. If the value is equal to or higher than an intermediate setting value that is an intermediate speed between the value and the low speed side setting value, the control operation of the control means is set so as to prevent the braking device from appearing in the braking state. The operation and effect are as follows.
[0008]
According to a fourth aspect of the present invention (Claim 4), in Claim 3, when the traveling speed discriminated by the vehicle speed discriminating means is less than the set value on the low speed side, the braking state of the braking device appears. The control operation of the control means is set so as to inhibit the control, and its operation and effect are as follows.
[0009]
According to a fifth feature of the present invention (Claim 5), in Claim 3 or 4, a mode in which only front wheel speed increase is allowed in conjunction with the steering operation of the front wheel, and the front wheel speed is turned simultaneously. The switch is provided with a switch for selecting one of the modes for braking the inner rear wheel, and its operation and effect are as follows.
[0010]
[Action]
According to the first feature, when the front wheel is steered, the front wheel is only operated when it is determined by the determining means that the traveling speed of the vehicle body is less than the high speed side set value and higher than the low speed side set value. The increase in speed is allowed. In other words, just set the high speed side set value to the upper limit of the running speed during work, and set the low speed side set value to the lower limit of the running speed during work. Even if the steering wheel is operated, the speed of the front wheel is prevented by the restraining means, and even if the steering wheel is operated at a very low speed, such as immediately after starting from a stopped state, the front wheel speed increases. A sudden turn will be prevented.
[0011]
According to the second feature, when the speed increase of the front wheel is blocked by the checking means, the notification means is notified that the speed increase of the front wheel is being checked, and the operator is notified by the notification means. It is possible to grasp that the vehicle is in a restrained state.
[0012]
According to the third feature, when the front wheel is steered, it is determined that the traveling speed of the vehicle body is equal to or higher than an intermediate set value that is an intermediate speed between the high speed side set value and the low speed side set value. Only when it is determined, braking of the braking device inside the turn is permitted. In other words, when the rear wheel inside the turn is braked at the same time as the acceleration of the front wheel, the vehicle turns sharply. Therefore, by setting the intermediate setting value that allows this braking to a lower speed than the high-speed side set value, the vehicle runs at high speed. It becomes possible to turn with a small radius while suppressing sudden turning at the time.
[0013]
According to the fourth feature, the rear wheel is not braked even during low-speed traveling in which the speed increase of the front wheel is prevented, and rapid turning is prevented.
[0014]
According to the fifth feature, any one of turning with a small radius only by increasing the speed of the front wheel at the time of working and turning at a very small radius by speeding up the front wheel and braking of the rear wheel inside the turning at the time of work. This selection can be performed by operating a switch, and an appropriate turning mode can be selected in accordance with the work mode and the state of the field.
[0015]
〔The invention's effect〕
Therefore, only when it is determined that the vehicle is in the working state based on the traveling speed, the speed of the front wheels is allowed to perform turning with a small radius to perform efficient work and travel on the road at high speed. And, when traveling in a non-working state such as traveling on the road at a very low speed, the turning of the vehicle body may be disrupted by preventing the turning with a small radius due to the acceleration of the front wheels, or turning against the will of the operator A work vehicle that does not carry out is rationally constructed (claim 1). In addition, not only can the operator recognize that the speed increase of the front wheels has been restrained by the lighting of the lamp, the display of the liquid crystal display, the operation of the buzzer, etc. as a notification means, but for example, the traveling speed can be increased too much during work Even when the traveling speed is reduced too much during work and the increase in the speed of the front wheels is prevented from being restrained, it is possible to set the traveling speed appropriately based on the notification (claim 2). At the same time, by setting the upper traveling speed that allows the braking operation of the rear wheel inside the turn to be lower than the speed at which the front wheel can be accelerated, only the front wheel is increased while suppressing sudden turning at high speed. This makes it possible to turn with a smaller radius than when the vehicle speed is increased, thereby enabling efficient work (Claim 3), and when the vehicle is traveling in a non-working state, such as traveling at a very low speed, Speed increase and rear wheel control This makes it possible to maintain turning with a stable posture of the vehicle body (Claim 4), and by selecting a turning mode required by switch operation, it is possible to turn in a form appropriate for the work. (Claim 5).
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
As shown in FIGS. 1 and 2, an engine 3 is disposed at the front of a vehicle body having a front wheel 1 and a rear wheel 2, and a main clutch in which power from the engine 3 is housed in a main clutch housing 4 is provided. The transmission case 5 is disposed at the rear part of the vehicle body, and is provided with a pair of left and right lift arms 7 that are driven up and down by a lift cylinder 6 at the rear end upper position of the transmission case 5, and a meter at the center part of the vehicle body A panel 8, a steering handle 9, and a driver seat 10 are disposed, and a position lever 11 for controlling the lift arm 7 is disposed on the right side of the driver seat 10. An agricultural tractor as an example of a work vehicle is configured by disposing a main transmission lever 12 for shifting the transmission of the transmission.
[0017]
A rotary tilling device 14 as a ground working device is detachably connected to the rear end position of the vehicle body via a two-point link mechanism 13, and between the two-point link mechanism 13 and the left and right lift arms 7. Further, the rotary tiller 14 can be moved up and down by the driving force of the lift cylinder 6 by interposing a pair of left and right lift rods 15. Further, a grounding type rear cover 14A is provided at the rear end position of the rotary tiller 14 so as to be swingable around the axis P in a lateral direction. The ground height of the rotary tiller 14 is determined from the swinging position of the rear cover 14A. A potentiometer type cover sensor 16 for measuring the thickness is provided.
[0018]
As shown in FIG. 2, a main clutch pedal 18 for disengaging the main clutch by a stepping operation is provided on the left side of step 17, and the right side of step 17 is independent of the left and right rear wheels 2 and 2. And a pair of left and right side brake pedals 19 for applying a braking force. Further, a steering operation system is formed which transmits the operating force of the steering handle 9 to the left and right front wheels 1 and 1 via a power steering mechanism (not shown).
[0019]
As shown in FIGS. 1, 3, and 4, a front wheel transmission device A is provided at a lower position of the transmission case 5, and the front wheel transmission device A has a peripheral speed of the front wheel 1 and a peripheral speed of the rear wheel 2. An equal speed drive state that equalizes, a speed increase drive state in which the peripheral speed of the front wheel 1 is increased from the peripheral speed of the rear wheel 2 in order to turn the vehicle body with a small radius, and a two-wheel drive that does not transmit power to the front wheel 1 It is configured to be able to reveal the state. That is, the front wheel transmission A is an intermediate shaft through which the power from the input shaft 21 for inputting the power from the transmission 5A of the transmission case 5 to the differential 2A of the rear wheel 2 is transmitted via the gear interlocking mechanism 22. 23, a front wheel drive shaft 24 supported in parallel with the intermediate shaft 22, a friction multi-plate standard clutch 26 for transmitting the power from the intermediate shaft 23 to the front wheel drive shaft 24 through a standard gear mechanism 25, A friction multi-plate type speed-up clutch 28 that transmits power from the shaft 23 to the front wheel drive shaft 24 through the speed-up gear mechanism 27 is provided. The standard clutch 26 and the speed-up clutch 28 are respectively engaged by supplying pressure oil. It is configured to reach the state and transmit power. Further, a transmission system is formed for transmitting the power from the front end of the front wheel drive shaft 24 to the differential device 1A of the front wheel 1 via the intermediate transmission shaft 29.
[0020]
In addition, friction side brakes 31 and 31 are provided as braking devices for applying a braking force to the axles 2B and 2B for the left and right rear wheels 2 and 2, and the side brake 31 is provided with an internal piston by supplying pressure oil. Is operated so that the friction plates are brought into pressure contact with each other, and a braking force proportional to the pressure oil pressure can be applied to the rear wheel 2. Then, an electromagnetically operated switching valve 32 for controlling pressure oil for the standard clutch 26 and the speed increasing clutch 27, left and right electromagnetic proportional brake valves 33, 33 for the left and right side brakes 31, 31, and the lift cylinder 6 And a hydraulic pump 35 driven by the engine 3 to supply pressure oil to these valves 32, 33, 33, 34. Yes. The brake valves 33 and 33 are each of an electromagnetic proportional type that obtains an opening degree proportional to the current value supplied to the solenoid.
[0021]
As shown in FIG. 3, a potentiometer-type steering sensor 36 for measuring the steering operation amount of the steering operation system is provided, and a brake for detecting these stepping operations in the vicinity of the left and right side brake pedals 19, 19 is provided. A pickup type vehicle speed sensor 38 (vehicle speed discrimination) that includes sensors 37 and 37 and measures the traveling speed of the vehicle body at a position close to the gear provided to the input shaft 21 that transmits power to the differential 2A of the rear wheels 2 and 2. And a pickup type engine speed sensor 39 for measuring the rotational speed of the engine 3 at a position close to the gear provided on the output shaft 3A of the engine 3.
[0022]
As shown in FIG. 5, the meter panel 8 includes a dial 41 for setting the driving mode of the front wheel 1, and the dial 41 maintains a state of driving only the rear wheel 2, and a “2WD” position. The “4WD” position where the constant speed drive state appears, and when the front wheel 1 is steered more than a set angle, the speed increasing power from the front wheel transmission A is transmitted to the front wheel 1 and the speed increasing drive. When the front wheel 1 is steered more than a set angle, the speed increasing power from the front wheel transmission device A is transmitted to the front wheel 1 and at the same time the rear wheel 2 inside the turn at the same time. The “double speed / AD” position for braking the side brake 31 can be set. The meter panel 8 includes a liquid crystal display 50, a lamp 51, and a speaker 52 as notification means.
[0023]
As shown in the figure, the tractor includes a control device 40 as a control means including a microprocessor. A mode switch 42 for determining the setting position of the dial 41 with respect to the control device 40; Steering sensor 36, left and right brake sensors 37, 37, vehicle speed sensor 38, engine speed sensor 39, potentiometer type lever sensor 43 that measures the operation position of position lever 11, and lift arm A potentiometer-type lift arm sensor 44 that measures the amount of swing of the rotary cultivator, a potentiometer-type plowing depth setting unit 45 that sets a target plowing depth of the rotary tiller 14, and a system to which signals from the cover sensor 16 are input. The standard solenoid on the operation side of the switching valve 32 with the standard clutch 26 is formed. 32L, a speed increasing solenoid 32H on the operation side of the speed increasing clutch 28 of the switching valve 32, a brake solenoid 33S, 33S of each of the brake valves 33, 33, and a lift solenoid 34U for operating the lift valve 34 to the lift side. In addition, a system for outputting a signal to each of the descending solenoids 34D for operating the elevating valve 34 to the lower side is formed, and an output signal system for the liquid crystal display 50, the lamp 51, and the speaker 42 is formed.
[0024]
Further, a coupler 46 that can be separated from the input signal system from the cover sensor 16 to the control device 40 is interposed so that the signal system can be separated when the rotary tiller 14 is separated from the vehicle body. When the coupler 46 is separated by providing a pull-up resistor 47 on the signal line on the control device 40 side, the voltage of the signal line is raised to the power supply voltage to indicate that the coupler 46 is in the separated state from the voltage state. It is configured so that it can be determined.
[0025]
When the position lever 11 is operated, the setting position of the position lever 11 measured by the lever sensor 43 is set to the target height with respect to the vehicle body, and the dead zone is formed based on the target height. The position control mode is set so that the rotary tiller 14 is moved up and down until the signal value measured by the lift arm sensor 44 is reached. When the position lever 11 is set to the lowest position, the automatic tilling control is performed. In addition, in this automatic plowing depth control, a target plowing depth based on the ground is set by the plowing depth setting unit 45, and the cover sensor 16 is within the dead zone formed based on the target plowing depth. The form of control is set so that the rotary tiller 14 is moved up and down until the signal value measured at is reached.
[0026]
The control device 40 is set with a program for controlling the front wheel transmission A and the side brake 31 in addition to the lifting control of the rotary tiller 14 described above. That is, as shown in the flowchart of FIG. 6, when the signal from the mode switch 42 is input to determine the driving mode of the front wheel 1 and the switch 2 is operated to the “2WD” position, the switching valve 32 is operated to the neutral position. Then, the state where the transmission to the front wheel 1 is cut off is maintained (steps # 101 to # 103), and when the mode switch 42 is operated to the position of “4WD”, the standard solenoid 32L of the switching valve 32 is turned off. The four-wheel drive state is maintained by driving (steps # 104 and # 105). Otherwise, the standard solenoid 32L of the switching valve 32 is driven to set the four-wheel drive state, and the lift arm sensor When it is determined that the lift arm 7 is not at the upper limit based on the signal from 44, the signal from the cover sensor 16 exists within a predetermined voltage range, and the rotary tiller 14 is If it is determined that the connection is established (steps # 106 to # 110), the turning control routine is processed (step # 200). If these conditions are not satisfied, the standard solenoid 32L of the switching valve 32 is used. The four-wheel drive state is maintained by maintaining this drive state, and this control is continued until reset (step # 111).
[0027]
The reason why the turning control routine is inhibited when the lift arm 7 is at the upper limit is that the traveling with the ground working device such as the rotary tiller being raised to the upper limit is in a non-working state, so the small radius is required. The reason why the turning control routine is blocked when the signal input from the cover sensor 16 does not exist within the predetermined voltage range is that the signal voltage input to the control device 40 is substantially the same. This is because, when the power supply voltage is increased, the rotary tiller 14 is separated from the vehicle body and is in a non-working state, and turning at a small radius is not necessary during such non-working.
[0028]
As shown in the flowchart of FIG. 7, in the turning control routine (# 200 step), a signal from the steering sensor 36 is input to determine whether or not the front wheel 1 has been operated by the set angle θ or more. If not, the brake valve 33 is held in the neutral position and the side brake 31 is kept in the non-braking state (only when the brake sensor 37 detects that the side brake pedal 19 has been operated) 31 is braked), and the driving state of the standard solenoid 32L of the switching valve 32 is maintained to maintain the four-wheel driving state (steps # 201 to # 204).
[0029]
Next, when the front wheel 1 is steered by an angle θ or more, as shown in the flowchart and FIG. 8, a signal from the vehicle speed sensor 38 is input and the traveling speed is set to the first set speed (high speed). Side set value · 5 km / hour) and whether it is the third set speed (low speed side set value · 0.2 km / hour) or more. If this condition is not satisfied, the liquid crystal display 50, The lamp 51 and the speaker 52 are driven to notify the operator that the front wheel 1 is not accelerated (steps # 205 to # 208). When this condition is satisfied, the operation position of the mode switch 42 is determined. If the travel speed is less than the second set speed (intermediate set value 3.6 Km / hr) based on the signal from the vehicle speed sensor 38 when the vehicle is at the “double speed / AD” position Of the side brake 31 The solenoid 33S of the key valve 33 is driven to brake the side brake 31 inside the turn, the rotary tiller 14 is raised to the upper limit, and the speed increasing solenoid 32H of the switching valve 32 is driven to increase the speed of the front wheel transmission A. The clutch 28 is operated to increase the speed of the front wheel 1, and the turning at this small radius is continued until the front wheel 1 is restored to a value less than the set angle θ (# 209-). Step # 214). In addition, the control means is configured by the program that performs the control operation for preventing the increase in the speed of the front wheel 1 based on the traveling speed.
[0030]
That is, as shown in FIG. 8, since the first set speed is set to the upper limit value of the travel speed used at the time of work, when traveling at a speed higher than the first set speed, the set angle θ or more is set. Because the front wheel 1 is not accelerated and the rear wheel 2 inside the turn is not braked even if the steering operation is performed, the third set speed is set to the lower limit value of the traveling speed used during the work. When the vehicle is traveling at a speed lower than the third set speed, the front wheel 1 is not accelerated and the rear wheel 2 on the inside of the turn is not braked even if the steering operation of the set angle θ or more is performed. The second set speed is set to the value of the traveling speed at which the turning is abrupt when the rear wheel 2 inside the turn is braked while the front wheel 1 is accelerated. Turning of steering operation with a set angle θ or more when traveling at a speed higher than the first set speed and lower than the first set speed When the operation is performed, the front wheel 1 is accelerated, but the rear wheel 2 on the inside of the turn is not braked. Further, it is less than the second set speed and greater than or equal to the third set speed. Only when the steering operation of the set angle θ or more is performed during traveling on the road, the acceleration of the front wheel 1 and the braking operation of the rear wheel 2 inside the turn are performed.
[0031]
The brake valve 33 is an electromagnetic proportional type that obtains an opening degree that is directly proportional to the value of the control current supplied to the solenoid 33S. Therefore, the braking force can be adjusted by the current supplied to the solenoid 33S. It has become a thing. And although not described in the flowchart, by increasing the current value supplied to the solenoid 33S as the traveling speed is higher, the piston of the brake 31 is rapidly operated to start braking. The control operation of the control device 40 is set so as to shorten the time and perform a turn at a desired position. The controller 40 is configured to send an intermittent signal to the solenoid 33S, and can adjust the current value supplied to the solenoid 33S in a PWM manner by setting the duty ratio of the intermittent signal. In order to accurately control this current value, the pressure of hydraulic oil supplied from the hydraulic pump 35 is obtained based on the rotational speed of the engine 3 measured by the engine speed sensor 39 and supplied to the solenoid 33s of the brake valve 33. The control operation of the control device 40 is set so as to correct the current value.
[0032]
Further, when the steering wheel is steered more than the set angle θ as described above, if the speed increase of the front wheel 1 is prevented from being restrained, the lamp 51 is blinked and at the same time the front wheel 1 with respect to the liquid crystal display 50. By displaying characters such as “The speed is fast, so double speed is being controlled” and that the same content is reported in an electronically synthesized human language from the speaker 52 The control operation of the control device 40 is set so as to notify the worker of the check state.
[0033]
As described above, in the present tractor, when the operation is performed in the state where the mode switch 42 is set to either the “double speed” position or the “double speed / AD double speed” position, the front wheel is automatically linked with the steering operation. 1 or the front wheel 1 and the rear wheel 2 on the inside of the turn are automatically braked to enable turning with a small radius. For example, the mode switch 42 is moved to the “double speed” position. Even when the road is driven at a very low speed without raising the rotary tiller 14 to the upper limit in the state of being set to the “double speed / AD double speed” position, the necessity of turning at a small radius based on the traveling speed is reduced. Judgment is made to prevent the speed increase of the front wheel 1 and the braking operation of the rear wheel 2 on the inside of the turn. Avoid turning and feel comfortable It has become a thing that enables a steering operation in good operation feeling.
[0034]
Further, when the rotary tiller 14 is not connected to the vehicle body, even when the rotary tiller 14 is connected to the vehicle body, the steering operation exceeding the set angle θ is performed even when the rotary tiller device 14 is raised to the upper limit position. Even if it is carried out, the front wheel 1 is not increased in speed, so that the operator's sense of operation of the steering wheel is not distorted, and the vehicle can run with a stable posture of the vehicle body. In particular, when the rotary tiller 14 is in contact with the ground during operation, the rotary tiller 14 is operated in conjunction with the steering operation without specially performing an operation of raising the rotary tiller 14. Since the control is performed to raise the upper limit, the labor of the operator is saved, and an easy work is enabled.
[0035]
[Another embodiment]
In addition to the above-described embodiment, the present invention can be configured so that the value of the first set speed or the second set speed can be artificially adjusted by an operating tool such as a dial, and the travel speed is determined. It is also possible to configure the vehicle speed discriminating means to be a program for obtaining the speed based on the operation position of the shift lever and the engine speed. In addition, a switch for selecting a mode in which only the front wheel speed is increased in conjunction with the steering operation and a mode in which the rear wheel is braked inside the turn at the same time as the speed increase may be configured as a push button. Is possible.
[Brief description of the drawings]
[Fig. 1] Overall side view of tractor [Fig. 2] Plan view of front of tractor [Fig. 3] Plan view of transmission system of vehicle body [Fig. 4] Hydraulic circuit diagram of vehicle body [Fig. 5] Block circuit diagram of control system [Fig. FIG. 6 is a flowchart of the main control operation. FIG. 7 is a flowchart of a turning control routine. FIG. 8 is a diagram showing a correspondence relationship between travel speed and control mode.
DESCRIPTION OF SYMBOLS 1 Front wheel 2 Rear wheel 31 Brake device 38 Vehicle speed discrimination means 40 Control means 42 Switch

Claims (5)

It has drive type left and right front wheels configured to be steerable and left and right drive type rear wheels, and the peripheral speed of the front wheels is derived from the peripheral speed of the rear wheels in conjunction with the steering operation of the front wheels. A work vehicle having a control means for increasing the speed,
Vehicle speed discriminating means for discriminating the travel speed of the vehicle body is provided, and when the travel speed discriminated by the vehicle speed discriminating means is equal to or higher than the high speed side set value, and a value smaller than the high speed side set value , A work vehicle provided with a check means for checking and preventing the appearance of the speed-up state of the front wheels when the speed is less than the low-speed set value set to about the lower limit of the traveling speed at . 2. The work vehicle according to claim 1, further comprising a notifying means for notifying that the front wheel is in a check-inhibited state when the appearance of the acceleration state of the front wheel is inhibited by the check-up means. A braking device that independently brakes the left and right rear wheels, and a traveling speed determined by the vehicle speed determining means is equal to or higher than an intermediate setting value that is an intermediate speed between the high-speed side setting value and the low-speed side setting value; 2. The work vehicle according to claim 1, wherein in some cases, the control operation of the control means is set so as to prevent the braking device from appearing in a braking state. The control operation of the control means is set so as to prevent the braking device from appearing when the traveling speed determined by the vehicle speed determining means is less than the low speed side set value. 3. Work vehicle according to 3. Provided with a switch for selecting either a mode that allows only the front wheel speed increase in conjunction with the steering operation of the front wheel or a mode that brakes the rear wheel inside the turn at the same time as the front wheel speed increase The work vehicle according to claim 3 or 4.

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