Transport vehicle

A control valve device (19) controls supply and discharge of pressurized oil to and from a hoist cylinder (12) which tilts a vessel (3). A throttle (24C) that limits a flow rate of hydraulic oil which is discharged from a hydraulic pump (13) and flows in a return line (16) through a pump line (15), a first directional control valve (24) and a center bypass oil passage (23) is provided in the floating position (F) of the first directional control valve (24). When the first directional control valve (24) is in the floating position (F), the upstream side of the throttle (24C) is connected to the pump line (15), and the downstream side of the throttle (24C) is connected to the return line (16). In addition, a rod-side actuator line (18) communicated with rod-side oil chambers (12E, 12F) in the hoist cylinder (12) is connected to the pump line (15).

TECHNICAL FIELD

The present invention relates to a transport vehicle such as a dump truck which is suitably used in transporting, for example, crushed stones or earth and sand excavated in an open-pit digging site, a stone quarry or a mine.

BACKGROUND ART

In general, a large-sized transport vehicle called a dump truck is provided with a vessel (loading platform) on a frame of a vehicle body tiltably in an upper-lower direction on a basis of the rear side. The transport vehicle carries and transports objects (for example, crushed stones or earth and sand) to be transported to a transportation destination (cargo unloading site or cargo collection site) in a state where the objects to be transported are loaded in a large amount on the vessel (Patent Document 1).

A transport vehicle of this type according to the conventional art comprises schematically an automotive vehicle body, a vessel which is provided on the vehicle body tiltably (liftably) in an upper-lower direction on a basis of the rear side and on which the objects to be transported are loaded, a hoist cylinder which is provided between the vessel and the vehicle body and tilts the vessel upward or downward with expansion or contraction of a rod therein, a hydraulic source comprising a hydraulic oil tank for reserving hydraulic oil and a hydraulic pump for delivering pressurized oil as the hydraulic oil to supply the pressurized oil to a bottom-side oil chamber or a rod-side oil chamber in the hoist cylinder for expanding or contracting the hoist cylinder, and a control valve device provided between the hydraulic source and the hoist cylinder to control supply and discharge of the pressurized oil to and from the bottom-side oil chamber or rod-side oil chamber in the hoist cylinder.

The control valve device used in the transport vehicle has a total of four switching positions composed of a raising position of supplying the pressurized oil from the hydraulic pump to the bottom-side oil chamber and discharging the hydraulic oil in the rod-side oil chamber to the hydraulic oil tank to expand the hoist cylinder for upward tilt (rotation) of the vessel, a lowering position of supplying the pressurized oil to the rod-side oil chamber and discharging the hydraulic oil in the bottom-side oil chamber to contract the hoist cylinder and lower down (downward rotate) the vessel, a floating position of discharging the hydraulic oil in the bottom-side oil chamber by self-weight of the side of the vessel to contract the hoist cylinder and allow for self-weight fall of the vessel, and a neutral position of stopping supply and discharge of the pressurized oil to and the from the hoist cylinder to stop movement of the hoist cylinder for holding the vessel.

Here, the control valve device is selectively switched to any switching position of the total of the four switching positions in response to a manual operation of an operating lever which an operator operates. The transport vehicle self-travels to the transportation destination in a state of loading the objects in the vessel and thereafter, expands the hoist cylinder to diagonally backward rotate the vessel. This raising operation allows the objects to be dumped to a cargo unloading site along the tilting direction of the vessel.

That is, when the operation of the operating lever by the operator switches the control valve device from the neutral position to the raising position, the pressurized oil from the hydraulic pump is supplied toward the bottom-side oil chamber in the hoist cylinder and the hydraulic oil in the rod-side oil chamber therein is discharged to the hydraulic oil tank. In consequence, the hoist cylinder expands to rotate the vessel in such a manner as to be greatly tilted to the vehicle body backward side, thus discharging (dumping) the loads in the vessel outside thereof in a sliding/falling manner.

On the other hand, after the dumping operation of the objects is completed, when the control valve device is switched from the raising position to, for example, the floating position by the operation of the operating lever, the hydraulic oil in the bottom-side oil chamber of the hoist cylinder is discharged by the self-weight of the side of the vessel to contract the hoist cylinder. Therefore, the vessel gradually goes down by the self-weight to a position of being seated on the vehicle body.

At traveling of the vehicle, for example, the operating lever is held to the floating position. Thereby, the vessel continues to be seated on the vehicle body by the self-weight, and the hoist cylinder also can be held in the contraction state by using the self-weight of the side of the vessel.

Incidentally, vehicle traveling paths of a crushed stone site or the like in a mine are dirt roads, and mostly have ragged road surfaces. In a transport vehicle traveling on such a road, vibrations caused by the bumpy road surface at traveling become larger in magnitude. At this time, in a case where the vessel is empty in loading, the vessel floats up from the vehicle body following vibrations caused by knocking-up from the road surface, and thereafter, the vessel collides with the vehicle body at the time when the vessel is again seated on the vehicle body. Therefore, uncomfortable feelings are given to the operator in the cab or repeated collisions of the vessel with the vehicle body degrade the durability and lifetime.

Therefore, in the transport vehicle according to Patent Document 1, in a case where the vehicle body is in the traveling state and the control valve device is in the floating position, when it is detected that the vessel moves in a direction of floating from the vehicle body, the control valve device is controlled from the floating position to the lowering position by a control unit. Thereby, when the vessel tends to float from the vehicle body, the control valve device is switched from the floating position to the lowering position, and thereby, the vessel can be pressed to the vehicle body side to restrict the floating-up of the vessel.

PRIOR ART DOCUMENT

Patent Document

SUMMARY OF THE INVENTION

In the transport vehicle according to the aforementioned Patent Document 1, for needing a sensor which detects that the vessel moves in the floating direction, there is a possibility of cost increase corresponding to a provision of the sensor. On the other hand, for example, depending on the bumpy degree of the road surface or the traveling speed, there are some cases where a floating height of the vessel is low and a floating time thereof is very short. In this case, when the floating of the vessel is detected, there is a possibility that the vessel falls from a point where the control valve device is switched from that detection position to the lowering position to a point of pressing the vessel to the side of the vehicle body, and then, collides with the vehicle body.

In view of the aforementioned problems in the conventional art, it is an object of the present invention to provide a transport vehicle which can stably seat a vessel on a vehicle body at the time of traveling in a state where a control valve device is in a floating position.

(1) In order to solve the above-described problems, the present invention is applied to a transport vehicle comprising an automotive vehicle body; a vessel which is tiltably provided on the vehicle body in an upper-lower direction on a basis of the rear side and on which objects to be transported are loaded; a hoist cylinder which is provided between the vessel and the vehicle body and tilts the vessel upward or downward with expansion or contraction of a rod; a hydraulic source comprising a hydraulic oil tank for reserving hydraulic oil therein and a hydraulic pump for delivering hydraulic oil as the pressurized oil; a control valve device for controlling supply and discharge of the pressurized oil to and from the hoist cylinder; a pump line for establishing connection between the hydraulic pump and the control valve device; actuator lines for establishing connection between the control valve device and the hoist cylinder; and a return line for returning the hydraulic oil which is delivered from the hydraulic pump and passes through the control valve device back to the hydraulic oil tank, wherein the control valve device has a plurality of switching positions composed of a raising position of expanding the hoist cylinder with supply and discharge of the pressurized oil to tilt the vessel upward, a lowering position of contracting the hoist cylinder with supply and discharge of the pressurized oil to lower down the vessel, a floating position of contracting the hoist cylinder with a self-weight of the side of the vessel to allow for self-weight fall of the vessel, and a neutral position of stopping movement of the hoist cylinder by stopping supply and discharge of the pressurized oil to hold the vessel.

The configuration adopted by the present invention is characterized in that a flow rate limiting section is provided in the floating position of the control valve device to limit a flow rate of the hydraulic oil flowing in the return line from the hydraulic pump, wherein when the control valve device is in the floating position, then upstream side of the flow rate limiting section is connected to the pump line, the downstream side of the flow rate limiting section is connected to the return line, and a rod-side actuator line communicated with rod-side oil chambers in the hoist cylinder among the actuator lines is connected to the pump line.

With this arrangement, when the control valve device is in the floating position, the hydraulic oil which is delivered from the hydraulic pump and flows in the return line through the pump line and the control valve device is limited in the flow rate by the flow rate limiting section. Therefore, a pressure (back pressure) corresponding to the flow rate of the hydraulic oil flowing in the flow rate limiting section and the degree of the flow rate limit is kept in the pump line upstream of the flow rate limiting section. In this case, since the rod-side actuator line is connected to the pump line, the rod-side oil chambers in the hoist cylinder are pressurized by the pressure kept by the flow rate limiting section.

Thereby, a thrust force (contraction force) in a direction of contracting the rod is generated in the hoist cylinder, and the vessel can be pressed against the vehicle body by the hoist cylinder. As a result, when the vehicle travels in a state where the control valve device is in the floating position, even if the force of a direction of floating the vessel from the vehicle body is applied to the vessel following passage of the vehicle on the bumpy road surface, the floating of the vessel can be restricted by the thrust force of the hoist cylinder. In this case, since the thrust force of the hoist cylinder can be regularly generated by the flow rate limiting section, it is possible to stably seat the vessel on the vehicle body. In addition, since the sensor which detects the movement of the vessel is not necessary, it is possible to restrict the cost increase.

(2) According to the present invention, the flow rate limiting section is configured of a throttle which reduces a flow passage area. With this arrangement, since the flow rate limiting section is configured of the throttle, a desired pressure (back pressure) can be kept upstream of this throttle by setting the flow passage area of this throttle to an appropriate magnitude. That is, a thrust force (contraction force) necessary for restricting the floating of the vessel can be generated in the hoist cylinder while restricting a load of the hydraulic pump.

(3) According to the present invention, the control valve device comprises first and second directional control valves connected in parallel between the hydraulic source and the hoist cylinder, wherein the first directional control valve is switched to any position of the neutral position, the raising position and the floating position, and the second directional control valve is switched to any position of the neutral position, the raising position and the lowering position.

With this arrangement, when the first directional control valve is in the floating position, the vessel can be pressed against the vehicle body by the hoist cylinder. Thereby, the vessel can be stably seated onto the vehicle body regardless of passage of the vehicle on the bumpy road surface.

(4) According to the present invention, an actuator connecting oil passage which connects the outflow side of the control valve device to the actuator line and the return line or the hydraulic oil tank is provided, and a check valve is provided midway of the actuator connecting oil passage to allow flow of the hydraulic oil only to the side of the hoist cylinder from the return line or the hydraulic oil tank. With this arrangement, the hydraulic oil resupplied through the check valve from the return line or the hydraulic oil tank can prevent the oil chamber in the hoist cylinder from becoming a vacuum pressure.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a transport vehicle according to an embodiment of the present invention will be in detail explained with reference to the accompanying drawings by taking a dump truck for transporting crushed stones, and earth and sand excavated in a mine, as an example.

In the figure, designated at1is a dump truck which is a large-sized transport vehicle, and the dump truck1schematically comprises a vehicle body2having a rigid frame structure, a vessel3(loading platform) mounted on the vehicle body2tiltably (liftably) on a basis of the rear side, and front wheels7and rear wheels9to be described later by which the vehicle body2travels.

The vessel3is formed as a large-sized container whose overall length reaches as much as 10 to 13 meters to load a large volume of heavy objects to be transported, such as crushed stones, and earth and sand (hereinafter, called earth and sand4). A rear side bottom portion of the vessel3is tiltably coupled to the rear end side of the vehicle body2by using a connecting pin5. In addition, a protector3A is integrally provided on a front side top portion of the vessel3in such a manner as to cover a cab6to be described later from the upper side.

That is, the bottom side of the vessel3is rotatably supported by the rear side of the vehicle body2by using the connecting pin5. When a hoist cylinder12to be described later is expanded or contracted, the front side (side of the protector3A) of the vessel3is raised or lowered (tilted) vertically on a basis of a position of the connecting pin5. In consequence, the vessel3is rotated between a traveling position shown inFIG. 1and a dumping position shown inFIG. 2. For example, at the dumping position shown inFIG. 2, a large number of the earth and sand4loaded in the vessel3is dumped at a predetermined unloading site so as to slide down in an arrow Y direction from the vessel3which has tilted backward.

Indicated at6is the cab which is provided in the front portion of the vehicle body2to be located under the lower side of the protector3A. The cab6defines an operator's room where an operator of the dump truck1gets in/off. An operator's seat, an accelerator pedal, a brake pedal and a steering handle (any thereof is not shown), an operating lever38A to be described later (refer toFIG. 3andFIG. 4), and the like are provided inside the cab6.

The protector3A of the vessel3protects the cab6from flying stones such as rocks by substantially completely covering the cab6therewith from the upper side. The protector3A of the vessel3has a function of protecting the operator inside the cab6at the time the vehicle (dump truck1) falls down.

Right and left front wheels7(only one is shown) are provided rotatably on the front side in the vehicle body2, and the front wheels7are configured as steered wheels which are steered by the operator of the dump truck1. Further, each of the front wheels7is formed with a tire diameter (outer diameter dimension) of, for example, as much as 2 to 4 meters in the same way as each of rear wheels9to be described later. A front suspension8composed of hydraulic shock absorbers and the like is provided between the front portion of the vehicle body2and the front wheels7. The front suspension8suspends the front side of the vehicle body2between the front wheels7.

Right and left rear wheels9(only one is shown) are rotatably provided on the rear side in the vehicle body2, and the rear wheels9are configured as drive wheels of the dump truck1which are driven and rotated by a traveling drive unit (not shown). A rear suspension10composed of hydraulic shock absorbers and the like is provided between the rear portion of the vehicle body2and the rear wheels9. This rear suspension10supports the rear side of the vehicle body2between the rear wheels9.

An engine11as a prime mover is configured, for example, by a large-sized diesel engine or the like. The engine11is provided in the vehicle body2to be located under the cab6, and drives/rotates a hydraulic pump13(refer toFIG. 3toFIG. 5) to be described later and the like.

Designated at12are a pair of right and left hoist cylinders (only one thereof is shown) telescopically provided between the vehicle body2and the vessel3. As shown inFIG. 3toFIG. 5, this hoist cylinder12is formed of a multi-stage (for example, two-stage) hydraulic cylinder, a so-called telescopic hydraulic cylinder5. That is, the hoist cylinder12is configured by a cylindrical outer tube portion12A positioned at the outer side, a cylindrical inner tube portion12B one end of which is telescopically inserted in the outer tube portion12A and the other end of which projects outside of the outer tube portion12A, a rod12C one end of which is telescopically inserted in the inner tube portion12B and the other end of which projects outside of the inner tube portion12B, and a piston12D which is provided in one end side of the rod12C and slides in the inner tube portion12B.

The inside of the outer tube portion12A of the hoist cylinder12are divided into three chambers composed of rod-side oil chambers12E and12F, and a bottom-side oil chamber12G defined by the inner tube portion12B, the rod12C and the piston12D. In this case, the rod-side oil chamber12F is communicated with the rod-side oil chamber12E through a port12H provided in the inner tube portion12B. The hoist cylinder12tilts the vessel3upward or downward on a basis of the rear side by expansion or contraction of the inner tube portion12B and the rod12C.

Specifically, the hoist cylinder12is configured such that, when pressurized oil is supplied into the bottom-side oil chamber12G from the hydraulic pump13, the inner tube portion12B expands together with the rod12C downward, and when the inner tube portion12B expands at the maximum, only the rod12C further expands to the maximum expansion position downward. Thereby, the hoist cylinder12rotates the vessel3to a raising position (dumping position) of being diagonally backward tilted on a basis of the connecting pin5.

On the other hand, the hoist cylinder12is configured such that, when the pressurized oil is supplied inside the rod-side oil chamber12E from the hydraulic pump13in a state where the rod12C expands at the maximum, only the rod12C first contracts, and thereafter, the inner tube portion12B contracts to the maximum contraction position together with the rod12C. Therefore, the hoist cylinder12rotates the vessel3to the lowering position (traveling position) of being downward lowered on a basis of the connecting pin5.

Next, the hydraulic circuit for driving the hoist cylinder12will be explained with reference toFIG. 3toFIG. 5.

Indicated at13is the hydraulic pump which delivers the hydraulic oil as pressurized oil. The hydraulic pump13configures a hydraulic source together with a hydraulic oil tank14for reserving the hydraulic oil therein to supply and discharge the pressurized oil to and from the hoist cylinder12. As shown inFIG. 1andFIG. 2, the hydraulic oil tank14is mounted on the side surface of the vehicle body2to be located under the vessel3.

Here, the hydraulic oil accommodated inside the hydraulic oil tank14is suctioned into the hydraulic pump13when the hydraulic pump13is rotated by the engine11. High-pressurized oil is delivered into a pump line15from the discharge side of the hydraulic pump13. The pump line15establishes connection between the hydraulic pump13and a control valve device19to be described later. On the other hand, the returned oil from the hoist cylinder12is discharged through a low-pressure return line16to the hydraulic oil tank14. The return line16returns the hydraulic oil which is delivered from the hydraulic pump13and passes through the control valve device19back to the hydraulic oil tank14.

Indicated at17and18are actuator lines which selectively connect the outflow side of the control valve device19to the bottom-side oil chamber12G and the rod-side oil chambers12E and12F in the hoist cylinder12, respectively. The actuator lines17and18are connected through the inflow side of the control valve device19to be described later respectively to the hydraulic sources (the hydraulic pump13and the hydraulic oil tank14). That is, the actuator lines17and18establish connection between the outflow side of the control valve device19and the hoist cylinder12through actuator connecting oil passages26A,26B,27A and27B to be described later. Specifically, the bottom-side actuator line17as one actuator line is connected to the bottom-side oil chamber12G in the hoist cylinder12from the actuator connecting oil passages26A and27A through the inside of the rod12C in the hoist cylinder12. The rod-side actuator line18as the other actuator line is connected to the rod-side oil chambers12E and12F in the hoist cylinder12from the actuator connecting oil passages26B and27B through the inside of the rod12C in the hoist cylinder12.

Thereby, the pressurized oil from the hydraulic pump13is supplied through the actuator lines17and18to the bottom-side oil chamber12G or the rod-side oil chambers12E and12F in the hoist cylinder12. The pressurized oil in the bottom-side oil chamber12G or the rod-side oil chambers12E and12F is discharged to the hydraulic oil tank14through any of the actuator lines17and18.

Designated at19is the control valve device which is provided between the hydraulic sources (the hydraulic pump13and the hydraulic oil tank14) and the hoist cylinder12. The control valve device19controls pressurized oil of supply and discharge to and from the hoist cylinder12. The control valve device19comprises, for example, a high-pressure side oil passage20, low-pressure side oil passages21and22, a center bypass oil passage23, a first directional control valve24, a second directional control valve25, the actuator connecting oil passages26A,26B,27A and27B, check valves28A,28B,30A and30B, and relief valves29A and29B. The first directional control valve24and the second directional control valve25have a parallel connection with each other through the high-pressure side oil passage20, the low-pressure side oil passages21and22, and the center bypass oil passage23. It should be noted that the control valve device19is configured, for example, as one valve block or by a combination of a plurality of valve blocks.

The high-pressure side oil passage20has one end which is connected through the pump line15to the delivery side of the hydraulic pump13and the other end which is connected to the second directional control valve25. A branch line32to be described later is connected to the midway section of the high-pressure side oil passage20. The low pressure-side oil passage21is arranged in the return side of the first directional control valve24and connects the actuator connecting oil passages26A and26B to be described later to the hydraulic oil tank14through the return line16. The low pressure-side oil passage22is arranged in the return side of the second directional control valve25and connects the actuator connecting oil passages27A and27B to be described later to the hydraulic oil tank14through the return line16. The center bypass oil passage23connects the pump line15and the return line16when the first and second directional control valves24and25are both in a neutral position (N), and when the first directional control valve24is in a floating position (F) and the second directional control valve25is in a neutral position (N).

In this case, as shown inFIG. 3, when the first and second directional control valves24and25are both in the neutral position (N), the hydraulic pump13is in an unloading state and a delivery pressure of the hydraulic pump13(pressure inside the pump line15) is kept in a low pressure state close to a tank pressure. On the other hand, as shown inFIG. 4, when the first directional control valve24is in the floating position (F) and the second directional control valve25is in the neutral position (N), a pressure (back pressure) in the pump line15is kept by a throttle24C to be described later to correspond to a flow rate of hydraulic oil flowing in the throttle24C and a flow passage area of the throttle24C. In consequence, when the control valve device19is in the floating position (F), as described later, the rod-side oil chambers12E and12F in the hoist cylinder12are pressurized by the pressure kept by the throttle24C.

A pair of the actuator connecting oil passages26A and26B are provided in the outflow side of the first directional control valve24. The actuator connecting oil passages26A and26B are connected through the actuator lines17and18respectively to the bottom-side oil chamber12G, and the rod-side oil chambers12E and12F in the hoist cylinder12and connected respectively to the return line16(hydraulic oil tank14). A pair of the actuator connecting oil passages27A and27B are provided in the outflow side of the second directional control valve25. The actuator connecting oil passages27A and27B are connected through the actuator lines17and18respectively to the bottom-side oil chamber12G, and the rod-side oil chambers12E and12F in the hoist cylinder12, and connected respectively to the return line16(hydraulic oil tank14). It should be noted that the respective actuator connecting oil passages26A,26B,27A and27B are formed as oil passages in the valve block configuring the control valve device19.

Each of the first and second directional control valves24and25is configured of a hydraulic pilot type directional control valve having six ports and three positions, for example. The first directional control valve24includes a pair of hydraulic pilot portions24A and24B. The first directional control valve24is switched from the neutral position (N) to a raising position (R) when a pilot pressure is supplied to the hydraulic pilot portion24A from a raising operation solenoid valve35to be described later, and is switched from the neutral position (N) to the floating position (F) when a pilot pressure is supplied to the hydraulic pilot portion24B from a floating operation solenoid valve37to be described later.

The second directional control valve25includes a pair of hydraulic pilot portions25A and25B. The second directional control valve25is switched from the neutral position (N) to the raising position (R) when a pilot pressure is supplied to the hydraulic pilot portion25A from the raising operation solenoid valve35, and is switched from the neutral position (N) to a lowering position (L) when a pilot pressure is supplied to the hydraulic pilot portion25B from a lowering operation solenoid valve36to be described later.

That is, as shown inFIG. 3andFIG. 4, the control valve device19has a plurality of switching positions composed of the neutral position (N), the raising position (R), the lowering position (L), and the floating position (F), and is switched to any position of the respective switching positions. Here, description will be made of a case where the control valve device19is in the neutral position (N). In this case, as shown inFIG. 3, by disposing the first and second directional control valves24and25both in the neutral position (N), the control valve device19becomes in a holding position to stop movement of the hoist cylinder12for holding the vessel3. In the neutral position (N) as the holding position, supply and discharge of pressurized oil to and from the hoist cylinder12through the actuator connecting oil passages26A and26B, and the actuator connecting oil passages27A and27B are stopped.

Description will be made of a case where the control valve device19is in the raising position. In this case, a pilot pressure is supplied to the hydraulic pilot portions24A and25A in the first and second directional control valves24and25from the raising operation solenoid valve35to be described later, and thereby the first and second directional control valves24and25both are switched from the neutral position (N) to the raising position (R). When the first and second directional control valves24and25are switched to the raising position (R), pressurized oil from the hydraulic pump13is supplied into the bottom-side oil chamber12G in the hoist cylinder12through the pump line15, the high-pressure side oil passage20, the first and second directional control valves24and25, the actuator connecting oil passages26A and27A and the bottom-side actuator line17. At this time, when the first directional control valve24is switched to the raising position (R), the hydraulic oil in the rod-side oil chambers12E and12F is returned to the hydraulic oil tank14through the rod-side actuator line18, the actuator connecting oil passage26B, the directional control valve24, the low pressure-side oil passage21and the return line16.

As a result, the inner tube portion12B and/or the rod12C in the hoist cylinder12expands by pressurized oil in the bottom-side oil chamber12G to rotate the vessel3to a dumping position shown inFIG. 2. That is, at this time, the first and second directional control valves24and25in the control valve device19both are arranged in the raising position (R), and the hoist cylinder12expands by a hydraulic force to tilt the vessel3upward.

On the other hand, description will be made of a case where the control valve device19becomes in the lowering position. In this case, a pilot pressure is supplied to the hydraulic pilot portion25B in the second directional control valve25from the lowering operation solenoid valve36to be described later, and thereby the second directional control valve25is switched from the neutral position (N) to the lowering position (L). The first directional control valve24is arranged in the neutral position (N). When the second directional control valve25is switched to the lowering position (L), pressurized oil from the hydraulic pump13is supplied to the rod-side oil chambers12E and12F in the hoist cylinder12through the pump line15, the high-pressure side oil passage20, the second directional control valve25, the actuator connecting oil passage27B, and the rod-side actuator line18. The hydraulic oil in the bottom-side oil chamber12G is returned to the hydraulic oil tank14through the bottom-side actuator line17, the actuator connecting oil passage27A, the second directional control valve25, the low pressure-side oil passage22and the return line16.

As a result, the inner tube portion12B and/or the rod12C in the hoist cylinder12contracts by the pressurized oil in the rod-side oil chambers12E and12F to downward rotate the vessel3to a traveling position shown inFIG. 1. That is, at this time, the second directional control valve25in the control valve device19is arranged in the lowering position (L), and the hoist cylinder12contracts by a hydraulic force to lower the vessel3to a position of being seated on the vehicle body2.

Next, description will be made of a case where the control valve device19becomes in the floating position. In this case, as shown inFIG. 4a pilot pressure is supplied to the hydraulic pilot portion24B in the first directional control valve24from the floating operation solenoid valve37to be described later, and thereby the first directional control valve24is switched from the neutral position (N) to the floating position (F). The second directional control valve25is arranged in the neutral position (N). When the first directional control valve24is switched to the floating position (F), the actuator connecting oil passage26A is connected to the low pressure-side oil passage21and the return line16through the directional control valve24. The actuator connecting oil passage26B is connected to the side of the return line16through the check valve28B to be described later, and also to the pump line15through the directional control valve24. Further, the other actuator connecting oil passage27B is connected to the low pressure-side oil passage22and the return line16through the check valve30B to be described later.

As a result, the hoist cylinder12contracts by the load (self-weight) from the vessel3, and the hydraulic oil in the bottom-side oil chamber12G is discharged to the hydraulic oil tank14through the bottom-side actuator line17, the actuator connecting oil passage26A, the directional control valve24, the low pressure-side oil passage21and the return line16. On the other hand, the hydraulic oil in the hydraulic oil tank14is resupplied to the rod-side oil chambers12E and12F through the actuator connecting oil passages26B and27B and the rod-side actuator line18from the check valves28B and30B to be described later. That is, at this time the directional control valve24in the control valve device19is arranged in the floating position (F) of allowing the self-weight fall of the vessel3.

In this case, the rod-side oil chambers12E and12F are connected also to the pump line15through the rod-side actuator line18, the actuator connecting oil passage26B and the directional control valve24. That is, as shown inFIG. 4andFIG. 5, in a case of the present embodiment, when the first directional control valve24is in the floating position (F), the pump line15is connected to the return line16through the first directional control valve24and the center bypass oil passage23, and is connected also to the rod-side actuator line18through the first directional control valve24and the actuator connecting oil passage26B.

Here, a throttle24C as a flow rate limiting section that limits a flow rate of hydraulic oil returning to the hydraulic oil tank14through the first directional control valve24from the hydraulic pump13is provided in the floating position (F) of the first directional control valve24. That is, the throttle24C which limits the flow rate of the hydraulic oil flowing in the return line16through the directional control valve24and the center bypass oil passage23from the pump line15is provided in the floating position (F) of the directional control valve24. The throttle24C reduces an area (flow passage area) of a flow passage (a line and a passage) of the hydraulic oil returning to the hydraulic oil tank14. For example, a notch may be formed as the throttle24C in a land portion of a spool in the directional control valve24, wherein it is possible to limit the flow rate of the hydraulic oil flowing from the pump line15to the return line16through the notch.

When the first directional control valve24in the control valve device19is in the floating position (F), the upstream side of the rod-side actuator line18as a line communicated with the rod-side oil chambers12E and12F in the hoist cylinder12is connected to the pump line15positioned between the hydraulic pump13and the throttle24C. That is, when the first directional control valve24is in the floating position (F), the upstream side of the throttle24C is connected to the pump line15and the downstream side of the throttle24C is connected to the return line16through the center bypass oil passage23. In addition, the rod-side actuator line18is connected to the pump line15through the actuator connecting oil passage26B and the directional control valve24.

Therefore, the hydraulic oil which is delivered from the hydraulic pump13and flows in the return line16through the pump line15, the first directional control valve24and the center bypass oil passage23is limited in flow rate by the throttle24C (a pressure loss is generated). Thereby, a high pressure (back pressure) corresponding to a flow rate of the hydraulic oil flowing through the throttle24C and a flow passage area (the degree of the flow rate limit) of the throttle24C is generated in the pump line15upstream of the throttle24C.

In this case, since the rod-side actuator line18is connected to the pump line15through the actuator connecting oil passage26B and the first directional control valve24, the rod-side oil chambers12E and12F in the hoist cylinder12are subjected to a pressure in the side of the pump line15which has become a high pressure by the throttle24C. In consequence, a thrust force (contraction force) in a direction of contracting the rod12C (and the inner tube portion12B) is generated in the hoist cylinder12, making it possible for the hoist cylinder12to push down the vessel3on the vehicle body2.

That is, as shown in an arrow F inFIG. 1, a thrust force F (contraction force F) as a downward force is applied to the vessel3from the hoist cylinder12. As a result, when the vehicle travels in a state where the control valve device19is in the floating position (F), even if a force in a direction of floating the vessel3from the vehicle body2is applied to the vessel3due to passage of the vehicle on the bumpy road surface, the thrust force F of the hoist cylinder12forcibly seats the vessel3on the vehicle body2, thus making it possible to restrict the floating (flopping) of the vessel3.

It should be noted that the flow passage area of the throttle24C is set as needed such that the thrust force F necessary for restricting the floating of the vessel3can be generated in the hoist cylinder12within a range where an excessive load is not applied to the engine11which drives the hydraulic pump13. That is, as the flow passage area of the throttle24C is made smaller, a pressure upstream of the throttle24C is the higher, thus making it possible to increase the thrust force F (force F pushing the vessel3to the side of the vehicle body2) of the hoist cylinder12. However, the load of the hydraulic pump13is increased by the extent of decreasing the flow passage area, and an efficiency of the engine11possibly degrades (energy losses increase). Therefore, the flow passage area of the throttle24C is set as needed such that a pressure which can restrict the floating of the vessel3can be increased (generated) upstream of the throttle24C by a delivery amount of the hydraulic pump13corresponding to an engine rotational speed at traveling in consideration of the efficiency of the engine11.

In the floating position (F) of the first directional control valve24, there are provided the throttle24C and besides, a different throttle24D which limits a flow rate of hydraulic oil flowing in the return line16from the bottom-side oil chamber12G through the bottom-side actuator line17, the actuator connecting oil passage26A, the first directional control valve24and the low pressure-side oil passage21. The different throttle24D limits the flow rate of the hydraulic oil flowing from the bottom-side oil chamber12G to the hydraulic oil tank14to restrict a descending speed (downward rotating speed) of the vessel3from being excessively fast.

The make-up check valves28A and28B are disposed in the control valve device19in the side of the first directional control valve24. The check valves28A and28B are provided midway through the actuator connecting oil passages26A and26B bypassing the outflow side of the first directional control valve24. One check valve28A allows the hydraulic oil in the hydraulic oil tank14to flow toward the bottom-side oil chamber12G in the hoist cylinder12through the actuator connecting oil passage26A and the actuator line17, and prevents the reverse flow of the hydraulic oil in the hydraulic oil tank14. This configuration prevents the bottom-side oil chamber12G in the hoist cylinder12from being a negative pressure by the hydraulic oil resupplied through the check valve28A during the expansion process of the hoist cylinder12.

On the other hand, the other check valve28B allows the hydraulic oil in the hydraulic oil tank14to flow toward the rod-side oil chambers12E and12F in the hoist cylinder12from the hydraulic oil tank14through the actuator connecting oil passage26B and the actuator line18, and prevents the reverse flow of the hydraulic oil in the hydraulic oil tank14. This configuration prevents the rod-side oil chambers12E and12F in the hoist cylinder12from being a negative pressure by the hydraulic oil resupplied through the check valve28B during the contraction process of the hoist cylinder12.

Relief valves29A and29B for excessive load prevention are provided in the control valve device19. The relief valves29A and29B are provided in parallel to the check valves28A and28B midway through the actuator connecting oil passages26A and26B bypassing the outflow side of the first directional control valve24. The relief valve29A as one of the relief valves29A and29B opens for relieving an excessive pressure in the side of the bottom-side oil chamber12G to the hydraulic oil tank14when an excessive load in the contraction direction is applied to the hoist cylinder12. The other relief valve29B opens for relieving an excessive pressure in the side of the rod-side oil chambers12E and12F when an excessive load in the expansion direction is applied to the hoist cylinder12.

The make-up check valves30A and30B are disposed in the control valve device19in the side of the second directional control valve25. The check valves30A and30B are provided midway through the actuator connecting oil passages27A and27B bypassing the outflow side of the second directional control valve25. One check valve30A allows the hydraulic oil in the hydraulic oil tank14to flow toward the bottom-side oil chamber12G in the hoist cylinder12from the low pressure-side oil passage22(hydraulic oil tank14) through the actuator connecting oil passage27A and the actuator line17, and prevents the reverse flow of the hydraulic oil in the hydraulic oil tank14. Thereby, the check valve30A resupplies the hydraulic oil to the bottom-side oil chamber12G in the hoist cylinder12to prevent the bottom-side oil chamber12G from being a negative pressure during the expansion process of the hoist cylinder12.

On the other hand, the other check valve30B, for example, allows the hydraulic oil in the hydraulic oil tank14to flow toward the rod-side oil chambers12E and12F in the hoist cylinder12from the low pressure side oil passage22(the hydraulic oil tank14) through the actuator connecting oil passage27B and the actuator line18, and prevents the reverse flow of the hydraulic oil in the hydraulic oil tank14. Thereby, the check valve30B resupplies the hydraulic oil to the rod-side oil chambers12E and12F in the hoist cylinder12to prevent the rod-side oil chambers12E and12F from being a negative pressure during the contraction process of the hoist cylinder12.

The main relief valve31is disposed between the pump line15(the high-pressure side oil passage20) and the hydraulic oil tank14(the low pressure-side oil passage21and the return line16). The relief valve31determines a maximum delivery pressure of the hydraulic pump13and restricts a pressure in the pump line15to the maximum delivery pressure or less. That is, the relief valve31opens when an excessive pressure exceeding the maximum delivery pressure is generated in the pump line15, and relieves the excessive pressure at this time to the side of the hydraulic oil tank14.

Next, an explanation will be made of solenoid valves35,36and37, an operating lever device38, a controller40and the like, which control the control valve device19.

Indicated at32is a branch line a base side of which is disposed to be branched from the high-pressure side oil passage20, and the branch line32is connected to a pilot pressure supply line34through a pressure reducing valve33. The pressure reducing valve33is switched to a valve closing position (a) and a valve opening position (b) to reduce a pressure of pressurized oil in the branch line32, which is supplied to the pilot pressure supply line34. Therefore, the pressure in the pilot pressure supply line34is kept in a setting pressure (that is, a pressure lower than that in the branch line32) in advance determined by the pressure reducing valve33. It should be noted that the branch line32may be branched, not from the high-pressure side oil passage20, but midway through the pump line15.

The solenoid valves35,36and37supply a pilot pressure to the hydraulic pilot portions24A,24B,25A and25B in the first and second directional control valves24and25in the control valve device19, respectively. The solenoid valves35to37are provided between the controller40to be described later and the control valve device19. The solenoid valves35to37opens and closes individually in response to an operation of the operating lever device38to be described later, and supply a pilot pressure for switching control to the control valve device19at valve opening.

The raising operation solenoid valve35thereof is switched from a valve closing position (c) to a valve opening position (d) in response to an excitation signal from the controller40, and supplies a raising operation pilot pressure from the pilot pressure supply line34toward the hydraulic pilot portions24A and25A in the first and second directional control valves24and25in the valve opening position (d). In consequence, the directional control valves24and25in the control valve device19are switched from the neutral position (N) to the raising position (R) shown inFIG. 3.

The lowering operation solenoid valve36is switched from the valve closing position (c) to the valve opening position (d) in response to an excitation signal from the controller40, and supplies a lowering operation pilot pressure from the pilot pressure supply line34toward the hydraulic pilot portion25B in the second directional control valve25in the valve opening position (d). In consequence, the second directional control valve25in the control valve device19is switched from the neutral position (N) to the lowering position (L) shown inFIG. 3. Since the solenoid valves35and37are demagnetized to be in the valve closing position (c) at this time, the first directional control valve24is arranged in the neutral position (N).

On the other hand, the floating operation solenoid valve37is switched from the valve closing position (c) to the valve opening position (d) in response to an excitation signal from the controller40, and supplies a floating operation pilot pressure from the pilot pressure supply line34toward the hydraulic pilot portion24B in the first directional control valve24in the valve opening position (d). In consequence, the first directional control valve24in the control valve device19is switched from the neutral position (N) shown inFIG. 3to the floating position (F) shown inFIG. 4andFIG. 5. Since the solenoid valves35and36are demagnetized to be in the valve closing position (c) at this time, the second directional control valve25is arranged in the neutral position (N).

Indicated at38is the operating device for performing a switching operation of the control valve device19. The operating lever device38is configured by an electric lever device, for example, and is disposed in a position close to the operator's seat in the cab6. The operating lever device38has an operating lever38A that is manually operated to be tilted by an operator in the cab6. The operating lever38A is tilted to any of the neutral position (N), the raising position (R), the lowering position (L) and the floating position (F) corresponding to the respective switching positions of the control valve device19, that is, the neutral position (N), the raising position (R), the lowering position (L) and the floating position (F).

A lever sensor39is attached to the operating lever device38. The lever sensor39detects an operating position (lever position) of the operating lever38A by the operator and outputs the detection signal to the controller40to be described later. Specifically, the lever sensor39detects in which position among the neutral position (N), the raising position (R), the lowering position (L) and the floating position (F) the operating lever38A in the operating lever device38is.

The controller40is composed of a microcomputer, and the controller40has an input side connected to the lever sensor39and the like, and an output side connected to the solenoid valves35to37and the like. The controller40determines an operating position of the operating lever38A based upon a detection signal from the lever sensor39, and outputs an excitation signal to the solenoid valves35to37to associate the operating position of the operating lever38A with the switching position of the control valve device19.

The dump truck1according to the present embodiment has the aforementioned configuration, and next, an operation thereof will be explained.

First, in a stone-crushing site such as amine, for example, a large-sized hydraulic excavator (not shown) is used to load earth and sand4as objects to be transported on the vessel3. At this time, the vessel3is placed in the traveling position shown inFIG. 1, and the dump truck1transports the earth and sand4to a cargo unloading site in a state where a large number of the earth and sand4is loaded on the vessel3.

In the cargo unloading site, the cargo unloading work starts. Here, when an operator in the cab6manually tilts the operating lever38A in the operating lever device38from the neutral position (N) to the raising position (R), the controller40outputs an excitation signal to the raising operation solenoid valve35. Thereby, the raising operation solenoid valve35is switched from the valve closing position (c) to the valve opening position (d), and a raising operation pilot pressure is supplied to the hydraulic pilot portions24A and25A in the directional control valves24and25in the control valve device19from the pilot pressure supply line34.

Therefore, the control valve device19is switched from the neutral position (N) to the raising position (R). As a result, the pressurized oil from the hydraulic pump13is supplied through the pump line15, the high-pressure side oil passage20, the first and second directional control valves24and25, the actuator connecting oil passages26A and27A and the bottom-side actuator line17into the bottom-side oil chamber12G in the hoist cylinder12. The hydraulic oil in the rod-side oil chambers12E and12F is returned through the rod-side actuator line18, the actuator connecting oil passage26B, the first directional control valve24, the low pressure-side oil passage21and the return line16back to the hydraulic oil tank14.

As a result, the inner tube portion12B and/or the rod12C in the hoist cylinder12expands in a direction of an arrow Z inFIG. 2by the pressurized oil in the bottom-side oil chamber12G to rotate the vessel3to the dumping position shown inFIG. 2in such a manner as to tilt the vessel3diagonally backward. At this time, the dump truck1rotates the vessel3to a tilting posture as shown inFIG. 2on a basis of the connecting pin5. Therefore, the earth and sand4in the vessel3can be dumped to the cargo unloading site in a direction of an arrow Y in such a manner as to slide downward.

At this time, when an operator releases its hand from the operating lever38A, the operating lever38A automatically returns back to the neutral position (N) by a returning spring (not shown). Therefore, a signal which is output to the raising operation solenoid valve35from the controller40is in a demagnetizing state (OFF), and the raising operation solenoid valve35returns back to the valve closing position (c) shown inFIG. 3. Therefore, the directional control valves24and25in the control valve device19are automatically returned to the neutral position (N) to stop supply and discharge of the pressurized oil to and from the bottom-side oil chamber12G and the rod-side oil chambers12E and12F in the hoist cylinder12. Accordingly, the hoist cylinder12can maintain the inner tube portion12B and the rod12C in the expanding state and temporarily stop the vessel3in a state of being maintained in the tilting posture shown inFIG. 2.

Next, when the dumping operation of the earth and sand4is completed, an operator manually tilts the operating lever38A from the neutral position (N) to the floating position (F), an excitation signal is output to the floating operation solenoid valve37from the controller40. Therefore, as shown inFIG. 4, the floating operation solenoid valve37is switched from the valve closing position (c) to the valve opening position (d), and a floating operation pilot pressure is supplied to the hydraulic pilot portion24B in the first directional control valve24from the pilot pressure supply line34.

Therefore, the first directional control valve24in the control valve device19is switched from the neutral position (N) to the floating position (F). Since the solenoid valves35and36are demagnetized to be in the valve closing position (c) at this time, the second directional control valve25is arranged in the neutral position (N). In consequence, the actuator connecting oil passage26A is connected to the low pressure-side oil passage21and the return line16through the first directional control valve24. The actuator connecting oil passage26B is connected to the side of the return line16through the check valve28B, and is also connected to the side of the pump line15through the first directional control valve24. Further, the other actuator connecting oil passage27B is connected to the low pressure-side oil passage22and the return line16through the check valve30B.

In consequence, the hoist cylinder12is operated to contract according to a load (self-weight) from the vessel3, and the hydraulic oil in the bottom-side oil chamber12G is discharged toward the hydraulic oil tank14and also the hydraulic oil in the hydraulic oil tank14is supplied into the rod-side oil chambers12E and12F through the check valve28B and/or the check valve30B. In this case, the hydraulic oil is supplied (resupplied) into the rod-side oil chambers12E and12F through the first directional control valve24also from the side of the pump line15. In this case, however, the cargo unloading work is completed, but the dump truck1is still in a stopping state. Therefore, the engine rotating speed is low. Accordingly, the pressure in the side of the pump line15kept by the throttle24C is low, and the amount of the hydraulic oil supplied into the rod-side oil chambers12E and12F from the side of the pump line15also corresponds to the pressure. In any way, the hoist cylinder12allows fall of the vessel3by the self-weight, thereby making it possible to lower down the vessel3to the traveling position shown inFIG. 1and seat the vessel3on the vehicle body2.

On the other hand, when the dump truck1is in a tilting state on the bumpy work site or on the sloping ground in the work site, even if the control valve device19is switched to the floating position (F), the vessel3does not possibly fall down by the self-weight. In such a case, however, an operator performs a tilting operation of the operating lever38A until the lowering position (L), and thereby the controller40outputs an excitation signal to the lowering operation solenoid valve36. Therefore, the lowering operation solenoid valve36is switched from the valve closing position (c) to the valve opening position (d), and a lowering operation pilot pressure is supplied from the pilot pressure supply line34to the hydraulic pilot portion25B in the second directional control valve25.

Therefore, the second directional control valve25in the control valve device19is switched from the neutral position (N) to the lowering position (L). At this time, the solenoid valves35and37are demagnetized to be in the valve closing position (c). Therefore, the first directional control valve24is arranged to be in the neutral position (N). In consequence, the pressurized oil from the hydraulic pump13is supplied into the rod-side oil chambers12E and12F in the hoist cylinder12through the pump line15, the high-pressure side oil passage20, the second directional control valve25, the actuator connecting oil passage273and the rod-side actuator line18. The hydraulic oil in the bottom-side oil chamber12G is returned back to the hydraulic oil tank14through the bottom-side actuator line17, the actuator connecting oil passage27A, the second directional control valve25, the low pressure-side oil passage22and the hydraulic oil return line16. Thereby, the hoist cylinder12is operated in such a manner that the inner tube portion12B and/or the rod12C contracts into the outer tube portion12A by the pressurized oil supplied into the rod-side oil chambers12E and12F, making it possible to rotate the vessel3downward to the traveling position as shown inFIG. 1by the hydraulic force of the hoist cylinder12to forcibly seat the vessel3on the vehicle body2.

However, when the control valve device19is switched to the lowering position (L), the hoist cylinder12is operated to contract with the hydraulic force, and therefore, there is a possibility of applying an extra load to the vessel3and the vehicle body2. When the control valve device19is left in a state switched to the lowering position (L) thereafter, the vessel3continues to be strongly pressed on the vehicle body2, and the hydraulic force from the hoist cylinder12acts on the abutment surface between the vessel3and the vehicle body2as an extra load. Therefore, the operator in the dump truck1causes the operating lever38A to be self-retained to the floating position (F) at the traveling of the vehicle. As a result, the control valve device19is switched to the floating position (F) and the vessel3continues to be seated on the vehicle body2by the self-weight and the hoist cylinder12also can be maintained in the contraction state using the self-weight of the side of the vessel3.

Incidentally, when the vehicle travels in a state where the control valve device19is in the floating position (F), in a case where the vessel3is in a vacant state of the object to be transported, the vessel3floats up from the vehicle body2following vibrations generated due to knocking-up from the road surface, and at the time the vessel3is to be seated on the vehicle body2, there is a possibility that the vessel3collides with the vehicle body2. Therefore, there is a possibility that uncomfortable feelings are given to an operator in the cab6or repeated collisions of the vessel3with the vehicle body2cause reduction in durability and lifetime.

Therefore, according to the present embodiment, in the floating position (F) of the first directional control valve24, there is provided the throttle24C that limits a flow rate of hydraulic oil returning to the hydraulic oil tank14through the first directional control valve24and the center bypass oil passage23from the hydraulic pump13. Therefore, when the first directional control valve24is in the floating position (F), the hydraulic oil which is delivered from the hydraulic pump13and flows in the return line16through the pump line15and the first directional control valve24is limited in flow rate by the throttle24C. Thereby, a high pressure (back pressure) corresponding to a flow rate of the hydraulic oil flowing through the throttle24C and a flow passage area of the throttle24C is kept in the pump line15upstream of the throttle24C.

In this case, since the rod-side actuator line18is connected to the pump line15through the first directional control valve24, the rod-side oil chambers12E and12F in the hoist cylinder12are pressurized by the pressure kept by the throttle24C. Here, the flow rate of the hydraulic oil flowing through the throttle24C, that is, the delivery rate of the hydraulic pump13corresponds to an engine rotating speed of the dump truck1at traveling. For example, as the engine rotating speed increases, the pressure in the side of the pump line15kept by the throttle24C becomes the higher. As a result, the pressure to be supplied from the pump line15into the rod-side oil chambers12E and12F through the actuator connecting oil passage26B and the rod-side actuator line18is also increased.

In consequence, a thrust force F (contraction force F) in a direction of contracting the rod12C (and the inner tube portion12B) is generated in the hoist cylinder12by the high pressure kept upstream of the throttle24C (the pump line15) by the throttle24C, making it possible for the hoist cylinder12to press the vessel3on the vehicle body2. As a result, when the vehicle travels in a state where the control valve device19is in the floating position (F), even if a force in a direction of floating the vessel3from the vehicle body2is applied to the vessel3following passage of the vehicle on the bumpy road surface, the thrust force F of the hoist cylinder12can seat the vessel3on the vehicle body2, thus restricting the floating-up of the vessel3.

At this time, the thrust force F of the hoist cylinder12can be all the time generated by the throttle24C. Therefore, the vessel3can be stably seated on the vehicle body2. In addition, since a sensor for detecting the movement of the vessel3is not necessary, an increase in cost can be restricted.

According to the present embodiment, the throttle24C limits the flow rate of the hydraulic oil which returns to the hydraulic oil tank14through the first directional control valve24and the center bypass oil passage23from the hydraulic pump13. Therefore, a desired pressure (back pressure) can be kept upstream of this throttle24C by setting the flow passage area of this throttle24C to an appropriate magnitude. That is, by setting the flow passage area of this throttle24C to the appropriate magnitude, the thrust force F necessary for restricting the floating of the vessel3can be generated in the hoist cylinder12while restricting the load of the hydraulic pump13.

Further, according to the present embodiment, the control valve device19is configured using the first and second directional control valves24and25connected in parallel between the hydraulic pump13and the hoist cylinder12. In this case, when the first directional control valve24is in the floating position (F), the vessel3can be pressed to the vehicle body2by the hoist cylinder12to stably seat the vessel3on the vehicle body2.

It should be noted that the aforementioned embodiment is explained by taking a case where the two-step hoist cylinder12is used, as an example. However, the present invention is not limited to that, and a multi-step hoist cylinder (hydraulic cylinder), for example, a three or more-step hoist cylinder or a one-step hoist cylinder (hydraulic cylinder) may be used. For example, in a case of a one-step hoist cylinder, the hoist cylinder may be configured of a single cylinder (cylindrical portion), a rod and a piston which defines an inside of the cylinder as a rod-side oil chamber and a bottom-side oil chamber.

Further, in the aforementioned embodiment, an explanation is made by taking a case of using the rear wheel drive type dump truck1as the transport vehicle, as an example. However, the present invention is not limited to that, the present invention may be applied to, for example, a front wheel drive type dump truck or a four-wheel drive type dump truck driving front and rear wheels both, and may be applied to a transport vehicle equipped with traveling wheels other than the dump truck. Further, the present invention may be applied to a crawler type transport vehicle.

DESCRIPTION OF REFERENCE NUMERALS