Hydraulic control apparatus for work machines

A hydraulic control apparatus which is used in order to prevent the blade from falling over on the pitch back side during a dual tilting operation, and to operate the left and right tilting cylinders in a uniform manner even in case where there is a large difference in the load pressure between the left and right tilting cylinders. In cases where it is desired to perform a dual tilting operation, the operator moves the operating lever either leftward or rightward while pressing the dual tilting switch of the operating lever. As a result of the switch being pressed, an electrical control signal that places the flow-combining/flow-dividing switching valve or flow-combining/flow-dividing valve in the flow-dividing position is generated by the controller, and the electrical control signal is output to the flow-combining/flow-dividing switching valve so that the flow-combining/flow-dividing switching valve or flow-combining/flow-dividing valve is switched to the flow-dividing position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hydraulic control apparatus for work machines, and more particularly relates to a hydraulic control apparatus used in a work machine equipped with a dozing blade that has left and right hydraulic cylinders used for tilting, such as bulldozers and the like.

2. Description of the Related Art

FIG. 2shows the peripheral parts of the blade installed on the front part of the vehicle body of a bulldozer in a perspective view.

Bulldozers perform work such as digging and transporting earth, and leveling the ground surface following such excavation by means of a blade3(dozing blade) that is attached to the front part of the vehicle main body.

A pair of tilting cylinders, i. e., left and right tilting cylinders4and5, are installed between the blade3and vehicle main body.

If both of the tilting cylinders4and5are simultaneously driven in the same direction (in extension or retraction), the blade3is placed in a pitch dump attitude (forward-inclined attitude) or pitch back attitude (rearward-inclined attitude).

Furthermore, if one of the tilting cylinders is placed in stopped state, and the other tilting cylinder is driven in extension or retraction, the blade3assumes an attitude in which the right end part or left end part of the blade3is tilted downward (right-tilted attitude or left-tilted attitude). This is called a single tilting operation. The performance of a single tilting operation is described in U.S. Pat. No. 5,799,737.

Furthermore, if one of the tilting cylinders is driven in extension or retraction at the same time that the other tilting cylinder is driven in extension or retraction, the operation speed of the tilting operation of the blade3is increased. This is called a dual tilting operation. The performance of a dual tilting operation is described in U.S. Pat. No. 4,802,537 and U.S. Pat. No. 6,481,506.

FIG. 5Ashows the hydraulic circuit in a case where two fixed displacement hydraulic pumps105and104are used as a pressurize oil supply source for the left and right tilting cylinders103and102.

As is shown inFIG. 5A, left and right tilting cylinders103and102are attached to the blade101. Fixed displacement hydraulic pumps105and104are installed corresponding to the left and right tilting cylinders103and102; furthermore, main operating valves107and106in which the direction and flow rate of the pressurized oil are controlled are installed respectively corresponding to the left and right cylinders103and102.

The pressurized oil that is discharged from the fixed displacement hydraulic pump105is supplied to the bottom end chamber103B or head end oil chamber103H of the left tilting cylinder103via the main operating valve107. Similarly, the pressurized oil that is discharged from the fixed displacement hydraulic pump104is supplied to the bottom end chamber102B or head end chamber102H of the right tilting cylinder102via the main operating valve106.

FIG. 6shows the hydraulic circuit110in a case where a single variable displacement type hydraulic pump111is used as the pressurized oil supply source of the left and right tilting cylinders103and102.

In order to prevent the construction of the hydraulic circuit from becoming complicated, the main operating valves107and105are connected in parallel to a single variable displacement hydraulic pump111.

Specifically, as is shown inFIG. 6, left and right tilting cylinders103and102are attached to the blade101. Main operating valves107and106in which the direction and flow rate of the pressurized oil are controlled are installed corresponding to the left and right tilting cylinders103and102. The discharge port of the variable displacement hydraulic pump111is caused to communicate with the inlet port of the main operating valve107via a pressure compensating valve113, and is caused to communicate with the inlet port of the main operating valve106via a pressure compensating valve112.

If the system is devised so that left and right tilting cylinders103and102are simultaneously driven by the single variable displacement hydraulic pump111without pressure compensating valves113and112, even if the opening areas of the main operating valves107and106are varied by the same amount by operating the operating levers, a large flow rate will be supplied on the side of the tilting cylinder with a smaller load (e. g., the left tilting cylinder103), and only a small flow rate will be supplied on the side of the tilting cylinder with a larger load (e. g., the tilting cylinder102).

Accordingly, pressure compensating valves113and112are installed for the respective main operating valves107and106so that flow rates corresponding to the amounts of operation of the operating levers are supplied to the left and right tilting cylinders103and102without being affected by the load.

Hydraulic pressure compensation is accomplished by the installation of the pressure compensating valves113and112. As a result, the differential pressure before and after the constriction on the side with a light load, e. g., the main operating valve107, is the same value as the differential pressure before and after the constriction of the main operating valve106on the side with a heavy load.

As a result of pressure compensation thus being performed, the differential pressures before and after the constrictions of both main operating valves107and106are the same value so that flow rates proportional to the degrees of opening of the main operating valves107and106, i. e., proportional to the amounts of operation of the operating levers, are supplied to the tilting cylinders103and102without being affected by the load.

SUMMARY OF THE INVENTION

A phenomenon in which the blade101falls over on the pitch back side occurs when a dual tilting operation is performed using the hydraulic circuit100described in the abovementioned Conventional art 2 (FIG. 5A).

FIG. 5Bshows how the stroke positions of the left and right tilting cylinders103and102shown inFIG. 5Avary.

Specifically, between the bottom end oil chambers103B and102B and head end oil chambers103H and102H of the left and right tilting cylinders103and102, the cross-sectional areas of the head end oil chambers103H and102H are smaller than the cross-sectional areas of the bottom end oil chambers103B and102B by an amount equal to the piston rods103aand102a,so that a difference in cross-sectional area exists between the two oil compartments. Furthermore, in the case of the hydraulic circuit shown inFIG. 5A, since fixed displacement hydraulic pumps105and104are used, if the opening areas of the main operating valves107and106are the same, then the supplied flow rates are the same when pressurized oil is supplied to the bottom end oil chambers103B and102B and when pressurized oil is supplied to the head end oil chambers103H and102H.

In the case of a dual tilting operation, pressurized oil is supplied to the bottom end oil chamber of one tilting cylinder of the left and right tilting cylinders103and102, and pressurized oil is supplied to the head end oil chamber of the other tilting cylinder.

Accordingly, from a state in which the stroke positions of the left and right tilting cylinders103and102are respectively the initial positions L0and R0, when the same flow rate is supplied from the fixed displacement hydraulic pumps105and104so that the left tilting cylinder103is driven in the direction of retraction, and the right tilting cylinder102is simultaneously driven in the direction of extension, the piston rod103aof the left tilting cylinder103moves by a stroke P in the direction of retraction from the initial position L0and reaches the position L1, but the piston rod102aof the right tilting cylinder102moves by a stroke Q which is smaller than the stroke P (Q<P) in the direction of extension from the initial position R0, and reaches the position R1, as a result of the abovementioned cross-sectional area difference. Subsequently, in order to return the blade101to the initial positions L0and R0, when the same flow rates are supplied to the left and right tilting cylinders103and102from the fixed displacement hydraulic pumps105and104, and the left tilting cylinder103is driven in the direction of extension while the right tilting cylinder102is driven in the direction of retraction, the same cross-sectional area difference causes the piston rod103aof the left tilting cylinder103to move by a stroke of Q in the direction of extension from the position L1so that the piston rod103areaches the position L2, while the piston rod102aof the right tilting cylinder102is caused to move by a stroke P which is larger than the stroke Q (P>Q) in the direction of retraction from the position R1, so that the piston rod102areaches the position R2.

As a result, the stroke positions of the left and right tilting cylinders103and102are shifted to the pitch back side of the blade101from the initial positions L0and R0by a stroke difference of (P−Q) between extension and retraction by a single dual tilting operation and return operation. In other words, the blade101falls over on the pitch back side. Furthermore, as a result of the repetition of a multiple number of dual tilting operations, the piston rods103aand102areach the stroke end on the pitch back side of the blade101, i. e., in the direction of retraction.

On the other hand, in cases where a dual tilting operation is performed using the hydraulic circuit110described in Conventional art 3 (FIG. 6), a phenomenon may occur in which the blade101tilt without returning to the initial position.

Specifically, in the case of a dual tilting operation, a difference in load pressure may be generated between the left and right tilting cylinders103and102. Here, even if there is a difference in the load pressure, the same flow rates can be supplied to the left and right tilting cylinders103and102if the pressure compensation performed by the pressure compensating valves103and102is perfect.

However, if the difference in the load pressure between the left and right tilting cylinders103and102is large, a deviation in pressure compensation may occur so that the same flow rate cannot be supplied to the left and right tilting cylinders103and102, thus making it impossible for the left and right tilting cylinders103and102to operate at a uniform speed. Accordingly, when a dual tilting operation is performed, the following problem arises: namely, the piston rods103aand102aof the left and right tilting cylinders103and102do not return to the same initial positions, so that the stroke positions deviate on the left and right, and the blade101tilts.

This is true not only in the case of a dual tilting operation, but also in cases where a pitch operation is performed.

If the difference in the load pressure between the left and right tilting cylinders103and102is large during a pitch operation, a deviation in pressure compensation may be generated, so that the same flow rates cannot be supplied to the left and right tilting cylinders103and102, thus making it impossible for the left and right tilting cylinders103and102to operate at a uniform speed. Accordingly, when a pitch operation is performed, the following problem arises: namely, the piston rods103aand102aof the left and right tilting cylinders103and102do not reach the same stroke position, so that the blade101tilts.

The present invention was devised in light of the above facts; a first problem to be solved by the present invention is to prevent the blade from falling over on the pitch back side during a dual tilting operation, and to allow the left and right tilting cylinders to operate in a uniform manner even in cases where there is a large difference in the load pressure between the left and right tilting cylinders.

Furthermore, a second problem to be solved by the present invention is to allow the left and right tilting cylinders to operate in a uniform manner even in cases where there is a large difference in the load pressure between the left and right tilting cylinders during a pitch operation, so that tilting of the blade can be prevented.

In the case of a bulldozer, the abovementioned left and right tilting cylinders are installed for the blade, and left and right lifting cylinders are also installed. Furthermore, ripper lifting cylinders, ripper tilting cylinders and the like are also installed for the ripper on the rear end of the vehicle body.

When earthmoving work or the like is performed, the tilting cylinders and other hydraulic cylinders (lifting cylinders) are simultaneously driven (in a composite operation). In such a composite operation, it is necessary to improve the working efficiency of the composite operation of the plurality of hydraulic actuators by efficiently supplying pressurized oil from a pressurized oil supply source in accordance with the loads that are applied to the respective hydraulic cylinders.

The present invention was devised in light of such facts; in addition to the abovementioned first problem to be solved and second problem to be solved, a third problem that is to be solved by the present invention is to improve the working efficiency during the composite operation of a plurality of hydraulic actuators in a work machine such as a bulldozer or the like equipped with tilting cylinders.

The first aspect of the present invention comprises a blade that is attached to a vehicle main body so that the blade is capable of a tilting operation; first and second variable displacement hydraulic pumps; left and right tilting hydraulic cylinders that are attached to left and right of the blade, and that are driven by a supply of pressurized oil that is discharged from the first and second variable displacement hydraulic pumps; first and second main operating valves in which direction and flow rate of the pressurized oil that is supplied to the left and right tilting hydraulic cylinders are controlled; first and second discharge oil passages that connect discharge ports of the first and second variable displacement hydraulic pumps and the first and second main operating valves; first and second pressure compensating valves that compensate differential pressures before and after the first and second main operating valves to specified values; a first flow-combining/flow-dividing valve that switches between a flow-combining position that causes communication between the first discharge oil passage and second discharge oil passage, and a flow-dividing position that cuts off the communication between the first discharge oil passage and the second discharge oil passage; and control means for controlling the switching of the flow-combining/flow-dividing valve so that a switching action is performed in which the flow-combining/flow-dividing valve is switched from the flow-combining position to the flow-dividing position in cases where it is judged that a dual tilting operation is to be performed in which pressurized oil is supplied to a bottom end oil chamber of one of the tilting hydraulic cylinders among the left and right tilting hydraulic cylinders, and pressurized oil is supplied to a head end oil chamber of the other tilting hydraulic cylinder.

The second aspect of the present invention comprises a blade that is attached to the vehicle main body so that the blade is capable of a tilting operation; first and second variable displacement hydraulic pumps; left and right tilting hydraulic cylinders that are attached to left and right of the blade, and that are driven by a supply of pressurized oil that is discharged from the first and second variable displacement hydraulic pumps; first and second main operating valves in which direction and flow rate of the pressurized oil that is supplied to the left and right tilting hydraulic cylinders are controlled; first and second discharge oil passages that connect discharge ports of the first and second variable displacement hydraulic pumps and the first and second main operating valves; first and second pressure compensating valves that compensate differential pressures before and after the first and second main operating valves to specified values; a first flow-combining/flow-dividing valve which switches between a flow-combining position that causes communication between the first discharge oil passage and second discharge oil passage, and a flow-dividing position that cuts off the communication between the first discharge oil passage and the second discharge oil passage; and control means for controlling switching of the flow-combining/flow-dividing valve so that a switching action is performed in which the flow-combining/flow-dividing valve is switched from the flow-combining position to the flow-dividing position in cases where it is judged that a pitch operation is to be performed in which pressurized oil is supplied to one of the oil chambers among a bottom end oil chamber and a head end oil chamber for the left and right tilting hydraulic cylinders.

The third aspect of the present invention is the first aspect of the present invention which further comprises flow rates control means for controlling the flow rates that are supplied to the left and right tilting hydraulic cylinders so that the stroke on an extension side and stroke on a retraction side of the left and right tilting hydraulic cylinders are the same during a dual tilting operation.

The fourth aspect of the present invention is the first aspect of the present invention which further comprises hydraulic actuators for a work implement that are driven by the supply of pressurized oil that is discharged from the first and second variable displacement hydraulic pumps, other than the left and right tilting hydraulic cylinders, wherein the switching control means control the flow-combining/flow-dividing valve so that an operation is performed in which the flow-combining/flow-dividing valve is switched from the flow-dividing position to the flow-combining position in cases where it is judged that the hydraulic actuators for a work implement are to be driven simultaneously with the left and right tilting hydraulic cylinders.

The fifth aspect of the present invention is the second aspect of the present invention which further comprises hydraulic actuators for a work implement that are driven by the supply of pressurized oil that is discharged from the first and second variable displacement hydraulic pumps, other than the left and right tilting hydraulic cylinders, wherein the switching control means control the flow-combining/flow-dividing valve so that an operation is performed in which the flow-combining/flow-dividing valve is switched from the flow-dividing position to the flow-combining position in cases where it is judged that the hydraulic actuators for a work implement are to be driven simultaneously with the left and right tilting hydraulic cylinders.

The first aspect of the present invention and third aspect of the present invention will be concretely described with reference to the accompanying drawings. As shown inFIG. 3, in cases where it is desired to perform a dual tilting operation, the operator moves the operating lever50in either the leftward or rightward direction C or D while pressing the dual tilting switch50c.

In a case where the abovementioned operation is performed in the hydraulic circuit shown inFIG. 1, the controller53generates an electrical control signal that is used to place a flow-combining/flow-dividing switching valve18in a flow-dividing position B as a result of the switch50cbeing pressed. This electrical control signal is output to the flow-combining/flow-dividing switching valve18, so that the flow-combining/flow-dividing switching valve18is switched to the flow-dividing position B, thus introducing pressurized oil from the oil passage66into the oil passages61and62. An flow-combining/flow-dividing valve17is connected ahead of the oil passage61, and flow-combining/flow-dividing valves48and148are connected ahead of the oil passage62. When pressurized oil is introduced into the oil passages61and62from the oil passage66, the flow-combining/flow-dividing valves17,48and148are switched to the flow-dividing position B. Furthermore, in cases where an electrical control signal is not output from the controller53, the flow-combining/flow-dividing switching valve18is in the flow-combining position A, the oil passages61and62communicate with the reservoir55, and the flow-combining/flow-dividing valves17,48and148are in the flow-combining position A.

As a result, the communicating passage16that connects the first hydraulic pump6and second hydraulic pump7is closed, so that the pressurized oil that is discharged from the first hydraulic pump6is discharged only into a first discharge oil passage14, and the pressurized oil that is discharged from the second hydraulic pump7is discharged only into a second discharge oil passage15.

Furthermore, a first load pressure detection oil passage90and a second load pressure detection oil passage91are cut off, and a first load pressure introduction oil passage163and a second load pressure introduction oil passage164(164′) are cut off, so that pressure compensation is canceled. Specifically, an own load pressure is applied to the pressure receiving part of a first pressure compensating valve9via a first load pressure detection port23, the first load pressure detection oil passage90, the first load pressure introduction oil passage163, and a shuttle valve63. As a result, the load pressure on the outlet cylinder port side of the first main operating valve8maintains this own load pressure.

Meanwhile, an own load pressure is applied to the pressure receiving part of the second pressure compensating valve12via a second load pressure detection port38, the second load pressure detection oil passage91, the second load pressure introduction oil passage164(164′), and a shuttle valve64. As a result, the load pressure on the outlet cylinder port side of the second main operating valve11maintains this own load pressure.

Thus, in the case of a dual tilting operation, the communicating passage16between the first hydraulic pump6and second hydraulic pump7is closed, and pressure compensation for the respective tilting cylinders4and5operates independently by the own load pressure. Accordingly, pressurized oil is independently supplied to the left and right tilting cylinders4and5from the first hydraulic pump6and second hydraulic pump7.

Accordingly, the flow rates of the pressurized oil supplied to the left and right tilting cylinders4and5can be independently adjusted by the servomechanisms71and72.

In the third aspect of the present invention, flow rate adjustment is performed as follows.

Specifically, in the controller53, as a result of the dual tilting switch50cbeing pressed, an electrical control signal that causes the stroke amounts of the tilting cylinders4and5during retraction and extension to be set at the same amount P is output to the servomechanisms71and72, and the swash angles of the swash plates6aand7aof the first and second hydraulic pumps6and7are controlled so that the flow rates supplied to the respective tilting cylinders4and5are adjusted.

Referring also toFIG. 5B, in the case of the left tilting cylinder4(tilting cylinder103inFIG. 5B), pressurized oil at a specified flow rate of QH is supplied to the head end oil chamber4b(head end oil chamber103H inFIG. 5B) during retraction, so that the tilting cylinder moves by a stroke of P in the direction of retraction from the initial position L0, and reaches the position L1. Then, during the subsequent extension, pressurized oil at a flow rate of QB which is larger than the flow rate QH during retraction is supplied to the bottom end oil chamber4a(bottom end oil chamber103B inFIG. 5B), so that the tilting cylinder moves from the stroke position L1in the direction of extension by the same stroke of P, and returns to the original initial position L0.

On the other hand, in the case of the right tilting cylinder5(tilting cylinder102inFIG. 5B), during extension, pressurized oil at a specified flow rate of QB is supplied to the bottom end oil chamber5a(bottom end oil chamber102B inFIG. 5B), so that the tilting cylinder moves in the direction of extension from the initial position R0by a stroke of P, and thus reaches the stroke position R3. Then, during the subsequent retraction, pressurized oil at a flow rate QH that is smaller than the flow rate QB during extension is supplied to the head end oil chamber5b(head end oil chamber102H inFIG. 5B), so that the tilting cylinder moves by the same stroke of P in the direction of retraction from the stroke position R3, and returns to the original initial position R0.

As a result, in a single dual tilting operation, the stroke positions of the left and right tilting cylinders4and5maintain the original initial positions without any deviation to the pitch back side from the initial positions L0and R0. In other words, a dual tilting operation can be performed without the blade3falling over on the pitch back side.

Furthermore, even if such a dual tilting operation is performed a multiple number of times, the piston rods do not reach the stroke end on the pitch back side of the blade3, i.e., in the direction of retraction.

Furthermore, since pressure compensation is canceled, the inconvenience of a deviation in pressure compensation occurring in cases where the difference in the load pressure between the left and right tilting cylinders4and5is large so that the same flow rate cannot be supplied to the left and right tilting cylinders4and5, thus making it impossible for the left and right tilting cylinders4and5to operate at a uniform speed, can be avoided. As a result, a state in which the piston rods of the left and right tilting cylinders4and5do not return to the initial positions in the case of a dual tilting operation can be prevented.

Next, the second aspect of the present invention will be described.

In cases where it is desired to perform a pitch operation, the operator moves the operating lever50in either the leftward or rightward direction C or D while pressing the pitch dump/pitch back switch50bof the operating lever50.

In case where the abovementioned operation is performed in the hydraulic circuit shown inFIG. 1, the controller53generates an electrical control signal that causes a switch to the flow-dividing position B as a result of the switch50bbeing pressed. The electrical control signal is output to the flow-combining/flow-dividing switching valve18, and the flow-combining/flow-dividing switching valve18is switched to the flow-dividing position B, so that pressurized oil from the oil passage66is introduced into the oil passages61and62. A flow-combining/flow-dividing valve17is connected ahead of the oil passage61, and flow-combining/flow-dividing valves48and148are connected ahead of the oil passage62. When pressurized oil is introduced into the oil passages61and62from the oil passage66, the flow-combining/flow-dividing valves17,48and148are switched to the flow-dividing position B. Furthermore, in cases where no electrical control signal is output from the controller53, the flow-combining/flow-dividing switching valve18is in the flow-combining position A, the oil passages61and62communicate with the reservoir55, and the flow-combining/flow-dividing valves17,48and148are in the flow-combining position A.

As a result, the communicating passage16that connects the first hydraulic pump6and second hydraulic pump7is closed, so that the pressurized oil that is discharged from the first hydraulic pump6is discharged only into a first discharge oil passage14, and the pressurized oil that is discharged from the second hydraulic pump7is discharged only into a second discharge oil passage15.

Furthermore, the first load pressure detection oil passage90and second load pressure detection oil passage91are cut off, and the first load pressure introduction oil passage163and second load pressure introduction oil passage164(164′) are cut off, so that pressure compensation is canceled. Specifically, an own load pressure is applied to the pressure receiving part of a first pressure compensating valve9via the first load pressure detection port23, the first load pressure detection oil passage90, the first load pressure introduction oil passage163, and shuttle valve63. As a result, the load pressure on the outlet cylinder port side of the first main operating valve8maintains this own load pressure.

Meanwhile, an own load pressure is applied to the pressure receiving part of the second pressure compensating valve12via a second load pressure detection port38, the second load pressure detection oil passage91, the second load pressure introduction oil passage164(164′), and a shuttle valve64. As a result, the load pressure on the outlet cylinder port side of the second main operating valve11maintains this own load pressure.

Thus, in a pitch back operation, the communicating passage16between the first hydraulic pump6and second hydraulic pump7is closed, and pressure compensation for the respective tilting cylinders4and5operates independently by the own load pressure. Accordingly, pressurized oil is independently supplied to the left and right tilting cylinders4and5from the first hydraulic pump6and second hydraulic pump7.

Accordingly, since pressure compensation is performed in a pitch operation, it is possible to avoid the inconvenience of a deviation in pressure compensation occurring in cases where the difference in the load pressure between the left and right tilting cylinders4and5is large so that the same flow rate cannot be supplied to the left and right tilting cylinders4and5, thus making it impossible for the left and right tilting cylinders4and5to operate at a uniform speed. As a result, a state in which the piston rods of the left and right tilting cylinders4and5do not reach the same stroke position so that the blade3tilts can be prevented.

Next, the fourth and fifth aspect of the present inventions will be described.

In the fourth and fifth aspect of the present inventions, pressure compensation is performed by introducing the maximum pressure among the load pressure detected in the respective main operating valves8,11,83and84into the respective pressure compensating valves9,12,85and86shown inFIGS. 1 and 4.

Furthermore, the pressurized oil discharged from the first and second hydraulic pumps6and7is supplied to the respective hydraulic cylinders4,5,81and82.

Here, in cases where a composite operation is performed in which the blade3is lifted, tilted and subjected to a pitch operation, the flow rate required by the lifting cylinders81and82may exceed the maximum flow rate of the pressurized oil that is discharged from one of the hydraulic pump6and7.

In the present invention, in the case of such a composite operation, the pressurized oil that is discharged from both hydraulic pumps6and7is caused to flow together and is supplied to the lifting cylinders81and82; accordingly, the operating speed of the lifting cylinders81and82is sufficiently guaranteed, so that the working efficiency can be improved.

Furthermore, since pressure compensation is performed during this composite operation, flow rates that are proportional to the amounts of operation of the tilting/pitch operating lever50and operating lever used for the lifting cylinders81and82can be supplied to the tilting cylinders4and5and lifting cylinders81and82, so that the operating characteristics of the composite operation can be improved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment of the hydraulic control apparatus for work machines provided by the present invention will be described below with reference to the accompanying drawings.

FIG. 1shows a hydraulic control apparatus for a bulldozer in terms of a hydraulic circuit.FIG. 2is a perspective view showing the construction of the peripheral parts of the bulldozer blade.

As is shown inFIG. 2, a blade3is installed on the front part of the vehicle main body not shown in the figures. Specifically, a pair of left and right straight frames1and2are supported at one end on the left and right outsides of a track frame not shown in the figures with trunnions as supporting points. The front ends of the respective straight frames1and2are respectively pivot-supported on the left and right of the back surface of the blade3.

A pair of left and right tilting cylinders (tilting hydraulic cylinders)4and5that tilt the blade3to the left and right are disposed between the blade3and the straight frames1and2. The rods of the tilting cylinders4and5are connected to the left and right of the back surface of the blade3, and the cylinder main bodies of the tilting cylinders4and5are connected to the straight frames1and2. Furthermore, although this is not shown inFIG. 2, a pair of left and right lifting cylinders that raise and lower the blade3are disposed on the bulldozer. Furthermore, a pair of left and right ripper lifting cylinders and a pair of left and right ripper tilting cylinders are installed corresponding to a ripper on the rear of the vehicle body.

As is shown inFIG. 1, the left and right tilting cylinders4and5are driven with two variable displacement hydraulic pumps, i. e., a first hydraulic pump6and second hydraulic pump7, as driving sources.

The first and second hydraulic pumps6and7are driven by an engine not shown in the figures.

The swash plate6aof the first hydraulic pump6is driven by a servomechanism71. The servomechanism71operates in accordance with a control signal (electrical signal, and varies the swash plate6aof the first hydraulic pump6to a position that corresponds to this control signal. As a result of the inclined position of the swash plate6aof the first hydraulic pump6being varied, the volume (cc/rev) of the first hydraulic pump6varies. Similarly, the swash plate7aof the second hydraulic pump7is driven by a servomechanism72. As a result of the inclined position of the swash plate7aof the second hydraulic pump7being varied, the volume (cc/rev) of the second hydraulic pump7varies.

The discharge port of the first hydraulic pump6communicates with a first discharge oil passage14. The first discharge oil passage14communicates with the pump ports19and20of the first main operating valve8used for the left tilting cylinder4. The reservoir ports21and22of the first main operating valve8respectively communicate with reservoirs28and29.

The first main operating valve8is a directional flow control valve that controls the direction and flow rate of the pressurized oil that is supplied to the left tilting cylinder4.

The cylinder port24of the first main operating valve8communicates with the head end oil chamber4bof the left tilting cylinder4via the first pressure compensating valve9and a check valve10, and the cylinder port25of the first main operating valve8communicates with the bottom end oil chamber4aof the left tilting cylinder4via the first pressure compensating valve9and a check valve10.

The outlet ports of the check valves10,10communicate with the reservoirs28and29via safety valves30and31and suction valves32and33.

The auxiliary cylinder ports26and27of the first main operating valve8respectively communicate with the head end oil chamber4band bottom end oil chamber4aof the left tilting cylinder4.

The first main operating valve8has a valve position A that causes the pump port20to communicate with the cylinder port25and the auxiliary cylinder port27, and causes the reservoir port21to communicate with the auxiliary cylinder port26, a neutral position, and a valve position B that causes the pump port19to communicate with the cylinder port24and auxiliary cylinder port26, and causes the reservoir port22to communicate with the auxiliary cylinder port27.

Pilot ports8aand8bare installed in the first main operating valve8. When pilot pressurized oil is supplied to the pilot port8a,the first main operating valve8moves to the side of the valve position A. Furthermore, when pilot pressurized oil is supplied to the pilot port8b,the first main operating valve8moves to the side of the valve position B.

Meanwhile, the outlet port of the second hydraulic pump7communicates with a second discharge oil passage15. The second discharge oil passage15communicates with the pump ports34and35of the second main operating valve11used for the right tilting cylinder5. The reservoir ports36and37of the second main operating valve11respectively communicate with the reservoirs28and29.

The second main operating valve11is a directional flow control valve that controls the direction and flow rate of the pressurized oil that is supplied to the right tilting cylinder5.

The cylinder port39of the second main operating valve11communicates with the head end oil chamber5bof the right tilting cylinder5via the second pressure compensating valve12and a check valve13, and the cylinder port40of the second main operating valve11communicates with the bottom end oil chamber5aof the right tilting cylinder5via the second pressure compensating valve12and a check valve13.

The outlet ports of the check valves13,13communicate with the reservoirs28and29via safety valves43and44and suction valves45and46.

The auxiliary cylinder ports41and42of the second main operating valve11respectively communicate with the head end oil chamber5band bottom end oil chamber5aof the right tilting cylinder5.

The second main operating valve11has a valve position A that causes the pump port35to communicate with the cylinder port40and the auxiliary cylinder port42, and causes the reservoir port36to communicate with the auxiliary cylinder port41, a neutral position, and a valve position B that causes the pump port34to communication with the5cylinder port39and the auxiliary cylinder port41, and causes the reservoir port37to communicate with the auxiliary cylinder port42.

Pilot ports11aand11bare installed in the second main operating valve11. When pilot pressurized oil is supplied to the pilot port11a,the second main operating valve11moves to the side of the valve position A. Furthermore, when pilot pressurized oil is supplied to the pilot port11b,the second main operating valve11moves to the side of the valve position B.

Pilot pressurized oil is supplied to the respective pilot ports8a,8b,11aand11bof the first and second main operating valves8and11via a pilot pressure signal circuit51.

As is shown inFIG. 3, a tilting operating lever50that can be operated in the leftward and rightward directions C and D is disposed in the driver's seat of the bulldozer. A pitch dump/pitch back switch50band a dual tilting switch50care disposed on the knob50aof the operating lever50.

A pilot valve49is attached to the operating lever50, and the pilot valve49operates in accordance with the operation of the operating lever50.

A pilot switching valve52is interposed in the pilot signal circuit51, and respective pilot oil passages51athrough51fare disposed in this circuit.

Original pressure is supplied to the inlet port of the pilot valve49attached to the operating lever50via the discharge port of the first hydraulic pump6, an oil passage56, an auto pressure reduction valve54, and an oil passage65. The outlet port of the pilot valve49is caused to communicate with the pilot oil passage51aor51bin accordance with the operating direction of the operating lever50. The pilot oil passage5la communicates with the pilot port8aof the first main operating valve8, and the pilot oil passage51bcommunicates with the pilot port8bof the first main operating valve8. The pilot oil passage51acommunicates with pilot oil passage51c,and the pilot passage51bcommunicates with the pilot passage51d. The pilot oil passages51cand51drespectively communicate with the inlet ports52aand52bof the pilot switching valve52.

The outlet ports52cand52dof the pilot switching valve52respectively communicate with the pilot ports11aand11bof the second main operating valve11via the pilot oil passages52eand52f.

The pilot switching valve52has a valve position A which causes the inlet port52ato communicate with the outlet port52cand causes the inlet port52bto communicate with the outlet port52d,a neutral position, and a valve position B which causes the inlet port52ato communicate with the outlet port52dand caused the inlet port52bto communicate with the outlet port52c. An electromagnetic solenoid52eis installed in the pilot switching valve52, and the pilot switching valve52operates in accordance with the electrical signal that is applied to the electromagnetic solenoid52eso that the valve position is switched. Electrical signals corresponding to the operating states of the switches50band50care applied to the electromagnetic solenoid52eof the pilot switching valve52.

As will be described later, switching to various operations such as the pitch dumping operation (forward tilting operation) of the blade3effected by the driving of both tilting cylinders4and5, the pitch back operation (rearward tilting operation) of the blade3effected by the driving of both tilting cylinders4and5, the single tilting operation of the blade3effected by the driving of only the left tilting cylinder4, and the dual tilting operation of the blade3effected by the driving of both tilting cylinders4and5, is accomplished in accordance with the operating direction of the operating lever50and the operating states of the switches50band50c.

Signals indicating the operating direction of the operating lever50and the operating states of the switches50band50care input into the controller53, and the electrical signals that are to be applied to the electromagnetic solenoid52eof the pilot switching valve52are generated on the basis of these input signals, and are output to the electromagnetic solenoid52eof the pilot switching valve52.

The first discharge oil passage14and second discharge oil passage15are connected by a communicating oil passage (flow-combining oil passage)16. A flow-combining/flow-dividing valve17is installed in the communicating oil passage16. The flow-combining/flow-dividing valve17is a switching valve which has a flow-combining position A that opens the communicating passage16and causes the first discharge oil passage14and second discharge oil passage15to communicate, and a flow-dividing valve position B which closes the communicating passage16and cuts off the communication between the first discharge oil passage14and second discharge oil passage15. The flow-combining/flow-dividing valve17performs a switching operation in accordance with hydraulic signals that are applied to the attached pilot valve17avia the pilot oil passage61. When the hydraulic signal is equal to or greater than a specified pressure, the valve is switched to the flow-dividing position B, and when the hydraulic signal is a pressure (reservoir pressure) that is less than this specified pressure, the valve is switched to the flow-combining position A.

The discharge port of the first hydraulic pump6communicates with the inlet port of the flow-combining/flow-dividing switching valve18via an oil passage56, auto pressure reduction valve54and oil passage66. The flow-combining/flow-dividing switching valve18causes the reservoir55and pilot oil passage61to communicate, and is a switching valve which has a flow-combining position A that outputs a hydraulic signal (reservoir pressure) that is smaller than the abovementioned specified pressure to the pilot oil passage61, and causes the first discharge oil passage14and second discharge oil passage15to communicate, and a flow-dividing position B that outputs a hydraulic signal that is equal to or greater than the abovementioned specified pressure to the pilot oil passage61, and cuts off the communication between the first discharge oil passage14and second discharge oil passage15. The flow-combining/flow-dividing switching valve18performs a switching operation in accordance with the electrical control signals that are applied to the attached electromagnetic solenoid18a.

The pilot oil passage61branches into a branch pilot oil passage62, and hydraulic signals are also output into the branch pilot oil passage62from the flow-combining/flow-dividing switching valve18.

First pressure compensating valves9,9that compensate the pressure difference before and after the constriction of the first main operating valve8to a specified value are installed in the first main operating valve8.

Meanwhile, second pressure compensating valves12,12that compensate the pressure difference before and after the constriction of the second main operating valve11to a specified value are installed in the second main operating valve11.

A pilot pressure on the side of the outlet port of the shuttle valve63is supplied to the pressure receiving parts of the first pressure compensating valves9,9.

One inlet port of the shuttle valve63communicates with the outlet port of the check valve10via a maintenance pressure introduction oil passage67, and the other inlet port of the shuttle valve63communicates with one input-output port of the flow-combining/flow-dividing valve148via a first load pressure introduction oil passage163.

Meanwhile, a pilot pressure on the side of the outlet port of the shuttle valve64is supplied to the pressure receiving parts of the second pressure compensating valves12,12.

One inlet port of the shuttle valve64communicates with the outlet port of the check valve13via a maintenance pressure introduction oil passage68, and the other inlet port of the shuttle valve64communicates with the other input-output port of the flow-combining/flow-dividing valve148via a second load pressure introduction oil passage164.

The cylinder ports24and25of the first main operating valve8communicate with a first load pressure detection port23, so that the load pressure of left tilting cylinder4is detected by the first load pressure detection port23. The first load pressure detection port23communicates with one input-output port of the flow-combining/flow-dividing valve48via a first load pressure detection oil passage190. Furthermore, the first load pressure detection oil passage90communicates with the first load pressure introduction passage163.

Meanwhile, the cylinder ports39and40of the second main operating valve11communicate with a second load pressure detection port38, and the load pressure of the right tilting cylinder5is detected by the second load pressure detection port38. The second load pressure detection port38communicates with the other input-output port of the flow-combining/flow-dividing valve48via a second load pressure detection oil passage91. Furthermore, the second load pressure detection port38communicates with the inlet port of the shuttle valve64via the second load pressure detection oil passage91and second load pressure introduction oil passage164(164′).

Specifically, the hydraulic circuits inside the flow-combining/flow-dividing valves48and148conduct pressurized oil from the first load pressure detection port23of the first main operating valve8to the flow-combining/flow-dividing valve48and148via the first load pressure detection oil passage90. Furthermore, the first load pressure detection oil passage90branches at the connection point M, and is connected to the left and right shuttle valves63,63via the load pressure introduction passage163. Furthermore, the hydraulic circuits outside the flow-combining/flow-dividing valves48and148conduct pressurized oil from the second load pressure detection port38of the second main operating valve11to the first load pressure detection oil passage91, and are constructed so as to branch into a three-way channel at the connection point Q. One branch oil passage of the connection Q is the oil passage92, which is connected to the flow-combining/flow-dividing valve48. Another branch oil passage is the oil passage93, which is connected to the flow-combining/flow-dividing valve148. The remaining branch oil passage is the second load pressure introduction oil passage164, which is connected to the shuttle valve64on the right side of the figure. The oil passage93is connected to the second load pressure introduction oil passage164′ by the inlet of the flow-combining/flow-dividing valve148, and the second load pressure introduction oil passage164′ is connected to the shuttle valve64on the left side of the figure.

Furthermore, the flow-combining/flow-dividing valves48and148are switching valves which have a flow-combining position that introduces the pilot pressurized oil with the highest load pressure among the respective load pressures detected by the first load pressure detection ports23and38into the first and second load pressure introduction oil passages163and164(164′), and a flow-dividing position B which respectively introduces the respective load pressures detected by the first load pressure detection ports23and38into the corresponding first and second load pressure introduction oil passages163and164(164′) via the corresponding first and second load pressure detection oil passages90and91. The flow-combining/flow-dividing valves48and148perform a switching operation in accordance with hydraulic signals that are applied via the branch pilot oil passage62to the attached pilot ports48aand148a. When the hydraulic signals are equal to or greater than a specified pressure, these valves are switched to the flow-dividing position B, and when the hydraulic signals are a voltage (reservoir voltage) that is smaller than this specified pressure; these valves are switched to the flow-combining position A.

In the controller53, signals indicating the operating direction of the operating lever50and the operating states of the switches50band50care input, the electrical control signals that are to be applied to the electromagnetic solenoid18aof the flow-combining/flow-dividing switching valve18are generated on the basis of these input signals, and these generated signals are output to the electromagnetic solenoid18aof the flow-combining/flow-dividing switching valve18.

Furthermore, in the controller53, signals that indicate the operating direction of the operating lever50and the operating states of the switches50band50care input, the electrical control signals that are to be applied to the servo valves71and72are generated on the basis of these input signals, and these generated signals are output to the servo valves71and72, so that the inclined positions of the swash plates6aand7aof the first and second hydraulic pumps6and7are controlled.

Furthermore, although this is not shown inFIG. 1, the control of the inclined positions of the swash plates2aand3aof the first and second hydraulic pumps6and7is based on the assumption that this control is accomplished by load sensing control.

Specifically, for example, the load pressure (designated as PL) that is introduced into the first load pressure introduction oil passage163is applied to the servomechanism71of the first hydraulic pump6, and the pressure (designated as Pp) of the pressurized oil flowing through the first discharge oil passage14is applied to the servomechanism71of the first hydraulic pump6.

Here, the difference between the two pressures Pp−PL is the pressure difference ΔP1before and after the constriction of the first main operating valve8. In the servomechanism71, the inclined position of the swash plate6aof the first hydraulic pump6is controlled so that the pressure difference ΔP1(=Pp−PL) before and after the first main operating valve8is maintained at a constant pressure.

In the case of a servomechanism using only a hydraulic circuit that has load sensing control, the pressure difference ΔP before and after the first main operating valve8is a constant value; however, in the present embodiment, the hydraulic pressure of a separate system is added to the hydraulic pressure of PL or Pp by the electrical signals from the controller53, so that the abovementioned before-and-after pressure difference ΔP is made variable.

Similarly, in regard to the side of the second hydraulic pump7as well, the load pressure (PL) that is introduced into the second load pressure introduction oil passage164is applied to the servomechanism72of the second hydraulic pump7, and the pressure (Pp) of the pressurized oil that flows through the second discharge oil passage15is applied to the servomechanism72of the second hydraulic pump7, so that load sensing control is similarly performed.

Next, the relationship between the tilting cylinders4and5and other tilting cylinders will be described with reference to the hydraulic circuit shown inFIG. 4. Furthermore, for convenience of description, inFIG. 4, the relationship between the left and right lifting cylinders81and82attached to the blade3and the left and right tilting cylinders4and5will be described, and a description of the pair of left and right ripper lifting cylinders and pair of left and right ripper tilting cylinders corresponding to the ripper installed on the rear of the vehicle body is omitted.

As is shown inFIG. 4, first and second main operating valves83and84are installed corresponding to the left and right lifting cylinders81and82in the same manner as the first and second main operating valves8and11installed corresponding to the left and right tilting cylinders4and5. Furthermore, first and second pressure compensating valves85and86are also respectively installed for the first and second main operating valves83and84in the same manner as the first and second pressure compensating valves9and12installed corresponding to the first and second main operating valves8and11.

The first main operating valve8for the left tilting cylinder and the first main operating valve83for the left lifting cylinder are connected in series to the first discharge oil passage14. Similarly, the second main operating valve11for the right tilting cylinder and the second main operating valve84for the right lifting cylinder are connected in series to the second discharge oil passage15.

Furthermore,FIG. 4is constructed as a series circuit, but working that uses a parallel circuit or tandem circuit is also possible.

The operation of the hydraulic circuit constructed as shown in the abovementionedFIG. 1andFIG. 4will be described below.

When the operator moves the key switch to the engine starting position, a voltage is applied to the controller53from the power supply, so that the controller53starts, and the engine is started. In the initial state of the controller53at the time of starting, an electrical control signal is output to the electromagnetic solenoid18aso that the flow-combining/flow-dividing switching valve18is positioned in the flow-combining position A.

When the flow-combining/flow-dividing switching valve18is positioned in the flow-combining position A, the respective flow-combining/flow-dividing valves17,48and148are positioned in the flow-combining position A, so that pressure compensation is performed.

Specifically, when the flow-combining/flow-dividing valves48and148are positioned in the flow-combining position A, the first load pressure detection oil passage90and second load pressure detection oil passage91are caused to communicate with each other, and the first load pressure introduction oil passage163and second load pressure introduction oil passage164(164′) also communicate. Here, assuming that the load pressure detected by the second load pressure detection port38of the second main operating valve11is higher than the load pressure detected by the first load pressure detection port23of the first main operating valve8, then the maximum load pressure is applied to the pressure receiving part of the first pressure compensating valve9via the second load pressure detection port38, second load pressure detection oil passage91, flow-combining/flow-dividing valve48, first load pressure introduction oil passage163and shuttle valve63. As a result, the load pressure on the outlet cylinder port side of the first main operating valve8varies from the own load pressure (a load pressure lower than the maximum load pressure) to the maximum load pressure in apparent terms.

Meanwhile, the maximum load pressure is applied to the pressure receiving part of the second pressure compensating valve12via the second load pressure detection port38, second load pressure detection oil passage91, and second load pressure introduction oil passage164(164′) and shuttle valve64. As a result, the load pressure on the outlet cylinder port side of the second main operating valve11maintains the own load pressure (maximum load pressure).

When pressure compensation is performed, the pressure difference before and after the constriction of the first main operating valve8on the side where the load is light is the same value as the pressure difference before and after the constriction of the second main operating valve11on the side where the load is heavy. Accordingly, in the pressure-compensated state, the pressure differences before and after the constructions of the first and second main operating valves8and11are the same value, so that the load has no effect, and flow rates that are proportional to the degrees of opening of the first and second main operating valves8and11, i. e., to the amount of operation of the operating lever50, can be supplied to the left and right tilting cylinders4and5.

Thus, a flow-combining state is created in the initial state. Subsequently, a judgment is made as to whether to place the system in a flow-combining state or flow-dividing state in accordance with the operating states of the switches50band50cdisposed on the operating lever50.

In cases where it is desired to perform a pitch operation, the operator moves the operating lever50in either the leftward direction or rightward direction C or D while pressing the pitch dumping/pitch back switch50bof the operating lever50.

In the controller53, as a result of the switch50bbeing pressed, electrical control signals that are used to place the flow-combining/flow-dividing switching valve18and the flow-combining/flow-dividing valves17,48and148in the flow-dividing position B are generated, and these electrical control signals are output to the flow-combining/flow-dividing switching valve18so that the flow-combining/flow-dividing switching valves18, the flow-combining/flow-dividing valves17,48and148are switched to the flow-dividing position B.

As a result, the communicating oil passage16is closed, so that the pressurized oil that is discharged from the first hydraulic pump6is discharged only into the first discharge oil passage14, and the pressurized oil that is discharged from the second hydraulic pump7is discharged only into the second discharge oil passage15.

Furthermore, the first load pressure detection oil passage90and second load pressure detection oil passage91are cut off, and the firsts load pressure introduction oil passage163and second load pressure introduction oil passage164(164′) are cut off, so that pressure compensation is canceled. Specifically, the own load pressure is applied to the pressure receiving part of the first pressure compensating valve9via the first load pressure detection port23, first load pressure detection oil passage90, first load pressure introduction oil passage163and shuttle valve63. As a result, the load pressure on the outlet cylinder port side of the first main operating valve8maintains the own load pressure.

On the other hand, the own load pressure is applied to the pressure receiving part of the second pressure compensating valve12via the second load pressure detection port38, second load pressure detection oil passage91, second load pressure introduction oil passage164, and shuttle valve64. As a result, the load pressure on the outlet cylinder port side of the second main operating valve11maintains the own load pressure.

In cases where it is desired to perform a pitch dumping operation, the operator moves the operating lever50in the “rightward direction D” while pressing the pitch dumping/pitch back switch50bof the operating lever50.

When the operating lever50is moved in the rightward direction D, the pilot pressure that is discharged from the outlet port of the pilot valve49is supplied to the pilot oil passage51a,and acts on the pilot port8aof the first main operating valve8via the pilot oil passage51a.

Furthermore, as a result of the switch50bbeing pressed, an electrical signal is output to the pilot switching valve52from the controller53, so that the pilot switching valve52is switched to the A position. Accordingly, the pilot pressure that is discharged from the outlet port of the of the pilot valve49acts on the pilot port11aof the second main operating valve11via the pilot oil passage51a,pilot oil passage51c,pilot switching valve52and pilot port oil passage51e.

Consequently, the first main operating valve8is switched to the A position, and the second main operating valve11is also switched to the A position. As a result, the pressurized oil that is discharged from the first hydraulic pump6passes through the first discharge oil passage14, pump port20of the first main operating valve8, and cylinder port25, and is supplied to the bottom end oil chamber4aof the left tilting cylinder4, so that the left tilting cylinder4is operated in the direction of extension. The return pressurized oil from the head end oil chamber4bof the left tilting cylinder4is recovered in the reservoir28via the auxiliary cylinder port26and reservoir port21of the first main operating valve8.

At the same time, the pressurized oil that is discharged from the second hydraulic pump7is supplied to the bottom end oil chamber5aof the right tilting cylinder5via the second discharge oil passage15, pump port35of the second main operating valve11, and cylinder port40, so that the right tilting cylinder5is operated in the direction of extension. The return pressurized oil from the head end oil chamber5bof the right tilting cylinder5is recovered in the reservoir28via the auxiliary cylinder port41and reservoir port36of the second main operating valve11. Thus, the left and right tilting cylinders4and5are simultaneously extended at an equal speed, so that the blade3performs a pitch dumping (forward tilting) operation.

In cases where it is desired to perform a pitch back operation, the operator moves the operating lever50in the “leftward direction C” while pressing the pitch dumping/pitch back switch50bof the operating lever50.

When the operating lever50is moved in the leftward direction C, the pilot pressure that is discharged from the outlet port of the pilot valve49is supplied to the pilot oil passage51b,and acts on the pilot port8bof the first main operating valve8via the pilot oil passage51b.

Furthermore, as a result of the switch50bbeing pressed, an electrical signal is output to the pilot switching valve52from the controller53, so that the pilot switching valve52is switched to the A position.

Accordingly, the pilot pressure that is discharged from the outlet port of the pilot valve49acts on the pilot port11bof the second main operating valve11via the pilot oil passage51b,pilot oil passage51d,pilot switching valve52, and pilot oil passage51f.

Consequently, the first main operating valve8is switched to the B position, and the second main operating valve11is also switched to the B position.

As a result, the pressurized oil that is discharged from the first hydraulic pump6is supplied to the head end oil chamber4bof the left tilting cylinder4via the first discharge oil passage14, pump port19of the first main operating valve8and cylinder port24, so that the left tilting cylinder4is operated in the direction of retraction. The return pressurized oil from the bottom end oil chamber4aof the left tilting cylinder4is recovered in the reservoir29via the auxiliary cylinder port27and reservoir port22of the first main operating valve8.

At the same time, the pressurized oil that is discharged from the second hydraulic pump7is supplied to the head end oil chamber5bof the right tilting cylinder5via the second discharge oil passage15, pump port34of the second main operating valve11, and cylinder port39, so that the right tilting cylinder34is operated in the direction of retraction. The return pressurized oil from the bottom end oil chamber5aof the right tilting cylinder5is recovered in the reservoir29via the auxiliary cylinder port42and reservoir port37of the second main operating valve11. Thus, the respective left and right tilting cylinders4and5are simultaneously retracted at an equal speed, so that the blade3performed a pitch back (rearward tilting) operation.

Thus, in the case of a pitch operation, pressure compensation is canceled, and pressurized oil is independently supplied from the first hydraulic pump6and second hydraulic pump7to the left and right tilting cylinders4and5.

Accordingly, the inconvenience of a deviation in pressure compensation being generated by the performance of pressure compensation during a pitch operation in cases where the difference in the load pressures of the left and right tilting cylinders4and5is large, so that the same flow rate cannot be supplied to the left and right tilting cylinders4and5, thus making it impossible to operate the left and right tilting cylinders4and5at a uniform operating speed, can be avoided. As a result, a state in which the piston rods of the left and right tilting cylinders4and5do not reach the same stroke position during a pitch operation, so that the blade3tilts, can be prevented.

In cases where it is desired to perform a dual tilting operation, the operator moves the operating lever50in either the leftward or rightward direction C or D while pressing the dual tilting switch50cof the operating lever50.

As a result of the switch50cbeing pressed, electrical control signals that are used to place the flow-combining/flow-dividing switching valve18and flow-combining/flow-dividing valves17,48and148in the flow-dividing position B are generated by the controller53, and these electrical control signals are output to the flow-combining/flow-dividing switching valve18so that the flow-combining/flow-dividing switching valves the flow-combining/flow-dividing valves17,48and148are switched to the flow-dividing position B.

As a result, the communicating oil passage16is closed, so that the pressurized oil that is discharged from the first hydraulic pump6is discharged only into the first discharge oil passage14, and the pressurized oil that is discharged from the second hydraulic pump7is discharged only into the second discharge oil passage15.

Furthermore, the first load pressure detection oil passage90and second load pressure detection oil passage91are cut off, and the first load pressure introduction oil passage163and second load pressure introduction oil passage164are cut off, so that pressure compensation is canceled. Specifically, the own load pressure is applied to the pressure receiving part of the first pressure compensating valve9via the first load pressure detection port23, first load pressure detection oil passage90, first load pressure introduction oil passage163and shuttle valve63. As a result, the load pressure on the outlet cylinder port side of the first main operating valve8maintains the own load pressure.

Meanwhile, the own load pressure is applied to the pressure receiving part of the second pressure compensating valve12via the second load pressure detection port38, the second load pressure detection oil passage91, the second load pressure introduction oil passage164and the shuttle valve64. As a result, the load pressure on the outlet cylinder port side of the second main operating valve11maintains the own load pressure.

In cases where it is desired to perform a right dual tilting operation, the operator moves the operating lever50in the “rightward direction D” while pressing the dual tilting switch50cof the operating lever50.

When the operating lever50is moved in the rightward direction D, the pilot pressure that is discharged from the outlet port of the pilot valve49is supplied to the pilot oil passage51a,and acts on the pilot port8aof the first main operating valve8via the pilot oil passage51a.

Furthermore, as a result of the switch50cbeing pressed, an electrical signal is output to the pilot switching valve52from the controller53, so that the pilot switching valve52is switched to the B position.

Accordingly, the pilot pressure that is discharged from the outlet port of the pilot valve49acts on the pilot port11bof the second main operating valve11via the pilot oil passage51a,pilot oil passage51c,pilot switching valve52and pilot oil passage51f.

Consequently, the first main operating valve8is switched to the A position, and the second main operating valve11is switched to the B position.

As a result, the pressurized oil that is discharged from the first hydraulic pump6is supplied to the bottom end oil chamber4aof the left tilting cylinder4via the first discharge oil passage14, pump port20of the first main operating valve8, and cylinder port25, so that the left tilting cylinder4is operated in the direction of extension. The return pressurized oil from the head end oil chamber4bof the left tilting cylinder4is recovered in the reservoir28via the auxiliary cylinder port26and reservoir port21of the first main operating valve8.

At the same time, the pressurized oil that is discharged from the second hydraulic pump7is supplied to the head end oil chamber5bof the right tilting cylinder5via the second discharge oil passage15, pump port34of the second main operating valve11, and cylinder port39, so that the right tilting cylinder5is operated in the direction of retraction. The return pressurized oil from the bottom end oil chamber5aof the right tilting cylinder5is recovered in the reservoir29via the auxiliary cylinder port42and reservoir port37of the second main operating valve11.

Thus, an extension operation of the left tilting cylinder4and a retraction operation of the right tilting cylinder5are simultaneously performed, so that the blade3performs a right dual tilting operation at a high speed (substantially twice the speed of a single tilting operation).

In cases where it is desired to perform a left dual tilting operation, the operator moves the operating lever50in the “leftward direction C” while pressing the dual tilting switch50cof the operating lever50.

When the operating lever503is moved in the leftward direction C, the pilot pressure that is discharged from the outlet port of the pilot valve49is supplied to the pilot oil passage51b,and acts on the pilot port8bof the first main operating valve8via the pilot oil passage51b.

Furthermore, as a result of the switch50cbeing pressed, an electrical signal is output to the pilot switching valve52from the controller53, so that the pilot switching valve52is switched to the B position.

Accordingly, the pilot pressure that is discharged from the outlet port of the pilot valve49acts on the pilot port11aof the second main operating valve11via the pilot oil passage51b,pilot oil passage51d,pilot switching valve52, and pilot oil passage51e.

Consequently, the first main operating valve8is switched to the B position, and the second main operating valve11is switched to the A position.

As a result, the pressurized oil that is discharged from the first hydraulic pump6is supplied to the head end oil chamber4bof the left tilting cylinder4via the first discharge oil passage14, pump port19of the first main operating valve8, and cylinder port24, so that the left tilting cylinder4is operated in the direction of retraction. The return pressurized oil from the bottom end oil chamber4aof the left tilting cylinder4is recovered in the reservoir29via the auxiliary cylinder port27and reservoir port22of the first main operating valve8.

At the same time, the pressurized oil that is discharged from the second hydraulic pump7is supplied to the bottom end oil chamber5aof the right tilting cylinder5via the second discharge oil passage15, pump port35of the second main operating valve11, and cylinder port40, so that the right tilting cylinder5is operated in the direction of extension. The return pressurized oil from the head end oil chamber5bof the right tilting cylinder5is recovered in the reservoir28via the cylinder port41and reservoir port36of the second main operating valve11.

Thus, a retraction operation of the left tilting cylinder4and extension operation of the right tilting cylinder5are simultaneously performed, so that the blade3performs a left dual tilting operation at a high speed (substantially twice the speed of a single tilting operation).

Thus, during a dual tilting operation, pressure compensation is canceled, so that pressurized oil is independently supplied to the left and right tilting cylinders4and5from the first hydraulic pump6and second hydraulic pump7.

Accordingly, the flow rates of the pressurized oil supplied to the left and right tilting cylinders4and5can be independently adjusted by means of the servomechanisms71and72.

In the controller53, as a result of the dual tilting switch50cbeing pressed, electrical control signals that are used to set the stroke amounts of the tilting cylinders4and5at the same amount P during retraction and during extension are output to the servomechanisms71and72, and the swash angles of the swash plates6aand7aof the first and second hydraulic pumps6and7are controlled so that the flow rates that are supplied to the respective tilting cylinders4and5are adjusted.

Referring now to the abovementionedFIG. 5Bas well, in the case of the left tilting cylinder4(tilting cylinder103inFIG. 5B), during retraction, pressurized oil at a specified flow rate QH is supplied to the head end oil chamber4b(head end oil chamber103H inFIG. 5B), so that the tilting cylinder moves in the direction of retraction by a stroke P from the initial position L0, and reaches the stroke position L1; then, during the subsequent extension, pressurized oil at a flow rate QB that is larger than the flow rate QH during retraction is supplied to the bottom end oil chamber4a(bottom end oil chamber103B inFIG. 5B), so that the tilting cylinder moves by the same stroke P in the direction of extension from the stroke position L1, and returns to the original initial position L0.

The amount of oil required in order to obtain the same stroke during retraction and extension (corresponding to the abovementioned QH and QB) is determined by the volumetric ratio of the head side and bottom side of the cylinder. Specifically, the reason for this is as follows: namely, inFIG. 5B, since the cylinder rod103aor102ais a rod that actually has a volume, if QH=QB, then a difference is generated in the movement stroke.

Accordingly, on the head side and bottom side, if an amount of oil that is proportional to the effective pressure receiving area that receives the hydraulic pressure is supplied to the cylinder during retraction and extension, the strokes during retraction and extension can be made equal.

In the present embodiment, the system is constructed so that this pressure receiving area ratio is stored beforehand in the controller53, and the swash angles of the swash plates6aand7aof the first and second hydraulic pump6and7are controlled so that an amount of oil that is reduced according to the stored pressure receiving area ratio is supplied to the head side during retraction (with the amount of pressurized oil supplied during extension (supply of pressurized oil to the bottom side) taken as 1).

Meanwhile, in the case of the right tilting cylinder5(tilting cylinder102inFIG. 5B, during extension, pressurized oil at a specified flow rate of QB is supplied to the bottom end oil chamber5a(bottom end oil chamber102B inFIG. 5B), so that this tilting cylinder moves by a stroke of P in the direction of extension from the initial position R0, and reaches the stroke position R3. Then, during the subsequent retraction, pressurized oil at a flow rate of QH that is smaller than the flow rate QB during extension is supplied to the head end oil chamber5b(head end oil chamber102H inFIG. 5B, so that the tilting cylinder moves by the same stroke P in the direction of retraction from the stroke position R3, and returns to the original initial position R0.

As a result, the stroke positions of the left and right tilting cylinders4and5maintain the original initial positions without being shifted toward the pitch back side from the initial positions L0and R0as a result of a single dual tilting operation. In other words, a dual tilting operation can be performed without causing the blade3to fall over on the pitch back side. Furthermore, even if a dual tilting operation is performed a multiple number of times, the piston rods do not reach the stroke end on pitch back side of the blade3, i. e., in the direction of retraction.

Furthermore, since pressure compensation is canceled, the inconvenience of a deviation in pressure compensation being generated in cases where the difference between the load pressures of the left and right tilting cylinders4and5is large during a dual tilting operation, so that the same flow rates cannot be supplied to the left and right tilting cylinders4and5, thus making it impossible for the left and right tilting cylinders4and5to operate at a uniform speed, can be avoided. As a result, a state in which the piston rods of the left and right tilting cylinders4and5do not return to the initial positions in a dual tilting operation, so that the blade3is tilted, can be prevented.

In cases where it is desired to perform a single tilting operation, the operating lever50is moved in either the leftward or rightward direction C or D without pressing either the pitch dumping/pitch back switch50bor dual tilting switch50cof the operating lever50.

If neither of the switches50bnor50cis pressed, then electrical control signals that are sued to place the flow-combining/flow-dividing switching valve18and flow-combining/flow-dividing valves17,48and148in the flow-combining position A are generated in the controller53, and these electrical control signals are output to the flow-combining/flow-dividing switching valve18so that the flow-combining/flow-dividing switching valve18and flow-combining/flow-dividing valves17,48and148are switched to the flow-combining position A.

As a result, the communicating oil passage16is closed, so that the pressurized oil that is discharged from the first and second hydraulic pumps6and7is discharged into the first discharge oil passage14, and the pressurized oil that is discharged from the first and second hydraulic pumps6and7is discharged into the second discharge oil passage15.

Furthermore, the first load pressure detection oil passage90and second load pressure detection oil passage91are caused to communicate with each other, and the first load pressure introduction oil passage163and second load pressure introduction oil passage164(164′) also communicate, so that pressure compensation is performed. Specifically, if the load pressure that is detected by the second load pressure detection port38of the second main operating valve11is higher than the load pressure that is detected by the first load pressure detection port23of the first main operating valve8, then the maximum load pressure is applied to the pressure receiving part of the first pressure compensating valve9via the second load pressure detection port38, second load pressure detection oil passage91, flow-combining/flow-dividing valve48, first load pressure introduction oil passage163and shuttle valve63. As a result, the load pressure on the outlet cylinder port side of the first main operating valve8varies from the own load pressure (a load pressure that is lower than the maximum load pressure) to the maximum load pressure in apparent terms.

Meanwhile, the maximum load pressure is applied to the pressure receiving part of the second pressure compensating valve12via the second load pressure detection port38, second load pressure detection oil passage91, second load pressure introduction oil passage164and shuttle valve64. As a result, the load pressure on the outlet cylinder port side of the second main operating valve11maintains the own load pressure (maximum load pressure).

In cases where it is desired to perform a right single tilting operation, the operating lever50is moved in the “rightward direction D” without pressing either the pitch dumping/pitch back switch50bor dual tilting switch50cof the operating lever50.

When the operating lever50is moved in the rightward direction, the pilot pressure that is discharged from the outlet port of the pilot valve49is supplied to the pilot oil passage51a,and acts on the pilot port8aof the first main operating valve8via the pilot oil passage51a.

Furthermore, when the switches50b,50bare not pressed, an electrical signal is output to the pilot switching valve52from the controller53, so that the pilot switching valve52is held in the neutral position N.

Accordingly, no pilot pressure is supplied to the pilot port8aor8bof the second main operating valve11.

Consequently, the first main operating valve8is switched to the A position, and the second main operating valve11is held in the N position.

As a result, the pressurized oil that is discharged from the first and second hydraulic pump6and7is supplied to the bottom end oil chamber4aof the left tilting cylinder4via the first discharge oil passage14, pump port20of the first main operating valve8, and cylinder port25, and left tilting cylinder4moves in the direction of extension. The return pressurized oil from the head end oil chamber4bof the left tilting cylinder4is recovered in the reservoir28via the auxiliary cylinder port26and reservoir port21of the first main operating valve8.

Meanwhile, since the second main operating valve11is in the neutral position, pressurized oil is not supplied to the right tilting cylinder5, so that the operation of the right tilting cylinder5is stopped.

Thus, in a state in which the right tilting cylinder5is stopped, only an extension operation of the left tilting cylinder4is performed, so that the blade3performs a right single tilting operation at the ordinary speed (low speed).

In cases where it is desired to perform a left single tilting operation, the operating lever50is moved in the “leftward direction C” without pressing either the pitch dumping/pitch back switch50bor dual tilting switch50cof the operating lever50.

When the operating lever50is moved in the leftward direction C, the pilot pressure that is discharged from the outlet port of the pilot valve49is supplied to the pilot oil passage51b,and acts on the pilot port8bof the first main operating valve8via the pilot oil passage51b.

Furthermore, if neither the switches50b,50bare not pressed, an electrical signal is output to the pilot switching valve52from the controller53, so that the pilot switching valve52is held in the neutral position N.

Accordingly, no pilot pressure is supplied to the pilot port8aor8bof the second main operating valve11.

Consequently, the first main operating valve8is switched to the B position, and the second main operating valve11maintains the neutral position.

As a result, the pressurized oil that is discharged from the first and second hydraulic pumps6and7is supplied to the head end oil chamber4bof the left tilting cylinder4via the first discharge oil passage14, the pump port19of the first main operating valve8, and the cylinder port24, so that the left tilting cylinder4is operated in the direction of retraction. The return pressurized oil from the bottom end oil chamber4aof the left tilting cylinder4is recovered in the reservoir29via the auxiliary cylinder port27and reservoir port22of the first main operating valve8.

Meanwhile, since the second main operating valve11is in the neutral position, no pressurized oil is supplied to the right tilting cylinder5, so that the operation of the right tilting cylinder5is stopped.

Thus, in a state in which the right tilting cylinder5is stopped, only a retraction operation of the left tilting cylinder4is performed, so that the blade3performs a left single tilting operation at the ordinary speed (low speed).

In cases where it is desired to cause the blade3to perform a tilting operation or pitch operation while lifting, the operator operates the operating lever50used for tilting/pitch operations, and also operates the operating lever used for the lifting cylinders81and82.

When a signal indicating that the operating lever50used for tilting/pitch operations and a signal indicating that the operating lever used for the lifting cylinders has been operated are input into the controller53, and it is judged that a tilting operation (single tilting operation, dual tilting operation) or pitch operation and a lifting operation are being performed at the same time, electrical control signals that are used to place the flow-combining/flow-dividing switching valve18and flow-combining/flow-dividing valves17,48and148in the flow-combining position A are generated, and these electrical control signals are output to the flow-combining/flow-dividing switching valve18so that the flow-combining/flow-dividing switching valve18and flow-combining/flow-dividing valves17,48and148are switched to the flow-combining position A.

As a result, the maximum pressure among the load pressures detected by the respective main operating valves8,11,83and84is introduced into the respective pressure compensating valves9,12,85and86, so that pressure compensation is performed.

Furthermore, the pressurized oil that is discharged from the first and second hydraulic pumps6and7is supplied to the respective hydraulic cylinders4,5,81and82.

Here, in the case of a composite operation in which the blade3is caused to perform a tilting operation of pitch operation while lifting, the flow rate required by the lifting cylinders81and82may in some cases exceed the maximum flow rate of the pressurized oil that is discharged from either one of the hydraulic pumps6and7. In the present embodiment, the pressurized oil that is discharged from both hydraulic pumps6and7is caused to flow together in the case of a composite operation, and is supplied to the lifting cylinders81and82; accordingly, the operating speeds of the lifting cylinders81and82can be sufficiently maintained, and the working efficiency can be improved.

Furthermore, since pressure compensation is performed in the case of a composite operation, flow rates that are proportional to the amounts of operation of the operating lever50used for tilting/pitch operations and the operating lever used for the lifting cylinders81and82can be supplied to the tilting cylinders4and5and lifting cylinders81and82regardless of differences in the magnitude of the load, so that the operating characteristics during a composite operation can be improved.

The present invention was described above in terms of several limited number of embodiments; however, other embodiment obtained by a person skilled in the art receiving the benefit of the disclosure of the present invention are also included in the scope of the technical spirit of the present invention.