HYDRAULIC CIRCUIT FOR HYDRAULICALLY DRIVEN WORKING VEHICLES

Abstract: The present invention relates to a hydraulic circuit for a hydraulically driven working vehicle which allows the vehicle to run at an almost constant speed during high-speed running, provides a great digging force during working, does not need charging pressure for preventing cavitation and loses a small amount of energy, and has a simple structure. To this end, the hydraulic circuit has a running hydraulic pump (2) and a working machine hydraulic pump (4) which are driven by power of an engine (1) for discharging pressure oil of a running HST circuit and a working machine-driving hydraulic circuit, respectively, wherein pressure oil from the working machine hydraulic pump (4) joins pressure oil in the running hydraulic pump (2) to effect high-speed running, while pressure oil from the running hydraulic pump (2) joins pressure oil in the working machine hydraulic pump (4) to generate a large digging force to effect digging.

Description: The present invention relates to a hydraulic circuit for a hydraulically driven working vehicle, and more particularly, to a hydraulic circuit for a hydraulically driven vehicle running by driving drive wheels with hydraulic pumps and a hydraulic motor driven by an engine, and having a working machine attached thereto. Hydraulically driven working vehicles have been known in which a running hydraulic pump and a working machine hydraulic pump are driven by an engine, a running motor is rotated by discharge pressure oil of the running hydraulic pump to drive a drive wheel for running, and a working machine cylinder is extended and contracted by discharge pressure oil of the working machine hydraulic pump to actuate a working machine. As a hydraulic circuit for the above-described hydraulically driven working vehicles, a hydraulic circuit disclosed in, for example, Japanese Unexamined Patent Publication No. 57-208349 has been known. In the hydraulic circuit, a running hydraulic pump and a running hydraulic motor are subjected to a closed-circuit connection by a first and a second main circuits, and the capacity of the running hydraulic pump (hereinafter, the capacity shows a discharge amount per one rotation, cc/rev) is changed to determine a running speed. At this time, discharge pressure oil of the working machine hydraulic pump is supplied to a working machine cylinder by a working machine valve, and when the working machine valve is placed in a neutral position, the discharge oil is supplied to one of the first and the second main circuits. In this hydraulic circuit, the discharge pressure oil of the working machine hydraulic pump is supplied to the running hydraulic motor, and the running hydraulic motor rotates at a speed higher than the rotation speed corresponding to a maximum discharge amount of the running hydraulic pump, thereby running the vehicle at a high speed. In this hydraulic circuit, however, a first directional control valve for supplying the discharge oil to one of the first and the second main circuits, a manually-operated second directional control valve, and a third directional control valve which is placed in a drain position when pressure oil to be supplied increases to a set pressure or higher are required. In addition, in this hydraulic circuit, a complicated mechanism for switching the first directional control valve by transmitting a movement of an operation member for changing the discharge direction of the running hydraulic pump to the control valve, and a mechanism for switching the second directional control valve are required, whereby the structure of the circuit becomes very complicated. Incidentally, the third directional control valve prevents the discharge pressure oil of the working machine hydraulic pump from being supplied to the first and the second main circuits when the pressure of the first main circuit or the second main circuit is the set pressure or higher, i.e., when a running resistance is high and the running hydraulic motor rotates at a low speed. In addition, in this hydraulic circuit, if the working valve is changed from the neutral position to an operating position when the running resistance is low and the running hydraulic motor is rotating at a high speed, the working machine hydraulic pump actuates the working machine, so that the running hydraulic motor is not supported by the working machine hydraulic pump, and a constant running speed cannot be obtained. For this reason, it is dangerous for an operator to actuate the working machine while running because the speed suddenly changes. In addition, for example, when earth and sand are loaded on a bucket by a loader, and the loader approaches a dump truck at a slow speed, a problem arises in that the number of rotations of the running hydraulic motor increases because the motor is supported by the working machine hydraulic pump, so that it becomes difficult for the operator to control the loader so as to approach the dump truck at a low speed. In addition, when digging resistance is high during digging with the working machine, the working machine stops. Thus, one of the following operations is required: the working machine is operated to decrease the digging resistance; the vehicle is moved forward by imparting a load to the running hydraulic motor; and the vehicle is moved backward. This creates a problem in that the operations required by the operator increase and cause fatigue, and the amount of work performed decreases. Further, since the closed circuit is used, it becomes necessary to supply a constant amount of oil to the first main circuit or the second main circuit in order to prevent cavitation, and a problem arises in that energy is lost. As an another embodiment, Japanese Unexamined Patent Publication No. 5-106245 has been known. According to the official gazette, a self-propelled working vehicle having an HST hydraulic running device includes a variable displacement hydraulic pump, and a running variable displacement hydraulic motor which is subjected to a closed circuit connection to the pump by a pair of main pipes, and obtains a running force using the output torque of the hydraulic motor. The self-propelled working vehicle decreases the discharge capacity of the variable displacement hydraulic motor until the detected operation speed of a front working machine reaches a predetermined value at least when the detected driving pressure of a front working machine hydraulic cylinder is a predetermined value or higher. Therefore, the running torque is reduced when a large front driving force is required, and a lifting force which is larger by the amount of the reduction of the running torque can be obtained, so that the front working machine positively starts operation in any type of earth and sand. However, although it is described that when the digging resistance is high during digging with the working machine, the discharge capacity of the running hydraulic motor is reduced to decrease the running tractive force, and the decreased engine output is used to increase the lifting force oil pressure for increasing, the lifting force of the working machine is not increased, so that the force for crushing a rock bed becomes weaker by the amount of reduction of the running tractive force, and the digging force cannot be increased. In addition, since the closed circuit is used, it becomes necessary to supply a constant amount of oil to the closed circuit in order to prevent cavitation, and a problem arises in that energy is lost. EP-A-0 381 328 discloses a hydraulic circuit for a working device comprising first and second hydraulic circuits each having a pump, and a flow joining/dividing device, wherein the flow of the first circuit is discharged into the second circuit when an operating lever is moved into a postition for lifting the working device at a high speed. The present invention pays attention to the problems of the prior arts, and relates to a hydraulic circuit for hydraulically driven working vehicles, and its object is to provide a hydraulic circuit which allows the vehicles to run at an almost constant speed during high-speed running, provides a great digging force during working, does not need charging pressure for preventing cavitation, loses a small amount of energy, and has a simple structure, particularly for a hydraulically driven working vehicle which is run by driving drive wheels with hydraulic pumps and a hydraulic motor driven by an engine, and to which a working machine is attached. According to the present invention, there is provided a hydraulic circuit for a hydraulically driven working vehicle, comprising: a running HST circuit driven by power of an engine to run the vehicle; a working machine-driving hydraulic circuit driven by power of the engine to drive a working machine, such as a bucket, attached to the vehicle, and having a predetermined relief pressure lower than that of the running HST circuit; a running hydraulic pump and a working machine-driving hydraulic pump for discharging pressure oil of the running HST circuit and for discharging pressure oil of the working machine-driving hydraulic circuit, respectively; a flow joining/dividing valve for joining or dividing discharge oil from the running hydraulic pump and the working machine-driving hydraulic pump with discharge oil in another circuit or into its own circuit ; and a load check valve disposed between the working machine-driving hydraulic pump and a working machine valve provided on the working machine-driving hydraulic circuit, wherein discharge oil from the running HST circuit joins discharge oil in the working machine-driving hydraulic circuit when the pressure of the running HST circuit is higher than the predetermined relief pressure of the working machine-driving hydraulic circuit. In addition, the pressure of the working machine-driving hydraulic circuit may preferably be reduced when discharge oil from the running HST circuit joins discharge oil in the working machine-driving hydraulic circuit. Further, pressure of the discharge oil from the running HST circuit which joins the discharge oil in the working machine-driving hydraulic circuit may preferably be the predetermined relief pressure or higher, and allowable pressure or lower of the working machine-driving hydraulic circuit. Still further, the hydraulic circuit may preferably be a running HST circuit of an open circuit comprising a tank for storing oil, a running variable displacement hydraulic pump for sucking oil and for discharging pressure oil, a running directional control valve for switching the pressure oil from the running variable displacement hydraulic pump, and a running hydraulic motor rotating clockwise or counterclockwise to produce an output upon receipt of the switched pressure oil from the running directional control valve. In addition, the joining may preferably be selected in operatively associated with a selector switch for switching high-speed running and low-speed running. By the described arrangements, since the controlled pressure of the running HST circuit is set higher than the predetermined relief pressure of the working machine-driving hydraulic circuit, the pressure of the discharge oil from the running HST circuit is joined with the pressure of the discharge oil in the working machine-driving hydraulic circuit when the pressure of the running HST circuit is higher than the pressure of the working machine-driving hydraulic circuit, or higher than the predetermined relief pressure of the working machine-driving hydraulic circuit, so that digging can be effected by high operation pressure of the running HST circuit even if the pressure of the working machine-driving hydraulic circuit is the predetermined relief pressure. For this reason, a digging force of the working cylinder is increased, thereby increasing the amount of work performed by the working machine. At this time, if the operator increases the pressure of the running HST circuit to increase a running tractive force, the digging force of the working cylinder is increased by further increased pressure, and the digging can be effected by the running tractive force while pushing, so that harder rock bed, etc. can easily be crushed, and workability is further improved. In addition, at this time, the engine can decrease the load exerted thereon by reducing the pressure of the working machine-driving hydraulic circuit. The output of the engine can be used for a lifting force, or a running tractive force of the working machine under the pressure of the running HST circuit, so that the output of the engine can be efficiently used for the working machine. In addition, the pressure of the discharge oil from the running HST circuit to be joined with the discharge oil in the working machine-driving hydraulic circuit is set to the allowable pressure or lower of the hydraulic equipment used for the working machine-driving hydraulic circuit, so that durability of the hydraulic equipment is ensured. Therefore, an inexpensive fixed gear pump having low allowable pressure can be used in the working machine-driving hydraulic circuit, swash plate control is not required, whereby the hydraulic circuit is simplified, and provided at low cost. In addition, since the open circuit is used, driving of a charge pump for preventing cavitation is not required, and energy loss is reduced. Further, when the frequency of working while running is high, or when digging a hard rock bed, etc., the working can be performed at a low speed while outputting a high digging force and tractive force by selecting low-speed running of a working mode of a Low position of the Hi/Low switch. For example, when loading heavy and hard rock bed, etc. on a loader and approaching a dump truck at a slow speed, the number of rotations of the running hydraulic motor decreases because it is not supported by the working machine hydraulic pump, so that the operator can easily bring the loader near the dump truck at a slow speed. In addition, when digging soft soil, earth and sand, and sand, etc., the sand, etc. can be transported by a high-speed running after digging by selecting a running mode of a Hi position of the Hi/Low switch, so that a work cycle is improved and the amount of work performed increases. Preferably, according to the present invention, there is provided a hydraulic circuit for a hydraulically driven working vehicle, wherein discharge oil from the working machine-driving hydraulic pump joins discharge oil in the running HST circuit when pressure of the running HST circuit is lower than first predetermined pressure, and engine speed is a predetermined value or higher, and the joining of the discharge oil from the working machine-driving hydraulic pump is cut off when the pressure of the running HST circuit is higher than the first predetermined pressure. In addition, the hydraulic circuit may preferably be a running HST circuit of an open circuit comprising a tank for storing oil, a running variable displacement hydraulic pump for sucking oil and for discharging pressure oil, a running directional control valve for switching the pressure oil from the running variable displacement hydraulic pump, and a running hydraulic motor rotating clockwise or counterclockwise to produce an output upon receipt of the switched pressure oil from the running directional control valve. Further, the joining may preferably be selected in operatively associated with a selector switch for switching high-speed running and low-speed running. By the described arrangements, the second directional control valve is placed in the communication position when the load of the running hydraulic motor is low and the pressure of the running hydraulic pump is low, and the first directional control valve is placed in the support position when the engine rotates at a high speed and the pressure of the pressure generating means is the switched pressure or high, so that the discharge pressure oil of the working machine hydraulic pump is supplied to the running hydraulic motor when running at a high engine speed and a low load, thereby increasing the speed. At this time, even if the working machine valve is operated, since the discharge pressure of the working machine hydraulic pump is shut off by the first directional control valve, the discharge pressure of the working machine hydraulic pump is always supplied to the running hydraulic motor and the vehicle runs at a constant speed, so that the operator can drive with safety. In addition, since the first directional control valve and the second directional control valve for controlling the support from the working machine hydraulic pump to the running circuit are automatically switched by pressure, a complicated connecting mechanism is not required, so that the structure is simplified. Incidentally, at this time, the operation of the working machine can be performed by using oil pressure of other circuits (for example, steering, etc.) and by operating the working machine valve, so that the working machine can be raised and lowered even during a high speed. In addition, since the open circuit is used, driving of a charge pump for preventing cavitation is not required, and energy loss is reduced. Further, when the frequency of working while running is high, or when digging a hard rock bed, etc., the working can be performed at a low speed while outputting a high digging force and tractive force by selecting low-speed running of a working mode of a Low position of the Hi/Low switch. For example, when loading heavy and hard rock bed, etc. on a loader and approaching a dump truck at a slow speed, the number of rotations of the running hydraulic motor decreases because it is not supported by the working machine hydraulic pump, so that the operator can easily bring the loader near the dump truck at a slow speed. In addition, when digging soft soil, earth and sand, and sand, etc., the sand, etc. can be transported by a high-speed running after digging by selecting a running mode of a Hi position of the Hi/Low switch, so that a work cycle is improved and the amount of work performed increases. By the described arrangements, the supply circuit for joining the discharge oil from the running HST circuit with the discharge oil in the working machine-driving hydraulic circuit is connected to the downstream of the check valves one of which is disposed on the running HST circuit, and the other one of which is disposed between the working machine-driving hydraulic pump and the working machine-driving directional control valve, and the flow-joining valve which opens when pressure of the working machine driving hydraulic circuit is a predetermined pressure value or higher is provided therebetween, so that the structure of the hydraulic circuit is simplified. The high pressure of the running HST circuit does not acts on the working machine-driving hydraulic pump due to the check valves, so that an inexpensive fixed gear pump of a simple structure can be used in the working machine-driving hydraulic circuit, and the control circuit is not required, whereby the structure of the hydraulic circuit is simplified. In addition, a lower limit value and an upper limit value of the pressure are provided for the flow-joining valve for joining discharge oil from the running HST circuit with the discharge oil in the working machine driving hydraulic circuit, so that a back flow from the working machine-driving hydraulic circuit to the running HST circuit can be prevented at the lower limit value, and the pressure of the working machine-driving hydraulic circuit can be held within an allowable range of the hydraulic equipment. The operation of the flow-joining valve is automatically switched by the pressure of the running HST circuit or of the working machine-driving hydraulic circuit, so that operability for the operator is improved. Or, the operation is switched by the selector switch attached to the operating lever of the working machine-driving hydraulic circuit, so that the operator can easily switch the operation. Since the pressure of the working machine-driving hydraulic circuit is automatically unloaded when reaching a predetermined pressure, the load exerted on the engine can be decreased similar to the above description, and the output of the engine can be used for a lifting force, or a running tractive force of the working machine under the pressure of the running HST circuit, so that the output of the engine can be efficiently used for the working machine. The flow-joining valve includes a first flow-joining valve for joining discharge oil from the running HST circuit with discharge oil in the working machine-driving hydraulic circuit, and a second flow-joining valve for joining discharge oil from the working machine-driving hydraulic circuit with discharge oil in the running HST circuit which are provided on a one-piece valve body, so that the structure is simplified. Further, since the unload valve is also provided on a one-piece valve body, the overall structure is further simplified, a space area can be reduced, and pipes for connecting respective devices are not required. In addition, since the open circuit is used, driving of a charge pump for preventing cavitation is not required, and energy loss is reduced. Further, when the frequency of working while running is high, or when digging a hard rock bed, etc., the working can be performed at a low speed while outputting a high digging force and tractive force by selecting low-speed running of a working mode of a Low position of the Hi/Low switch. For example, when loading heavy and hard rock bed, etc. on a loader and approaching a dump truck at a slow speed, the number of rotation of the running hydraulic motor decreases because it is not supported by the working machine hydraulic pump, so that the operator can easily bring the loader near the dump truck at a slow speed. In addition, when digging soft soil, earth and sand, and sand, etc., the sand, etc. can be transported by a high-speed running after digging by selecting a running mode of a Hi position of the Hi/Low switch, so that a work cycle is improved and the amount of work performed increases. Fig. 1 is a hydraulic circuit diagram showing a first embodiment of a hydraulically driven working vehicle according to the present invention; Fig. 2 is a hydraulic circuit diagram showing a second embodiment of a hydraulically driven working vehicle according to the present invention ; Fig. 3 is a hydraulic circuit diagram showing a third embodiment of a hydraulically driven working vehicle according to the present invention ; Fig. 4 is a diagram showing control pressure of a running hydraulic motor according to the present invention; Fig. 5 is a diagram showing acceleration and deceleration during high speed, or during low speed; Fig. 6 is a hydraulic circuit diagram showing a fourth embodiment of a hydraulically driven working vehicle according to the present invention; Fig. 7 is a hydraulic circuit diagram showing a fifth embodiment of a hydraulically driven working vehicle according to the present invention; Fig. 8 is a sectional view of a flow joining/dividing valve according to the fifth embodiment; Fig. 9 illustrates an operation of a third directional control valve according to the fifth embodiment, and shows a state in which a solenoid for a pilot valve is not excited; Fig. 10 illustrates the operation of the third directional control valve according to the fifth embodiment, and shows a state in which the solenoid for the pilot valve is excited, and a supporting spool has not moved yet; Fig. 11 illustrates the operation of the third directional control valve according to the fifth embodiment, and shows a state in which the solenoid for the pilot valve is excited, and the supporting spool is moving; Fig. 12 illustrates the operation of the third directional control valve according to the fifth embodiment, and shows a state in which the solenoid for the pilot valve is excited, and the supporting spool is moving further rightward in the drawing; Fig. 13 is a hydraulic circuit diagram showing a sixth embodiment of a hydraulically driven working vehicle according to the present invention;; Fig. 14 is a flowchart of supporting from a working circuit to a running circuit according to the sixth embodiment; Fig. 15 is a flowchart of supporting from the running circuit to the working circuit according to the sixth embodiment; and Fig. 16 is a flowchart of supporting from the running circuit to the working circuit according to the sixth embodiment. The preferred embodiments of a hydraulic circuit diagram for a hydraulically driven working vehicle according to the present invention will be described in detail with reference to the attached drawings. Fig. 1 is a hydraulic circuit diagram of a first embodiment of the present invention. As shown in Fig. 1, a running hydraulic pump 2, a working machine-driving hydraulic pump 3 for driving a working machine (hereinafter, referred to as a working machine hydraulic pump 3), and a controlling hydraulic pump 4 are driven by an engine 1. A discharge path 2a of the running hydraulic pump 2 for a running HST circuit is connected to one of a first main circuit 6 and a second main circuit 7 by switching a running valve 5, and the first and the second main circuits 6 and 7 are connected to a normal rotation port 8a and a reverse rotation port 8b of a running hydraulic motor 8, respectively. An output torque of the running hydraulic motor 8 connected to the first main circuit 6 and the second main circuit 7 drives a drive wheel 9. A discharge path 3a of the working machine hydraulic pump 3 of a working machine-driving hydraulic circuit is controlled by a first directional control valve 10 of a flow joining/dividing valve for joining with another circuit or driving into its own circuit, so as to be connected to either one of a working machine circuit 11 and a support circuit 12, the working machine circuit 11 is connected to a pump port 14 of a working machine valve 13 through a load check valve 15, and the support circuit 12 is connected to the discharge path 2a of the running hydraulic pump 2. The first directional control valve 10 is held in a first position A by a spring 16, and placed in a second position B when pressure of a prescribed switching pressure P1 or higher is applied to a pressure receiving portion 17, and pressure proportional to engine speed is supplied to the pressure receiving portion 17 by a second directional control valve 18. The second directional control valve 18 is held in a supply position C by a spring 19, and placed in a drain position D when pressure of a set first switching pressure P2 (hereinafter, referred to as first switching pressure 2) or higher is supplied to a pressure receiving portion 20. A restrictor 21 and a drain circuit 23 including a low-pressure relief valve 22 are connected to a discharge path 4a of the controlling hydraulic pump 4, and a detection circuit 24 is divided from the upstream of the restrictor 21, thereby constituting a pressure generation means 25 for generating pressure proportional to the engine speed. The detection circuit 24 is connected to an inlet port 18a of the second directional control valve 18. In other words, upstream pressure P3 of the restrictor 21 is proportional to the square of a passing flow rate of the restrictor 21, the passing flow rate of the restrictor 21 is proportional to a discharge flow rate of the controlling hydraulic pump 4, and the discharge flow rate is proportional to the speed of the engine 1, so that the upstream pressure P3 of the restrictor 21 is proportional to the square of the speed of the engine 1. The working machine valve 13 connects the working machine circuit 11 to an another valve or tank when placed in a neutral position E, supplies pressure oil to a contraction chamber 26a of the working machine cylinder 26 when placed in a first position F, and supplies pressure oil to an extension chamber 26b of the working machine cylinder 26 when placed in a second position G. A supply circuit 30 is connected to the discharge path 2a of the running hydraulic pump 2, the supply circuit 30 is connected between the pump port 14 and the load check valve 15 of the working machine hydraulic valve 13, and the supply circuit 30 is provided with an open/close valve 31. The open/close valve 31 is placed in a shutoff position a by a spring 32, and in a communication position b having a restrictor 34 when energized by a solenoid 33, and the solenoid 33 is energized by external operating members. For example, when the open/close valve 31 is connected to a power-supply circuit through a switch 36 provided on an operating lever 35 of the working machine valve 13, and the switch 36 is turned on (enter), the solenoid 33 is energized. Next, a running operation will be described. In normal, the working machine valve 13 is placed in the neutral position E, and the running valve 5 is placed in a forward position H, discharge pressure oil of the running hydraulic pump 2 is supplied to the normal rotation port 8a of the running motor 8, and the drive wheel 9 is driven in a normal rotation to allow a vehicle to effect forward running. In addition, backward running may be effected by operating the running valve 5 in the direction opposite to the forward position H, and by supplying the discharge pressure oil of the running hydraulic pump 2 to the reverse rotation port 8b of the running motor 8 to rotate the running motor 8 in the reverse direction. When running resistance of the drive wheel 9 is low under the above-described condition, a load of the running hydraulic motor 8 is reduced, and a pump pressure of the running hydraulic pump 2 becomes low pressure in accordance therewith. The low pump pressure acts on the pressure receiving portion 20 of the second directional control valve 18 as pressure of the first switching pressure P or lower, and the second directional control valve 18 is placed in the supply position C. This allows upstream pressure P3 of the restrictor 21 to be supplied to the pressure receiving portion 17 of the first directional control valve 10 by way of the supply position C of the second directional control valve 18. When an operator increases the speed of the engine 1 to high speed under the above-described condition, the discharge flow rate of the running hydraulic pump 2 increases, the running hydraulic motor 8 rotates at a high speed, and the drive wheel 9 is driven at a high speed, so that the vehicle runs at a high speed and a low load. The vehicle speed at this time increases with the increase in the speed of the engine 1. The discharge flow rate of the controlling hydraulic pump 4 increases with the increase in the speed of the engine 1. When the speed of the engine 1 increases to a predetermined speed or higher, the upstream pressure P3 of the restrictor 21 is increased to the switching pressure P1 or higher of the first directional control valve 10 by the increased discharge flow rate of the controlling hydraulic pump 4. By the pressure of the switching pressure P1 or higher, the first directional control valve 10 is placed in the second position B, the discharge pressure oil of the working machine hydraulic pump 3 is supplied from the support circuit 12 to the normal rotation port 8a of the running hydraulic motor 8 by way of the discharge path 2a, the running valve 5, and the first main circuit 6, and the running hydraulic motor 8 rotates at a higher speed to increase the vehicle speed. In this way, when the running resistance of the running hydraulic motor 8 is low (driving pressure is low) on a level ground, the engine 1 rotates at a high speed, and the vehicle runs at a high speed, the discharge pressure oil of the working machine hydraulic pump 3 is supplied to the running hydraulic motor 8 without becoming a resistance unlike a conventional manner, so that the engine output can effectively be used. When the running resistance of the drive wheel 9 increases under the above-described condition, the load of the running hydraulic motor 8 also increases, so that pressure acting on the running hydraulic pump 2 becomes high pressure. This allows the pressure acting on the pressure receiving portion 20 of the second directional control valve 18 to increase to the first switching pressure P2 or higher, and the second directional control valve 18 is switched to the drain position D. In the drain position D, since the pressure receiving portion 17 of the first directional control valve 10 communicates with a tank, the first directional control valve 10 is placed in the first position A, so that the discharge pressure oil of the working machine hydraulic pump 3 is flown into the tank by the working machine valve 13. At this time, by setting resistance of the circuit to low, the discharge pressure of the working machine hydraulic pump 3 becomes low pressure and at the same time, input torque reduces to substantially zero, so that the output of the engine 1 can effectively be used as the input torque of the running hydraulic pump 2. In addition, even if the engine 1 rotates at a high speed, the first directional control valve 10 is automatically switched when a load acts on the working machine and a load of a predetermined value or higher acts on the running hydraulic motor 8, so that the discharge pressure oil of the working machine hydraulic pump 3 flows into the working machine valve 13. This enables an operation, such as digging. As described above, since the discharge pressure oil of the working machine hydraulic pump 3 is supplied to the running hydraulic motor 8 when running with a high speed of the engine 1 and a low load of the running hydraulic motor 8 (when running at a high speed and a low load), the speed can be increased higher than a running speed corresponding to the maximum discharge amount of the running hydraulic pump 2. In short, the discharge amount (the discharge amount per unit time) of the running hydraulic pump 2 is determined by the engine speed, and becomes maximum during high-speed rotation. Incidentally, even if the running hydraulic pump 2 is of a variable displacement type, the discharge amount is determined by the engine speed x capacity (the discharge amount per one rotation), and becomes maximum during high-speed rotation because the capacity is usually set to the maximum when the discharge pressure is low. Incidentally, when the running hydraulic pump 2 is of the variable displacement type and the horse power is controlled to be constant, the capacity is increased or decreased by the pump pressure to control the input torque (capacity x engine speed x pump pressure) to be constant, so that the capacity per one rotation increases and the discharge amount becomes maximum during a low load as described above. Thus, when the engine 1 rotates at a high speed and the load is low, the discharge amount (discharge amount per unit time) of the running hydraulic pump 2 becomes maximum and the running speed also becomes maximum. Further, according to the present invention, the discharge pressure oil of the working machine hydraulic pump 3 is supplied at this time to further increase the speed. In addition, when running with a low speed rotation of the engine 1, and the low load of the running hydraulic motor 8, the pressure acting on the pressure receiving portion 17 of the first directional control valve 10 decreases to the switching pressure P1 or lower, and the first directional control valve 10 is placed in the first position A, so that by placing the working machine valve 13 in the first position F, and the second position G, the pressure oil can be supplied to the contraction chamber 26a and the extension chamber 26b of the working machine cylinder 26 to operate the working machine. This allows the working machine to be operated while running at a low speed. Thus, when the hydraulically driven working vehicle is a wheel loader, the vehicle can run at a low speed to perform a crane operation while suspending a load by a bucket. In addition, when loading a load on a dump truck, the bucket can be raised while running at a low speed and the vehicle can approach the dump truck while running at a slow speed, so that operability is improved and the operation is facilitated, whereby a cycle time of the loading operation, etc. can be shortened. In addition, since the discharge pressure oil of the working machine hydraulic pump 3 is supplied from the inlet side of the working machine valve 13 to the running hydraulic motor 8, even if the working machine valve 13 is placed in the first and the second positions F and G, the discharge pressure oil of the working machine hydraulic pump 3 can be supplied to the running hydraulic motor 8 at once when running with the high-speed rotation of the engine and the low load, whereby the running speed can be increased. For example, when the engine is rotated at a high speed by an accelerator, etc. during the above-described crane operation, the load suspension can be stopped to increase the running speed without operating the working machine valve 13, and the original operation can be continued when the running speed is decelerated. Next, a digging operation will be described. When the hydraulically driven working vehicle is the wheel loader, an operator switches the running valve 5 in the forward position H to effect the forward running of the vehicle as described above, and digs a non-illustrated bucket into the natural ground. When the bucket is dug into the natural ground, the operator places the working machine valve 13 in the second position G to supply the discharge pressure of the working machine hydraulic pump 3 to the extension chamber 26b of the working machine cylinder 26, and raises the bucket to effect digging. In short, when the bucket is dug into the natural ground, the running resistance of the drive wheel 9 increases and the pump pressure of the running hydraulic pump 2 increases near-controlled pressure (for example, 420 kg/cm 2 ) of a running relief valve 37. By this pressure, the second directional control valve 18 is placed in the drain position D, so that the first directional control valve 10 is placed in the first position A regardless of the engine speed, whereby the discharge pressure oil of the working machine hydraulic pump 3 is supplied to the working machine circuit 11. During the above-described digging operation, the pressure in the extension chamber 26b of the working machine cylinder 26 increases only up to controlled pressure (for example, 210 kg/cm 2 ) of a working machine relief valve 38, so that the magnitude of thrust of the working machine cylinder 26 corresponds to the controlled pressure of the working machine relief valve 38. For this reason, a force for moving the bucket upward may be insufficient, so that the bucket cannot be raised. In this case, the operator turns on the switch 36, energizes the solenoid 33 to place the open/close valve 31 in the communication position b, supplies a high pump pressure of the running hydraulic pump 2 to the extension chamber 26b of the working machine cylinder 26 to increase the thrust, and increases raising force to raise the bucket. Fig. 2 shows a hydraulic circuit diagram of a second embodiment. In the second embodiment, the discharge path 4a of the controlling hydraulic pump 4 is connected to the inlet port 18a of the second directional control valve 18, and the switching pressure P1 of the first directional control valve 10 is controlled pressure of a relief valve 39 provided on the discharge path 4a of the controlling hydraulic pump 4. This allows the discharge pressure oil of the working machine hydraulic pump 3 to be supplied to the running hydraulic motor 8 during the low-load running regardless of the engine speed, whereby high-speed rotation can be effected. Fig. 3 is a hydraulic circuit diagram of a third embodiment. In the first embodiment, the electromagnetic open/close valve 31 actuated by the manually-operated working machine valve 13 and the solenoid 33 is used, and the supply circuit 30 from the running hydraulic pump 2 is connected between the pump port 14 and the load check valve 15 of the working machine valve 13. In contrast, in the third embodiment, as shown in Fig. 3, pilot oil pressure from a pressure-proportional pressure reduction valve 41 linked to an operating lever 41a controls a working machine hydraulic valve 42 and a hydraulic open/close valve 43, and a first supply circuit 40 from the running hydraulic pump 2 is connected between the pump port 14 and the load check valve 15 of the working machine hydraulic valve 42 similar to the first embodiment. In addition, although the working machine relief valve 38 is disposed between the working machine valve 13 and the first directional control valve 10 in the first embodiment, an another one working machine circuit-allowing relief valve 44 is additionally disposed between the hydraulic open/close valve 43 and the working machine hydraulic valve 42 in the third embodiment. To one end of the hydraulic open/close valve 43, a first pressure receiving portion 43a for receiving the switching pressure from the first supply circuit 40 and a second pressure receiving portion 43b for receiving the switching pressure from the pressure-proportional pressure reduction valve 41 are attached, and to the other end, a third pressure receiving portion 43c for receiving the switching pressure from the working machine hydraulic pump 3, and a spring 43d are attached. The hydraulic open/close valve 43, when the controlled pressure of the working machine relief valve 38 acts on the third pressure receiving portion 43c, pressure oil of the running hydraulic pump 2 increases to the controlled pressure (for example, 210 kg/cm 2 ) or higher of the working machine relief valve 38 to act on the first pressure receiving portion 43a, and one-step higher pilot pressure from the pressure-proportional pressure reduction valve 41 acts on the second pressure receiving portion 43b, presses spring force of the spring 43b to be switched from the shutoff position a to the communication position b. The one-step higher pilot pressure from the pressure-proportional pressure reduction valve 41 is generated when the operator fully operates the operating lever 41a up to the stroke end. In addition, even if the pressure oil of the running hydraulic pump 2 reaches the controlled pressure of 420 kg/cm 2 of the running relief valve 37, the hydraulic open/close valve 43 is not switched from the shutoff position a to the communication position b because pressing force of the first pressure receiving portion 43a is set lower than the spring force of the spring 43d. In addition, even if the operating lever 41a is operated when the working machine relief valve 38 reaches the controlled pressure, the hydraulic open/close valve 43 is not switched from the shutoff position a to the communication position due to a spring force of the spring 43d and reactive force of the third pressure receiving portion 43c. In the above-described arrangements, in order to simplify the apparatus, the first pressure receiving portion 43a and the third pressure receiving portion 43c may not be provided so that the hydraulic open/close valve 43 presses the spring force of the spring 43d to be changed from the shutoff position a to the communication position b when the operator fully operates the operating lever 41a and the one-step higher pilot pressure is generated from the pressure-proportional pressure reduction valve 41. The working machine circuit-allowing relief valve 44 reduces and restricts the high pressure from the running hydraulic pump 2 to the pressure allowed by working machine hydraulic equipment. For example, in the case where the working vehicle is a wheel loader, the controlled pressure of the running relief valve 37 is controlled to 420 kg/cm 2 , the controlled pressure of the working machine relief valve 38 is controlled to 210 kg/cm 2 , and the controlled pressure of the working machine circuit-allowing relief valve 44 is controlled to 230 kg/cm 2 . In addition, a circuit for sending the switching pressure P1 of the first directional control valve 10 from the second directional control valve 18 to the pressure receiving portion 17 of the first directional control valve 10 is provided with a Hi/Low electromagnetic open/close valve 46 (hereinafter, referred to as electromagnetic open/close valve 46). The electromagnetic open/close valve 46 is connected to a Hi/Low switch 47 for selecting a running mode for a high-speed running of the vehicle or an operation mode for a low-speed operation, and the operator selects high-speed or low-speed of the vehicle. The electromagnetic open/close valve 46 is communicated when the operator selects Hi (high-speed), and is shut off when Low is selected. When the operator selects Hi (high-speed), the running resistance is low, and the speed of the engine 1 is high, the discharge pressure oil of the working machine hydraulic pump 3 is supplied to the running hydraulic motor 8 to increase the speed. This allows the vehicle to obtain high-speed running. When the operator selects Low (low-speed), the discharge pressure oil of the working machine hydraulic pump 3 is not supplied to the running hydraulic motor 8 but supplied only to the working machine valve 13. A swash plate control means 8c for varying the discharge capacity of the running hydraulic motor 8 is provided with a motor swash plate electromagnetic open/close valve 48 (hereinafter, referred to as swash plate electromagnetic open/close valve 48). The swash plate electromagnetic open/close valve 48 is connected to the Hi/Low switch 47. The swash plate electromagnetic open/close valve 48 outputs high pressure Pmh to a swash plate control device 8a of the running hydraulic motor 8 when the operator selects Hi (high-speed) of the Hi/Low switch 47, as shown in Fig. 4, and outputs low pressure Pmu to the swash plate control device 8a when Low (low-speed) is selected. Incidentally, in Fig. 4, the horizontal axis represents a stroke of the swash plate electromagnetic open/close valve 48, and the vertical axis represents control pressure supplied to the swash plate control device 8a of the running hydraulic motor 8. By this, when the operator selects Hi (high-speed), the running hydraulic motor 8 reduces an inclination-rotation angle of the swash plate and the discharge capacity, so that the vehicle can obtain higher-speed running. At this time, for example, acceleration and deceleration during high speed, or low speed shown in Fig. 5 are performed. In Fig. 5, the horizontal axis represents the speed of the engine 1, and the vertical axis represents the vehicle speed. Solid lines in the drawing represents during high speed, dotted lines represents during low speed, and the maximum discharge amount of the running hydraulic pump 2 and the minimum discharge amount of the running hydraulic motor 8 are illustrated. When the speed of the engine 1 is increased, the vehicle speed suddenly increases in a curve of secondary degree, and when the speed of the engine 1 is decreased, the vehicle speed linearly decreases. This allows the acceleration property during the increase of the speed to be improved, and allows shock to be reduced because the speed is slowly decreased during deceleration, whereby driving property is improved. An operation in the above arrangement will be described. First, a case will be described in which a work is performed in low-speed running. The operator set the Hi/Low switch 47 to Low to select the low-speed running. This enlarges the inclination-rotation angle of a non-illustrated swash plate of the running hydraulic motor 8, increases the discharge capacity (cc/rev) of the running hydraulic motor 8, decreases the number of rotations, and increases the output torque. In addition, the discharge pressure oil of the working machine hydraulic pump 3 is supplied only to the working machine cylinder 26 by way of the working machine hydraulic valve 42. In addition, the pressure oil of the running hydraulic pump 2 is supplied to the running hydraulic motor 8. When the operator operates the operating lever 41a attached to the pressure-proportional pressure reduction valve 41 under this condition, the pilot pressure generated at the pressure-proportional pressure reduction valve 41 acts on the working machine hydraulic valve 42 to switch the working machine hydraulic valve 42 in a desired direction. This allows the discharge pressure oil of the working machine hydraulic pump 3 to be supplied to the contraction chamber 26a and the extension chamber 26b of the working machine cylinder 26 by placing the working machine hydraulic valve 42 is in the first position F and in the second position G. At this time, similar to the above description, the pressure of the working machine cylinder 26 increases only up to the controlled pressure (for example, 210 kg/cm 2 ) of the working machine relief valve 38, so that the magnitude of thrust of the working machine cylinder 26 corresponds to the controlled pressure of the working machine relief valve 38. However, during digging, etc., the controlled pressure of the working machine relief valve 38 is often insufficient for a digging force of the bucket. In this case, the operator moves forward the vehicle at a low speed, and raises the bucket to increase the digging force. For this operation, when the operator fully operates the operating lever 41a to the stroke end, the pressure-proportional pressure reduction valve 41 generates one-step higher pilot pressure, and the pilot pressure acts on the second pressure receiving portion 43b of the hydraulic open/close valve 43. In addition, the pressure oil from the running hydraulic pump 2 acts on the first pressure receiving portion 43a of one end of the hydraulic open/close valve 43, and the discharge pressure oil of the working machine hydraulic pump 3 and the spring force of the spring 43d act on the third pressure receiving portion 43c of the other end. At this time, since the controlled pressure of the working machine relief valve 38 acts on the third pressure receiving portion 43c of the hydraulic open/close valve 43, when the pressure oil of the running hydraulic pump 2 increases to the controlled pressure of the working machine relief valve 38 (for example, 210 kg/cm 2 ) or higher, and the one-step higher pilot pressure from the pressure-proportional pressure reduction valve 41 acts on the second pressure receiving portion 43b, the hydraulic open/close valve 43 presses the spring force of the spring 43d to be switched from the shutoff position a to the communication position b. This allows the high pressure oil from the running hydraulic pump 2 to be supplied to the working machine hydraulic valve 42 through the hydraulic open/close valve 43. The high pressure from the running hydraulic pump 2 is controlled to 230 kg/cm 2 by the working machine circuit-allowing relief valve 44, the working machine can be actuated with this pressure by supplying pressure oil to the extension chamber 26b of the working machine cylinder 26, and the digging force of the working machine cylinder 26 can be increased. At this time, in case of digging a hard rock bed, etc., if the digging force for raising the bucket is insufficient, the operating lever 41a is operated to actuate the bucket by high pressure oil from the running hydraulic pump 2, and when the bucket is slightly raised to perform digging, the operating lever 41a is returned to move the vehicle forward by the high pressure oil of the running hydraulic pump 2, and digging is performed by a tractive force. By this, the hard rock bed, etc. are easily crushed by raising and pressing of the bucket. In addition, at this time, the pressure of 230 kg/cm 2 controlled by the working machine circuit-allowing relief valve 44 is shut off by the load check valve 15 so as not to act on the working machine hydraulic pump 3. For this reason, the working machine hydraulic pump 3 is restricted to the controlled pressure (for example, 210 kg/cm 2 ), which is within an allowable range of the working machine relief valve 38. At this time, even if the pressure oil of the running hydraulic pump 2 reaches the controlled pressure of 420 kg/cm 2 of the running relief valve 37, the hydraulic open/close valve 43 is not switched from the shutoff position a to the communication position b because the pressing force of the first pressure receiving portion 43a is set weaker than the spring force of the spring 43d. Therefore, unless the operator fully operates the operating lever 41a, the pressure oil is not supplied from the running hydraulic pump 2 to the working machine hydraulic valve 42 even if the pressure oil of the running hydraulic pump 2 reaches the controlled pressure of 420 kg/cm 2 of the running relief valve 37 during running. In addition, even if the hydraulic open/close valve 43 is changed from the shutoff position a to the communication position b, and the pressure oil of the running hydraulic pump 2 reaches the controlled pressure of 420 kg/cm 2 of the running relief valve 37, this high pressure is controlled to 230 kg/cm 2 by the working circuit-allowing relief valve 44, so that the pressure acting on the working machine cylinder 26 is restricted within the allowable pressure range. In this way, the pressure oil from the running hydraulic pump 2 is distributed and supplied to the working machine cylinder 26 and the running hydraulic motor 8, and the discharge capacity of the running hydraulic motor 8 is set large, so that the vehicle can move forward at a slow speed, and output torque increases. Therefore, the digging force of the bucket is composed of a large output torque output from the running hydraulic motor 8 which rotates upon receipt of the pressure oil distributed from the running hydraulic pump 2 while moving the vehicle forward at a slow speed, and a large force of the working machine cylinder 26 due to the distributed high pressure, so that hard rock bed, etc. can be dug without slipping a tire of the drive wheel 9. Next, the high-speed running will be described. The operator connects the Hi/Low switch 47 to Hi to select the high-speed running. This allows the swash plate electromagnetic open/close valve 48 to be communicated, reduces the inclination-rotation angle of the non-illustrated swash plate of the running hydraulic motor 8, decreases the discharge capacity, and increases the number of rotations. In addition, by connecting the Hi/Low switch 47 to Hi, the electromagnet open/close valve 46 is switched to the communication position b, and supplies the pressure oil from the second directional control valve 18 to the pressure receiving portion 17 of the first directional control valve 10. By this, the first directional control valve 10 is controlled by the switching pressure P1 of the controlling hydraulic pump 4. Therefore, if the pressure of the running hydraulic motor 8 is low, and the speed of the engine 1 is high, the first directional control valve 10 is switched, and the discharge pressure oil of the working machine hydraulic pump 3 supports the running hydraulic pump 2. The running hydraulic motor 8 receives the discharge pressure oil of both of the running hydraulic pump 2 and the working machine hydraulic pump 3, and the number of rotations is increased by the reduction of the discharge capacity (cc/rev) of the running hydraulic motor 8, thereby running the vehicle at high speed. At this time, since the discharge pressure oil of the working machine hydraulic pump 3 supports the running hydraulic pump 2, the working machine cylinder 26 actuates upon receipt of pressure oil from a non-illustrated steering pump. The non-illustrated steering pump is constructed so that it is driven by the engine 1, and it supplies the pressure oil to a non-illustrated steering cylinder swinging the vehicle, and to the working machine cylinder 26. In addition, the discharge pressure oil of the working machine hydraulic pump 3, supplies the pressure oil to the working machine cylinder 26 by way of the working machine hydraulic valve 42, or supports the running hydraulic pump 2 in accordance with the pressure of the running hydraulic motor 8 and the speed of the engine 1 similar to the first embodiment. Fig. 6 is a hydraulic circuit diagram of a fourth embodiment. In the first embodiment, the supply circuit 30 from the running hydraulic pump 2 is connected between the pump port 14 of the working machine hydraulic valve 13 and the load check valve 15 through the open/close valve 31 having the restrictor 34. However, in the fourth embodiment, as shown in Fig. 6, a second supply circuit 50 from the running hydraulic pump 2 is, similar to the first embodiment, connected between the pump port 14 of the working machine hydraulic valve 13 and the load check valve 15 by way of a first open/close valve 51 and a first check valve 52. On one end of the first open/close valve 51, a first pressure receiving chamber 51a connected to the extension chamber 26b of the working machine cylinder 26 by a pilot pipe 53 is provided, and on the other end, the first pressure receiving chamber 51b connected to the second supply circuit 50, and a spring 51c are provided. The first open/close valve 51 is actuated against a force adding the force of the spring 51c and a force by a predetermined pressure (for example, 210 kg/cm 2 ) acting on the first pressure receiving chamber 51b, switched from the shutoff position a to the communication position b to communicate with the second supply circuit 50 when the pressure oil of the extension chamber 26b of the working machine cylinder 26 increases to controlled pressure (for example, 210 kg/cm 2 ) or higher of a working machine circuit allowing relief/unload valve (hereinafter, referred to as relief/unload valve 54), which is described later. In addition, the first open/close valve 51 is switched from the shutoff position a to the communication position b to communicate with the second supply circuit 50 when the pressure oil of the extension chamber 26b of the working machine cylinder 26 increases to the controlled pressure of the relief/unload valve 54 or higher, and the pressure acting on the first pressure receiving chamber 51b is a predetermined pressure (for example, 210 kg/cm 2 ) or lower. However, the pressure oil of the extension chamber 26b of the working machine cylinder 26 does not flow in the second supply circuit 50 because it is shut off by the first check valve 52. In addition, the first open/close valve 51, when the pressure acting on the first pressure receiving portion 51b increases to the second predetermined pressure (for example, 230 kg/cm 2 ) or higher, is switched to the shutoff position a in response to the pressure so as to shut off the second supply circuit 50. This allows the working machine circuit to be restricted to allowable pressure of the hydraulic equipment. In addition, the high pressure of the second supply circuit 50 can supply pressure oil to the second extension chamber 26b of the working machine cylinder 26 to operate the working machine, whereby a digging force of the working machine cylinder 26 can be increased. In the above-described embodiment, the first pressure receiving chamber 51b connected to the second supply circuit 50 is provided. However, the first open/close valve 51 may be provided with a variable restrictor 51d so that the second supply circuit 50 is shut off by the first open/close valve 51 when the pressure of the second circuit 50 increases to the second predetermined pressure (for example, 230 kg/cm 2 ) or higher. In addition, although the working machine relief valve 38 is arranged between the load check valve 15 and the first directional control valve 10 in the first embodiment, the relief/unload valve 54 is disposed on the same position in the fourth embodiment. The relief/unload valve 54 restricts the pressure acting on the hydraulic equipment of the working machine circuit side to the allowable pressure of the hydraulic equipment, and unloads the discharge pressure of the working machine hydraulic pump 3 at the time of supporting. To this relief/unload valve 54, the pressure divided from the second supply circuit 50 is supplied by the pilot pipe 50a after passing through an open/close valve 56 for the relief valve and the first open/close valve 51. The open/close valve 56 for the relief valve includes a shutoff position e and a communication position f, and the pressure of the second supply circuit 50 acts on a pressure receiving chamber 56a provided on one end, and force of a spring 56b acts on the other end. The open/close valve 56 for the relief valve, when the second supply circuit of the running circuit increases to 210 kg/cm 2 or higher, is switched from the shutoff position e to the communication position f. The relief/unload valve 54 is usually controlled, for example, to 210 kg/cm 2 . When the pressure of the extension chamber 26b of the working machine cylinder 26 reaches 210 kg/cm 2 , the first open/close valve 51 is switched from the shutoff position a to the communication position b, and when the pressure of the second supply circuit 50 of the running circuit increases to 210 kg/cm 2 or higher, the open/close valve 56 for the relief valve is also switched from the shutoff position e to the communication position f. When the first open/close valve 51 and the open/close valve 56 for the relief valve are switched to the communication positions, the pressure of the relief/unload valve 54 is decreased by the pilot pressure (210 kg/cm 2 or higher) received through the communication position e of the open/close valve 56 for the relief valve and the communication position b of the first open/close valve 51 so as to be controlled to substantially 0 kg/cm 2 . By this, the working machine increases digging force of the working machine cylinder 26 by high pressure of 210 kg/cm 2 or higher from the running hydraulic pump 2, and the engine 1 can reduce a load because the pressure oil of the working machine hydraulic pump 3 decreases to substantially 0 kg/cm 2 . In the above-described arrangement, for example, when the working vehicle is a wheel loader, the controlled pressure of the running relief valve 37 is controlled to 420 kg/cm 2 , and the controlled pressure of the relief/unload valve 54 is controlled in two steps of higher controlled pressure of 210 kg/cm 2 , and the controlled pressure of substantially 0 kg/cm 2 at the time of unloading. In addition, the switching pressure P1 of the first directional control valve 10 is set so as to be switched at the switching pressure of 10 kg/cm 2 which is obtained when the speed of the working vehicle is the speed Na of the engine 1 equivalent to high speed of 21 km/hour. In addition, the first switching pressure P2 of the second directional control valve 18 is set so as to be switched when the discharge capacity of the running hydraulic motor 8 is equivalent to a low speed of 12 km/hour of the working vehicle, and output torque Ta of the running hydraulic motor 8 is the second switching pressure of 180 kg/cm 2 . Next, an operation will be described. When the running resistance of the drive wheel 9 is low and the load of the running hydraulic motor 8 is the output torque Ta or lower, i.e., when the pump pressure of the running hydraulic pump 2 is low (the second switching pressure of 180 kg/cm 2 or lower), the pressure of the pressure receiving portion 20 of the second directional control valve 18 decreases to the first switching pressure P2 or lower to be switched to the supply position C, whereby upstream pressure P3 of the restrictor 21 is supplied to the pressure receiving portion 17 of the first directional control valve 10. In this state, when the engine 1 is rotating at low speed and the switching pressure P1 of the first directional control valve 10 is the switching pressure of 10 kg/cm 2 or lower, or when the Hi/Low switch 47 is selected to Low, the first directional control valve 10 is placed in the first position A, the discharge pressure oil of the working machine hydraulic pump 3 is supplied to the working machine valve 13, so that the working machine can be operated by the operation of the working machine valve 13. When the Hi/Low switch 47 is selected to Low, since the first directional control valve 10 is placed in the first position A, the discharge pressure oil of the working machine hydraulic pump 3 is supplied only to the working machine valve 13 similar to the third embodiment, so that the working machine can always be operated. In addition, when the Hi/Low switch 47 is selected to Hi for working, the operation is as follows. When the working vehicle is a wheel loader, the condition is such that the speed of the working vehicle is low (the speed of 12 km/hour) and the bucket is dug into the natural ground, and the working machine such as the bucket is in an operable condition. The operator digs the working machine into the natural ground at low speed under this condition, and depresses an accelerator pedal 55 to increase the speed of the engine 1. When the working vehicle is dug into the natural ground, the running resistance of the drive wheel 9 becomes high to decrease the speed, and the load loaded on the running hydraulic motor 8 increases. The pressure applied to the running hydraulic motor 8 at this time is set to increase to the second switching pressure of 180 kg/cm 2 or higher, the pressure acts as the load on the running hydraulic pump 2 and acts on the pressure receiving portion 20 of the second directional control valve 18. This allows the second directional control valve 18 to be placed in the drain position D, so that the first directional control valve 10 is placed in the first position A regardless of the engine speed, whereby the discharge pressure oil of the working machine hydraulic pump 3 is supplied to the working machine circuit 11. Therefore, even if the operator depresses the accelerator pedal 55 to increase the speed of the engine 1, the discharge pressure oil of the working machine hydraulic pump 3 is supplied to the working machine circuit 11, so that the working machine is operated by the operation of the working machine valve 13. In addition, at this time, even if digging resistance increases during the digging operation, and the pressure in the extension chamber 26b of the working machine cylinder 26 reaches the controlled pressure of the relief/unload valve 54 of 210 kg/cm 2 , there may be a case where a force for raising the bucket is insufficient. In this case, the pressure of 210 kg/cm 2 in the extension chamber 26b of the working machine cylinder 26 acts on the first open/close valve 51 to automatically switch the first open/close valve 51 to the communication position b. At this time, when the pressure of the second supply circuit 50 of the running circuit is 210 kg/cm 2 or lower, the open/close valve 56 for the relief valve is shut off at the shutoff position e, so that the pressure from the pilot pipe 50 divided from the second supply circuit 50 does not act on the relief/unload valve 54. For this reason, the controlled pressure of the relief/unload valve 54 is maintained at the pressure of 210 kg/cm 2 . At this time, the operator depresses the pedal 55 and moves forward the working vehicle to increase the pressure of the second supply circuit 50 of the running circuit to 210 kg/cm 2 or higher. When the pressure of the second supply circuit 50 of the running circuit increases to 210 kg/cm 2 or higher, the open/close valve 56 for the relief valve is switched from the shutoff position e to the communication position f, so that the pressure from the pilot pipe 50a divided from the second supply circuit 50 acts on the relief/unload valve 54. For this reason, the controlled pressure of the relief/unload valve 54 decreases to the pressure of substantially 0 kg/cm 2 , and the pressure of the second supply circuit 50 increases to the pressure of 210 kg/cm 2 or higher of the running circuit, so that this pressure acts on the extension chamber 26b of the working machine cylinder 26 by way of the communication position b of the first open/close valve 51, the first check valve 52, and the second position G of the working machine valve 13, whereby the digging force can be increased. In addition, when the pressure is 230 kg/cm 2 or higher, the first open/close valve 51 is placed in the shutoff position a. This allows the hydraulic equipment for use in the working machine to be protected because even higher pressure does not act on the working machine valve 13 and the working machine cylinder 26. In addition, when the pressure is 230 kg/cm 2 or higher, the controlled pressure of the relief/unload valve 54 is substantially from 0 kg/cm 2 to 210 kg/cm 2 , so that the digging force is maintained. In addition, when the variable restrictor 51d is used, the variable restrictor 51d of the first open/close valve 51 is restricted when the pressure of the second supply circuit 50 of the running circuit is below 230 kg/cm 2 , and the variable restrictor 51d of the first open/close valve 51 is closed when the pressure is 230 kg/cm 2 or higher. This allows the hydraulic equipment for the working machine to be protected similar to the above description. When running with earth and sand loaded on the bucket after the digging has completed, the operator operates the operating lever 35 to return the working machine valve 13 to the neutral position E. This allows the discharge pressure oil of the working machine hydraulic pump 3 to return to the tank by way of the neutral position E of the working machine valve 13. At this time, when the operator depresses the accelerator pedal 55 to increase the speed of the engine 1, the discharge pressure of the controlling hydraulic pump 4 is increased by the restrictor 21. In addition, even if the wheel loader loads earth and sand on the bucket, the load loaded on the running hydraulic motor 8 is small because running resistance of the drive wheel 9 is low on the level ground, and the pressure to be applied to the running hydraulic motor 8 is set to be the second switching pressure of 180 kg/cm 2 or lower. Therefore, since the pressure acting on the pressure receiving portion 20 of the second directional control valve 18 is the second switching pressure of 180 kg/cm 2 or lower, the second directional control valve 18 is placed in the position C. By this, the first directional control valve 10 is placed in the second position B, the discharge pressure oil of the working machine hydraulic pump 3 is supplied from the support circuit 12 to the normal rotation port 8a of the running hydraulic motor 8 by way of the discharge path 2a, the running valve 5, and the first main circuit 6, and the running hydraulic motor 8 rotates at higher speed to increase the vehicle speed. When running with earth and sand loaded on the bucket, and approaching to a dump truck to load the earth and sand, the operator eases up to the depressed accelerator pedal 55. By this, the speed of the engine 1 drops, and the number of rotations of the controlling hydraulic pump 4 becomes low, whereby the discharge amount decreases. By this, the pressure of the restrictor 21 decreases, the switching pressure P1 of the first directional control valve 10 decreases to the switching pressure of 10 kg/cm 2 or lower, and the first directional control valve 10 is placed in the first position A, whereby the discharge pressure oil of the working machine hydraulic pump 3 is supplied to the working machine cylinder 16 through the working machine valve 13, so that the working machine can be operated by the operation of the working machine valve 13. In this way, when Hi is selected, work can be performed, and particularly, when the digging force for digging soft soil, earth and sand, and sand is low, a fast working cycle can be obtained. In addition, as described above, the working machine cylinder 26 may be operated upon receipt of pressure oil supplied from the non-illustrated steering pump. In addition, when running on the level ground at high speed such as a third speed or a fourth speed, the running resistance of the drive wheel 9 is low on the level ground similar to the above description, so that the load loaded on the running hydraulic motor 8 is low, the pressure applied on the running hydraulic motor 8 is the second switching pressure of 180 kg/cm 2 or lower, and the second directional control valve 18 is placed in the position C. In addition, the operator depresses the accelerator pedal 55 in order to run at the third speed, or the fourth speed. For this reason, the speed of the engine 1 increases, and the discharge pressure of the controlling hydraulic pump 4 is increased by the restrictor 21. By this, the first directional control valve 10 is placed in the second position B, the discharge pressure oil of the working machine hydraulic pump 3 is supplied to the normal rotation port 8a of the running hydraulic motor 8 from the support circuit 12 by way of the discharge path 2a, the running valve 5, and the first main circuit 6, and the running hydraulic motor 8 rotates at higher speed to increase the vehicle speed. When the Hi/Low switch 47 is selected to Hi even during this high speed, running at still higher speed than that of the above description can be effected because the discharge capacity of the running hydraulic motor is small. In addition, even if the engine 1 rotates at high-speed number of rotations to rotate the working machine hydraulic pump 3 at a high speed, the discharge pressure oil of the working machine hydraulic pump 3 does not become resistance because it is supplied to the running hydraulic motor 8 without racing as usual, so that the engine output can effectively be used. In addition, since the discharge pressure oil of the working machine hydraulic pump 3 is always supplied to the running hydraulic motor 8, running at constant speed responsive to the accelerator can be effected. Fig. 7 is a hydraulic circuit diagram of a fifth embodiment. The flow joining/dividing valve 60 is composed of the first directional control valve 10 for joining to support from the working machine circuit 11 to a running circuit 61, a third directional control valve 62 for joining to support from the running circuit 61 to the working machine circuit 11, and an unload valve 66. The third directional control valve 62 is composed of a working machine supporting valve 64 one of which is connected to the first directional control valve 10 and the other one of which is connected to the pump port 14 of the working machine valve 13, and a pilot valve 65 for switching the working machine supporting valve 64. The pilot valve 65 comprises a solenoid operated valve of two positions, and the pilot valve 65 is connected to a selector switch 68 for supporting the working machine circuit 11 from the running circuit 61 by way of an electric AND circuit 67. The AND circuit 67 is connected to the working machine circuit 11 through a working machine pressure sensor 69. In addition, the pump port 14 of the working machine hydraulic valve 13 is connected to the load check valve 15 by a support pipe 71. The support pipe 71 is connected to the working machine supporting valve 64, the pilot valve 65, and a pressure receiving chamber 66a of the unload valve 66. In addition, a support circuit check valve 72 is disposed on the support pip 71, and between the first directional control valve 10 and the working machine supporting valve 64. In the above-described arrangement, for example, when the working vehicle is a wheel loader, the controlled pressure of the running relief valve 37 is set to 420 kg/cm 2 , the controlled pressure of the working machine relief valve 38 is set to 210 kg/cm 2 , and the unload valve 66 is constructed so as to be changed at 220 kg/cm 2 . In addition, the working machine supporting valve 64 is constructed so as to be changed from position J to position K at 210 kg/cm 2 , and from the position K to position L at 250 kg/cm 2 . Next, an operation will be described. As regards the first directional control valve 10 for joining the flow to support from the working machine circuit 11 to the running circuit 61, the description will be omitted because it is the same as that of the fourth embodiment. The third directional control valve 62 for supporting from the running circuit 61 to the working machine circuit 11, and the unload valve 66 will be described. First, similar to the fourth embodiment, it is assumed that the first directional control valve 10 is placed in the first position A with the load of the running hydraulic motor 8 being the output torque Ta or higher. By this, the discharge pressure oil of the working machine hydraulic pump 3 is supplied to the working valve 13, and the working machine is in the operable condition by the operation of the working valve 13. When the pressure in the extension chamber 26b of the working machine cylinder 26 is the controlled pressure of 210 kg/cm 2 or lower of the working machine relief valve 38 under this condition, the discharge pressure oil of the working machine hydraulic pump 3 is supplied to the extension chamber 26b of the working machine cylinder 26 by way of the second position B of the first directional control valve 10, and the position G of the working valve 13 so as to extend the working machine cylinder 26. In addition, the pressure reaches the working machine supporting valve 64, the pilot valve 65, and the pressure receiving chamber 66a of the unload valve 66 by the support pipe 71. Since the pilot valve 65 is shut off at a shutoff position M, the pressure in the extension chamber 26b from the support pipe 71 does not reach the pressure receiving chamber 66a of the working machine supporting valve 64. For this reason, the working machine supporting valve 64 is placed in a shutoff position J, and the running circuit 61 and the working machine circuit are shut off, so that the running hydraulic pump 2 does not support the working machine hydraulic pump 3. In addition, the pressure in the extension chamber 26b from the support pipe 71 reached the pressure receiving portion 66a of the unload valve 66 is 220 kg/cm 2 or lower, so that the unload valve 66 stops in the shutoff position P, and the discharge pressure oil of the working machine hydraulic pump 3 is shut off and not unloaded. Next, when the pressure in the extension chamber 26b of the working machine cylinder 26 is the controlled pressure of 210 kg/cm 2 or higher of the working machine relief valve 38, the working machine relief valve 38 is operated to control the pressure of the working machine circuit 11 to controlled pressure of 210 kg/cm 2 . There may be a case where the force for raising the bucket is insufficient even if the controlled pressure of the working machine circuit 11 reaches 210 kg/cm 2 . At this time, the operator turns on the selector switch 68 for supporting. In addition, at this time, a signal from the working machine pressure sensor 69 connected to the working machine circuit 11 enters the AND circuit 67. By two signals from the selector switch 68 and the working machine pressure sensor 69, the AND circuit 67 outputs a signal to the solenoid 65b of the pilot valve 65 to switch the pilot valve 65 from the shutoff position M to the communication position N. This allows the pressure of 210 kg/cm 2 or higher of the extension chamber 26b from the support pipe 71 to reach the pressure receiving chamber 64a of the working machine supporting valve 64, thereby switching the working machine supporting valve 64 from the shutoff position J to the communication position K. For this reason, the running circuit 61 is in communication with the working machine circuit 11, and the running hydraulic pump 2 supports the working machine hydraulic pump 3. At this time, when the pressure of the running hydraulic pump 2 is 210 kg/cm 2 or lower, the running hydraulic pump 2 does not support the working machine hydraulic pump 3, and back flow from the working machine hydraulic pump 3 to the running hydraulic pump 2 is prevented by the support circuit check valve 72. In addition, at this time, the operator increases forward force obtained by rotating the engine 1 at a high speed, i.e., the pressure of the running hydraulic motor 8 to raise the output torque with holding the bucket in abutment with the rock base, etc., and supports this pressure from the running hydraulic pump 2 to the working machine hydraulic pump 3. By this, the rock base, etc. can easily be dug by a resultant force of bucket raising force and forward force. When the pressure supported by the running hydraulic pump 2 increases to 220 kg/cm 2 or higher, the unload valve 66 is switched to a communication position Q, and the discharge pressure oil of the working machine hydraulic pump 3 is communicated with the tank to be unloaded. This allows the discharge pressure oil of the working machine hydraulic pump 3 to decrease substantially to 0 kg/cm 2 , so that the load of the engine 1 is lightened, and a surplus force is generated in the output. Further, when the pressure in the extension chamber 26b of the working machine cylinder 26 increases to reach 250 kg/cm 2 , the pressure of 250 kg/cm 2 in the extension chamber 26b from the support pipe 71 reaches the pressure receiving chamber 66a of the working machine supporting valve 64, and switches the working machine supporting valve 64 from the communication position K to the shutoff position L. For this reason, the working machine supporting valve 64 shuts off the running circuit 61 and the working machine circuit 11 again, and reduces the pressure of the support pipe 71. For this reason, the force for raising the bucket increases until the pressure in the extension chamber 26b of the working machine cylinder 26 reaches 250 kg/cm 2 , and even higher pressure does not go from the running circuit 61 to the working machine circuit 11. Thus, the allowable pressure of the hydraulic equipment for the working machine circuit 11 is maintained. In addition, when the pressure in the extension chamber 26b of the working machine cylinder 26 increases to 250 kg/cm 2 or higher, the unload valve 66 is switched to the shutoff position P to reduce the pressure of the support pipe 71. For this reason, the pressure of the discharge pressure oil of the working machine hydraulic pump 3 decreases to 220 kg/cm 2 again, so that the pressure in the extension chamber 26b of the working machine cylinder 26 is maintained at 220 kg/cm 2 . In the above-described embodiment, the pilot valve 65 is switched in the AND circuit 67 by two signals from the selector switch 68 and the working machine pressure sensor 69. However, the pilot valve 65 may be switched by respective signals alone. Fig. 8 is a sectional view of the flow joining/dividing valve 60 in the fifth embodiment. The flow joining/dividing valve 60 includes the first directional control valve 10, the third directional control valve 62 and the unload valve 66 accommodated in a one-piece body 60A. Figs. 9 to 12 illustrate the operation of the third directional control valve 62. Referring to Fig. 8, a piston 17a of the pressure receiving portion 17 is disposed on right-side one end of the first directional control valve 10. A spool 10a is disposed in abutment with the piston 17a, and a pump port 10b from the working machine hydraulic pump 3 is disposed in the center of the spool 10a. In addition, on the right of the spool 10a, a working machine port groove 10c to the working machine valve 13 and the unload valve 66 is disposed between the pump port 10b and the piston 17a, and on the left side, a running port groove 10d to the running valve 5 and the working machine supporting valve 64 is disposed. The third directional control valve 62 and the unload valve 66 are arranged on the same line, and the third directional control valve 62 is arranged on the left of the drawing, while the unload valve 66 is arranged on the right of the drawing. A tank groove 66b is provided in left-side one end of the unload valve 66, an unloading port groove 66c connected to the working machine port groove 10c of the first directional control valve 10 is provided in its right side, and further, a pressure receiving chamber 66a is provided on its right side. In the third directional control valve 62 including the working machine supporting valve 64 and the pilot valve 65, the pilot valve 65 is accommodated inside of the working machine supporting valve 64 to be integrally formed. In addition, on the left side of the drawing opposite to the unload valve 66, a solenoid 65a for the pilot valve 65 is disposed. On the right side of the solenoid 65a, a supporting first port 64b linked to the support pipe 71 connected to the working machine valve 13 is arranged, and on its right side, a supporting second port 64c linked to the running port groove 10d of the first directional control valve 10 is arranged. Referring to Fig. 9, in a hole linked to the supporting first port 64b and the supporting second port 64c, a supporting spool 64d is disposed, and a spool hole 64e having a large diameter, and a spool hole 64f having a small diameter are formed inside the supporting spool 64d. A pressure receiving area is provided by the difference between the diameters so as to constitute a pressure receiving portion 64a. In a fixed sleeve 65b of the pilot valve 65 inserted into the inner diameters of the spool holes 64e and 64f, a first drill hole 65c linked to the support pipe 71 and a first drill hole 65d linked to the pressure receiving chamber 64a are formed. Next, an operation of the flow joining/dividing valve 60 will be described. Referring to Fig. 8, the first directional control valve 10, when the pressure oil does not act on the piston 17a of the pressure receiving portion 17, is opened at a position Ha of the spool 10a, and shut off at a position Hb of the spool 10a, whereby the pump port 10b and the working machine port groove 10c are connected. This is equivalent to the first position A of the first directional control valve 10 shown in Fig. 7. When the pressure oil acts on the piston 17a, the first directional control valve 10 is opened at the position Hb of the spool 10a, whereby the pump port 10b and the running port groove 10d are connected, and is shut off at the position Ha of the spool 10a, whereby the pump port 10b and the working machine port groove 10c are shut off. This is equivalent to the second position B of the first directional control valve 10 shown in Fig. 7. The unload valve 66 is mainly composed of a check valve 66d and a spring 66e, and the check valve 66d is constructed to have a small diameter on the left side of the pressure receiving chamber 66a, and to have a large diameter on the right side, and a pressure receiving area for receiving the pressure of the pressure receiving chamber 66a acting on the check valve 66d is provided. When the pressure of the pressure receiving chamber 66a linked to the support pipe 71 is predetermined pressure or lower, the check valve 66d is pressed leftward of the drawing by the spring 66e, whereby the tank groove 66b and the unloading port groove 66c are shut off at a position Hc. This is equivalent to the shutoff position P of the unload valve 66 shown in Fig. 7. When the pressure of the pressure receiving chamber 66a increases to the predetermined pressure or higher, the check valve 66d moves rightward of the drawing against the spring 66e, whereby the tank groove 66b and the unloading port groove 66c are opened at the position Hc. This is equivalent to the communication position Q of the unload valve 66 shown in Fig. 7. An operation of the third directional control valve 62 will be described with reference to Figs. 9 to 12. In Fig. 9, a drawing is shown in which the solenoid 65a for the pilot valve 65 is not excited, and the first drill hole 65c linked to the support pipe 71 and the first drill hole 65d linked to the pressure receiving chamber 64a are shut off by a valve rod 65e. This position is the shutoff position M of the pilot valve 65 in Fig. 7. By this, since the pressure oil does not acts on the pressure receiving chamber 64a of the working machine supporting valve 64, the supporting spool 64d does not move, so that the supporting first port 64b linked to the support pipe 71 and the running port groove 10d of the first directional control valve 10 are shut off by the supporting spool 64d at the position Hd. This state shows the shutoff position J of the working machine supporting valve 64 in Fig. 7. In Fig. 10 a state is shown in which the solenoid 65a for the pilot valve 65 is excited, and the supporting spool 64d has not moved yet. The first drill hole 65c is in communication with the first drill hole 65d by a slit 65f of the valve rod 65e. This position is the communication position N of the pilot valve 65 in Fig. 7. This allows the pressure oil to act on the pressure receiving chamber 64a of the working machine supporting valve 64, so that the supporting spool 64d starts to move when pressure becomes a first predetermined pressure. In Fig. 11, a state is shown in which the solenoid 65a of the pilot valve 65 is excited, and the supporting spool 64d is moving. The first drill hole 65c is in communication with the first drill hole 65d by the slit 65f of the valve rod 65e, and this position is the communication position N of the pilot valve 65 in Fig. 7. This allows the pressure oil of the first predetermined pressure to act on the pressure receiving chamber 64a of the working machine supporting valve 64 to move the supporting spool 64d, so that the supporting first port 64b linked to the support pipe 71 is in communication with the running port groove 10d of the first directional control valve 10 by a slit 64e at the position Hd. This state shows the communication position K of the working machine supporting valve 64 in Fig. 7. Fig. 12 shows a state in which the solenoid 65a for the pilot valve 65 is excited, and the supporting spool 64d moves further rightward of the drawing. The first drill hole 65c is in communication with the first drill hole 65d by the slit 65f of the valve rod 65e, and this position is the communication position N of the pilot valve 65 in Fig. 7. This allows the pressure oil of the second predetermined pressure to act on the pressure receiving chamber 64a of the working machine supporting valve 64 to move the supporting spool 64d further rightward of the drawing, so that the supporting first port 64b linked to the support pipe 71 and the running port groove 10d of the first directional control valve 10 are shut off by the slit 64e of the supporting spool 64d at the position He. This state shows the shutoff position L of the working machine supporting valve 64 in Fig. 7. This allows the working machine supporting valve 64, when the pressure oil of the first predetermined pressure is operated, to communicate the supporting first port 64b linked to the support pipe 71 with the running port groove 10d of the first directional control valve 10, and the pressure oil (arrow Qm) is fed from the running port groove 10d to the supporting first port 64b. When the pressure oil of the second predetermined pressure is operated, the supporting first port 64b linked to the support pipe 71 and the running port groove 10d of the first directional control valve 10 are shut off, so that the pressure oil from the running port groove 10d to the supporting first port 64b stops again. This allows the circuit of the working machine valve 13 to be maintained to the pressure oil of the second predetermined pressure or lower. Fig. 13 is a hydraulic circuit diagram of a sixth embodiment. Although the pilot oil pressure is used for controlling the valves in the fifth embodiment, an example is shown in the sixth embodiment in which the valves are controlled by an electric connection. Therefore, the number of ports, the positions, and the functions of respective valves are the same as those of the fifth embodiment. An electromagnetic flow joining/dividing valve 80 is composed of an electromagnetic first directional control valve 81 for joining to support from the working machine circuit 11 to the running circuit 61, an electromagnetic third directional control valve 82 for supporting from the running circuit 61 to the working machine circuit 11, and an electromagnetic unload valve 83. To the engine 1, an engine speed sensor 85 for measuring the engine speed, a fuel injection amount sensor 86 for measuring the fuel injection amount of the engine 1, or an accelerator lever position sensor 87 for measuring an accelerating amount of the accelerator lever are attached. In addition, to the running hydraulic motor 8, a running speed sensor 88 for measuring the running speed by the running hydraulic motor 8, and a running pressure sensor 89 for measuring running torque applied to the running hydraulic motor 8 are attached. In addition, a controller 90 is provided for controlling the electromagnetic flow joining-dividing valve 80 upon receipt of signals from these sensors. The controller 90 is provided with a speed change lever position sensor 91 attached to a speed change lever. Next, an operation will be described by a flowchart of Fig. 14. In step 1, the running pressure sensor 89 measure the discharge pressure of the running hydraulic pump 2 in order to measure the running torque Ta exerted on the running hydraulic motor 8. In step 2, it is judged whether or not the pressure applied to the running hydraulic motor 8 exceeds a predetermined value. When exceeds in step 2, a procedure advanced to step 3. In step 3, when the pressure applied to the running hydraulic motor 8 exceeds the predetermined value, the controller 90 does not output a switching command to the electromagnetic first directional control valve 81. By this, in step 4, the working machine circuit 11 drives the working machine without joining to support from the working machine circuit 11 to the running circuit 61. Incidentally, when the pressure does not exceed the predetermined value in step 2, the procedure advances to step 5. In step 5, the engine speed sensor 85 measures the speed of the engine 1, the fuel injection amount of the engine 1 is measured by the fuel injection amount sensor 86, or the accelerating amount of the accelerator lever is measured by the accelerator lever position sensor 87. In step 6, it is judged whether or not the speed of the engine 1 is a predetermined speed. When the speed of the engine 1 is the predetermined speed or low, the procedure advances to step 7. In step 7, the controller 90 does not output a switching command to the electromagnet first directional control valve 81. By this, in step 8, the working machine circuit 11 drives the working machine without joining to support from the working machine circuit 11 to the running circuit 61. When the speed of the engine 1 is the predetermined speed or higher, the procedure advances to step 9. In step 9, it is judged whether or not the speed change lever position sensor 91 is placed in the high speed of the fourth speed, or the fifth speed. In step 9, when the speed change lever position sensor 91 is placed in the high speed of the fourth speed, or the fifth speed, the procedure advances to step 10. In step 10, the controller 90 outputs a switching command to the electromagnet first directional control valve 81. By this, in step 11, the running hydraulic motor 8 joining to support from the working machine circuit 11 to the running circuit 61 rotates at high speed. In step 9, when the speed change lever position sensor 91 is not placed in the high speed of the fourth speed, or the fifth speed, the procedure advances to step 12. In step 12, the controller 90 does not output a switching command to the electromagnetic first directional control valve 81. By this, in step 13, the working machine circuit 11 drives the working machine without joining to support from the working machine circuit 11 to the running circuit 61. Incidentally, although the speed of the engine 1 and the speed change position of the speed change lever position sensor 91 are detected and judged in the above description, the speed of the running hydraulic motor 8 may be detected and judged by the running speed sensor 85. That is to say, in place of step 6 and step 9, whether or not the running hydraulic motor 8 rotates at the predetermined speed or higher is judged. When the predetermined speed or higher, the procedure may advance to step 10, and the procedure may advance to step 12 when the predetermined speed or lower. In addition, although it is judged whether or not the speed change lever position sensor 91 is placed in the high speed of the fourth speed, or the fifth speed in step 9, it may be judged by the Hi/Low switch 47 in the third embodiment. In addition, in the above description, when the flow is not joined to support from the working machine circuit 11 to the running circuit 61, and the pressure in the extension chamber 26b of the working machine cylinder 26 is the controlled pressure of 210 kg/cm 2 or higher of the working machine relief valve 38 in step 4 and step 10, or step 12, the electromagnetic third directional control valve 82 is switched to support the working machine circuit 11 from the running circuit 61 similar to the fourth embodiment or the fifth embodiment, and the electromagnetic unload valve 83 is switched to unload the working machine hydraulic pump 3, thereby reducing the load acting on the working machine hydraulic pump 3. Next, a case in which the working machine circuit 11 is supported by the running circuit 61 will be described with reference to Fig. 15 and Fig. 16. In step 21, it is judged whether or not the controlled pressure of the working machine circuit 11 exceeds a predetermined pressure (for example, 210 kg/cm 2 ). This is measured by the working machine pressure sensor 69 connected to the working machine circuit 11, and is used for the judgement whether or not the force for raising the bucket is insufficient even if the controlled pressure exceeds the predetermined pressure (210 kg/cm 2 ). In the case of NO in step 21, the procedure returns to step 21 again. In step 22, the operator watches the movement of the working machine (for example, bucket) to judge whether or not the working machine has stopped. When stopped, it is judged that the force for raising the working machine is insufficient. Therefore, in the case of NO, the procedure returns to step 21. When stopped, the procedure advances to step 23. In step 23, the selector switch 68 is operated to be turned on. In step 24, it is judged whether or not oil pressure of the running circuit 61 exceeds predetermined pressure (for example, 220 kg/cm 2 ). In step 25, the controller 90 outputs a switching command to the electromagnetic third directional control valve 82 and the electromagnetic unload valve 83 by a signal of exceeding the predetermined pressure (210 kg/cm 2 ) from the working machine pressure sensor 69, an ON signal operated from the selector switch 68, and a signal of exceeding the predetermined pressure (220 kg/cm 2 ) from the running pressure sensor 89. In step 26, the electromagnetic third directional control valve 82 and the electromagnetic unload valve 83 are switched, the running hydraulic pump supports the working machine circuit 11, and the working machine hydraulic pump 3 is unloaded to reduce a load acting on the working machine hydraulic pump 3. The pressure supporting from the running hydraulic pump 2 increases to 220 kg/cm 2 or higher, whereby a force for raising the bucket increases. In step 27, the pressure supported from the running hydraulic pump 2 increases, and it is judged whether or not the increased pressure reaches 250 kg/cm 2 . In case of NO, the procedure returns to step 21. When the pressure reaches 250 kg/cm 2 in step 27, the procedure advances to step 28. In step 28, the controller 90 outputs a command for stopping the support from the running hydraulic pump 2 to the working machine circuit 11 to the electromagnetic third directional control valve 82. In step 29, the electromagnetic third directional control valve 82 is switched to stop the support. By the steps as described above, the pressure in the extension chamber 26b of the working machine cylinder 26 increases a force for raising the bucket of 250 kg/cm 2 , and even higher pressure does not go from the running circuit 61 to the working machine circuit 11, so that allowable pressure of hydraulic equipment for the working machine circuit 11 is maintained. In the above embodiment, the switching effected by two signals from the selector switch 68 and the working machine pressure sensor 69 is described. However, the electromagnetic unload valve 83 may be energized by only a signal from the working machine pressure sensor 69, only a signal from the switch 36 attached to the operating lever 35, or by two signals from the switch 36 and the working machine pressure sensor 69. In addition, although the description is given by the electromagnetic hydraulic equipment, it is appreciated that valves can be similarly controlled by the hydraulically operated hydraulic equipment like the third embodiment. In addition, in the above embodiment, the description is given of the allowable pressure for the working machine by providing a numerical values, for example, 250 kg/cm 2 , etc. However, it is appreciated that the allowable pressure is not restricted thereto and can be selected in accordance with its circuit. The present invention is useful as a hydraulic circuit for a hydraulically driven working vehicle, such as a wheel loader, a crane truck, and a construction vehicle, etc. which allows the vehicle to run at an almost constant speed during high-speed running, provides a large digging force during working, does not need charging pressure for preventing cavitation and loses small amount of energy, and has a simple structure.

Claims: A hydraulic circuit for a hydraulically driven working vehicle, comprising: a running HST circuit driven by power of an engine (1) to run the vehicle; a working machine-driving hydraulic circuit driven by power of the engine (1) to drive a working machine, such as a bucket, attached to the vehicle, and having a predetermined relief pressure lower than that of the running HST ciruit; a running hydraulic pump (2) and a working machine-driving hydraulic pump (3) for discharging pressure oil of the running HST circuit and for discharging pressure oil of the working machine-driving hydraulic circuit, respectively; a flow joining/dividing valve (10) for joining or dividing discharge oil from the running hydraulic pump (2) and the working machine-driving hydraulic pump (3) with discharge oil in another circuit or into its own circuit; and a load check valve (15) disposed between the working machine-driving hydraulic pump (3) and a working machine valve (13) provided on the working machine-driving hydraulic circuit, wherein discharge oil from the running HST circuit joins discharge oil in the working machine-driving hydraulic circuit when the pressure of the running HST circuit is higher than the predetermined relief pressure of the working machine-driving hydraulic circuit. A hydraulic circuit for a hydraulically driven working vehicle according to claim 1, wherein the pressure of the working machine-driving hydraulic circuit is reduced when discharge oil from the running HST circuit joins discharge oil in the working machine-driving hydraulic circuit. A hydraulic circuit for a hydraulically driven working vehicle according to claim 1, wherein pressure of the discharge oil from the running HST circuit which joins the discharge oil in the working machine-driving hydraulic circuit is the predetermined relief pressure or higher, and allowable pressure or lower of the working machine-driving hydraulic circuit. A hydraulic circuit for a hydraulically driven working vehicle according to claim 1, wherein discharge oil from the working machine-driving hydraulic circuit joins discharge oil in the running HST circuit when pressure of the running HST cirucit is lower than first predetermined pressure, and engine speed is a predetermined value or higher, and the joining of the discharge oil from the working machine-driving hydraulic circuit is cut off when the pressure of the running HST circuit is higher than the first predetermined pressure. A hydraulic circuit for a hydraulically driven working vehicle according to any one of claims 1 to 4, wherein the circuit includes a running HST circuit of an open circuit comprising a tank for storing oil, a running variable displacement hydraulic pump (2) for sucking oil and for discharging pressure oil, a running directional control valve (52) for switching the pressure oil from the running variable displacement hydraulic pump (2), and a running hydraulic motor (8) rotating clockwise or counterclockwise to produce an output upon receipt of the switched pressure oil from the running directional control valve (5). A hydraulic circuit for a hydraulically driven working vehicle according to any one of claims 1 to 4, wherein the joining is selected in operatively associated with a selector switch (68) for switching high-speed running and low-speed running.

Inventors: HAYASHI SEITA, NUNOTANI SADAO, HAYASHI, SEITA, NUNOTANI, SADAO, HAYASHI, Seita, Komatsu Ltd. Oyama Plant, NUNOTANI, Sadao, Komatsu Ltd. Oyama Plant
Applicants: KOMATSU MFG CO LTD, KOMATSU LTD.
Application year: 1996
Main classification: E02F9
IPC codes: E02F9