JP2000281275A - Oil pressure control circuit for hydraulic grab bucket - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oil pressure control circuit for a hydraulic grab bucket continuously operable for a long time, by suppressing temperature up caused by heating working oil due to relief actuation, etc., in setting pressure in a pressure control valve for setting conventional hydraulic cylinder working pressure when closing the bucket to grip cargoes and the like. SOLUTION: In an oil pressure control circuit for a hydraulic grab bucket equipped with a hydraulic cylinder 15 for opening/closing a bucket, a hydraulic pump for driving the cylinder 15, and an electromagnetic switch valve 11 linked to the discharge pipe of the hydraulic pump to switchingly supply pressure oil to the head and rod side chambers 15a and 15b of the cylinder 15; the hydraulic pump is adopted as a variable discharge type hydraulic pump 19 equipped with a discharge amount control mechanism for making the discharge amount Q of the hydraulic pump minimum when hydraulic pump discharge pressure is increased to set pressure PS, that is, the discharge Q is restricted to the minimum discharge amount needed to pressurize the cylinder 15.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydraulic control circuit for a hydraulic grab bucket such as an electro-hydraulic grab bucket, and more particularly to an operation of a hydraulic cylinder for opening and closing a bucket such as garbage such as municipal refuse or combustion ash. When the bucket is opened and closed, such as when the bucket is closed and gripped, or when the bucket is opened and the gripped load is released, the heat generation of the hydraulic oil of the hydraulic cylinder, temperature rise, etc. are effectively suppressed to continuously operate the hydraulic grab bucket. The present invention relates to a hydraulic control circuit of a hydraulic grab bucket that can be operated by being operated.

[0002]

2. Description of the Related Art Conventionally, for example, in a cleaning plant, a pressure control valve such as a hydraulic pump or a relief valve driven by a motor such as an electric motor is mounted on a girder of a grab bucket body for transporting refuse such as municipal refuse or incineration ash. In addition, a hydraulic grab bucket that incorporates an electric hydraulic pump unit including a hydraulic device such as a direction control valve and opens and closes a grab bucket is used.

[0003] This type of hydraulic grab bucket has, for example, a pair of left and right buckets openably and closably mounted by a hydraulic cylinder on a girder that can be suspended by a lifting device such as a crane, and the hydraulic cylinder is mounted on the girder. A hydraulic pump to be driven, a prime mover such as an electric motor to drive the hydraulic pump, an electromagnetic switching valve for sending hydraulic oil discharged from the hydraulic pump to the hydraulic cylinder and switching a driving (operating) direction of the hydraulic cylinder, and a hydraulic cylinder operation A pressure adjusting valve for setting pressure and a hydraulic pump unit having a built-in hydraulic pipe for connecting the respective devices and an oil tank are provided.

[0004] When the hydraulic pump unit of the grab bucket is taken in and immersed in garbage or incineration ash which is a grasped object or in water, the incineration ash, water and the like enter the unit. In some cases, the periphery of a hydraulic device including a motor such as an electric motor, a hydraulic pump, or the like is covered with a panel and sealed so as to be sealed against the external atmosphere.

A hydraulic control circuit of such a conventional hydraulic pump unit will be described with reference to FIG. 1 is an oil tank, 2 is a suction pipe, and 3 is a hydraulic pump rotated and driven by an electric motor 4 as a prime mover, and is configured as a multiple pump in which a large capacity pump 3a and a small capacity pump 3b are integrated. 5
Is a discharge pipe connected to the oil discharge port of the large capacity pump 3a;
Is a relief valve of a balance piston type or the like as a pressure control valve for setting a circuit pressure branched from the discharge pipe 5 and connected to the oil tank 1 at an end, that is, for setting an operating pressure of the hydraulic cylinder 15 for opening and closing the bucket. Reference numeral 7 denotes a discharge pipe connected to the oil discharge port of the small capacity pump 3b, and reference numeral 8 denotes a bypass line communicating between the discharge pipe 7 and the tank line 10 and having a check valve (check valve) 9 interposed therebetween. The check valve 9 acts to raise a predetermined pilot pressure and supply it to a pilot switching valve 11B described later.

Reference numeral 11 designates an electromagnetic switching valve for switching and supplying the hydraulic oil discharged from the large-capacity hydraulic pump 3a and sent to the discharge pipe 5 to the head side chamber 15a and the rod side chamber 15b of the hydraulic cylinder 15 to switch the operating direction of the hydraulic cylinder 15. The electromagnetic pilot switching valve, the electromagnetic pilot switching valve 11
Is composed of a main switching valve 11A and a pilot switching valve 11B. The main switching valve 11A has a center portion at a neutral position shown in the figure and a pump port P to which the end of the large capacity hydraulic pump discharge pipe 5 is connected and the tank line 10
Is connected to the tank port T to which the end of
A tandem center type switching valve in which ports A and B are blocked is provided.

On the other hand, the pilot switching valve 11B has a center portion at a neutral position shown in the drawing, and a pump port connected to an end of the small-capacity hydraulic pump discharge pipe 7 is blocked, and an end of the tank line 12 is connected. A pressure port block type solenoid-operated directional control valve in which the tank port T, the A port, and the B port are in communication with each other. One end of each of the pressure oil supply pipes 13 and 14 is connected to the A port and the B port of the main switching valve 11A, and the other end of the pressure oil supply pipe 13 is connected to the rod side chamber 1 of the hydraulic cylinder 15.
5b, and the other end of the pressure oil supply pipe 14 is connected to the head side chamber 15a of the hydraulic cylinder 15.

A port of the pilot switching valve 11B and one end of the main switching valve 11A are connected to a pilot pressure supply pipe 11
a port B of the pilot switching valve 11B is connected to the pilot pressure supply pipe 1 at the other end of the main switching valve 11A.
1b.

In the hydraulic control circuit of the hydraulic grab bucket configured as described above, when the electric motor 4 is started in the state shown in FIG. 11, the hydraulic pump 3 is driven and the large capacity pump 3a is driven.
The oil discharged from the tank is bypassed at the center of the main switching valve 11A of the electromagnetic pilot switching valve 11 through the discharge pipe 5, returned to the tank line 10, and circulated idle. The oil discharged from the small-capacity pump 3b flows through the discharge pipe 7 and returns to the tank line 10 through the bypass line 8 with the check valve 9 interposed therebetween, and is circulated idle.

In this state, when the garbage, which is a grasping object, is grasped by the hydraulic grab bucket, the garbage is suspended with the hydraulic grab bucket suspended by a crane as a lifting device, positioned above the garbage, and opened. , And the bucket is closed. At this time, SOL.b of the pilot switching valve 11B of the electromagnetic pilot switching valve 11 is excited. Then, the pressure oil discharged from the small capacity pump 3b flows into the pilot pressure supply pipe 11b through the discharge pipe 7 and the B port of the pilot switching valve 11B, and flows into the main switching valve 1b.
It is supplied to the other end of 1A, the main switching valve 11A is switched, and its pump port P and B port are communicated.

Then, the oil discharged from the large-capacity pump 3a flows from the pump port P of the main switching valve 11A to the B port via the discharge pipe 5 and passes through the hydraulic oil supply pipe 14 to the hydraulic cylinder 15 for opening and closing the bucket. The air is supplied to the head side chamber 15a, the piston rod is extended, the grab bucket is closed, and dust is grasped. At this time, the oil in the rod side chamber 15b of the hydraulic cylinder 15 flows from the A port of the main switching valve 11A to the tank line 10 via the tank port T via the pressure oil supply pipe 13, and is returned to the oil tank 1.

After the dust is thus grasped by the bucket, the excitation of SOL.b of the pilot switching valve 11B is released, the pilot switching valve 11B is neutralized as shown, and the main switching valve 11A is returned to the neutral state. After lifting the hydraulic grab bucket with a crane, it is transported by moving it to the unloading point, opening the bucket and dropping the grabbed garbage to unload. At this time, SOL.a of the pilot switching valve 11B of the electromagnetic pilot switching valve 11 is excited. Then, the pressure oil discharged from the small-capacity pump 3b flows through the discharge pipe 7 into the pilot pressure supply pipe 11a via the A port of the pilot switching valve 11B, and is supplied to one end of the main switching valve 11A. Is switched so that the pump port P and the A port communicate with each other.

Then, the oil discharged from the large-capacity pump 3a flows through the discharge pipe 5 from the pump port P of the main switching valve 11A to the A port, and passes through the hydraulic oil supply pipe 13 to the hydraulic cylinder 15 for opening and closing the bucket. The piston rod is supplied to the rod side chamber 15b, the piston rod is retracted, the grab bucket is opened, and dust is unloaded. At this time, the oil in the head side chamber 15a of the hydraulic cylinder 15 flows from the B port of the main switching valve 11A to the tank line 10 via the tank port T via the pressure oil supply pipe 14, and is returned to the oil tank 1.

The hydraulic pressure of the pressure oil supplied to the hydraulic cylinder 15 necessary for closing the bucket for grasping the debris with the bucket and for opening the bucket when unloading the debris which has been grasped is controlled by a pressure control valve. The pressure is set within the pressure set by the relief valve 6.

[0015]

When a load such as dust is grasped by a hydraulic grab bucket using a hydraulic control circuit as shown in FIG. 11, the bucket is closed or the load is interposed between the buckets. Is trapped or cannot be closed any more, the pressure of the discharge oil discharged from the large-capacity pump 3a of the hydraulic pump 3 rises and reaches the set pressure of the relief valve 6, and the discharge oil of the large-capacity pump 3a The whole amount is relieved from the relief valve 6, flows to the tank line 10 and is returned to the oil tank 1. By this relief operation, the entire energy of the hydraulic oil is converted into heat, the hydraulic oil is heated, and the temperature of the hydraulic oil rises.

Further, the closing operation of the bucket when the load is grasped by the bucket is rarely completed by one operation. For example, if the load is grasped too much, the bucket is not completely closed. In such a case, when the first closing operation is completed, the hydraulic grab bucket is slightly lifted by the crane and the main switching valve 11A of the electromagnetic pilot switching valve 11 is lifted.
To a neutral state, the pressurization of the head chamber side 15a is stopped, and the load is gradually dropped from the bucket while the pressure oil of the head chamber side 15a and the rod chamber side 15b of the hydraulic cylinder 15 is held, and then, again. The SOL.b of the pilot switching valve 11B of the electromagnetic pilot switching valve 11 is excited to put the main switching valve 11A into the bucket closed state, pressurize the closing side (the head chamber 15a side) of the hydraulic cylinder 15, and complete the bucket. Or lifted so that it can be transported.

Such an additional pressurizing operation, that is, an operation called “grabbing”, usually needs to be repeated several times, such as three times. Each time such a catching operation is performed, the relief operation of the discharge oil of the large-capacity pump 3a by the relief valve 6 is performed, and the oil temperature is further increased. The relief operation of the relief valve 6 increases the power consumption of the electric motor 4 as the driving motor of the hydraulic pump 3 and causes heat generation.

When the hydraulic pump unit has a hermetic structure as described above, heat is not satisfactorily dissipated due to ventilation, so that the release of the relief valve 6 causes the generation of hydraulic oil and the operation of the hydraulic pump 3. Heat of the motor 4 cannot be released to the outside, and the hydraulic oil such as hydraulic oil, the hydraulic pump 3, the electromagnetic pilot switching valve 11, the piping, and the hydraulic motor 4 and the temperature of the motor 4 are excessively increased. Had trouble.

With the recent increase in the amount of municipal solid waste in waste management plants and the introduction of continuous combustion of waste incinerators as a measure against dioxins, the frequency of use of grab buckets in waste treatment plants is increasing, and these severe conditions are increasing. There is a demand for a grab bucket that can be operated continuously for a long time even under various use conditions.

The present invention has been made to eliminate the above-mentioned drawbacks of the conventional hydraulic control circuit of a hydraulic grab bucket, and a load such as municipal waste or combustion ash is transferred to a bucket by a bucket opening / closing hydraulic cylinder. When the load is closed and gripped, or the bucket is opened to release the load, the temperature rise due to the heat generated by the hydraulic oil caused by the relief operation of the pressure control valve for setting the operating pressure of the hydraulic cylinder as described above. And a hydraulic control circuit for a hydraulic grab bucket capable of suppressing the heat generation of a prime mover such as an electric motor for driving a hydraulic pump by the relief operation and enabling the hydraulic grab bucket to be continuously operated for a long time. I do.

[0021]

In order to achieve the above object, a hydraulic control circuit for a hydraulic grab bucket according to the present invention comprises:

(1) A hydraulic cylinder for opening and closing the bucket, a hydraulic pump for driving the hydraulic cylinder, and a hydraulic oil connected to a discharge pipe of the hydraulic pump for switching and supplying hydraulic oil to the head side chamber and the rod side chamber of the hydraulic cylinder. In a hydraulic control circuit of a hydraulic grab bucket provided with an electromagnetic switching valve for switching the operation direction of a cylinder, when the hydraulic pump discharge pressure is increased to a set pressure, the discharge amount of the hydraulic pump becomes minimum, and the hydraulic cylinder The variable discharge type hydraulic pump is provided with a discharge amount control mechanism for stopping the discharge amount to a minimum necessary for pressurizing the oil.

(2) In the hydraulic control circuit of the hydraulic grab bucket of the above (1), a pressure detector attached to a discharge pipe of the hydraulic pump, and the discharge pressure of the hydraulic pump is controlled by the pressure detector. A timer for starting the timing by detecting that the set pressure has been reached and for setting the electromagnetic switching valve to a neutral state after a predetermined time, and by setting the electromagnetic switching valve to a neutral state, the hydraulic pump discharge oil is supplied to the tank. The idle circulation is performed between them.

(3) In the hydraulic control circuit for a hydraulic grab bucket according to the above (1) or (2), the electromagnetic switching valve is an electromagnetic pilot switching valve comprising a main switching valve and a pilot switching valve. A dedicated pump for securing the operating pilot pressure of the hydraulic pump is mounted coaxially with or separately from the hydraulic pump.

[0025]

In the configuration of (1), the hydraulic pump for driving the hydraulic cylinder for opening and closing the bucket is, for example, a variable displacement piston pump as a variable discharge hydraulic pump,
The variable displacement type piston pump includes a control mechanism for minimizing the discharge amount of the pump when the discharge pressure of the pump is increased to a set pressure, and stopping the discharge amount to the minimum discharge amount required to pressurize the hydraulic cylinder. Therefore, when the pump discharge pressure exceeds the set pressure of the pump, for example, when the bucket grasps the load and closes or when the bucket for releasing the load opens, the pump discharges while maintaining the pressure at that point. The quantity is minimal (theoretical zero), so that the power requirement of the pump is minimal (theoretical zero). This is because the power required for the pump is the product of the discharge pressure and the discharge amount. Further, heat generation from the pump is suppressed. Therefore, an increase in the oil temperature is effectively suppressed.

In the configuration of the above (2), when the discharge pressure of the pump reaches the set pressure of the pump, the electromagnetic switching valve is switched to the neutral state after a lapse of a predetermined time, and the oil discharged from the pump is idle-circulated to and from the oil tank. Therefore, the pressure oil held at the pump set pressure does not act on the pipeline from the pump discharge port to the electromagnetic switching valve. For this reason, for example, there is no drain from the internal drain (leakage) of the pump or the spool of the electromagnetic switching valve, so that the heat generated by the drain is prevented, and the effect of further reducing the heat generation of the hydraulic oil is obtained.

In the configuration of the above (3), a dedicated pump for securing the operating pilot pressure of the electromagnetic pilot switching valve is attached. It is not necessary to provide a means for securing (standing) the pilot pressure in the discharge pipe, for example, a check valve (check valve) or the like, so that a large capacity (large flow rate) can be obtained when the bucket is closed or the bucket is opened. The heat generated by the oil in (2) passing through the check valve of the means, that is, the heat generated by the cracking pressure of the check valve is suppressed.

[0028]

FIG. 1 is a front view of an embodiment of an electro-hydraulic grab bucket as a hydraulic grab bucket applied to the present invention. FIG. 2 is a right side view of the electro-hydraulic grab bucket. 3 is a first embodiment of a hydraulic control circuit applied to the electro-hydraulic grab bucket, FIG. 4 is an operation principle diagram of a variable discharge (variable displacement) type hydraulic pump applied to the hydraulic control circuit of FIG. 3, and FIG. 5 is a control characteristic curve showing the relationship between the discharge amount and the discharge pressure of the variable discharge hydraulic pump applied to the present invention, and FIG. 6 is a variable discharge hydraulic pump according to the progress of opening and closing operations of the hydraulic grab bucket of the present invention. FIG. 7 is a graph showing a change state of the required power of the variable discharge type hydraulic pump according to the progress of the opening and closing operation of the hydraulic grab bucket by the hydraulic control circuit of the present invention, and FIG. Constant FIG. 9 is a graph showing how the required power of the gear hydraulic pump changes with the passage of the opening and closing operation of the hydraulic grab bucket by the hydraulic control circuit having a discharge type gear hydraulic pump and a relief valve attached to its discharge pipe. 7
8 is a graph in which the difference in required power of the pump between the present invention and the conventional hydraulic control circuit is visually expressed by superimposing the graphs of FIG. 8 and FIG. 10. FIG. 10 is a second embodiment of the hydraulic control circuit of the present invention. .

First, an overall structure of an electro-hydraulic grab bucket according to an embodiment as a hydraulic grab bucket applied to the present invention will be described with reference to FIGS. The electro-hydraulic grab bucket 50 includes a pair of left and right buckets 52, which are rotatably supported on the upper part of a shaft 51 a supported and fixed to the lower part of the girder 51.
A hydraulic cylinder 15 for opening and closing the bucket, which is pivotally supported at both ends by a pin between the left and right buckets at the upper end portion of the bucket 52 located above the pivot point of the shaft, and which opens and closes; The main part is constituted by a hydraulic pump unit PN installed on the upper part of the hydraulic pump.

1 is an oil tank, 53 is left and right buckets 52
A connection bar is mounted between the two ends and pivotally supported by pins so as to open and close a pair of buckets symmetrically with respect to a vertical center line. 52a is a claw provided at the bottom of the bucket 52, and 54 is a buffer made of an elastic body such as a tire attached to both side surfaces of the bucket 52 and capable of absorbing shock even when the bucket 50 collides with a wall or the like.

Reference numeral 55 denotes a suspension member fixed to the girder 51. A chain is connected to a pin attached to the suspension member 55, and the grab bucket 50 is moved by a crane.
Is suspended or suspended. Reference numeral 57 denotes a cabtire cable for supplying electric power to a motor for driving a variable displacement hydraulic pump, an electromagnetic switching valve, an operation panel, and a control panel incorporated in the hydraulic pump unit PN. Hydraulic pump unit PN can be used to externally handle garbage,
The periphery is hermetically sealed with a panel PNP so that dust and the like do not enter. Reference numeral 56 denotes a protector (cover) for protecting the hydraulic pump unit PN and the like from the surroundings.

With such a configuration, the grab bucket 5
The reference numeral 0 indicates that the pair of left and right buckets 52, 52 are opened and closed by rotating the pair of left and right buckets symmetrically with respect to the vertical center line about the shafts 51a, 51a by operating the hydraulic cylinder 15. In FIG. 1, the fully closed position (state) is indicated by a solid line, and the fully open position (state) is indicated by a two-dot chain line. When the buckets 52, 52 are completely closed, the claws 52a at the tips of the left and right buckets are engaged with each other.

Next, a hydraulic control circuit for opening and closing the bucket 52 applied to the electrohydraulic grab bucket 50 will be described. First, a first embodiment of the hydraulic control circuit will be described with reference to FIG. FIG. 3 shows a hydraulic control circuit corresponding to the conventional example of FIG. In FIG. 3, FIG.
The same or corresponding parts are denoted by the same reference numerals and description thereof will be omitted.

In the hydraulic control circuit of the present embodiment, a variable discharge (also referred to as variable displacement) type hydraulic pump 19 is applied as a hydraulic pump. An electromagnetic pilot switching valve 11 composed of a main switching valve 11A and a pilot switching valve 11B as shown in FIG.
Has been applied. However, in the present embodiment, a check valve (check valve) 18 is interposed in a P (pump port) line of the main switching valve 11A of the electromagnetic pilot switching valve 11 which communicates with the discharge pipe 5 of the hydraulic pump 19. . The check valve 18 secures (establishes) a predetermined pilot pressure required for operating a main spool for switching the main switching valve 11A of the electromagnetic pilot switching valve 11.

A pipe 7 is branched from the discharge pipe 5 of the variable displacement hydraulic pump 19, and the end of the pipe 7 is connected to the P port of the pilot switching valve 11B. Further, a pressure switch 16 as a pressure detector is attached to the discharge pipe 5 at a position upstream of the branch point of the pipe 7. Reference numeral 17 denotes a drain pipe of the hydraulic pump, and reference numeral 21 denotes a line filter interposed in the tank line 19.

Here, a variable displacement type piston pump 19 as an embodiment of a variable discharge type hydraulic pump applied to the hydraulic control circuit of the present invention will be described with reference to FIGS. As shown in the operation principle diagram of FIG. 4, the variable discharge type hydraulic pump 19 includes a pump body 19A and a discharge amount control mechanism 1.
9B. In FIG. 4, the pump body 19A is
Although not shown in the drawings, as is well known, a drive shaft to which the output shaft of the electric motor 4 as a prime mover is connected, and an axis line connected to the distal end of the drive shaft via a universal joint to connect the drive shaft to the drive shaft A center pin, which is obliquely connected to the axis, and a plurality of pistons (also called plungers) held by the center pin and whose ends are connected to the tip of the drive shaft via a universal joint, in a circumferential direction. It is composed of cylinder blocks arranged and included at intervals. As described above, the axis of the cylinder block is arranged obliquely with respect to the axis of the drive shaft, and is also called an oblique axis.

When the drive shaft is driven to rotate by the electric motor 4, each piston slides in the cylinder of the cylinder block and reciprocates to suck and discharge oil. By changing the tilt angle (tilt angle) of the cylinder block, the moving distance (stroke) of the piston changes, and the discharge amount can be increased or decreased. That is, when the cylinder block tilt angle is increased, the stroke of the piston is increased and the discharge amount can be increased. Conversely, when the cylinder block tilt angle is reduced, the discharge amount can be reduced. If the axis of the cylinder block coincides with the axis of the drive shaft, that is, if the tilt angle is zero, the discharge amount can be zero.

The pump body is provided with a minimum tilt adjustment screw and a maximum tilt adjustment screw for adjusting the position of the tilt angle of the cylinder block, and the tilt of the cylinder block is adjusted between the two adjustment screws. By changing the angle, the discharge amount can be changed steplessly between the maximum and minimum ranges.

As shown in FIG. 4, the discharge amount control mechanism 19B is attached to the pump body 19A inside the pump.
Is provided. The discharge amount control mechanism 19B includes a control piston 30, a spring 31, a connecting rod 32, a control bush 40, a pilot piston 41 slidably fitted to the control bush 40, a control spring 42, and communication conduits 35, 36 therebetween. , 37, 38. The control piston 30 is slidably provided inside the pump, a connecting rod (connection rod) 32 is connected to the control piston 30, and an end of the connecting rod 32 is further connected to an end of the cylinder block. The control piston 30 is configured to be able to change the tilt angle of the cylinder block via the connecting rod 32 by displacing and moving.

The spring 31 urges the cylinder block via the connecting rod 32 so as to maintain the cylinder block at the maximum tilt angle (in the direction in which the cylinder block is maintained). That is, when the pump discharge pressure is lower than the set pressure of the pump, the cylinder block
The load is stopped by the maximum tilt angle adjusting screw to maintain the maximum tilt angle. The pump set pressure is set by adjusting the deflection of a control spring 42 that urges a slidably provided pilot piston 41 that is fitted into a control bush 40 inside the pump.

In the variable displacement piston pump 19 having such a configuration, the pressure oil on the pump discharge side acts on the small diameter side 30b of the control piston 30 through the pipe line 35, and the pressure oil on the small diameter side of the control piston 30 further increases. The oil passes through the line 36 and the primary side F of the control bush 40 and the pilot piston 41.
Acts on the head of The secondary side S of the control bush 40 is connected to the large-diameter side 30a of the control piston 30 via a conduit 37, and further connected to the tank via a throttle 38a.

When the pump discharge pressure is equal to or lower than the set pressure, for example, when the bucket 52 is closing or opening, as shown in the principle diagram of FIG. Is shut off, and the pressure oil on the pump discharge side passes through the pipe line 35 and acts only on the small-diameter side 30b of the control piston 30, so that the pump keeps its cylinder block at the maximum tilt angle position and the maximum discharge amount. Will be kept.

On the other hand, when the pump discharge pressure becomes equal to or higher than the set pressure, for example, when the bucket 52 is completely closed or fully opened, the pressure oil which is conducted to the pipe 36 via the pipe 35 is released by the pilot piston 41. The pilot piston 41 overcomes the pressing force of the control spring of FIG.
And the pilot piston 41a moves to the position 41b, and the primary side F and the secondary side S of the control bush 40 move.
Becomes conductive. Thereby, the large diameter side 30 of the control piston 30
The pilot pressure also acts on line a through the pipe 37, and the control piston 30 moves to the force equilibrium position against the urging force of the spring 31. As a result, the tilt angle of the cylinder block decreases, and the discharge amount decreases. Be reduced.

Thus, the variable displacement hydraulic pump 19 composed of the pump body 19A and the discharge amount control mechanism 19B has a control characteristic as shown in FIG. 5, that is, a constant pressure holding control characteristic. ing. That is, the variable displacement hydraulic pump 19 sends the maximum oil amount according to the maximum tilt angle of the cylinder block to the hydraulic circuit [point (a)], and the pump discharge pressure, that is, the operating pressure of the hydraulic cylinder 15 for opening and closing the bucket. When the pressure exceeds the set pressure PS of the pump [point (b)], the cylinder block automatically operates in the direction of decreasing the tilt angle, and the pump discharge pressure (operating pressure) is maintained so as to maintain the set pressure value. The amount Q is reduced and the ejection amount becomes zero [point (c)].

More specifically, in FIG. 5, at point (a), the pump 19 is operated at the maximum discharge amount with the cylinder block at the maximum tilt angle, and the control piston 30 and the cylinder block receive the spring force to reach the maximum. Stopped by the tilt angle adjustment screw. And bucket 5
When the operation load of the hydraulic cylinder 15 rises after 2 starts grasping the load, the pump discharge pressure P rises from the point (a) to the point (b). For example, when the bucket 52 is closed or the load is excessively grasped, Eventually, the pressure reaches the set pressure PS at the point (b). At this point (b), the cylinder block of the pump 19 is stopped at the maximum tilt position. Thereafter, the pump discharge pressure (operating pressure) P slightly increases, but the discharge amount Q is increased to a point ( c), and at the point (c), the pump 3 is operated with the discharge amount Q substantially zero.

The operation of the hydraulic control circuit of the electrohydraulic grab bucket having the structure shown in FIGS. 3, 4 and 5 will be described with reference to the graphs shown in FIGS.
At the start of the operation, the electromagnetic pilot switching valve 11 is in the state shown in FIG. 3. When the electric motor 4 is started, the variable displacement hydraulic pump 19 is driven, and oil flows through the discharge pipe 5. After passing through the check valve 18, the oil flowing through the discharge pipe 5 passes through the center of the main switching valve 11 </ b> A of the electromagnetic pilot switching valve 11, returns to the tank line 10, and is circulated idle.

When the bucket 52 is closed from the idle circulation state to grab garbage as a load, the grab bucket 50 is hung by a crane, positioned above the garbage, and lands on the garbage with the bucket open. At this time, SOL.b of the pilot switching valve 11B of the electromagnetic pilot switching valve 11 is excited. Then, a part of the pressure oil discharged from the hydraulic pump 19 is returned to the check valve 18.
At the pressure of the cracking pressure plus α, flows into the pilot pressure supply pipe 11b through the B port of the pilot switching valve 11B through the pipe 7, and is supplied to the other end side of the main switching valve 11A, and the main switching valve 11A is switched. The pump port P and the B port communicate with each other.

Then, the oil discharged from the hydraulic pump 19 and flowing through the discharge pipe 5 flows from the P port to the B port of the main switching valve 11A, passes through the pressure oil supply pipe 14, and the head side chamber of the hydraulic cylinder 15 for opening and closing the bucket. 15a, the piston rod is extended, the grab bucket is closed, and debris begins to be grabbed. At this time, the hydraulic pump 19
Is operated at a set pressure or less, the cylinder block is maintained at the maximum tilt angle, and the discharge amount Q is the maximum discharge amount at the point (a) of the discharge amount / discharge pressure characteristic curve in FIG. The oil in the rod side chamber 15 b of the hydraulic cylinder 15 is
And flows from the A port of the main switching valve 11A to the tank line 10 via the tank port T and returns to the oil tank 1.

When the bucket 52 starts closing at time t1 in FIG. 6 and begins to grab dust, the discharge pressure of the hydraulic pump 19 increases as shown in FIG. That is, the pump discharge pressure increases from point (a) to point (b) in FIG. Then, at time t2, the bucket 52 grasps the dust and closes, or the bucket 52 grasps the garbage too much,
If it can no longer be closed, the hydraulic cylinder 1
5 increases, and reaches a pressure set by the discharge amount control mechanism 19B of the hydraulic pump 19, that is, a so-called pump set pressure PS (which is also a cylinder block tilting start pressure) [point of the control characteristic (FIG. b)], the discharge oil amount of the hydraulic pump 19 changes from the point (b) in FIG. 5 to (c) while maintaining the pressure, and reaches a minimum of almost zero at the point (c) (this state Dead head condition).

When the pump discharge amount becomes substantially zero at the time t2, the required power of the pump becomes substantially zero as shown in the graph of FIG. This is because the required pump power is calculated by the product of the discharge amount and the discharge pressure.
Thereby, the required power of the electric motor 4 is suppressed and the heat generation of the hydraulic pump 19 is suppressed.

At time t3 when a time T1 has elapsed from time t2 when the dead head state is reached, the excitation of the pilot switching valve 11B of the electromagnetic pilot switching valve 11 is released, the main switching valve 11A is set to the neutral state, and the hydraulic cylinder 15 The pressurization of the head side chamber 15a is stopped, and the oil discharged from the hydraulic pump 19 is circulated idle. At this time, since the ports A and B of the main switching valve 11A are blocked, the hydraulic pressure of the head side chamber 15a and the rod side chamber 15b of the hydraulic cylinder 15 is maintained.

When the idle circulation state is entered, the hydraulic pump 19
The discharge pressure P is a pressure corresponding to a resistance of a pipe such as the discharge pipe 5 or a resistance as low as the cracking pressure of the check valve 18.
The operation is performed with the discharge amount Q being substantially the maximum discharge amount at the point (a) in FIG. In the present embodiment, when the time t2 is reached, the pressure switch 16 serving as a pressure detector detects that the discharge pressure of the hydraulic pump 19 has reached the dead head pressure, and accordingly, a first timer (not shown) measures the time. Is started, and the time T1 is set by the first timer. This time T1 is set to, for example, 1 to 2 seconds.

As described above, in this embodiment, the hydraulic pump 1
After a predetermined time when the discharge pressure of No. 9 reaches the set pressure, the pump discharge oil is circulated through the PT port of the main switching valve 11A in an idle state. The pressurized oil does not act. For this reason, for example, the drain from the internal drain (leakage) of the pump 19 and the drain from the spool of the electromagnetic pilot switching valve 11 are eliminated, and the heat generation accompanying the drain is prevented, and the effect of further reducing the heat generation of the hydraulic oil is obtained. Can be.

It is rare that the above-described grasping of the dust by closing the bucket is completed only by the first grasping operation. In most cases, the bucket is not completely closed due to excessive grasping of the dust, and the bucket is lifted. It cannot be transported. In such a case, the grab bucket 50 is slightly lifted by a crane to drop garbage little by little, for example, at the same time as the start of the idle circulation at the time t3 when the first gripping operation is completed. Adjusts the grip load so that it can be fully closed, and the bucket closing operation, that is,
The pressurization of the head side chamber 15a of the hydraulic cylinder 15 is performed again.

Also, even if the fully closed state can be achieved by the first gripping operation, the crane operator (grab bucket operator) may not be able to completely confirm the distant bucket in some cases. It is necessary to repeatedly pressurize the closing side chamber 15a of the hydraulic cylinder 15 in order to complete the closing of the bucket. Such an operation is also called catching.

That is, at time t4, this catching is started, and the solenoid-operated pilot switching valve 11 is started as described above.
SOL.b of the pilot switching valve 11B is excited, and the head side chamber 1 of the hydraulic cylinder 15 is moved in the direction to close the bucket 52.
5a is pressurized. Almost simultaneously with the start of pressurization (time t4), the hydraulic pump 19 reaches a dead head state. When time T1 has elapsed from time t4, that is, at time t5
The pilot switching valve 11B of the electromagnetic pilot switching valve 11
Is released, the main switching valve 11A is set to the neutral state, the pressurization of the head side chamber 15a of the hydraulic cylinder 15 is stopped, and the discharge oil of the hydraulic pump 19 is circulated idle. Such a catching operation is usually repeated 1 to 5 times.

When the catching operation is completed, the hydraulic circuit is brought into an idle circulation state, the grab bucket 50 is lifted by the crane and transported to a predetermined unloading position, and the time t6 is reached.
, The bucket is opened and unloading is started.

When the bucket is opened for unloading, the pilot switching valve 11 of the electromagnetic pilot switching valve 11
Excite B SOL.a. Then, the hydraulic pump 19
The pressure oil discharged from the tank flows through the discharge pipe 7 into the pilot pressure supply pipe 11a through the A port of the pilot switching valve 11B, is supplied to one end of the main switching valve 11A, and the main switching valve 11 is switched. The pump port P and the A port are communicated.

Then, the hydraulic oil discharged from the hydraulic pump 19 and flowing through the discharge pipe 5 flows from the pump port P of the main switching valve 11A to the A port, passes through the hydraulic oil supply pipe 13, and opens and closes the hydraulic cylinder 15 for opening and closing the bucket. Rod side chamber 15
b, the piston rod is retracted, the grab bucket is opened, and refuse is unloaded. At this time, the oil in the head side chamber 15a of the hydraulic cylinder 15 flows from the B port of the main switching valve 11A to the tank line 10 via the tank port T via the pressure oil supply pipe 14, and is returned to the oil tank 1.

When the bucket opening operation by such an operation reaches time t7, the bucket 52 is fully opened (opened completely), and the discharge pressure P of the hydraulic pump 19 is controlled by the discharge amount control mechanism 19B of the hydraulic pump 19. 5 (point (b) in FIG. 5), the discharge oil amount Q of the hydraulic pump 19 becomes the minimum (dead head state) while maintaining the set pressure [((heading state in FIG. 5)). c) point], the required power of the electric motor 4 is suppressed, and the heat generation from the hydraulic pump 19 is suppressed.

At a time t8, which is a time T2 after the time t7 when the dead head state is reached, the excitation of the pilot switching valve 11B of the electromagnetic pilot switching valve 11 is released, and the main switching valve 11A is returned to the neutral state. The pressurization of the head side chamber 15a is stopped, and the discharge oil of the hydraulic pump 19 is put into an idle circulation state, and the unloading is completed. At this time, since the ports A and B of the main switching valve 11A are blocked, the head side chamber 15a of the hydraulic cylinder 15 is
The hydraulic pressure of the rod side chamber 15b is maintained.

In the present embodiment, the time T2 is, for example, 0.3 to 0.5 seconds and is set by a second timer (not shown). In this embodiment, the discharge pressure of the hydraulic pump 3 is controlled by the pressure switch 16 in the present embodiment, similarly to the first timer for setting the T1 time when the bucket is closed, in the present embodiment. This is performed by detecting that the dead head pressure has been reached.

As described above, in this embodiment, even during the bucket opening operation, the pump discharge oil is circulated through the PT port of the main switching valve 11A after a predetermined time when the discharge pressure of the hydraulic pump 19 reaches the set pressure. Therefore, the pressure oil held at the pump set pressure does not act on the pipeline from the pump discharge port to the main switching valve 11A. For this reason, for example, the drain from the internal drain (leakage) of the pump 19 and the drain from the spool of the electromagnetic pilot switching valve 11 are eliminated, and the heat generation accompanying the drain is prevented, and the effect of further reducing the heat generation of the hydraulic oil is obtained. Can be. Thereafter, the grab bucket 50 is returned to the load gripping position by the crane, and the same gripping operation (opening / closing operation of the bucket) as described above is repeatedly performed.

Thus, as described above, the hydraulic control circuit of the present embodiment employs the variable displacement hydraulic pump 19 provided with the discharge amount control mechanism 19B, and operates in the dead head state in the opening and closing operation of the bucket 52. By doing so, the amount of heat generated by the hydraulic oil when closing and opening is greatly reduced compared to a conventional hydraulic control circuit that employs a fixed (constant discharge capacity) pump and a relief valve as a pressure control valve. Can be done.

Here, the required power of the pump in the system in which the conventional fixed pump and the relief valve are combined is as shown in the graph of FIG. 8, and includes the graph of FIG. 8 and the discharge amount control mechanism 19B of FIG. When the graph of the required power of the hydraulic pump 19 of the present embodiment employing the variable displacement hydraulic pump 19 is superimposed, a graph shown in FIG. 9 is obtained. As is clear from the graph of FIG. 9, in this embodiment, the hydraulic pump 1
The required power of No. 9 can be reduced in the hatched portion compared to the conventional embodiment.

As described above, in the hydraulic control circuit of the present embodiment, when the bucket 52 is closed or opened, the relief operation of the relief valve as in the conventional hydraulic control circuit employing the fixed pump and the relief valve is performed. When the pressure reaches the set pressure PS, the pump discharge amount is made substantially zero (dead head state) while maintaining the predetermined pressure, and the power required for the pump can be made substantially zero. In addition, it is possible to reduce the temperature rise of the drive motor 4 of the hydraulic pump 19 excessively. Therefore, even if the hydraulic pump unit PN has a closed structure, continuous operation of the hydraulic unit becomes possible, and continuous operation of the electrohydraulic grab bucket 50 becomes possible.

Next, a second embodiment of the hydraulic control circuit of the electrohydraulic grab bucket according to the present invention will be described with reference to FIG. 10, the same reference numerals are given to the same or corresponding portions as those of the circuit of FIG. 3 of the first embodiment, and description thereof will be omitted. In this embodiment, the operating pressure for spool switching of the main switching valve 11A of the electromagnetic switching valve 11 (so-called,
To ensure the pilot pressure), the hydraulic pump 20
It comprises a variable displacement hydraulic pump 20a as a main pump for opening and closing the bucket, and a pilot-dedicated small displacement pump 20b mounted coaxially with the hydraulic pump 20a.

The structure of the electromagnetic pilot switching valve 11 is the same as that of the first embodiment shown in FIG. The pipe end of the discharge pipe 7 of the small pilot pump 20 b is connected to the pump port P of the pilot switching valve 11 B of the electromagnetic pilot switching valve 11. The discharge pipe 7 is connected to the tank line 10 by a pipe 7a provided with a check valve 7b. Check valve 7
The b prevents the return oil from the hydraulic cylinder 15 of the main circuit and the backflow of the oil during the idle circulation of the main pump 20a. Other configurations are the same as those of the first embodiment in FIG.

With such a configuration, it is not necessary to provide the check valve 18 for establishing (securing) pilot pressure in the discharge pipe 5 of the main pump as in the circuit of FIG. Main pump 20a as occurs in the circuit
The heat generated from the check valve due to the large discharge flow rate, that is, the heat generated by flowing the oil by overcoming the cracking pressure of the check valve is eliminated, and the rise in the oil temperature can be further suppressed.

The above is the description of the pilot-use small capacity pump 20.
Although b shows the aspect provided coaxially with the main pump 20a, it may be a pump 20b provided with a single electric motor separately from the main pump 20a.

In the first and second embodiments described above, the main switching valve 11A of the electromagnetic pilot switching valve 11 has a neutral port with PT connection, and the dead time of the hydraulic pump 19 (main hydraulic pump 20a) during idle circulation. Although the head operation is not required and the heat generation reducing effect is further measured, in the present invention, the main switching valve 11A of the type in which the pump port P is blocked while the main switching valve 11A of the electromagnetic pilot switching valve 11 is in a neutral state. Alternatively, when the hydraulic pumps 19 and 20a reach the dead head state during the opening and closing operation of the bucket, the dead head state may be maintained without idle circulation. Also in this method, in the dead head state, the discharge amount of the pump is substantially zero, and the required power is also substantially zero. Therefore, the effect of suppressing the oil temperature rise and the effect of reducing the amount of heat generated by the pump driving motor can be obtained.

In the present invention, the variable displacement type hydraulic pump may be a variable displacement type hydraulic pump provided with a control mechanism that provides a horsepower (discharge pressure × discharge flow rate is substantially constant) in a discharge pressure range equal to or lower than a set pressure. That is, as one control method of the variable displacement hydraulic pump, a pump having a constant horsepower and a flow rate characteristic is used, and by adding a sequence valve to this, a normal horsepower constant characteristic is obtained below the set pressure of the sequence valve. However, when the set pressure of the sequence valve is reached, the discharge flow rate can be minimized.

In this case, the bucket opening / closing speed can be adjusted to the load while suppressing the heat generated by the oil and the electric motor, as in the first embodiment. Work efficiency can be improved. That is, for example, when gripping a soft load or a light load, the discharge pressure of the variable displacement hydraulic pump is low because the bucket gripping load is small, and the pump is constant in horsepower, so the discharge amount can be increased accordingly. Therefore, the opening and closing speed of the bucket can be increased, and the cycle time of the load grasping operation can be reduced.

Further, according to the present invention, the variable displacement type hydraulic pump is provided with a variable displacement type having a control mechanism for minimizing the discharge amount by an external signal (electricity, oil pressure, other signals or commands) regardless of the set pressure. It may be a type hydraulic pump. With such a configuration, since the hydraulic pump can be forcibly put into the dead head state by a signal or command from outside the grab bucket, for example, the dead head operation without load is performed instead of the idle circulation operation. It is possible to further save energy and suppress heat generation. In other words, separately from or as an alternative to deadhead control in place of the relief valve, an electromagnetic valve that selects an electromagnetic proportional valve or a sequence valve is controlled by an external electric signal, and the hydraulic pump is dead even at a low pressure during idle. Heating of the hydraulic circuit can be further reduced by, for example, setting the head state.

In the present invention, the variable displacement hydraulic pump may be a variable displacement hydraulic pump provided with a control mechanism having stepwise discharge pressure and discharge amount characteristics. For example, a two-pressure, two-flow control mode is adopted, and fast-forwarding at low load is possible, that is, the speed at the time of opening the bucket can be increased, and the working efficiency can be improved.

In the above embodiment, the case where the hydraulic pump unit PN is hermetically enclosed by the panel PNP has been described. However, the present invention is applicable even if the hydraulic pump unit PN is open to the atmosphere. Things.

In the above embodiment, the case where the hydraulic grab bucket is an electro-hydraulic grab bucket in which the prime mover for driving the hydraulic pump is an electric motor has been described.
The present invention can be applied to an engine hydraulic grab bucket in which the prime mover is an engine such as a diesel engine or a gasoline engine.

Further, in the above embodiment, the bucket type shown in FIGS. 1 and 2 has been described. However, the present invention is not limited to this, and may be applied to a hydraulic grab bucket having any type of bucket. Can be.

In the above embodiment, the load (grab)
However, the present invention is not limited to this, and the present invention can be applied to various other objects such as combustion ash, powdered material, combustion ash, sludge, and the like.

[0080]

As is clear from the above description, the present invention has the following advantages because it is configured as described in the claims.

According to the first aspect of the present invention, the load on the grab bucket increases, for example, when the grab bucket grips the load and closes or when the grab bucket releases the load, and the load on the grab bucket increases. When the pressure exceeds the set pressure, the pump maintains the pressure at that point and the discharge amount becomes the minimum, almost zero. Therefore, the so-called dead head operation is performed, so that the required power of the pump is reduced to a minimum of almost zero. Thus, there is no case where the relief operation of the pressure oil is performed as in a conventional circuit having a pressure control valve such as a relief valve. As a result, the rise in oil temperature can be greatly reduced, and when the prime mover for driving the pump is an electric motor, the excessive rise in temperature and power consumption can be greatly suppressed. Further, when the prime mover is an engine such as a diesel engine, the fuel consumption can be suppressed.

Therefore, even if the hydraulic pump unit, that is, the hydraulic control circuit has a closed structure, or if the hydraulic grab bucket frequently uses the catching operation. Thus, continuous operation of the hydraulic control circuit becomes possible and continuous operation of the hydraulic grab bucket becomes possible.

According to the second aspect of the invention, when the discharge pressure of the pump reaches the set pressure of the pump, the electromagnetic switching valve is switched to the neutral state after a predetermined time has elapsed, and the pump discharge oil is circulated idlely with the oil tank. Therefore, the pressure oil held at the pump set pressure does not act on the pipeline from the pump discharge port to the electromagnetic switching valve. For this reason, for example, the drain from the drain port of the pump, the drain from the spool of the electromagnetic switching valve, etc. can be eliminated, the heat generation accompanying the drain can be prevented, and the heat generation of the hydraulic oil can be further reduced. Can be planned.

According to the third aspect of the present invention, a dedicated pump for securing the operating pilot pressure of the electromagnetic pilot switching valve is mounted. There is no need to provide a means such as a check valve (check valve) for securing the pilot pressure in the pipe, and a large amount of oil passes through the check valve when the bucket is closed or the bucket is opened. As a result, heat generation due to cracking pressure of the check valve or the like can be suppressed, and a rise in oil temperature can be further suppressed.

[Brief description of the drawings]

FIG. 1 is a front view of an embodiment of an electrohydraulic grab bucket as a hydraulic grab bucket applied to the present invention.

FIG. 2 is a right side view of the electrohydraulic grab bucket.

FIG. 3 is a first embodiment of a hydraulic control circuit applied to the electrohydraulic grab bucket.

4 is an operation principle diagram of a variable discharge type hydraulic pump of the hydraulic control circuit of FIG. 3;

FIG. 5 is a control characteristic curve showing a relationship between a discharge amount and a discharge pressure of a variable discharge amount type hydraulic pump applied to the present invention.

FIG. 6 is a graph showing a change state of the discharge pressure of the variable discharge type hydraulic pump according to the progress of the opening and closing operation of the hydraulic grab bucket of the present invention.

FIG. 7 is a graph showing a change in required power of the variable discharge hydraulic pump according to the progress of opening and closing operations of the hydraulic grab bucket by the hydraulic control circuit of the present invention.

FIG. 8 is a diagram showing a change in required power of the constant discharge type hydraulic pump due to the progress of the opening and closing operation of the hydraulic grab bucket by the conventional constant discharge amount type hydraulic pump and a hydraulic control circuit having a relief valve attached to its discharge pipe. FIG.

9 is a graph in which the graphs of FIGS. 7 and 8 are superimposed to visually represent the difference in required power between the pumps of the present invention and the conventional hydraulic control circuit.

FIG. 10 is a second embodiment of the hydraulic control circuit of the present invention.

FIG. 11 is a conventional hydraulic control circuit applied to an electro-hydraulic grab bucket.

[Explanation of symbols]

Reference Signs List 1 oil tank 3 constant discharge type (fixed) hydraulic pump 4 electric motor 5 pump oil discharge pipe 7 small capacity pump oil discharge pipe 10 tank line 11 electromagnetic pilot switching valve 11A main switching valve 11B pilot switching valve 15 hydraulic cylinder 7b, 18 reverse Stop valve (check valve) 19 Variable discharge rate (variable capacity) type hydraulic pump 19A Pump body 19B Discharge rate control mechanism 20a Variable discharge rate (variable capacity) type hydraulic pump (main pump) 20b Pilot dedicated small capacity pump PN hydraulic pump unit 50 electrohydraulic grab bucket 51 girder 52 bucket

Claims (3)

[Claims] 1. A hydraulic cylinder for opening and closing a bucket, a hydraulic pump for driving the hydraulic cylinder, and a hydraulic cylinder connected to a discharge pipe of the hydraulic pump for switching and supplying pressure oil to a head side chamber and a rod side chamber of the hydraulic cylinder. In the hydraulic control circuit of a hydraulic grab bucket provided with an electromagnetic switching valve for switching the operation direction of the hydraulic pump, when the hydraulic pump discharge pressure is increased to a set pressure, the discharge amount of the hydraulic pump is minimized, and the hydraulic cylinder is operated. A hydraulic control circuit for a hydraulic grab bucket, comprising a variable discharge type hydraulic pump provided with a discharge amount control mechanism for stopping the discharge amount to a minimum required for pressurizing. 2. A pressure detector attached to a discharge pipe of a hydraulic pump, and the pressure detector detects that the discharge pressure of the hydraulic pump has reached a set pressure of the hydraulic pump, starts timing, and starts a predetermined time. A timer for setting the solenoid-operated switching valve to a neutral state later, wherein the solenoid-operated switching valve is set to a neutral state to idle-circulate hydraulic pump discharge oil between the tank and the tank. Hydraulic grab bucket hydraulic control circuit. 3. The electromagnetic switching valve is an electromagnetic pilot switching valve including a main switching valve and a pilot switching valve, and a dedicated pump for securing an operating pilot pressure of the electromagnetic pilot switching valve is coaxial with or separate from the hydraulic pump. 3. The hydraulic control circuit for a hydraulic grab bucket according to claim 1, wherein the hydraulic control circuit is mounted on a hydraulic grab bucket.