HYDRAULIC DRIVE CIRCLE

Abstract

To reduce manufacturing costs, a bridge oil channel 30, connecting a first bridge port 23c and a second bridge port 23g, and a connecting oil channel 31, connecting the bridge oil channel 30 and a middle bridge port 23a, are provided. When a slide valve 22 is in a first position, pump oil supply ports 23b and 23f and the middle bridge port 23a are connected, the first bridge port 23c is blocked, and the second bridge port 23g and a second actuator port 23h are connected. When the slide valve 22 is in a second position, the pump oil supply ports 23b and 23f and the middle bridge port 23a are connected, the second bridge port 23g is blocked, and the first bridge port 23c and a first actuator port 23d are connected. A pressure relief valve 33 is provided on the bridge oil channel 30.

Description

TECHNICAL AREA

[0001] The present invention relates to a hydraulic drive circuit which performs a supply control of oil from a hydraulic pump to a hydraulic actuator by actuating a slide valve. STATE OF THE ART

[0002] For example, in a hydraulic drive circuit that controls the supply of oil to a hydraulic cylinder, a first actuator port and a second actuator port are provided in a spool orifice of a reversing valve. The first actuator port is connected to a bottom chamber of the hydraulic cylinder, and the second actuator port is connected to a rod chamber. The spool is actuated from this position. In this case, the first and second actuator ports are selectively connected to a hydraulic pump, and the hydraulic cylinder moves back and forth between an extended and a retracted position. In this type of hydraulic drive circuit, a pressure relief valve is inserted into an oil channel that supplies oil from the spool to the hydraulic cylinder.This prevents the supply of oil to the hydraulic cylinder at a pressure equal to or higher than a set pressure (see, for example, patent literature 1). LITERATURE LIST Patent literature

[0003] Patent Literature 1: JP 2003-202003 A BRIEF DESCRIPTION OF THE INVENTION Technical Problem

[0004] The pressure relief valve described above must function both when oil is supplied to the bottom chamber and when oil is supplied to the rod chamber. In other words, in the hydraulic drive circuit described above, providing a pressure relief valve in each oil channel supplying oil from the slide to the hydraulic cylinder prevents situations where excessive hydraulic pressure is applied to both the bottom chamber and the rod chamber. However, prior art designs require a large number of pressure relief valves, inevitably increasing manufacturing costs. This problem is not limited to hydraulic actuators in the form of hydraulic cylinders but can similarly occur with other hydraulic actuators, such as hydraulic motors.

[0005] In view of the circumstances described above, one object of the present invention is to provide a hydraulic drive circuit in which the manufacturing costs can be reduced. Solution to the problem

[0006] To solve the aforementioned problem, a hydraulic drive circuit according to the present invention is a hydraulic drive circuit that performs a supply control of oil from a hydraulic pump to a hydraulic actuator by opening a spool of a reversing valve arranged between the hydraulic pump and the hydraulic actuator. The reversing valve is provided with a pump oil supply port connected to the hydraulic pump, a first actuator port and a second actuator port connected to the hydraulic actuator, a first bridge port, a second bridge port and a third bridge port, a bridge oil channel connecting the first bridge port and the second bridge port, and a connecting oil channel connecting the bridge oil channel and the third bridge port.When the slide valve is in the first position, the pump oil supply port and the third bridge port are connected, the first bridge port is blocked, and the second bridge port and the second actuator port are connected. When the slide valve is in the second position, the pump oil supply port and the third bridge port are connected, the second bridge port is blocked, and the first bridge port and the first actuator port are connected. A pressure relief valve is provided at any one of the bridge oil channels and the connecting oil channels. Advantageous effects of the invention

[0007] According to the present invention, the oil flows through a connecting oil channel and a bridge oil channel, either when oil is supplied from the first actuator port or when oil is supplied from the second actuator port. Therefore, if only one pressure relief valve is provided in the connecting oil channel or the bridge oil channel, pressure relief can be achieved when the hydraulic pressure is equal to or higher than a set pressure, even when oil is supplied from either of the two actuator ports. This prevents the application of excessive pressure to a hydraulic actuator while simultaneously reducing manufacturing costs. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a circuit diagram of a state in which a slide is in a neutral position in a hydraulic drive circuit, which forms a first embodiment of the present invention. Fig. 2 is a cross-sectional view showing a structure of the in Fig. 1 illustrated hydraulic drive circuit. Fig. 3 is a circuit diagram of a state in which the slider is in a first position in which Fig. 1 illustrated hydraulic drive circuit is located. Fig. 4 is a cross-sectional view showing a structure of the in Fig. 3 illustrated hydraulic drive circuits. Fig. 5 is a circuit diagram of a state in which the slider is in a second position in which Fig. 1 illustrated hydraulic drive circuit is located. Fig. 6 is a cross-sectional view showing a structure of the in Fig. 5 illustrated hydraulic drive circuits. Fig. Figure 7 is a circuit diagram of a state in which a slide is in a neutral position in a hydraulic drive circuit, which forms a second embodiment of the present invention. Fig. 8 is a circuit diagram of a state in which the slider is in a first position in which Fig. 7 illustrated hydraulic drive circuit is located. Fig. 9 is a circuit diagram of a state in which the slider is in a second position in which Fig. 7 illustrated hydraulic drive circuit is located. DESCRIPTION OF EXECUTION FORMS

[0008] Preferred embodiments of a hydraulic drive circuit according to the present invention are described in detail below with reference to the accompanying drawings. First embodiment

[0009] Fig. 1 and Fig. Figure 2 illustrates a hydraulic drive circuit that constitutes a first embodiment of the present invention. The hydraulic drive circuit illustrated herein by way of example drives a hydraulic cylinder (hydraulic actuator) 10 using oil supplied from a hydraulic pump 1. The hydraulic pump 1 is a variable displacement pump in which a displacement volume is varied by changing the tilt angle of a swashplate or a swashplate shaft. The hydraulic cylinder 10 is of the double-acting type in which an actuating rod 12 moves back and forth with respect to a cylinder body 11 by selectively supplying oil to a rod chamber 11a and a bottom chamber 11b. The hydraulic cylinder 10 of this type can be used as at least one of a boom hydraulic cylinder, an arm hydraulic cylinder, or a bucket hydraulic cylinder, which are provided, for example, on a working machine.

[0010] A hydraulic valve assembly is provided between the hydraulic pump 1 and the hydraulic cylinder 10. The hydraulic valve assembly includes a reversing valve 20, which selectively supplies oil from the hydraulic pump 1 to the rod chamber 11a and the bottom chamber 11b of the hydraulic cylinder 10, and a valve body 21 equipped with a spool 22. The spool 22 has a cylindrical shape with a plurality of web sections separated by annular grooves and is arranged in a spool bore 23 of the valve body 21 so that it is movable in an axial direction.

[0011] The slide hole 23 of the valve main body 21, which receives the slide 22, is connected to a central bridge connection (third bridge connection) 23a at a substantially central part in the axial direction. Fig. 2. In the valve hole 23, a first pump oil supply connection 23b, a first bridge connection 23c, a first actuator connection 23d and a first tank connection 23e are successively provided in one part which is located in Fig. 2 is located on the left side of the middle bridge connection 23a. In the valve hole 23, a second pump oil supply connection 23f, a second bridge connection 23g, a second actuator connection 23h and a second tank connection 23i are provided successively in a section that is located in Fig. 2 is located on the right side of the middle bridge connection 23a.

[0012] When the slide 22 moves in an axial direction, a connection state is switched between the first pump oil supply port 23b and the second pump oil supply port 23f and the middle bridge port 23a, and a connection state is switched between the first bridge port 23c and the second bridge port 23g and the first actuator port 23d and the second actuator port 23h.

[0013] More precisely, the middle bridge port 23a is connected to the second tank port 23i when the slide 22 is in a neutral position. Meanwhile, the first pump oil supply port 23b, the second pump oil supply port 23f, the first bridge port 23c, the second bridge port 23g, the first actuator port 23d, and the second actuator port 23h are all in a blocked state. When the slide 22 moves to the right from the neutral position... Fig. When 1 is moved, the middle bridge connection 23a and the first pump oil supply connection 23b are connected to each other, and the second bridge connection 23g and the second actuator connection 23h are connected to each other, while the first actuator connection 23d is in a state of being connected to the first tank connection 23e, as in Fig. 3 and Fig. Figure 4 illustrates this. The first bridge connection 23c and the second tank connection 23i are all held in a blocked state (a first position of the slide 22). When the slide 22 moves from the neutral position to the left Fig. When 1 is moved, the middle bridge connection 23a and the second pump oil supply connection 23f are connected to each other, and the first bridge connection 23c and the first actuator connection 23d are connected to each other, while the second actuator connection 23h is in a state of being connected to the second tank connection 23i, as shown in Fig. 5 and Fig. Figure 6 illustrates that the second bridge connection 23g and the first tank connection 23e are all held in a blocked state (a second position of the slide 22).

[0014] The second pump oil supply port 23f of the valve hole 23 is connected to the hydraulic pump 1 via an oil supply channel 24. The oil supply channel 24 is equipped with a main relief valve 24a. The first actuator port 23d of the valve hole 23 is connected to the bottom chamber 11b of the hydraulic cylinder 10 via a bottom oil channel 25, and the second actuator port 23h is connected to the rod chamber 11a of the hydraulic cylinder 10 via a rod oil channel 26. Suction safety valves 25a and 26a are provided in the central section of the bottom oil channel 25 and in the central section of the rod oil channel 26, respectively. The suction safety valves 25a and 26a connect the oil channels 25 and 26 to a tank T only when the respective pressures of the oil channels 25 and 26 exceed specified values.If, on the other hand, the respective pressures of the oil channels 25 and 26 are negative pressures, the suction safety valves 25a and 26a allow oil to pass from tank T to the bottom oil channel 25 and to the rod oil channel 26. The first tank connection 23e and the second tank connection 23i of the valve hole 23 are each connected to tank T via an oil drain channel 27.

[0015] The first pump oil supply port 23b and the second pump oil supply port 23f are connected to each other by a pump supply oil channel 28, which is provided on the valve main body 21. The pump supply oil channel 28 extends from the first pump oil supply port 23b to the outer circumferential side along the radial direction of the slide 22, is then curved and extends in a direction that approaches the slide 22 along the radial direction of the slide 22. This connects the pump oil supply channel 28 to the second pump oil supply port 23f.

[0016] A bridge oil channel 30 provided on the valve main body 21 connects the first bridge port 23c and the second bridge port 23g. The bridge oil channel 30 extends from the first bridge port 23c to the outer circumferential side along the radial direction of the slide 22, and is then in Fig. 2 curved to the right and extends along the axial direction of the slide 22. Furthermore, the extending end section extends in a direction that approaches the slide 22 along the radial direction of the slide 22.

[0017] This connects the bridge oil channel 30 to the second bridge port 23g. In the illustrated example, the pump supply oil channel 28 and the bridge oil channel 30 are located at positions 180 degrees apart on the circumference of the slide 22. A connecting oil channel 31 is provided between the bridge oil channel 30 and the middle bridge port 23a. The connecting oil channel 31 extends from the middle bridge port 23a to the outer circumferential side along the radial direction of the slide 22. This connects the extending edge section to the central section of the bridge oil channel 30.

[0018] The connecting oil channel 31 is provided with a pressure equalization valve 32, and the bridge oil channel 30 is provided with a pressure relief valve 33. The pressure equalization valve 32 is configured to increase its opening degree when the pressure on the side of the bridge oil channel 30 is higher than the pressure on the side of the middle bridge port 23a. The pressure relief valve 33 opens when the pressure of the bridge oil channel 30 exceeds a preset relief pressure (< a set pressure of the main relief valve 24a) and discharges oil into the tank T. In the first embodiment, the pressure relief valve 33 is of an electromagnetic type, which is capable of changing a relief pressure according to a specific control signal.

[0019] The slide 22 is provided with a neutral spring 40 at one end section and with pressure chambers 41a and 41b at both end sections. The neutral spring 40 holds the slide 22 in the neutral position and is housed within a spring housing 42. The pressure chambers 41a and 41b accommodate the end sections of the slide 22 and are filled with oil. Although not clearly illustrated in the drawing, pilot oil channels for applying a pilot pressure output from an actuating valve are individually connected to the respective pressure chambers 41a and 41b. When a pilot pressure output in connection with the actuating valve is applied to the pressure chambers 41a and 41b via the pilot oil channels, the slide 22 can be moved axially against the spring force of the neutral spring 40.By removing a pilot pressure applied to the pressure chambers 41a and 41b, the slide 22 returns to the neutral position due to the spring force of the neutral spring 40.

[0020] When the valve 22 is in the neutral position in the hydraulic drive circuit configured as described above, the first pump oil supply port 23b, the second pump oil supply port 23f, the first bridge port 23c, the second bridge port 23g, the first actuator port 23d, and the second actuator port 23h are all in a blocked state, as described above. Therefore, no oil flows to the rod chamber 11a and the bottom chamber 11b of the hydraulic cylinder 10, and the actuating rod 12 maintains its current position relative to the cylinder main body 11.

[0021] If the slide 22 moves from this state to the right by actuating the actuating valve Fig. 1 and Fig. When 2 is moved, oil supplied to the oil supply channel 24 from the hydraulic pump 1 is supplied via the second pump oil supply port 23f, the middle bridge port 23a, the connecting oil channel 31, the pressure equalization valve 32, the bridge oil channel 30, the second bridge port 23g, the second actuator port 23h and the rod oil channel 26 of the rod chamber 11a of the hydraulic cylinder 10, as shown in Fig. 3 and Fig. Figure 4 illustrates this. Simultaneously, oil in the bottom chamber 11b is discharged into tank T via the bottom oil channel 25, the first actuator connection 23d, the first tank connection 23e, and the oil drain channel 27. Therefore, in the hydraulic cylinder 10, the actuating rod 12 is retracted relative to the cylinder main body 11.

[0022] If, on the other hand, the slide 22 moves to the left in the drawing by actuating the actuating valve, oil supplied to the oil supply channel 24 from the hydraulic pump 1 is fed via the second pump oil supply port 23f, the middle bridge port 23a, the connecting oil channel 31, the pressure equalization valve 32, the bridge oil channel 30, the first bridge port 23c, the first actuator port 23d and the bottom oil channel 25 of the bottom chamber 11b of the hydraulic cylinder 10, as shown in Fig. 5 and Fig. Figure 6 illustrates this. Simultaneously, oil in the rod chamber 11a is discharged into tank T via the rod oil channel 26, the second actuator connection 23h, the second tank connection 23i, and the oil drain channel 27. Therefore, in the hydraulic cylinder 10, the actuating rod 12 is extended relative to the cylinder main body 11.

[0023] During such operation, according to the hydraulic drive circuit, oil flows through the connecting oil channel 31 and the bridge oil channel 30 either when oil is supplied to the bottom chamber 11b of the hydraulic cylinder 10 or when oil is supplied to the rod chamber 11a. Therefore, although only the pressure relief valve 33 is provided for the bridge oil channel 30, pressure relief can also be achieved when oil is supplied to either the bottom chamber 11b or the rod chamber 11a if the oil pressure is equal to or higher than the set pressure. This prevents excessive pressure from being applied to the hydraulic cylinder 10 while simultaneously reducing manufacturing costs.

[0024] It should be noted that in the first embodiment described above, the bridge oil channel 30 is provided with the pressure relief valve 33. However, the present invention is not limited to this. The pressure relief valve 33 can be provided on a section between the pressure equalization valve 32 and the bridge oil channel 30 in the connecting oil channel 31. Second embodiment

[0025] Fig. Figure 7 illustrates a hydraulic drive circuit representing a second embodiment of the present invention. Similar to the first embodiment, the hydraulic drive circuit illustrated here by way of example drives the hydraulic cylinder (hydraulic actuator) 10 by means of oil supplied from the hydraulic pump 1, and differs only in the configuration of the hydraulic valve device. Configurations that differ from those of the first embodiment are described below, and other common configurations are identified by the same reference numerals.

[0026] In the hydraulic valve device of the second embodiment, a spool hole of a valve body, which receives a spool 52 of a directional changeover valve 50, is provided with a first pump oil supply port 53a and a second pump oil supply port 53b, a first actuator port 53c and a second actuator port 53d, a first bridge port 53e and a second bridge port 53f, as well as a first tank port 53g and a second tank port 53h. When the spool 52 moves in the axial direction, a connection state is switched between the first pump oil supply port 53a and second pump oil supply port 53b and the first bridge port 53e, second bridge port 53f, first actuator port 53c and second actuator port 53d.

[0027] More precisely, the first pump oil supply port 53a, the second pump oil supply port 53b, the first bridge port 53e, the second bridge port 53f, the first actuator port 53c, and the second actuator port 53d are all in a blocked state when the slide 52 is in the neutral position. When the slide 52 moves from the neutral position to the right Fig. 7 moves, the first pump oil supply port 53a and the first bridge port 53e are connected to each other, and the second bridge port 53f and the second actuator port 53d are connected to each other, while the first actuator port 53c is in a state of being connected to the first tank port 53g, as in Fig. Figure 8 illustrates this. When the slide 52 moves from the neutral position to the left in the drawing, the second pump oil supply port 53b and the second bridge port 53f are connected together, and the first bridge port 53e and the first actuator port 53c are connected together, while the second actuator port 53d is in a state of being connected to the second tank port 53h.

[0028] The second pump oil supply port 53b of the valve hole is connected to the hydraulic pump 1 via an oil supply channel 54. The oil supply channel 54 is equipped with a main relief valve 54a. The first actuator port 53c of the valve hole is connected to the bottom chamber 11b of the hydraulic cylinder 10 via a bottom oil channel 55, and the second actuator port 53d is connected to the rod chamber 11a of the hydraulic cylinder 10 via a rod oil channel 56. Suction safety valves 55a and 56a are provided in the central section of the bottom oil channel 55 and in the central section of the rod oil channel 56, respectively. The first tank port 53g and the second tank port 53h of the valve hole are each connected to the tank T via an oil drain channel 57. The first pump oil supply port 53a and the second pump oil supply port 53b are connected to each other by a pump supply oil channel 58 which is provided on the main valve body.

[0029] The first bridge port 53e and the second bridge port 53f are connected to each other by a bridge oil channel 60, which is provided on the main valve body. The bridge oil channel 60 is equipped with a pressure relief valve 61. The pressure relief valve 61 opens when the pressure of the bridge oil channel 60 exceeds a preset relief pressure (< a set pressure of the main relief valve 54a) and releases oil into the tank T. In the second embodiment, the pressure relief valve 61 is of an electromagnetic type, which is capable of changing a relief pressure according to a specific control signal.

[0030] When the valve 52 is in the neutral position in the hydraulic drive circuit configured as described above, the first pump oil supply port 53a, the second pump oil supply port 53b, the first bridge port 53e, the second bridge port 53f, the first actuator port 53c, and the second actuator port 53d are all in a blocked state, as described above. Therefore, no oil flows to the rod chamber 11a and the bottom chamber 11b of the hydraulic cylinder 10, and the actuating rod 12 maintains its current position relative to the cylinder body 11.

[0031] If the slide 52 moves from this state to the right by actuating the actuating valve Fig. When 7 is moved, oil supplied to the oil supply channel 54 from the hydraulic pump 1 is fed via the first pump oil supply port 53a, the first bridge port 53e, the bridge oil channel 60, the second bridge port 53f, the second actuator port 53d and the rod oil channel 56 of the rod chamber 11a of the hydraulic cylinder 10, as shown in Fig. Figure 8 illustrates this. Simultaneously, oil in the bottom chamber 11b is discharged into tank T via the bottom oil channel 55, the first actuator connection 53c, the first tank connection 53g, and the oil drain channel 57. Therefore, in the hydraulic cylinder 10, the actuating rod 12 is retracted relative to the cylinder main body 11.

[0032] If, on the other hand, the slide 52 moves to the left by actuating the actuating valve Fig. When 7 is moved, oil supplied to the oil supply channel 54 from the hydraulic pump 1 is supplied via the second pump oil supply port 53b, the second bridge port 53f, the bridge oil channel 60, the first bridge port 53e, the first actuator port 53c and the bottom oil channel 55 of the bottom chamber 11b of the hydraulic cylinder 10, as shown in Fig. Figure 9 illustrates this. Simultaneously, oil in the rod chamber 11a is discharged into tank T via the rod oil channel 56, the second actuator connection 53d, the second tank connection 53h, and the oil drain channel 57. Therefore, in the hydraulic cylinder 10, the actuating rod 12 is extended relative to the cylinder main body 11.

[0033] During such operation, according to the hydraulic drive circuit, oil passes through the bridge oil channel 60 either when oil is supplied to the bottom chamber 11b of the hydraulic cylinder 10 or when oil is supplied to the rod chamber 11a. Thus, although only the pressure relief valve 61 is provided for the bridge oil passage 60, pressure relief can be achieved even when oil is supplied to either the bottom chamber 11b or the rod chamber 11a, provided the oil pressure is equal to or higher than the set pressure. This prevents excessive pressure from being applied to the hydraulic cylinder 10, while simultaneously reducing manufacturing costs.

[0034] It should be noted that in both embodiments described above, the hydraulic cylinder 10 is illustrated as the hydraulic actuator by way of example; however, the invention can also be applied to embodiments in which the oil supply is controlled with respect to other hydraulic actuators, such as a hydraulic motor. Furthermore, an electromagnetic pressure relief valve is illustrated by way of example as pressure relief valves 33 and 61; however, the present invention is not limited to these. Reference symbol list 1 hydraulic pump 10 hydraulic excavators 20, 50 Directional changeover valve 22, 52 sliders 23a middle bridge connection (third bridge connection) 23b, 53a first pump oil supply connection 23c, 53e first bridge connection 23d, 53c first actuator connection 23e, 53g first tank connection 23f, 53b second pump oil supply connection 23g, 53f second bridge connection 23h, 53d second actuator connection 23i, 53h second tank connection 28, 58 Pump supply oil channel 30, 60 Bridge Oil Canal 31 Connecting oil channel 32 Pressure equalization valve 33, 61 Pressure relief valve T Tank QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] JP 2003-202003 A

[0003]

Claims

[1] Hydraulic drive circuit which performs a supply control of oil from a hydraulic pump with respect to a hydraulic actuator by opening a slide of a reversing valve arranged between the hydraulic pump and the hydraulic actuator, wherein the reversing valve is provided with: a pump oil supply connection that is connected to the hydraulic pump; a first actuator connection and a second actuator connection, which are connected to the hydraulic actuator; a first bridge connection, a second bridge connection and a third bridge connection; a bridge oil channel connecting the first bridge approach and the second bridge approach; and a connecting oil channel that connects the bridge oil channel and the third bridge connection, wherein, when the slide is in a first position, the pump oil supply port and the third bridge port are connected, the first bridge port is blocked, and the second bridge port and the second actuator port are connected, when the slide is in a second position, the pump oil supply port and the third bridge port are connected, the second bridge port is blocked, and the first bridge port and the first actuator port are connected, and A pressure relief valve is provided at any one of the bridge oil channels and the connecting oil channel. [2] Hydraulic drive circuit according to claim 1, wherein the connecting oil channel is provided with a pressure equalization valve which controls a supply flow rate of oil to the hydraulic actuator according to a load pressure of the hydraulic actuator. [3] Hydraulic drive circuit according to claim 1, wherein the directional switching valve is provided with the third bridge connection on a central part in an axial direction, The pump oil supply connection, the first bridge connection, the first actuator connection and a tank connection connected to a tank are provided one after the other from the third bridge connection to an end section, The pump oil supply port, the second bridge port, the second actuator port and a tank port connected to a tank are provided sequentially from the third bridge port to the other end section, and the pump oil supply port provided on one end section side with respect to the third bridge port and the pump oil supply port provided on the other end section side with respect to the third bridge port are each connected to each other by a pump connection oil channel. [4] Hydraulic drive circuit which performs a supply control of oil from a hydraulic pump with respect to a hydraulic actuator by opening a slide of a reversing valve arranged between the hydraulic pump and the hydraulic actuator, wherein the reversing valve is provided with: a pump oil supply connection that is connected to the hydraulic pump; a first actuator connection and a second actuator connection, which are connected to the hydraulic actuator; a first bridge connection and a second bridge connection; and a bridge oil channel connecting the first bridge abutment and the second bridge abutment, wherein, when the slide is in a first position, the pump oil supply port and the first bridge port are connected to each other, and the second bridge port and the second actuator port are connected to each other, when the slide is in a second position, the pump oil supply port and the second bridge port are connected to each other, and the first bridge port and the first actuator port are connected to each other, and A pressure relief valve is provided on the bridge oil channel.

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