Hydraulic system of construction machine

A hydraulic system includes: a slewing motor; a mechanical brake; and a slewing control valve interposed between a main pump and the slewing motor. A first pilot port of the slewing control valve is connected to a first solenoid proportional valve by a pilot line. A second pilot port of the slewing control valve is connected to a second solenoid proportional valve by a second pilot line. The first solenoid proportional valve and the second solenoid proportional valve are connected to an auxiliary pump by a primary pressure line. A switching valve is interposed between the auxiliary pump and the mechanical brake. The switching valve includes a pilot port that is connected to the first pilot line by a switching pilot line. The valve switches from a closed to an open position when a pilot pressure led to the pilot port becomes higher than or equal to a setting value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is national stage application of International Application No. PCT/JP2020/029482, filed on Jul. 31, 2020, which designates the United States, and claims the benefit of Japanese Patent Application No. 2019-152661, filed on Aug. 23, 2019, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a hydraulic system of a construction machine.

BACKGROUND ART

In construction machines such as hydraulic excavators and hydraulic cranes, the components thereof are driven by a hydraulic system. The hydraulic system includes, for example, a slewing motor and a boom cylinder as hydraulic actuators. The slewing motor slews a slewing unit, and the boom cylinder luffs a boom provided on the slewing unit. These hydraulic actuators are supplied with hydraulic oil from a main pump via control valves.

Generally speaking, each control valve includes: a spool disposed in a housing; and a pair of pilot ports for moving the spool. In a case where an operation device that outputs an electrical signal is used as an operation device to move the control valve, solenoid proportional valves are connected to the respective pilot ports of the control valve, and the control valve is driven by the solenoid proportional valves.

The slewing motor may be provided with a mechanical brake (in the case of a self-propelled construction machine, the mechanical brake may be called a “parking brake”) to prevent the slewing unit from slewing, for example, when the construction machine is parked on a slope (see Patent Literature 1, for example). When supplied with pressurized oil, the mechanical brake is switched from a brake-applied state, in which the mechanical brake prevents the rotation of the output shaft of the slewing motor, to a brake-released state, in which the mechanical brake allows the rotation of the output shaft. The mechanical brake is supplied with the pressurized oil from an auxiliary pump via a solenoid switching valve.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, the above-described configuration requires not only the solenoid valves for driving the control valve (i.e., the solenoid proportional valves), but also the solenoid valve dedicated for the mechanical brake (i.e., the solenoid switching valve).

In view of the above, an object of the present invention is to provide a hydraulic system of a construction machine, the hydraulic system making it possible to reduce the number of solenoid valves.

Solution to Problem

In order to solve the above-described problems, a hydraulic system of a construction machine according to the present invention includes: a slewing motor; a mechanical brake that is, when supplied with pressurized oil, switched from a brake-applied state, in which the mechanical brake prevents rotation of an output shaft of the slewing motor, to a brake-released state, in which the mechanical brake allows the rotation of the output shaft; a slewing control valve interposed between a main pump and the slewing motor, the slewing control valve including a first pilot port for a first slewing operation and a second pilot port for a second slewing operation; a first solenoid proportional valve connected to the first pilot port by a first pilot line; a second solenoid proportional valve connected to the second pilot port by a second pilot line; an auxiliary pump connected to the first solenoid proportional valve and the second solenoid proportional valve by a primary pressure line; and a switching valve interposed between the auxiliary pump and the mechanical brake, the switching valve including a pilot port that is connected to the first pilot line by a switching pilot line, the switching valve switching from a closed position to an open position when a pilot pressure led to the pilot port becomes higher than or equal to a setting value.

According to the above configuration, the pilot port of the switching valve for the mechanical brake is connected to the first pilot line between the first solenoid proportional valve and the slewing control valve. Therefore, when the first solenoid proportional valve outputs a secondary pressure higher than or equal to the setting value of the switching valve, the switching valve switches to an open state, and braking by the mechanical brake is released. That is, a pilot-type switching valve can be used as a switching valve for the mechanical brake, and the switching valve can be operated by utilizing the first solenoid proportional valve, which is intended for driving the slewing control valve. This makes it possible to reduce the number of solenoid valves.

Advantageous Effects of Invention

The present invention provides a hydraulic system of a construction machine, the hydraulic system making it possible to reduce the number of solenoid valves.

DESCRIPTION OF EMBODIMENTS

FIG.1shows a hydraulic system1A of a construction machine according to Embodiment 1 of the present invention.FIG.2shows a construction machine10, in which the hydraulic system1A is installed. Although the construction machine10shown inFIG.2is a hydraulic excavator, the present invention is applicable to other construction machines, such as a hydraulic crane.

The construction machine10shown inFIG.2is a self-propelled construction machine, and includes a traveling unit11. The construction machine10further includes: a slewing unit12slewably supported by the traveling unit11; and a boom that is luffed relative to the slewing unit12. An arm is swingably coupled to the distal end of the boom, and a bucket is swingably coupled to the distal end of the arm. The slewing unit12is equipped with a cabin16including an operator's seat. In the present embodiment, the traveling unit11includes crawlers as traveling means. Alternatively, the traveling means of the traveling unit11may be wheels. The construction machine10need not be of a self-propelled type.

The hydraulic system1A includes, as hydraulic actuators20, a boom cylinder13, an arm cylinder14, and a bucket cylinder15, which are shown inFIG.2, a slewing motor81shown inFIG.1, and an unshown pair of travel motors (a left travel motor and a right travel motor). The boom cylinder13luffs the boom. The arm cylinder14swings the arm. The bucket cylinder15swings the bucket. The slewing motor81slews the slewing unit12. The left travel motor rotates the left crawler of the traveling unit11, and the right travel motor rotates the right crawler of the traveling unit11.

As shown inFIG.1, the hydraulic system1A further includes a main pump22, which supplies hydraulic oil to the aforementioned hydraulic actuators20. InFIG.1, the hydraulic actuators20other than the slewing motor81are not shown for the purpose of simplifying the drawing.

The main pump22is driven by an engine21. Alternatively, the main pump22may be driven by an electric motor. The engine21also drives an auxiliary pump23. The number of main pumps22may be more than one.

The main pump22is a variable displacement pump (a swash plate pump or a bent axis pump) whose tilting angle is changeable. The delivery flow rate of the main pump22may be controlled by electrical positive control, or may be controlled by hydraulic negative control. Alternatively, the delivery flow rate of the main pump22may be controlled by load-sensing control.

Control valves4are interposed between the main pump22and the hydraulic actuators20. In the present embodiment, all the control valves4are three-position valves. Alternatively, one or more of the control valves4may be two-position valves.

All the control valves4are connected to the main pump22by a supply line31, and connected to a tank by a tank line33. Each control valve4is connected to a corresponding one of the hydraulic actuators20by a pair of supply/discharge lines. In a case where the number of main pumps22is more than one, the same number of groups of the control valves4as the number of main pumps22are formed. In each group, the control valves4are connected to the corresponding main pump22by the supply line31.

For example, the control valves4include: a boom control valve that controls supply and discharge of the hydraulic oil to and from the boom cylinder13; an arm control valve that controls supply and discharge of the hydraulic oil to and from the arm cylinder14; and a bucket control valve that controls supply and discharge of the hydraulic oil to and from the bucket cylinder15. The control valves4also include a slewing control valve4t, which controls supply and discharge of the hydraulic oil to and from the slewing motor81.

The aforementioned supply line31includes a main passage and branch passages. The main passage extends from the main pump22. The branch passages are branched off from the main passage, and connect to the control valves4. In the present embodiment, a center bypass line32is branched off from the main passage of the supply line31, and the center bypass line32extends to the tank. The control valves4are disposed on the center bypass line32. The center bypass line32may be eliminated.

A relief line34is branched off from the main passage of the supply line31, and the relief line34is provided with a relief valve35for the main pump22. The relief line34may be branched off from the center bypass line32at a position upstream of all the control valves4.

Each control valve4includes: a spool disposed in a housing; and a pair of pilot ports for moving the spool. For example, the housings of all the control valves4may be integrated together to form a multi-control valve unit. The pilot ports of each control valve4are connected to respective solenoid proportional valves6by respective pilot lines5.

Each solenoid proportional valve6is a direct proportional valve outputting a secondary pressure that indicates a positive correlation with a command current. Alternatively, each solenoid proportional valve6may be an inverse proportional valve outputting a secondary pressure that indicates a negative correlation with the command current.

All the solenoid proportional valves6are connected to the auxiliary pump23by a primary pressure line41. The primary pressure line41includes a main passage and branch passages. The main passage extends from the auxiliary pump23. The branch passages are branched off from the main passage, and connect to the solenoid proportional valves6. A relief line42is branched off from the main passage of the primary pressure line41, and the relief line42is provided with a relief valve43for the auxiliary pump23.

Operation devices7to move the control valves4are disposed in the aforementioned cabin16. Each operation device7includes an operating unit (an operating lever or a foot pedal) that receives an operation for moving a corresponding one of the hydraulic actuators20, and outputs an electrical signal corresponding to an operating amount of the operating unit (e.g., an inclination angle of the operating lever).

Specifically, the operation devices7include: a boom operation device7a, an arm operation device7b, a bucket operation device7c, and a slewing operation device7d, each of which includes an operating lever; and a left travel operation device7eand a right travel operation device7f, each of which includes a foot pedal. Some of the operation devices7may be combined together and may share the same operating lever. For example, the boom operation device7aand the bucket operation device7cmay be combined together, and the arm operation device7band the slewing operation device7dmay be combined together.

The operating lever of the boom operation device7areceives a boom raising operation and a boom lowering operation. The operating lever of the arm operation device7breceives an arm crowding operation and an arm pushing operation. The operating lever of the bucket operation device7creceives a bucket excavating operation and a bucket dumping operation. The operating lever of the slewing operation device7dreceives a first slewing operation and a second slewing operation. Each of the foot pedal of the left travel operation device7eand the foot pedal of the right travel operation device7freceives a forward travel operation and a backward travel operation.

One of the first and second slewing operations is a left slewing operation, and the other is a right slewing operation. The left slewing operation may be either the first slewing operation or the second slewing operation. When the operating lever of the slewing operation device7dreceives the first slewing operation (i.e., when the operating lever is inclined in a first slewing direction), the slewing operation device7doutputs a first slewing electrical signal whose magnitude corresponds to the operating amount of the operating lever (i.e., the inclination angle of the operating lever). When the operating lever of the slewing operation device7dreceives the second slewing operation (i.e., when the operating lever is inclined in a second slewing direction), the slewing operation device7doutputs a second slewing electrical signal whose magnitude corresponds to the operating amount of the operating lever (i.e., the inclination angle of the operating lever).

The electrical signal outputted from each operation device7is inputted to a controller70. The controller70controls solenoid proportional valves6based on the electrical signals outputted from the operation devices7.FIG.1shows only part of signal lines for simplifying the drawing. For example, the controller70is a computer including memories such as a ROM and RAM, a storage such as a HDD, and a CPU. The CPU executes a program stored in the ROM or HDD.

Next, the slewing control valve4tinterposed between the main pump22and the slewing motor81is described in more detail.

The slewing control valve4tincludes a first pilot port for the first slewing operation and a second pilot port for the second slewing operation. The first pilot port is connected to a first solenoid proportional valve6a(one of the aforementioned solenoid proportional valves6) by a first pilot line5a(one of the aforementioned pilot lines5), and the second pilot port is connected to a second solenoid proportional valve6b(one of the aforementioned solenoid proportional valves6) by a second pilot line5b(one of the aforementioned pilot lines5).

When the first slewing electrical signal is outputted from the slewing operation device7d, the controller70feeds a command current to the first solenoid proportional valve6a, and increases the command current in accordance with increase in the first slewing electrical signal. Similarly, when the second slewing electrical signal is outputted from the slewing operation device7d, the controller70feeds a command current to the second solenoid proportional valve6b, and increases the command current in accordance with increase in the second slewing electrical signal.

The slewing control valve4tis connected to the slewing motor81by a pair of supply/discharge lines91and92. The supply/discharge lines91and92are connected to each other by a bridging passage93. The bridging passage93is provided with a pair of relief valves94, which are directed opposite to each other. A portion of the bridging passage93between the relief valves94is connected to the tank by a make-up line97. Each of the supply/discharge lines91and92is connected to the make-up line97by a corresponding one of bypass lines95. Alternatively, the pair of bypass lines95may be provided on the bridging passage93in a manner to bypass the pair of relief valves94, respectively. The bypass lines95are provided with check valves96, respectively.

The slewing motor81is provided with a mechanical brake83to prevent the slewing unit12from slewing when the construction machine is parked, for example, on a slope. The mechanical brake83has a structure in which a spring thereof blocks an output shaft82of the slewing motor81from rotating. To release the blocking by the spring, hydraulic pressure is used. Specifically, when supplied with pressurized oil, the mechanical brake83is switched from a brake-applied state, in which the mechanical brake83prevents the rotation of the output shaft82of the slewing motor81, to a brake-released state, in which the mechanical brake83allows the rotation of the output shaft82. A drain line84extends from the mechanical brake83to the tank through the slewing motor81.

The mechanical brake83is connected to a switching valve52by a supply/discharge line53. The switching valve52is connected to the auxiliary pump23by a pump line51, and to the tank by a tank line54. The upstream portion of the pump line51and the upstream portion of the primary pressure line41merge together to form a shared passage.

The switching valve52interposed between the auxiliary pump23and the mechanical brake83includes a pilot port, and when a pilot pressure led to the pilot port becomes higher than or equal to a setting value α, the switching valve52switches from a closed position, which is a neutral position, to an open position. When the switching valve52is in the closed position, the switching valve52blocks the pump line51, and brings the supply/discharge line53into communication with the tank line54. When the switching valve52is in the open position, the switching valve52brings the pump line51into communication with the supply/discharge line53. The pilot port of the switching valve52is connected to the first pilot line5aby a switching pilot line61.

Next, with reference toFIGS.3to5, the control of the first solenoid proportional valve6aand the second solenoid proportional valve6bby the controller70is described in detail. InFIGS.3to5, the first pilot port side of the slewing control valve4tis referred to as “A side” and the second pilot port side of the slewing control valve4tis referred to as “B side.”

As shown inFIG.3, in a case where the pilot pressure at one of the first and second pilot ports is zero, when the pilot pressure at the other one of the first and second pilot ports becomes a predetermined value β, the slewing control valve4tstarts opening (i.e., one of or both supply/discharge passages start communicating with a pump passage). The predetermined value β is greater than the setting value α of the switching valve52.

When the first slewing operation is performed (i.e., while the first slewing electrical signal is being outputted from the slewing operation device7d), the controller70feeds no command current to the second solenoid proportional valve6b, but feeds a command current whose magnitude corresponds to the first slewing electrical signal to the first solenoid proportional valve6aas previously described. However, as indicated by solid line inFIG.4, the controller70controls the first solenoid proportional valve6a, such that the first solenoid proportional valve6aoutputs a secondary pressure higher than or equal to the setting value α of the switching valve52. To be more specific, at the start of the slewing operation, the controller70feeds a command current to the first solenoid proportional valve6a, such that the secondary pressure from the first solenoid proportional valve6aincreases to the predetermined value β (i.e., the pilot pressure when the slewing control valve4tstarts opening). Consequently, the switching valve52switches to the open state, and the braking by the mechanical brake83is released.

On the other hand, when the second slewing operation is performed (i.e., while the second slewing electrical signal is being outputted from the slewing operation device7d), the controller70feeds a command current to the first solenoid proportional valve6a, such that the secondary pressure from the first solenoid proportional valve6abecomes a predetermined value ε as indicated by two-dot chain line inFIG.4, and feeds a command current whose magnitude corresponds to the second slewing electrical signal to the second solenoid proportional valve6bas previously described. The predetermined value c is greater than or equal to the setting value α of the switching valve52, and is less than the aforementioned predetermined value β.

Since the pressure at the first pilot port of the slewing control valve4tis the predetermined value c, the slewing control valve4tdoes not open until the pressure at the second pilot port becomes a predetermined value γ (=β+ε). Accordingly, at the start of the slewing operation, the controller70feeds a command current to the second solenoid proportional valve6b, such that the secondary pressure from the second solenoid proportional valve6bincreases to the predetermined value γ. Consequently, the switching valve52switches to the open state, and the braking by the mechanical brake83is released.

As described above, both when the first slewing operation is performed and when the second slewing operation is performed, the controller70controls the first solenoid proportional valve6a, such that the first solenoid proportional valve6aoutputs a secondary pressure higher than or equal to the setting value α of the switching valve52.

Further, in the present embodiment, also when a boom operation, an arm operation, or a bucket operation (hereinafter, each of these operations is referred to as a “work-related operation”) is performed, the controller70controls the first solenoid proportional valve6a, such that the first solenoid proportional valve6aoutputs a secondary pressure higher than or equal to the setting value α of the switching valve52. Whether or not a boom operation is being performed is determined based on whether or not the boom operation device7ais outputting a boom electrical signal. Whether or not an arm operation is being performed is determined based on whether or not the arm operation device7bis outputting an arm electrical signal. Whether or not a bucket operation is being performed is determined based on whether or not the bucket operation device7cis outputting a bucket electrical signal.

To be more specific, as shown inFIG.5, at the start of the work-related operation, the controller70feeds a command current to the first solenoid proportional valve6a, such that the secondary pressure from the first solenoid proportional valve6aincreases to the predetermined value ε. Consequently, the switching valve52switches to the open state, and the braking by the mechanical brake83is released. The secondary pressure from the first solenoid proportional valve6ais kept to the predetermined value c while the work-related operation is being performed, and becomes zero when the work-related operation is ended.

Therefore, when the first slewing operation is performed while the work-related operation is being performed, as indicated by solid line inFIG.5, at the start of the slewing operation, the secondary pressure from the first solenoid proportional valve6aincreases from the predetermined value ε to the predetermined value β. On the other hand, when the second slewing operation is performed while the work-related operation is being performed, the second solenoid proportional valve6bis controlled in the same manner as in the case shown inFIG.4where the second slewing operation is performed alone.

As described above, in the hydraulic system1A of the present embodiment, the pilot port of the switching valve52for the mechanical brake83is connected to the first pilot line5abetween the first solenoid proportional valve6aand the slewing control valve4t. Therefore, when the first solenoid proportional valve6aoutputs a secondary pressure higher than or equal to the setting value α of the switching valve52, the switching valve52switches to the open state, and the braking by the mechanical brake83is released. That is, the pilot-type switching valve52can be used as a switching valve for the mechanical brake83, and the switching valve52can be operated by utilizing the first solenoid proportional valve6a, which is intended for driving the slewing control valve4t. This makes it possible to reduce the number of solenoid valves.

Next, with reference toFIG.6andFIG.7, a hydraulic system according to Embodiment 2 of the present invention is described. The hydraulic system of the present embodiment is different from the hydraulic system of Embodiment 1 only in terms of the control of the first solenoid proportional valve6aand the second solenoid proportional valve6b. That is, the configuration of the hydraulic system of the present embodiment is as shown inFIG.1.

In the present embodiment, both when the first slewing operation is performed and when the second slewing operation is performed, the controller70controls the first solenoid proportional valve6aand the second solenoid proportional valve6b, such that each of the first solenoid proportional valve6aand the second solenoid proportional valve6boutputs a secondary pressure higher than or equal to the setting value α of the switching valve52.

To be more specific, when the first slewing operation is performed (i.e., while the first slewing electrical signal is being outputted from the slewing operation device7d), the controller70feeds a command current to the second solenoid proportional valve6b, such that the secondary pressure from the second solenoid proportional valve6bbecomes the predetermined value ε as indicated by solid line inFIG.6, and feeds a command current whose magnitude corresponds to the first slewing electrical signal to the first solenoid proportional valve6aas previously described. As described in Embodiment 1, the predetermined value ε is greater than or equal to the setting value α of the switching valve52. In the present embodiment, the predetermined value c need not be less than the aforementioned predetermined value β (the predetermined value β is, in a case where the pilot pressure at one of the first and second pilot ports is zero, the pilot pressure at the other one of the first and second pilot ports when the slewing control valve4tstarts opening). However, desirably, the predetermined value ε is less than the predetermined value β.

Since the pressure at the second pilot port of the slewing control valve4tis the predetermined value c, the slewing control valve4tdoes not open until the pressure at the first pilot port becomes the predetermined value γ (=β+ε). Accordingly, at the start of the slewing operation, the controller70feeds a command current to the first solenoid proportional valve6a, such that the secondary pressure from the first solenoid proportional valve6aincreases to the predetermined value γ. Consequently, the switching valve52switches to the open state, and the braking by the mechanical brake83is released.

When the second slewing operation is performed (i.e., while the second slewing electrical signal is being outputted from the slewing operation device7d), the control of the first solenoid proportional valve6aand the second solenoid proportional valve6bis performed in the same manner as the control described in Embodiment 1 as indicated by two-dot chain line inFIG.6.

Further, in the present embodiment, when a boom operation, an arm operation, or a bucket operation is performed (i.e., when a work-related operation is performed), the controller70controls the first solenoid proportional valve6aand the second solenoid proportional valve6b, such that each of the first solenoid proportional valve6aand the second solenoid proportional valve6boutputs a secondary pressure higher than or equal to the setting value α of the switching valve52.

To be more specific, as shown inFIG.7, at the start of the work-related operation, the controller70feeds a command current to the first solenoid proportional valve6a, such that the secondary pressure from the first solenoid proportional valve6aincreases to the predetermined value c, and feeds a command current to the second solenoid proportional valve6b, such that the secondary pressure from the second solenoid proportional valve6bincreases to the predetermined value ε. Consequently, the switching valve52switches to the open state, and the braking by the mechanical brake83is released. Each of the secondary pressure from the first solenoid proportional valve6aand the secondary pressure from the second solenoid proportional valve6bis kept to the predetermined value c while the work-related operation is being performed, and becomes zero when the work-related operation is ended.

Therefore, when the first slewing operation is performed while the work-related operation is being performed, as indicated by solid line inFIG.7, at the start of the slewing operation, the secondary pressure from the first solenoid proportional valve6aincreases from the predetermined value ε to the predetermined value γ. On the other hand, when the second slewing operation is performed while the work-related operation is being performed, as indicated by two-dot chain line inFIG.7, at the start of the slewing operation, the secondary pressure from the second solenoid proportional valve6bincreases from the predetermined value ε to the predetermined value γ.

The secondary pressure from the second solenoid proportional valve6bwhen the first slewing operation is performed may be zero as in Embodiment 1. In this case, however, the pressure difference between the pilot pressure for switching the switching valve52(i.e., the predetermined value ε inFIG.4) and the pilot pressure when the slewing control valve starts opening (i.e., the predetermined value β inFIG.4) is small. Therefore, it is desirable to take malfunction preventative measures, such as strengthening a return spring in the slewing control valve4t. In this respect, when the first slewing operation is performed, if the second solenoid proportional valve6boutputs a secondary pressure higher than or equal to the setting value α of the switching valve52as in the present embodiment, the pressure difference between the pilot pressure for switching the switching valve52(i.e., the predetermined value ε inFIG.6) and the pilot pressure when the slewing control valve4tstarts opening (i.e., the predetermined value γ inFIG.6) becomes great. Therefore, taking malfunction preventative measures is unnecessary.

Other Embodiments

The present invention is not limited to the above-described embodiments. Various modifications can be made without departing from the scope of the present invention.

For example, when a work-related operation is performed, the controller70need not feed a command current to the first solenoid proportional valve6a. However, when the work-related operation is performed, if the secondary pressure from the first solenoid proportional valve6ais higher than or equal to the setting value α of the switching valve52as in Embodiment 1 and Embodiment 2, the mechanical brake83is switched to the brake-released state not only when a slewing operation is performed, but also when a boom operation is performed, when an arm operation is performed, and when a bucket operation is performed. For this reason, during a boom operation, an arm operation, or a bucket operation being performed, when force that causes the slewing unit12to slew is exerted, for example, from the ground, the mechanical brake83does not receive the force. Consequently, a situation where excessive force is applied to the mechanical brake83and thereby the mechanical brake83gets damaged is prevented. That is, the torque capacity of the mechanical brake83can be set to a torque capacity dedicated for stationary braking. Therefore, the mechanical brake83can be made compact.

Alternatively, as in a hydraulic system1B shown inFIG.8, the pilot port of the switching valve52may be connected, by the switching pilot line61, not only to the first pilot line5a, but also to the second pilot line5b. In the example shown inFIG.8, the switching pilot line61includes: a high pressure selective valve64; a pair of input lines62and63, which connects a pair of input ports of the high pressure selective valve64to the first pilot line5aand the second pilot line5b, respectively; and an output line65, which connects an output port of the high pressure selective valve64to the pilot port of the switching valve52. In other words, the switching pilot line61leads a higher one of the secondary pressure from the first solenoid proportional valve6aor the secondary pressure from the second solenoid proportional valve6bto the pilot port of the switching valve52. According to this configuration, even if the first solenoid proportional valve6afails, the mechanical brake83can be switched to the brake-released state by utilizing the second solenoid proportional valve6b.

As shown inFIG.8, the switching valve52may be connected to the drain line84of the mechanical brake83by the tank line54.

Further, in the configuration shown inFIG.8, similar to Embodiment 1 and Embodiment 2, both when the first slewing operation is performed and when the second slewing operation is performed, the first solenoid proportional valve6amay output a secondary pressure higher than or equal to the setting value α of the switching valve52. However, by performing the control described below, the control of the first solenoid proportional valve6aand the second solenoid proportional valve6bafter the braking by the mechanical brake83is released can be simplified.

For example, as shown inFIG.9, in a case where the first slewing operation is performed, after the start of the slewing operation, the secondary pressure from the second solenoid proportional valve6bmay be brought to zero, whereas in a case where the second slewing operation is performed, conversely toFIG.9, after the start of the slewing operation, the secondary pressure from the first solenoid proportional valve6amay be brought to zero. In this manner, after the start of the slewing operation, normal control of controlling only one of the first and second solenoid proportional valves6aand6bthat corresponds to the slewing operation being performed may be performed.

SUMMARY

As described above, the hydraulic system of the construction machine according to the present invention includes: a slewing motor; a mechanical brake that is, when supplied with pressurized oil, switched from a brake-applied state, in which the mechanical brake prevents rotation of an output shaft of the slewing motor, to a brake-released state, in which the mechanical brake allows the rotation of the output shaft; a slewing control valve interposed between a main pump and the slewing motor, the slewing control valve including a first pilot port for a first slewing operation and a second pilot port for a second slewing operation; a first solenoid proportional valve connected to the first pilot port by a first pilot line; a second solenoid proportional valve connected to the second pilot port by a second pilot line; an auxiliary pump connected to the first solenoid proportional valve and the second solenoid proportional valve by a primary pressure line; and a switching valve interposed between the auxiliary pump and the mechanical brake, the switching valve including a pilot port that is connected to the first pilot line by a switching pilot line, the switching valve switching from a closed position to an open position when a pilot pressure led to the pilot port becomes higher than or equal to a setting value.

According to the above configuration, the pilot port of the switching valve for the mechanical brake is connected to the first pilot line between the first solenoid proportional valve and the slewing control valve. Therefore, when the first solenoid proportional valve outputs a secondary pressure higher than or equal to the setting value of the switching valve, the switching valve switches to an open state, and braking by the mechanical brake is released. That is, a pilot-type switching valve can be used as a switching valve for the mechanical brake, and the switching valve can be operated by utilizing the first solenoid proportional valve, which is intended for driving the slewing control valve. This makes it possible to reduce the number of solenoid valves.

For example, the above hydraulic system may further include: a slewing operation device that outputs a first slewing electrical signal when receiving the first slewing operation, the first slewing electrical signal corresponding to an operating amount of the first slewing operation, and outputs a second slewing electrical signal when receiving the second slewing operation, the second slewing electrical signal corresponding to an operating amount of the second slewing operation; and a controller that controls the first solenoid proportional valve and the second solenoid proportional valve based on the first slewing electrical signal and the second slewing electrical signal. Both when the first slewing operation is performed and when the second slewing operation is performed, the controller may control the first solenoid proportional valve, such that the first solenoid proportional valve outputs a secondary pressure higher than or equal to the setting value.

Both when the first slewing operation is performed and when the second slewing operation is performed, the controller may control the first solenoid proportional valve and the second solenoid proportional valve, such that each of the first solenoid proportional valve and the second solenoid proportional valve outputs a secondary pressure higher than or equal to the setting value. The secondary pressure from the second solenoid proportional valve when the first slewing operation is performed may be zero. In this case, however, the pressure difference between the pilot pressure for switching the switching valve and the pilot pressure when the slewing control valve starts opening is small. Therefore, it is desirable to take malfunction preventative measures, such as strengthening a return spring in the slewing control valve. In this respect, when the first slewing operation is performed, if the second solenoid proportional valve outputs a secondary pressure higher than or equal to the setting value of the switching valve, the pressure difference between the pilot pressure for switching the switching valve and the pilot pressure when the slewing control valve starts opening becomes great. Therefore, taking malfunction preventative measures is unnecessary.

In a case where the first solenoid proportional valve outputs a secondary pressure higher than or equal to the setting value both when the first slewing operation is performed and when the second slewing operation is performed, the construction machine may be a self-propelled hydraulic excavator, and when a boom operation, an arm operation, or a bucket operation is performed, the controller may control the first solenoid proportional valve, such that the first solenoid proportional valve outputs a secondary pressure higher than or equal to the setting value.

Alternatively, in a case where each of the first solenoid proportional valve and the second solenoid proportional valve outputs a secondary pressure higher than or equal to the setting value both when the first slewing operation is performed and when the second slewing operation is performed, the construction machine may be a self-propelled hydraulic excavator, and when a boom operation, an arm operation, or a bucket operation is performed, the controller may control the first solenoid proportional valve and the second solenoid proportional valve, such that each of the first solenoid proportional valve and the second solenoid proportional valve outputs a secondary pressure higher than or equal to the setting value.

According to the above configurations, the mechanical brake is switched to the brake-released state not only when a slewing operation is performed, but also when a boom operation is performed, when an arm operation is performed, and when a bucket operation is performed. For this reason, during a boom operation, an arm operation, or a bucket operation being performed, when force that causes the slewing unit to slew is exerted, for example, from the ground, the mechanical brake does not receive the force. Consequently, a situation where excessive force is applied to the mechanical brake and thereby the mechanical brake gets damaged is prevented. That is, the torque capacity of the mechanical brake can be set to a torque capacity dedicated for stationary braking. Therefore, the mechanical brake can be made compact.

The pilot port of the switching valve may be connected to the first pilot line and the second pilot line by the switching pilot line. The switching pilot line may lead a higher one of a secondary pressure from the first solenoid proportional valve or a secondary pressure from the second solenoid proportional valve to the pilot port of the switching valve. According to this configuration, even if the first solenoid proportional valve fails, the mechanical brake can be switched to the brake-released state by utilizing the second solenoid proportional valve.