Patent Description:
In recent years, in construction machines such as hydraulic excavators and wheel loaders, energy saving has been an important development item. For energy saving of a construction machine, energy saving of a hydraulic system itself is important, and it has been examined to apply a hydraulic closed circuit system in which a hydraulic actuator is connected in a closed circuit manner to and directly controlled by a hydraulic pump. This system does not suffer from pressure loss by a control valve and does not suffer from flow rate loss either because the pump delivers hydraulic fluid only of a required flow rate. Also, it is possible for the system to regenerate positional energy of the actuator and energy upon deceleration. Therefore, energy saving is possible.

As a background art of a construction machine that includes a hydraulic closed circuit combined therein, a configuration is described in Patent Document <NUM>. In the configuration, a plurality of variable displacement hydraulic pumps are branched and connected to a plurality of hydraulic actuators via a solenoid switching valve so as to configure a closed circuit, to thereby make it possible to achieve a combined operation and a high-speed operation of the actuators.

In the hydraulic circuit disclosed in Patent Document <NUM>, at the time of no-operation (the engine is operating) of the construction machine or at the time of traveling, it is necessary to control the pump delivery volume of the hydraulic pumps for a closed circuit to zero such that the flow rate is not delivered. This is because, at the time of no-operation or traveling, the solenoid switching valve that connects the hydraulic pumps for a closed circuit and the actuators to each other is in a closed state and, if the flow rate should be delivered, then the delivery pressure rises to a relief pressure. Consequently, the pumps are acted upon by a high load, and the reliability degrades. Further, since also the load to the engine increases, the energy saving performance of the construction machine deteriorates.

As the hydraulic pump for a closed circuit, a variable displacement swash plate hydraulic pump is commonly used, and in order to control the pump delivery volume to zero, it is necessary to control the tilting angle of the swash plate to zero. In the state in which the tilting angle is zero, the piston in the pump is little displaced with respect to the cylinder and is pressed against a cylinder wall surface by the centrifugal force of the piston itself. Therefore, if this state continues, then there is the possibility that an oil film at a sliding portion may be broken to cause abrasion of or damage to the piston or the cylinder, resulting in decrease of the reliability. Especially in such a hydraulic pump for a closed circuit of a large delivery volume as is used in a large construction machine, since the piston is heavy and besides long term reliability is also demanded, the foregoing is a big problem.

The present invention has been made in view of the problem described above, and it is an object of the present invention to provide a construction machine configured such that a hydraulic actuator is driven by a hydraulic pump for a closed circuit, by which oil film breakage of the hydraulic pump for a closed circuit at the time of no-operation or traveling is prevented to improve the reliability and a high operation rate can be obtained.

In order to attain the object described above, the present invention provides a construction machine comprising a closed circuit pump consisting of a bidirectionally tiltable hydraulic pump having two suction/delivery ports, a hydraulic actuator connected in a closed circuit manner to the closed circuit pump, a charge pump, a charge line connected to a delivery port of the charge pump, a check valve that is provided in a hydraulic line connecting the charge line and the closed circuit pump to each other and permits hydraulic operating fluid to flow from the charge line into the closed circuit pump, a charge relief valve provided in the charge line, an operation lever for instructing an operation of the hydraulic actuator, and a controller that controls a delivery volume of the closed circuit pump according to an input from the operation lever. The construction machine comprises a switching valve that is provided in a hydraulic line connecting one of delivery ports of the closed circuit pump and the charge line to each other and is opened and closed according to a control signal from the controller. The controller is configured to, in a case where a state in which the delivery volume of the closed circuit pump is kept to zero continues for a predetermined time period or more, open the switching valve and keep the delivery volume of the closed circuit pump to a predetermined delivery volume or more, the predetermined delivery volume being greater than zero.

According to the present invention configured in such a manner as described above, in a case where a state in which a tilting amount of the closed circuit pump is zero continues for a predetermined time period or more, keeping the tilting amount of the closed circuit pump to a predetermined tilting amount that is equal to or greater than zero causes a piston in the closed circuit pump to be displaced with respect to a cylinder. Therefore, oil is introduced to a sliding portion between the piston and the cylinder to thereby assure an oil film, and consequently, abrasion of the piston or the cylinder can be prevented. Further, by establishing communication of the delivery port of the closed circuit pump with the charge line via the switching valve, it is possible to suppress a delivery pressure of the closed circuit pump to a low level equal to or lower than a charge pressure. Consequently, it is possible to prevent deterioration in fuel consumption and improve durability of the closed circuit pump.

According to the present invention, provided is a construction machine configured such that a hydraulic actuator is driven by a hydraulic pump for a closed circuit, by which oil film breakage of the hydraulic pump for a closed circuit at the time of no-operation or traveling is prevented to improve the reliability and a high operation rate can be obtained.

In the following, a construction machine according to an embodiment of the present invention is described with reference to the drawings, taking a hydraulic excavator as an example. It is to be noted that, in the figures, equivalent members are denoted by the same reference character, and overlapping description of them is suitably omitted.

<FIG> is a side elevational view of a hydraulic excavator according to the present embodiment.

Referring to <FIG>, the hydraulic excavator <NUM> includes a lower track structure <NUM> having crawler type track devices <NUM> on the opposite left and right sides thereof, and an upper swing structure <NUM> mounted swingably on the lower track structure <NUM> through a swing device <NUM>. The swing device <NUM> is driven by a swinging hydraulic motor (not depicted).

To the front side of the upper swing structure <NUM>, a front implement <NUM> for performing excavation work and so forth is mounted. The front implement <NUM> includes a boom <NUM> coupled pivotably in the upward and downward direction to the front side of the upper swing structure <NUM>, an arm <NUM> coupled pivotably in the upward and downward direction and in the forward and rearward direction to a distal end portion of the boom <NUM>, and a bucket <NUM> coupled pivotably in the upward and downward direction and in the forward and rearward direction to a distal end portion of the arm <NUM>. The boom <NUM>, the arm <NUM>, and the bucket <NUM> are driven by a boom cylinder <NUM>, an arm cylinder <NUM>, and a bucket cylinder <NUM>, respectively, which are single rod type hydraulic cylinders.

A cab <NUM> in which an operator is to board is provided on the upper swing structure <NUM>. In the cab <NUM>, an operation lever 56a (depicted in <FIG>) for issuing an instruction for operation of the arm <NUM> and the upper swing structure <NUM>, an operation lever 56d (depicted in <FIG>) for issuing an instruction for operation of the boom <NUM> and the bucket <NUM>, and so forth are arranged.

<FIG> is a hydraulic circuit diagram of the hydraulic excavator <NUM>. It is to be noted that, in <FIG>, only elements related to driving of the boom cylinder <NUM> and the arm cylinder <NUM> are depicted while elements related to driving of the other actuators are omitted.

Referring to <FIG>, an engine <NUM> that is a power source is connected to a power transmission device <NUM> that distributes power. To the power transmission device <NUM>, a charge pump <NUM> formed from a fixed displacement hydraulic pump, closed circuit pumps <NUM> and <NUM> each formed from a bidirectionally tiltable variable displacement hydraulic pump, and open circuit pumps <NUM> and <NUM> each formed from a unidirectionally tiltable variable displacement hydraulic pump are connected.

The charge pump <NUM> is connected at a suction port thereof to a tank <NUM> and at a delivery port thereof to a charge line <NUM>. The charge line <NUM> is connected to the tank <NUM> via a charge relief valve <NUM>. The charge relief valve <NUM> holds a delivery pressure of the charge pump <NUM> (pressure of the charge line <NUM>) to a substantially fixed low pressure.

The closed circuit pump <NUM> is connected at one of suction/delivery ports thereof to a bottom side hydraulic chamber 1a of the boom cylinder <NUM> via a switching valve 43a and through a bottom side hydraulic line 91a, and at the other of the suction/delivery ports thereof to a rod side hydraulic chamber 1b of the boom cylinder <NUM> via the switching valve 43a and through a rod side hydraulic line 91b. The switching valve 43a switches the flow line between conduction and interruption in accordance with a signal supplied from a controller <NUM>, and is in the interruption state when no signal is supplied. The closed circuit pump <NUM> is connected in a closed circuit manner to the boom cylinder <NUM> when the switching valve 43a is placed into the conduction state.

Further, the closed circuit pump <NUM> is connected at one of the suction/delivery ports thereof to a bottom side hydraulic chamber 3a of the arm cylinder <NUM> via a switching valve 43b and through a bottom side hydraulic line 92a, and at the other of the suction/delivery ports thereof to a rod side hydraulic chamber 3b of the arm cylinder <NUM> via the switching valve 43b and through a rod side hydraulic line 92b. The switching valve 43b switches the flow line between conduction and interruption in accordance with a signal supplied from the controller <NUM>, and is in the interruption state when no signal is supplied. The closed circuit pump <NUM> is connected in a closed circuit manner to the arm cylinder <NUM> when the switching valve 43b is placed into the conduction state.

The closed circuit pump <NUM> is connected at one of suction/delivery ports thereof to the bottom side hydraulic chamber 1a of the boom cylinder <NUM> via a switching valve 45a and through the bottom side hydraulic line 91a, and at the other of the suction/delivery ports thereof to the rod side hydraulic chamber 1b of the boom cylinder <NUM> via the switching valve 45a and through the rod side hydraulic line 91b. The switching valve 45a switches the flow line between conduction and interruption in accordance with a signal supplied from the controller <NUM>, and is in the interruption state when no signal is supplied. The closed circuit pump <NUM> is connected in a closed circuit manner to the boom cylinder <NUM> when the switching valve 45a is placed into the conduction state.

The closed circuit pump <NUM> is connected at one of the suction/delivery ports thereof to the bottom side hydraulic chamber 3a of the arm cylinder <NUM> via a switching valve 45b and through the bottom side hydraulic line 92a, and at the other of the suction/delivery ports thereof to the rod side hydraulic chamber 3b of the arm cylinder <NUM> via the switching valve 45b and through the rod side hydraulic line 92b. The switching valve 45b switches the flow line between conduction and interruption in accordance with a signal supplied from the controller <NUM>, and is in the interruption state when no signal is supplied. The closed circuit pump <NUM> is connected in a closed circuit manner to the arm cylinder <NUM> when the switching valve 45b is placed into the conduction state.

The open circuit pump <NUM> is connected at a suction port thereof to the tank <NUM> and at a delivery port thereof to a delivery hydraulic line <NUM>. The delivery hydraulic line <NUM> is connected to the tank <NUM> via a bleed-off valve <NUM>. The bleed-off valve <NUM> changes its opening area in accordance with a signal supplied from the controller <NUM>, and is in a fully open state when no signal is supplied. Further, the delivery hydraulic line <NUM> is connected to the bottom side hydraulic line 91a of the boom cylinder <NUM> via a switching valve 44a and is connected to the bottom side hydraulic line 92a of the arm cylinder <NUM> via a switching valve 44b. The switching valves 44a and 44b switch the flow line between conduction and interruption in accordance with a signal supplied from the controller <NUM>, and are in the interruption state when no signal is supplied.

The open circuit pump <NUM> is connected at a suction port thereof to the tank <NUM> and at a delivery port thereof to a delivery hydraulic line <NUM>. The delivery hydraulic line <NUM> is connected to the tank <NUM> via a bleed-off valve <NUM>. The bleed-off valve <NUM> changes its opening area in accordance with a signal supplied from the controller <NUM>, and is in a fully open state when no signal is supplied. Further, the delivery hydraulic line <NUM> is connected to the bottom side hydraulic line 91a of the boom cylinder <NUM> via a switching valve 46a and is connected to the bottom side hydraulic line 92a of the arm cylinder <NUM> via a switching valve 46b. The switching valves 46a and 46b switch the flow line between conduction and interruption in accordance with a signal supplied from the controller <NUM>, and are in the interruption state when no signal is supplied.

The closed circuit pump <NUM> is connected at one of the suction/delivery ports thereof (on the side connected to the rod side hydraulic chamber 1b of the boom cylinder <NUM> and also to the bottom side hydraulic chamber 3a of the arm cylinder <NUM>) to the charge line <NUM> through a branch hydraulic line <NUM>, and a switching valve <NUM> is provided in the branch hydraulic line <NUM>. Further, the closed circuit pump <NUM> is connected at one of the suction/delivery ports thereof (on the side connected to the rod side hydraulic chamber 1b of the boom cylinder <NUM> and also to the bottom side hydraulic chamber 3a of the arm cylinder <NUM>) to the charge line <NUM> through a branch hydraulic line <NUM>, and a switching valve <NUM> is provided in the branch hydraulic line <NUM>. The switching valves <NUM> and <NUM> switch the flow line between conduction and interruption in accordance with a signal supplied from the controller <NUM>, and are in the interruption state when no signal is supplied.

The bottom side hydraulic line 91a and the rod side hydraulic line 91b of the boom cylinder <NUM> are connected to the charge line <NUM> via check valves 37a and 37b and a flushing valve <NUM>, and the bottom side hydraulic line 92a and the rod side hydraulic line 92b of the arm cylinder <NUM> are connected to the charge line <NUM> via check valves 38a and 38b and a flushing valve <NUM>. The closed circuit pump <NUM> is connected at the suction/delivery ports thereof to the charge line <NUM> via check valves 30a and 30b and main relief valves 80a and 80b, and the closed circuit pump <NUM> is connected at the suction/delivery ports thereof to the charge line <NUM> via check valves 31a and 31b and main relief valves 81a and 81b. The check valves 30a and 30b are built in the closed circuit pump <NUM>, and the check valves 31a and 31b are built in the closed circuit pump <NUM>.

Each of the check valves 30a, 30b, 31a, 31b, 37a, 37b, 38a, and 38b sucks, when the pressure in the closed circuit decreases, hydraulic operating fluid from the charge line <NUM> into the circuit to thereby prevent cavitation of the circuit. The flushing valves <NUM> and <NUM> are low pressure selecting valves that connect the low pressure side of the closed circuit and the charge line <NUM> to each other, and keep the balance in hydraulic fluid amount in the closed circuit by discharging surplus hydraulic operating fluid in the closed circuit to the charge line <NUM> or by sucking hydraulic operating fluid lacking in the closed circuit from the charge line <NUM>. Each of the main relief valves 80a, 80b, 81a, and 81b relieves, when the pressure in the closed circuit exceeds a predetermined pressure (main relief pressure), hydraulic operating fluid to the tank <NUM>, to thereby protect the circuit.

The controller <NUM> issues a command to the pumps <NUM> to <NUM> and the switching valves 43a to 46b, <NUM>, and <NUM> in response to an input from the operation lever 56a or 56d and sensor information such as the engine speed and the pressures to the individual portions. Further, the controller <NUM> includes an unload controlling section 57a for performing unload control to be hereinafter described. The unload controlling section 57a is implemented, for example, as one function of a program executed by the controller <NUM>.

<FIG> is a cross sectional view of the closed circuit pump <NUM> (<NUM>).

Referring to <FIG>, the closed circuit pump <NUM> (<NUM>) includes a casing <NUM>, a rear case <NUM>, a shaft <NUM>, a cylinder <NUM>, pistons <NUM>, shoes <NUM>, a valve plate <NUM>, a swash plate <NUM>, a cradle <NUM>, suction/delivery ports <NUM> and <NUM>, a charge port <NUM>, and check valves 30a (31a) and 30b (31b).

Rotational power from the engine <NUM> is inputted to the shaft <NUM>, and the cylinder <NUM>, the plurality of pistons <NUM> accommodated in the cylinder <NUM>, and so forth operate rotationally together with the shaft <NUM>. The pistons <NUM> slidably rotate in contact with the swash plate <NUM>. Since the swash plate <NUM> has an angle α, the pistons <NUM> are displaced in an axial direction with respect to the cylinder <NUM>. For example, the pistons <NUM> suck hydraulic operating fluid from the suction/delivery port <NUM> and delivers the hydraulic operating fluid to the suction/delivery port <NUM>.

The swash plate <NUM> is provided tiltably through the cradle <NUM> in the casing <NUM>. The front surface side of the swash plate <NUM> forms a smooth surface 308a that guides the shoes <NUM> slidably. In contrast, the rear surface side of the swash plate <NUM> is supported tiltably (slidably) on the cradle <NUM>. The cradle <NUM> is provided fixedly on the casing <NUM> and positioned around the shaft <NUM>.

The tilting angle α of the swash plate <NUM> can be adjusted by a regulator and a servo piston which are not depicted. When the tilting angle α is zero, the pump delivery flow rate is zero, and when the tilting angle α has a negative value, the hydraulic operating fluid is sucked from the suction/delivery port <NUM> and is delivered to the suction/delivery port <NUM>.

The charge line <NUM> is connected to the charge port <NUM>. If the pressure in the suction/delivery ports <NUM> and <NUM> becomes equal to or lower than a charge pressure, then the check valves 30a and 30b (check valves 31a and 31b) are opened, and the hydraulic operating fluid from the charge pump <NUM> is sucked, to thereby prevent cavitation in the closed circuit pump <NUM> (<NUM>).

An example of operation of the actuators in the configuration described above is described first.

Referring to <FIG>, when an extension operation of the boom cylinder <NUM> is to be performed, the switching valves 43a and 44a are placed into a conduction state, and the hydraulic operating fluid is delivered from the closed circuit pump <NUM> and the open circuit pump <NUM>. Consequently, a flow rate corresponding to the two pumps is fed into the bottom side hydraulic chamber 1a of the boom cylinder <NUM>, and so that the boom cylinder <NUM> extends. Although the discharge flow rate from the rod side hydraulic chamber 1b of the boom cylinder <NUM> is sucked into the closed circuit pump <NUM>, when the flow rate becomes surplus, the hydraulic operating fluid is discharged from the flushing valve <NUM> to the charge line <NUM>, but when the flow rate becomes insufficient, the hydraulic operating fluid is sucked conversely from the charge line <NUM> into the closed circuit via the flushing valve <NUM> or the check valves 30a and 37a.

When a pull-in operation of the arm cylinder <NUM> is to be performed, the switching valves 43b and 44b are placed into a conduction state, the hydraulic operating fluid is delivered from the closed circuit pump <NUM> in a direction opposite to that in the case described above, and the bleed-off valve <NUM> is opened. Consequently, the hydraulic operating fluid is discharged from the bottom side hydraulic chamber 3a of the arm cylinder <NUM>, so that the arm cylinder <NUM> performs a pull-in operation.

When there is no lever input from the operator at the time of waiting for work or the like, all of the switching valves 43a to 46b are placed into an interruption state, and even in a case where the front implement <NUM> of the hydraulic excavator <NUM> is in the air as depicted in <FIG>, the front implement <NUM> is held by the actuators <NUM> and <NUM> such that it does not move down in the direction of the own weight. The closed circuit pumps <NUM> and <NUM> have a tilting angle controlled to zero such that no delivery flow rate occurs.

In this case, as depicted in <FIG>, the pistons <NUM> in each of the closed circuit pumps <NUM> and <NUM> are pressed against a cylinder wall surface 304b by a centrifugal force, the tilting angle α is zero, and the pistons <NUM> and the cylinder <NUM> do not have relative displacement therebetween. Therefore, the hydraulic operating fluid as lubricating oil is less likely to be introduced into a contact portion between the pistons <NUM> and the cylinder <NUM>. Therefore, there is the possibility that, if this state continues, then the oil film at the contact portion may be broken to cause abrasion between or damage to the pistons <NUM> and the cylinder <NUM>, resulting in deterioration of the reliability.

Therefore, in the present embodiment, in order to solve the problem described above, the switching valves <NUM> and <NUM> are provided in the branch hydraulic lines <NUM> and <NUM>, respectively, and the unload controlling section 57a is provided in the controller <NUM>.

<FIG> is a flow chart of the unload controlling section 57a. Although unload control of the closed circuit pump <NUM> is described here, the description similarly applies also to the closed circuit pump <NUM>.

The controller <NUM> first decides whether or not the required pump delivery volume to the closed circuit pump <NUM> is zero (step S1).

When it is decided in step S1 that the required pump delivery volume is not zero (No), the pistons <NUM> are in a state displaced with respect to the cylinder <NUM>, and the pistons <NUM> and the cylinder <NUM> are not to be abraded. Therefore, the state of the pump delivery volume being zero is continued. In particular, the controller <NUM> sets a zero tilt duration Tzero to zero (step S2), and while keeping the command for the switching valve <NUM> to interruption (Close), the controller <NUM> applies the required delivery volume to the command delivery volume for the closed circuit pump <NUM> (step S3).

When it is decided in step S1 that the required pump delivery volume is zero (Yes), the controller <NUM> decides whether or not the engine speed N exceeds a predetermined rotation speed Nhigh (step S4).

When it is decided No (the engine speed N is equal to or lower than the predetermined rotation speed Nhigh) in step S4, since the centrifugal force acting upon the piston <NUM> of the closed circuit pump <NUM> is small and the possibility that the pistons <NUM> or the cylinder <NUM> may be abraded is sufficiently low, the state of the pump delivery volume being zero is continued as it is. In particular, the controller <NUM> sets the zero tilt duration Tzero to zero (step S2), and while keeping the command for the switching valve <NUM> to interruption (Close), the controller <NUM> applies the required delivery volume to the command delivery volume for the closed circuit pump <NUM> (step S3).

When it is decided Yes (the engine speed N exceeds the predetermined rotation speed Nhigh) in step S4, the controller <NUM> adds a control cycle ΔT to the zero tilt duration Tzero in the preceding control cycle to calculate the zero tilt duration Tzero at the present point of time (step S5).

Subsequently to step S5, the controller <NUM> decides whether or not the zero tilt duration Tzero exceeds a predetermined waiting time period Tlimit (step S6). In the present embodiment, the waiting time period Tlimit is defined as a function of the engine speed N and is set such that it decreases as the engine speed N increases with respect to the predetermined rotation speed Nhigh as depicted in <FIG>. This is because, as the engine speed N increases, the centrifugal force acting upon the piston <NUM> of the closed circuit pump increases and the time period until the oil film at the sliding portion between the pistons <NUM> and the cylinder <NUM> is broken decreases.

When the controller <NUM> decides No (the zero tilt duration Tzero is equal to or shorter than the predetermined waiting time period Tlimit) in step S6, it continues the state of the pump delivery volume being zero. In other words, while keeping the command for the switching valve <NUM> to interruption (Close), the controller <NUM> applies the required delivery volume to the command delivery volume for the closed circuit pump <NUM> (step S3).

When the controller <NUM> decides YES (the zero tilt duration Tzero exceeds the waiting time period Tlimit) in step S6, it sets the command to the switching valve <NUM> to open (Open) and sets the command delivery volume for the closed circuit pump <NUM> to a predetermined delivery volume Vset (step S7).

Subsequently to step S3 or step S7, the controller <NUM> outputs a command to the switching valve <NUM> and the closed circuit pump <NUM> (step S8), thereby ending the flow.

<FIG> depicts flows of the hydraulic operating fluid when the engine speed N exceeds the predetermine rotation speed Nhigh and the zero tilt duration Tzero of the closed circuit pump <NUM> exceeds the waiting time period Tlimit. The closed circuit pump <NUM> delivers a flow rate according to the delivery volume Vset. The hydraulic operating fluid delivered from the closed circuit pump <NUM> flows to the charge line <NUM> via the switching valve <NUM> and returns to the tank <NUM> via the charge relief valve <NUM> as indicated by a thick solid line in <FIG>. To the suction side of the closed circuit pump <NUM>, the delivery flow rate of the charge pump <NUM> is supplied via the check valve 30b as indicated by a thick broken line in <FIG>.

As a result, the following advantageous effects are achieved.

Since the pistons <NUM> in the closed circuit pump <NUM> are displaced with respect to the cylinder <NUM> with the closed circuit pump delivery volume kept to a level equal to or higher than the predetermined delivery volume Vset, the hydraulic fluid is introduced to the sliding portion to thereby assure an oil film, by which abrasion can be prevented. Further, since the suction/delivery ports of the closed circuit pump <NUM> are connected to the charge line <NUM> via the switching valve <NUM>, the delivery pressure of the closed circuit pump <NUM> is suppressed to a low level equal to or lower than the charge pressure. Consequently, degradation of fuel consumption can be prevented, and the durability of the closed circuit pump <NUM> itself can be improved.

Further, since the waiting time period Tlimit until unload control is started is provided and the waiting time period Tlimit is changed according to the rotation speed N, for example, in such a case where the probability of abrasion is low as upon engine idling, the waiting time period Tlimit is set to infinity such that unload control is not performed. Since this results in significant reduction in the number of times of operation of the switching valves <NUM> and <NUM>, the reliability of the switching valves <NUM> and <NUM> can be assured readily.

Further, as depicted in <FIG>, the present embodiment adopts a configuration in which, out of the pieces of hydraulic equipment connected to the charge line <NUM>, the check valves 30a, 30b, 31a, and 31b are arranged nearest from the delivery port of the charge pump <NUM> while the charge relief valve <NUM> and the switching valves <NUM> and <NUM> are arranged farther than them. Consequently, since, during unload control, hydraulic operating fluid of a comparatively low temperature in the tank <NUM> is sucked into the closed circuit pumps <NUM> and <NUM> via the charge pump <NUM>, temperature rise of the closed circuit pumps <NUM> and <NUM> is suppressed, and the reliability and the durability can be improved. If the charge relief valve <NUM>, the check valves 30a, 30b, 31a, and 31b, and the switching valves <NUM> and <NUM> should be arranged in this order or the switching valves <NUM> and <NUM>, the check valves 30a, 30b, 31a, and 31b, and the charge relief valve <NUM> should be arranged in this order from the nearest side to the delivery port of the charge pump <NUM>, the hydraulic operating fluid delivered from the closed circuit pumps <NUM> and <NUM> is sucked back into the closed circuit pumps <NUM> and <NUM> via the switching valves <NUM> and <NUM> and the check valves 30a, 30b, 31a, and 31b. Therefore, since the hydraulic operating fluid is circulated without passing the tank <NUM>, there is the possibility that the temperature of the hydraulic operating fluid may rise locally and the viscosity of the hydraulic operating fluid may decrease, resulting in promotion of abrasion of the sliding portion.

Further, the present embodiment adopts a configuration in which the switching valves <NUM> and <NUM> for unload control are connected to only either one of the bottom side hydraulic chambers 1a and 3a and the rod side hydraulic chambers 1b and 3b of the hydraulic cylinders <NUM> and <NUM> and are thus connected to the side on which a high pressure by the own weight of the front implement <NUM> does not act. In particular, in the present embodiment, as a frequently used aerial posture, such a scene as depicted in <FIG> is supposed in which the bucket <NUM> is driven to perform a warm-up operation. In this posture, since a high pressure by the own weight of the front implement <NUM> acts upon the bottom side of the boom cylinder <NUM> and the rod side of the arm cylinder <NUM>, the present embodiment adopts a configuration in which the switching valves <NUM> and <NUM> are connected to the rod side of the boom cylinder <NUM> and the bottom side of the arm cylinder <NUM> upon which a high pressure does not act, as depicted in <FIG>.

Consequently, even in a case in which the switching valve <NUM> is stuck open (stuck-open failure) at a point of time at which the operator intends to perform a very small boom-raising operation, for example, from the aerial posture of <FIG> and places the switching valve 43a into the conduction state, the boom <NUM> does not move down suddenly in the own weight direction. Therefore, a movement that is not intended by the operator can be suppressed.

In the present embodiment, provided is the hydraulic excavator <NUM> comprising the closed circuit pumps <NUM> and <NUM> each consisting of a bidirectionally tiltable hydraulic pump having two suction/delivery ports, the actuators <NUM> and <NUM> each connected in a closed circuit manner to the closed circuit pumps <NUM> and <NUM>, the charge pump <NUM>, the charge line <NUM> connected to the delivery port of the charge pump <NUM>, the check valves 30a, 30b, 31a, and 31b that are provided in the hydraulic lines each connecting the charge line <NUM> and the closed circuit pump <NUM> or <NUM> to each other and permit the hydraulic operating fluid to flow from the charge line <NUM> into the closed circuit pumps <NUM> and <NUM>, the charge relief valve <NUM> provided in the charge line <NUM>, the operation levers 56a and 56d for instructing operations of the actuators <NUM> and <NUM>, and the controller <NUM> that controls the delivery volumes of the closed circuit pumps <NUM> and <NUM> according to inputs from the operation levers 56a and 56d. The hydraulic excavator <NUM> includes the switching valves <NUM> and <NUM> that are provided in the branch hydraulic lines <NUM> and <NUM> each connecting one of the suction/delivery ports of the closed circuit pump <NUM> or <NUM> and the charge line <NUM> to each other and are opened and closed according to a control signal from the controller <NUM>. The controller <NUM> is configured to, in a case where the state in which the delivery volumes of the closed circuit pumps <NUM> and <NUM> are kept to zero continues for the predetermined time period Tlimit or more, open the switching valves <NUM> and <NUM> and keep the delivery volumes of the closed circuit pumps <NUM> and <NUM> to the predetermined delivery volume Vset or more, the predetermined delivery volume being greater than zero.

According to the present embodiment configured in such a manner as described above, keeping the delivery volumes of the closed circuit pumps <NUM> and <NUM> equal to or greater than the predetermined delivery volume Vset causes the pistons <NUM> in each of the closed circuit pumps <NUM> and <NUM> to be displaced with respect to the cylinder <NUM>, and therefore, oil is introduced into the sliding portion to thereby assure an oil film and can prevent abrasion. Further, connecting the delivery ports of the closed circuit pumps <NUM> and <NUM> to the charge line <NUM> via the switching valves <NUM> and <NUM> suppresses the pump pressure to a low level (to a pressure equal to or lower than the charge pressure), and therefore, it is possible to prevent deterioration in fuel consumption and improve the durability of the closed circuit pumps <NUM> and <NUM>. As a result, in the construction machine configured such that a hydraulic actuator is driven by a hydraulic pump for a closed circuit, oil film breakage of the hydraulic pump for a closed circuit that is the most important equipment can be prevented. Therefore, it is possible to provide a construction machine in which the reliability of the closed circuit pump is improved and a high operation rate is achieved. It is to be noted that, while, in the present embodiment, the switching valves <NUM> and <NUM> are provided on the rod side of the boom cylinder <NUM> and the bottom side of the arm cylinder <NUM>, the switching valves <NUM> and <NUM> may otherwise be provided on the bottom side of the boom cylinder <NUM> and the rod side of the arm cylinder <NUM>.

Further, the controller <NUM> in the present embodiment is configured to set the predetermined time period Tlimit shorter as the rotation speed N of the closed circuit pumps <NUM> and <NUM> increases.

Consequently, since the waiting time period Tlimit until unload control is started changes according to the rotation speed N of the closed circuit pumps <NUM> and <NUM>, in a case where the possibility of abrasion is low as upon engine idling, for example, the unload control is not performed. As a result, the number of times of operation of the switching valves <NUM> and <NUM> decreases, and therefore, assurance of the reliability of the switching valves <NUM> and <NUM> is facilitated.

Further, in the present embodiment, out of the check valves 30a, 30b, 31a, and 31b, the charge relief valve <NUM>, and the switching valves <NUM> and <NUM> all connected to the charge line <NUM>, the check valves 30a, 30b, 31a, and 31b are arranged nearest from the delivery port of the charge pump <NUM>.

Consequently, since, during unload control, oil of a comparatively low temperature is sucked from the tank <NUM> into the closed circuit pumps <NUM> and <NUM> via the charge pump <NUM>, temperature rise of the closed circuit pumps <NUM> and <NUM> is suppressed, and the reliability and the durability can be improved.

Further, the hydraulic excavator <NUM> according to the present embodiment comprises the front implement <NUM> including the boom <NUM> and the arm <NUM>. The closed circuit pumps <NUM> and <NUM> include the first closed circuit pump <NUM> and the second closed circuit pump <NUM>, and the actuators <NUM> and <NUM> include the boom cylinder <NUM> that drives the boom <NUM> and the arm cylinder <NUM> that drives the arm <NUM>. The switching valves <NUM> and <NUM> include the first switching valve <NUM> provided in the first branch hydraulic line <NUM> that connects one of the suction/delivery ports of the first closed circuit pump <NUM> and the charge line <NUM> to each other, and the second switching valve <NUM> provided in the second branch hydraulic line <NUM> that connects one of the suction/delivery ports of the second closed circuit pump <NUM> and the charge line <NUM> to each other. Both the switching valve <NUM> and the switching valve <NUM> are arranged on the rod side of the boom cylinder <NUM> and the bottom side of the arm cylinder <NUM>.

Consequently, even in a case where the switching valves <NUM> and <NUM> are stuck open (stuck-open failure), such a movement of the front implement <NUM> as to suddenly move down in its own weight direction can be prevented, and therefore, a motion not intended by the operator can be suppressed.

Claim 1:
A construction machine comprising:
a closed circuit pump (<NUM>, <NUM>) consisting of a bidirectionally tiltable hydraulic pump having two suction/delivery ports;
a hydraulic actuator (<NUM>, <NUM>) connected in a closed circuit manner to the closed circuit pump (<NUM>, <NUM>);
a charge pump (<NUM>);
a charge line (<NUM>) connected to a delivery port of the charge pump (<NUM>);
a check valve (30a, 30b, 31a, 31b) that is provided in a hydraulic line connecting the charge line (<NUM>) and the closed circuit pump (<NUM>, <NUM>) to each other and permits hydraulic operating fluid to flow from the charge line (<NUM>) into the closed circuit pump (<NUM>, <NUM>);
a charge relief valve (<NUM>) provided in the charge line (<NUM>) ;
an operation lever (56a, 56d) for instructing an operation of the hydraulic actuator (<NUM>, <NUM>); and
a controller (<NUM>) that controls a delivery volume of the closed circuit pump (<NUM>, <NUM>) according to an input from the operation lever (56a, 56d); characterised in that
the construction machine includes a switching valve (<NUM>, <NUM>) that is provided in a hydraulic line (<NUM>, <NUM>) connecting one of the suction/delivery ports of the closed circuit pump (<NUM>, <NUM>) and the charge line (<NUM>) to each other and is opened and closed according to a control signal from the controller (<NUM>), and in that
the controller (<NUM>) is configured to, in a case where a state in which the delivery volume of the closed circuit pump (<NUM>, <NUM>) is kept to zero continues for a predetermined time period or more, open the switching valve (<NUM>, <NUM>) and keep the delivery volume of the closed circuit pump (<NUM>, <NUM>) to a predetermined delivery volume or more, the predetermined delivery volume being greater than zero.