Patent Application
20150330415
The present invention relates generally to a hydraulic drive system for construction machines, such as hydraulic excavators including travel hydraulic motors and variable displacement hydraulic pumps. More particularly, the present invention relates to a load sensing control hydraulic drive system that controls displacement of a hydraulic pump such that a delivery pressure of the hydraulic pump is higher than a maximum load pressure of a plurality of actuators by a predetermined value (a target differential pressure).
A hydraulic drive system of this type for a construction machine is disclosed in patent document 1. The hydraulic drive system disclosed in patent document 1 includes a travel detection unit and a setting change unit. The travel detection unit detects travelling operation in which a travel hydraulic motor is driven. On the basis of the detection result of the travel detection unit, the setting change unit sets a target differential pressure of load sensing control at a first specified value during any time other than the travelling operation and sets the target differential pressure of load sensing control at a second specified value smaller than the first specified value during the travelling operation. In addition, in response to the target differential pressure of load sensing control set to be smaller during the travelling operation, an opening area of a spool of a travel flow control valve is set to be greater than before over an entire spool stroke. This arrangement allows a flow rate required for traveling to be supplied to the travel hydraulic motor during the travelling operation, thereby achieving a travel speed as usual and reducing energy loss and improve energy efficiency.
In order to reduce the target differential pressure of load sensing control in accordance with reduction in engine speed thereby to improve fine operability during reduction in engine speed, the hydraulic drive system disclosed in patent document 1 is configured to introduce an output pressure from an engine speed sensing valve unit to a load sensing control section of a pump control unit, as the target differential pressure of load sensing control. The engine speed sensing valve unit includes a flow sensing valve and a differential pressure reducing valve. The flow sensing valve varies a differential pressure thereacross in accordance with a delivery flow rate of a pilot pump driven by the engine. The differential pressure reducing valve generates and outputs the differential pressure across the flow sensing valve as an absolute pressure.
In one embodiment (the embodiment of FIG. 8) of the hydraulic drive system disclosed in patent document 1, on the assumption that the system includes the engine speed sensing valve unit, a travel pilot pressure from a travel control lever unit is introduced to an open side end of the spool of the flow sensing valve. This causes the travel pilot pressure to act in a direction in which a variable restrictor of the flow sensing valve opens, thereby generating the target differential pressure of load sensing control as the second specified value.
Patent Document 1: JP, A 2011-247301
In the hydraulic drive system disclosed in patent document 1, the target differential pressure of load sensing control is set at the second specified value smaller than the first specified value during the travelling operation and, in response to the setting of the smaller target differential pressure of load sensing control, the opening area of the spool of the travel flow control valve is set to be greater than usual over an entire spool stroke. This reduces energy loss and achieves improved energy efficiency in the travelling operation.
In the prior art, however, since the opening area of the spool of the travel flow control valve is set at a greater value than usual over the entire spool stroke, when the travel control lever is operated in the range of stroke less than a half to perform travelling operation, in particular upon travel fine operation, etc., the flow rate supplied from the hydraulic pump to the travel hydraulic motors are apt to be affected by variations in travelling load and changes in the pump delivery pressure, and this raises a problem to avoid favorable operability.
It is an object of the present invention to provide a hydraulic drive system for a construction machine in which a travel speed as usual is achieved and energy loss is reduced and energy efficiency is improved, while when the travel control lever is operated in the range of stroke less than a half to perform travelling operation, the flow rate supplied from the hydraulic pump to the travel hydraulic motors are hard to be affected by variations in travelling load and changes in the pump delivery pressure thereby to achieve favorable travel operability.
(1) To solve the foregoing problem, an aspect of the present invention provides a hydraulic drive system for a construction machine, the system comprising: a variable displacement main pump driven by a prime mover; a plurality of actuators including travel hydraulic motors and driven by a hydraulic fluid delivered from the main pump; a plurality of flow control valves including travel flow control valves, that controls flow rates of a hydraulic fluid supplied from the main pump to the plurality of actuators; a plurality of operating units including travel operating units, that instructs operating directions and operating speeds of the plurality of the actuators and outputs commands for operating the plurality of flow control valves; a plurality of pressure compensation valves for controlling differential pressures across the plurality of flow control valves; and a pump control unit for performing load sensing control of a displacement of the main pump such that a delivery pressure of the main pump becomes higher by a target differential pressure than a maximum load pressure of the actuators, the plurality of pressure compensation valves being configured to control the differential pressures across the respective flow control valves such that the differential pressure across each of the flow control valves is maintained at a differential pressure between the delivery pressure of the main pump and the maximum load pressure of the actuators, wherein the hydraulic drive system further comprises: a travel detection unit that detects travelling operation in which the travel hydraulic motors are driven; and a target differential pressure setting unit that, based on a result of detection by the travel detection unit, sets the target differential pressure of load sensing control at a first specified value at any time other than the travelling operation and sets the target differential pressure of load sensing control at a second specified value smaller than the first specified value during the travelling operation, wherein the travel flow control valves each has such an opening area characteristic that an opening area at a spool stroke when the corresponding travel operating unit is fully operated is large enough to obtain a predetermined flow rate required for traveling when the target differential pressure of load sensing control is set at the second specified value, and an opening area in a spool stroke range when the corresponding travel operating unit is finely operated is approximate to an opening area of a travel flow control valve having a maximum opening area that can obtain a predetermined flow rate required for traveling when the target differential pressure of load sensing control is set at the first specified value.
The travel flow control valve is set to have an opening area at the spool stroke when the travel operating unit is fully operated large enough to obtain the predetermined flow rate required for traveling even when the target differential pressure of load sensing control is the second specified value smaller than the first specified value. This arrangement enables a travel speed known in the art during travelling operation to be achieved and energy efficiency to be improved by reducing energy loss.
The favorable operability can be achieved in the following method. The opening area in the spool stroke range when the travel operating unit is finely operated is adapted to be approximate to the opening area of the travel flow control valve. The opening area has the maximum area where a predetermined flow rate required for traveling when the target differential pressure of load sensing control is the first specified value (the opening area on a smaller side) can be obtained. When the travel lever is operated in the stroke range over which the travel lever is operated halfway or less, including fine operation, to perform the travelling operation, the system will be less susceptible to effects from variations in a travel load and changes in a pump delivery pressure.
(2) Preferably, in (1) above, the target differential pressure setting unit comprises: a pilot pump driven by the prime mover; a prime mover speed sensing valve unit including: a flow sensing valve disposed in a line through which a hydraulic fluid delivered from the pilot pump flows, for varying a differential pressure across the flow sensing valve in accordance with a delivery flow rate of the pilot pump; and a differential pressure reducing valve that generates the differential pressure across the flow sensing valve as an absolute pressure and outputs the absolute pressure as the target differential pressure of load sensing control; and a variable restrictor valve disposed in parallel with the flow sensing valve in a line through which the hydraulic fluid delivered from the pilot pump flows, wherein the variable restrictor valve is in a fully closed position at any time other than the travelling operation and is in a restricting position during the travelling operation and continuously increases an opening area thereof from a full closure up to a maximum as an input amount of the travel operating unit increases from a minimum to a maximum.
The arrangements in which the variable restrictor valve is disposed in parallel with the flow sensing valve and in which the opening area of the variable restrictor valve increases continuously from the fully closed position to the maximum allow an output pressure of the differential pressure reducing valve (target differential pressure of load sensing control) to a minimum, the output pressure being at the time that the travel operating unit is fully operated to decrease at a rate identical to an input amount of the travel operating unit throughout an entire prime mover speed range from a maximum. For this reason, when the prime mover speed is reduced to a low speed to thereby finely operate the travel operating unit, the output pressure of the differential pressure reducing valve (target differential pressure of load sensing control) can be reduced in accordance with the input amount of the travel operating unit. Accordingly, the differential pressure across the travel flow control valve can be similarly reduced.
An operation in which the travel operating unit is finely operated (e.g., a finely operated downhill travelling operation) often involves reduction in the prime mover speed to a low speed. In the aspect of the present invention, the output pressure of the differential pressure reducing valve (target differential pressure of load sensing control) decreases at the rate identical to the input amount of the travel operating unit in the finely operated downhill travelling operation. The differential pressure across the travel flow control valve can be similarly reduced as a result.
When the prime mover speed is reduced to a low value to thereby perform fine operation in travel, the opening area of the travel flow control valve is made small as described in above (1) and the differential pressure across the travel flow control valve is made to decrease at the rate identical to the input amount of the travel operating unit. This enables a rate of flow supplied to the travel hydraulic motor to be finely adjusted in accordance with the input amount. This adjustment eliminates an excessive travel speed unexpected by an operator and significantly improves operability.
The present invention achieves a travel speed known in the art during travelling operation and improves energy efficiency by reducing energy loss while obtaining favorable operability less susceptible to effects from variations in a travel load and changes in a pump delivery pressure when travelling operation is performed through operation of a travel lever over a half stroke range or less.
When the prime mover speed is reduced to a low speed to thereby perform fine operation in travel, the present invention allows the rate of flow supplied to the travel hydraulic motor to be finely adjusted in accordance with the input amount, thus eliminating the likelihood that an excessive travel speed unexpected by the operator will be produced and significantly improving operability.
An embodiment of the present invention will be described below with reference to the accompanying drawings.
In
The control valve 4 includes a second hydraulic fluid supply line 4a (internal path), a plurality of flow control valves 6a, 6b, 6c, 6d, 6e . . . , pressure compensation valves 7a, 7b, 7c, 7d, 7e . . . , shuttle valves 9a, 9b, 9c, 9d, 9e . . . , a differential pressure reducing valve 11, a main relief valve 14, and an unloading valve 15. More specifically, the second hydraulic fluid supply line 4a is connected to a first hydraulic fluid supply line 5 (piping) to which a delivered fluid from the main pump 2 is supplied. The flow control valves 6a, 6b, 6c, 6d, 6e . . . of a closed center type are each connected to a corresponding one of hydraulic lines 8a, 8b, 8c, 8d, 8e . . . that branch off from the second hydraulic fluid supply line 4a. The flow control valves 6a, 6b, 6c, 6d, 6e . . . each control a flow rate and a direction of a hydraulic fluid supplied from the main pump 2 to a corresponding one of the actuators 3a, 3b, 3c, 3d, 3e . . . . The pressure compensation valves 7a, 7b, 7c, 7d, 7e . . . are each disposed upstream of a corresponding one of the flow control valves 6a, 6b, 6c, 6d, 6e . . . . The pressure compensation valves 7a, 7b, 7c, 7d, 7e . . . each control a differential pressure across a meter-in restrictor of a corresponding one of the flow control valves 6a, 6b, 6c, 6d, 6e . . . . The shuttle valves 9a, 9b, 9c, 9d, 9e . . . each select the greatest pressure (maximum load pressure) of load pressures of actuators 3a, 3b, 3c, 3d, 3e . . . and output the greatest pressure to a signal hydraulic line 27. The differential pressure reducing valve 11 receives the pressure of the second hydraulic fluid supply line 4a (the delivery pressure of the main pump 2) and the pressure of the signal hydraulic line 27 (the maximum load pressure) introduced thereto and outputs as an absolute pressure PLS a differential pressure between the main pump 2 delivery pressure (pump pressure) and the maximum load pressure. The main relief valve 14 is connected to the second hydraulic fluid supply line 4a. When the pressure of the second hydraulic fluid supply line 4a (the main pump 2 delivery pressure) becomes greater than or equal to a set pressure, the main relief valve 14 opens to return the hydraulic fluid of the second hydraulic fluid supply line 4a to a tank T, thereby preventing the pressure of the second hydraulic fluid supply line 4a (the main pump 2 delivery pressure) from exceeding the set pressure. The unloading valve 15 is connected to the second hydraulic fluid supply line 4a. When the main pump 2 delivery pressure becomes greater than the maximum load pressure to which a set pressure of a pressure receiving portion 15a and a spring 15b is added, the unloading valve 15 opens to return the main pump 2 delivered fluid back to the tank T, thereby preventing the main pump 2 delivery pressure from building up relative to the maximum load pressure.
The flow control valves 6a, 6b, 6c, 6d, 6e . . . have load ports 26a, 26b, 26c, 26d, 26e . . . , respectively. When the flow control valves 6a, 6b, 6c, 6d, 6e . . . are each in a neutral position, the load ports 26a, 26b, 26c, 26d, 26e . . . each communicate with the tank T to thereby output a tank pressure as a load pressure. When the flow control valves 6a, 6b, 6c, 6d, 6e . . . are each placed in the right or left operated position shown in
The shuttle valves 9a, 9b, 9c, 9d, 9e . . . are connected in a tournament format and, together with the load ports 26a, 26b, 26c, 26d, 26e . . . and the signal hydraulic line 27, constitute a maximum load pressure detection circuit. The shuttle valve 9a selects and outputs the higher pressure among a pressure at the load port 26a of the flow control valve 6a and another one at the load port 26b of the flow control valve 6b. The shuttle valve 9b selects and outputs the higher pressure among an output pressure from the shuttle valve 9a and a pressure at the load port 26c of the flow control valve 6c. The shuttle valve 9c selects and outputs the higher pressure among an output pressure from the shuttle valve 9b and a pressure at the load port 26d of the flow control valve 6d. The shuttle valve 9d selects and outputs the higher pressure among an output pressure from the shuttle valve 9c and a pressure at the load port 26e of the flow control valve 6e. The shuttle valve 9e selects and outputs the higher pressure among an output pressure from the shuttle valve 9d and an output pressure from a similar shuttle valve (not shown). The shuttle valve 9e is disposed at a last stage. The output pressure from the shuttle valve 9e serves as a maximum load pressure output to the signal hydraulic line 27 and introduced to the differential pressure reducing valve 11 and the unloading valve 15.
The pressure compensation valves 7a, 7b, 7c, 7d, 7e . . . respectively have valve opening-side pressure receiving portions 28a, 28b, 28c, 28d, 28e . . . for setting target differential pressures. An output pressure from the differential pressure reducing valve 11 is introduced to the pressure receiving portions 28a, 28b, 28c, 28d, 28e . . . . A target compensation differential pressure is set depending on the absolute pressure of the differential pressure between the hydraulic pump pressure and the maximum load pressure (hereinafter referred to as the absolute pressure PLS). Controlling to bring the differential pressures across the flow control valves 6a, 6b, 6c, 6d, 6e . . . to the same absolute pressure PLS value regulates the pressure compensation valves 7a, 7b, 7c, 7d, 7e . . . such that the differential pressures across the flow control valves 6a, 6b, 6c, 6d, 6e . . . equal the absolute pressure PLS. This control allows, during a combined operation that simultaneously drives multiple actuators, the delivery flow rate of the main pump 2 to be distributed in accordance with an opening area ratio of the flow control valves 6a, 6b, 6c, 6d, 6e . . . regardless of a magnitude of the load pressure of each of the actuators 3a, 3b, 3c, 3d, 3e . . . so as to achieve high combined operationality. When a saturation condition develops in which the main pump 2 delivers a short supply of delivery flow rate that falls short of a required flow rate, the absolute pressure PLS decreases in accordance with the degree of the short supply. The differential pressures across the flow control valves 6a, 6b, 6c, 6d, 6e . . . controlled by the pressure compensation valves 7a, 7b, 7c, 7d, 7e . . . are accordingly reduced at the same rate. Consequently, the flow rates of the flow control valves 6a, 6b, 6c, 6d, 6e . . . decrease at the same rate. In this case too, the delivery flow rate of the main pump 2 is distributed in accordance with the opening area ratio of the flow control valves 6a, 6b, 6c, 6d, 6e . . . so as to achieve nigh combined operationality.
The unloading valve 15 includes the pressure receiving portion 15a, the spring 15b, a pressure receiving portion 15c, and a pressure receiving portion 15d. Specifically, the pressure receiving portion 15a and the spring 15b are operative in a closing direction to establish a set pressure Pun0 for the unloading valve 15. The pressure receiving portion 15c is operative in an opening direction to receive the pressure of the second hydraulic fluid supply line 4a (the delivery pressure of the main pump 2) introduced thereto. The pressure receiving portion 15d is operative in a closing direction to receive the maximum load pressure detected by the shuttle valves 9a, 9b, 9c, 9d, 9e . . . introduced thereto via the signal hydraulic line 27. The pressure receiving portion 15a receives an output pressure Pa (to be described later) of a differential pressure reducing valve 51 of the engine speed sensing valve unit 13 introduced thereto via a hydraulic line 41. When the delivery pressure of the main pump 2 becomes higher than the sum of the maximum load pressure and the set pressure Pun0 of the pressure receiving portion 15a and the spring 15a, the unloading valve 15 opens to thereby return the hydraulic fluid of the main pump 2 to the tank T to thereby keep the delivery pressure of the main pump 2 below the sum of the maximum load pressure and the set pressure Pun0. When all control levers are in their neutral positions and the maximum load pressure detected by the shuttle valves 9a, 9b, 9c, 9d, 9e . . . is the tank pressure, the delivery pressure of the main pump 2 is controlled to the set pressure Pun0 of the unloading valve 15.
The actuators 3a, 3b, 3c, 3d, 3e) are, for example, a swing motor, a boom cylinder, an arm cylinder, a left track motor, and a right track motor, respectively, of the hydraulic excavator. The flow control valves 6a, 6b, 6c, 6d, 6e) are, for example, swing, boom, arm, left track, and right track flow control valves, respectively. For convenience' sake, a bucket cylinder, a swing cylinder, and other actuators and flow control valves relating to these actuators are not shown.
By operating the gate lock lever 24, the gate lock valve 100 is allowed to be switched between a position to connect the pilot hydraulic line 31c to the pilot hydraulic line 31b and a position to connect the pilot hydraulic line 31c to the tank T. When the gate lock valve 100 is placed in the position to connect the pilot hydraulic line 31c to the pilot hydraulic line 31b and any control lever of the control lever units 60a, 60b, 60c, 60d, 60e . . . is operated, the control lever unit generate an operating pilot pressure using the hydraulic pressure of the pilot hydraulic fluid source 33 as a primary pressure in accordance with an input amount of the control lever. When the gate lock valve 100 is placed in the position to connect the pilot hydraulic line 31c to the tank T, the control lever units 60a, 60b, 60c, 60d, 60e . . . are incapable of generating the operating pilot pressure even when the corresponding control lever is operated.
The engine speed sensing valve unit 13 includes a flow sensing valve 50 and the differential pressure reducing valve 51. Specifically, the flow sensing valve 50 is disposed between the hydraulic fluid supply line 31a and the pilot hydraulic line 31b of the pilot pump 30. The differential pressure reducing valve 51 outputs a differential pressure across the flow sensing valve 50 as an absolute pressure. The flow sensing valve 50 includes a variable restrictor 50a that increases an opening area with a rise in the flow rate of the flow sensing valve 50 (the delivery flow rate of the pilot pump 30). The hydraulic fluid of the pilot pump 30 flows past the variable restrictor 50a of the flow sensing valve 50 toward the side of the pilot hydraulic line 31b. At this time, a differential pressure that increases with an increasing flow rate is generated at the variable restrictor 50a of the flow sensing valve 50. The differential pressure reducing valve 51 outputs the differential pressure across the variable restrictor 50a as the absolute pressure Pa. The delivery flow rate of the pilot pump 30 varies with the speed of the engine 1. Thus, detecting the differential pressure across the variable restrictor 50a allows the delivery flow rate of the pilot pump 30 and the speed of the engine 1 to be detected. Additionally, the variable restrictor 50a increases the opening area with an increasing rate flow of the area (with an increasing differential pressure thereacross). The variable restrictor 50a exhibits characteristics of a mild increase in the differential pressure at increasing flow rate of the area.
The main pump 2 includes a pump control unit 12 for controlling a tilting angle (capacity or displacement volume). The pump control unit 12 includes a horsepower control tilting actuator 12a, an LS control valve 12b, and an LS control tilting actuator 12c.
When the delivery pressure of the main pump 2 increases, the horsepower control tilting actuator 12a reduces the tilting angle of the main pump 2 to thereby prevent input torque of the main pump 2 from exceeding predetermined maximum torque. The horsepower consumption of the main pump 2 can be limited and the engine 1 can be prevented from stalling due to overload accordingly.
The LS control valve 12b has pressure receiving portions 12d and 12e that face each other. The absolute pressure Pa (a first specified value) as an output pressure of the differential pressure reducing valve 51 of the engine speed sensing valve unit 13 is introduced via a hydraulic line 40 to the pressure receiving portion 12d serving as a target differential pressure of load sensing control (target LS differential pressure). The absolute pressure PLS serving as the output pressure of the differential pressure reducing valve 11 is introduced to the pressure receiving portion 12e. When the absolute pressure PLS becomes higher than the absolute pressure Pa (PLS>Pa), the pressure of the pilot hydraulic fluid source 33 is introduced to the LS control tilting actuator 12c to thereby reduce the tilting angle of the main pump 2. When the absolute pressure PLS becomes lower than the absolute pressure Pa (PLS<Pa), the LS control tilting actuator 12c is brought into communication with the tank T to thereby increase the tilting angle of the main pump 2. Consequently, the tilting angle of the main pump 2 is controlled such that the delivery pressure of the main pump 2 becomes higher by the absolute pressure Pa (target differential pressure) than the maximum load pressure. The LS control valve 12b and the LS control tilting actuator 12c constitute load sensing pump control means that controls tilting of the main pump 2 such that the delivery pressure of the main pump 2 becomes higher by the target differential pressure of load sensing control than the maximum load pressure of the actuators 3a, 3b, 3c, 3d, 3e . . . .
It is here noted that the absolute pressure Pa varies according to the engine speed. An actuator's speed in keeping with the engine speed can therefore be controlled in the following method: using the absolute pressure Pa as the target differential pressure of load sensing control to set the target compensation differential pressure of the pressure compensation valves 7a, 7b, 7c, 7d, 7e . . . in accordance with the absolute pressure PLS of the differential pressure between the delivery pressure of the main pump 2 and the maximum load pressure. As described earlier, the variable restrictor 50a of the flow sensing valve 50 of the engine speed sensing valve unit 13 has such a characteristic that the greater the flow rate of the flow sensing valve 50 becomes, the milder the increase in the differential pressure thereacross becomes. This characteristic leads to improvement in a saturation phenomenon in accordance with the engine speed and favorable operability can be achieved when the engine speed is set low.
The absolute pressure Pa (the first specified value), the output pressure of the differential pressure reducing valve 51 of the engine speed sensing valve unit 13, is introduced to the pressure receiving portion 12d as the target differential pressure of load sensing control (the target LS differential pressure). The same absolute pressure Pa is introduced to the pressure receiving portion 15a of the unloading valve 15. The pressure receiving portion 15a and the spring 15b together establish the set pressure for the unloading valve 15. Thus, the set pressure for the unloading valve 15 is set at a value higher by a set portion achieved by the spring 15b than the target LS differential pressure. Additionally, the set portion achieved by the spring 15b is such a value small enough to retain the unloading valve 15 in a closed position when pressure of the pressure receiving portion 15d equals the tank pressure before the engine 1 is started. This reduces engine load when the engine 1 is started, achieving high startability of the engine 1.
In addition, the hydraulic drive system according to the embodiment is characterized by including shuttle valves 70a, 70b, and 70c (travel detection unit) and a variable restrictor valve 80. Specifically, the shuttle valves 70a, 70b, and 70c are disposed at delivery ports of remote control valves 60d1, 60d2, 60e1, and 60e2 of the travel control lever units 60d and 60e. The shuttle valves 70a, 70b, and 70c are incorporated in a tournament format so as to detect, of the operating pilot pressures d1, d2, e1, and e2 generated by the remote control valves 60d1, 60d2, 60e1, and 60e2, the highest pressure to thereby output the highest pressure as a travel pilot pressure to a signal hydraulic line 71. The variable restrictor valve 80 is disposed in the hydraulic fluid supply line 31a and pilot hydraulic line 31b, through which the delivery fluid of the pilot pump 30 flows, in parallel with the flow sensing valve 50. The variable restrictor valve 80 includes a spring 80a and a pressure receiving portion 80b. The spring 80a acts in a closing direction. The pressure receiving portion 80b receives the travel pilot pressure output from the shuttle valves 70a, 70b, and 70c and introduced thereto via the signal hydraulic line 71 and acts in an opening direction.
Shuttle valves 37a, 37b, and 37c constitute a travel detection unit that detects travelling operation in which traveling motors 3d and 3e are driven. The travel pilot pressure detected by the shuttle valves 70a, 70b, and 70c corresponds to an input amount (operating stroke) of the travel control lever unit 60d or 60e.
When the travel lever is operated from the neutral position to the fully operated position, the output pressure of the differential pressure reducing valve 51 (target LS differential pressure) is reduced by functioning of the variable restrictor valve 80 from a first specified value Pa4 to a second specified value Pa3. When the travel lever is in the neutral position, the output pressure of the differential pressure reducing valve 51 (target LS differential pressure) decreases from the first specified value Pa4 to Pa2 as the engine speed decreases from Nrate to Nmin. As the travel lever is operated with an increasing input amount, the output pressure of the differential pressure reducing valve 51 (target LS differential pressure) decreases at a ratio identical to the change in the input amount of the travel lever (travel pilot pressure) throughout the entire engine speed range. When the travel lever is fully operated, the output pressure of the differential pressure reducing valve 51 (target LS differential pressure) decreases from the second specified value Pa3 to Pa1 as the engine speed decreases from Nrate to Nmin. The arrangements in which the variable restrictor valve 80 is disposed in parallel with the flow sensing valve 50 and in which the opening area of the variable restrictor valve 80 increases continuously from the fully closed position to the maximum allow the output pressure of the differential pressure reducing valve 51 (target LS differential pressure), when the travel lever is fully operated, to decrease at the rate identical to the change in the input amount of the travel lever (travel pilot pressure) throughout the entire engine speed range from the maximum Nrate to the minimum Nmin (to state the foregoing differently, similarly decrease throughout the entire engine speed range). In
The travel lever of comparative example 1 has an opening area of Aamax at a spool stroke of Stmax when the travel lever is fully operated. Because comparative example 1 includes no variable restrictor valve 80, Aamax represents the opening area of the travel flow control valve capable of supplying the traveling motors 3d and 3e with the predetermined flow rate QT required for traveling when the output pressure of the differential pressure reducing valve 51 (target LS differential pressure) is the first specified value Pa4 (see
The travel lever of comparative example 2 has an opening area of Abmax at a spool stroke of Stmax when the travel lever is fully operated. Abmax represents the opening area of the travel flow control valve capable of supplying the traveling motors 3d and 3e with the predetermined flow rate QT required for traveling even when the output pressure of the differential pressure reducing valve 51 (target LS differential pressure) is decreased to the second specified value Pa3 (see
With the travel flow control valves 6d and 6e in the embodiment, the opening area at the spool stroke Stmax when the travel lever is fully operated is, as in comparative example 2, Abmax (which is large enough to obtain the predetermined flow rate QT required for traveling even when the output pressure of the differential pressure reducing valve 51 [target LS differential pressure] is decreased to the second specified value Pa3 [see
The expressions “opening area approximate to” or “opening area substantially identical to” in the first half of the spool stroke, as used herein, refers to a condition in which the opening area is identical to that in comparative example 1 or differs from that in comparative example 1 by 15% or less, but preferably by 10% or less. In addition, the opening area characteristic in the first half of the spool stroke may be defined as being different by 15% or less from a characteristic represented by a straight line connecting between an opening start and an opening area Aamax in the spool stroke range of ⅓ of the maximum stroke Stmax.
In
The upper swing structure 300 includes a cabin (operator chamber) 313. The cabin 313 includes an operator seat 121, left and right control lever units 122 and 123 for front implement/swing (
Operation of the embodiment will be described with reference to
(a) All control levers including the travel levers are in their neutral position:
When all control levers of the control lever units 60a, 60b, 60c, 60d, 60e . . . are in their neutral positions, the travel levers are also in the neutral position so that the travel pilot pressure detected by the shuttle valves 70a, 70b, and 70c is the tank pressure. For this reason, the tank pressure is introduced to the pressure receiving portion 80b of the variable restrictor valve 80, making the variable restrictor valve 80 maintained in the fully closed position by the spring 80a.
Because the variable restrictor valve 80 is in the fully closed position, the differential pressure reducing valve 51 of the engine speed sensing valve unit 13 outputs the absolute pressure Pa4 in accordance with the flow rate delivered from the pilot pump 30 (engine speed) as usual when the engine speed is the rated Nrate. The absolute pressure Pa4 is introduced to the pressure receiving portion 12d of the LS control valve 12b as the first specified value of the target LS differential pressure.
When all the control levers are in their neutral positions, all of the flow control valves 6a, 6b, 6c, 6d, 6e . . . are in their neutral positions as well. Thus, no hydraulic fluid is supplied to the actuators 3a, 3b, 3c, 3d, 3e . . . and the maximum load pressure detected by the shuttle valves 9a, 9b, 9c, 9d, 9e . . . is the tank pressure. The delivery pressure of the main pump 2 is consequently maintained at the minimum pressure corresponding to the set pressure of the unloading valve 15. Additionally, the output pressure of the differential pressure reducing valve 11 introduced to the pressure receiving portion 12e of the LS control valve 12b is the delivery pressure of the main pump 2 (pressure corresponding to the set pressure of the unloading valve 15) and the set pressure of the unloading valve 15 is higher than the output pressure of the differential pressure reducing valve 51 introduced to the pressure receiving portion 12e of the LS control valve 12b. Thus, the delivery flow rate of the main pump 2 is maintained at the minimum flow rate by the function of the LS control valve 12b.
(b) The travel levers are operated
(b1) When the travel levers are operated gradually from the neutral position to the fall stroke position
The following describes a case in which the control levers of the travel control lever units 60d and 60e are operated gradually from the neutral position to the full stroke position.
When the travel levers are operated gradually from the neutral position to the full stroke position, the travel pilot pressure is detected by the shuttle valves 70a, 70b, and 70c and introduced to the pressure receiving portion 80b of the variable restrictor valve 80. As described earlier with reference to
The reduced differential pressure across the flow sensing valve 50 causes the delivery pressure of the pilot pump 30 disposed upstream of the flow sensing valve 50 to be smaller by the amount of its reduction.
By contrast, when the control levers of the travel control lever units 60d and 60e are operated in the left direction shown in
A difference between the delivery pressure of the main pump 2 and the maximum load pressure is detected by the differential pressure reducing valve 11. The absolute pressure PLS that is the output pressure from the differential pressure reducing valve 11 is set in the pressure compensation valves 7a to 7e as the target compensation differential pressure. For these reasons, the differential pressure across each of the flow control valves 6d and 6e is controlled to be equal to the target LS differential pressure. As described earlier, the output pressure of the differential pressure reducing valve 51 (target LS differential pressure) gradually decreases from Pa4 (the first specified value) to Pa3 (the second specified value) as the travel pilot pressure increases. This causes the differential pressure across each of the flow control valves 6d and 6e to be decreased similarly.
(b2) When the travel levers are fully operated
When the travel levers are fully operated with the engine speed at the rated Nrate, the output pressure of the differential pressure reducing valve 51 (target LS differential pressure) decreases to the minimum pressure Pa3 (the second specified value) and the differential pressure across each of the flow control valves 6d and 6e is also reduced to the minimum pressure Pa3 (the second specified value).
As described earlier with reference to
As described above, even when the travel levers are fully operated and the differential pressure across each of the flow control valves 6d and 6e is reduced to the minimum pressure Pa3 (the second specified value), the flow control valves 6d and 6e are set to have a large opening area accordingly. Thus, the traveling motors 3d and 3e can be supplied with the predetermined flow rate QT required for traveling.
On top of that, the differential pressure across each of the travel flow control valves 6d and 6e is reduced to the minimum pressure Pa3 (the second specified value). This reduces internal loss of the flow control valves 6d and 6e so that energy loss during travelling operation is improved.
(b3) When the travel levers are returned from the fully operated position to the neutral position
In contrast to the case of (b1), the opening area of the variable restrictor valve 80 gradually decreases. Accordingly, when the engine speed is the rated Nrate, the output pressure of the differential pressure reducing valve 51 (target LS differential pressure) gradually increases from Pa3 (the second specified value) to Pa4 (the first specified value). The differential pressure across each of the flow control valves 6d and 6e increases similarly.
(b4) When the travel levers are operated in a stroke range over which the travel levers are operated halfway or less
When the travel levers are operated in the stroke range over which the travel levers are operated halfway or less with the engine speed at the rated Nrate, the output pressure of the differential pressure reducing valve 51 (target LS differential pressure) decreases from the maximum pressure Pa4 (the first specified value) in accordance with the lever input amount. The differential pressure across each of the flow control valves 6d and 6e decreases accordingly. Meanwhile, the travel flow control valves 6d and 6e are set to offer the opening area characteristic in such a manner that: in the spool stroke range corresponding to the stroke range over which the travel levers are operated halfway or less, that is, the first half of the spool stroke, the travel flow control valves 6d and 6e have an opening area approximate to the opening area of comparative example 1. The opening area of the flow control valves 6d and 6e is therefore smaller than in comparative example 2. When the travelling operation is performed by operating the travel levers in the stroke range over which the travel levers are operated halfway or less, the rate of flow from the main pump 2 to the traveling motors 3d and 3e is less affected by variations in the travel load and changes in the pump delivery pressure. Favorable travel operability can thus be achieved.
As described earlier, arrangements are made in which the variable restrictor valve 80 is disposed in parallel with the flow sensing valve 50 and the opening area of the variable restrictor valve 80 increases continuously from the fully closed position to the maximum. Thus, as described earlier with reference to
An exemplary type of operation performed in which the travel levers are operated in the stroke range over which the travel levers are operated halfway or less includes a finely operated downhill travelling operation. In a case where a hydraulic excavator is unloaded from the cargo deck of a truck or trailer for hauling a hydraulic excavator, two planks would be placed across one end of the cargo deck of the truck or trailer and the ground and the hydraulic excavator would be driven to move slowly along the planks to be unloaded from the cargo deck. In this operation, the operator would need to drive the hydraulic excavator slowly. In most cases the operator would reduce the engine speed to a range between the minimum (Nmin) and a medium speed, e.g., to low speed.
As described earlier with reference to
In comparative example 2, the output pressure of the differential pressure reducing valve 51 when the travel levers are fully operated changes as indicated by the dash-double-dot line in
In comparative example 2, in order to unload the hydraulic excavator from the cargo deck of the hydraulic excavator-hauling truck or trailer, the engine speed may be reduced to a speed that falls within the Nmin-to-medium speed range and the travel levers may then be operated. In this case, the opening area of the flow control valves 6d and 6e is greater than in comparative example 1 to be on the open side; moreover, the output pressure of the differential pressure reducing valve 51 (target LS differential pressure) is substantially identical to that when the travel levers are not operated as indicated by, for example, the low speed Na in
By contrast, in the present embodiment, as described with reference to
Additionally, in the present embodiment, the output pressure of the differential pressure reducing valve 51 when the travel levers are fully operated with the engine speed reduced to a speed that falls within the range between Nmin and medium speed, e.g., the low speed Na, is reduced at the rate identical to the change in the travel pilot pressure. If the travel levers are finely operated, the output pressure of the differential pressure reducing valve 51 is reduced according to the input amount of the travel levers.
The engine speed is reduced to a low speed that falls within the Nmin-to-medium speed range. The travel levers are then finely operated in order to unload the hydraulic excavator from the cargo deck of the hydraulic excavator-hauling truck or trailer. Therefore, the rate of flow supplied to the traveling motors 3d and 3e can be finely adjusted in accordance with the input amount of the travel levers. This eliminates the likelihood that an excessive travel speed unexpected by the operator will be produced, thus significantly improving the operability.
(c) Control levers other than those for travel are operated
When the control levers of the control lever units 60a, 60b, 60c . . . other than those for travel are operated, since the travel levers are placed in their neutral positions, the output pressure of the differential pressure reducing valve 51 of the engine speed sensing valve unit 13 is Pa4 (the first specified value). This Pa4 is introduced as the target LS differential pressure to the pressure receiving portion 12d of the LS control valve 12b when the engine speed is the rated Nrate as in the case of (a) described above.
When the control lever of the boom control lever unit 60b is operated in the left direction shown in
A condition in which the delivery flow rate of the main pump 2 is in short supply (saturation) can occur when a plurality of control levers is operated to intend combined operations for simultaneously driving a plurality of actuators in any operations other than causing the hydraulic excavator to travel, such as in combined operations of boom raising and arm crowding. In this case, the delivery pressure of the main pump 2 decreases to a level lower than the target LS differential pressure (Pa4) and the absolute pressure PLS as the output pressure of the differential pressure reducing valve 11 becomes lower than the target LS differential pressure (absolute pressure PLS<Pa4). Reductions in the target compensation differential pressures as a result of the foregoing reduction in the absolute pressure PLS occur in all pressure compensation valves relating to the combined operations (e.g., the boom pressure compensation valve 7b and the arm pressure compensation valve 7c). A flow rate ratio in keeping with an opening area ratio of a plurality of flow control valves (e.g., the boom flow control valve 6b and the arm flow control valve 6c) is thus maintained, which enables smooth combined operations in accordance with the ratios of the lever input amounts of the control lever units.
As described heretofore, in the present embodiment, the travel speed known in the art can be achieved during the travelling operation and energy efficiency can be improved by reducing energy loss. When the travel levers are operated in the stroke range over which the travel levers are operated halfway or less to perform the travelling operation, effects from variations in the travel load and changes in the pump delivery pressure can be reduced so that favorable travel operability can be achieved.
When the engine speed is reduced to a low speed to thereby perform fine operation in travel, the rate of flow supplied to the traveling motors 3d and 3e can be finely adjusted in accordance with the input amount of the travel levers. This eliminates a possible excessive travel speed unexpected by the operator, thus significantly improving travel operability.
Various changes in form and detail of the embodiment may be made therein without departing from the spirit and scope of the present invention. For example, in the embodiment, the output pressure of the differential pressure reducing valve 11 (the absolute pressure of the differential pressure between the main pump 2 delivery pressure and the maximum load pressure) is introduced to the pressure receiving portions 28a to 28e . . . of the pressure compensation valves 7a to 7e . . . . Alternatively, pressure receiving portions that face the pressure compensation valves 7a to 7e . . . may be provided and the main pump 2 delivery pressure and the maximum load pressure may be introduced individually to these pressure receiving portions to thereby set the target compensation differential pressure.
The embodiment has been described for a case in which the construction machine is a hydraulic excavator. The present invention can nonetheless be applied to any type of construction machine other than the hydraulic excavator (e.g., a hydraulic crane and a wheel type excavator) and can achieve the same advantageous effects as long as the construction machine includes a travel hydraulic motor.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-012665 | Jan 2013 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2013/080929 | 11/15/2013 | WO | 00 |
