Patent Application
20120031088
The present invention relates to hydraulic excavators and other construction machines in general and particularly to a hydraulic drive system for a construction machine which allows changes in the operational characteristics of a boom directional control valve.
A hydraulic excavator, a construction machine, typically comprises the following components: an undercarriage; an upper swing structure mounted swingably atop the undercarriage; a multi-joint front arm structure including a boom, an arm, and a bucket, the arm structure being attached to the upper swing structure in a vertically movable manner; and multiple hydraulic cylinders designed to actuate the boom, the arm, and the bucket. The hydraulic drive system of the excavator includes the following components: a hydraulic pump; multiple operating devices for controlling the operation (operational direction and speed) of the boom and the like; and multiple directional control valves for controlling the flow (flow direction and flow rate) of pressurized oil routed from the hydraulic pump to a hydraulic boom cylinder and the like in response to the operation of the operating devices. An open-center directional control valve includes a center bypass oil passage(s) and meter-in and meter-out oil passages, and the orifice areas of these oil passages determine the operational characteristics of the directional control valve, thereby also determining the operational performance of components to be actuated.
Thus far, a method has been proposed in which either of first and second boom directional control valves, both being open center valves but differing in operational characteristics, is selected (see Patent Document 1). The hydraulic drive system of Patent Document 1 includes the following components: a hydraulic pilot operating device; a solenoid switch valve placed on the pilot line of the operating device; and a manual switch for controlling the solenoid switch valve. When the operator turns the manual switch off, the solenoid switch valve is placed in a first switch position, allowing the operating device to output a spool-control pilot pressure to a pressure receiver of a first boom directional control valve. When, on the other hand, the operator turns the manual switch on, the solenoid switch valve is placed in a second switch position, allowing the operating device to output a spool-control pilot pressure to a pressure receiver of a second boom directional control valve. This allows selection of the operational performance suitable for the work at hand.
By using the technique of Patent Document 1, it would be possible that the orifice area of a center bypass oil passage of the first boom directional control valve is allowed to become larger than that of a meter-in oil passage of the first boom directional control valve when the spool of the first boom directional control valve is in the maximum position of a boom-lowering spool stroke and that the orifice area of a center bypass oil passage of the second boom directional control valve is allowed to become smaller than that of a meter-in oil passage of the second boom directional control valve (or the center bypass oil passage of the second boom directional control valve is allowed to close completely) when the spool of the second boom directional control valve is in the maximum position of a boom-lowering spool stroke. In that case, the operator can be allowed to turn the manual switch off to select the first boom directional control valve while the bucket is in the air without touching the ground at the time of lowering the boom, whereby the amount of oil supplied to the rod side of the hydraulic boom cylinder can be made relatively small. As a result, the own weight of the front arm structure helps to drive the hydraulic boom cylinder, thereby reducing the power required of the hydraulic pump. When, on the other hand, the bucket reaches the ground to start excavation at the time of lowering the boom, the operator can be allowed to turn the manual switch on to select the second boom directional control valve, so that the amount of oil supplied to the rod side of the hydraulic boom cylinder can be made relatively large. As a result, driving pressure (i.e., high hydraulic pressure) is generated on the rod side of the hydraulic boom cylinder, thereby allowing a powerful boom descending motion.
However, excavation requires repetitions of boom ascending and descending motions, forcing the bucket to repeatedly move from the ground into the air and vice versa. Thus, every time the boom is lowered, the operator is required to operate the manual switch right after the bucket has touched the ground (in other words, at the timing when the hydraulic boom cylinder requires driving pressure). This is not only bothersome to the operator but could lead to a decrease in labor efficiency.
An object of the present invention is thus to provide a hydraulic drive system for a construction machine which allows automatic changes in the operational characteristics of a boom directional control valve by judging whether or not a hydraulic boom cylinder needs driving pressure at the time of a boom-lowering operation.
(1) To achieve the above object, the invention provides a hydraulic drive system for a construction machine, the system comprising: a hydraulic pump; a hydraulic boom cylinder for actuating a boom; an operating device for controlling the operation of the boom; and a boom directional control valve for controlling the flow of pressurized oil routed from the hydraulic pump to the hydraulic boom cylinder in response to the operation of the operating device, the boom directional control valve being an open center valve, the system having characteristics that allow the orifice area of a center bypass oil passage of the boom directional control valve to become larger than the orifice area of a meter-in oil passage of the boom directional control valve when a spool of the boom directional control valve is in the middle position of a boom-lowering spool stroke and that allow the orifice area of the center bypass oil passage to become smaller than the orifice area of the meter-in oil passage or allow the center bypass oil passage to completely close when the spool is in the maximum stroke position of the boom-lowering spool stroke. The system further comprises: stroke limit varying means for selecting either the middle position or the maximum stroke position as the limit of a boom-lowering spool stroke of the boom directional control valve; pressure judging means for detecting or receiving an oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom and for judging whether or not the oil-feeding-side pressure is equal to or greater than a predetermined threshold value; and control means for controlling the stroke limit varying means such that the limit of the boom-lowering spool stroke of the boom directional control valve is set to the middle position when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is less than the threshold value and such that the limit of the boom-lowering spool stroke of the boom directional control valve is set to the maximum stroke position when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is equal to or greater than the threshold value.
(2) In the above hydraulic drive system (1), the stroke limit varying means preferably includes: a first pilot oil passage for outputting a spool-control pilot pressure generated based on a boom-lowering operation by the operating device to a pressure receiver of the boom directional control valve without any change to the spool-control pilot pressure; a second pilot oil passage for reducing, with the use of a pressure-reducing valve, a spool-control pilot pressure generated based on a boom-lowering operation by the operating device and then outputting the reduced pressure to the pressure receiver of the boom directional control valve; and pilot-oil-passage selecting means for selecting either the first pilot oil passage or the second pilot oil passage. Preferably, the control means controls the pilot-oil-passage selecting means such that the second pilot oil passage is selected when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is less than the threshold value and such that the first pilot oil passage is selected when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is equal to or greater than the threshold value.
(3) In the above hydraulic drive system (1), the stroke limit varying means preferably includes: a pilot oil passage for outputting a spool-control pilot pressure generated based on a boom-lowering operation by the operating device to a pressure receiver of the boom directional control valve; and a variable pressure-reducing valve, located on the pilot oil passage, for limiting the maximum value of the spool-control pilot pressure in a variable manner. Preferably, the control means controls a limit value set for the variable pressure-reducing valve such that the limit value becomes a predetermined first limit value when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is less than the threshold value and such that the limit value becomes a predetermined second limit value larger than the first limit value when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is equal to or greater than the threshold value.
(4) To achieve the above object, the invention also provides a hydraulic drive system for a construction machine, the system comprising: a hydraulic pump; a hydraulic boom cylinder for actuating a boom; an operating device for controlling the operation of the boom; and a first boom directional control valve for controlling the flow of pressurized oil routed from the hydraulic pump to the hydraulic boom cylinder in response to the operation of the operating device, the first boom directional control valve being an open center valve, the system having characteristics that allow the orifice area of a center bypass oil passage of the first boom directional control valve to become larger than the orifice area of a meter-in oil passage of the first boom directional control valve when a spool of the first boom directional control valve is in the middle position of a boom-lowering spool stroke and that allow the orifice area of the center bypass oil passage to become smaller than the orifice area of the meter-in oil passage or allow the center bypass oil passage to completely close when the spool is in the maximum stroke position of the boom-lowering spool stroke. The system further comprises: a second boom directional control valve, the second boom directional control valve being an open center valve, the orifice area of a center bypass oil passage of the second boom directional control valve being larger than the orifice area of a meter-in oil passage of the second boom directional control valve when a spool of the second boom directional control valve is in the middle position and the maximum stroke position of a boom-lowering spool stroke; directional-control-valve selecting means for selecting either the first boom directional control valve or the second boom directional control valve and actuating the selected boom directional control valve in response to the operation of the operating device; pressure judging means for detecting or receiving an oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom and for judging whether or not the oil-feeding-side pressure is equal to or greater than a predetermined threshold value; and control means for controlling the directional-control-valve selecting means such that the second boom directional control valve is selected when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is less than the threshold value and such that the first boom directional control valve is selected when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is equal to or greater than the threshold value.
(5) In the above hydraulic drive system (4), the directional-control-valve selecting means preferably includes: a first pilot oil passage for outputting a spool-control pilot pressure generated based on a boom-lowering operation by the operating device to a pressure receiver of the first boom directional control valve; a second pilot oil passage for outputting a spool-control pilot pressure generated based on a boom-lowering operation by the operating device to a pressure receiver of the second boom directional control valve; and pilot-oil-passage selecting means for selecting either the first pilot oil passage or the second pilot oil passage. Preferably, the control means controls the pilot-oil-passage selecting means such that the second pilot oil passage is selected when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is less than the threshold value and such that the first pilot oil passage is selected when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is equal to or greater than the threshold value.
In accordance with the invention, it is possible to automatically change the operational characteristics of a boom directional control valve by judging whether or not a hydraulic boom cylinder needs driving pressure at the time of a boom-lowering operation.
Embodiments of the present invention will now be described with reference to the accompanying drawings.
The hydraulic excavator of
The undercarriage 2 includes the following components: a substantially H-shaped track frame 9; right and left drive wheels 10 attached rotatably to the right and left rear sides of the track frame 9; right and left hydraulic travel motors 11 for driving the right and left drive wheels 10, respectively; and right and left follower wheels 12 (idler wheels) attached rotatably to the right and left front sides of the track frame 9 and driven by the drive force transmitted from the drive wheels 10 via the trackbelts 1.
Attached to the front side of the track frame 9 is a soil-removal blade 13 which is vertically moved by a hydraulic blade cylinder 14. Between a central portion of the track frame 9 and the swing frame 4 is a rotary wheel, not illustrated. Radially inside this rotary wheel is a hydraulic swing motor 15 which is designed to rotate the swing frame 4 relative to the track frame 9.
The horizontal movement of the swing post 5 relative to the swing frame 4 is achieved by a vertical pin, not illustrated, and by a hydraulic swing cylinder 16. The horizontal movement of the swing post 5 causes the front arm structure 6 to swing rightward or leftward.
The front arm structure 6 includes the following components: a boom 17 attached movably to the swing post 5; an arm 18 attached movably to the distal end of the boom 17; and a bucket 19 attached movably to the distal end of the arm 18. The boom 17, the arm 18, and the bucket 19 are actuated by a hydraulic boom cylinder 20, a hydraulic arm cylinder 21, and a hydraulic bucket cylinder 22, respectively. Note that the bucket 19 can be replaced by an optional attachment (e.g., a crusher).
The cab 7 is provided with a cab seat 23 on which the operator is seated. Located in front of the seat 23 are right and left travel levers 24 which are operable with hands or feet and designed to actuate the right and left hydraulic travel motors 11, respectively, so as to move the hydraulic excavator forward or backward. Located to the left of the left travel lever 24 (at the bottom left section of the cab 7) is an attachment control pedal, not illustrated, for controlling a hydraulic attachment actuator. Located to the right of the right travel lever 24 (at the bottom right section of the cab 7) is a swing control pedal, not illustrated, for actuating the hydraulic swing cylinder 16 to swing rightward or leftward the swing post 5 (that is, the entire front arm structure 6).
Located on the left side of the seat 23 are the following components: a crosswise-movable swing/arm control lever 25 for actuating the hydraulic swing motor 15 to swing the upper swing structure 3 right or left when the lever 25 is moved right or left and for actuating the hydraulic arm cylinder 21 to cause the arm 18 to perform a dump or crowd operation when the lever 25 is moved forward or backward; and a lock lever 27, provided as an anti-false operation lever, for blocking the supply of source pressure from a pilot pump 26 (see
The above-mentioned right and left trackbelts 1, upper swing structure 3, swing post 5, blade 13, boom 17, arm 18, and bucket 19 are those components driven by a hydraulic drive system installed in the hydraulic excavator.
The hydraulic drive system of
The operating device 30 includes a pair of pressure reducing valves 34a and 34b for generating a spool-control pilot pressure (a second pilot pressure) by reducing a first pilot pressure supplied from the pilot pump 26 based on how much forward or backward the lever 28 has been moved. When the lever 28 is moved backward (toward the left side of
The boom directional control valve 31 includes the following components: a center bypass oil passage A; meter-in oil passages B1 and B2 (oil-feeding passages); and meter-out oil passages C1 and C2 (oil-return passages). These oil passages A, B1, B2, C1, and C2 can change their orifice areas based on the stroke amount of the spool of the boom directional control valve 31. When the spool is in its neutral position, the center bypass oil passage A opens fully whereas the meter-in oil passages and the meter-out oil passages close completely. In this case, the pressurized oil supplied from the hydraulic pump 29 is not routed to the hydraulic boom cylinder 20 but returned to a tank. When the spool moves in the boom-raising direction, the meter-in oil passage B1, designed to supply the pressurized oil from the hydraulic pump 29 to the bottom side of the hydraulic boom cylinder 20, and the meter-out oil passage C1, designed to return the oil from the rod side of the hydraulic boom cylinder 20 to the tank, increase in orifice area in response to the stroke amount of the spool. At the same time, the center bypass oil passage A decreases in orifice area; it closes completely at the maximum stroke position. This allows oil the amount of which is proportional to the stroke amount to be supplied to the bottom side of the hydraulic boom cylinder 20, causing the hydraulic boom cylinder 20 to expand. As a result, the boom 17 is raised.
In contrast, when the spool moves in the boom-lowering direction, the meter-in oil passage B2, designed to supply the pressurized oil from the hydraulic pump 29 to the rod side of the hydraulic boom cylinder 20, and the meter-out oil passage C2, designed to return the oil from the bottom side of the hydraulic boom cylinder 20 to the tank, increase in orifice area in response to the stroke amount of the spool. At the same time, the center bypass oil passage A decreases in orifice area. This allows oil the amount of which is proportional to the stroke amount to be supplied to the rod side of the hydraulic boom cylinder 20, causing the hydraulic boom cylinder 20 to contract. As a result, the boom 17 is lowered. Note that Embodiment 1 is designed not to completely close the center bypass oil passage A when the spool is placed in the maximum stroke position in the boom-lowering direction but allows it to partially open. This prevents the descending motion of the boom 17 from becoming much faster than the ascending motion of the boom 17 due to the area difference between the rod side and bottom side of the hydraulic boom cylinder 20.
As illustrated in
With reference again to
The hydraulic drive system of
When the rod-side pressure is less than the threshold value (i.e., when driving pressure is not necessary), the controller 42 does not output a drive signal to the solenoid of the solenoid switch valve 40, placing the solenoid switch valve 40 in the right-side switch position of
When, on the other hand, the rod-side pressure is equal to or greater than the threshold value (i.e., when driving pressure is necessary), the controller 42 outputs the drive signal to the solenoid of the solenoid switch valve 40, placing the solenoid switch valve 40 in the left-side switch position of
The operation of the hydraulic drive system of Embodiment 1 will now be described with reference to
After the operator moves the lever 28 furthest forward (at time t1) to lower the boom 14 for excavation or the like, the solenoid switch valve 40 selects the pilot oil passage 38b because the rod-side pressure of the hydraulic boom cylinder 20 stays smaller than the threshold value while the bucket 19 is in the air without touching the ground (from time t1 to time t2). In other words, a limit is placed on the spool-control pilot pressure so that the limit of the boom-lowering spool stroke of the boom direction control valve 31 can be set to the middle position L1. This reduces the amount of oil supplied to the rod side of the hydraulic boom cylinder 20, keeping the rod-side pressure low. As a result, the own weight of the front arm structure 6 helps to drive the hydraulic boom cylinder 20, thereby reducing the power required of the hydraulic pump 29.
After the bucket 19 touches the ground to start excavation or the like (after time t2), the rod-side pressure of the hydraulic boom cylinder 20 starts to increase. When the rod-side pressure of the hydraulic boom cylinder 20 reaches the threshold value, the controller 42 outputs the drive signal, allowing the solenoid switch valve 40 to select the pilot oil passage 38a. In other words, no limit is placed on the spool-control pilot pressure, and the limit of the boom-lowering spool stroke of the boom direction control valve 31 is set to the maximum stroke position L2. This increases the amount of oil supplied to the rod side of the hydraulic boom cylinder 20, increasing the rod-side pressure further. As a result, driving pressure is generated on the rod side of the hydraulic boom cylinder 20, thereby allowing a powerful boom descending motion.
As above, Embodiment 1 of the present invention makes it possible to automatically change the operational characteristics of the boom directional control valve 31 by judging whether or not the hydraulic boom cylinder 20 needs driving pressure at the time of lowering the boom 17. This is not bothersome to the operator and leads to high labor efficiency, compared with when the operator has to do the above with the use of a manual switch as in Patent Document 1.
As stated above, Embodiment 1 is designed such that the judgment of whether or not the hydraulic boom cylinder 20 needs driving pressure at the time of lowering the boom 17 is made through the examination of whether or not the rod-side pressure of the hydraulic boom cylinder 20 is equal to or greater than the predetermined threshold value. On the other hand, the above judgment may instead be made by, for example, examining whether or not the bottom-side pressure of the hydraulic boom cylinder 20 (i.e., the oil-exhaust-side pressure at the time of lowering the boom 17) is less than a predetermined threshold value. This method, however, leaves room for improvement as discussed below. The bottom-side pressure (back pressure) of the hydraulic boom cylinder 20 at the time of lowering the boom 17 increases in proportion to the operational speed of the hydraulic boom cylinder 20 (i.e., the speed of a descending motion of the boom 17). Assume now that an excavation is judged to have started when the bottom-side pressure of the hydraulic boom cylinder 20 has become less than the threshold value, and the controller 42 then changes the switch position of the solenoid switch valve 40 to set the limit of the boom-lowering spool stroke of the boom directional control valve 31 to the maximum stroke position L2 so that a powerful boom descending motion can be achieved. Even so, the bottom-side pressure of the hydraulic boom cylinder 20 will exceed the threshold value when the speed of the descending motion of the boom 17 exceeds a given value during subsequent excavations. Thus, it is likely that the controller 42 may change the switch position of the solenoid switch valve 40 to set the limit of the boom-lowering spool stroke of the boom directional control valve 31 to the middle position L1 even when the hydraulic boom cylinder 20 does need driving pressure. Consequently, a limit is placed on the speed of the descending motion of the boom 17. In contrast, Embodiment 1 is designed such that the judgment of whether or not the hydraulic boom cylinder 20 needs driving pressure at the time of lowering the boom 17 is made through the examination of whether or not the rod-side pressure of the hydraulic boom cylinder 20 is equal to or greater than the threshold value. Thus, there is no need to limit the speed of the descending motion of the boom 17. Accordingly, a powerful boom descending motion can be achieved, irrespective of the operational speed of the boom 17.
As stated above, the hydraulic drive system of Embodiment 1 includes the solenoid switch valve 40 for selecting either of the pilot oil passages 38a and 38b, the pressure sensor 41 for detecting the rod-side pressure of the hydraulic boom cylinder 20, and the controller 42 for outputting the drive signal to the solenoid of the solenoid switch valve 40 when the rod-side pressure is equal to or greater than the threshold value. Note, however, that the invention is not limited to such an electrical configuration. For instance, as in the modification of
As another modification (not illustrated), it is also possible for the hydraulic drive system not to have the control valve 44 and instead route the rod-side pressure of the hydraulic boom cylinder 20 to a pressure receiver of a switch valve 43A and set a threshold value for the rod-side pressure using the spring of the switch valve 43A. When the rod-side pressure is less than the threshold value, the switch valve 43A is placed in a first switch position (same as the right-side switch position of the switch valve 43 of
As also stated above, the hydraulic drive system of Embodiment 1 includes the pilot oil passages 38a and 38b and the solenoid switch valve 40 for selecting either of the pilot oil passages 38a and 38b as stoke limit varying means for setting the limit of the boom-lowering spool stroke of the boom directional control valve 31 to either of the middle position L1 and the maximum stroke position L2. The invention is of course not limited to this configuration but can be modified in various forms without departing from the technical scope of the invention. For instance, when the invention is applied to a hydraulic excavator which includes an operating device having an electrical lever (i.e., an operating device for outputting an electric control signal based on how much its lever is moved), a controller may be provided in order to either limit or not limit the electrical control signal output from the operating device. This modification as well leads to the same advantages of Embodiment 1.
Embodiment 2 of the present invention will now be described with reference to
The hydraulic drive system of Embodiment 2 includes the following components: a pilot oil passage 45 for routing the spool-control pilot pressure generated by the pressure reducing valve 34b of the operating device 30 to the pressure receiver 36b of the boom directional control valve 31; and a solenoid-driven variable pressure-reducing valve 46, placed on the pilot oil passage 45, for limiting the maximum value of the spool-control pilot pressure in a variable manner.
Similar to Embodiment 1, the hydraulic drive system of Embodiment 2 also includes the pressure sensor 41 and the controller 42. The pressure sensor 41 detects the rod-side pressure of the hydraulic boom cylinder 20. The controller 42 examines whether or not the rod-side pressure of the hydraulic boom cylinder 20 detected by the pressure sensor 41 is equal to or greater than the predetermined threshold value, thereby judging whether or not the hydraulic boom cylinder 20 needs driving pressure upon lowering the boom 17. Based on that judgment, the controller 42 controls the variable pressure-reducing valve 46.
When the rod-side pressure is less than the threshold value (i.e., when driving pressure is not necessary), the controller 42 does not output a drive signal to the solenoid of the variable pressure-reducing valve 46. Thus, a limit value for the variable pressure-reducing valve 46 is set to a predetermined first limit value by the spring. This limits the maximum of the spool-control pilot pressure generated by the pressure reducing valve 34b of the operating device 30 to the first limit value. The limited spool-control pilot pressure is then output to the pressure receiver 36 of the boom directional control valve 31. As a result, the limit of the boom-lowering spool stroke of the boom directional control valve 31 is set to the middle position L1 of
When, on the other hand, the rod-side pressure is equal to or greater than the threshold value (i.e., when driving pressure is necessary), the controller 42 outputs the drive signal to the solenoid of the variable pressure-reducing valve 46, thereby setting the limit value for the variable pressure-reducing valve 46 to a predetermined second limit value which is larger than the first limit value. This limits the maximum of the spool-control pilot pressure generated by the pressure reducing valve 34b of the operating device 30 to the second limit value. The limited spool-control pilot pressure is then output to the pressure receiver 36b of the boom directional control valve 31 (normally, the spool-control pilot pressure generated by the pressure reducing valve 34b of the operating device 30 is output to the pressure receiver 36b without any change to the pressure). As a result, the limit of the boom-lowering spool stroke of the boom directional control valve 31 is set to the maximum stroke position L2 of
Similar to Embodiment 1, Embodiment 2 of the invention also makes it possible to automatically change the operational characteristics of the boom directional control valve 31 by judging whether or not the hydraulic boom cylinder 20 needs driving pressure at the time of lowering the boom 17. This is not bothersome to the operator and leads to high labor efficiency, compared with when the operator has to do the above with the use of a manual switch as in Patent Document 1.
As stated above, the hydraulic drive system of Embodiment 2 includes the solenoid-driven variable pressure-reducing valve 46 placed on the pilot oil passage 45; the pressure sensor 41 for detecting the rod-side pressure of the hydraulic boom cylinder 20; and the controller 42 for outputting the drive signal to the solenoid of the variable pressure-reducing valve 46 when the rod-side pressure is equal to or greater than the threshold value. Note, however, that the invention is not limited to such an electrical configuration. For example, as in the modification of
Embodiment 3 of the present invention will now be described with reference to
The hydraulic drive system of Embodiment 3 includes the boom directional control valve 31 (open center valve) and a boom directional control valve 48 (open center valve) that differs from the boom directional control valve 31 in operational characteristics. The swing directional control valve 32, the boom directional control valves 31 and 48, and the bucket directional control valve 33 are connected in series in this order.
The boom directional control valve 48 includes the following components: a center bypass oil passage D; meter-in oil passages E1 and E2 (oil-feeding passages); and meter-out oil passages F1 and F2 (oil-return passages). These oil passages D, E1, E2, F1, and F2 can change their orifice areas based on the stroke amount of the spool of the boom directional control valve 48. When the spool is in its neutral position, the center bypass oil passage D opens fully whereas the meter-in oil passages and the meter-out oil passages close completely. When the spool moves in the downward direction of
In contrast, when the spool moves in the upward direction of
When the lever 28 is moved backward (toward the left side of
The pilot circuit 50 includes the following components: a pilot oil passage 51a for routing the spool-control pilot pressure generated by the pressure reducing valve 34b of the operating device 30 to the pressure receiver 36b of the boom directional control valve 31; a pilot oil passage 51b for routing the spool-control pilot pressure generated by the pressure reducing valve 34b of the operating device 30 to the pressure receiver 49b of the boom directional control valve 48; and a solenoid switch valve 52 for selecting either of the pilot oil passages 51a and 51b.
As in Embodiments 1 and 2, the hydraulic drive system of Embodiment 3 also includes the pressure sensor 41 and the controller 42. The pressure sensor 41 detects the rod-side pressure of the hydraulic boom cylinder 20. The controller 42 examines whether or not the rod-side pressure of the hydraulic boom cylinder 20 detected by the pressure sensor 41 is equal to or greater than the predetermined threshold value, thereby judging whether or not the hydraulic boom cylinder 20 needs driving pressure upon lowering the boom 17. Based on that judgment, the controller 42 controls the switch valve 52.
When the rod-side pressure is less than the threshold value (i.e., when driving pressure is not necessary), the controller 42 does not output a drive signal to the solenoid of the solenoid switch valve 52, placing the solenoid switch valve 52 in the right-side switch position of
When, on the other hand, the rod-side pressure is equal to or greater than the threshold value (i.e., when driving pressure is necessary), the controller 42 outputs the drive signal to the solenoid of the solenoid switch valve 52, placing the solenoid switch valve 52 in the left-side switch position of
Similar to Embodiments 1 and 2, Embodiment 3 of the invention also makes it possible to automatically change the operational characteristics of the boom directional control valves by judging whether or not the hydraulic boom cylinder 20 needs driving pressure at the time of lowering the boom 17. This is not bothersome to the operator and leads to high labor efficiency, compared with when the operator has to do the above with the use of a manual switch as in Patent Document 1.
As stated above, the hydraulic drive system of Embodiment 3 includes the solenoid switch valve 52 for selecting either of the pilot oil passages 51a and 51b, the pressure sensor 41 for detecting the rod-side pressure of the hydraulic boom cylinder 20, and the controller 42 for outputting the drive signal to the solenoid of the solenoid switch valve 52 when the rod-side pressure is equal to or greater than the threshold value. Note, however, that the invention is not limited to such an electrical configuration. For instance, the solenoid switch valve 52 can be replaced by a hydraulic pilot switch valve (not illustrated), and the pressure sensor 41 and the controller 42 by a hydraulic pilot control valve (not illustrated) for outputting a hydraulic pressure signal to a pressure receiver of that switch valve. The control valve can include a pressure receiver for receiving the rod-side pressure of the hydraulic boom cylinder 20 and a spring for setting a threshold value for the rod-side pressure. When the rod-side pressure is less than the threshold value, the control valve is placed in a first switch position, allowing the pressure receiver of the switch valve to communicate with the tank (that is, the hydraulic pressure received by the pressure receiver of the switch valve becomes the tank pressure, thus becoming smaller). As a result, the switch valve is placed in a first switch position to select the pilot oil passage 51b. When, on the other hand, the rod-side pressure is equal to or greater than the threshold value, the control valve is placed in a second switch position, allowing the pressure receiver of the switch valve to communicate with the pilot pump 26 (that is, the hydraulic pressure received by the pressure receiver of the switch valve becomes the pump pressure, thus becoming larger). As a result, the switch valve is placed in a second switch position to select the pilot oil passage 51a. The above modification also leads to the same advantages of Embodiment 3.
As another modification (not illustrated), it is also possible for the hydraulic drive system not to have the control valve and instead route the rod-side pressure of the hydraulic boom cylinder 20 to a pressure receiver of a switch valve and set a threshold value for the rod-side pressure using the spring of the switch valve. When the rod-side pressure is less than the threshold value, the switch valve is placed in a first switch position, thereby selecting the pilot oil passage 51b. When, on the other hand, the rod-side pressure is equal to or greater than the threshold value, the switch valve is placed in a second switch position, thereby selecting the pilot oil passage 51a. This modification also leads to the same advantages of Embodiment 3.
As also stated above, the hydraulic drive system of Embodiment 3 includes the pilot oil passages 51a and 51b and the solenoid switch valve 52 for selecting either of the pilot oil passages 51a and 51b as directional-control-valve selecting means for selecting either of the boom directional control valves 31 and 48. The invention is of course not limited to this configuration but can be modified in various forms without departing from the technical scope of the invention. For instance, when the invention is applied to a hydraulic excavator which includes an operating device having an electrical lever, a controller may be provided in order to select the destinations of the electrical control signal. This modification as well leads to the same advantages of Embodiment 3.
We have also stated that, in all the foregoing embodiments 1 to 3 and modifications, the center bypass oil passage of the boom directional control valve 31 is allowed to completely close when its spool is in the maximum position of a boom-raising stroke and to partially open when the spool is in the maximum position of a boom-lowering stroke. The invention is not limited to the above, however. The center bypass oil passage may instead close completely also when the spool is in the maximum position of a boom-lowering stroke. This also leads to the same advantages of the invention.
It should also be noted that the invention is not limited to the above-described examples in which the invention is applied to a small-sized hydraulic excavator.
The invention is of course applicable to medium- or large-sized hydraulic excavators and to other construction machines as well.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-263172 | Nov 2009 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP10/65872 | 9/14/2010 | WO | 00 | 5/3/2011 |
