Power plants – Pressure fluid source and motor – Unsafeness – unreadiness or disarray prevent manual change or...
Reexamination Certificate
2001-09-18
2003-03-04
Lopez, F. Daniel (Department: 3745)
Power plants
Pressure fluid source and motor
Unsafeness, unreadiness or disarray prevent manual change or...
C091S446000
Reexamination Certificate
active
06526747
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a hydraulic drive system for a construction machine, such as a hydraulic excavator, in which a delivery pressure of a hydraulic pump is held higher than a maximum load pressure of a plurality of actuators by a target differential pressure under load sensing control, and differential pressures across a plurality of directional control valves are controlled by respective associated pressure compensating valves. More particularly, the present invention relates to a hydraulic drive system including a safety device to lock an actuator when it is in an inoperative condition while an engine is being driven, thereby preventing a malfunction.
BACKGROUND ART
A construction machine, such as a hydraulic excavator, includes a safety device for making an actuator immobile even with a control lever manipulated, thereby preventing the machine from malfunctioning, when an operator is not boarded on the machine while an engine is being driven, or when an operator is boarded on the machine, but no work is carried out. When a directional control valve has a pilot-operated spool, a safety device is generally constructed such that a pilot lock switching valve is provided between a pilot pump and a pilot valve of a control lever device, and by shifting the pilot lock switching valve, supply of a hydraulic fluid to the pilot valve of the control lever device is cut off to make the directional control valve locked. One example of that type of the pilot lock switching valve is disclosed in, e.g., Japanese Patent No. 2567720.
Also, as a hydraulic pump control system, there is known the so-called load sensing system (hereinafter referred to also as the “LS system”) in which a delivery pressure of a hydraulic pump is held higher than a maximum load pressure of a plurality of actuators by a target differential pressure. Usually, in the LS system, differential pressures across a plurality of directional control valves are controlled by respective associated pressure compensating valves so that a hydraulic fluid can be supplied at a ratio depending on opening areas of the directional control valves regardless of the magnitudes of load pressures during the combined operation in which a plurality of actuators are driven at the same time. Hydraulic drive systems including LS systems are disclosed in, e.g., JP,A 60-11706 and JP,A 10-196604. In such a hydraulic drive system including an LS system, when a directional control valve has a pilot-operated spool, it is also general that a pilot lock switching valve similar to the above-mentioned one is provided as a safety device.
DISCLOSURE OF INVENTION
As described above, a conventional safety device (pilot lock switching valve) for a hydraulic drive system is based on an assumption of a directional control valve being pilot-shifted, and is constructed so as to cut off supply of a hydraulic fluid to a pilot valve of a control lever device, whereby the directional control valve is locked to make an associated actuator locked. However, the directional control valve is not limited to the pilot-shifted one, but may be mechanically shifted by transmitting a motion of a control lever directly to a spool for operating it.
For example, in many of small-sized hydraulic excavators having small swing bodies, such as mini-shovels, a directional control valve for travel is mechanically shifted. Also, in hydraulic excavators, a bucket is usually mounted as a front attachment of a front operating mechanism. With increasing versatility of work, however, it is now general that the bucket is replaceable by another front attachment such as a crusher. In many of such cases, a directional control valve associated with a front attachment other than the bucket is also designed as a mechanically shifted valve. Further, the directional control valve associated with the front attachment other than the bucket is either assembled in a valve unit beforehand or retrofitted to the valve unit.
Thus, when the hydraulic drive system includes a mechanically shifted directional control valve, or when a mechanically operated directional control valve is retrofitted to the hydraulic drive system, the conventional safety device cannot lock the directional control valve and hence cannot make the associated actuator locked.
Another conceivable solution for locking a mechanically shifted directional control valve is to fix a control lever mechanically, but this solution would entail a complicated mechanism.
An object of the present invention is to provide a hydraulic drive system including pressure compensating valves controlled by an LS system, in which an actuator can be locked with a simple construction and can be prevented from malfunctioning in an inoperative condition while an engine is being driven, even when the hydraulic drive system includes a mechanically shifted directional control valve, or even when a mechanically shifted directional control valve is retrofitted to the hydraulic drive system.
(1) To achieve the above object, according to the present invention, there is provided a hydraulic drive system comprising a variable displacement hydraulic pump, a plurality of actuators driven by a hydraulic fluid delivered from the hydraulic pump, a plurality of directional control valves for controlling respective flow rates of the hydraulic fluid supplied from the hydraulic pump to the plurality of actuators, a plurality of pressure compensating valves for controlling respective differential pressures across the plurality of directional control valves, and pump control means for performing load sensing control to hold a delivery pressure of the hydraulic pump higher than a maximum load pressure of the plurality of actuators by a target differential pressure, the plurality of pressure compensating valves including a first pressure compensating valve provided in association with a particular one of the plurality of directional control valves and a second pressure compensating valve provided in association with the other directional control valve than the particular one, wherein the hydraulic drive system further comprises a first lock switching valve having first and second shift positions and outputting a pressure of a hydraulic supply source when the first lock switching valve is shifted from the first position to the second position; and a first pressure receiving section provided at an end of the first pressure compensating valve on the si de acting in the closing direction, and connected to the output side of the first lock switching valve, the first pressure compensating valve being fully closed when the first lock switching valve is shifted to the second position and the pressure of the hydraulic supply source is introduced to the first pressure receiving section.
Thus, the first lock switching valve is provided, the first pressure receiving section is provided in the first pressure compensating valve to be connected to the output side of the first lock switching valve, and the pressure of the hydraulic supply source is introduced to the first pressure receiving section when the first lock switching valve is shift ed to the second position, thereby fully closing the first pressure compensating valve. With such an arrangement, even when the particular directional control valve is a mechanically shifted valve, the actuator associated with the particular directional control valve can be locked and hence prevented from malfunctioning in an inoperative condition while an engine is being driven. Also, since the first pressure receiving section can be provided by utilizing a pressure receiving section that is originally provided in an ordinary pressure compensating valve for a drain passage, the actuator can be locked with a simple construction. Moreover, since a main passage for supplying the hydraulic fluid to the actuator therethrough is cut off by the first pressure compensating valve, the actuator can be reliably locked.
Further, even when a mechanically shifted directional control valve for a front attachment is added to employ an additional attach
Hamamoto Satoshi
Kanai Takashi
Kawamoto Junya
Nagao Yukiaki
Nakatani Kenichiro
Hitachi Construction Machinery Co. Ltd.
Lopez F. Daniel
Mattingly Stanger & Malur, P.C.
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