This invention relates to a hydraulic device for use in construction machinery.
A typical hydraulic shovel is comprised of an arm, a boom, a hydraulic cylinder for driving a bucket, an actuator such as a hydraulic motor, and an operating lever for an operator to control the above, wherein hydraulic fluid is .supplied to the actuator in accordance with the operation of the operating lever to thereby drive the actuator. In such devices, a directional control valve is usually installed between the actuator and the hydraulic pump as means for changing the operating direction of the actuator. This directional control valve is switched, thereby switching the direction of the flow of hydraulic fluid through the actuator, in accordance with the direction of operation of the operating lever.
In these kinds of hydraulic devices, the employment of a so-called load sensing system is commonly known. This system works such that the supply of hydraulic fluid to the actuator is in accordance with the degree of operation of the operating lever regardless of the load on the actuator thereby achieving a actuator drive speed which corresponds to the degree of operation of the operating lever.
In this load sensing system, the position of the spool of the directional control valve is typically changed in proportion to the degree of operation of the operating lever, such that the opening area of the directional control valve is changed in proportion to the degree of operation of the operating lever. Furthermore, a pressure compensator comprising for example a pressure compensator valve is also provided such that the pressure difference between hydraulic pressure on the exit port side of the directional valve which is linked to the inlet chamber of the actuator, (equivalent to the load pressure on the actuator) and the hydraulic pressure on the inlet port side of the directional control valve is controlled to a predetermined value, thereby ensuring that the amount of flow of fluid through the actuator is in proportion to the opening area and therefore in proportion to the degree of operation of the operating lever.
By using this kind of load sensing system, even if the load on the actuator changes during operation, a flow of hydraulic fluid corresponding to the degree of operation of the operating lever is supplied to the actuator, therefore making it possible to achieve an actuator drive speed corresponding to the degree of operation of the operating lever and hence the controllability of the actuator is enhanced.
However, in these kinds of hydraulic devices for construction machinery, the hydraulic fluid on the exit side of the actuator flows to the fluid tank through an exit passage formed in the directional control valve. In this exit passage there is formed a restriction whose restriction area changes in accordance with the position oil said spool, in other words, in proportion to the degree of operation of the operating lever. As a consequence, in the case where for example the degree of operation of the operating lever is held constant and the drive speed of the actuator is maintained constant, and if the change in load on the actuator is only slight or if the change in load over time is smooth, then the amount of fluid flowing from the actuator is almost identical to the amount of fluid flowing to the actuator, which is in proportion to the degree of control of the operating lever, and a stable actuator drive speed can be achieved.
However, as a result of the nature of the work of these kinds of construction machinery, there are many cases of the load on the actuator suddenly changing. For example a load acting in the opposite direction to that of the direction of drive of the actuator may suddenly change to act in the same direction as the direction of drive of the actuator. In these kinds of cases, irrespective of the amount of flow of fluid flowing to the actuator, (which is in proportion to the degree of control of the operating lever), a force works to increase the drive speed of the actuator and if the drive speed continues to increase the fluid pressure in the exit chamber of the actuator decreases. It is well known that if the fluid pressure in the exit chamber of the actuator falls below a critical pressure, cavitation occurs and causes erosion of the actuator etc. and furthermore that under the kind of conditions where cavitation occurs, the operation of the actuator can easily become unstable.
As mentioned above, in existing hydraulic devices for construction machines, the restriction area simply changes in accordance with the degree of operation of the operating lever. Thus when said restriction area is relatively large corresponding to a relatively large degree of operation of the operating lever, and the drive speed of the actuator starts to increase as a result of a change in load on the actuator as mentioned above, since the resistance to the flow of fluid from the actuator is small, the amount of fluid flowing from the actuator increases with a consequent increase in the drive speed of the actuator. As a result, these devices were prone to cavitation and to the operation of the actuator often became unstable.
In an attempt to resolve these problems, there are devices in which the restriction area of the exit passage of the directional control valve is set to a small value even when the degree of operation of the control lever is relatively large. However, in these kinds of devices, because the restriction area of the exit passage is set to be small regardless of the degree of operation of the operating lever, for various drive conditions of the actuator, the resistance to the flow of fluid from the actuator is large thereby causing an increase in the loss of pressure with a consequent decrease in the energy efficiency of the device.
There are also hydraulic devices for construction machinery, which employ so-called "Bleed-off" control which do not use pressure compensators. In these devices, as a result of the particular construction cavitation does not easily occur. However, because the amount of fluid supplied to the actuator can change even when the degree of operation of the operating lever is maintained constant it becomes difficult to obtain an actuator drive speed corresponding to the degree of operation of the operating lever.