Power plants – Pressure fluid source and motor – Having condition responsive control in a system of distinct...
Reexamination Certificate
2000-08-24
2002-06-25
Look, Edward K. (Department: 3745)
Power plants
Pressure fluid source and motor
Having condition responsive control in a system of distinct...
C091S446000
Reexamination Certificate
active
06408622
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a hydraulic drive system for a construction machine including a swing control system, such as a hydraulic excavator. More particularly, the present invention relates to a hydraulic drive system wherein, when a hydraulic fluid from a hydraulic pump is supplied to a plurality of actuators, including a swing motor, through respective associated directional control valves, a delivery rate of the hydraulic pump is controlled by a load sensing system and differential pressures across the directional control valves are controlled by respective associated pressure compensating valves.
BACKGROUND ART
JP, A, 60-11706 discloses a hydraulic drive system for controlling a delivery rate of a hydraulic pump by a load sensing system (hereinafter referred to also as an LS system). Also, JP, A, 10-37907 discloses a hydraulic drive system for a construction machine including a swing control system, the hydraulic drive system including an LS system and being intended to realize independence and operability of the swing control system. A 3-pump system mounted on an actual machine is also disclosed as an open-center hydraulic drive system for a construction machine including a swing control system, the hydraulic drive system being intended to realize independence of the swing control system. Further, JP, A, 10-89304 discloses a hydraulic drive system wherein a delivery rate of a hydraulic pump is controlled by an LS system and a pressure compensating valve is given a load dependent characteristic.
In the hydraulic drive system disclosed in JP, A, 60-11706, a plurality of pressure compensating valves each include means for setting, as a target compensation differential pressure, a differential pressure between a delivery pressure of the hydraulic pump and a maximum load pressure among a plurality of actuators. In the combined operation where a plurality of actuators are simultaneously driven, there may occur a saturation state that the delivery rate of the hydraulic pump is not enough to supply flow rates demanded by a plurality of directional control valves. In such a saturation state, the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure is lowered, and correspondingly the target compensation differential pressure of each pressure compensating valve is reduced. As a result, the delivery pressure of the hydraulic pump can be distributed again in accordance with a ratio between the respective flow rates demanded by the actuators.
In the hydraulic drive system disclosed in JP, A, 10-37907 and the 3-pump system mounted on an actual machine, an independent open-center circuit using an independent hydraulic pump is constructed for a swing section, which includes a swing motor, separately from a circuit for the other actuators, whereby independence and operability of the swing control system is ensured.
In the hydraulic drive system disclosed in JP, A, 10-89304, a plurality of pressure compensating valves each have hydraulic pressure chambers constructed as follows. A pressure bearing area of a hydraulic pressure chamber, to which an input side pressure of a directional control valve is introduced and which produces a force acting in the valve-closing direction, is set to be greater than a pressure bearing area of a hydraulic pressure chamber, to which an output side pressure of the directional control valve is introduced and which produces a force acting in the valve-opening direction. The pressure compensating valve is given such a load dependent characteristic that, as a load pressure of each associated actuator rises, the target compensation differential pressure of the pressure compensating valve is reduced (i.e., the pressure compensating valve is throttled) to decrease a supply flow rate to the actuator. As a result, the actuators on both the lower and higher load sides can be operated with good operability in a stable manner without hunting. Further, a ratio of the pressure bearing area of the hydraulic pressure chamber, to which the output side pressure of the directional control valve is introduced, to the pressure bearing area of a hydraulic pressure chamber, to which the input side pressure of the directional control valve is introduced, is specified to fall in the range of 0.97-0.94.
DISCLOSURE OF THE INVENTION
The conventional hydraulic drive systems described above however have the following problems with the swing control system.
JP, A, 60-11706: following problems {circle around (1)} and {circle around (2)}
JP, A, 10-89304: following problem {circle around (2)}
JP, A, 10-37907: following problem {circle around (3)}
Open-center 3-pump system mounted on actual machine: following problem {circle around (3)}
{circle around (1)} jerky feel in operation at start-up of swing
{circle around (2)} occurrence of energy loss, vibration, heat, etc. at start-up of swing
{circle around (3)} increase in cost and space and complicated circuit configuration due to provision of a separate circuit
(1) JP, A, 60-11706
When the hydraulic drive system including the LS system, disclosed in JP, A, 60-11706, is applied to the swing control system, it is difficult to keep balance between load sensing control (hereinafter referred to also as an LS control) of the hydraulic pump and a flow rate compensating function of the pressure compensating valve due to an inertial load of the swing control system. This is because a difficulty occurs in keeping balance between response of the pressure compensating valve and response of the LS control for the hydraulic pump due to the following reasons when a swing driving pressure is controlled in a stage of shift from swing acceleration to steady rotation.
(1) In a swing start-up and acceleration mode, the pump LS control is performed so as to raise a delivery pressure of the hydraulic pump depending on the swing start-up pressure for holding a constant flow rate.
(2) To hold constant a differential pressure across a throttling element of the directional control valve, the pressure compensating valve is operated in a direction to increase a flow rate passing itself that tends to reduce upon a rise of the load pressure.
(3) When the swing reaches a steady speed, the swing driving pressure is lowered and therefore the pump LS control is not required to control the delivery pressure of the hydraulic pump so high as in the swing start-up and acceleration mode. Hence the pump LS control is performed in a direction to lower the delivery pressure of the hydraulic pump.
(4) Upon a lowering of the swing driving pressure, the pressure compensating valve is operated in a direction to reduce the flow rate passing itself that tends to increase.
Because of quick shift from (1) to (4), the swing operation becomes jerky (above problem {circle around (1)}).
Further, in the above steps (1) and (2) of the swing start-up and acceleration mode, the hydraulic fluid is supplied to the swing motor at a flow rate larger than a necessary level. As a result, the load pressure of the swing motor rises to a pressure set by an overload relief valve that serves as a swing safety valve, and a large amount of hydraulic fluid corresponding to an extra flow rate is drained to a reservoir through the swing safety valve. The extra flow rate results in energy loss, thereby deteriorating energy efficiency, and also gives rise to vibration, heat and noise (above problem {circle around (2)}).
(2) JP, A, 10-89304
In the hydraulic drive system disclosed in JP, A, 10-89304, since the pressure compensating valve is given a load dependent characteristic, the target compensation differential pressure of the pressure compensating valve is reduced in response to a rise of the load pressure of the swing motor when swing is solely started up, and when the swing motor shifts to a steady sate, the target compensation differential pressure of the pressure compensating valve is also returned to the original value in response to a lowering of the load pressure of the swing motor. As a result, swing can be started up without causin
Hamamoto Satoshi
Kanai Takashi
Kawamoto Junya
Nagao Yukiaki
Okazaki Yasuharu
Hitachi Construction Machinery Co. Ltd.
Lazo Thomas E.
Look Edward K.
Mattingly Stanger & Malur, P.C.
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