Power plants – Pressure fluid source and motor – Having condition responsive control in a system of distinct...
Reexamination Certificate
2000-08-02
2002-06-04
Look, Edward K. (Department: 3745)
Power plants
Pressure fluid source and motor
Having condition responsive control in a system of distinct...
C091S516000, C091S446000
Reexamination Certificate
active
06397591
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 occurs 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 rate of the hydraulic pump can be redistributed 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 thereby 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.
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: problems {circle around (1)} and {circle around (2)}
JP, A, 10-89304: problems {circle around (2)} and {circle around (3)}
JP, A, 10-37907: problem {circle around (4)}
Open-center 3-pump system mounted on actual machine: problem {circle around (4)}
{circle around (1)} jerky feel in operation at start-up of swing alone
{circle around (2)} change of the swing speed at shift from operation of swing alone to combined operation including swing and vice versa.
{circle around (3)} extreme drop of the swing speed at start-up of combined operation including swing
{circle around (4)} 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 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 in the LS control of 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)}).
In the combined operation, as described above, there occurs 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. Corresponding to such a saturation state, the target compensation differential pressure of each pressure compensating valve is reduced, and the delivery rate of the hydraulic pump is redistributed in accordance with a ratio between the respective flow rates demanded by the actuators. With that function, even in the combined operation, the actuators are operated, although slightly slowed down, by the hydraulic fluid distributed at the ratio depending on the intended operations, whereby a feel in the operation is not impaired.
However, such slowdown likewise occurs in the swing operation, and during the combined operation including swing, the swing speed is also reduced as with one or more other actuators. This slowdown gives rise to change of the swing speed at shift from the swing-combined operation to the swing-alone operation and vice versa, thus causing the operator to feel awkward (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 at the start-up
Hamamoto Satoshi
Kanai Takashi
Kawamoto Junya
Nagao Yukiaki
Nakatani Kenichiro
Hitachi Construction Machinery Co. Ltd.
Lazo Thomas E.
Look Edward K.
Mattingly Stanger & Malur, P.C.
LandOfFree
Hydraulic driving unit does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Hydraulic driving unit, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Hydraulic driving unit will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2970267