Power plants – Pressure fluid source and motor – Having condition responsive control in a system of distinct...
Reexamination Certificate
1998-08-03
2001-04-10
Lopez, F. Daniel (Department: 3745)
Power plants
Pressure fluid source and motor
Having condition responsive control in a system of distinct...
C060S445000, C060S468000
Reexamination Certificate
active
06212886
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a hydraulic drive system equipped in construction machines such as hydraulic excavators and hydraulic cranes, and a directional control valve apparatus for use in the hydraulic drive system. More particularly, the present invention relates to a hydraulic drive system and a directional control valve apparatus each of which includes a closed center directional control valve and an unloading circuit for connecting a hydraulic fluid supply line of a hydraulic pump to a reservoir.
BACKGROUND ART
Directional control valves for use in a hydraulic drive system of hydraulic machines such as hydraulic excavators and hydraulic cranes are grouped into the open center type and the closed center type. One example of conventional directional control valve apparatus including an open center directional control valve is shown in FIG.
11
.
In
FIG. 11
, a housing
101
includes a spool
102
inserted therein and displacing (through its stroke) depending on an input amount by which a control lever unit is operated. A feeder port
103
connected to a hydraulic pump and a bypass port
105
connected to a reservoir port
104
are formed in the housing
101
around a central portion of the spool
102
. When the spool
102
is in a neutral position, the bypass port
105
is fully opened, causing all of a hydraulic fluid from the hydraulic pump to flow into a reservoir. As the spool
102
is moved to the left or right on the drawing, an opening area of the bypass port
105
is reduced, enabling part of the hydraulic fluid from the hydraulic pump to be supplied to an actuator through a meter-in throttle
102
a
or
102
b
and a load port
109
a
or
109
b
. When the spool
102
is in a full stroke position, the bypass port
105
is blocked off, enabling all of the hydraulic fluid from the hydraulic pump to be supplied to the actuator.
Additionally, denoted by
107
a
,
107
b
are overload relief valves and
108
is a load check valve, these valves being parts which are usually necessary for constructing the directional control valve apparatus. A not-shown main relief valve is also attached to the valve apparatus.
In the valve apparatus using such an open center directional control valve, since the bypass port
105
is throttled to have an opening corresponding to the input amount of the control lever unit, the so-called bleed control under which an actuator is driven while bleeding part of the delivery rate of a hydraulic pump can be effected at start-up of the actuator. The bleed control does not abruptly change the delivery pressure of the hydraulic pump and therefore provides a good operation feeling free from shocks imposed on the actuator.
On the other hand, a closed center directional control valve is a valve in which a feeder port is fully closed when a spool is in a neutral position. When combined with a pressure compensating valve, the closed center directional control valve can supply a hydraulic fluid to an actuator at a certain flow rate corresponding to the valve opening area regardless of variation in load pressure. However, since the closed center directional control valve is designed to always supply the actuator with the hydraulic fluid at a flow rate corresponding to the valve opening area, the delivery pressure of the hydraulic pump tends to change abruptly and the actuator tends to move so quickly. In view of such a problem, JP, A, 7-63203 proposes a hydraulic drive system which enables the bleed control to be effected in a hydraulic circuit including the closed center directional control valve as with the case of using the open center directional control valve.
The hydraulic drive system disclosed in JP, A, 7-63203 comprises a variable displacement hydraulic pump, a plurality of actuators driven by a hydraulic fluid delivered from the hydraulic pump, a plurality of closed center directional control valves for controlling flows of the hydraulic fluid supplied to the plurality of actuators, a plurality of control lever units for driving the plurality of directional control valves to operate, a bypass line connected to a hydraulic fluid supply line of the hydraulic pump, a bleed valve for returning the hydraulic fluid delivered from the hydraulic pump to a reservoir when the plurality of directional control valves are in neutral positions, and a controller for controlling the bleed valve to have an opening corresponding to an input amount by which the plurality of control lever units are operated.
In the hydraulic drive system thus constructed, the bleed valve functions like a center bypass throttle because the bleed valve is controlled to have an opening corresponding to the input amount of the control lever units. Accordingly, an equivalent operation feeling to that obtained in the bleed control using the open center directional control valve can be obtained while using a closed center valve as the directional control valve, thus resulting in good operability.
In the above-mentioned related arts, the bypass port
105
of the open center directional control valve and the bypass line described in JP, A, 7-63203 are both used for connecting the hydraulic fluid supply line of the hydraulic pump to the reservoir. Therefore, those port and line are referred to together as “an unloading circuit” in this Description.
DISCLOSURE OF THE INVENTION
The above-mentioned related arts however have the problems below.
First, in the valve apparatus using the open center directional control valve shown in
FIG. 11
, because the bypass port (unloading circuit)
105
is blocked off upon the operation of the spool
102
, the spool
102
has an overall length extended corresponding to the presence of the bypass port
105
, thus raising a difficulty in machining the spool with high accuracy, and the housing
101
has an increased length in its portion where the bypass port
105
is provided, thus increasing weight and cost of the housing and also raising a difficulty in manufacture of a molding. Particularly, in a construction machine having a plurality of driven members such as a hydraulic excavator and hydraulic crane, the valve apparatus is usually constructed as a multiple valve wherein respective spools of a plurality of directional control valves are arranged side by side, and in this case the additional housing weight resulted from the presence of the bypass port is increased in proportion to the number of the spools. Accordingly, the housing weight is so increased as to cause a considerable effect upon a manufacture cost.
On the other hand, in the hydraulic drive system disclosed in JP, A, 7-63203, the above-stated problems attributable to the open center directional control valve are not encountered because the directional control valve is itself a closed center valve. However, the controller for controlling the bleed valve provided in the unloading circuit is required to develop an equivalent function to that obtained in the bleed control using the open center directional control valve by using the closed center valve, thus resulting in a higher cost.
A first object of the present invention is to provide a hydraulic drive system and a directional control valve apparatus with which a spool can have a shorter axial length than required in an open center directional control valve, an improvement in machining accuracy and a more compact structure can be realized, and an equivalent function to that obtained by the open center directional control valve can be developed with no need of using a controller.
A second object of the present invention is to provide a directional control valve apparatus which is constructed as a multiple valve including a plurality of spools arranged side by side, which can develop an equivalent function to that obtained by the open center directional control valve, which can make a housing lighter and more compact than the case of using an open center directional control valve, and which can be manufactured at a reduced cost.
Features of the present invention to achieve the above objects and other associated features are as f
Hirata Toichi
Sugiyama Genroku
Toyooka Tsukasa
Hitachi Construction Machinery Co. Ltd.
Lopez F. Daniel
Mattingly, Stangler & Malur
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