Patent Publication Number: US-6212886-B1

Title: Hydraulic drive system and directional control valve apparatus in hydraulic machine

Description:
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 follows. 
     (1) To achieve the above first object, according to the present invention, in a hydraulic drive system comprising a hydraulic pump, an actuator driven by a hydraulic fluid delivered from the hydraulic pump, a closed center directional control valve connected to the hydraulic pump through a hydraulic fluid supply line and controlling a flow of the hydraulic fluid supplied to the actuator, an unloading circuit interconnecting the hydraulic fluid supply line of the hydraulic pump and a reservoir, and throttle and cutoff means installed in the unloading circuit for throttling and cutting off the unloading circuit depending on an amount by which the directional control valve is operated, the throttle and cutoff means comprises a logic valve, a pilot circuit including a pilot variable throttle to control a rate of a pilot flow for controlling an opening area of the logic valve depending on the rate of the pilot flow, and operation interlock means for changing an opening of the pilot variable throttle depending on the operation amount of the directional control valve. 
     In the present invention thus constructed, when the directional control valve is operated, the opening of the pilot variable throttle is changed by the operation interlock means depending on the operation amount of the directional control valve, and the rate of the pilot flow passing the pilot circuit is also changed. When the rate of the pilot flow is reduced, the opening of the logic valve is also reduced, whereupon the hydraulic fluid drained to the reservoir through the unloading circuit is gradually throttled to raise the delivery pressure of the hydraulic pump. When the pump delivery pressure becomes higher than the load pressure of the actuator, part of the hydraulic fluid delivered from the hydraulic pump is drained to the reservoir through the unloading circuit while the remaining hydraulic fluid is supplied to the actuator. When the logic valve is fully closed, all of the hydraulic fluid delivered from the hydraulic pump is supplied to the actuator. 
     Thus, with the present invention, an equivalent function to that obtained in the bleed control using an open center directional control valve can be obtained by using the closed center directional control valve, and that function can be achieved without using a controller. 
     Since the throttle and cutoff means in a combination of the logic valve and the pilot variable throttle is used, even the case of developing an equivalent function to that obtained by open center directional control valves in a hydraulic circuit including a plurality of actuators can be easily coped with by connecting a plurality of pilot variable throttles in series without using a controller. 
     Further, since the directional control valve is itself a closed center valve while developing an equivalent function to that obtained by an open center directional control valve, it is possible to shorten the overall length of a spool of the directional control valve to such an extent as corresponding to no need of a bypass port, improve the machining accuracy, and make a housing of the directional control valve more compact. 
     (2) In the above (1), the logic valve of the throttle and cutoff means comprises, for example, a valve body and a feedback slit formed in the valve body so as to change an opening area thereof depending on an amount by which the valve body is moved, the feedback slit being connected to said pilot circuit such that the pilot flow in the pilot circuit passes the feedback slit. 
     With that construction of causing the pilot flow to pass the feedback slit of the logic valve, the opening of the logic valve is controlled to increase and decrease depending on an increase and decrease in the rate of the pilot flow. 
     (3) In the above (1), preferably, the operation interlock means reduces the opening of the pilot variable throttle as the operation amount of the directional control valve increases. 
     With that feature, as the operation amount of the directional control valve increases, the opening of the pilot variable throttle is reduced and the rate of the pilot flow passing the feedback slit of the logic valve is controlled to decrease. 
     (4) In the above (1), preferably, the hydraulic drive system further comprises a pilot valve for outputting a pilot pressure as an operation signal and driving the directional control valve, and the operation interlock means comprises means for introducing the same pilot pressure as output from the pilot valve to the directional control valve and the pilot variable throttle of the throttle and cutoff means. 
     By constructing the directional control valve and the pilot variable throttle of the throttle and cutoff means to be of the hydraulic pilot type, the directional control valve and the pilot variable throttle can be easily operated in an interlock manner. 
     (5) In the above (1), preferably, the actuator and the directional control valve are each provided in plural number, and the pilot variable throttle of the throttle and cutoff means is provided in plural number corresponding to the plurality of directional control valves, the plurality of pilot variable throttles being connected in series. 
     With that feature, an equivalent function to that obtained by an open center directional control valve can be achieved, as stated in the above (1), for each of combinations of the directional control valves and the pilot variable throttles. In addition, an equivalent function to that obtained by a conventional multiple open center directional control valve can be realized. 
     (6) In the above (1), preferably, flow rate detecting means for detecting the rate of the pilot flow is disposed between the pilot variable throttle of the throttle and cutoff means and said reservoir, a delivery rate of the hydraulic pump being regulated by using a signal generated by the flow rate detecting means. 
     With that feature, an equivalent function to that obtained by the conventional open center directional control valve can be achieved. In addition, since the pilot flow passes the flow detecting means at a small rate, the flow rate detecting means can be reduced in size and improved in reliability. 
     (7) In the above (1), preferably, a relief valve is disposed in parallel to the pilot variable throttle of the throttle and cutoff means. 
     By providing the relief valve to release the pilot flow instead of a conventional large-sized relief valve which has been used to release a large flow rate supplied from the hydraulic pump, the relief valve can be reduced in size and the construction of the hydraulic drive system can be further simplified. 
     (8) Also, to achieve the above first object, according to the present invention, in a directional control valve apparatus comprising a valve housing, a main spool slidably disposed in the housing and communicating a feeder port connected to a hydraulic pump with load ports connected to an actuator, an unloading circuit interconnecting the feeder port and a reservoir port connected to a reservoir, and throttle and cutoff means installed in the unloading circuit for throttling and cutting off the unloading circuit depending on an amount by which the main spool is operated, the throttle and cutoff means comprises a logic valve including a feedback slit of which opening area is changed depending on an amount by which a valve body is moved, a sub-spool including a pilot variable throttle to control a rate of a pilot flow passing the feedback slit for reducing an opening of the pilot variable throttle as the sub-spool displaces from a neutral position, and operation interlock means for displacing the sub-spool depending on the operation amount of the main spool. 
     In the present invention thus constructed, when the main spool is operated, the sub-spool is displaced by the operation interlock means to reduce the opening of the pilot variable throttle, whereupon the rate of the pilot flow passing the feedback slit of the logic valve is reduced and the opening of the logic valve is also reduced. Accordingly, as stated in the above (1), part of the hydraulic fluid delivered from the hydraulic pump is drained to the reservoir through the unloading circuit while the remaining hydraulic fluid is supplied to the actuator. Thus, an equivalent function to that obtained by an open center directional control valve can be achieved without using a controller. 
     Further, as stated in the above (1), even the case of employing a hydraulic circuit including a plurality of actuators can be easily coped with, and an improvement in machining accuracy and a more compact structure can be realized. 
     (9) In the above (8), preferably, the operation interlock means comprises means for communicating pilot pressure bearing chambers defined at both ends of the main spool respectively with pilot pressure bearing chambers defined at both ends of the sub-spool. 
     With that feature, the directional control valve and the pilot variable throttle can be easily operated in an interlock manner by using the same pilot pressure. 
     (10) In the above (8), preferably, the main spool and the sub-spool are arranged in the housing parallel to each other. 
     With that feature, opposite ends of the main spool and the sub-spool are positioned to face the same opposite housing surfaces so that the operation interlock means can be mounted to each of those housing surfaces and the interlock structure for both the spools can be more easily realized. 
     (11) In the above (8), preferably, the directional control valve apparatus further comprises a load check valve disposed in an inlet portion of the main spool for preventing a hydraulic fluid from flowing backward from the load ports to the feeder port, the sub-spool being arranged parallel to the main spool on the side opposite to the load check valve with the main spool positioned therebetween. 
     With that feature, when installing the sub-spool, the sub-spool can be rationally arranged in a free space inside the housing without causing interference with the load check valve. In addition, as stated in the above (10), the operation interlock means for the main spool and the sub-spool can be easily constructed. 
     (12) To achieve the above second object, according to the present invention, in the above (8), the valve apparatus is constructed as a multiple valve including a plurality of main spools arranged side by side, and corresponding plural sets of combination of the main spools and sub-spools are installed in the valve housing, the plurality of sub-spools being connected in series. 
     With that feature, as stated in the above (4), an equivalent function to that obtained by an open center directional control valve can be achieved for each of combinations of the main spools and the sub-spools, and in addition an equivalent function to that obtained by a conventional multiple open center directional control valve can be realized. 
     Further, in the valve apparatus constructed as a multiple valve including a plurality of main spools arranged side by side, a weight reduction of the housing to such an extent as corresponding to no need of a bypass port can be achieved in proportion to the number of the main spools. Accordingly, the housing can be greatly reduced in weight and size and the manufacture cost can be considerably cut down. 
     (13) In the above (8), preferably, flow rate detecting means for detecting the rate of the pilot flow is disposed between the sub-spool and the reservoir port, a delivery rate of the hydraulic pump being regulated by using a signal generated by the flow rate detecting means. 
     With that feature, as stated in the above (5), the flow rate detecting means can be reduced in size and improved in reliability. 
     (14) In the above (8), a relief valve is disposed in parallel to the sub-spool. 
     With that feature, as stated in the above (6), the relief valve can be reduced in size and the construction of the hydraulic drive system can be further simplified. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a hydraulic circuit diagram of a hydraulic drive system and a directional control valve apparatus according to a first embodiment of the present invention. 
     FIG. 2 is a sectional view of a directional control valve apparatus, shown in FIG. 1, sectioned along a plane including a main spool and a sub-spool. 
     FIG. 3 is a sectional view of the directional control valve apparatus, shown in FIG. 1, sectioned along a plane including a valve body of a logic valve. 
     FIG. 4 is a hydraulic circuit diagram of a hydraulic drive system and a directional control valve apparatus according to a second embodiment of the present invention. 
     FIG. 5 is a hydraulic circuit diagram of a hydraulic drive system and a directional control valve apparatus according to a third embodiment of the present invention. 
     FIG. 6 is a sectional view of a directional control valve apparatus, shown in FIG. 5, sectioned along a plane including one main spool and one sub-spool. 
     FIG. 7 is a sectional view taken along the line VII—VII in FIG. 6, the view showing the correlation of main spools in arrangement. 
     FIG. 8 is a sectional view showing the connection of pilot passages. 
     FIG. 9 is a hydraulic circuit diagram of a hydraulic drive system and a directional control valve apparatus according to a fourth embodiment of the present invention. 
     FIG. 10 is a hydraulic circuit diagram of a hydraulic drive system and a directional control valve apparatus according to a fifth embodiment of the present invention. 
     FIG. 11 is a sectional diagram showing the structure of a conventional valve apparatus including an open center directional control valve. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereunder, embodiments of the present invention will be described with reference to the drawings . 
     To begin with, a first embodiment of the present invention will be described with reference to FIGS. 1-3. 
     In FIG. 1, a hydraulic drive system of this embodiment comprises a hydraulic pump  50 , a hydraulic actuator  51  driven by a hydraulic fluid delivered from the hydraulic pump  50 , a directional control valve apparatus  52  for controlling a flow of the hydraulic fluid supplied from the hydraulic pump  50  to the actuator  51 , and a reservoir  53 . 
     The directional control valve apparatus  52  comprises a main spool  2  constituting a closed center directional control valve, a feeder port  3  and an inlet port  3   a  both connected to the hydraulic pump and constituting a hydraulic supply line, a reservoir port  4  connected to the reservoir  53 , load ports  5   a ,  5   b  connected to the actuator, a load check valve  6  for preventing the hydraulic fluid from flowing backward from the load ports  5   a ,  5   b  to the feeder port  3 , overload relief valves  7   a ,  7   b  connected respectively to the load ports  5   a ,  5   b , and a main relief valve  8  for limiting a maximum pressure in the circuit. Note that although the reservoir port  4  associated with the main spool  2  and the reservoir ports  4  associated with the overload relief valves  7   a ,  7   b  are shown separately from the reservoir port  4  downstream of the main relief valve  8  and correspondingly the reservoirs are also shown separately from the reservoir  53  downstream of the main relief valve  8  in FIG. 1 for convenience of illustration, those reservoir ports and reservoirs are in fact formed respectively as a single reservoir port and a single reservoir. 
     The main spool  2  communicates the feeder port  3  with the load port  5   a  or  5   b  and the load port  5   b  or  5   a  with the reservoir port  4  depending on the direction in which it is operated. Also, the main spool  2  is provided with meter-in and meter-out variable throttles (described later) so that the flow rate of the hydraulic fluid flowing from the feeder port  3  to the load port  5   a  or  5   b  and the flow rate of the hydraulic fluid flowing from the load port  5   a  or  5   b  to the reservoir port  4  are controlled depending on the displacement (stroke) of the main spool  2 . 
     In this embodiment, the main spool  2  is operated by using a pilot pressure given as an output of a pilot valve  54 , and when a control lever  54   a  of the pilot valve  54  is operated, the output pressure of the pilot valve  54  is applied to a corresponding end of the main spool  2  for shifting the main spool  2 . 
     The directional control valve apparatus  52  further includes an unloading circuit  10  interconnecting the feeder port  3  and the reservoir port  4 . The unloading circuit  10  comprises a passage  10   a  as part of the feeder port  3  and a passage  10   b  connected to the reservoir port  4 . Between the passages  10   a  and  10   b , there is provided throttle and cutoff means  11  for throttling and cutting off the unloading circuit  10  depending on an amount (stroke) by which the main spool  2  is operated. 
     In this embodiment, the throttle and cutoff means  11  is made up of a logic valve  12  and a pilot circuit  15  having a sub-spool  13 . 
     The logic valve  12  has a valve body  12   a  of the seat valve type for opening and closing the unloading circuit. The valve body  12   a  is provided with a feedback slit  12   b  whose opening area increases and decreases depending on an amount by which the valve body  12   a  is moved. An end of the valve body  12   a  opposite to an outlet of the logic valve  12  (the passage  10   b ) is positioned to face a back pressure chamber  12   c , and an inlet of the logic valve  12  (the passage  10   a ) is communicated with the back pressure chamber  12   c  through both a passage  12   d  formed in the valve body  12   a  and the feedback slit  12   b.    
     The pilot circuit  15  has a pilot passage  14  and the sub-spool  13  is disposed between passage portions  14   a  and  14   b  of the pilot passage  14 . The passage portion  14   a  of the pilot passage  14  is connected to the back pressure chamber  12   c , and the passage portion  14   b  of the pilot passage  14  is connected to the reservoir port  4 . The same output pressure of the pilot valve  54  (pilot pressure) as applied to the main spool  2  is also applied to both ends of the sub-spool  13  through passages  22   a ,  22   b  serving as operation interlock means. The pilot pressure acts to move the sub-spool  13  depending on the operation amount of the main spool  2  to change openings (opening areas) of pilot variable throttles  13   a ,  13   b  (see FIG. 2, described later) provided in the sub-spool  13 . Stated otherwise, when the sub-spool  13  is in a neutral position as shown, the pilot variable throttles each have a maximum opening, and when the sub-spool  13  is moved in either direction from the neutral position as shown, the opening of the pilot variable throttle is reduced depending on the amount of movement of the sub-spool  13 . 
     In accordance with change in opening of the pilot variable throttle of the sub-spool  13 , a pilot flow passing the feedback slit  12   b  of the logic valve  12  is changed and the opening area of the logic valve  12  in communication with the reservoir port  4  is controlled. After passing the feedback slit  12   b  of the logic valve  12 , the pilot flow is drained to the reservoir port  4  through the back pressure chamber  12   c , the pilot passage  14   a , the pilot variable throttle provided in the sub-spool  13 , and the pilot passage  14   b . By throttling the opening of the pilot variable throttle to restrict a rate of the pilot flow, the pressure in the back pressure chamber  12   c  of the logic valve  12  is raised, whereupon the valve body  12   a  of the logic valve  12  is moved downward on the drawing to restrict an opening of the logic valve  12  (an opening amount of the unloading circuit  10 ). When the pilot variable throttle of the sub-spool  13  is brought into a closed state, the logic valve  12  is substantially closed and the unloading circuit  10  is closed. 
     It is to be noted that the above-stated operation of the logic valve  12  and the sub-spool  13  is known from JP, A, 6-193604. 
     One example of the structure of the directional control valve apparatus  52  is shown in FIGS. 2 and 3. 
     FIG. 2 shows a section of the directional control valve apparatus  52  sectioned along a plane passing the axes of the main spool  2  and the sub-spool  13 . The main spool  2  and the sub-spool  13  are slidably fitted in a housing  1 . The housing  1  has formed therein the feeder port  3  connected to the hydraulic pump  50  (see FIG.  1 ), the load ports  5   a ,  5   b  connected to the actuator  51  (see FIG.  1 ), and reservoir ports  4   a ,  4   b  through which the return hydraulic fluid from the load ports  5   a ,  5   b  are drained upon the operation of the main spool  2 . A load check valve  6  is disposed between the feeder port  3  and the inlet port  3   a , and the overload relief valves  7   a ,  7   b  are disposed respectively in the load ports  5   a ,  5   b . The feeder port  3  is extended in a direction perpendicular to the drawing sheet. 
     The main spool  2  is provided with meter-in variable throttles (notches)  2   a - 1 ,  2   a - 2  and meter-out variable throttles (notches)  2   b - 1 ,  2   b - 2  formed as shown. When the main spool  2  is moved to the right on the drawing, the hydraulic fluid in the feeder port  3  pushes up the load check valve  6  and flows into the inlet port  3   a . From the inlet port  3   a , the hydraulic fluid flows into the load port  5   a  through the meter-in variable throttle  2   a - 1  and is then supplied to the actuator  51  (see FIG.  1 ). The return hydraulic fluid from the actuator  51  flows from the load port  5   b  into the reservoir port  4   b  through the meter-out variable throttle  2   b - 1  and is then returned to the reservoir  53  (see FIG.  1 ). The above explanation is also equally applied to when the main spool  2  is moved to the left on the drawing. In this case, however, the hydraulic fluid from the inlet port  3   a  is supplied to the actuator  51  (see FIG. 1) through the meter-in variable throttle  2   a - 2  and the return hydraulic fluid from the actuator  51  is returned to the reservoir  53  (see FIG. 1) through the meter-out variable throttle  2   b - 2  and the reservoir port  4   a.    
     The sub-spool  13  is provided with the pilot variable throttles (notches)  13   a ,  13   b  formed in a central portion thereof. The variable throttle  13   a  is positioned at one end of a pilot passage  14   a - 2  extended in the direction perpendicular to the drawing sheet, and the variable throttle  13   b  is positioned at one end of a pilot passage  14   b - 1  formed along an outer periphery of the sub-spool  13 , the other end of the pilot passage  14   b - 1  being connected to the reservoir port  4   b  through a pilot passage  14   b - 2 . 
     The main spool  2  and the sub-spool  13  are fitted to extend parallel to each other. The output pressure of the pilot valve  54  (see FIG. 1) is introduced to one of main pressure bearing chambers  21   a ,  21   b  defined in pilot caps  20   a ,  20   b  for being applied to the main spool  2 , and at the same time introduced to one of sub-pressure bearing chambers  23   a ,  23   b  through passages  22   a ,  22   b  provided in the pilot caps  20   a ,  20   b  for being applied to the sub-spool  13 . 
     FIG. 3 is a sectional view sectioned along a plane passing the logic valve  12 . A portion of the feeder port  3  is extended as the passage  10   a  to the inlet of the logic valve  12  of the throttle and cutoff means  11 , and the valve body  12   a  of the logic valve  12  is positioned between the passage  10   a  and the passage  10   b  communicating with a reservoir port  4   c . The valve body  12   a  has the feedback slit  12   b  and the passage  12   d . An end of the valve body  12   a  opposite to the passage  10   b  is positioned to face the back pressure chamber  12   c  which is connected to the pilot passage  14   a - 2  through a pilot passage  14   a - 1 . 
     The pilot passages  14   a - 1 ,  14   a - 2  correspond to the passage portion  14   a  of the pilot passage  14  shown in FIG. 1, and the pilot passages  14   b - 1 ,  14   b - 2  correspond to the passage portion  14   b  of the pilot passage  14  shown in FIG.  1 . Moreover, the reservoir ports  4   a ,  4   b ,  4   c  are all connected to the reservoir port  4  shown in FIG.  1 . 
     As explained above, when the opening of the pilot variable throttle  13   a ,  13   b  of the sub-spool  13  shown in FIG. 2 is changed, the pilot flow passing the feedback slit  12   b  of the logic valve  12  shown in FIG. 3 is changed and the position of the valve body  10   a  of the logic valve  12  in the vertical direction on the drawing is changed and hence the opening of the logic valve  12 , i.e., the opening amount of the unloading circuit  10 , is controlled. 
     After passing the feedback slit  12   b  of the logic valve  12  shown in FIG. 3, the pilot flow is drained to the reservoir port  4   b  through the back pressure chamber  12   c , the pilot passages  14   a - 1  and  14   a - 2  ( 14   a ), the pilot variable throttle  13   a  or  13   b  provided in the sub-spool  13 , and the pilot passages  14   b - 1  and  14   b - 2  ( 14   b ). 
     When the sub-spool  13  is operated to reduce the opening of the pilot variable throttle  13   a  or  13   b , the pilot flow is restricted and the pressure in the back pressure chamber  12   c  of the logic valve  12  is raised, whereupon the valve body  12   a  of the logic valve  12  is moved downward on the drawing to restrict the opening amount of the unloading circuit  10 . When the pilot variable throttle  13   a  or  13   b  of the sub-spool  13  is brought into a closed state, the unloading circuit  10  is substantially closed. 
     Further, in FIG. 3, the relief valve  8  is disposed between the feeder port  3  and the reservoir port  4   c.    
     In this embodiment thus constructed, when the control lever  54   a  of the pilot valve  54  is not operated, the meter-in variable throttles  2   a - 1  and  2   a - 2  of the main spool  2  of the directional control valve are closed, the pilot variable throttles  13   a  and  13   b  of the sub-spool  13  are opened at a maximum opening, and the valve body  12   a  of the logic valve  12  is in a maximum opening position. Therefore, all of the hydraulic fluid delivered from the hydraulic pump  50  flows into the reservoir  53  through the logic valve  12  and the reservoir port  4 . 
     When the operator manipulates the control lever  54   a  of the pilot valve  54  in the above condition, the pilot pressure is produced depending on the direction and amount in and by which the control lever  54   a  is operated, and applied to the main spool  2  of the directional control valve. At the same time, the pilot pressure is also applied to the sub-spool  13  of the throttle and cutoff means  11 . Accordingly, the meter-in variable throttle  2   a - 1  or  2   a - 2  of the main spool  2  is opened and the opening of the pilot variable throttle  13   a  or  13   b  of the sub-spool  13  is reduced such that the main spool  2  and the sub-spool  13  are operated to provide respective openings corresponding to the same pilot pressure. Thus, the opening of the logic valve  12  is reduced and as the output pressure of the pilot valve  54  rises, the hydraulic fluid drained to the reservoir  53  from the hydraulic pump  50  is gradually throttled to raise the delivery pressure of the hydraulic pump  50 . When the pump delivery pressure becomes higher than the load pressure of the actuator  51 , part of the hydraulic fluid delivered from the hydraulic pump  50  is supplied to the actuator  51  through the meter-in variable throttle  2   a - 1  or  2   a - 2  of the main spool  2 . As the opening of the logic valve  12  reduces, the flow rate supplied to the actuator  51  is increased. When the pilot variable throttle  13   a  or  13   b  of the sub-spool  13  is fully closed and the logic valve  12  is also fully closed, all of the hydraulic fluid delivered from the hydraulic pump  50  is supplied to the actuator  51 . 
     With this embodiment, therefore, an equivalent function to that obtained by the open center directional control valve can be achieved by using the closed center directional control valve (the main spool  2 ) and good operability is resulted. Further, that function can be realized without using a controller. 
     Since a bleed valve of the unloading circuit  10  is constituted by the throttle and cutoff means  11  in a combination of the logic valve  12  and the sub-spool  13 , even the case of developing an equivalent function to that obtained by the open center directional control valves in a hydraulic circuit including a plurality of actuators can be easily coped with, as will be apparent from an embodiment described later, by connecting a plurality of sub-spools  13  in series without using a controller. 
     Also, since the directional control valve is itself a closed center valve while developing an equivalent function to that obtained by the conventional open center directional control valve, it is possible to improve the problems attributable to the open center directional control valve (i.e., such problems that the main spool has an increased overall length, thus raising a difficulty in machining the main spool with high accuracy, and that the housing  1  has an increased length in its portion where the unloading circuit is arranged, thus increasing weight and cost of the housing and also raising a difficulty in manufacture of a molding). As a result, an improvement in machining accuracy and a more compact structure can be realized and the manufacture cost of the directional control valve apparatus can be cut down in combination with no need of a controller. 
     Further, since the valve apparatus can be designed to have a smaller ratio L/D of an outer diameter D to an overall length L of the main spool  2 , effects caused by bending of a b ore occur red in machining of the housing and bending of the spool itself can be made smaller. As a result, the valve apparatus can be designed with a smaller clearance in design drawings than conventional, and hence an oil-tight ability, which has been a technical-problem specific to directional control valves of the spool type, can be remarkably improved. In addition, since the housing can be designed to have a smaller size and heat is more quickly propagated, it is also possible to eliminate a trouble of spool seizure that may occur by a heat shock due to thermal unbalance. 
     Moreover, in this embodiment, the directional control valve is of the hydraulic pilot type and both the main spool  2  and the sub-spool  13  can be easily operated in an interlock manner by using the same pilot pressure. 
     Especially, since the main spool  2  and the sub-spool  13  are arranged parallel to each other, opposite ends of the main spool  2  and the sub-spool  13  are positioned to face the same opposite housing surfaces so that the pilot cap  20  common to both the spools can be mounted to each of those housing surfaces and the interlock structure for both the spools can be more easily realized. 
     A second embodiment of the present invention will be described with reference to FIG.  4 . In this embodiment, the main spool and the sub-spool are mechanically interlocked with each other. In FIG. 4, equivalent members to those shown in FIG. 1 are denoted by the same reference numerals. 
     Referring to FIG. 4, denoted by  52 A is a directional control valve apparatus for use in a hydraulic drive system of this embodiment. The directional control valve apparatus  52 A has a mechanical control lever unit  54 A instead of the pilot valve  54  shown in FIG.  1 . The control lever unit  54 A comprises a control lever  54   b  mechanically coupled to the main spool  2  and an operation interlock mechanism  54   c  for transmitting motion of the control lever  54   b . When the control lever  54   b  is operated, the main spool  2  is mechanically operated depending on the direction and amount in and by which the control lever  54   b  is operated, and at the same time the movement of the main spool  2  is transmitted to the sub-spool  13 . Thus, the sub-spool  13  is also mechanically operated depending on the operation amount of the main spool  2 . 
     This embodiment can also provide similar advantages as obtainable with the first embodiment. 
     A third embodiment of the present invention will be described with reference to FIGS. 5 to  8 . This embodiment intends to realize the function of a conventional multiple directional control valve by combinations of plural main spools and plural sub-spools. In FIGS. 5 to  8 , equivalent members to those shown in FIGS. 1 and 2 are denoted by the same reference numerals. 
     Referring to FIG. 5, a hydraulic drive system of this embodiment includes, in addition to the actuator  51 , another actuator  51 - 2  as a hydraulic actuator driven by the hydraulic fluid delivered from the hydraulic pump  50 . Correspondingly, a directional control valve apparatus  52 B is constructed as follows. 
     The main spool  2  and a main spool  2 - 2  are connected in parallel to the feeder port  3  connected to the hydraulic pump  50  of the directional control valve apparatus  52 B. In a pilot circuit  15 B of throttle and cutoff means  11 B, the sub-spool  13  operated in interlock with the main spool  2  and a sub-spool  13 - 2  operated in interlock with the main spool  2 - 2  are connected to a pilot passage  14 B in series. As with the main spool  2 , an inlet port  3   a - 2 , load ports  5   a - 2 ,  5   b - 2 , a load check valve  6 - 2 , and overload relief valves  7   a - 2 ,  7   b - 2  are provided for the main spool  2 - 2 . 
     Here, the sub-spool  13 - 2  is disposed between passage portions  14   a  and  14   c  of the pilot passage  14 B, and the sub-spool  13  is disposed downstream of the sub-spool  13 - 2  between passage portions  14   c  and  14   a  of the pilot passage  14 B. The same output pressure of a pilot valve  54 - 2  (pilot pressure) as applied to the main spool  2 - 2  is also applied to both ends of the sub-spool  13 - 2 . The pilot pressure acts to move the sub-spool  13 - 2  depending on the operation amount of the main spool  2 - 2  to change openings (opening areas) of pilot variable throttles (described later) provided in the sub-spool  13 - 2 . Stated otherwise, when the sub-spool  13 - 2  is in a neutral position as shown, the pilot variable throttles each have a maximum opening, and when the sub-spool  13 - 2  is moved in either direction from the neutral position as shown, the opening of the pilot variable throttle is reduced depending on the amount of movement of the sub-spool  13 - 2 . 
     In the above construction, when the control lever  54   a  or  54   a - 2  of one of the pilot valves  54 ,  54 - 2  is operated to shift one of the main spools  2 ,  2 - 2 , the corresponding sub-spool  13 ,  13 - 2  is moved to restrict the pilot flow and raise the pressure in the back pressure chamber  12   c  of the logic valve  12 , whereupon the valve body  12   a  of the logic valve  12  is moved downward on the drawing to restrict the opening of the logic valve  12  (the opening amount of the unloading circuit  10 ). When the pilot variable throttle of the sub-spool  13  or  13 - 2  is brought into a closed state, the logic valve  12  is substantially closed and the unloading circuit  10  is closed. 
     Also, when both the control levers  54   a ,  54   a - 2  of the pilot valves  54 ,  54 - 2  are half-operated to operate the main spools  2 ,  2 - 2 , the sub-spools  13 ,  13 - 2  are also operated correspondingly. At this time, because of the control levers  54   a ,  54   a - 2  being half-operated, the sub-spools  13 ,  13 - 2  are each likewise half-operated to restrict the pilot flow in accordance with the opening of the pilot variable throttle corresponding to the amount of movement of the sub-spool, thereby restricting the opening of the logic valve  12  (the opening amount of the unloading circuit  10 ). The above operation is equivalent to the operation obtained, in a system in which bypass ports of a plurality of open center directional control valves are connected in series, from the presence of the bypass ports when half-operating directional control valves. Thus, even when both the actuators  51 ,  51 - 2  are driven for the combined operation, equivalent control to that obtained in the bleed control using the open center directional control valves can be made by using the closed center directional control valves (the main spool  2 - 2 ). 
     One example of the structure of the directional control valve apparatus  52 B will be described with reference to FIGS. 6 to  8 , as well as FIGS. 2 and 3 in connection with the first embodiment. 
     The structure of portions relating to the main spool  2  and the sub-spool  13  of the directional control valve apparatus  52 B is essentially the same as that in the first embodiment explained above with reference to FIG.  2 . However, the pilot passage  14   a - 2  in FIG. 2 is replaced by a pilot passage  14   a - 3  described later. 
     Also, the structure of a portion relating to the logic valve  12  of the throttle and cutoff means  11 B of the directional control valve apparatus  52 B is the same as that in the first embodiment explained above with reference to FIG.  3 . 
     The structure of portions relating to the main spool  2 - 2  and the sub-spool  13 - 3  of the directional control valve apparatus  52 B is shown in FIG.  6 . 
     FIG. 6 shows a section of the directional control valve apparatus  52 B sectioned along a plane passing the axes of the main spool  2 - 2  and the sub-spool  13 - 2 . The main spool  2 - 2  and the sub-spool  13 - 2  are slidably fitted in the housing  1 . The housing  1  has formed therein the load ports  5   a - 2 ,  5   b - 2  connected to the actuator  51 - 2  (see FIG. 5) and reservoir ports  4   a - 2 ,  4   b - 2  through which the return hydraulic fluid from the load ports  5   a - 2 ,  5   b - 2  are drained upon the operation of the main spool  2 - 2 . The load check valve  6 - 2  is disposed between the feeder port  3  and the inlet port  3   a - 2 , and the overload relief valves  7   a - 2 ,  7   b - 2  are disposed respectively in the load ports  5   a - 2 ,  5   b - 2 . The feeder port  3  is extended in a direction perpendicular to the drawing sheet and connected to the feeder port  3  shown in FIG.  2 . 
     The structure of the main spool  2 - 2  is essentially the same as that of the main spool  2  in the first embodiment shown in FIG.  2 . 
     The sub-spool  13 - 2  is provided with pilot variable throttles (notches)  13   a - 2 ,  13   b - 2  formed in a central portion thereof. The variable throttle  13   a - 2  is positioned at one end of the pilot passage  14   a - 2  extended in the direction perpendicular to the drawing sheet, and the variable throttle  13   b - 2  is positioned at one end of the pilot passage  14   a - 3 . 
     The main spool  2 - 2  and the sub-spool  13 - 2  are fitted to extend parallel to each other. The output pressure of the pilot valve  54 - 2  (see FIG. 5) is introduced to one of main pressure bearing chambers  21   a - 2 ,  21   b - 2  defined in pilot caps  20   a - 2 ,  20   b - 2  for being applied to the main spool  2 - 2 , and at the same time introduced to one of sub-pressure bearing chambers  23   a - 2 ,  23   b - 2  through passages  22   a - 2 ,  22   b - 2  provided in the pilot caps  20   a - 2 ,  20   b - 2  for being applied to the sub-spool  13 - 2 . 
     FIG. 7 is a sectional view taken along the line VII—VII in FIG. 6, the view showing the correlation of the main spools  2 ,  2 - 2  and the logic valve  12 . The main spools  2 ,  2 - 2  are arranged in the housing  1  along with other main spools parallel to one another, thereby constructing a multiple valve. The reservoir port  4   c  is formed in the upper side of the housing  1  on the drawing, and the logic valve  12  is positioned in the reservoir port  4   c  above the drawing sheet, as indicated by a two-dot-chain line. 
     FIG. 8 is a view showing the correlation of the pilot passages  14   a - 2 ,  14   a - 3 ,  14   b - 1 . The pilot passage  14   a - 2  is connected to the back pressure chamber  12   c  of the logic valve  12  through the pilot passage  14   a - 1  shown in FIG. 3, and the pilot passage  14   a - 3  connects the variable throttle  13   b - 2  of the sub-spool  13 - 2  and the variable throttle  13   a  of the sub-spool  13  with each other. The pilot passages  14   a - 1 ,  14   a - 2  correspond to the passage portion  14   a  of the pilot passage  14 B shown in FIG. 5, the pilot passage  14   a - 3  corresponds to the passage portion  14   c  of the pilot passage  14 B shown in FIG. 5, and the pilot passages  14   b - 1 ,  14   b - 2  correspond to the passage portion  14   b  of the pilot passage  14 B shown in FIG.  5 . Moreover, the reservoir ports  4   a ,  4   b ,  4   c ,  4   a - 2 ,  4   b - 2  are all connected to the reservoir port  4  shown in FIG.  1 . 
     As explained above, when one of the sub-spool  13  shown in FIG.  2  and the sub-spool  13 - 2  shown in FIG. 6 is operated, the pilot flow passing the feedback slit  12   b  of the logic valve  12  shown in FIG. 3 is changed and the position of the valve body  10   a  of the logic valve  12  in the vertical direction on the drawing is changed and hence the opening of the logic valve  12 , i.e., the opening amount of the unloading circuit  10 , is controlled. 
     After passing the feedback slit  12   b  of the logic valve  12  shown in FIG. 3, the pilot flow is drained to the reservoir port  4   b  through the back pressure chamber  12   c , the pilot passages  14   a - 1  and  14   a - 2  ( 14   a ), the pilot variable throttle  13   a - 2  or  13   b - 2  provided in the sub-spool  13 - 2 , the pilot passage  14   a - 3  ( 14   c ), the pilot variable throttle  13   a  or  13   b  provided in the sub-spool  13 , and the pilot passages  14   b - 1  and  14   b - 2  ( 14   b ). 
     When the sub-spool  13  or  13 - 1  is operated to reduce the opening of one of the pilot variable throttles  13   a ,  13   b ,  13   a - 2 ,  13   b - 2 , the pilot flow is restricted and the pressure in the back pressure chamber  12   c  of the logic valve  12  is raised, whereupon the valve body  12   a  of the logic valve  12  is moved downward on the drawing to restrict the opening amount of the unloading circuit  10 . When the variable throttle of the sub-spool  13  or  13 - 2  is brought into a closed state, the unloading circuit  10  is substantially closed. 
     With this embodiment, as described above, the equivalent function, explained in connection with the first embodiment, to that obtained by the open center directional control valve can be achieved for each of combinations of plural main spools and plural sub-spools; hence similar advantages as obtainable with the first embodiment can be obtained. In addition, the function of a conventional multiple open center directional control valve can be realized by combinations of plural main spools and plural sub-spools. 
     Further, with this embodiment wherein the valve apparatus  52 B is constructed as a multiple valve, the weight of the housing  1  is reduced in proportion to the number of the spools corresponding to a weight reduction of the unloading circuit for each of the conventional open center directional control valves. As a result, the housing  1  can be greatly reduced in weight and size and the manufacture cost can be considerably cut down. 
     Note that, by adding the combination of the main spool and the sub-spool shown in FIG. 2, the valve apparatus can be constructed to include any desired number of directional control valves. 
     A fourth embodiment of the present invention will be described with reference to FIG.  9 . In FIG. 9, equivalent members to those shown in FIGS. 1 and 5 are denoted by the same reference numerals. 
     In a conventional combination of an open center directional control valve and flow rate control for a hydraulic pump, it is general that a throttle with a relief function, for example, is provided as flow rate detecting means in an unloading circuit through which all of a hydraulic fluid delivered from a hydraulic pump passes, and the displacement of the hydraulic pump is controlled in response to a signal from the flow rate detecting means. 
     Such a conventional method has however had a problem that the hydraulic fluid delivered from the hydraulic pump passes the flow rate detecting means (throttle) at a full flow rate sometimes, which results in a larger size and poor reliability of the flow rate detecting means itself. 
     This embodiment intends to solve the above problem as well. A pilot circuit  15 C of throttle and cutoff means  11 C included in a directional control valve apparatus  52 C shown in FIG. 9 is arranged such that flow rate detecting means, e.g., a throttle  30 , for detecting a rate of the pilot flow passing the pilot passage  14 B is disposed in the pilot passages  14   b  between the downstream sub-spool  13  and the reservoir port  4 . A pressure signal generated by the throttle  30  is introduced to a regulator  50   a  for the hydraulic pump  50  through a signal line  31  so that the delivery rate of the hydraulic pump  50  can be adjusted. 
     With this embodiment thus constructed, an equivalent function to that obtained by the conventional open center directional control valve can be achieved. In addition, since the pilot flow passes the throttle  30  as the flow rate detecting means at a small rate, a relief valve which has been conventionally provided in parallel to the throttle can be dispensed with, and the flow rate detecting means can be reduced in size and improved in reliability. 
     A fifth embodiment of the present invention will be described with reference to FIG.  10 . In FIG. 10, equivalent members to those shown in FIGS. 1 and 5 are denoted by the same reference numerals. 
     Referring to FIG. 10, a pilot circuit  15 D of throttle and cutoff means  11 C included in a directional control valve apparatus  52 D of this embodiment has a relief valve  40  connected in parallel to the sub-spool  13 - 2  as shown. When the pressure in the passage portion  14   a  of the pilot passage  14 B rises in excess of the setting pressure of the relief valve  40 , part or all of the pilot flow is drained to the reservoir  53  through the reservoir port  4 . 
     With this embodiment thus constructed, when the pressure in the pilot passage  14   a  in which the pressure rises in relation to the feeder port  3  rises in excess of the setting pressure, the relief valve  40  is opened to lower the pressure in the back pressure chamber  12   c  of the logic valve  12 , whereupon the valve body  12   a  of the logic valve  12  cutting off the communication between the feeder port  3  and the reservoir port  4  is moved upward on the drawing. Accordingly, the relief valve  40  can set an upper limit of the maximum pressure imposed on the feeder port  3 . 
     Thus, by providing the relief valve  40  in the pilot passage  14   a  instead of a conventional large-sized relief valve which has been used to release a large flow rate supplied from the hydraulic pump  50 , and releasing the pilot flow through the relief valve  40 , an equivalent function to that obtained by the conventional relief valve can be achieved and the construction of the hydraulic drive system and the directional control valve apparatus can be further simplified. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, the axial length of the spool of the directional control valve can be shortened without impairing the function of an open center directional control valve, and an improvement in machining accuracy and a more compact structure can be realized. In addition, an equivalent function to that obtained by the open center directional control valve can be developed with no need of using a controller, and the manufacture cost of the directional control valve can be cut down. 
     Also, according to the present invention, in a directional control valve apparatus which is constructed as a multiple valve including a plurality of main spools arranged side by side, a weight reduction of the housing corresponding to need of no bypass port is achieved in proportion to the number of the main spools. As a result, the housing can be greatly reduced in weight and size and the manufacture cost can be considerably cut down.