Abstract:
When a hydraulic circuit for driving actuators of a construction machine or the like includes a single hydraulic pump and a plurality of direction changeover valves connected to the hydraulic pump for controlling the respective actuators, operation of one of the direction changeover valves may result in undesirable change in the output oil pressure of the other direction changeover valves. The inventive hydraulic circuit includes means for driving a regulator of the hydraulic pump in response to any oil puressure change at the inlets of the direction changeover valves corresponding to the output oil pressure change, thereby compensating for the pressure change.

Description:
BACKGROUND OF INVENTION 
     This invention relates to a hydraulic circuit used in construction machines, industrial vehicles and likes. 
     A typical example of such hydraulic circuit is disclosed in U.S. Pat. No. 4,693,272 and it will be described first with reference to FIGS. 1 and 2. As shown in the drawings, the circuit is arranged such that a plurality of direction changeover valves 54 connected with an actuator 53 are connected to the outlet of a pump 52 of variable discharge type whose discharge quantity varies with a pilot pressure acting to a regulator 51 and each direction changeover valve 54 is provided with a pressure compensation valve 55 and a shuttle valve 56 for selecting the maximum load pressure, thereby causing the load pressure selected by the shuttle valve 56 to act to the regulator 51 and a spring chamber 57 of the pressure compensation valve 55. 
     In operation of the above-mentioned hydraulic circuit, the shuttle valve 56 of each direction changeover valve 54 is connected through passages 59a, 59b and 59c of a spool 58 to a tank passage 60 when the spool 58 is in its neutral position as shown in FIG. 1. Accordingly, a tank pressure acts to the regulator 51 of the variable discharge pump 52 and, therefore, the pump 52 produces at its outlet an oil pressure responsive to a pressing force of a spring disposed in the regulator 51. Accordingly, in the state of FIG. 1, the circuit is held in such a condition that low pressure oil is confined within passages 61 and 62 of the direction changeover valve 54. 
     When the spool 58 of the direction changeover valve 54 is moved rightwards from the position as shown, the passage 59a is first closed by a slide hole of the spool 58 and a load passage 63 and a first supply passage 64 are connected with each other. With such connection of the load passage 63 and the first supply passage 64, the load pressure of the actuator 53 acting on the load passage 63 acts also on the first supply passage 64. This load pressure further acts through the shuttle valve 56, a high pressure selection passage 65 and a passage 68 to the regulator 51. Therefore, the variable discharge pump 52 delivers an oil pressure which is higher than the load pressure of the actuator 53 by an amount corresponding to the spring force in the regulator 51. If the spool 58 is moved further rightwards, the entrance passage 61 is connected to a second supply passage 67 through a metering iris 66 and, at this time, the oil pressure of the second supply passage 67 in the upstream side of the pressure compensation valve 55 becomes a pressure corresponding to the load pressure of the actuator 53 acting on the pressure compensation valve 55 since the load pressure of the actuator 53 has acted already through the high pressure selection passage 65 and the passage 68 to the spring chamber 57 of the pressure compensation valve 55. Accordingly, the pressure difference between the side of the entrance passage 61 (upstream side) and the side of the second supply passage 67 (downstream side) of the metering iris 66 becomes a pressure difference responsive to the spring force of the regulator 51. Therefore, the oil quantity passing the metering iris 66 becomes to have a value responsive to the aperture of the metering iris 66 (or the amount of movement of the spool 58). 
     While the above description has been made on the operation of the direction changeover valve as shown when its spool 58 is moved, similar operation will take place when a plurality of direction changeover valves are operated at the same time. More particularly, at this time, the highest one of the actuator loads connected to the respective direction changeover valves is selected and applied from the high pressure selection passage 65 through the passages 68 and 69 to the spring chamber 57 of the pressure compensation valve 55 of each direction changeover valve 54. Accordingly, the oil pressure difference between the upstream and downstream of the metering iris 66 of each direction changeover valve becomes a differential pressure responsive to the spring force of the regulator 51 and, therefore, the oil quantity passing each direction changeover valve has a value responsive to the amount of operation of each direction changeover valve. 
     When such hydraulic circuit is applied to an actuator of a construction or industrial machine, the variable discharge pump 52 which is used as its oil pressure source is generally driven by an engine and located near the engine. However, the direction changeover valve for feeding pressurized oil discharged from the pump to a plurality of actuators are often located far from the pump. Therefore, the outlet of the pump and the inlet of each direction changeover valve are connected through a hydraulic piping which produces a substantial pressure loss. Thus, there has been such a problem in that the pressure loss may affect the flow control function of each direction changeover valve and this problem will be described in more detail below. 
     When any of the direction changeover valves is operated in the above-mentioned hydraulic circuit, the load pressure of the operated actuator 53 acts on the regulator 51 of the variable discharge pump 52 to produce an oil pressure corresponding to the load pressure. As the pressurized oil is supplied through a piping to the direction changeover valve, the oil pressure of the supply passage of the direction changeover valve is lower than the oil pressure at the outlet of the variable discharge pump 52 by a value corresponding to the piping resistance. Accordingly, the pressure difference formed by the spool 58 of the direction changeover valve across the metering iris 66 becomes lower than a pressure corresponding to the spring force of the regulator 51 by the above-mentioned value corresponding to the piping resistance. The piping resistance increases with increase of the rate of flow therethrough and, therefore, if the aperture of the metering iris 66 increases, the pressure difference across the metering aris 66 will decrease correspondingly. Accordingly, it becomes impossible to control the flow rate in response to the movement of the spool 58. This problem may result in the following trouble, for example. When a plurality of direction changeover valves are operated at the same time and the amount of operation is constantly maintained, if the amount of operation of one of the direction changeover valves is increased (or decreased), the oil quantity flowing from the variable discharge pump to the direction changeover valve will increase (or decrease) correspondingly. Therefore, as described above, the piping resistance will increase (or decrease) to decrease (or increase) the pressure difference across the metering iris 66 of each direction changeover valve, thereby decreasing (or increasing) the flow rate. Accordingly, when two direction changeover valves are operated at the same time, the operation of one direction changeover valve may result in instantaneous reduction (or increase) of the speed of the actuator 53 connected to the other direction changeover valve. 
     Accordingly, an object of this invention is to solve the above-mentioned problem in the prior art and provide an improved hydraulic circuit which enables stable flow rate control. 
     SUMMARY OF INVENTION 
     This object can be attained by a hydraulic circuit according to this invention, which comprises a composite valve connected to the outlet of a hydraulic pump of variable discharge type and composed of a plurality of direction changeover valves having a spool for controlling direction and rate of flow of pressurized oil fed to an actuator. Each direction changeover valve comprises an entrance passage connecting with the outlet of the hydraulic pump, a first supply passage caused to connect with the actuator by the spool, a second supply passage connecting with the entrance passage through a metering iris formed by the spool, and a pressure compensation valve having a pressure chamber disposed between the first and second supply passages. The composite valve includes high pressure selecting means for selecting the highest one of the pressures in the first supply passages of the respective direction changeover valves to apply it to the pressure chambers of ther respective pressure compensation valves and apply a pressure depending upon the highest pressure to a regulator of the hydraulic pump. 
     As a feature of this invention, the hydraulic circuit further comprises a pressure reducing valve disposed between the inlet of the composite valve and an outlet of the hydraulic pump, and the pressure reducing valve is provided with a spring chamber subjected to the oil pressure of the regulator or the hydraulic pump and a pressure chamber subjected to the oil pressure of the entrance passages of the composite valve, thereby maintaining the oil pressure of the entrance passages of the composite valve higher than the oil pressure of the regulator of the hydraulic pump by a valve corresponding to the spring force of the spring chamber. 
     As another feature of this invention, the regulator of the hydraulic pump includes a spring chamber subjected to a pressure depending upon the highest pressure and a pilot chamber opposed thereto and the pilot chamber is connected to the entrance passages of the composite valve. 
     These objects and features of this invention will be described in more detail below in conjunction with its preferred embodiments with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     In the drawings: 
     FIG. 1 is a partly sectional side view of a composite valve used in the prior art hydraulic circuit; 
     FIG. 2 is a partly sectional plan view of the composite valve of FIG. 1; 
     FIG. 3 is a schematic diagram showing an embodiment of the hydraulic circuit according to this invention; and 
     FIG. 4 is a schematic diagram showing another embodiment of the hydraulic circuit according to this invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the first embodiment shown in FIG. 3, a composite valve 2 is composed of a pair of direction changeover valves 1a and 1b having the same structure and a pressure reducing valve 4 and disposed between a hydraulic pump 3 of variable discharge type and a pair of actuators 5a and 5b. Due to the same structure, the structural components of the direction changeover valves 1a and 1b are expressed by the same numerals suffixing &#34;a&#34; and &#34;b&#34;, respectively, and the description will be made mainly about the first direction changeover valve 1a. 
     The direction changeover valve 1a includes a main body 10a having a plurality of internal passages and a spool 12a having major diameter portions 20a, 21a, 22a and 23a, minor diameter portions 24a, 25a and 26a and taper portions 27a and 28a, the major diameter portions being slidably fit in a through-hole 11a in the body 10a. The through-hole 11a has openings of a pair of load passages 13a and 14a connecting with an actuator 5a through a pair of conduits 6a and 7a, respectively, an entrance passage 15a connecting with the hydraulic pump 3a and an entrance passage 15b of the direction changeover valve 1b, first and second supply passages 16a and 17a located between the load passages 13a and 14a and the entrance passage 15a, and a pair of discharge passages 18a and 19a connecting with a tank 8. In the position as shown (hereinunder referred to as &#34;neutral position&#34;), the spool 12a shuts off the load passages 13a and 14a from the discharge passages 18a and 19a, and the first and second supply passages 16a and 17a from the entrance passage 15a, respectively, by its major diameter portions 20a, 23a, 21a and 22a. 
     When the spool 12a is moved leftwards from the neutral position to a position hereunder referred to as &#34;first changeover position&#34;, the minor diameter portion 24a connects the load passage 13a to the discharge passage 18a and the minor diameter portion 26a connects the load passage 14a to the first supply passage 16a. Then, the taper portion 27a forms an iris responsive to movement of the spool 12a between the second supply passage 17a and the entrance passage 15a. 
     When the spool 12a is moved rightwards from the neutral position to a position hereinunder referred to &#34;second changeover position&#34;, the minor diameter portion 26a connects the load passage 14a to the discharge passage 19a and the minor diameter portion 24a connects the load passage 13a to the first supply passage 16a. Then, the taper portion 28a forms an iris responsive to movement of the spool 12a between the second supply passage 17a and the supply passage 15a. 
     A peripheral groove 29a formed in the major diameter portion 22a serves to connect a pilot passage 30a formed in the main body 10a and opened to the through-hole 11a to the tank 8 when the spool 12a is in the neutral position and to disconnect the former from the latter when the spool is in the other positions. The first and second supply passages 16a and 17a have pilot passages 31a and 42, respectively, branching away therefrom. 
     A pressure compensation valve 33a includes a valve seat 34a disposed between the first and second supply passages 16a and 17a, a valve body 37a put in contact with the valve seat 34a to form a pilot chamber 35a and a spring 36a tensed in the pilot chamber 35a to urge the valve body 37a. The pilot chambers 35a and 35b of the pressure compensation valves 33a and 33b are connected through a pilot passage 39 and a high pressure selection passage 38 to either of the pilot passages 31a and 31b. The high pressure selection passage 38 has two inlets 38a and 38b connected to the pilot passages 31a and 31b, respectively, and an outlet 38c connected to the pilot passage 39 and serves to connect one of the pilot passages 31a and 31b having a higher fluid pressure to the pilot passage 39. Therefore, the pressure compensation valves 33a and 33b operate when the spools 12a and 12b of the direction changeover valves 1a and 1b are moved to the first or second changeover position, so that the higher one of the fluid pressures in the first supply passages 16a and 16b acts to both pilot chambers 35a and 35b to make the fluid pressures in the second supply passages 17a and 17b equal to respective sums of the fluid pressures in the pilot chambers 35a and 35b and the urging pressures of the springs 36a and 36b. 
     The hydraulic pump 3 of variable discharge type includes a regulator 3b and a hydraulic pump 3d and has its discharge port 3e connected through a pump passage 15 to the pressure reducing valve 4 disposed in the upstream of the composite valve 2. When the regulator 3b is in changeover position 3n as shown, a pressure chamber 3c of a control cylinder 3a of the hydraulic pump 3d is connected to the tank 8 and, therefore, a pressure at the discharge port 3e is kept at a minimum value by a spring force of the control cylinder 3a. The regulator 3b includes a spring chamber 3f having a spring 3k and connecting with a pilot passage PL1 for transmitting an oil pressure responsive to the output of the high pressure selection passage 38 of the composite valve 2, and a pressure chamber 3g connecting with the discharge port 3e of the hydraulic pump 3d, and serves to control the discharge pressure of the hydraulic pump 3d in accordance with a quantitative relationship between the urging forces of the spring chamber 3f and the pressure chamber 3g. The spring 3k of the regulator 3b is selected to be strong enough to compensate for a pressure loss produced at the maximum flow rate in the pump passage 15, as compared with a spring 4b of the pressure reducing valve 4. 
     When no hydraulic signal is transmitted through the pilot passage PL1 to the regulator 3b, the urging force of the spring chamber 3f is attributable only to the spring 3k. Therefore, if the discharge pressure of the hydraulic pump 3d exceeds the urging force of the spring 3k, the regulator 3b turns to another changeover position 3m to connect the pressure chamber 3c to the discharge port 3e. Thus, the discharge pressure acts to the pressure chamber 3c to stop increase of the discharge pressure of the hydraulic pump 3b. Accordingly, the discharge pressure of the hydraulic pump 3d is kept at a low value corresponding to the urging force of the spring 3k when no hydraulic pressure acts on the pilot passage PL1. 
     When an oil pressure acts in the pilot passage PL1, this pressure acts to the spring chamber 3f of the regulator 3b in addition to the urging force of the spring 3k. Accordingly, the discharge pressure of the hydraulic pump 3d is maintained at a value which is higher than the oil pressure in the pilot passage PL1 by a value corresponding to the urging force of the spring 3k. 
     The pressure reducing valve 4 disposed between the discharge port 3e of the hydraulic pump 3d and the entrance passage 15b of the composite valve 2 includes a spring chamber 4a having a spring 4b at an end thereof and connecting with the pilot passage PL1 and a pressure chamber 4c subjected to the oil pressure of the entrance passage 15b for opposing the spring chamber 4a, and has two change-over positions 4n and 4m. While the pressure reducing valve 4 is generally in the fully opened position 4n, it turns to the partially opened position 4m, when the pressure of the pressure chamber 4c exceeds the pressure of the spring chamber 4a, for keeping the pressure in the entrance passage 15b at a valve corresponding to the pressure of the spring chamber 4a. Thus, the oil pressure in the entrance passage 15b is kept at a value which is higher than the oil pressure in the pilot passage PL1 by a value corresponding to the urging force of the spring 4b of the spring chamber 4a. 
     The raised oil pressure in the entrance passage 15a is transmitted through the iris to the second supply passage 17a to open the pressure compensation valve 33a. Accordingly, the pressurized oil in the second supply passage 17a flows through the first supply passage 16a and the conduit 6a into the actuator 5a and the oil discharged from the actuator 5a flows through the conduit 7a, the load passage 14a and the discharge passage 19a into the tank 8 to initiate operation of the actuator 5a. At the same time, the oil pressure in the pressure chamber 35a of the pressure compensation chamber 33a acts as a load pressure of the actuator 5a. 
     In the above-mentioned operation, the pressure difference across the metering iris formed by the taper portion 28a of the spool 12a of the direction changeover valve 1a assumes a value corresponding to the urging force of the spring 4b of the spring chamber 4a of the pressure reducing valve 4. When the spool 12a is moved further rightwards, the aperture of the metering iris is increased and, therefore, the oil pressure in the entrance passage 15a is instantaneously reduced. However, this instantaneous reduction is transmitted to the pressure chamber 4c of the pressure reducing valve 4 to cause the pressure reducing valve 4 to supply the deficit of pressurized oil immediately. Thus, the oil pressure in the entrance passage 15a is recovered soon and maintained at a value corresponding to the urging force of the spring 4b of the pressure reducing valve 4. 
     It is obvious that a similar operation is effected to move the actuator 5a in the opposite direction when the spool 12a is moved leftwards, and it is also the case when only the other spool 12b is moved. 
     While the above description has been made on the case where only one of the direction changeover valves of the composite valve 2 is operated, a similar operation is effected also when both direction changeover valves 1a and 1b are operated concurrently. Now, the description will be made below about this case. 
     When the spools 12a and 12b of the direction changeover valves 1a and 1b are moved concurrently rightwards from the neutral position, the pilot passage 39 is first closed by displacement of the grooves 29a and 29b and, next, the load passages 13a and 13b are connected respectively to the first supply passages 16a and 16b through the minor diameter portions 24a and 24b to apply the load pressures of the actuators 5a and 5b to the first supply passages 16a and 16b, respectively. Then, higher one of the load pressures is applied through the pilot passage 31a or 31b, the high pressure selection passage 38 and the pilot passage 39 to the pressure chambers 35a and 35b of both pressure compensation valves 33a and 33b. Therefore, the output pressure of the hydraulic pump 3d increases in response to the oil pressure in the pilot passage PL1 and the pressure reducing valve 4 controls this output pressure and applies it to the supply passage 15a. 
     When both spools 12a and 12b are moved further rightwards, the taper portions 28a and 28b form metering irises between the entrance passages 15a and 15b and the second supply passages 17a and 17b, respectively. Therefore, the pressurized oils in the entrance passages 15a and 15b  flow into the second supply passages 17a and 17b through these metering irises to apply the oil pressure through the pilot passage PL1 to the spring chamber 4a of the pressure reducing valve 4, thereby increasing the oil pressure in the pump passage 15 and the entrance passages 15a and 15b. When the oil pressure in the second supply passages 17a and 17b exceeds the pressure in the pressure chambers 35a and 35b of the pressure compensation valves 33a and 33b, it opens the pressure compensation valves 33a and 33b to flow the pressurized oil into the first supply passages 16a and 16b and then through the load passages 13a and 13b into the actuators 5a and 5b, respectively. The rate of flow at this time is kept at a value corresponding to the movement of the spool of each direction changeover valve, since the pressure difference across the metering iris of each direction changeover valve is kept in response to the urging force of the spring 4b of the pressure reducing valve 4. 
     In the above-mentioned operation, if one spool 12a, for example, is moved further rightwards to increase the aperture of its metering iris, the amount of oil flowing into the actuator 5a increases correspondingly to cause instantaneous reduction of the oil pressure in the entrance passages 15a and 15b. However, this pressure reduction is compensated by the pressure reducing valve 4 and the pressure difference across the iris is kept constant. 
     If the amount of oil required by the actuators 5a and 5b exceeds the amount of discharge of the hydraulic pump 3d upon movement of the spools 12a and 12b of the direction changeover valves 1a and 1b, the pressure compensation valves 33a and 33b confine the rate of flow from the second supply passages 17a and 17b to the first supply passages 16a and 16b to keep the oil pressure in the second supply passages 17a and 17b at the pressure of the spring chamber 4a of the pressure reducing valve 4. Accordingly, if the amount of discharge of the variable discharge hydraulic pump 3d becomes lacking with respect to the demand of the actuators 5a and 5b, the amount of flow to the actuators reduces concurrently. However, the rate of reduction corresponds to the aperture of the metering irises formed by the taper portions 28a and 28b of the spools 12a and 12b. 
     As described above, the pressure reducing valve disposed in the upstream of the composite valve keeps the oil pressure of the supply passages of the composite valve higher than the maximum load pressure of the actuators by a value corresponding to the urging force of the spring of the spring chamber. Thus, the piping resistance between the hydraulic pump and the composite valve can be avoided from affecting the flow rate control. 
     FIG. 4 shows another embodiment of this invention, which differs from the above mentioned embodiment of FIG. 3 in that the pressure reducing valve is removed and, instead, another pilot passage is provided for leading the output pressure of the hydraulic pump acting on the pilot chambers of the regulators directly from the supply passages of the composite valve. 
     In the drawing, a composite valve 2 is provided with a port PS which enables to directly take out the pressure of the upstream of the metering iris of the spool 12a or 12b from the entrance passage 15a or 15b. On the other hand, the variable discharge hydraulic pump 3 is provided with a port PS&#39; which enables to take the output pressure of the pump into the pressure chamber 3g of the regulator 3b which is opposed to the spring chamber 3f thereof from the outside. Then, the port PS of the composite valve 2 and the port PS&#39; of the pump 3 are connected through a pilot passage PL2. 
     In operation, the downstream pressure of the metering irises formed by the spools 12a and 12b of the composite valve 2 is transmitted through the second supply passage 17a or 17b to the port LS to act directly to the spring chamber 3f of the regulator 3b through the pilot passage PL1. On the other hand, the upstream pressure of the metering iris is transmitted from the supply circuit 15a or 15b to the port PS to act directly to the pressure chamber 3g of the regulator 3b through the pilot passage PL2. For example, when the aperture of the metering iris formed by the spool 12a increases, the pressure difference across the iris tends to decrease. However, the regulator 3b actuates the control cylinder 3a of the hydraulic pump 3 to increase its amount of discharge so that the pressure difference corresponds to the urging force of the spring 3k. Therefore, the controlled rate of flow becomes to have a value corresponding to the movement of the spool of each direction changeover valve. Thus, influence of the pressure loss in the pump passage 15 can be fully avoided by only adding the thin pilot passage PL2 between the hydraulic pump 3 and the composite valve 2. 
     The above embodiments are provided for illustrative purpose only and do not limit the invention. It should be obvious for those skilled in the art that various modifications and changes can be added to the embodiments without leaving the scope of invention which is defined in the appended claims.