|5996623||Hydraulic directional-control valve|
|6318079||Hydraulic control valve system with pressure compensated flow control|
This invention relates to a hydraulic directional control valve. More particularly, the invention relates to a spool valve in which a flow path controlled by the valve spool and extending between an inlet and a tank port is kept open when the valve spool is in a neutral position so that hydraulic fluid entering through the inlet can flow direct to the tank port with a very low pressure drop (open-center valve), according to the characteristics of the preamble of independent claim 1.
Although the applicability of the invention is not necessarily limited to mobile hydraulic systems, the invention is of particular interest in such systems, especially in mobile hydraulic systems in which two or more directional control valves are connected to a common hydraulic fluid source and form a valve block.
Open-center spool valves are common in mobile hydraulic systems, because in such systems it is often desirable to be able to control several functions simultaneously and supply the hydraulic actuators or motors (such as cylinders or rotary motors) from a common pump. Examples of such valves are described in
A disadvantage of open-center valves becomes apparent when several functions (actuators) are operated simultaneously, because then the functions influence one another such that the control of each function becomes both load-dependent and dependent on other functions which are activated at the same time.
The load-dependency occurs because the rate of flow to and from the work ports of the valve, and thus to the actuators, is determined not only by the position of the valve spool, but also by the load, that is, the pressure required by the load.
When several functions are operated simultaneously, the pressure generated by the pump is determined by the function that requires the highest pressure, that is, the function that moves the greatest load. The fluid flows to the simultaneously operated functions therefore become dependent on one another. As a result, the control of the flows becomes more difficult. A solution for reducing this dependency, the so-called load interference, that is often used is to design the valve spools such that the pump will have to generate a high pressure before the valve opens the flow path to the work ports of the valve. The reduced load interference comes at a price, however, namely high energy losses due to the high pump pressure required to open the flow path.
To meet the requirement for proper control with small energy losses, so-called load-sensing valves have been developed, which are normally also pressure compensated to provide load-independent control of the functions.
However, the load sensing and the pressure compensation is associated with a high degree of complexity in load-sensing valves. Systems with load-sensing valves also are prone to oscillations and therefore require some kind of damping, but damping causes energy losses.
An object of the invention is to provide spool valve that meets the requirement for proper control and small energy losses without having the disadvantages described above which are associated with load-sensing valves.
To this end, the invention provides a hydraulic spool valve of the open-center type which comprises a pressure compensating valve disposed such that the flow of hydraulic fluid from the work port to the actuator is pressure compensated and thus load-independent.
In the directional control valve according to the invention, the pressure compensating valve forms a restriction that limits the pressure drop across the restriction which meters the flow of hydraulic fluid to the work port (the "meter-in" restriction), so that the flow of hydraulic fluid to the work port, and thus to the actuator, can be limited to a selected value determined by the position of the valve spool.
If the valve has a pair of work ports to control a double-acting device, such as a double-acting cylinder or some other reversible actuator, a pressure compensating valve can be associated with each of the two work ports. In operation of the valve according to the invention, only the pressure compensating valve associated with the work port that is receiving the fluid flow from the pump operates to control the fluid flow to the actuator to keep the flow independent of the load, while the other pressure compensating valve does not carry out any flow controlling function.
The invention can be embodied in conventional open-center valves with relatively small modifications.
The invention will be described in greater detail below with reference to the accompanying drawings.
The directional control spool valve 10 shown in Fig. 1 may form one of two or more control valve sections which are united to form a valve block and supplied with hydraulic fluid from a common source. It has a valve housing 11 comprised of two housing parts 12, 13.
Housing part 12 with the components therein can be regarded as essentially corresponding to a conventional spool valve of the open-center type. A bore formed in the housing part 12 accommodates a valve spool 14. This valve spool is spring-biased toward a central neutral position and can be displaced axially in either direction, manually by means of a lever 15, or hydraulically by means of a pair of electrohydraulic valves 16.
As is common practice, valve spool 14 is provided with lands which control, in cooperation with the housing, the flow of hydraulic fluid (liquid) in passageways and recesses which communicate with the bore. Two branches of a feed passageway 17 communicate via a check valve 18 with a first inlet port 19 and can be connected with a first work passageway 20 by displacing the valve spool 14 to the right and connected with a second work passageway 21 by displacing the spool to the left.
When the valve spool 14 is displaced to the right and connects the left branch of the feed passageway 17 with the first work passageway 20, the second work passageway 21 is connected with a recess 22 which is in constant open communication with a first tank port 24 (hidden in Figs. 1 and 3). Similarly, when the valve spool 14 is displaced to the left and connects the right branch of the feed passageway 17 with the second work passageway 21, the first work passageway 20 is simultaneously connected with a recess 25 which, like the recess 22, is in constant open communication with the first tank port 24.
Whether the valve spool 14 is displaced to the right or to the left from the illustrated neutral position, the "open-center" position, it will block or at least drastically reduce a passageway, hereafter referred to as the "open-center" passageway, which in the neutral position allows hydraulic fluid to flow substantially without pressure drop through the valve housing part 12 between a second inlet port 26 and a second tank port 27 (ports 26 and 27 are hidden in Figs. 1 and 3 but indicated in Figs. 2 and 4.
If an abnormally high pressure should appear in any of the work passageways 21 and 22, that work passageway is connected through an associated, normally closed chock valve 28, 29 (shown in Figs. 1 and 3 only) with the adjacent recess 22, 25 and thus with the first tank port 24.
To the extent it has been described so far, the spool valve 10 shown in Fig. 1 is conventional in its basic structure and function. However, in conventional spool valves of the kind described, each work passageway opens into a work port which is connected direct to the actuator. Moreover, in conventional valves, the feed passageway 17 is not open to the outer face of the first housing part 12 as shown in Fig. 1.
In the directional spool valve shown in Fig. 1, work ports 30, 31 are provided in the housing part 13 and communicate with the work passageways 20 and 21, respectively, by way of respective pressure compensating valves 32 and 33.
Each of these pressure compensating valves 32, 33 comprises a compensator piston 34, 35 which is urged toward an open position by a spring 36, 37. In the open position of the compensator piston, hydraulic fluid can flow past the pressure compensating valve 32, 33 without any substantial pressure drop. Fluid pressure acting on an end face of the compensator piston 34, 35 from the direction of the work passageway 20, 21 urges the compensator piston 34, 35 in the same direction as the spring 36, 37, that is, toward the open position.
The surface area of the opposite face of the compensator piston 34, 35 is equal to that of the first face and constantly subjected to the fluid pressure in the first inlet port 19 and the feed passageway 17 through a passageway 38 in the housing part 13, an opening 39 in the housing part 12 and a passageway 40 in the check valve 18. Thus, this pressure urges the compensator piston 34, 35 toward the closed position in opposition to the spring force and in opposition to the fluid pressure the work passageway 20, 21.
Together with the housing part 13 the compensator piston 34, 35 forms a flow-through passageway 41, 42 between the work passageway 20, 21 and the work port 30, 31. In Fig. 1, this flow-through passageway 41 of the pressure compensating valve 32 at the first work port 30 is shown in a closed or almost closed position (the end face of the compensator piston 34, 35 which is exposed to the passageway 38 in the housing part 13 is presumed to be subjected to operating fluid pressure), while the flow-through passageway 42 of the pressure compensating valve 33 at the other work port 31 is shown in a completely open position.
Movement of the compensator piston 34, 35 toward the closed position in opposition to the force of the spring 36, 37 is limited by a rod 43, 44 on the spring-biased side of the compensator piston.
In operation of the valve 10, when the valve spool 14 is displaced to the right, the compensator piston 34 of the pressure compensating valve 32 maintains the pressure differential between the passageway 38 and the work port 30 at a value that is determined by the force applied by the spring 36. The pressure differential between the left branch of the feed passageway 17 and the work passageway 20 is maximised to a certain value. Consequently, the fluid flow to the work port 30 will be limited.
Naturally, if the valve spool 14 is displaced to the left, a corresponding action of the compensator piston 35 of the other pressure compensating valve 33 is obtained.
Fig. 2 shows a hydraulic system which comprises a valve assembly formed by an inlet valve section 45, two identical directional control valve sections, each of them being a spool valve 10 as shown in Fig. 1, a fixed-displacement pump 46 and a tank 47 for hydraulic fluid (liquid). The three valve sections 10 and 45 are joined to form a valve block in which the valve sections communicate directly with one another through internal passageways.
The inlet valve section 45 is supplied with hydraulic fluid from the pump 46 through an inlet port 48. It comprises an electrically operated shut-off valve 49, which communicates with the inlet port 48 through a pair of restrictors 50, 51, and a two-position directional control valve 52, which is connected direct to the inlet port 48. In one of the positions the outlet of the two-position valve 52 is connected to the tank 47 and in the other position the outlet is connected to the first inlet port 19 of the adjacent directional control valve 10. Moreover, the inlet valve section comprises a proportional two-position valve 53 for diverting a portion of the pump output flow to the tank 47 and also a relief valve 54.
When the hydraulic system is idling, pump 46 delivers hydraulic fluid through the two inlet restrictors 50, 51 to and through the shut-off valve 49 and back to the tank 47, and also through a restrictor 55 located in the "open-center" passageway 56 and through the directional control valves 10 back to the tank 47, and moreover through the valve 52, the spool of which is held in its upper position in opposition to the force of a biasing spring to pass hydraulic fluid to the tank 47 without any significant pressure drop. Valve 53 is in the closed position.
Closing of the shut-off valve 49 (by the operator) activates the hydraulic system. When the valve 49 is closed, the spool of valve 52 is displaced to its lower position so that the first inlet ports 19 of the directional control valves 10 are pressurised and valve 53 opens to pass a substantial portion of the output flow of the pump 46 to the tank 47. The "open-center" passageway 56 still passes a portion of the pump output flow direct to the tank 47.
When any of the directional control valves 10 is operated - it is here assumed that the directional control valve 10 closest to the inlet valve section 45 is operated, namely by displacement of its valve spool 14 to the left, as viewed in Fig 1 - hydraulic fluid is supplied by way of the valve 52 to the operated directional control valve 10 through its check valve 18 and through its pressure compensating valve 32 and work port 30 to the associated double-acting actuator C. In Fig. 2, this actuator is shown as a double-acting cylinder. From that actuator hydraulic fluid flows back to the operated directional control valve 10 and through it by way of its work port 31, which now serves as a return port, a check valve 58 associated with the pressure compensating valve 33 (a corresponding check valve 57 is associated with the other pressure compensating valve 32), and the tank port 24 back to the tank 47.
The "open-center" inlet of the directional control valve - this inlet is the second inlet port 26 - is now completely or at least almost blocked by the valve spool 14 of the directional control valve 10. If the pump output flow is greater than the flow required by the actuator C plus that portion, if any, of the pump output flow which is still flowing through the "open-center"-passageway 56, the excess is passed direct back to the tank 47 through the valve 53 of the inlet section 45.
If the valve spool 14 is instead displaced to the right, the function of course will be similar, the only difference being that the work port 30, instead of being a feeding work port, serves as a return port with the aid of the check valve 57 and the work port 31 serves as a feeding work port.
The modified directional control spool valve 10A shown in Fig. 3 is essentially identical with the valve 10 shown in Fig. 1 except in respect of the hydraulic control inputs, represented in the drawings by the passageways 38, 40 (Figs. 1 , 2) and 38, 40A (Figs. 3, 4), of the pressure compensating valves 32, 33. As shown in Fig. 2 by the broken lines representing the passageways 38 and 40, the control inputs of the pressure compensating valves 32, 33 of the valve 10 of Fig. 1 are separately connected to the downstream side of the check valves 18.
In the valve 10A of Fig. 3, on the other hand, as shown in Fig. 4, the control inputs of the pressure compensating valves 32, 33 are connected to the downstream side of the check valves 18 through the passageway 38, a common passageway 40A and separate check valves 59. Accordingly, in the hydraulic system shown in Fig. 4, all pressure compensating valves 32, 33 are always controlled in respect of the greatest load, that is, the greatest work port pressure.
Owing to the pressure compensation provided by the pressure compensating valves 32, 33, as long as the pump capacity is adequate, the control of the load represented by the actuator C is not affected if the other directional control valve 10, or any additional similar directional control valve 10, is operated simultaneously with the operation of the first directional control valve 10. Moreover, a change of the valve spool position of one of the directional control valves 10 being operated does not affect the function of any simultaneously operated directional control valve 10.
If the pump capacity should become inadequate when several directional control valves 10 are operated simultaneously, none of the functions will be disabled completely. Instead, the available pump capacity will be allocated with equal percentages to all simultaneously activated functions.
The illustrated and described location of the pressure compensating valves 30 and 31 in a separate housing part 13 joined with another housing part 12 is beneficial, but of course is not essential to the operability of the directional control valve according to the invention. Moreover, it is not necessary to provide pressure compensating valves in accordance with the invention for both or all work ports.
Moreover, while the disposition of the pressure compensating valves shown in the drawings and described above is beneficial, other locations along the flow path from the upstream or inner end of the work passageways to the work port or ports may be contemplated and suitable, depending on the size or design of the pressure compensating valves, for example.