Abstract:
A hydraulic system has high priority hydraulic functions connected to a primary supply line that receives pressurized fluid from a source and low priority hydraulic functions connected to secondary supply line. A priority valve couples the primary supply line to the secondary supply line. The priority valve detects when the source is unable to furnish enough pressurized fluid to satisfy the demands of all the high and low priority hydraulic functions. In that case the priority valve reduces or eliminates fluid flow between the primary and secondary supply lines.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not Applicable 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a hydraulic system in which pressurized fluid from a source is applied in a controlled manner to a plurality of hydraulic actuators that produce movement of different components on a machine, and in particular to devices that determine which of the hydraulic actuators are to be operable when insufficient fluid is available from the source to operate all the hydraulic actuators. 
   2. Description of the Related Art 
   Modern aircraft employ hydraulic systems to operate various mechanical components, such as ailerons, elevators and the rudder which are parts of the flight control system, as well as doors and landing gear. One or more hydraulic pumps furnish pressurized fluid to a plurality of valve assemblies, each controlling the application of the pressurized fluid to a hydraulic actuator that moves a component on the aircraft. A given valve may be mechanically operated by a member of the flight crew or may be electrically operated either by a crew member or by an electronic controller. 
   Normally, the pumps furnish sufficient hydraulic fluid so that as many of the hydraulic actuators can be operated simultaneously as is necessary. However, conditions occur in which the pumps are incapable of furnishing enough hydraulic fluid to operate all the desired actuators at the same time. At those times, it is desirable that the hydraulic actuators associated with flight control be able to operate as normally as possible. Therefore, when a limited amount of hydraulic fluid is available, that fluid should be allocated to the flight controls on a priority basis before being made available to less critical functions. 
   For that purpose, a priority control valve was incorporated in the hydraulic system to enable flight control actuators to operate as normally as possible, while limiting fluid flow to other less critical hydraulic actuators. Prior priority control valves sometimes exhibited an adverse condition commonly called “thrashing.” That condition occurred when the priority control valve attempted to close in response to the flow to the secondary actuators that caused a reduction in pressure to the primary actuators. The closing action resulted in an increase of the pressure for the flight control actuators to which the priority control valve reacted by attempting to reopen. It is possible for the response time of the hydraulic system to be such that this open-close-open cycle became a continuous, resonant cycling that was harmful to the system. 
   As a consequence, it is desirable to provide a device that automatically recognizes when insufficient hydraulic fluid is available for operating all the hydraulic actuators and allocating the available fluid only to high priority actuators. It is further desired to reduce or eliminate the thrashing condition encountered with previous priority control valves. 
   SUMMARY OF THE INVENTION 
   A hydraulic system has a plurality of hydraulic functions divided into a primary section and a secondary section. A primary supply line receives pressurized fluid from a source and conveys that fluid to the hydraulic functions in the primary section and a secondary supply line provides pressurized fluid to the hydraulic functions in the secondary section. 
   A priority valve controls the flow of fluid from the primary supply line to the secondary supply line. The priority valve has a valve bore with a valve seat therein. An inlet port, connected to the primary supply line, communicates with the valve bore on one side of the valve seat. An outlet port is connected to the secondary supply line and is in communication with the valve bore on another side of the valve seat. 
   A poppet is slideably received in the valve bore thereby defining a control chamber on a side of the poppet remote from the valve seat. Upon sliding in the valve bore, the poppet engages and disengages the valve seat. The poppet includes a spool bore that opens into the control chamber. A first passage provides a conduit for fluid to flow between the inlet port and the spool bore and an end passage creates another conduit for fluid from the inlet port to flow to adjacent the closed end of the spool bore. A second passage extends between the spool bore and the control chamber, while a third passage provides a conduit for fluid to flow between the spool bore and the outlet port. 
   A control spool is slideably received in the spool bore with a surface exposed to pressure adjacent the closed end of the spool bore. In a first position, the control spool creates a first path between the first and second passages and in a second position a second path is provided between the second and third passages. A spring mechanism, such as one or more springs for example, biases the control spool toward the first position. 
   When pressure at the inlet port is below a predefined level, the spring mechanism holds the control spool in the first position which keeps the poppet against the valve seat and the priority valve closed. When sufficient fluid becomes available for powering all the hydraulic functions, pressure at the inlet port increases above the predefined level. That pressure is conveyed adjacent the closed end of the spool bore which creates a force that moves the control spool into the second position. In this state, pressure in the control chamber is relieved through the third passage to the outlet port enabling the inlet port pressure to drive the poppet away from the valve seat to open the priority valve. Thereafter, if an inadequate amount of fluid becomes available, the inlet port decreases below the predefined level causing the control spool to return to the first position. This results in the poppet moving back against the valve seat closing the priority valve. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of a hydraulic system incorporating a priority valve according to the present invention; 
       FIG. 2  is a longitudinal cross sectional view through the priority valve in a closed state; 
       FIGS. 3 through 6  depict the priority valve in sequential stages of opening; and 
       FIGS. 7 through 9  depict the priority valve in sequential stages of closing. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Although the present invention is being described in the context of a hydraulic system for an aircraft, it can be implemented on other types of hydraulically operated equipment where certain hydraulic functions have a higher operational priority than other functions. 
   With initial reference to  FIG. 1 , a hydraulic system  10  for a machine, such as an aircraft, has a reservoir  12  that holds hydraulic fluid. A pump  14  furnishes that fluid under pressure into a daisy chain of supply lines  15  and  16  connected to a plurality of hydraulic functions  17 ,  18 ,  19  and  20 . The first three hydraulic functions  17 ,  18  and  19  are part of a primary section  21  and have a high operational priority functions as compared to the other hydraulic function  20  in a secondary section  22 . For example, the hydraulic functions in the primary section  21  relate to the flight controls that are essential for the aircraft to fly, whereas the hydraulic functions in the secondary section  22  are less critical wherein the aircraft is able to fly without those functions being operational. It should be understood that there may be more functions in both the primary and secondary sections  21  and  22  than those illustrated in  FIG. 1 . 
   Each hydraulic function  17 - 20  controls motion of a machine member and comprises a control valve  24  and a hydraulic actuator  26 , which may be a cylinder/piston assembly or a hydraulic motor, for example. The control valves  24  govern application of pressurized fluid from the primary supply line  16  to the respective actuator  26  and the return flow of fluid from the actuator to a return line  25  connected to the reservoir  12 . The control valves  24  are illustrated as being electrically operated, three-position, four-way spool valves, however manual mechanically operated valves and other types of valves or combinations of valves may be used to control the fluid flow. By selectively operating a control valve  24  into different positions, the direction and speed of the associated actuator  26  is variably controlled. 
   The hydraulic system  10  incorporates a unique priority valve  28  which interfaces the primary supply line  15  in the primary section  21  to the secondary supply line  16  in the secondary section  22  and controls the fluid flow there between. When the pump  14  is unable to furnish sufficient fluid to adequately power all the functions  17 - 20 , the priority valve  28  limits the flow of fluid to the low priority functions in the secondary section  22  to the extent necessary to enable the high priority functions primary section  21  to operate as fully as possible with the available amount of fluid. 
   With reference to  FIG. 2 , the priority valve  28  is a passive device in that it opens and closes in response to pressure levels in the hydraulic system and is not acted on by an electrical actuator, such as a solenoid, or by an external mechanical actuator operated manually or by another mechanism. The priority valve  28  has a body  30  with an inlet port  32  directly connected to the primary supply line  15  and an outlet port  34 , directly connected to the secondary supply line  16 . The term “directly connected” as used herein means that the associated components are connected together by a conduit or coupling without any intervening element, such as a valve, an orifice or other device, which restricts or controls the flow of fluid beyond the inherent restriction of any conduit. The inlet port  32  opens into a side of a valve bore  36  within the body  30  and the outlet port  34  opens into one end of that bore. A valve seat  48  is formed within the valve bore  36  between the inlet port  32  and the outlet port  34 . The end of the valve bore  36  remote from the outlet port  34  is closed by a plug  49  threaded into that bore. 
   A poppet  40  is slideably received within the valve bore  36  without being biased by spring that acts directly on the poppet. The poppet has a nose  47  that selectively engages the valve seat  48  to open and close fluid communication between the inlet and outlet ports  32  and  34  and thereby control the flow of fluid through the priority valve  28 . The pressure at the inlet port  32  thus is applied to the sides of the poppet  40  and the pressure at the outlet port  34  is applied to the nose  47  of the poppet. A control chamber  42  is formed within the valve bore  36  on a remote side of the poppet from the valve seat  48 . A spool bore  44  extends part way into the poppet from the control chamber  42 . A first passage  46  extends transversely through the poppet  40  from an external location that is in constant communication with the inlet port  32  to an intermediate location along the spool bore  44 . An end passage  50  conveys fluid between the inlet port  32  and an opening adjacent the closed end of the spool bore  44 . A second passage  52  extends from another intermediate location along the spool bore  44  to the control chamber  42 . A third passage  54  extends from the poppet nose  47  on the side facing the outlet port  34  to an opening in the spool bore  44  between the opening of the second passage  52  and the control chamber  42 . 
   A valve spool  62  is slideably received within the spool bore  44  in the poppet  40  and has an interior end that abuts the closed end of the spool bore in the illustrated closed state of the priority valve  28 . A portion of the valve spool  62  at the interior end has a reduced diameter providing an end surface  60  on which pressure from the end passage  50  acts even when the spool end abuts the closed end of the spool bore. A second reduced diameter portion is located along the length of the spool forming an annular groove  64 . In the closed state of the priority valve  28 , the groove  64  provides a first path between the first and second passages  46  and  52 , thereby creating a first passageway between the inlet port  32  and the control chamber  42 . 
   The valve spool  62  extends out of the spool bore  44  in the poppet  40  and has an external end that is captured in a recess in a spring shaft  68 , which combined function as a control spool  63 . This two-piece construction of the valve and control spools  62  and  63  is preferred to reduce friction misalignment. Alternatively for less critical applications, the valve spool  62  and the spring shaft  68  can be integrated as a single piece. The remote upper end  74  of the spring shaft  68  extends through an aperture in the bore plug  49  and is exposed to the ambient pressure outside the priority valve  28 . The control spool  63  is passive, meaning that it is not operated by an electrical actuator, such as a solenoid, or by an external mechanical actuator. 
   The spring shaft  68  projects through a spring retainer  70  that is engaged by one end of a spring  72  which has an second end abutting the bore plug  49 . The force of the spring  72  biases the spring shaft  68  and the valve spool  62  toward the poppet  40 . 
   Referring still to  FIG. 2 , when the hydraulic system  10  starts from rest, the pump  14  had been deactivated and the supply lines  15  and  16  are at the relatively low pressure level of the reservoir  12 . As a consequence, the priority valve  28  initially is held in the illustrated closed position shown in  FIG. 2  by the force of the spring  72 . Specifically, the spring force acting on the spring shaft  68  pushes the control spool  63  inward until abutting the closed end of the spool bore  44 . This applies a force that holds the poppet  40  against the valve seat  48 . At this time, the spring force is greater than the forces exerted on the valve by pressures from the supply lines  15  and  16  applied to the inlet and outlet ports  32  and  34 . In this closed state of the priority valve  28 , the poppet groove  64  provides the first path between the first and second passages  46  and  52  which creates a first passageway between the inlet port  32  and the control chamber  42 . 
   As the pump begins operating, pressure in the primary supply line  15  increases, but pressure in the secondary supply line  16  remains at the initial relatively low level, because the priority valve  28  is closed. The primary supply line pressure is applied from the inlet port  32  through the first passageway to the control chamber  42  which further acts to hold the poppet  40  against the valve seat  48 . Eventually the primary supply line pressure at the inlet port  32  increases to the point that exerts a force on the interior end surface  60  of the control spool  63  which balances against the opposing force applied by the spring  72 . Because the upper end  74  of the control spool  63  extends out of the body  30 , it is exposed to the lower ambient pressure at the location of the priority valve  28  in the aircraft. Therefore, pressure in the control chamber  42  does not act on the control spool  63  in a manner that counteracts the pressure at the interior end surface  60 . Thus pressure at the closed end of the spool bore  44  that is applied to the lower end of the control spool, essentially acts only against the force of the spring  72 . 
   Further pressure increase in the primary supply line  15  moves the control spool  63  relative to the poppet  40  and away from the closed end of the spool bore  44 , as shown in  FIG. 3 . At in this position, the first passage  46  does not open into the annular groove  64  thereby terminating communication of pressure between the first passage  46  and the second passage  52  leading to the control chamber  42 . Thus pressure at the inlet port pressure no longer is applied to the control chamber  42  and a constant pressure remains trapped in the control chamber. The trapped pressure in the control chamber  42  holds the poppet  40  against the valve seat  48  keeping the priority valve  28  closed. 
   Continued movement causes the control spool  63  to travel far enough to reach the position shown in  FIG. 4  at which the upper section of the annular groove  64  opens into the third passage  54  that leads to the outlet port  34 . In this position, the second passage  52  still opens into the annular groove  64 , thereby providing a second path between the second and third passages  52  and  54 . This now provides a second passageway between the control chamber  42  and the outlet port  34 . 
   In this state of the priority valve  28 , the higher pressure from the primary supply line  15  at the inlet port  32  is cut off from being applied to the control chamber  42 . The pressure in the control chamber  42 , however, is relieved through the third passage  54 , control spool groove  64  and the second passage  52  into the outlet port  34  and the secondary supply line  16 . With the control chamber pressure released in this manner, the net force, from the inlet port pressure acting on a poppet shoulder  65 , the outlet port pressure acting on the poppet nose  47  and pressure in the control chamber  42 , causes the poppet  40  to follow the control spool  63  and move away from the valve seat  48  as shown in  FIG. 5 . This enables fluid flow between the inlet and outlet ports  32  and  34  and thus from the primary supply line  15  into the secondary supply line  16  in  FIG. 1 . Therefore, a significant pressure change in the primary supply line  15  must occur before the control spool  63  moves enough distance to open the second passageway between the control chamber  42  and the outlet port  34  and enable the poppet  40  to move away from the valve seat. Therefore minor pressure fluctuations are insufficient to open the priority valve  28 . 
   The poppet continues to move away from the valve seat, further enlarging the opening between the inlet and outlet ports  32  and  34 , as illustrated in  FIG. 6 . Increasing pressure continues to move the control spool until it reaches a balanced force intermediate position, as shown in  FIG. 6 . The poppet follows the control spool until passage  54  is blocked. At this time the passageways to and from the control chamber  42  are closed thereby trapping pressure therein that resists further motion of the poppet  40 . Additional pressure increase in the primary supply line  15  as applied to the inlet port  32  may result in the control spool  63  and poppet  40  moving farther upward as a unit against the force of the spring  72 . 
   In this final opened state, the poppet  40  is held open by the equilibrium of forces from the port pressures and the spring  72 . The priority valve  28  remains in this stated depicted in  FIG. 6  under normal operating conditions of the hydraulic system  10  in which pressurized fluid is supplied to the hydraulic functions in both the primary and secondary sections  21  and  22 . 
   Thereafter, if the pump  14  is incapable of furnishing enough hydraulic fluid to operate all the actuators  26  in the system, the priority valve  28  limits the amount of hydraulic fluid that is made available to the secondary section  22 , while allocating as much of the available fluid as is needed to the high priority functions in the primary section  21 . Specifically, when the total demand for fluid exceeds the amount available from the pump  14 , the priority valve  28  closes to the extent necessary to maintain the pressure in the primary supply line  15  at an optimum level. At that time, pressure in the primary supply line  15  is below a level that keeps the priority valve  28  fully open, so that the force of the spring  72  moves the control spool  63  back into the spool bore  44  in the poppet  40  as shown in  FIG. 7 . That action moves the upper edge of the control spool groove  64  below the opening of the third passage  54  maintaining closed the second passageway between the control chamber  42  and the outlet port  34 . However, the poppet does not move with respect to the valve seat  48 . 
   As the inlet pressure continues to decrease, the spring force moves the control spool  63  farther into the spool bore  44  in the poppet  40  as shown in  FIG. 8 . At this new position, the control spool groove  64  communicates with the first passage  46  and still is aligned with the second passage  52 , which again opens the first passageway between the inlet port  32  and the control chamber  42 . This results in the greater primary supply line pressure being applied to the control chamber  42  which forces the poppet  40  toward the valve seat  48  reducing the fluid flow through the priority valve  28  to the secondary supply line  16 . The poppet  40  assumes a partially closed position illustrated in  FIG. 9  at which the amount that the reduction of flow is proportional to the difference between demand for fluid and the amount of fluid available fluid from the pump  14 . 
   If the amount of fluid demanded by the priority hydraulic functions  17 - 19  in the primary section  21  exceeds the amount of fluid available from the pump  14 , the priority valve  28  closes completely returning to the state shown in  FIG. 2 , where all the available fluid is allocated only to the high priority functions. In order to change the position of the poppet  40  (to close), the control spool  63  must open passage  46 . Because of the overlap of the control spool between the first and third passages  46  and  54 , a significant pressure change in the primary supply line  15  must occur before the control spool  63  moves enough distance to close the second passageway between the control chamber  42  and the outlet port  34  and enable the poppet  40  to move toward the valve seat. Therefore, minor pressure fluctuations are insufficient to close the priority valve  28 . 
   The poppet  40  and the control spool  63  form a two-stage priority valve  28  that has hysteresis with respect to the pressure levels at which the valve closes and opens. That hysteresis is provided by the control spool  63  having to travel some distance within the spool bore  44  before a new passageway through the poppet  40  is opened to allow the poppet to move. As a result, a significant pressure change must occur in the hydraulic system in order to affect the fluid flow through the priority valve  28 , in effect adds damping which eliminate the thrashing cycle encountered with previous priority control techniques. In other words, the present priority valve  28  is resistant to oscillating between open and closed states due to minor pressure fluctuations. 
   The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.