Patent Publication Number: US-2003221730-A1

Title: Multi-stage multi-piston valve

Description:
[0001] This is a continuation-in-part of application Ser. No. 10/051,884, filed Jan. 17, 2002. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The invention is in the field of fluid control. More particularly, the invention is a multi-piston spool valve capable of controlling the flow of fluid to a dynamic hydraulic component, such as a hydraulic actuator or motor. The valve enables either zero, restricted or full fluid flow to the hydraulic component, thereby allowing multiple stages or levels of control of the component. A user of the invention is thereby provided with the ability to cause the controlled component to make rapid, major movements, or slower, more precise movements.  
       [0003] The valve&#39;s functionality is achieved using a system of multiple pilot valves that act on associated pistons located within the valve. The valve&#39;s spool is moved through the action of at least one pilot piston. One of more stop pistons are employed to limit the movement of the pilot piston(s).  
       BACKGROUND OF THE INVENTION  
       [0004] The flow of fluid to a hydraulic actuator, hydraulic motor, or other dynamic hydraulic device is often controlled through the use of a pilot-operated spool valve. In most cases, the spool valve is used solely to provide directional control whereby the controlled device either receives no flow, maximum flow in a first direction, or maximum flow in a reverse direction. To accomplish this functionality, the pilot acts to cause a maximum movement of the valve&#39;s spool. Once the spool has moved fully in one direction, maximum fluid flow is enabled to the controlled hydraulic device. To cause the controlled hydraulic device to stop or reverse direction, a reverse movement of the valve&#39;s spool is required. It should be noted that when maximum flow is enabled, the controlled device moves at its maximum speed.  
       [0005] There are some applications where a pilot-operated spool valve is employed to provide proportional control of a hydraulic component. In this type of application, it is usually desired to cause the controlled hydraulic component to move at speeds greater than zero but less than the component&#39;s maximum speed. In some applications, proportional control is achieved using a spool-type servovalve.  
       [0006] One example of a servovalve designed to give a user proportional control of a dynamic hydraulic device is taught by Sloate in U.S. Pat. No. 4,674,539. The Sloate servovalve makes use of an electric motor in combination with threadedly-engaged members to slowly cause the translation of the servovalve&#39;s spool. However, the speed of operation of such a unit is severely limited. Sloate notes that changing the thread ratios employed in the device can change the speed of operation.  
       [0007] Proportional control of a hydraulic component enables precision control of the component. However, there are times when it would be desirable to have multi-speed control of a hydraulic device. This type of control would offer both simple directional control and precision proportional control of the hydraulic component.  
       [0008] A first example where multi-speed control is desirable is found when a hydraulic motor is connected to a winch. It is often advantageous to initially lift a load at a low speed, giving one a chance to assess the security of the lifting harness, before lifting the load at full speed.  
       [0009] A second example may be found when a hydraulic motor is used to operate a cooling fan. A typical arrangement would employ a control valve that enables the fan to run at full speed, or not at all. There may be certain conditions or situations where one or more intermediate speeds are desirable.  
       [0010] A third example is presented in some marine steering systems, where a hydraulic actuator is connected to a rudder or water deflector. In this type of application, it is desirable during relatively high-speed operation of the vessel for the rudder or water deflector to move fairly slowly. This enables a precise steering control of the vessel. When traveling at a relatively low speed, such as during docking maneuvers, one needs to move the rudder or water deflector at a very high rate in order to obtain the necessary movements of the vessel in an appropriate amount of time. In addition, when the vessel is docked, it may be beneficial to rapidly move the rudder/water deflector to a predetermined storage position.  
       [0011] There are many other situations where multi-speed control of a hydraulic component would be advantageous. The situations would usually also require the control system to be relatively low in cost, extremely durable and highly reliable.  
       SUMMARY OF THE INVENTION  
       [0012] The invention is a multi-piston spool valve capable of controlling a dynamic hydraulic component, such as a hydraulic actuator or motor. The valve allows a user to enable either a zero, restricted or full fluid flow to the hydraulic component. When a restricted flow of fluid is enabled, the user can achieve slow, precise movements of the component. When full fluid flow is enabled, the user can cause major, maximum-speed movements of the controlled component.  
       [0013] The operation of the valve is accomplished using a system of pilot valves. The system comprises a primary pilot valve arrangement (primary pilot) and at least one secondary pilot valve (secondary pilot). The primary pilot is operatively connected to at least one pilot piston located in the spool valve. The spool is operatively connected to the pilot piston(s) whereby the pilot piston(s) function to cause a translation of said spool. The secondary pilot is operatively connected to at least one stop piston located in the spool valve. The stop piston(s) function to oppose/limit the full movement of the pilot piston(s).  
       [0014] When full fluid flow to the component is desired, the primary pilot directs pressurized fluid into a chamber in the spool valve that is located adjacent a pilot piston. The fluid then applies pressure on one end of said pilot piston. This causes the pilot piston, and the operatively-connected spool, to move. Without any opposition from the stop piston(s), the pilot piston and spool can move to their maximum extent. This results in an outlet port in the spool valve being fully uncovered, thereby enabling the maximum rate of fluid flow to the controlled hydraulic component.  
       [0015] When a restricted flow of fluid to the component is desired, the primary pilot and at least one secondary pilot are actuated. When only one secondary pilot is employed, the secondary pilot sends pressurized fluid into a chamber associated with a stop piston. This causes the stop piston to be positioned at a predetermined location. At the same time, the primary pilot acts in the same manner as previously described, sending pressurized fluid into a chamber in the spool valve and causing a pilot piston to move the spool. However, the movement of the pilot piston and the spool is stopped short by the stop piston.. As a result, a fluid outlet in the spool valve that leads to the controlled component will be only partially uncovered, resulting in a restriction in the fluid flow path. This leads to an intermediate flow of pressurized fluid to the controlled hydraulic component.  
       [0016] When the spool valve includes multiple stop pistons, multiple secondary pilot valves (secondary pilots) are employed to control the movement of the stop pistons. The multiple stop pistons interact to provide multiple limit stops that affect a pilot piston&#39;s allowed travel. When two stop pistons are employed, the spool valve will be capable of providing five levels or stages of fluid flow to the controlled hydraulic component.  
       [0017] A fluid flow control valve and system in accordance with the invention is relatively low in cost and requires a minimal number of solenoids to control the valve&#39;s operation. The system&#39;s simple design enables it to be highly reliable and extremely durable. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0018]FIG. 1 is a cross-sectional view of a multi-piston spool valve in accordance with the invention.  
     [0019]FIG. 2 is a cross-sectional view of a portion of the valve shown in FIG. 1. The valve portion is shown when the valve is in a no-flow condition.  
     [0020]FIG. 3 is a cross-sectional view identical to that shown in FIG. 2 except that the valve portion is shown when the valve is in a full-flow condition.  
     [0021]FIG. 4 is a cross-sectional view identical to that shown in FIG. 2 except that the valve portion is shown when the valve is in a limited-flow condition.  
     [0022]FIG. 5 is a system diagram showing the valve of FIG. 1 connected to a primary pilot, a secondary pilot and a hydraulic actuator.  
     [0023]FIG. 6 is a cross-sectional view of a portion of a second embodiment of a multi-piston spool valve in accordance with the invention. The valve portion is shown when the valve is in a no-flow condition.  
     [0024]FIG. 7 is a cross-sectional view identical to that shown in FIG. 6 except that the valve portion is shown when the valve is in a first limited-flow condition.  
     [0025]FIG. 8 is a cross-sectional view identical to that shown in FIG. 6 except that the valve portion is shown when the valve is in a second limited-flow condition.  
     [0026]FIG. 9 is a cross-sectional view identical to that shown in FIG. 6 except that the valve portion is shown when the valve is in a third limited-flow condition.  
     [0027]FIG. 10 is a cross-sectional view identical to that shown in FIG. 6 except that the valve portion is shown when the valve is in a full-flow condition.  
     [0028]FIG. 11 is a system diagram showing a valve modified per FIG. 6 connected to a primary pilot, two secondary pilots and a hydraulic actuator.  
     [0029]FIG. 12 is a cross-sectional view of a third embodiment of a multi-piston spool valve in accordance with the invention.  
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
     [0030] Referring now to the drawings in greater detail, wherein like reference numbers refer to like parts throughout the several figures, there is shown by the numeral  1  a multi-piston spool valve in accordance with the invention.  
     [0031] The valve  1  includes a central spool  2  slidably received within a sleeve/body  4 . The spool is spring-centered by springs  6  located adjacent each end of the spool. The sleeve/body  4  is shown having a center-located port  8  that can be connected to a source of pressurized fluid, two ports  10  and  12  that can be connected to a fluid return, and two ports  14  and  16  that can be connected to a load. The load would typically be a dynamic hydraulic component such as a hydraulic actuator or hydraulic motor.  
     [0032] Located at each end of the valve  1  are identical piston assemblies  20  and  22 . Piston assembly  20  is seen in more detail in FIGS.  2 - 4  wherein the valve  1  is shown in three different flow configurations.  
     [0033] Piston assembly  20  includes a body  24  that features a shaped cavity  26  at one end for receiving one of the springs  6 . While one end of the spring  6  presses on the body  24 , the other end of the spring  6  presses on a flange member  28 . Member  28  presses against an end  30  of the spool. Whenever the spool moves to the left of the null, no-flow position shown in FIG. 2, it pushes member  28  to the left, thereby compressing the adjacent spring  6 .  
     [0034] Operatively connected to the spool  2  is a movable pusher member  32 . As can be seen in FIG. 2, a major portion of the pusher member is located within the piston assembly  20 . The pusher member includes an elongated, cylindrical body  34  that has an outwardly-extending flange portion  36  at one end. The body  34  is guided in its movement by a complementary thru-bore  38  in a tubular portion  40  of the piston assembly&#39;s body  24 .  
     [0035] The flange portion  36  of the pusher member is located adjacent a pilot piston  42 . The pilot piston is movably secured within a complementary thru-bore  44  of a guide unit  46 . The guide unit is releasably-engaged to the inner wall  48  of the body  24  by a conventional fastening system, such as by the threaded engagement shown.  
     [0036] It should be noted that the diameter of thru-bore  44  is only nominally larger than the diameter of the cylindrical body of the pilot piston. The thru-bore  44  thereby functions to guide the pilot piston  42  as said piston moves back and forth in a direction parallel to the valve&#39;s longitudinal axis. It should also be noted that one or more seal members, such as o-rings (not shown), may be employed to form a seal between the pilot piston  42  and the thru-bore  44 .  
     [0037] Located at one end of the guide unit  46  is a plug  50 . The plug forms one wall of a variable volume chamber  52 . The opposite wall of the chamber is formed by the rear surface  54  of the pilot piston. Fluid may travel into, or out of, the chamber  52  via a fluid passage  56  that extends through the body  24  of the piston assembly and via a connecting passage  58  in the guide unit. In this manner, when pressurized fluid is directed into chamber  52  via passages  56  and  58 , the fluid will apply force on the piston  42  and thereby cause it to move toward the pusher member  32 . Once the piston engages the pusher member, any forward movement of the piston will cause an equal forward movement of the pusher member and the connected spool  2 .  
     [0038] The piston assembly&#39;s body  24  includes another fluid passage  60  that extends into a variable volume chamber  62 . A movable stop piston  64  forms one wall of said chamber  62 , and is slidable in a complementary bore  66  in the body  24  and along the body portion  34  of the pusher member. It should be noted that one or more seal members, such as o-rings (not shown), may be employed to form a seal between the stop piston  64  and bore  66 . Similar or different conventional seal members may also be employed between the stop piston&#39;s bore  68  and the body portion  34  of the pusher member.  
     [0039] When pressurized fluid is directed into chamber  62 , said fluid will apply pressure on the stop piston and cause the stop piston to move toward the guide unit  46 . Once the stop piston contacts the forward end  70  of the guide unit, it will cease moving.  
     [0040] Since the flange portion  36  of the pusher member is larger in diameter than the thru-bore  68  of the stop piston, the stop piston can act to stop/limit the travel of the pusher member. Once the flange portion contacts the stop piston, it cannot move to the right unless the stop piston also moves to the right. A first stop for the pusher member occurs when the stop piston is pressed against the guide unit  46  by pressurized fluid in chamber  62 . A second stop can occur when the stop piston is pressed against vertical wall  72  of the piston assembly&#39;s body  24 . It should also be noted that since the pilot piston contacts and moves the pusher member, by stopping the pusher member, the stop piston also effectively stops/limits the travel of the pilot piston.  
     [0041]FIG. 5 provides an example of a typical system in which the valve  1  would be employed. A hydraulic actuator  76  is connected to ports  14  and  16  of valve  1  via two fluid lines,  78  and  80 . Valve  1  is shown connected to primary and secondary pilots. Two solenoid-actuated valves  82  and  84  form the primary pilot.  
     [0042] Valve  82  is connected to a source of pressurized fluid, such as a pump (not shown) or pressurized reservoir (not shown) via fluid line  86 . The valve is also connected to a fluid return, such as a fluid sump (not shown), by a fluid line  90 . The valve&#39;s fluid outlet line  92  connects to passage  56  of piston assembly  20 . A user-actuable solenoid  94  is attached to the valve and functions to operate the valve. The valve enables the fluid outlet line  92  to be connected to either pressurized fluid from line  86  or to the fluid return via line  90 .  
     [0043] Valve  84  is preferably identical to valve  82  and is connected to a source of pressurized fluid, such as a pump (not shown) or pressurized reservoir (not shown) via a fluid line  96 . The valve is also connected to a fluid return, such as a fluid sump (not shown), by a fluid line  98 . The valve&#39;s fluid outlet line  100  connects to passage  56  of piston assembly  22 . A user-actuable solenoid  102  is attached to the valve and functions to operate the valve. The valve enables the fluid outlet line  100  to be connected to either pressurized fluid from fluid line  96  or to the fluid return via line  98 .  
     [0044] It should be noted that when valves  82  and  84  are separate units from valve  1 , the fluid lines  92  and  100  that connect them to the valve  1  would be pipes or hoses that extend between the associated valves. Alternatively, the valves  82  and  84  may be incorporated into a single valve block that would also contain valve  1 . In the latter situation, fluid lines  92  and  100  would be passages in said valve block extending between the associated valves. While two separate valves  82  and  84  are shown forming the primary pilot, said valves can be replaced by a single four-way valve (not shown), such as a solenoid-operated spool valve.  
     [0045] A secondary pilot, in the form of a solenoid-operated valve  104 , is also connected to the valve  1 . Valve  104  is connected to a source of pressurized fluid, such as a pump (not shown) or pressurized reservoir (not shown) via a fluid line  106 . The valve is also connected to a fluid return, such as a sump (not shown), by a fluid line  110 . The valve&#39;s fluid outlet line  112  connects to passage  60  of both piston assemblies  20  and  22 . A user-actuable solenoid  114  is attached to the valve and functions to operate the valve. The valve enables the outlet line  112  to be connected to either pressurized fluid from line  106  or to the fluid return via line  110 .  
     [0046] Port  8  of valve  1  connects the valve to a source of pressurized fluid, such as a pump (not shown) or pressurized reservoir (not shown), via fluid line  116 . This line would be used as the source of pressurized fluid for the actuator  76 .  
     [0047] Ports  10  and  12  of valve  1  connect the valve to a fluid return, such as a sump (not shown), via a fluid return line  118 . This return line is used to direct fluid expelled from the actuator to the fluid return.  
     [0048] FIGS.  2 - 4  will now be described for a valve  1  operating in a system per FIG. 5.  
     [0049]FIG. 2 shows the piston assembly  20  and the center portion of the valve  1  when the valve is in a null, no-flow state whereby no pressurized fluid is being directed to the actuator  76 . Passage  56  of the assembly is connected to return line  90  via line  92  and valve  82 . Passage  60  of the assembly is connected to return line  110  via line  112  and valve  104 . The spool is centered as the flange member  28  of both assemblies  20  and  22  presses against the spool due to the action of the springs  6 . As one can see in this figure, the spool&#39;s land  120  completely blocks the valve&#39;s port  14  leading to the actuator  76  via fluid line  80 . As one can also see in FIG. 1, the spool&#39;s land  122  completely blocks the valve&#39;s port  16  that leads to-the actuator  76  via fluid line  78 .  
     [0050]FIG. 3 shows the piston assembly  20  when the valve  1  is in a full, or maximum flow condition. The spool has been shifted to the right an amount whereby the spool&#39;s lands  120  and  122  have moved to a point where the valve&#39;s ports  14  and  16  are completely open/unblocked. At this point, pressurized fluid can readily flow from the high-pressure fluid line  116  into the valve  1  via port  8 , and then into the hydraulic actuator&#39;s fluid line  80  via port  14 . As the pressurized fluid enters the actuator, the actuator&#39;s piston  124  will move to the left (per FIG. 5) and cause fluid to be expelled from the actuator via fluid line  78 . The fluid moves through line  78  and goes into the valve  1  via port  16 . The returning fluid then flows to a fluid return via line  118  and the valve&#39;s port  10 .  
     [0051] As one can see in FIG. 3, the full movement of the spool was achieved via a maximum movement to the right of the pilot piston  42 . This was accomplished by sending power to solenoid  94  of valve  82 . Once solenoid  94  was actuated, valve  82  enabled pressurized fluid to travel from line  86 , through valve  82 , through line  92 , and then to chamber  52  via passages  56  and  58  in the piston assembly  20 . The pressurized fluid applied force to the rear surface  54  of the pilot piston and pushed the pilot piston to the right. The forward end of the piston applied pressure on the spool  2  via the pusher member  32 , and caused the spool to move to the right. It should be noted that the other piston assembly  22  enabled the spool to move to the right since its chamber  52  is open to the fluid return via its passages  56 ,  58 , lines  98  and  100 , and valve  84 .  
     [0052] An important feature to note in FIG. 3 is that the pilot piston can only move to the right a limited distance. Its rightward travel preferably comes to a limit/stop when the flange portion  36  of the pusher member contacts stop piston  64  and presses said stop piston against vertical wall  72 . Alternatively, the spool&#39;s travel can be limited by flange member  28  of piston assembly  22  contacting that assembly&#39;s tubular portion  40 .  
     [0053]FIG. 4 shows the piston assembly  20 , and a center portion of the valve  1 , when the valve is in a limited-flow condition. At the point shown, the spool&#39;s lands  120  and  122  are only partially covering the fluid ports  14  and  16  respectively. The resultant restriction in the fluid path significantly reduces the rate of fluid flow to the actuator  76  from fluid line  80 , and from the actuator via fluid line  78 .  
     [0054] As can be seen in FIG. 4, the pilot piston  42  has only moved approximately half the distance it was allowed to move per FIG. 3. This reduction in its movement was the result of a leftward movement of the stop piston  64 .  
     [0055] To achieve the limited flow to the actuator  76 , valve  82  was actuated in the same manner as discussed previously relative to the full-flow condition shown in FIG. 3. However, at the same time, solenoid  114  of valve  104  was actuated. This enabled pressurized fluid to flow from fluid line  106 , through valve  104 , through fluid line  112 , and then into the chambers  62  of both piston assemblies  20  and  22  via their associated passages  60 .  
     [0056] Once the pressurized fluid entered the chamber  62  of each piston assembly, the fluid applied force against the rear face  130  of stop piston  64 . This force caused the stop piston to move in a direction away from the spool until it&#39;s forward surface  132  contacted end  74  of the guide unit  46 .  
     [0057] Once the stop piston is in the position shown in FIG. 4, the pilot piston can only move the pusher member until the flange portion  36  of the pusher member contacts surface  132  of the stop piston. Since the area of surface  130  of the stop piston is, greater than the area of the rearward-facing surface  54  of the pilot piston, the force applied to the stop piston by the pressurized fluid in chamber  62  is greater than the force applied to the pilot piston by the pressurized fluid in chamber  52  (assuming the same fluid pressure in both chambers). As a result, the pilot piston cannot move the stop piston to the right of the position shown in FIG. 4. In this manner, the stop piston stops/limits the pilot piston&#39;s travel, and will only allow the pilot piston to move the spool to the right by the distance shown in FIG. 4. It should be noted that as the stop piston in piston assembly  20  was stopping the rightward movement of the assembly&#39;s pilot piston, the stop piston in piston assembly  22  also moved to abut end  74  of that assembly&#39;s guide unit. However, since the body of the pusher member is slidable in the bore of the stop piston, the movement of the stop piston in assembly  22  had no effect on the spool&#39;s movement. If the spool was being moved to the left through the action of the pilot piston of piston assembly  22 , the stop piston in piston assembly  20  would similarly allow said movement.  
     [0058]FIG. 6 provides a cross-sectional view of a portion of an alternate embodiment of a multi-piston spool valve  200  in accordance with the invention. The valve is shown in a no-flow condition.  
     [0059] Valve  200  is basically identical to valve  1 , and includes a center-located spool  2  that has lands  120  and  122  that can cover or block ports  14  and  16  respectively. The spool can be shifted by the action of a pair of identical piston assemblies  202  and  204 . The piston assemblies are located at opposite ends of the spool.  
     [0060] The difference between valve  1  and valve  200  lies in the structure and functionality of the piston assemblies. Piston assemblies  202  and  204  are very similar to the piston assemblies  20  and  22  of the first embodiment of the invention, with the primary exceptions being that each employs two stop pistons  206  and  208 , and an additional fluid passage  210 . The structure and functionality associated with the assembly&#39;s pilot piston is unchanged.  
     [0061] As can be seen in FIG. 6, the piston assembly  202  includes many of the same components as were employed in piston assembly  20 . This includes the pilot piston  42 , guide unit  46 , pusher member  34 , centering spring  6 , and fluid passages  56 ,  58  and  60 . All of the ports  8 - 16  in the center portion of the valve  200  can also be connected in the same manner as described in the first embodiment of the invention. While only land  120  can be seen in FIGS.  6 - 10 , land  122  (note FIG. 1) will move in the same manner as land  120  and will cover or uncover its respective port  16  accordingly.  
     [0062] The stop pistons  206  and  208  are preferably tubular in shape and are located in a stacked, concentric relation. In this manner, and as will be described, the stop pistons can interact with each other and limit singly, or in combination, the movement of the pilot piston  42 .  
     [0063] The first stop piston  206  has a flange portion  211  and an elongated body portion  212 . As can be seen in the figure, two seal members, in the form of o-rings  214  and  216 , provide a sealing engagement with the adjacent surface of the second stop piston  208 . A third sealing member, o-ring  220 , provides a sealing engagement with the outer surface of the pusher member  34 . One should note in the figure that there is a small chamber  222  located between the first and second stop pistons. Fluid passage  60  in the body  224  of the piston assembly opens into an elongated groove  226 . The groove  226  faces a complementary groove  228  in the second stop piston. The second stop piston includes a fluid passage  230  that connects groove  228  with the chamber  222 .  
     [0064] The second stop piston  208  employs three seal members in the form of o-rings  232 ,  234  and  236  to seal the outer surface of the second stop piston to the adjacent inner wall of the body  224  of the piston assembly. One should note the depending lip  240  located at the end of the second stop piston. When the two stop pistons are in the position shown in the figure, lip  240  engages surface  242  of the first stop piston. This functions to stop/limit the travel of the first stop piston. When pressurized fluid is directed into chamber  222 , the fluid will longitudinally force apart the two stop pistons until surface  242  engages lip  240 .  
     [0065] One should also note that there is a chamber  244  located between the second stop piston and the inner wall of the body  224 . When pressurized fluid is directed into this chamber via fluid passage  210  in the body  224 , the fluid will push the second stop piston to the left until the end of the stop piston engages the end of the guide unit.  
     [0066]FIG. 11 provides an example of a system diagram that would be used in conjunction with the valve  200 . It should be noted that the only significant difference between this diagram and the diagram shown in FIG. 5 is that an additional valve  250  is employed to supply fluid to chamber  244  within each of the piston assemblies. Valve  250  is connected to a source of pressurized fluid, such as a pump (not shown) or pressurized reservoir (not shown) by fluid line  252 . Fluid line  254  connects valve  250  to a fluid return, such as a sump (not shown). Fluid line  256  connects the output of the valve to fluid passage  210  in the body  224  of both piston assemblies  202  and  204 . A user-actuable solenoid  258  is attached to the valve and functions to operate the valve. The valve enables the outlet line  256  to be connected to either pressurized fluid from line  252  or to the fluid return via line  254 . It should be noted that the output line  112  of valve  104  is used to connect to fluid passage  60  in the body  224  of both piston assemblies  202  and  204 . As noted previously, the passage  60  is employed in the second embodiment to provide a fluid connection to chamber  222 .  
     [0067] Unlike the valve shown in FIG. 1, valve  200  has five different flow positions. FIGS.  6 - 10  show the different positions for the valve  200 . The description of these figures is made in conjunction with a description of the valve&#39;s operation per the system shown in FIG. 11.  
     [0068]FIG. 6 shows the configuration of the piston assembly  202  when the valve is in a null, no-flow condition. At the time shown, there is no pressurized fluid being directed to any of the passages  56 ,  60  and  210 . Springs  6  are centering the spool. At this point, ports  14  and  16  are completely covered/blocked by the spool&#39;s lands  120  and  122  and there is no flow of fluid to, or from, the actuator  76 .  
     [0069]FIG. 7 shows piston assembly  202  at a point when valve  200  is in a low-flow condition. This condition is achieved when all three of valves  82 ,  104  and  250  are enabling pressurized fluid to travel to passages  56 ,  60  and  210  respectively. Valve  84  is positioned to enable a return fluid flow from chamber  52  of piston assembly  204 . The pressurized fluid entering chamber  52  of assembly  202  has caused the pilot piston to move to the right. As the pilot piston moved, it pushed the pusher member and spool to the right. The pilot piston&#39;s travel was stopped when the flange portion  36  of the pusher member contacted vertical surface  260  of the first stop piston  206 .  
     [0070] It should be noted in FIG. 7 that the pressurized fluid flowing into chambers  222  and  244  caused the first and second stop pistons respectively to move to their maximum extent to the left. One should note that the vertical surface  262  of the second stop piston  208  is spaced from the adjacent vertical wall  264  of the body  224 . The travel of the first stop piston relative to the second stop piston was stopped when the lip  240  of the second stop piston engaged vertical surface  242  of the first stop piston. At this point, the spool&#39;s lands  120  and  122  no longer completely block their respective ports  14  and  16 , whereby said ports are now slightly open/unblocked. As a result, a low rate of fluid flow is enabled to the actuator  76  via line  80  and from the actuator via line  78 .  
     [0071]FIG. 8 shows piston assembly  202  at a point when valve  200  is in a medium-flow condition. This condition is achieved when valves  82  and  250  are enabling pressurized fluid to travel to passages  56  and  210  respectively. At the same time, valve  104  is connecting passage  60  to the fluid return via line  110 . Also at this time, valve  84  is positioned to enable a return fluid flow from chamber  52  of piston assembly  204 .  
     [0072] As can be seen in FIG. 8, the pressurized fluid that has flowed into chamber  52  of the piston assembly  202  has caused the pilot piston to move to the right. As the pilot piston moved, it pushed the pusher member and spool to the right. The pilot piston&#39;s travel was stopped when the flange portion  36  of the pusher member contacted surface  260  of the first stop piston  206 . It should be noted in the figure that the pressurized fluid from passage  210  has caused the second stop piston  208  to move to the left by its maximum extent, whereby the piston&#39;s surface  262  is spaced from the adjacent vertical wall  264 . The lack of pressurized fluid to passage  60  has enabled the first stop piston to slide to the right whereby its vertical surface  266  now contacts the adjacent vertical surface  268  of the second stop piston. At this point, the spool&#39;s lands  120  and  122  have moved a small distance to the right from their positions of FIG. 7, thereby allowing a slightly greater opening of the ports  14  and  16  respectively. A medium rate of fluid flow, slightly greater than that allowed by the piston assembly configuration shown in FIG. 7, is now enabled to the actuator via line  80  and from the actuator via line  78 .  
     [0073]FIG. 9 shows piston assembly  202  at a point when valve  200  is in a moderately high-flow condition. This condition is achieved when valves  82  and  104  are enabling pressurized fluid to travel to passages  56  and  60  respectively. At the same time, valve  250  is connecting passage  210  to the fluid return via line  254 . Also at this time, valve  84  is positioned to enable a return fluid flow from chamber  52  of piston assembly  204 . The pressurized fluid has caused the pilot piston to move to the right. As the pilot piston moved, it pushed the pusher member and spool to the right. The pilot piston&#39;s travel was stopped when the flange portion  36  of the pusher member contacted surface  260  of the first stop piston  206 . It should be noted that the pressurized fluid from passage  60  caused the first stop piston to move to the left by its maximum extent. The lack of pressurized fluid to passage  210  has enabled the second stop piston to slide to the right whereby its vertical surface  262  contacts the adjacent vertical surface  264  of the body  224 . At this point, the spool&#39;s lands  120  and  122  have moved a small distance to the right from their positions of FIG. 8, thereby allowing a slightly greater opening of the ports  14  and  16  respectively. Ports  14  and  16  are now almost completely unobstructed. As a result, a moderately high rate of fluid flow, slightly greater than that allowed by the piston assembly configuration shown in FIG. 8, is enabled to the actuator via line  80  and from the actuator via line  78 .  
     [0074]FIG. 10 shows piston assembly  202  at a point when valve  200  is in a maximum-flow condition. This condition is achieved when valve  82  enables pressurized fluid to travel to chamber  52  via passage  56 . At the same time, valves  104  and  250  are connecting passages  60  and  210  respectively to the fluid return via lines  110  and  254  respectively. Also at this time, valve  84  is positioned to enable a return fluid flow from chamber  52  of piston assembly  204 . The pressurized fluid has caused the pilot piston to move to the right to the maximum extent possible. As the pilot piston moved, it pushed the pusher member and spool to the right. The pilot piston&#39;s travel was stopped when the flange portion  36  of the pusher member contacted surface  260  of the first stop piston  206 . The lack of pressurized fluid in passages  60  and  210  has enabled both stop pistons to slide to the right, to their maximum extent. In the position shown, the vertical surface  262  of the second stop piston is contacting the adjacent vertical surface  264  of the inner wall of the body  224 . Also in the position shown, the vertical surface  266  of the first stop piston is contacting the adjacent vertical surface  268  of the second stop piston. At this point, the spool&#39;s lands  120  and  122  have moved a small distance to the right from their positions of FIG. 9. Ports  14  and  16  are now completely unobstructed and enable a completely unrestricted flow of fluid to actuator  76  via line  80  and from the actuator via line  78 .  
     [0075] While the functionality of the piston assemblies  20  and  202  have been shown and described, the functionality of piston assemblies  22  and  204  is basically identical. To enable piston assembly  22  or assembly  204  to move the spool to the left, valve  84  would be actuated in lieu of valve  82 . The restricted flow positions would be caused in the same manner as previously described via the actuation of the secondary pilot valve(s).  
     [0076]FIG. 12 provides a cross-sectional view of a multi-piston spool valve  300  in accordance with the invention. This type of valve would typically be employed when a reversible fluid flow is not required. An example of such an application is to control fluid flow to a hydraulic motor that is operating a cooling fan.  
     [0077] Valve  300  is very similar to valve  1 , except that it only employs a single piston assembly. As shown, piston assembly  20  is located proximate one end of the valve&#39;s spool  302  and functions in the same manner as assembly  20  of the first embodiment. The piston assembly could be connected in the same manner as shown in FIG. 5 and cause rightward movements of the spool  302 .  
     [0078] In operation, the valve&#39;s port  8  could be connected to a source of pressurized fluid, while the valve&#39;s port  10  could be connected to a load, such as a hydraulic motor. To uncover port  8 , the piston assembly  20  moves the spool to the right. As the spool moves, its end  304  moves into a cavity  306  located at the opposite end of the valve. Two centering springs  6  are employed to bias the spool to a centered position. It should be noted that depending on the location of the pilot piston  42  and the operatively connected spool  302 , land  120  will either prevent any fluid from flowing to the load, allow a restricted fluid flow to the load, or allow maximum fluid flow to the load. If one desires to allow multiple restricted fluid flows, the piston assembly  20  can be replaced by piston assembly  202 , wherein piston assembly  202  would be connected much in the same manner as described in FIG. 11.  
     [0079] It should be noted in all embodiments of the invention that when pressurized fluid is directed into a fluid chamber associated with a pilot piston or stop piston, the fluid acts as a force applicator that causes the piston to move. While not shown, other conventional force applicators, such as a solenoid, spring, etc. may be used in lieu of a pressurized chamber to cause the movement of a pilot piston or stop piston. The use of other types of force applicators may not provide the simplicity or durability of the preferred fluid chambers. It should also be noted that the pusher member  32  is optional and can be replaced by a pilot piston that is shaped to incorporate the function of the pusher member and thereby apply pressure directly on the spool.  
     [0080] The previously-described fluid chambers  52 ,  62 ,  222  and  244  are all variable in volume. It should be noted that depending on the chamber configuration, the chamber&#39;s minimum volume may approximate zero. At such a point, the chamber would comprise the outlet of the fluid passage leading to said chamber.  
     [0081] The preferred embodiments of the invention disclosed herein have been discussed for the purpose of familiarizing the reader with the novel aspects of the invention. Although preferred embodiments of the invention have been shown and described, many changes, modifications and substitutions may be made by one having ordinary skill in the art without necessarily departing from the spirit and scope of the invention as described in the following claims.