Abstract:
Two stage spool valves having, in a single assembly, a first stage spool and a second stage spool. The first stage spool may have a solenoid actuator with a return spring, with the second stage spool having either a hydraulic return or a spring return, or both. The solenoid actuator may be comprised of a stationary structure defining a magnetic flux path, including a stationary structure pole face, that together with a moveable structure, including a moveable structure pole face, define a substantially zero air gap magnetic circuit when the solenoid actuator is actuated, the pole faces of the stationary structure and the moveable structure each being defined in part by soft magnetic iron and in part by hardened steel, the hardened steel of the moveable structure contacting the hardened steel of the stationary structure and the soft magnetic iron of the moveable structure being face to face with the soft magnetic iron of the stationary structure when the solenoid actuator is actuated.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 11/703,303 filed Feb. 6, 2007 which claims the benefit of U.S. Provisional Patent Application No. 60/771,112 filed Feb. 7, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of solenoid actuated spool valves. 
     2. Prior Art 
     Solenoid actuated spool valves are well known in the prior art. Such valves include single solenoid spring return valves and double solenoid valves, either of which may or may not incorporate magnetic latching. An example of such valves may be found in U.S. Pat. No. 5,640,987. Also known are two-stage spool valve systems, the first stage being a solenoid valve that hydraulically controls a second stage spool valve. See for instance U.S. Pat. No. 6,739,293. 
     In certain applications, such as in fuel injectors and hydraulic valve actuation systems, solenoid actuated spool valves must have a useful life of billions of cycles. This requires that the wear of the various parts be held to a minimum, in turn requiring hardened steels, such as by way of example, 52100 or 440C. These materials, however, have a relatively low magnetic field saturation density in comparison to the saturation density of physically soft magnetic steels (iron), such as annealed 1020. These steels may have a saturation density of as much as twice the saturation density of the hardened steels, and since magnetic forces are proportional to the square of the flux density, may provide approximately four times the maximum actuation force provided by the hardened steels for the same pole area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross section of a preferred embodiment of the present invention in the unactuated state. 
         FIG. 2  is a cross section of the preferred embodiment of the two stage valve of  FIG. 1  in the partially actuated state. 
         FIG. 3  is a cross section of the preferred embodiment of the two stage valve of  FIG. 1  in the fully actuated state. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First referring to  FIG. 1 , a cross-section of one embodiment of the present invention may be seen. This embodiment is a two-stage spool valve, the first stage being electromagnetically or solenoid actuated and the second stage being hydraulically actuated by the first stage. The first stage includes the combination of hardened steels for improved wear characteristics, and soft magnetic irons, for improved magnetic characteristics. The first stage also uses a single solenoid coil with a spring return, the spring return comprising two springs, one active over the entire return stroke and one active only over approximately one-half of the return stroke, to effectively provide a stepped spring return force of some fraction of the solenoid actuation force to provide fast action both on actuation and on return. The second stage uses a hydraulic return with the actuation of the first stage providing a hydraulic actuating force greater than the hydraulic return force, both forces being dependent on the pressure of a control fluid being supplied by a source of fluid under pressure. 
     As may be seen in  FIG. 1 , a body member  20  forms the body or housing for a pilot spool  22 , as well as a second stage spool  24 . At one end of the body member  20  is a solenoid coil  26 , a pole piece  28  and a cover  29 . Between the pole piece  28  and the body member  20  is an insert  30 . Also on an extension  34  of the pilot spool  22  is another insert  32 . Preferably insert  30  is press fitted into the pole piece  28  and insert  32  is press fitted onto an extension  34  on the pilot spool  22 . In general, inserts  30  and  32  are fabricated of a high saturation density material such as annealed  1020  or high saturation density irons specifically intended for use in magnetically actuated devices. Also in general, the other parts of the two-stage valve shown in  FIG. 1  are preferably hardened steel parts which, while having some magnetic characteristics, are chosen primarily for their wear resistance, such as 52100 or 440C. In that regard, insert  30  is assembled into pole piece  28  and insert  32  is assembled onto extension  34  of the pilot spool  22  before the end faces thereof are finished so that in the finished parts, the face of insert  30  is coplanar with the face of pole piece  28  and the face of insert  32  is coplanar with the face of the extension  34 . 
       FIG. 1  represents the unactuated condition of the two-stage spool valve. In this condition, port  36 , connected to a source S of fluid under pressure, is blocked by the land on the pilot spool  22 . Any leakage into groove  38  in the body member  20  is coupled through passage  40  to the center region  42  of the pilot spool  22 , and from there, into spring region  44  to the vent V port  46 . Consequently, region  48  behind push pins  50  will be unpressurized. However, port  51 , also coupled to source S of fluid under pressure, is coupled to region  52  behind push pins  54 . This pressure forces the second stage spool  24  to its right-most position as shown in  FIG. 1 . At the same time, spring  56  pushes push member  58  to the left, forcing the pilot spool  22  to the left until the left end thereof abuts the corresponding face of the second stage spool  24 . If desired, a separate stop could be provided for the leftmost motion of the pilot spool  22 . 
     In the preferred embodiment, there are three push pins  50  and only two push pins  54 , with all push pins being of the same diameter. Consequently, with push pins  54  being pressurized and push pins  50  not being pressurized, the assembly will seek the position shown in  FIG. 1 . However, when an actuating current is applied to the solenoid coil  26 , the pilot spool  22 , with the insert  32  thereon, will be magnetically attracted toward the right, initially being resisted only by spring  56  until push member  58  abuts member  62 , as shown in  FIG. 2 . In the preferred embodiment this occurs at approximately the mid position of the pilot spool  22  where the source S of fluid under pressure coupled to port  36  begins to be coupled to region  48  behind push pins  50 . With the air gap in the magnetic circuit being smaller than in  FIG. 1 , the total magnetic force on the pilot spool  22  will have typically increased. At the same time, further motion of the pilot spool  22  causes spring  60  to also start to compress as a result of push member  58  engaging member  62  and moving the same to the right against the force of spring  60 . This, in essence, provides a step in the spring force, better approaching the typical shape of the magnetic force versus pilot spool position. This provides fast actuation, while also providing a boost force on deactivation to assure a fast return to the unactuated position. 
     Finally, when the solenoid actuator is fully actuated as illustrated in  FIG. 3 , both springs  56  and  60 , both of which were initially preloaded, have been further compressed, with the face of extension  34  on the pilot spool  22  and insert  32  resting against the adjacent face of pole piece  28  and insert  30 . In this position, fluid is coupled from the source S through port  36  and annular passage  38  to the region  48  behind push pins  50 , hydraulically forcing push pins  50  to the left. As stated before, in the preferred embodiment, push pins  50  and  54  are of the same diameter, though the second stage spool  24  only has two push pins  54 , whereas the pilot spool  22  has three push pins  50 . Accordingly, when region  48  behind push pins  50  is pressurized, the force pushing second stage spool  24  to the left is greater than the force encouraging second stage spool  24  to the right. In that regard, note that as the pressure of the source S of fluid under pressure increases, the actuating forces and return forces on the second stage spool  24  similarly increase. Also in that regard, spring  70 , operative between end cap  72  and second stage spool  24 , determines the position of the second stage spool when the pressure of the source S is not present, such as will occur when the pump or other fluid pressurizing means is off. In particular, that position would be the position shown in  FIG. 1 , wherein the second stage spool  24  couples the control port C  68  to vent port V  46 , whereas when the solenoid is actuated as in  FIG. 3 , the source S of fluid under pressure in port  51  is coupled to control port C  68 , with flow between control port C  68  and vent port V  46  being blocked. Thus the two-stage valve of this embodiment is a two-stage three-way valve coupling a control port to a source of fluid under pressure or to a vent, depending on the state of actuation of the solenoid actuator in the pilot valve. 
     As previously described, inserts  30  and  32  are high saturation density magnetic materials, chosen not primarily for their wear characteristics, but for their high saturation density. Wear on these materials is minimized by the fact that when the two-stage valve is actuated, the hardened extension  34  on the pilot spool  22  is stopped by portion  70  on the hardened steel pole piece  28 . The high saturation density inserts  30  and  32 , however, concentrate the flux to provide a high flux density between the two members in the actuated state of  FIG. 3 , providing both a high attraction force during actuation as well as a high holding force after actuation, provided the cross-section of the hardened steel members, such as pole piece  28  and body member  20 , have sufficient cross-section to not saturate even at their lower saturation densities. Thus inserts  30  and  32 , having approximately twice the saturation density of pole piece  28  and body member  20 , allow the concentration of the same flux over approximately one-half the area. Since the magnetic force of attraction between inserts  30  and  32  is proportional to the square of the flux divided by the area, the use of inserts  30  and  32  may at least approximately double the actuation force of the solenoid actuator, allowing stronger springs for springs  56  and  60 , thereby increasing both the actuation speed and the release speed of the two-stage valve. In that regard, the advantage of using a two-stage valve in many applications is the fact that larger flow porting between a control port and the source and vent ports may be obtained while still using a relatively small, low power solenoid actuator for the pilot spool. Also it should be noted that magnetic latching of the pilot spool in the actuated position can be used, if desired, primarily as a result of selection of the force of springs  56  and  60  and/or the presence or absence of a holding current in the solenoid coil  26 . In that regard, if magnetic latching is not used, a full actuation current may be maintained in the solenoid coil  28  throughout each actuation period, or a full current pulse may be used for actuation purposes, followed by a substantially reduced holding current, which will maintain substantially the full magnetic field strength in the magnetic circuit because of the fact that, on actuation, the air gap in the magnetic circuit goes to a substantially zero gap. Also, while the present invention has been disclosed with respect to a three-way, two-stage valve, it will be obvious to those skilled in the art that simple changes may be made to provide some other type of valve, such as by way of example, a two-stage two-way valve. 
     In the illustrations presented herein, the assembly or manner of holding together of end cap  72 , body member  20 , pole piece  28  and cover  29  is not shown. The various parts may be joined in any manner, such as is known in the art, such as by way of example, by screws, by threaded parts, by placement in and closure of an outer housing, or by assembly and entrapment in the housing or body of a higher assembly. 
     The present invention as described herein has been described with respect to various unique features thereof. It is to be understood, however, that various subcombinations of the features described may also be advantageously incorporated in single or two-stage valves. Thus while certain embodiments of the present invention have been disclosed and described herein, it is to be understood that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.