Abstract:
A wide variety of machines have moveable members that are operated by an hydraulic actuator. For example, an internal combustion engine has a camshaft which is mechanically coupled to rotate with the crankshaft and which opens and closes cylinder intake and exhaust valves. Traditionally the camshaft timing was fixed at a setting that produced the best operation for all engine operating conditions. However, it has been recognized that engine performance can be improved if the valve timing varies as a function of engine speed, engine load, and other factors. Thus a hydraulic actuator is being used on some engines to vary the coupling relationship of the camshaft to the crankshaft. A solenoid operated valve controls the application of pressurized fluid to operate the hydraulic actuator.

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 hydraulic valves, and more particularly to spool type valves that have an integral filter for fluid flowing through the valve. 
     2. Description of the Related Art 
     A wide variety of machines have moveable members that are operated by an hydraulic actuator. For example, an internal combustion engine has a camshaft which is mechanically coupled to rotate with the crankshaft and which opens and closes cylinder intake and exhaust valves. Traditionally the camshaft timing was fixed at a setting that produced the best operation for all engine operating conditions. However, it has been recognized that engine performance can be improved if the valve timing varies as a function of engine speed, engine load, and other factors. Thus a hydraulic actuator is being used on some engines to vary the coupling relationship of the camshaft to the crankshaft. A solenoid operated valve control the application of pressurized fluid to operate the hydraulic actuator. 
     Over time, the hydraulic fluid flowing through a machine carries small particles, such as pieces of metal from the engine components. Those particles can block orifices in the valve or can become lodged so as to impede motion of valve components. The particles also may adversely affect operation of other elements of the hydraulic system. Some prior valves incorporated screens to prevent the small particles from entering the valve. 
     SUMMARY OF THE INVENTION 
     A valve arrangement comprises a body with a longitudinal bore within which is formed an interior annular recess. The annular recess has a circumferential surface through which a fluid port opens. A filter band with a plurality of apertures there through abuts the circumferential surface of the recess. The filter band extends over an opening of a fluid port into the annular recess. Preferably the filter band is shaped in a ring and specifically for example may be a strip of material bent into a cylinder with overlapping ends. 
     A helical spring, located within the recess, retains the filter band against the circumferential surface. The helical spring has spaced apart convolutions that engage the filter band and exert a outward radial force which maintains the filter band abutting the circumferential surface. 
     A valve element, such as a spool, for example, is slideably received within the longitudinal bore to control flow of fluid through the fluid port. 
     In one embodiment of the valve arrangement, the annular recess has two side surfaces on opposite sides of the circumferential surface, and the helical spring engages both of the side surfaces. For example, the helical spring may have a first end at which two convolutions abut each other, and has an opposite second end at which another pair of convolutions abut each other. The first and second ends of the spring engage the two side surfaces of the recess and thus the spring extends across the entire width of that recess. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal cross sectional view through an electrohydraulic valve that incorporates a filter assembly according the present invention; 
         FIG. 2  is a cross sectional view through the valve along line  2 - 2  in  FIG. 1 ; 
         FIG. 3  is an enlarged section of  FIG. 1  showing the area where an input port opens into the valve bore and a filter assembly at that location; 
         FIG. 4  illustrates a filter sheet that is a component of the filter assembly; 
         FIG. 5  shows the filter sheet bent into a band as occurs upon being inserted into the valve; and 
         FIG. 6  shows a retention spring used to hold the filter band in position inside the valve. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described in the context of an exemplary electrohydraulic valve  10  depicted in  FIGS. 1 and 2 , however it should be understood that the invention can be practiced with other types of valves, The electrohydraulic valve  10  has a tubular valve body  20  that during use is inserted into an aperture  22  in a manifold  24 . The tubular valve body  20  has a longitudinal bore  21  into which a plurality of ports open. A supply passage  26  in the manifold  24  conveys pressurized fluid from a pump or other source to a plurality of inlet ports  28  in the valve body  20 . Although the exemplary valve  10  has six inlet ports  28  in  FIG. 2 , other amounts of ports can be provided. Each inlet port  28  opens through an inner circumferential surface  27  into a first annular recess  29  formed in the curved surface of the valve body&#39;s longitudinal bore  21 . A plurality of first and second workports  30  and  32  in the tubular valve body  20  provide fluid paths between the longitudinal bore  21  and manifold passages  34  and  36  that lead to a hydraulic actuator which is driven by the fluid. The first and second workports  30  and  32  open into second and third annular recesses  31  and  33 , respectively, in the curved surface of the longitudinal bore  21 . As with the inlet ports, there may be a plurality of first workports  30  and a plurality of second workports  32  spaced radially around the bore  21 . At the inner end of the manifold aperture  22 , a return passage  25  communicates with an outlet port  35  in the valve body to convey fluid back to a tank of the hydraulic system. 
     A valve element in the form of a spool  44  is slideably received within the longitudinal bore  21  in the valve body  20  and has an exterior annular notch  46 . In selected positions of the spool, the exterior annular notch  46  provides a fluid path between the inlet and outlet ports  28  and  35  and the two workports  30  and  32 , and thus between the associated manifold passages. In a middle position of the spool travel that is illustrated in  FIG. 1 , the inlet port  28  is closed from both workports  30  and  32  which also are blocked by lands on the spool  44 . A central passage  48  extends though the spool  44  between the opposite ends  47  and  49  and in leftward spool positions provides a path between the first workport  30  and the outlet port  35 . A head  54  projects from the outward end  49  of the valve spool  44  and has an aperture  53  there through. A valve spring  50  biases the inward end of the spool  44  away from a nose piece  52  at one end of the valve body  20  at which the outlet port  35  is located. 
     The valve  10  also includes a linear actuator  51  attached to the opposite end of the valve body  20 . The linear actuator  51  has a metal outer housing  55  that encloses a solenoid coil  58  wound in a non-magnetic bobbin  60 . Two magnetically conductive pole pieces  64  and  66  that extend into opposite ends of the bobbin  60  and both have a central aperture extending there through. An actuator plunger  70  is slideably received within central apertures of the pole pieces  64  and  66  and thus within the central opening of the solenoid coil  58 . The actuator plunger  70  includes a cylindrical armature  72  of ferromagnetic material and a tubular push member  74  that is secured in an aperture through the armature  72 . The push member  74  projects outward from the linear actuator  51  and abuts the head  54  of the valve spool  44 . 
     When electric current is applied to the solenoid coil  58 , an electromagnetic field is produced that drives the armature  72  and the push member  74  toward the valve spool  44 . That action causes the valve spool to move against the bias force of the valve spring  50  and thereby slide in the longitudinal bore  21  of the valve body  20 . For example, the solenoid coil  58  can be driven by a pulse width modulated (PWM) electrical signal having a duty cycle that is varied in a conventional manner to move the spool  44  to different desired positions in the valve body  20 . The PWM signal is applied to the linear actuator  51  via a connector  57 . 
     With continuing reference to  FIG. 1 , a separate filter  80  is located in each annular recess  29 ,  31  and  33  in the longitudinal bore  21  of the valve body  20  to filter fluid flowing through the inlet ports  28  and the first and second workports  30  and  32 . With particular reference to  FIG. 4 , each filter  80  comprises a thin (e.g., 0.1 mm thick), rectangular strip  81  of metal that has a plurality of apertures  84  between its two major surfaces. For example, a standard photolithographic etching process can be employed to form apertures of a size small enough to prevent undesirable particles from entering and adversely affecting operation of the valve  10 . The strip  81  is bent into a curve with a first end section  86  overlapping a second end section  88 , thereby forming an annular filter band  82 , as specifically shown in  FIG. 5 . 
     As shown in  FIG. 1 , a separate filter band  82  is held against the curved circumferential surface  27  of each annular recesses  29 ,  31  and  33  by a helical retention spring  90 . The details of one of the retention springs  90  are illustrated in  FIG. 6 . The center convolutions, or turns,  92  of the helical spring are spaced apart, whereas two convolutions  94  and  96  at each end of the helix abut each other, thereby forming a generally flat end surface  97  and  98 , respectively. The outermost convolution is closed meaning that the end  99  of the wire, which forms the spring, touches the wire near the beginning of the outermost convolution and is not spaced apart as are the center convolutions  92 . Alternatively, each flat end surface  97  and  98  could be formed by only a single closed convolution. 
     With additional reference to  FIGS. 1 ,  2  and  3 , when the retention spring  90  is installed in one of the annular recesses  29 ,  31  and  33 , the flat ends engage the annular side walls  95  on opposite sides of the circumferential surface  27  of the recess. As a result, the retention spring  90  extends across the entire width of the respective recess  29 ,  31  or  33  and holds the edges of the associated filter band  82  against the circumferential surface  27 . Thus the opening of the respective inlet port  28  or workports  30  or  32  into the recess is tightly covered by the filter band  82 . This restraint of the filter band  82  by the helical retention spring  90  inhibits fluid pressure and flow from the respective port from collapsing the band away from the circumferential surface  27  and opening a fluid path around the filter  80 . The helical retention spring  90  also inhibits that pressure and flow from moving a filter band  82  partially out of the respective recess  29 ,  31  or  33  and into the annular notch  46 , where the filter band would interfere with the sliding motion of the valve spool  44 . This retention is achieved by the convolutions of the helical retention spring  90  applying force evenly across the entire width of the filter band  82 . The flat ends of the retention spring  90  engaging the annular side walls  95  on opposite sides of the circumferential surface  27  of the recess prevent the retention spring from sliding across the width of the filter band  82 . 
     The filters  80  are inserted one at a time into the valve body  20  before the spool  44  in placed into the longitudinal bore  21 . A funnel shaped tool may be employed for that process. The tool has a long tube that is inserted into the longitudinal bore  21  with an open end of the tube positioned adjacent the particular recess  29 ,  31  or  33  into which the filter is to be placed. A cylindrical filter band  82  in inserted into the funnel and pushed inward into the tube of the tool, thereby contracting the diameter of the band, enabling the band to slide through the tube. The filter band  82  ultimately is pushed out of the end of the tool and into the recess in the longitudinal bore  21 . At that time, the resiliency of the filter band  82  causes it to expand diametrically into the recess until the band rests against the curved circumferential surface  27 . Then a similar process is used to place a retention spring  90  into the same recess. When the retention spring  90  expands diametrically upon exiting the insertion tool, the outer circumferential edges of each convolution of the helix exerts an outward radial force against the filter band  82 , further holding the band against the curved circumferential surface  27  of the recess. The flat ends of the installed retention spring  90  engage the opposite annular side walls  95  of the recess to center the spring in the recess. In the installed state, the abutting convolutions  94  and  96  at those ends of the retention spring secure the edges of the filter band  82  against the recess&#39;s circumferential surface  27 . The force of that securing inhibits pressure in the respective port from bending the band away from the circumferential surface  27   
     Although the present filter band has been described in the context of use on an electrohydraulic spool valve, it should be understood that the band can be used on other types of valves. Furthermore, the valve may have a greater or lesser number of ports and thus recesses in the valve body&#39;s longitudinal bore. 
     The foregoing description was primarily directed to preferred embodiments 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.