Abstract:
A direct drive servo-valve wherein rotational motion of a drive motor rotor is converted into linear motion of a spool valve. The drive motor includes a shaft which has affixed to the end thereof a ball which engages the spool valve. The ball is affixed to the shaft by providing a bore internally of the shaft which is disposed eccentrically to the longitudinal axis of the shaft. The ball has a portion thereof ground of so as to provide an integral protrusion which is received within the bore in the end of the shaft. The ball is permanently affixed to the shaft by brazing the protrusion into the bore and a planar surface which surrounds the base of the protrusion to the end of the shaft against which it mates.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to direct drive servo-valves and more particularly to a direct drive servo-valve in which rotational motion of a motor rotor is converted into linear motion of a spool valve and specifically to a novel drive connection between a shaft of the motor rotor and the spool valve. 
     BACKGROUND OF THE INVENTION 
     Torque motor driven spool valves are well known in the art including such which operate through the utilization of a rotary torque motor having a drive member extending from the rotor thereof into contact with the spool valve to directly reciprocate the spool valve within a bore provided in the valve housing to thereby control the flow of fluid from a source thereof to the load in response to electrical signals applied to the drive motor. Such devices are commonly referred to as direct drive valves and there are various ways of interconnecting the shaft to the spool valve in an eccentric manner so as to convert the limited angle rotary motion of the motor rotor shaft to linear motion of the spool valve. 
     One example of such a prior art connection is illustrated in U.S. Pat. No. 4,793,377 which discloses the utilization of a spherical tip which is formed integrally with the shaft of the motor rotor and engages the spool valve to control the fluid flow through the valve housing. 
     Another prior art direct drive valve utilizing a spherical ball drive mechanism is shown in U.S. Pat. No. 4,573,494. Therein disclosed is a spherical bearing assembly which includes an outer race disposed upon the spool valve and a spherical bearing member disposed upon the end of the motor rotor shaft in a slip-fit manner with the outer surface thereof being received within the inner surface of the outer race member. 
     A further prior art connecting device for the drive member is shown in U.S. Pat. No. 5,052,441 which discloses a ball having a hole drilled therein which receives the end of the eccentrically disposed shaft on the motor rotor with the ball and the shaft brazed together. Typically devices of this type utilize a hardened ball which is required to withstand the frictional wear between the ball and the opening in the spool valve. Because the ball is of hardened material, the hole therein is typically machined by electron-discharged machining (EDM). The shaft extending from the motor rotor is machined to provide a cylindrical post having a diameter such that it is received within the opening formed by EDM in the ball. The utilization of the EDM operation is expensive and leaves a re-melt layer on the ball which must be removed before the post is permanently affixed to the ball by a brazing operation. The removal of the re-melt layer is usually done by lapping which is an expensive operation as is the EDM machining. If the re-melt layer is not completely removed, then the brazing will not accomplish adherence of the ball material to the post on the shaft resulting in a weak braze joint which can in turn cause failures of the direct drive valve during use. 
     There is thus needed a simple way of providing a spherical ball drive mechanism at the end of the motor rotor shaft for engagement as the drive member for a spool valve on a rotary direct drive valve. 
     SUMMARY OF THE INVENTION 
     A direct drive servo-valve including a valve housing having a valve spool reciprocally received within a bore provided therein for controlling fluid flow therethrough along with a motor means having a drive member for engagement with the valve spool which drive member includes a shaft having a longitudinal axis and an end defining an eccentrically disposed bore with a ball having an integral protrusion thereon sized such that the protrusion is adapted to be received within the bore with the ball being permanently secured to the shaft. 
     In accordance with a further aspect of the present invention there is provided a method of manufacturing a direct drive servo valve which includes forming an eccentrically disposed bore in one end of a shaft of a drive member which engages a spool for controlling the flow of fluid, providing a ball, removing a portion of the ball to provide a protrusion thereon and then securing the ball to the end of the shaft in a permanent fashion such as by brazing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a direct drive servo valve which incorporates the drive member of the present invention; 
     FIG. 1 a  is a magnified view of a portion of the structure illustrated in FIG. 1; 
     FIG. 2 is a partial cross-sectional view of a drive member assembly constructed in accordance with the principles of the present invention; 
     FIG. 3 is a cross-sectional view of a shaft taken about the lines  3 — 3  of FIG. 2; 
     FIG. 3A is a magnified view of a portion of the drive shaft similar to that shown in FIG. 2 but without the ball; 
     FIG. 4 is a view of the ball which is used as a part of the drive member illustrated in FIG.  2  and constructed in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     By reference to FIG. 1 there is shown generally at  10  a direct drive servo valve of the type which utilizes the direct drive valve ball drive mechanism constructed in accordance with the principles of the present invention. As is therein shown a valve housing  12  defines a bore  14  therein. A spool valve  16  is disposed within the bore  14  and reciprocates within that bore. 
     Through the spool valve  16  reciprocation the flow of fluid under pressure to and from a load (not shown) is controlled. The fluid under pressure designated “P” is present within the conduits or passageways  18  which communicate with the bore  14  as shown. The return or sump designated by “R” is present within the conduit or passageways  20 . When the spool  16  is caused to moved to a position such as that shown in FIG. 1, fluid under pressure flows from the conduits  18  through the port which is opened by the spool valve  16  and into the control conduit or passageway designated “C 1 ” at  22  to the load and returns from the load through the control conduit or passageway “C 2 ” at  24  and returns to the return or sump R through the port opened by the spool valve  16 . As known by those skilled in the art if the spool  16  moves towards the left the flow to and from the load is reversed from that just described. 
     Attached to the valve housing  12  is a motor means  26  which includes a drive member  28  which engages the spool  16 . The drive member  28  rotates about a limited angle and such rotation is translated into reciprocal linear movement of the spool valve  16 . The rotation of the drive member  28  is controlled by command signals which are applied to electrical leads  30  which are connected to an appropriate amplifier  32  having an appropriate source of electrical power  34  connected thereto. The output of the amplifier  32  is applied to the stator of the drive motor  26 . If desired an appropriate position feedback signal may be applied to the amplifier  32  as shown by the dashed line  36 . The position feedback signal may be generated by a position transducer  37  which provides an electrical signal proportional to the rotary position of the rotor of the drive motor  26  or alternatively the linear position of the spool valve  16  depending upon the particular application and structure of the device being controlled. 
     As shown more specifically in FIG. 1 a  the drive member  28  may include a motor rotary shaft  38  which has an eccentrically mounted ball  40  affixed thereto. The ball  40  fits within an appropriate opening which may be a bore or a slot or a groove formed within the spool  16 . As is well known to those skilled in the art as the shaft rotates in either direction as shown by the arrows  42  and  44 , the ball positioned within the opening  42  causes the spool valve to linearly move as indicated by the arrows  46  and  48 , respectively, so as to control the fluid flow as above described. 
     As indicated above, the ball  40  may be fixed to the shaft  38  in various ways such as by being formed integrally with the shaft, by having the shaft end extend through an opening formed within the ball or by the utilization of a spherical bearing assembly or the like. In each instance these structures are relatively expensive to manufacture and thus add to the overall cost of the rotary direct drive valve. 
     In accordance with the principles of the present invention the drive member which engages the spool is constructed by a totally different method and configuration which substantially reduces the manufacturing cost of the structure. 
     By reference now to FIG. 2 there is illustrated the assembly of a shaft utilized within a motor means of the direct drive servo valve constructed in accordance with the principles of the present invention. As is illustrated in FIG. 2 a shaft  50  has eccentrically disposed on one end  52  thereof a ball  54 . The shaft  50  is adapted to be supported within the rotor of the motor means by appropriate bearings or the like as is well known to those skilled in the art. Such structure does not form a part of the present invention but is clearly illustrated in prior art U.S. Pat. No. 5,052,441 which is incorporated herein by this reference and as a result, further detailed description of the motor means will not be provided herein. By reference to FIGS. 2 through 5 the manner in which the drive member is constructed in accordance with the principles of the present invention will be described in further detail. As is shown the shaft  50  defines a bore  52  which is eccentrically offset from the longitudinal axis  54  of the shaft  50 . As is seen particularly in FIG. 3A the shaft  50  defines a substantially planar surface  56  which surrounds the bore  52 . There is also provided a cross bore  58  which intersects the bore  52 . The purpose of the cross bore  58  will be described more in detail hereinafter. 
     As above pointed out the spool valve  16  is contacted by a ball disposed within the opening  42  to cause it to reciprocate within the bore  14 . In accordance with the principles of the present invention such a ball is shown in FIG.  4 . As is therein illustrated there is provided a ball  56  which has a diameter  58 . A protrusion  60  is formed integrally with the ball  56  and extends therefrom. A planar surface  62  surrounds the base of the protrusion  60 . As will be recognized by those skilled in the art, the protrusion  60  formed as an integral part of the ball  56  is best formed by removing that portion of the ball which is shown within the volumes  64  and  66  defined by the dashed lines. It will be recognized that the “volumes” is really a continuous volume defined by the planar surface  62  and the outer perimeter of the protrusion  60  and the surface of the ball  56  as defined by the dashed lines  67  and  68 . The volume may be removed from the ball  56  to provide the protrusion  60  in various manners. The most desirable in accordance with the preferred form of the present invention is to secure the ball within a proper fitting and then grind away the volume above referred to leaving the protrusion  60  as a post extending from the planar surface  62 . The amount of material removed in order to provide the protrusion  60  may vary according to the particular application and strength of the structure as desired. It is however important that the length of the protrusion  60  be less than a radius of the ball  56 . Such construction is necessary so that a complete surface for the ball  56  is provided about its equator so that there will be line contact between the ball and the opening  42  in the spool valve  16  in order to obtain appropriate operation of the direct drive servo valve. In accordance with a preferred embodiment of the present invention the length of the protrusion  60  measured along a diameter of the ball  56  is between 20% and 60% of the radius of the ball  56 . 
     The ball  56  is permanently secured to the shaft  50 . Such permanent attachment may be accomplished by various means known to those skilled in the art. Preferably a brazing compound is applied to the protrusion  60  so as to substantially cover the outer perimeter or outer radial surface  69  thereof and to the planar surface  62  of the ball and the ball is then inserted into the bore  52 . Thereafter the combination of the shaft and the ball along with the brazing compound contained thereon is elevated to an appropriate temperature and maintained at that temperature for a period sufficient to cause brazing of the ball to the shaft by permanently adhering the outer perimeter  69  of the protrusion  60  within the bore  52  and to secure the planar surface  62  to the planar surface  56 . In this manner the drive member can adequately support the tension load as the shaft rotates to reciprocate the spool valve  16  within its bore  14 . By having appropriate brazing of the planar surfaces  56  and  62  as well as the outer perimeter  69  of the protrusion  60  to the inner surface of the bore  52  adds surprising strength to the overall structure. As a result of this strength, the shaft  50  maybe constructed of material which is less expensive than that used in the past. For example, in the past where a post was formed on the shaft and the post was inserted into a hole formed in the ball the shaft was made of much stronger material such as Inconel. In utilizing the structure of the present invention the shaft may be constructed for example of 300 series stainless steel. 
     Where the ball is permanently secured to the shaft by way of brazing the cross bore  58  is utilized to provide a creep hole so that any excess brazing material which may be present on the ball can travel through the interior of the bore  52  and find relief within the cross bore  58 . Also the cross bore  58  provides a vent for gases formed during the brazing process to preclude the same being trapped within the bore  52 . It has also been found that the cross bore  58  provides a convenient inspection port which may be utilized after the manufacture of the drive member, as above described, to determine whether it has been properly formed, that is, that the ball  56  has been fully secured to the shaft  50  in the manner above described.