Abstract:
An encapsulated stator for use in solenoid-actuated liquid metering device has a bobbin assembly and a plunger guide which are substantially surrounded by a plastic overmold to hold the assembly together and provide a protective housing. Upper and lower ends of the plunger guide are not covered by the overmold, but rather are left exposed so that additional manufacturing and assembly steps may be performed upon the stator. The plunger guide is formed integrally with a support plate and two threaded body for receiving bolts to secure the stator in its operating position. A plastic cap with an integrally molded electrical connector housing for mating engagement with an electrical power supply is secured to the bobbin assembly and retains the core and plunger guide in proper positions relative to the bobbin prior to the assembly being encased in the overmold. A method of manufacturing the encapsulated stator is also disclosed.

Description:
FIELD OF THE INVENTION 
     The present relates to solenoid-type fuel metering devices such as those used on diesel and other direct fuel injection internal combustion engines, and more particularly to an improved stator for use in such a solenoid. 
     BACKGROUND OF THE INVENTION 
     In modern direct fuel injection engines, such as diesels, it is known to use one or more solenoid actuated devices to control the volume and timing of the fuel supplied to the fuel injectors associated with each combustion chamber. Such solenoid fuel metering devices are preferably electronically controlled and allow the extremely precise metering of fuel necessary to achieve optimum engine performance and low emissions. Fuel metering solenoids must be extremely rugged in order to function properly in the engine environment over a long service life, and they must be able to operate at relatively high fuel pressures. 
     In order to ensure the factory-set performance characteristics of the engine are not altered by a mechanic or operator after the engine is sold, it is desirable to construct the fuel metering devices to prevent or discourage any tampering therewith. 
     A prior art fuel metering solenoid stator, as seen in FIG. 1, comprises two large metal components: a casing  100  which houses a solenoid bobbin assembly, and a tubular metal plunger guide  102  with an integral top plate  104 . The stator is assembled by inserting the bobbin assembly into a cavity formed in the lower surface of the casing  100 , placing mold cores in alignment with three apertures  106 , 108  in the top of the casing, and pouring or injecting a liquid plastic into the cavity to surround the bobbin assembly and mold cores. The mold cores are removed to leave three holes  110 , 112  passing completely through the casing  100 , and the plunger guide  102  is inserted downwardly through the large, center hole  110  so that it passes through the center of the bobbin assembly. Bolts are inserted upwardly through the two smaller holes  112  passing adjacent the outside of the bobbin and engage threaded holes  114  in the top plate  104  to secure the top plate to the casing and the entire assembly to an engine (not shown). An electrical connector  116  for supplying current to the bobbin wire exits the side of the casing. 
     A metering plunger (not shown) is inserted upwardly into the bore  118  of the plunger guide  102  from below and a spring (not shown) is disposed in the upper end of the guide  102  to bias the plunger downwardly. When the solenoid stator is energized, the magnetic field generated thereby draws the plunger upward to alternatively open and close a metering orifice below the stator assembly. 
     This prior art stator assembly requires a significant amount of precision machining and assembly, and therefore is relatively expensive to produce. 
     SUMMARY OF THE INVENTION 
     It is an objective of this invention to provide a stator for a liquid metering solenoid that is less costly to produce than prior art stators. It is a further objective of this invention to provide a stator assembly that is substantially encapsulated within a plastic material and so is rugged and tamper resistant. 
     According to the invention, an encapsulated stator comprises a bobbin assembly and a plunger guide which are substantially surrounded by a plastic overmold to hold the assembly together and provide a protective housing. Upper and lower ends of the plunger guide are not covered by the overmold, but rather are left exposed so that additional manufacturing and assembly steps may be performed on the stator. An electrical conductor is connected to the coil and passes through the overmold so that electrical power may be supplied to the coil. 
     According to another feature of this invention, the encapsulated stator further includes at least one threaded body having a threaded hole for receiving a fastener such as a bolt. In the preferred embodiment, two threaded bodies are utilized and are positioned on opposite sides of the bobbin assembly with the threaded holes parallel to a longitudinal axis of the plunger guide. The threaded bodies are substantially surrounded by the overmold with the holes exposed at the lower surface of the stator and are used to fasten the stator in its operating position. 
     According to a further feature of the invention, the plunger guide and the threaded body are connected with one another by a support plate which extends substantially normal to the longitudinal axis of the plunger guide and is disposed above the bobbin assembly. The support plate serves to positively position the threaded bodies relative to the plunger guide so as to ensure dimensional accuracy of the finished part. In a preferred embodiment of the invention, the support plate, plunger guide and threaded bodies are formed as an integral unit by casting. 
     According to yet another feature of this invention, a cap is secured to the bobbin assembly adjacent its upper end such that the plunger guide projects through a hole in the cap. The engagement between the cap and the bobbin retains the core and plunger guide in proper positions relative to the bobbin prior to the assembly being encased in the overmold. In a preferred embodiment of the invention, the cap comprises an electrical connector housing for mating engagement with the electrical power supply which powers the solenoid. 
     The present invention also provides a method of manufacturing an encapsulated stator having the structure described above. In the invention method, a plunger guide is inserted through a center passage of a bobbin assembly to form a stator assembly, and a plastic material is molded over and around the stator assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein: 
     FIG. 1 is an exploded perspective view of a prior art stator; 
     FIG. 2 is a perspective view of an encapsulated stator according to the present invention; 
     FIG. 3 is a bottom view of the encapsulated stator of FIG. 2; 
     FIG. 4 is a cross section of the stator taken along lines  4 — 4  in FIG. 2; 
     FIG. 5 is a cross section of the stator taken along lines  5 — 5  in FIG. 2; 
     FIG. 6 is an assembly view of a stator assembly according to the present invention prior to being encased in an overmold; 
     FIG. 7 is an exploded view of the stator assembly shown in FIG. 6; 
     FIG. 8 is an exploded perspective view of an alternative embodiment of a fuel metering solenoid stator; and 
     FIG. 9 is a partially cut-away view of the casing and plunger guide of the stator of FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS.  2 - 7 , an encapsulated liquid metering stator  10  according to the present invention comprises a stator assembly  12  encased within and substantially surrounded by a plastic overmold  14 . A generally tubular plunger guide  16  extends through the overmold  14  from top to bottom such that its upper and lower ends are exposed at the upper and lower surfaces respectively of the overmold. The exposed lower end of the plunger guide  16  allows a plunger (not shown) to be inserted upwardly into the plunger guide, and the exposed upper end allows a plunger biasing spring (not shown) to be inserted therein. When the invention stator  10  is installed in its operating environment, a lower end of the plunger extends from the plunger guide  16  and alternatively opens and closes a metering orifice depending upon the axial position of the plunger as controlled by energization of the solenoid. 
     The stator assembly  12  includes a bobbin  18  with a length of electrically conductive wire  20  wrapped therearound to form a coil in the manner well known in the solenoid art. The bobbin  18  is preferably rectangular in overall configuration and is formed from a non-ferrous plastic material. First and second end plates  22  project upwardly from the top surface of the bobbin  18  adjacent opposite ends thereof. Guide ridges  23  extend vertically along the opposite inner surface of the bobbin  18 . First and second retaining arms  24  project upwardly from the top surface of the bobbin  18  and have inwardly extending barbs  26  at their distal ends. 
     First and second cores  28  are formed of a ferrous metal and are inserted downwardly from the top surface of the bobbin  18 , one adjacent each of the end plates  22 . Each core  28  is generally E-shaped, having a center arm  28   a  passing through the center passageway of the bobbin  18  and two side arms  28   b  passing along opposite outer surfaces of the bobbin. The cores  28  contact and are positioned by the end plates  22  and guide ridges  23  when they are properly positioned within the bobbin  18 . As is well known in the solenoid art, the cores  28  are preferably of laminar construction and are manufactured by tack welding or otherwise bonding together a plurality of identical thin sheets of magnetic steel. 
     The plunger guide  16  is generally cylindrical and has an internal bore  30  extending along its longitudinal axis. The bore  30  is of the proper diameter adjacent its lower end to receive the metering plunger (not shown), and is of larger diameter adjacent its upper end to receive the spring (not shown) which biases the plunger downwardly. After the spring has been inserted into the bore  30  from its upper end, a plug (not shown) is inserted into the bore to retain the spring. 
     The plunger guide  16  is preferably formed integrally with a support plate  32  which lies in a plane generally normal to the longitudinal axis of the plunger guide. Preferably, first and second threaded bodies  34  are disposed at opposite ends of the support plate  32  and project downwardly parallel with the plunger guide  16 . Threaded holes  36  extend upwardly into the threaded bodies  34  from their lower ends. The plunger guide  16 , support plate  32 , and threaded bodies  34  are preferably formed of a substantially non-ferrous metal and are cast as an integral unit. Alternatively, the threaded bodies may be formed separately from the plunger guide  16  and/or support plate  32 . If formed separately, the threaded bodies may be connected with the plunger guide  16 /support plate  32  assembly prior to being surrounded by the overmold  14 , or may be placed in the mold separately from the other components. 
     A connector cap  38  is disposed adjacent a top end of the bobbin  18  assembly and is preferably formed from a thermoplastic material, such as glass-filled nylon, by injection molding. The connector cap  38  has a hole  40  passing vertically through its center and an electrical connector housing  42  is disposed on an upper surface of the cap and extends outwardly therefrom. A pair of electrically conductive wires  44  are molded into the cap so that first ends thereof are disposed adjacent the lower surface of the cap and opposite second ends are disposed within the electrical connector housing  42 . One or more electrical circuit components (such as resistors, diodes, transistors, or semiconductors) may be provided to make electrical connection with the coil  20 . A circuit component  45  is shown in FIG. 4 molded into the connector housing  42  and connected with wires  44 , but such components may also be located elsewhere in or on the encapsulated stator  10  and connect with the circuit formed by the wires  44  and the coil wire  20  at any point. A narrow perimeter ridge  46  extends outwardly around the outer edge of the cap  38 , and an annular ridge  48  extends upwardly surrounding the hole  40  in the cap. 
     After the cores  28  have been properly assembled with the bobbin  18 , the plunger guide  16  is inserted downwardly through the center of the bobbin so that it passes between the center arms  28   a  of the two cores  28 , and the threaded bodies  34  pass adjacent opposite outer surfaces of the bobbin  18 . The support plate  32  contacts the top surfaces of the cores  28  and fits between the end plates  22  of the bobbin  18 . 
     The connector cap  38  is then placed over the top of the plunger guide  16  so that the upper end of the guide projects upwardly through the hole  40  and the lower surface of the cap contacts the upper surface of the support plate  32 . The barbs  26  at the distal ends of the retaining arms  24  of the bobbin  18  snap into engagement with notches  50  formed in corresponding positions on the lower surface of the cap  38 . The ends of the coil wire  20  are soldered to the first ends of the wires  44  extending downwardly from the cap  38 . 
     When assembled in this fashion, the stator assembly  12  forms a unitary component in which all of the parts are securely retained in their proper positions relative to one another. The unitary stator assembly  12  may be handled and, if necessary, shipped prior to its being inserted into a mold (not shown) for formation of the overmold  14 . 
     The overmold  14  may be formed by a thermoplastic material, in an injection molding process, or by a thermoset plastic. The overmold  14  surrounds the upper surface of the cap  38 , leaving the connector housing  42  and the top end of the plunger guide  16  extending upwardly therethrough. The perimeter ridge  46  around the edge of the cap  38  and the annular ridge  48  around the hole  40  provide for a good mechanical bond between the overmold  14  and the cap. The plastic utilized to form the overmold  14  flows into the spaces between the various components of the stator; i.e., into the spaces between the threaded bodies  34  and the bobbin  18 , and into the spaces inside the center of the bobbin that are not occupied by the core center arms  28   a  and the plunger guide  16 . The overmold  14  preferably leaves the lower ends of the plunger guide  16 , the threaded bodies  34 , and the cores  28  exposed on the lowermost surface of the encapsulated stator  10 . 
     After the plastic of the overmold  14  has set and the encapsulated stator  10  is removed from its mold, the only machining steps remaining to be performed are those necessary to ensure that the bottom surface of the encapsulated stator is flat and true. The bottom surface is ground flat, including the lower ends of the plunger guide  16 , threaded bodies  34 , and cores  28 . 
     In a preferred embodiment of the invention manufacturing method, the plunger guide  16  is initially cast or otherwise fabricated such that it is solid adjacent its lower end, the central bore  30  extending downwardly into the guide from the upper end but stopping short of the lower end of the guide. The lower end of the bore  30  is then bored or ground to the correct internal diameter to receive the plunger. This step is preferably performed substantially simultaneously with the grinding of the lower surface of the encapsulated stator. This sequence of manufacturing steps has been found to be an effective way to achieve the very high degree of precision necessary in forming the diameter of the lower, plunger receiving portion of bore  30  and in its perpendicularity of the to the bottom surface of the encapsulated stator. 
     An alternative embodiment of a liquid metering solenoid stator  52  is depicted in FIGS. 8 and 9 and comprises a casing  53  formed of a non-ferrous metal and having a lower cavity  54  for receiving a stator assembly  56  and an integrally formed plunger guide  58  extending downwardly through the cavity  54 . The unitary casing  53  and plunger guide  58  are preferably formed by casting and are subsequently machined to form an interior bore  58   a  through the plunger guide  58 , an electrical connector aperture  60  in the upper surface of the casing  53 , and first and second blind, threaded holes  62  in the lower edge of the casing  53 . 
     The stator assembly  56  comprises a bobbin  64 , a coil  66  wrapped on the bobbin, and a pair of cores  68  substantially similar to those used in the encapsulated stator  10  described hereinabove. The ends of the coil wire  66  are soldered or otherwise connected to terminals  70 . 
     During assembly of the stator  52 , the stator assembly  56  is inserted upwardly into the lower cavity  54  of the casing  53  such that the plunger guide  58  passes through the center of the bobbin  64  and the terminals  70  project through the connector aperture  60 . A connector housing  72  is then fastened to the upper surface of the casing  53  to make electrical connection with the terminals  70  and is secured to the casing  53  by bolts  74 . A thermoplastic or thermoset plastic material is then poured or injected into the cavity  54  to surround the stator assembly  56  and secure it in place therein. The lower surface of the resulting solenoid stator is ground flat as in the previously described embodiment, and the lower end of the plunger guide bore  58   a  may be simultaneously bored or ground to the correct diameter. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.