Abstract:
Improved adhesion, electrical connection, and thermal connection between an electrical heater and a wax motor element are obtained by using multiple zones of different adhesives optimized for different properties. The location of the zones and the area of the zones may be a controlled to effect a trade off between different properties and to limit the expression of the conductive adhesive.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     This invention relates to the assembly of wax motors of the type used in household appliances such as washing machines and dishwashers. 
     Wax motors are disclosed in U.S. Pat. No. 5,572,869 assigned to the assignee of the present invention and hereby incorporated by reference. 
     In such wax motors, a metal housing holds wax and a piston so that expansion of the wax when the housing is heated, drives the piston outward. The housing may be heated by a positive temperature coefficient (PTC) resistor attached to the housing. The PTC resistor may have one face in both thermal and electrical contact with the metal housing so that the metal housing may both receive heat from the heated element and serve as one electrode for that resistor. 
     In order to ensure good thermal contact between the housing and the PTC resistor, and to increase the robustness of the assembly, an adhesive may be placed between the housing and resistor. This adhesive may be a thin layer of thermally conductive adhesive that is nominally electrically insulating, but perforated by the operating voltage of 120 volts to allow conduction between the housing and PTC resistor. Alternatively, the adhesive may be a thermally conductive adhesive with electrically conductive particles dispersed therein. 
     The use of a thin layer of electrically insulating adhesive that is perforated by the line voltage does not always establish a reliable electrical contact between the housing and resistor resulting in an unnecessarily high rejection rate for tested units. On the other hand, electrically conductive adhesives have proven difficult to apply with a propensity to create short circuit paths across the sides of the resistor when the housing and resistor are pressed together and adhesive is squeezed out from between them. 
     The selection of possible adhesives is limited by the fact that the adhesive must provide some gap filling qualities and the ability to accommodate different coefficients of expansion of the metal housing of the wax motor and the resistor element over a wide temperature range of heating and cooling. It is difficult to find an electrically conductive adhesive that provides the necessary space filling characteristics, compliance to permit thermal expansion, thermal conductivity, and adhesive strength. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides an adhesive system in which multiple distinct zones of different types of adhesives are used to assemble PTC resistor and wax motor together. These distinct zones allow the selection of individual adhesives optimized for different properties. Further, by placing the conductive adhesive within zones of non-conductive adhesive, the non-conductive adhesive can be used to corral the conductive adhesive limiting its potential for creating short circuit paths. 
     Specifically, the present system provides a thermal actuator of a type having a wax motor element with a face attached to a PTC resistor where a first adhesive material attaching the PTC resistor to the face of the wax motor is placed in a first zone on the face and a second adhesive material attaching the PTC resistor to the face of the wax motor is placed in a second zone on the face. The first and second adhesive materials have substantially different properties of electrical conduction. The first and second adhesives may also have different thermal conduction properties. 
     Thus, it is one object of at least one embodiment of the invention to optimize properties of adhesion and thermal conductivity independently of electrical conductivity through the use of multiple adhesives with independently adjustable zones. 
     The first adhesive may have substantially greater electrical conductivity than the second adhesive and the second adhesive may have stronger adhesive properties than the first adhesive. 
     Thus, it is another object of at least one embodiment of the invention to provide improved adhesion over that which can be obtained by suitable electrically conductive adhesives. 
     The first adhesive may have substantially greater electrical conductivity than the second adhesive and the first zone may be substantially smaller an area than the second zone. 
     Thus, it is another object of at least one embodiment of the invention to limit the amount of conductive adhesive used to eliminate problems of inadvertent short circuits. 
     The adhesives may be elastic to accommodate different coefficients of thermal expansion between the face of the wax motor and the PTC resistor. 
     Thus, it is another object of at least one embodiment of the invention to provide a system that can accommodate the wide range of temperatures required of the interface between a heater and a metal housing. 
     The first adhesive may have greater electrical conductivity than the second adhesive and the second adhesive may be placed in two independent areas flanking an area of the first zone. 
     Thus, it is another object of at least one embodiment of the invention to provide control of the conductive material by corralling it with non-conductive material. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a right side view in elevation of an actuator assembly of the present invention; 
         FIG. 2  is a top plan view of bottom casing section of the actuator assembly of  FIG. 1 ; 
         FIG. 3  ms sectional view of the assembly of  FIG. 1  taken in the plane indicated by line  3 — 3  in  FIG. 1 ; 
         FIG. 4  is a sectional view taken in the plane indicated by line  4 — 4  in  FIG. 2 ; 
         FIG. 5  is a sectional view taken in the plane indicated by line  5 — 5  in  FIG. 2 ; 
         FIG. 6  is a sectional view taken in the plane indicated by line  6 — 6  in  FIG. 2 ; 
         FIG. 7  is a sectional view taken in the plane indicated by line  7 — 7  in  FIG. 3 ; 
         FIG. 8  is a right end view taken in the plane indicated by line  8 — 8  in  FIG. 2 ; 
         FIG. 9  is a left side view of the top casing section of  FIG. 1  turned upside down; 
         FIG. 10  is top plan view of the top casing section of  FIGS. 1 and 9 ; 
         FIG. 11  is a top sectional view of the actuator seen in  FIG. 3 ; 
         FIG. 12  is a sectional view taken in the plane indicated by line  12 — 12  in  FIG. 11 ; 
         FIG. 13  is a side plan view of one of the terminals seen in  FIG. 12 ; 
         FIG. 14  is an exploded, sectional view of the assembly of  FIG. 1 , with the actuator removed; 
         FIG. 15  is a sectional view taken in the plane indicated by line  15 — 15  in  FIG. 1 ; 
         FIG. 16  is an exploded perspective view of the wax motor and the PTC resistor used in the actuator of the present invention showing placement of the adhesive on the upper surface of the wax motor; and 
         FIG. 17  is a top plan view in phantom showing the spreading of the adhesives of  FIG. 16  when the PTC resistor is compressed against the housing. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows an actuator assembly  10  for a soap dispenser in a dishwasher. The assembly  10  includes a top casing section  11  and a bottom casing section  12 . The casing sections  11 ,  12  extend longitudinally, which is from left to right as seen in  FIG. 1 . Apertured lugs  13 ,  14  (also seen in  FIGS. 1 ,  4 ,  5  and  12 ) are integrally formed with casing sections  11 ,  12 , near the front end, for attachment of the assembly  10  within a larger piece of apparatus. Contact blades  15 ,  16  on electrical terminals  201  extend downwardly near the rear or base end wall  17 , for plugging the assembly  10  into a source of electrical energy. The casing sections  11 ,  12  are made of a synthetic polyester or polyamide material, such as Celanese 1503-2; Celanex 3310; Celanex 7700 or IMPET 530. 
     As seen in  FIG. 3 , a positive temperature coefficient (PTC) resistor  20  is mounted inside the casing sections  11 ,  12 . The resistor  20  receives electrical current supplied through contact blades  15 ,  16  and heats up to apply heat to a wax motor element  18 . The wax motor element  18 , is mounted inside the casing sections  11 ,  12 . As seen in more detail in  FIG. 11 , the wax motor element  18  has a metal housing  19 . As seen in  FIG. 11 , the housing  19  has first counterbore  106  in which an O-ring  104  of temperature resistant material is positioned. A brass washer  102  and a washer  103  made of a fluorocarbon material, such as Teflon, are stacked in a second counterbore  107  of larger diameter than the first counterbore  106 . The brass washer  102 , being of softer material than the hardened shaft  101 , is retained by a crimp in the housing  19 . A hardened shaft  101 , preferably of stainless steel, provides an operating stem  21  that extends out of the housing  19  through openings in the centers of O-ring  104 , Teflon washer  103 , and brass washer  102 . The Teflon washer  103  is provided with an interference fit with shaft  101 . A thermally expansive material  109 , such as wax or another suitable material, is contained within a main cavity  108  in the housing. 
     Returning to  FIGS. 3 and 12 , the positive temperature coefficient (PTC) resistor  20  with ohmic layers  209 ,  210  is attached to one side of the thermal actuator housing  19  by one of several methods to be described. Current is supplied through terminals to resistor  20  to produce resistive heating. Heat is conducted to the housing  19  from resistor  20 , and from there, is conducted to the material  109 , causing it to expand and causing the operating stem  21  of shaft  101  to move out from crimped end of the housing  100 . O-ring  104  is sized relative to the counterbore  106 , such that it is allowed to float in the manner of a hydraulic seal. The operating stem  21  is moved through an operating stroke of approximately ¼ inch. 
     Operating stem  21  ( FIG. 7 ) has a tip that is received in a hole  52  in a base  51  of a plunger  22 . The plunger  22  is made of one of the materials specified above for the casing sections  11 ,  12  and has a shaft end  50  that extends out of the casing sections  11 ,  12 . This shaft end  50  is formed with a nose  57 , and first and second annular flanges  54  and  56  are separated by an annular groove  55  for connection to apparatus controlling the soap dispenser door. Ribs  58  extend from the base up the sides of the shaft end  50 . The ribs  58  extend along the inner diameter of the spring, for a portion of their length, to support the inner diameter of the spring  23  and prevent the spring  23  from buckling. 
     As seen in  FIGS. 2 ,  4 ,  5 ,  10  and  12 , the casing sections  11 ,  12  are formed with vents  24 ,  25  to vent heat from the casing sections  11 ,  12 . The casing sections  11 ,  12  also provide cowl sections  26 ,  27  ( FIGS. 1 ,  8 ,  9  and  10 ) which together form a cowl extending from one end of the casing. 
     The top casing section  11  has a connecting pin  28  ( FIG. 9 ) extending down from a left side wall to be received in a slot  29  ( FIGS. 2 and 3 ) in a left sidewall of the bottom casing section  12 . On the right sidewall, a pin  34  extends up from the bottom section to be received in a slot (not shown) in the top section similar to slot  37 . Inside the cowl sections  26 ,  27  are arcuate sections  43 ,  44  ( FIGS. 1 ,  2 ,  3  and  5 ) which form a projection within a circular groove  47  inside the cowl sections  26 ,  27 . One end of the coiled compression spring  23  is received in this groove  47 , with an interference fit over arcuate sections  43 ,  44  to radially locate the inner diameter of the spring  23  and to hold the casing halves together. Cowl sections  26 ,  27  form an axial bore  46  through the end wall  49  to allow extension of the plunger  22  outside the casing sections  11 ,  12 . 
     The actuator stem also has a slot  53  transverse to its longitudinal axis for receiving a metal pin  60  ( FIGS. 3 and 7 ) during assembly. Pin  60  is received horizontally through casing slots  61  in the casing sections  11 ,  12  during assembly, as seen in  FIG. 3 . The casing sections  11 ,  12  are put together and the pin  60  is then removed to allow one end of the spring to move forward over the internal arcuate sections  43 ,  44  formed on the opposing end of the casing sections  11 ,  12 . 
     A web  45  in an H-shape, as seen from the top in  FIG. 2 , is positioned towards the base end wall  17  of the two casing sections  11 ,  12 . A similar web (not shown) is formed on the top casing section  11 , so that the webs will enclose the housing  19  and form a frame for holding the housing  19 . Just inside the base end wall  17 , the lower casing section  11  provides two stab connecting fingers  64 ,  65  ( FIGS. 2 ,  3 ,  4 ,  6 ,  8 ) with tapered tips which fit in slots  76 ,  77  alternating with like fingers  74 ,  75  (FIG.  6 ) on the opposing casing section  12 . Fingers  74 ,  75  are received in slots  66 ,  67  ( FIG. 10 ). The combination of four meshed fingers  64 ,  65 ,  74 ,  75  forms additional support just inside the base end wall  17 , and fingers  64 ,  65 ,  74 ,  75  are held against lateral deformation by the base end wall  17 . 
     In addition, each casing section  11 ,  12  has a third stab connecting finger  68 ,  78  spaced from one edge of the end wall  17  and located on one corner of the casing section  11 ,  12 . This finger  68 ,  78  engages a tab  70 ,  80  extending laterally from the base end wall  17  on an opposing corner of the other casing section  11 ,  12 . There is a relief  69 ,  79  next to each third finger  68 ,  78  to allow the finger  68 ,  78  to flex as it slides around the tab  70 ,  80 . A projection  71 ,  81  adjacent to the end then protects the finger  68 ,  78  against reversing direction around the tab  70 ,  80 . 
     During assembly, spring  23  is compressed on plunger  22  and pin  60  is inserted in slot  53  to retain spring  23  in a compressed state. Casing sections  11  and  12  are assembled with pin  60  projecting through casing slots  61 , and when the pin  60  is removed, the end of the spring  23  moves forward over arcuate sections  43 ,  44  as seen in  FIG. 1 . It should be noted that the connecting pins  28 ,  34  on opposite sides are located on opposite casing sections  11 ,  12 , and that fingers  64 ,  65 ,  74  and  75  mesh together to provide symmetrical and balanced loading across the casing joint. After the casing sections have been joined, the pin  60  is removed to allow the spring  23  to fit over arcuate sections  43 ,  44  and into groove  47 . The end coils of the spring  23  capture and contain the arcuate sections  43 ,  44  of the casing sections  11 ,  12  to positively hold the casing sections  11 ,  12  together. 
     Along the sidewalls, there are overlapping flanges  91 ,  92 ,  93  and  94  seen in  FIGS. 2 ,  9 ,  11  and  12  to provide a mating interengagement of the sidewalls along a portion of the casing joint. Member  95 , seen best in  FIGS. 11 and 14 , encloses one corner of the housing near the base end wall  17 . 
     Referring to  FIGS. 12 and 13 , terminals  201  are each provided with bifurcated, flexible leg contacts  203  to distribute the current flow onto the ohmic layer  209  of resistor  20 . This improves heating response because the ohmic layers  209 ,  210  are of relatively low conductivity compared to the terminal  201 . The bifurcated contacts  203  provide increased reliability in the event that a fracture occurs in the resistor  20  because the remaining portions would then continue to function unimpeded. 
     Referring to  FIG. 13 , each of the two terminals  201  has a conventional contact blades  15 ,  16  and a longitudinal beam section  202  containing a longitudinal rib  206  to provide reinforcement and stiffness. The rib  206  is extended around bend section  204  as shown by reference  205  in  FIG. 12 . The terminal  201  is thinned at section  207  by coining or other means to permit deflection of the flexible leg contacts  203  and to enable a resilient connection of the leg contacts  203  against ohmic layer  209  and housing  19  as seen in  FIG. 12 . 
     Referring to  FIG. 12 , PTC resistor  20  is shown with conventional ohmic layers  209  and  210 , shown in exaggerated thickness in the drawing. The purpose of these layers is to conduct current to the resistor  20  to cause the heating. The efficiency of ohmic layer  209  can be improved by applying current at multiple locations from a terminal of lower resistivity than the ohmic layer  209 . This is done with bifurcated flexible leg contacts  203 . Further, a fracture of the ceramic resistor  20  will be tolerated without a loss of reliability and performance by supplying power to both sides of the fracture  213 . 
     Referring to  FIGS. 3 and 14 , bulkhead sections are formed by overlapping side members  82 – 85  and upper and lower walls  86 ,  87  which form a barrier having a window  90 , the barrier fitting closely around the housing  19  to isolate and separate the sealed end of actuator housing  19  from the PTC resistor  20 . Leakage of thermally expansive material from the sealed end of housing  19 , if allowed to contact resistor  20 , could impair performance of the resistor  20  and cause it to overheat. The bulkhead sections prevent any leaked material from migrating back to the region of the resistor  20 . 
     The resistor  20  is rectangular in shape to fit the sidewall of actuator housing  19 . The PTC resistor  20  is attached to the housing using one of three materials, either a) a thermally conductive grease, b) a thin layer of thermally conductive adhesive that is perforated by an operating voltage of 120 volts to allow conduction between the PTC resistor and actuator housing or c) a thermally conductive adhesive with electrically conductive particles dispersed therein. This adhesive  212  is shown in exaggerated thickness in  FIG. 12 . 
     Each of these three materials provides a thermally conductive adhesive  212  allowing multiple conductive paths between the resistor  20 , ohmic layer  210  and the wall of the actuator housing  19 . In the case of the adhesive with conductive particles, the conductive particles can also be uniformly sized to more evenly distribute the current conducted therethrough. 
     Referring to  FIGS. 2 ,  12  and  13  another feature of the construction assists in correct and easy assembly of the terminals  201  in the casing sections  11  and  12 . The casing sections  11  and  12  are formed with pockets  214  located between T-shaped apertures  215  for receiving the contact blades  15 ,  16  and respective sidewalls of the casing sections  11 ,  12 . The upper end  216  of each terminal  201  is narrower than each contact blade  15 ,  16  and is laterally offset from the contact blade  15 ,  16 . The contact blades  15 ,  16  are assembled by insertion from the inside of one casing section  12  used as the lower housing section, and when the top casing section  11  is placed on top of lower casing section  12 , the upper end  216  of the terminals  201  will be received and held in proper position, providing the terminals  201  are assembled with the bifurcated contacts  203  facing to the inside of the casing section  11 ,  12 . The pockets  214  are offset from the apertures  215  to account for the offset in the terminals  201  caused by the bend at reference  205 . 
     As seen in  FIG. 2 , the casing sections  11 ,  12  each have a pair of apertures  215  for receiving the contact blades and pockets  214  disposed between respective apertures  215  and respective walls of each respective casing section  11 ,  12  to allow the casing sections  11 ,  12  to be used interchangeably in assembly with the terminals  201 . 
     The pockets  214  include ramp surfaces  217  for selectively guiding chamfered upper ends  216  of terminals  201  into pockets  214 . The pockets  214  also include ramps  218 , seen best in  FIG. 5 , for guiding the chamfered lower ends  219  of contact blades  15 ,  16  away from pockets  214  and into apertures  215 . Additional ramp surfaces  220 ,  221  are provided around T-shaped apertures  215  to guide the lower ends of contact blades  15 ,  16  into apertures  215 . 
     Referring now to  FIG. 16 , the PTC resistor  20  may be assembled to the metal housing  19  to contact an upper face  300  of the metal housing  19 . Before this assembly, adhesive  212  is placed at the interface between the PTC resistor  20  and face  300 , preferably, but not necessarily on the face  300 . 
     In the present invention, two different adhesives  310  and  312  are placed in distinct zones  302   a ,  302   b  and  304  on face  300  spaced along the length of the face  300  generally aligned with the shaft  101  of the wax motor element  18 . A first adhesive  310  may be placed in two locations indicated by zones  302   a  and  302   b  flanking zone  304  at which a second adhesive  312  is placed. 
     The first adhesive  310 , in a preferred embodiment is a two part silicon adhesive that incorporates thermally conductive material to increase its thermal conductivity. A suitable adhesive  310  provides a thermal conductivity of 0.83 watts/° C. meter and a strength of 700 psi. This adhesive provides strength to the joint and ensures good thermal connection between the resistor  20  and the face  300 . 
     As described above, the second adhesive  312  is placed at zone  304  and is a one part silicon adhesive that incorporates electrically conductive material such as carbon or silver particles. A suitable adhesive  312  provides an electrical conductivity of 150 ohms centimeters and a strength of 475 psi. This adhesive  312  provides principally conduction between the resistor  20  and the face  300  and is limited to that required amount necessary to provide electrical conduction yet to reduce any extrusion of the adhesive as the PTC resistor  20  is pressed against face  300 . The localized area of conduction provided by the second adhesive  312  at zone  304  is acceptable because of the current spreading effect of ohmic layer  210  on the surface of the PTC resistor  20 . 
     It will be understood that adhesive  312  provides some adhesion and thermal conductivity and adhesive  310  provides some electrical conductivity through “punch through” of adhesive  310  under line voltages. 
     The amount of adhesive  310  at zones  302   a  and  302   b  is substantially greater than the amount of adhesive  312  at zone  304   
     Referring now to  FIG. 17 , when the PTC resistor  20  is clamped against the housing  19  the zones  302   a  and  302   b  of adhesive  310  expand to corral the adhesive  312  at zone  304 . This corralling property plus the carefully metered amount of the adhesive at zone  304  reduces the risk of excess extrusion of the conductive adhesive in zone  304  such as could create short circuits between ohmic layers  210  and  209  of the PTC resistor  20 . 
     The PTC resistor  20  is held clamped to the housing  19  for sufficient time for the adhesive in zones  302   a  and  302   b  to cure at which time the wax motor and PTC resistor may be handled as a unit for subsequent manufacturing steps as described above. 
     It will be understood that the adhesive may alternatively be placed on the ohmic layer  210  of the PTC resistor prior to compression or that some adhesive may be placed on the housing  19  and some on the PTC resistor  20 . 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.