Abstract:
A self-contained thermal actuator positions the piston return member inside the actuator cup. The inventive configuration eliminates the guide and diaphragm seal that closed the open end of the wax filled cup in the typical thermal actuator configuration. The piston and an O-ring seal seated in an annular recess defined by the piston contain the thermally responsive wax. A return member within the cup and engaged between axially spaced, radially overlapping shoulders on the cup and piston biases the piston toward its pre-actuation position. The return member may be a spring or an elastomeric O-ring that also seals the thermal actuator against intrusion of contaminants from the use environment. The piston is closely received in the cup for guided axial reciprocation therein.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a thermally actuated control device, particularly of the type wherein a thermally responsive wax generates a force to move an actuator in the form of a push rod or the like. 
   2. Description of the Related Art 
   Actuators of this type are often used for thermal control valves, such as described in U.S. Pat. Nos. 2,873,633, 4,036,433 and 5,176,317. Typically, such actuators are designed to produce a predetermined actuating movement in response to a change in temperature. One typical configuration for a thermal actuator includes a rigid cup filled with the thermally responsive wax, a resilient diaphragm wax seal covering the wax, a rigid guide member covering the diaphragm wax seal, and an actuator rod or piston received in the guide member to bear on a plug extending from the wax seal. An increase in temperature ΔT causes the thermally responsive wax to expand, generating a force against the diaphragm wax seal, plug and piston to do some work. An actuator of this type can be used to open or close a valve, for example. The typical thermal actuator installation also includes a return spring arranged to bias the piston, plug and diaphragm toward their pre-actuation (return) position. The return spring ensures that upon a decrease in temperature the shifted valve or element returns to its pre-actuation position. The necessity for an external biasing member complicates assemblies utilizing the wax filled thermal actuator and precludes provision of a self-contained actuator of this type. The present inventors have recognized a need to provide a thermal actuator of the thermally responsive wax type which does not require an external biasing element. Such a thermal actuator simplifies assemblies using the device and expands possible applications for the actuator. 
   SUMMARY OF THE INVENTION 
   The present invention relocates the return member for a thermal actuator inside the rigid cup and eliminates the guide and diaphragm seal typical of the prior art. The piston of the inventive thermal actuator is configured to fill the top portion of the cup. A seal between the piston and the cup contains the thermally responsive wax in the bottom of the cup. The inside surface of the cup and the outside surface of the piston cooperate to guide the reciprocal axial movement of the piston between pre-actuation and actuated positions. 
   The inventive thermal actuator eliminates the separately manufactured guide member of the typical actuator design. Further, a simple O-ring seal replaces the more complex diaphragm seal and its associated plug. One benefit of the inventive design is a more direct force delivery from the expanding wax to the piston due to elimination of the intervening diaphragm seal and plug of the prior art. O-ring seals are standard elements whose mechanical behavior and sealing characteristics are well understood. Therefore, an effective wax containment seal can be accomplished at reduced cost compared to the previous diaphragm type seal. 
   The return member may be a second O-ring-shaped elastomeric member engaged between an annular shoulder on the cup and an annular shoulder on the piston to return the piston to its pre-actuation position when the thermally responsive wax cools. This configuration is particularly suited to a use environment for the thermal actuator that requires relatively short plunger travel. Alternatively, the return member may be a conventional coil spring or a stack of Belleville-type washer springs. Other configurations and materials for an internal return member may be compatible with the present invention. 
   The invention is particularly useful in applications requiring a miniature, self-contained thermal actuator. The disclosed thermal actuator configuration provides a simple, durable, reliable and relatively inexpensive thermally responsive actuator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a longitudinal sectional view through a first embodiment of a thermal actuator according to aspects of the present invention; 
       FIG. 2  is an exploded preassembly sectional view of the thermal actuator of  FIG. 1 ; 
       FIG. 3  is an exterior side view of the thermal actuator of  FIGS. 1 and 2 ; 
       FIG. 4  is a longitudinal sectional view of a cup suitable for use in the thermal actuator of  FIGS. 1-3 ; 
       FIG. 5  is a top end view of the cup of  FIG. 4 ; 
       FIG. 6  is an exterior side view of a piston compatible with the thermal actuator of  FIGS. 1-3 ; 
       FIG. 7  is a bottom end view of the piston of  FIG. 6 ; 
       FIG. 8  is an exploded preassembly sectional view of a first alternative embodiment of a thermal actuator according to aspects of the present invention; 
       FIG. 9  is a partial sectional view of the thermal actuator of  FIG. 8  as assembled; 
       FIG. 10  is an exploded preassembly sectional view of a second alternative embodiment of a thermal actuator according to aspects of the present invention; 
       FIG. 11  is a partial sectional view of the thermal actuator of  FIG. 10  as assembled; 
       FIG. 12  is an exploded preassembly sectional view of a third alternative embodiment of a thermal actuator according to aspects of the present invention; and 
       FIG. 13  is a partial sectional view of the thermal actuator of  FIG. 12  as assembled. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A first embodiment of a thermal actuator according to aspects of the present invention will now be described with reference to  FIGS. 1-7 . The thermal actuator  10  includes a rigid cup  12  defining a longitudinal bore  14  extending from a closed bottom portion  16  of the cup to an opening  18  defined by a top portion  20  of the cup. A wall  22  surrounds the opening  18  at the top of the cup. As best seen in  FIGS. 2 and 4 , the bore  14  in the cup may be a stepped bore having a first diameter D 1  at the bottom portion  16  of the cup and expanding in stepwise fashion D 2 , D 3 , D 4  as the bore progresses toward the open top portion  20  of the cup. The largest diameter D 4  of the bore  14  thins the cup wall  22  to form a lip  24 . After the parts shown in  FIG. 2  are assembled together and installed within the cup as shown in  FIG. 1 , the lip  24  is bent (formed) inwardly to define a first annular shoulder  26 . The inwardly formed lip  24  retains the parts within the cup  12  as a substantially sealed, self-contained unit. 
   The bottom portion  16  of the cup is filled with a predetermined quantity of thermally responsive wax  28 . The thermally responsive wax  28  is selected to provide a desired actuation force F by expansion in response to a known increase in temperature ΔT. As is known in the art, thermally responsive wax can be formulated to expand to generate the actuation force F over a broad range of temperatures. 
   A piston  30  (which also may be referred to as a plunger herein) is arranged in the longitudinal bore  14  of the cup  12  for axial reciprocation between pre-actuation and actuated positions. The piston  30  extends from a first end  32  adjacent the thermally responsive wax  28  to a second end  34  having a head  36 . The piston head  36  includes an actuation surface  38 . The actuation surface  38  of the illustrated piston  30  is a conical surface centered on a longitudinal axis A of the piston  30 . The conical actuation surface has a shallow angle α of approximately 7° relative to a perpendicular P to the longitudinal axis A. Piston diameters D 5  and D 6  cooperate with longitudinal bore diameters D 1  and D 2  to guide the piston  30  during axial movement. 
   The piston  30  includes a radially outwardly projecting second annular shoulder  40  having a diameter D 6 . The second annular shoulder  40  is configured to radially overlap the first annular shoulder  26  (provided by the inwardly formed lip  24 ) in axially spaced relationship. A return member  42  in the form of an elastomeric O-ring is engaged between the first and second annular shoulders  26 ,  40  to bias the piston  30  toward the thermally responsive wax  28 . As best shown in  FIGS. 1 and 2 , diameters D 2  and D 3  of the longitudinal bore  14  are configured to accommodate axial movement of the piston shoulder  40  and compression of the return member  42 . The return member  42  is compressed when the piston  30  is in its actuated position and expands to return the piston to its pre-actuation position when the thermally responsive wax cools. The predetermined quantity of thermally responsive wax  28  is selected so its volume does not interfere with seating of the piston  30  in its pre-actuation position (at the cold temperature). In other words, the wax is measured by volume and is left lower than the bottom of the piston  30 . Therefore, the return member  42  will return the piston  30  all the way to a hard stop on shoulder  52  after each cooling of the thermal actuator. Thus, the pre-actuation position of the actuation surface  38  relative to the cup  12  will be the same after each actuation cycle. 
   The first end  32  of the piston defines an annular recess  44  in which is seated an O-ring seal  46 . The O-ring seal  46  is selected with a thickness T that will be radially compressed between D 7  of the piston  30  and the inside surface  48  of the longitudinal bore to contain the thermally responsive wax  28  below the piston  30 . Force F generated by the expanding wax is delivered to the first end piston and seal  46  to move the piston toward the open end portion  20  of the cup, e.g., toward an actuated position. This piston movement compresses the return member  42  and moves the actuation surface  38  of the head of the piston above the formed lip  24  of the cup to an actuated position (shown in dashed lines on  FIG. 3 ). In the piston return position shown in  FIGS. 1 and 3 , the center of the actuation surface  38  projects above the inwardly formed lip  24 . The particular shape of the piston head  36  and its actuation surface  38  may be modified to suit a particular use environment. Movement of the actuation surface  38  of the head from the position shown in  FIG. 1  to the actuated position shown in dashed lines in  FIG. 3  can be used to perform work such as actuating a switch, closing or opening a valve, or the like. 
   The configuration of the return member  42  and/or the properties of its material can be altered to control movement of the piston  30 . For example, changing the durometer of the return member material will have an effect similar to placement of a weaker or stronger spring in the assembly. Material durometer also affects the speed of movement of the piston  30 . Another variable in piston movement for the thermal actuator  10  is the configuration of the piston groove  54  in which the return member  42  is seated, as well as the space  50  surrounding the return member  42 . The space  50  around the return member  42  accommodates its elastic deformation during piston actuation. In thermal actuator  10 , this space is provided by the diameters D 3  and D 4  of the top portion  20  of the cup as shown in  FIG. 1 . Alternatively, the return member  42  may be selected so that it is loosely seated in piston groove  54  or the piston groove  54  configuration may be altered to provide the necessary expansion space. The configuration of the piston groove  54  may also be altered to provide space to accommodate return member deformation. 
   As best shown in  FIG. 3 , the assembled thermal actuator is a self-contained unit  10 . The return member  42  is arranged inside the cup  12  and secondarily functions to seal the inside of the actuator  10  against the intrusion of foreign matter. The self-contained and substantially sealed configuration of the thermal actuator  10  simplifies incorporating the actuator into an assembly and dramatically expands the number of uses for such actuators. 
     FIGS. 8-13  illustrate three alternative embodiments of a thermal actuator according to aspects of the present invention. Each of the three alternative embodiments  10   a ,  10   b ,  10   c  function in a manner substantially similar to the thermal actuator  10  described above. The alternative embodiments will be described in detail only where they differ from thermal actuator  10 . 
     FIGS. 8 and 9  illustrate a first alternative embodiment of thermal actuator  10   a . The cup  12   a  shown in  FIG. 8  is of a simplified configuration at its top portion  20   a . The wall  22   a  surrounding the top portion  20   a  is of a constant thickness, eliminating the stepped bore of the top portion  20  shown in  FIG. 2 . The diameter D 8  of the top portion  20   a  of the cup  12   a  is selected to correspond roughly to diameter D 3  of cup  12  shown in  FIG. 4 . Diameter D 8  leaves some extra space  50  in the top portion  20   a  of the cup  12   a  after inward deformation of the lip  24  to define the first annular shoulder  26 . This space  50  accommodates elastic deformation of an elastomeric return member such as that illustrated in  FIGS. 1 and 13 . Elimination of the stepped bore shown in  FIG. 4  simplifies manufacture of the thermal actuator. Thermal actuator  10   a  employs a return member  42   a  in the form of a coil spring. Piston  30   a  is configured with a second annular shoulder  40  and head  36   a  complimentary to the coil spring return member  42   a . The components illustrated in  FIG. 8  are assembled as shown in  FIG. 9  and lip  24  is inwardly formed to define a first annular shoulder  26  in axially spaced relation to the second annular shoulder  40 . The coil spring return member  42   a  is compressed between the first and second annular shoulders. The actuation surface  38   a  of piston  30   a  is a simple flat surface exemplary of the many alternative actuation surface configurations compatible with the present invention. 
     FIGS. 10 and 11  illustrate a second alternative embodiment of a thermal actuator  10   b  employing the same simplified cup  12   a  as described with respect to  FIGS. 8 and 9 . The return member  42   b  of thermal actuator  10   b  is in the form of a stack of belleville-type washer springs engaged between first and second annular shoulders  26 ,  40 . Thermal actuator  10   b  employs a simple flat actuation surface  38   a.    
     FIGS. 12 and 13  illustrate a third alternative embodiment of a thermal actuator  10   c . Thermal actuator  10   c  employs the simplified cup  12   a  as discussed with respect to  FIGS. 8 and 9 . The diameter D 9  of the top portion  20   a  of the cup  12   a  is selected to include an annular space  50  which accommodates elastic deformation of the elastomeric return member  42  during piston movement between return and actuated positions. The piston  30   c  illustrates a still further alternative configuration for an actuation surface  38   b . Actuation surface  38   b  includes a steeply angled conical surface with a flat tip. 
     FIGS. 9 ,  11  and  13  illustrate the pistons  30   a ,  30   b ,  30   c  in their return or pre-actuation positions. The pre-actuation positions are defined by a hard stop between the underside of the second annular shoulder  40  and a shoulder  52  between the upper and lower cup portions  20 ,  16 . This hard stop-defined pre-actuation position ensures that the piston  30   a ,  30   b ,  30   c  will be in the same position relative to the cup  12   a  at sub-actuation temperatures. 
   While a preferred embodiment of the foregoing invention has been set forth for purposes of illustration, the foregoing description should not be deemed a limitation of the invention herein. Accordingly, various modifications, adaptations and alternatives may occur to one skilled in the art without departing from the spirit and the scope of the present invention.