Abstract:
A seal assembly for a wax motor or the like provides an elastomeric seal, fitting against an actuator shaft that may slide through the elastomeric seal, where the elastomeric seal has mold flash regions that are displaced from the contact area between the shaft and the elastomeric seal significantly increasing elastomeric seal life.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application 61/816,892 filed Apr. 29, 2013 and hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to wax motors used to provide mechanical actuation in response to heating, and specifically to a wax motor having an extended life. 
       BACKGROUND OF THE INVENTION 
       [0003]    Wax motors provide a simple and reliable linear actuator used in a variety of applications including home appliances. Typically, a wax motor provides a metal housing holding wax and a piston so that expansion of the wax drives the piston outward. The expansion of wax occurs when the housing is heated, for example, by electrical resistance such as a positive temperature coefficient thermistor. Typically, a return spring is present in the mechanism associated with the wax motor to return the piston into the housing when the wax cools. Such wax motors develop large forces over a short stroke distance such as one-quarter inch and may be operated for many heating and cooling cycles. 
         [0004]    The design and construction of a wax motor is described in U.S. Pat. No. 5,572,869 assigned to the assignee of the present invention and hereby incorporated by reference. 
         [0005]    The wax motor piston may be a smooth cylindrical rod passing out of the housing through a seal that retains the wax in the housing. A common seal includes an elastomeric O-ring supported at its outer face by one or more washers, for example, a Teflon washer followed by a brass washer. The outer washer maybe held in place by a crimping of the housing around the outer washer. 
         [0006]    The O-ring may fit within a counter bore in the housing and allows it to float in the manner of a hydraulic seal effectively preventing the escape of the thermally expansive wax. 
         [0007]    The seal formed by the O-ring operates to reliably prevent leakage of the wax motor during initial operation. Over time with extended use, the O-ring may begin to wear at its interface with the piston shaft ultimately leading to leakage and failure of the wax motor. 
       SUMMARY OF THE INVENTION 
       [0008]    The present inventors have determined that seal leakage that develops after extensive use of the wax motor is strongly dependent on the molding process used to produce the O-ring of the seal. In particular, O-rings molded so as to displace the molding flash away from the inner diameter of the O-ring that would normally be in contact with the piston provide extensively longer lifetimes. This is despite the fact that these specially molded o-rings and standard O-rings both seal acceptably for many thousands of initial cycles and any flash actually increases the material in the wear zone. This displacement of the flash may be accompanied by design is decreases the peak pressure of the contact region by providing a more uniform inner diameter. 
         [0009]    Specifically then, the present invention provides a thermally responsive actuator having a housing having a bore extending along an axis and opening at a first end of the housing. A piston is slidable along the axis and has a shaft extendable through the opening of the housing with movement of the piston and a thermal expansion material is positioned within the bore in contact with the piston. A seal assembly is attached to the first end of the housing to cover the opening and surround a portion of the shaft to allow the shaft to slide through the seal while preventing the escape of the thermal expansion material Importantly, the seal assembly includes an elastomeric seal surrounding a portion of the shaft and contacting the portion of the shaft at a contact region, the elastomeric seal having mold flash regions displaced from a center of the contact region. 
         [0010]    It is thus a feature of at least one embodiment of the invention to increase the operating life of a wax motor or similar device by accommodating possible structural weakness in the mold flash regions and displacing them away from a center of the contact region with the shaft subject to the highest contact pressures for an O-ring. The standard molding process of the prior art results in flash at center of contact between the O-ring and shaft caused by a seam where the core and cavity sections of the O-ring mold meet. The flash is usually removed by tumbling or secondary operation or in handling. While the inventors do not wish to be bound by a particular theory, it is believed that the result of this flash removal is an area where there are negative and/or positive surface defects which are in the center of the sealing region of the o-ring. 
         [0011]    The contact region of the elastomeric seal in a relaxed state may have a have a substantially constant cross-section. 
         [0012]    It is thus a feature of at least one embodiment of the invention to increase the contact area between the elastomeric seal and the shaft without increasing distention of the elastomeric seal, the peak pressures of contact and thus wear forces, thereby decreasing wear rate. 
         [0013]    The elastomeric seal may fit within a counter bore in the housing having a diameter larger than a relaxed diameter of the elastomeric seal. 
         [0014]    It is thus a feature of at least one embodiment of the invention to provide increased seal life in a floating seal design. 
         [0015]    The counter bore may be coaxially about a bore having a diameter extending substantially midway between a diameter of an outer periphery of the elastomeric seal and an inner periphery of the elastomeric seal as installed in the counter bore. 
         [0016]    It is thus a feature of at least one embodiment of the invention to increase seal life in the design with minimized axial elastomeric seal compression. 
         [0017]    The elastomeric seal may further include a gate mark removed from the contact region. 
         [0018]    It is thus a feature of at least one embodiment of the invention to also accommodate possible structural weakness in the elastomeric seal near where an elastomer is introduced into the elastomeric seal mold. 
         [0019]    The elastomeric seal may include witness marks in the mold flash regions removed from the contact region. 
         [0020]    It is thus a feature of at least one embodiment of the invention to increase the wear life of an elastomeric seal even where there is no appreciable flash. 
         [0021]    The elastomeric seal in a relaxed state may be a torus having a cross-section lying within a circle along a plane including the axis and wherein the mold flash regions are diametrically opposed across the circle at an angle to the axis. 
         [0022]    It is thus a feature of at least one embodiment of the invention to shift the mold flash regions away from the contact region while preserving symmetry in the mold. 
         [0023]    Alternatively, the elastomeric seal in a relaxed state is a torus may have a rectangular cross-section in a plane including the axis and wherein the mold flash regions are at corners of the rectangular cross-section. 
         [0024]    It is thus a feature of at least one embodiment of the invention to shift the mold flash regions away from the contact region and to provide a large contact region of uniform pressure. 
         [0025]    Alternatively, the elastomeric seal in a relaxed state may include a frustoconical portion. 
         [0026]    It is thus a feature of at least one embodiment of the invention to provide a tapered seal where axial compression can be used to control radial seal compression. 
         [0027]    Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings in which like numerals are used to designate like features. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a perspective view of a wax motor suitable for use with the present invention showing the housing having an end that crimps about an opening through which the wax motor piston may extend; 
           [0029]      FIG. 2  is a cross-section taken along lines  2 - 2  of  FIG. 1  showing receipt of the piston in a cavity within the housing holding wax, the latter end within the cavity by an O-ring followed by one or more washers, the last being held by the crimping of the housing; 
           [0030]      FIG. 3  is an exploded view of the piston, O-ring and washers showing a cross-sectional detail of the O-ring with bias-displaced mold flash; 
           [0031]      FIG. 4  is a fragmentary view of an example mold for fabricating the O-ring of  FIG. 3 ; 
           [0032]      FIG. 5  is an exploded cross-sectional view of the O-ring of  FIG. 3  and a portion of the wax motor housing and piston shaft in an alternative embodiment that also provides a flatter inner diameter of the O-ring; 
           [0033]      FIG. 6  is a Fig. similar to that of  FIG. 3  showing an alternative design where the mold flash is displaced from the inner diameter region only; 
           [0034]      FIG. 7  is a fragmentary, cross-sectional view similar to  FIG. 2  showing a cylindrical seal design providing mold flash displaced from the inner diameter region; 
           [0035]      FIG. 8  is a perspective view of the cylindrical seal design of  FIG. 7 ; 
           [0036]      FIG. 9  is a figure similar to that of  FIG. 4  showing a cavity design for molding the cylindrical seal of  FIG. 7 ; 
           [0037]      FIG. 10  is a figure similar to that of  FIG. 7  showing a frustoconical seal design also having mold flash displaced from the inner diameter region; 
           [0038]      FIG. 11  is a figure similar to that of  FIG. 8  for the frustoconical seal of  FIG. 10 ; 
           [0039]      FIG. 12  is a figure similar to that of  FIG. 9  showing a cavity design for the molding of the frustoconical seal of  FIG. 10 ; 
           [0040]      FIG. 13  is a figure similar to that of  FIG. 7  showing a flanged, frustoconical seal also having mold flash displaced from the inner diameter region; 
           [0041]      FIG. 14  is a figure similar to that of  FIG. 8  for the flanged frustoconical seal of  FIG. 13 ; and 
           [0042]      FIG. 15  is a figure similar to that of  FIG. 9  showing a cavity design for molding the flanged frustoconical seal of  FIG. 13   
           [0043]    Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items and equivalents thereof. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0044]    Referring now to  FIG. 1 , a wax motor  10  may provide a metal housing  12 , for example, having a rectangular body  13  and an opening  14  in one end through a machine cylindrical neck  19 . A polished cylindrical shaft  16  may slidably extend along an axis  15  through a seal assembly  17 . The shaft  16  may, for example, be a hardened stainless steel material. 
         [0045]    Referring also to  FIG. 2 , the metal housing  12  may provide for blind bore  18  along axis  15  into which the shaft  16  extends. A first counter bore  20  at an end of the blind bore  18  near the opening  14  may hold an O-ring  22  positioned coaxially about the shaft  16  and constructed of a temperature resistant elastomer. A second counter bore  24  of larger diameter than the first counter bore  20  and closer to the opening  14  may hold a Teflon washer  26  and a brass washer  28  stacked concentrically about the shaft  16  so that the Teflon washer  26  is adjacent to the O-ring  22  and the brass washer  28  is positioned on an outer side of the Teflon washer  26 . The Teflon washer  26  has an interference fit with the shaft  16  while the brass washer  28  is sized to have a sliding fit with the shaft within the intended operating temperature range of the wax motor  10 . 
         [0046]    A lip of the cylindrical neck  19  around the second counter bore is crimped radially inward about the outer face of the brass washer  28  to retain the brass washer  28  in position with slight compression of the Teflon washer  26  against a floor of the second counter bore  24  and minimal or no compression of the O-ring  22  between the Teflon washer  26  and a floor of the first counter bore  20 . 
         [0047]    A thermally expansive material  30 , such as wax or another suitable material, is contained within the bore  18  surrounding the shaft  16 . 
         [0048]    Referring now to  FIG. 3 , the O-ring  22  may approximate a ring torus and be constructed of the elastomeric material having a radial symmetry about axis  15 . Mold flash regions  32 , being locations where elastomeric material may leak into the seam between mold portions, may be located in diametric opposition about the cross-sections  33  of the O-ring  22  in a plane perpendicular to the axis  15 . In particular, the mold flash region  32  at each cross-section  33  of the O-ring  22  will define diameter lines  34  through the cross-sections  33  where the diameter lines  34  intersect the axis  15  at an angle, for example 45 degrees. When the mold flash region  32  is displaced along these canted diameter lines  34 , the mold flash region  32  will be removed from the inner radius  36  of the O-ring  22  and thus away from contact with the shaft  16  passing through the O-ring  22 . The invention contemplates that there may be no actual mold/at the mold flash regions  32  either because of mold precision or post-molding/-removal steps. Independent of the existence of mold flash, the O-ring  22  may exhibit “witness lines” caused by slight irregularities at the mold part lines. Such witness lines, may represent minor misalignment between mold portions or other discontinuities. 
         [0049]    O-rings  22  of this characteristic have been determined to provide for sealing of the thermal expansion material  30  in a wax motor  10  for many more actuation cycles than conventional O-rings in which the mold flash regions are in contact with the shaft  16 . This phenomenon does not appear to be related to the existence of the mold flash which arguably increases the material in the wear region and therefore might be expected to postpone wear and leakage. While the inventors do not wish to be bound to a particular theory, it is believed that displacement of the mold flash region may be independent of any mold flash but instead reflective of the microstructure of the O-ring  22  in the vicinity of a mold seam. Accordingly the invention is not limited to O-rings that actually exhibit mold flash. 
         [0050]    The O-ring  22  may further have a gate mark  38  being excess elastomeric material from an attachment of the O-ring  22  to material of a feeding sprue during the molding process. The gate mark  38  may also be displaced away from the wear region, for example, to an outer diameter of the O-ring  22  away from contact with the shaft  16 . 
         [0051]    Referring now to  FIG. 4 , O-rings of this type may be manufactured by a mold  40  having a bottom plate  42  with an upwardly extending frustoconical projection  44  that narrows with elevation. The projection  44  may have sidewalls formed at an angle, for example, of  45  degrees to the surface of plate  42  generally conforming to the desired diameter lines  34 . Centered along this upward extension of the frustoconical projection  44 , an annular groove  46  may circumscribe the frustrum so as to define one-half of the mold cavity having a toroidal cross-section. An upper plate  48  may have a frustroconical cavity  50  with the same general external dimensions as upwardly frustoconical projection  44  to mate therewith during the molding process. Machined into the surface of this cavity  50  is another groove  52  which mates with the groove  46  of projection  44  when the upper plate  48  and the bottom plate  42  are in the closed, seated relationship of  FIG. 4 . 
         [0052]    Elastomeric material may be forced in molten form and at high pressure into the cavity formed by groove  46  and groove  52  (through a gate channel  57 ) intersecting the cavity in a radial direction from outside of the cavity. Methods of molding O-rings suitable for use in the present invention are described generally in U.S. Pat. No. 4,043,727 and in the Aug. 2, 1945 issue of American Machinists cited therein and hereby incorporated in its entirety by reference. 
         [0053]    Referring now to  FIG. 5 , the cross-section  33  of the O-ring  22  in its relaxed state before installation on the shaft  16  may have a flattened inner radius  36  diverging from a perfectly circular cross-section  60  having a radius  62 . This form may be produced by proper shaping of the cavities described above with respect to  FIG. 4 . Specifically the inner radius  64  of the cross-section  33  of the O-ring  22  perpendicular to the axis  15  may be smaller than radius  62  of a circular cross-section  60 , the smaller radius  64  hence producing a larger radius of curvature. 
         [0054]    In the relaxed state, the O-ring  22  has an inside diameter  66  slightly smaller than the outside diameter  68  of the shaft  16  to seal tightly thereon. The outside diameter  70  of the O-ring  22  in the relaxed state, however, is slightly smaller than the internal diameter  72  of the counter bore  20  so that the O-ring  22  may essentially float with respect to the housing  12 . A radial wall  76  between counter bore  20  and the blind bore  18  extends inward to a point  65  approximately midway between the radially outer periphery  69  of the O-ring  22  and its inner periphery  71  when placed on the shaft  16  to restrict axial motion of the O-ring  22  within the housing  12  in a direction away from the opening  14  with minimal compression in the axial direction on the O-ring  22 . 
         [0055]    Referring now to  FIG. 6 , it will be appreciated that when the O-ring  22  is installed on the shaft  16 , it will contact the shaft  16  over a contact region  80  that will be increased with the flattened design shown in  FIG. 5  without the need to increase the contact force between the O-ring  22  and the shaft  16  to elastically flatten the O-ring  22 . This contact region  80  is of principal concern with respect to the displacement of inner mold flash region  32  which may be displaced along diameter line  34  angled with respect to axis  15  to be removed from the contact region  80  and its sealing interface against shaft  16 . In contrast, outer mold flash region  32 ′ may lie along the equatorial diameter line of the cross-section perpendicular to axis  15  being removed from the sealing interface of the inner radius  36 . This irregular part line is possible because of the elasticity of the material of the O-ring  22  which could allow it to be removed from the mold in without typical mold relief. 
         [0056]    Referring now to  FIGS. 7 , and  8 , the O-ring  22  of the wax motor  10  may be replaced with a cylindrical elastomeric seal  90  being generally a circular cylinder having a central, axial bore  92  of substantially constant circular cross-section when the cylindrical elastomeric seal  90  is in a relaxed state. The cylindrical elastomeric seal  90  may be constructed of the same materials as the O-ring  22  and may fit within the first counter bore  20 , the latter extended slightly in axial length to accommodate a greater axial length of the elastomeric seal  90 . In this case, the Teflon washer  26  may be omitted, with the inner surface of the brass washer  28  abutting the outer circular base of the cylindrical elastomeric seal  90 . A tapered transition  94  may be made between the first counter bore  20  and the blind bore  18  to allow axial compression of the elastomeric seal  90  inward by the brass washer  28  such as increases a sealing force between elastomeric seal  90  and the shaft  16 . 
         [0057]    As with the O-ring  22 , any witness lines caused by molten elastomeric material migrating into the part line of the mold during molding will be displaced from the center of the axial bore  92  to its edge. Referring to  FIG. 9 , as with the O-ring  22 , the part line between a mold bottom plate  42  and a mold top plate  48  may be displaced away from the center of the contact region defined by a substantially cylindrical central mold pin  100  forming the axial bore  92 . A gate channel  57  may enter the mold cavity at one circular face of the seal  90  (when molded) away from the contact region with the shaft  16  (when the cylindrical elastomeric seal  90  is assembled on the shaft  16 ) 
         [0058]    Referring now to  FIGS. 10 ,  11 , and  12 , an alternative design provides a frustoconical seal  102  fitting within a corresponding frustoconical first counter bore  20  that tapers toward its connection with the blind bore  18 . Compression by the brass washer  28  against the larger base of the frustoconical seal  102  provides increased compression of the frustoconical seal  102  over the contact region between the axial bore  92  and the shaft  16 . A similar mold  40  as described with respect to  FIG. 9 , but where the central mold pin  100  includes an outer flair at one and forming a concave countersink  104  in the face of the seal  102  about the shaft  16  on the side which abuts the brass washer  28 . As with the previous design, the contact region during the shaft  16  and the frustoconical seal  102  is spread over a larger constant cross-section central bore  92  (defined when the frustoconical seal  102  is in the relaxed state). 
         [0059]    Referring now to  FIGS. 13 and 14 , the frustoconical seal  102  may have an integrally molded flange  106  attached at its outer face as abuts the brass washer  28 . A rear face of this molded flange  106  abuts the radial wall separating the first counter bore  20  from the second counter bore  24  so that the molded flange  106  may be compressed between that wall and a rear face of the brass washer  98 . Again axial compression tends to tighten the frustoconical portion  102  about the shaft  16  caused by the interaction of the frustoconical portion  102  with the conical first counter bore  20 . Referring to  FIG. 15 , a mold similar to that shown and described with respect to  FIG. 12  may be used for molding the frustoconical seal  102  with the integrally molded flange  106  by introducing a radially extending rim  108  to form that molded flange  106 . 
         [0060]    Various features of the invention are set forth in the following claims. It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention.