Abstract:
An improved construction for a heating element for an appliance is disclosed. A coiled resistance wire extending coaxially along the length of an elongate sheath is surrounded by an electrically insulating, high thermally conductive material that fills the sheath around the wire. The resistance wire is secured to a terminal pin at a connection comprising a connection insert that is securely affixed to the terminal pin, intermediate the terminal pin and resistance wire. The resistance wire is then able to be welded to the connector to provide a superior mechanical connection between a terminal pin and the resistance wire, even though the terminal pin is made from copper. The disclosed construction, therefore, provides good electrical and thermal conductivity and resists the tendency to separate during manufacture.

Description:
FIELD 
       [0001]    The present disclosure relates to a heating element for an appliance. In particular, the present disclosure relates to an improved construction for a heating element such as a water-immersed heating element, for example. 
       BACKGROUND 
       [0002]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0003]    Appliances, such as dishwashers, clothes washers and water heaters, for example, employ a heating element for heating water or other liquid that is used in the appliance. The heating element is immersed in the water to be heated. When the heating element is energized, it produces heat that is transferred to the surrounding water. 
         [0004]    Such heating elements generally comprise a resistance heater that produces heat when an electrical current is passed through it. A typical tubular heating element comprises a coiled resistance wire extending coaxially along the length of an elongate metal sheath. An electrically insulating material having a relatively high thermal conductivity is used to fill the space between the coil and the inner wall of the sheath. The resistance wire is commonly made from metals such as Fe/Cr/Al or Ni/Cr. Granulated magnesium oxide (MgO) is one substance known to be suitable for serving as the filler material. 
         [0005]    During the heating element&#39;s manufacturing process, the granulated magnesium oxide is introduced into the sheath. The sheath is subsequently subjected to a compression force, which causes the sheath to reduce in diameter, increase in length and compact the granulated magnesium oxide inside. In the compacted state, the magnesium oxide&#39;s dielectric and thermal conductive properties are improved. As a result of the compacting process, the heating element may be “partially compacted” (e.g., the diameter of the heating element is reduced by approximately 15% or less of its original diameter, such as from 0.375 in. to 0.334 in. (8.5 mm)) or “fully compacted” (e.g., the diameter of the heating element is reduced by approximately 15% or more of its original diameter, such as from 0.375 in. to 0.315 in. (8.0 mm)). Fully compacted heating elements are generally preferred over partially compacted heating elements due to performance and reliability advantages, such as increased efficiency of heat transfer from the resistance wire to the sheath and increased ability to manipulate or bend the heating assembly to fit particular applications, for example. 
         [0006]    The heating elements of the type described also include a thermal protection device, such as a thermally-actuated cutoff switch or a thermally-actuated fuse. The thermal protection device allows current to pass to the resistance wire at normal operating temperatures, but it prevents or “cuts off” the current to the resistance wire if the temperature of the heating element exceeds a predetermined threshold temperature. The thermal protection device is typically embedded in the heating element adjacent to and in a thermally conductive relationship with, the resistance wire. This is accomplished via a metal terminal pin that is connected to the resistance wire on one end and the thermal protection device on the other. 
         [0007]    During operation of the heating element, heat generated by the resistance wire is conducted through the terminal pin to the thermal protection device. In instances where the heating element approaches and/or exceeds the predetermined temperature, the thermal protection device cuts off the current to the resistance wire. 
         [0008]    One condition under which the heating element may exceed the predetermined threshold temperature is a “dry start;” that is, when the heating element is energized but, the heating element is not immersed in liquid. When a dry start occurs, the heating element quickly heats up to a temperature beyond its normal operating temperature such that the heating element or the appliance in which it is installed may be damaged or rendered inoperable. Therefore, it is important for the thermal protection device of the heating element to react very quickly (e.g., less than 80 seconds) to cut off the current to the heating element when the predetermined threshold temperature is reached so as to eliminate or minimize any damage to the appliance or its components. 
         [0009]    In order to achieve the desired reaction time in the thermal protection device, the efficient transfer of thermal energy from the resistance wire to the thermal protection device is desired. In this regard, it is important to securely fasten the terminal pin to the resistance wire. The construction of known water-immersed heating elements incorporates a metal terminal pin (usually made from steel) that is welded to the resistance wire. Even better heat transfer characteristics and reaction times, however, can be achieved with terminal pins that are made from copper, since copper has superior electrical and thermal conductivity as compared to steel. A copper terminal pin, though, is not easily welded to the heating element. This is so because the material composition of the resistance wire, e.g., Fe/Cr/Al or Ni/Cr, is not suitable for welding to copper without the use of advanced welding techniques, like laser welding or ultrasonic welding, for example, which generally are not considered to be cost-effective in this application. Consequently, construction of heating elements having a copper terminal pin has employed a connection method less robust than welding. There, the coiled resistance wire of the heating element is typically attached to the terminal pin by being “screwed onto” groves that are formed in the end of the terminal pin. 
         [0010]    While marginally acceptable in the manufacture of partially compacted heating elements, the “screw on” connection method has proved less suitable for consistent and reliable production of fully compacted heating elements. In this regard, the forces applied to the heating element for compacting the magnesium oxide are known to degrade the physical and electrical connections between the resistance wire and the terminal pin. It is not uncommon in the manufacture of fully compacted heating elements that the resistance wire and the terminal pin become fully detached. In other cases, though the heating element and terminal pin do not completely separate during compaction, the resulting heating elements exhibit a high incidence of electrical arcing at the connection between the terminal pin and the resistance wire, thereby resulting in premature failure of the heating element. 
         [0011]    Thus, there is opportunity for improvement of known water-immersed heating elements. For example, it is desirable to provide a heating element utilizing a copper terminal pin that provides a superior connection between the terminal pin and the resistance wire even after compaction. 
       SUMMARY 
       [0012]    A heating element for an appliance comprising a resistance wire and a terminal pin in electrical and thermal contact is disclosed. A connection insert is securely affixed to the terminal pin and the resistance wire is, in turn, secured to the connection insert, such as by welding. The heating element disclosed provides the ability to use a copper terminal pin in the heating element while at the same time achieving a robust electrical, thermal and mechanical connection between the terminal pin and the resistance wire which is a significant advantage over prior known heating element constructions. Optionally, a thermal protection device is located between the resistance wire and the terminal pin. In such a configuration the terminal pin permits the efficient transfer of thermal energy from the resistance wire to the thermal protection device. The thermal protection device operates to cut-off current to the resistance wire when the heating element exceeds a predetermined temperature. 
         [0013]    Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0014]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0015]      FIG. 1  is a plan view of a heating element according to the present teachings, partially in cross-section, and particularly showing the connection between a resistance wire and a thermal protection device of the heating element; 
           [0016]      FIG. 2  is an exploded perspective view of the connection between the resistance wire and the thermal protection device shown in  FIG. 1 ; 
           [0017]      FIG. 3  is an enlarged view of detail A of  FIG. 1 ; and 
           [0018]      FIG. 4  is a plan view similar to that of  FIG. 1 , of an alternate embodiment of a heating element according to the present teachings. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
         [0020]    With initial reference to  FIG. 1 , an exemplary heating element for an appliance according to one embodiment the present teachings is illustrated at  10 . As shown, this embodiment of the heating element  10  generally includes two leg portions  11  and a serpentine portion  13  extending between the leg portions  11 . Each leg portion  11  terminates in an electrical connector  15  that enables the heating element  10  to be installed for use in a household appliance, for example. In one appliance installation, the heating element  10  may be used for heating water or another liquid in the appliance. There, the heating element can be immersed in the liquid to be heated. When the heating element is electrically energized, it produces heat that is transferred to the surrounding liquid. Of course, it should be understood that the foregoing merely represents one possible application for a heating element according to the present teachings, and that the heating element may be employed in many other applications. 
         [0021]    As shown in  FIG. 1 , the heating element  10  comprises the components of a resistance wire  12 , a terminal pin  14 , a socket  16 , a thermal protection device  18 , and a terminal assembly  20 . The resistance wire  12 , the terminal pin  14 , the socket  16 , and the thermal protection device  18  are surrounded by an outer sheath  24 . A suitable electrically insulating, thermally conductive material  22  fills the space between the resistance wire  12  and the sheath  24 . An example of a thermally conductive material  22  that can be used includes granulated magnesium oxide (MgO). Particularly shown in the cross-sectioned portion of  FIG. 1  is the connection between the resistance wire  12  and the thermal protection device  18  of the heating element  10 . 
         [0022]    The resistance wire  12  is any suitable resistance wire capable of acting as a heating element. Known resistance wires made from metals such as Fe/Cr/Al or Ni/Cr are suitable for use in the heating element  10 . As illustrated throughout the figures, the resistance wire  12  is wound into a coil. The resistance wire  12  receives electrical current from a current source (not shown) and is in thermal and electrical contact with the terminal pin  14 . The connection between the resistance wire  12  and the terminal pin  14  is illustrated in greater detail in  FIG. 3 . 
         [0023]    The terminal pin  14  is any suitable electrical and thermal conductor for conducting electrical current and thermal energy between the terminal pin  14  and the resistance wire  12 . The terminal pin  14  can be manufactured from a metal, such as steel. Preferably, however, the terminal pin  14  is made from solid copper to take advantage of copper&#39;s superior electrical and thermal conductivity. Alternatively, the terminal pin  14  can comprise a bimetallic construction such as a copper core steel pin, for example. 
         [0024]    With additional reference to the exploded perspective view of  FIG. 2 , the terminal pin  14  is shown to have a generally stepped, cylindrical configuration. In particular, the terminal pin  14  includes a first portion  26  having a first outer diameter, a second portion  28  having a second outer diameter, a third portion  30  having a third outer diameter, and a stem portion  32  having a stem outer diameter. The first outer diameter is larger than the second outer diameter, the second outer diameter is larger than the third diameter, and the third outer diameter is larger than the stem outer diameter. Moreover, the stem outer diameter is conically tapered such that it progressively decreases from an end of the stem portion  32  proximate to the third portion  30  of the terminal pin  14  to an end of the stem portion  32  that is distal to the third portion  30 . 
         [0025]    The terminal pin  14  is connected to the thermal protection device  18  with any suitable connector capable of conducting electrical current and thermal energy, such as the socket  16 . The socket  16  includes a first receptacle  34  and a second receptacle  36 . The first receptacle  34  is sized and configured to securely receive the terminal pin  14 . The second receptacle  36  is sized and configured to securely receive the thermal protection device  18 . The socket  16  is made of any suitable material that possesses good electrical and thermal conductivity. Preferably, the socket  16  is made from copper. 
         [0026]    The thermal protection device  18  is any suitable device that will terminate current flow to the resistance wire  12  when the resistance wire  12  exceeds a temperature that may cause damage to the heating element  10  or surrounding areas. For example, the thermal protection device  18  can be a thermally-actuated cutoff switch, a thermally-activated fuse, a PTC device, or the like, as are well-known in the art. 
         [0027]    The terminal assembly  20  generally includes a connection terminal  38 , a mechanical connector  40 , and a sleeve  42 . The mechanical connector  40  includes a main body  44  and a flange portion  46 . The main body  44  defines an aperture  48 . An exterior of the main body  44  include threads  50 . The flange portion  46  is cylindrically-shaped and extends beyond the main body  44 . 
         [0028]    The connection terminal  38  and the thermal protection device  18  extend to within the aperture  48  where the connection terminal  38  and the thermal protection device  18  are electrically connected. The connection terminal  38  and the thermal protection device  18  are inserted into opposite ends of a spacer or tubular jacket  42  seated within the aperture  48 . The jacket  42  facilitates alignment of the connection terminal  38  with the thermal protection device  18 . The jacket  42  also facilitates electrically connecting the connection terminal  38  to the thermal protection device  18 . 
         [0029]    A second terminal assembly (see  FIG. 1 ) that is similar to the terminal assembly  20  is provided at an opposite end of the resistance wire  12 . 
         [0030]    Portions of the terminal pin  14  and the resistance wire  12  are surrounded by the conductive layer  22 . The conductive layer  22  is any suitable electrically insulating, thermally conductive material. The conductive layer  22  conducts thermal energy generated by the resistance wire  12  to the outer sheath  24  and the environment surrounding the heating element  10 . A suitable material for use as the conductive layer  22  is magnesium oxide (MgO). 
         [0031]    The outer sheath  24  is any suitable material capable of transferring thermal energy generated by the resistance wire  12  from within the outer sheath  24  to the environment surrounding the heating element  10 . For example, the outer sheath  24  can be made of a metal, such as steel. 
         [0032]    With continued reference to  FIGS. 1 and 2  and additional reference to  FIG. 3 , details of the connection between the resistance wire  12  and the terminal pin  14  are shown. The connection between the resistance wire  12  and the terminal pin  14  is facilitated by an insert or collar, illustrated as a sleeve  52 , for example. The sleeve  52  is installed over the third portion  30  of the terminal pin  14 . The sleeve  52  is designed having an inner diameter that is smaller than the outer diameter of the third portion  30 . In particular, the inner diameter of the sleeve  52  has a diameter sufficient to provide a slight interference fit with the third portion  30  of the terminal pin  14  on the order several thousands of an inch (e.g., 0.002 in.). 
         [0033]    The sleeve  52  comprises any suitable electrically and thermally conductive material. It is generally preferred that the sleeve  52  is made from steel so that the resistance wire  12  is can be easily welded to the sleeve  52  with conventional and cost-effective welding techniques. One method presently contemplated for securing the steel sleeve  52  to the terminal pin  14  is facilitated by first heating the sleeve  52 , causing its inner diameter to expand so that the sleeve can pass over the outer diameter of the third portion  30 . The inner diameter of the steel sleeve  52  then contracts as the sleeve  52  cools, thereby becoming securely attached to the terminal pin  14  with an interference fit. Of course, other manufacturing methods and techniques for attaching the sleeve  52  to the terminal pin  14  may be employed, as desired; pressing the sleeve  52  onto the terminal pin  14  being one example. 
         [0034]    The resistance wire  12  is secured to an exterior portion of the sleeve  52 . The resistance wire  12  is secured to the sleeve  52  using any device or method that will provide a secure electrical, thermal, and mechanical connection between the resistance wire  12  and the sleeve  52 , and ultimately to the terminal pin  14 . For example, as indicated above, the resistance wire  12  is preferably welded to the sleeve  52  to provide an extremely robust electrical, thermal, and mechanical connection. In a preferred construction, such welding may be achieved by conventional welding techniques in a cost-effective manner because the sleeve  52  and the resistance wire  12  can be made from materials that are compatible for welding. 
         [0035]    A strong mechanical connection between the resistance wire  12  and the sleeve  52  (and terminal pin  14 ) is particularly important to insure that the resistance wire  12  does not separate or otherwise become completely or partially detached from the sleeve  52  (and terminal pin  14 ) during the manufacture of the heating element  10 . In particular, during its manufacture, the heating element  10  is subjected to a reduction rolling process to fully compact or partially compact the heating element  10 . Fully compacting the heating element  10  provides a number of advantages, such as: superior heat transfer characteristics; a superior ability to manipulate, form, or bend the heating element  10  to fit a particular application; superior strength of the heating element  10 ; and superior lifespan of the heating element  10 . 
         [0036]    One of ordinary skill in the art will appreciate that the insert or collar can take the form of any device that will provide a mechanical, electrical, and thermal connection between the resistance wire  12  and the terminal pin  14  that will not degrade under the forces generated during the reduction rolling process. For example, the insert or collar need not be a sleeve  52 , but can take the form of, for example, a tab or a plate. Further, while the sleeve  52  is described as a steel sleeve, the sleeve  52  can be made of any suitable material that will provide or permit a mechanical, electrical, and thermal connection between the resistance wire  12  and the terminal pin. 
         [0037]    To further enhance the mechanical connection between the sleeve  52  and the terminal pin  14 , after the sleeve is installed on the terminal pin  14  the terminal pin  14  can be deformed to create a protrusion portion  54 . In such an instance, the protrusion portion  54  is provided between the third portion  30  and the stem portion  32 . As shown in  FIG. 3 , the protrusion portion  54  projects beyond the inner diameter of the sleeve  52 , but not as far as the outer diameter of the sleeve  52 . The protrusion portion  54 , as shown, can be formed by applying pressure to the terminal pin  14  at opposite sides the third portion  30 . Doing so deforms the terminal pin  14 , creating an indentation in the third portion  30  in one direction while creating the protrusion portion  54  in a direction perpendicular to the indentation. Alternatively, the protrusion portion  54  can extend from the terminal pin  14  at discrete points about the circumference of the terminal pin  14 , or it can take the form of an annular rim that extends completely around the circumference of the terminal pin  14 . 
         [0038]    In an alternate construction of the heating element  10  incorporating a terminal pin  14  having a bimetallic construction, such as a copper core steel pin, the resistance wire  12  may be attached in a secure manner directly to the terminal pin  14  preferably, by conventional welding techniques. In this regard, the exterior surface material the bimetallic terminal pin  14  and the resistance wire  12  can be made from materials that are compatible for welding. 
         [0039]    In operation, the heating element  10  is connected to a circuit of an appliance at its terminal assemblies  20 . A current source (not shown) such that the connection terminal  38  is in electrical contact with the current source. The mechanical connection between the terminal assembly  20  and the circuit of the appliance is enhanced through cooperation between the threads  50  and corresponding threads of the appliance. 
         [0040]    Current is conducted through the connection terminal  38 , the thermal protection device  18 , the socket  16 , the terminal pin  14 , and the sleeve  52  to the resistance wire  12 . The high resistance of the resistance wire  12  causes the wire  12  heat up (e.g., I 2 r heating) when current is applied to the wire  12 . The thermal energy generated by the resistance wire  12  is conducted by the conductive layer to the outer sheath  24 . A heat transfer then takes place between the outer sheath  24  and the environment in which the heating element  10  is operating. 
         [0041]    Thermal energy generated by the resistance wire  12  is also conducted through the sleeve  52 , to the terminal pin  14  and the socket  16 , and to the thermal protection device  18 . If the thermal energy detected at the thermal protection device  18  exceeds a predetermined threshold, the thermal protection device opens the circuit to interrupt the flow of electrical current to the resistance wire  12 . When the heating element  10  is used in a dishwasher for example, the predetermined threshold can be set to a temperature at which the heating element  10  or other portions of the dishwasher may be damaged under dry start conditions. 
         [0042]    An alternate construction for a heating element without an integrated thermal protection device is shown in  FIG. 4  at  100 . Similar to the heating element  10  shown in  FIG. 1 , the heating element  100  is shown to generally include two leg portions  11  and a serpentine portion  13  extending between the leg portions  11 . Each leg portion  11  terminates in an electrical connector  15  having a connection terminal  38  for connection to a current source (not shown). 
         [0043]    The heating element  100  is illustrated as further comprising a resistance wire  12  and a terminal pin  14  surrounded by an outer sheath  24 . A electrically insulating, thermally conductive material  22 , such as magnesium oxide, fills the space between the resistance wire  12  and the sheath  24 . Shown in the cross-sectioned portion of  FIG. 4  is the connection between the resistance wire  12  and the thermal protection device  18  of the heating element  10  substantially as described.