Abstract:
A bimetallic switch comprising a bimetallic element having an outer edge and being adapted to snap between an open mode and a closed mode; a movable contact disposed on the bimetallic element; a first terminal in electrically conductively coupled to the movable contact; a fixed contact disposed adjacent the movable contact such that, when the switch is in a closed position, the fixed contact and the movable contact are in engagement with one another, and when the switch is in an open position, the fixed contact and the movable contact define an open contact gap therebetween; a second terminal electrically conductively coupled to the fixed contact; a step disposed adjacent the outer edge of the bimetallic element such that a clearance is defined therebetween when the bimetallic element is in its closed mode, the clearance being positioned and dimensioned such that, when the outer edge of the bimetallic element deforms prior to a snapping open thereof, the clearance isolates a deformation of the outer edge thereby keeping the switch closed until the snapping open of the bimetallic element.

Description:
FIELD OF THE INVENTION 
     The invention relates to a bimetallic switch having a bimetal disc as the switching element. 
     BACKGROUND OF THE INVENTION 
     Temperature controlled bimetallic switches are well known in the art. The bimetal supplies the force to open or close the contact system of the switch. These switches are used to prevent overheating or overcurrent conditions in a great number of electrical appliances, both large and small, such as household appliances, automobile components, and office automation equipment. 
     Various shapes of bimetallic elements are available, such as discs or cantilever strips. The thermal deflection of a strip element is usually easier to predict because formulas to predict deflection due to temperature are readily available. On the other hand, snap-acting bimetallic discs are also useful. These tend to change from a concave shape to a convex shape in response to changes in temperature. 
     There are three classifications to the type of bimetallic switches available in the art. These are: automatic reset, manual reset and one shot. Automatic resets have two distinct temperature points. A normally closed switch will open when exposed to heat at the higher of the two temperature points. When the switch cools, it will then automatically close at the lower temperature threshold. Manual reset switches, on the other hand, are typically closed, and have only an open threshold temperature. They require an outside force (such as, for example, a push button), to reset the bimetal disc after it has been opened. One shots will snap open only once and will not reset under normal conditions. They are actually automatic devices, except that the disc has been formed to have a low temperature point below −100° Celsius. While one shots and manuals are typically used to protect a process, automatics are used on the other hand to control a process. There is, in addition, a hybrid version of an automatic switch, which is called a power down reset. The device includes a heat source which is activated when the contacts are opened because of increased temperature. This heat source produces enough heat to keep the bimetal above the low reset temperature threshold. In order for the device to reset, power to the switch must be removed and the temperature be low enough for the device to reset. 
     Bimetallic discs are said to “snap” when they change state from a crown up or dome shape to a crown down or dish shape when exposed to increased temperature. The time that the disc takes to snap is related, among other things, to the temperature change that it is exposed to. 
     One problem associated with the use of bimetallic switches is creep. The snapping over of the disc does not occur evenly across the snap time. There is in fact a transition state where the disc moves very slowly when compared to the total action time. This slow rate of movement is called “creep”. Thus, when a formed snap acting bimetallic element gets close to its snap temperature (i.e. the threshold temperature at which the bimetallic element snaps in order to open or close the switch), it begins to deflect slightly. Depending on how the contact system is arranged, this small deflection or creep may lead to a contact gap prior to a snapping over of the bimetallic element. It is to be noted that the size of the arc is additionally related to electrical load and voltage as well as to other environmental factors. As a result, some thermostats may not arc because of the loads they switch. 
     Among examples of bimetallic switches of the prior art which may exhibit the problem of creep are those described in U.S. Pat. Nos. 4,862,133, 4,517,541, 4,424,506, 3,577,111, 3,067,306, 2,724,753 and 2,340,056. 
     Various designs for bimetallic switches have been proposed to resolve the problem of creep. 
     Japanese Patent Number 63-292539 discloses a bimetallic switch designed to prevent malfunction and vibrations during normal service. Here, as seen in the figures of that patent, the bimetallic disc  4  is supported by support pieces  7  having supporting surfaces  6 , which are formed alongside the locus of the ends  8  of disc  4  under the service temperature thereof. Even though the disc deforms within the temperature range of its service temperature, the convex side of the central part  5  of bimetal  4  does not push the base  1  or movable contact piece  3 , or separate therefrom, to a great extent. In this way, malfunction due to creep, and vibrations, may be prevented. 
     U.S. Pat. No. 5,121,095 uses an elaborate spring member independent of the contact arms to remove creep. The switch contacts are actuated via the bimetal to a spring member, to an insulated pin, and to a contact arm system. 
     Other current designs remove creep through an expensive measurement and custom part assembly process which involves the installation of a pin measured to very small tolerances, such as 0.001″, in each thermostat assembly. In such a case, a manufacturer must stock numerous pins to allow for the tolerance stack of the final assembly. 
     The above devices, which address the problem of creep, nevertheless do so at the cost of having to provide complicated and difficult to manufacture designs for bimetallic switches. Attention must be paid to the costs of manufacture including both labor and material costs. 
     It is therefore an object of the invention to provide a simple and cost effective bimetallic switch which allows for a creepless contact system. It is a further object to provide a bimetallic switch having a small footprint. It is a further object of the invention to provide a bimetallic switch where the disc is placed vertically, thus allowing for a relatively larger sized disc. 
     SUMMARY OF THE INVENTION 
     The above objects, and others to become apparent as the description progresses, is achieved by the provision of a bimetallic switch comprising a bimetallic element having an outer edge and being adapted to snap between a first mode and a second mode; a movable contact disposed on the bimetallic element; a first terminal in electrically conductively coupled to the movable contact; a fixed contact disposed adjacent the movable contact such that, when the switch is in a closed position, the fixed contact and the movable contact are in engagement with one another, and when the switch is in an open position, the fixed contact and the movable contact define an open contact gap therebetween; a second terminal electrically conductively coupled to the fixed contact; a step disposed adjacent the outer edge of the bimetallic element such that a clearance is defined therebetween when the bimetallic element is in its first mode, the clearance being positioned and dimensioned such that, when the outer edge of the bimetallic element deforms prior to a snapping thereof out of its first mode, the clearance isolates a deformation of the outer edge until the snapping of the bimetallic element. Advantageously, the bimetallic element is a bimetallic disc. 
     According to one embodiment of the invention, the switch further comprises a housing having inner walls defining an enclosure therein, wherein: the bimetallic element, the movable contact and the fixed contact are disposed within the enclosure; and the step is integral with the inner walls of the housing. 
     According to another embodiment, the bimetallic switch further comprises a cantilever, movable arm supporting the bimetallic element at one end thereof; and a stationary arm disposed adjacent the cantilever arm and supporting the fixed contact thereon. 
     Optionally, the bimetallic element is oriented such that a snapping direction thereof is perpendicular to a direction of current flow into and out of at least one of the first terminal and the second terminal. Preferably, the first terminal and the second terminal are substantially planar, and the bimetallic element is a bimetallic disc oriented such that a plane including its outer edge is orthogonal relative to a plane of the first terminal and the second terminal. According to a further embodiment, the step is configured such that, when the outer edge of the bimetallic element has deformed just prior to a snapping thereof, the outer edge rests upon an engaging surface of the step substantially without a clearance therebetween. 
     Preferably, the housing is parallelepiped shaped, the bimetallic switch further including a cover for closing the enclosure. 
     According to another embodiment, where the bimetallic switch is a disc, the step is dimensioned and shaped to extend in a plane that is substantially parallel to a plane including the outer edge of the bimetallic disc and which intersects the bimetallic disc at a point within a predetermined range of 65 to 95 percent of a radius thereof when the bimetallic disc is in its closed mode. 
     According to one embodiment, the first mode of the bimetallic element corresponds to a closed position of the switch, and the second mode of the bimetallic element corresponds to an open position of the switch. In such a case, the movable contact may be positioned with respect to the fixed contact for maintaining a minimum open contact gap therebetween thereby isolating creep during contact closure. Alternatively, the first mode may correspond to an open position of the switch, and the second mode to a closed position of the switch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded, perspective top view of an embodiment of a bimetallic switch according to the invention. 
     FIG. 2 is an exploded perspective bottom view of the bimetallic switch of FIG.  1 . 
     FIG. 3 a  is a side-elevational view of the switch of FIG. 1 along direction S with the switch lid removed, showing the switch in a closed position. 
     FIG. 3 b  is an enlarged view of portion A in FIG. 3 a.    
     FIG. 4 a  is a view similar to FIG. 3 a , showing the switch in an open position. 
     FIG. 4 b  is an enlarged view of portion B in FIG. 4 a.   
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, as seen in FIGS. 1 and 2, an exemplary embodiment of a bimetallic switch according to the invention includes a housing or base  1  made of an electrically non-conductive material, for example of steatite or alumina, which houses therein a stationary arm  3  and a movable arm  5  secured to housing  1  by way of rivets  7  which additionally secure substantially planar terminals  9  to arms  3  and  5 . Rivets  7  are preferably made of steel. Arms  3  and  5  and terminals  9  are also, as would be well recognized by a person skilled in the art, made of an electrically conductive material. Stationary arm  3  includes thereon a fixed contact  11 , which interacts with a movable counter-contact  13  disposed on a bimetallic disc  15  affixed to the movable arm  5 . The contacts  11  and  13  may be made of a silver alloy on a copper alloy base, and the bimetallic element is made of a bimetal material dictated by temperature. A cover or lid plate  17  made, for example, from steel, aluminum or brass, is provided for enclosing the movable arm, stationary arm and contacts within housing  1 , the cover being secured to the housing via conventional means, such as, for example, a snap-fit mechanism as shown in the figures. Movable arm  5  is supported in a cantilever manner above stationary arm  3  for supporting the bimetallic disc  15  and contact  13  thereon. The stationary arm and the movable arm are preferably made of a copper alloy. The stationary arm may be about 0.024″ thick, and the movable arm about 0.006″ thick. Contacts  11  and  13  may be made, by way of example, from brass with optional plating, and may be about 0.020″ thick, as is well known in the art. The switch is preferably rated (i.e., by the National Electrical Manufacturer&#39;s Association (NEMA)) for 120/240 Volts AC, 15 Amps (at 120 Volts), 10 Amps (at 240 Volts), with a temperature range of 32 to 450 degrees Fahrenheit (depending on the bimetallic element selected). 
     As can be appreciated in FIGS. 3 a  and  4   a  in particular, the bimetallic switch as shown provides an assembly of the individual components shown in an exploded view in FIGS. 1 and 2 such that a snapping direction of the bimetallic disc (that is, the direction in which contact  13  moves in relation to contact  11  in order to open or close the switch) is orthogonal to a direction of current flow between the terminals  9  and the arms  3  and  5 . Advantageously, the bimetallic disc may be oriented in this manner such that a plane including therein the disc&#39;s outer edge  19  (FIG. 3 b ) is orthogonal relative to the plane of the terminals  9 . This orientation advantageously enables the disc  15  to be disposed in a vertical plane in typical installations. 
     As seen in FIG. 3 a , the switch as seen in a closed position involves the contacting engagement of contacts  11  and  13  with one another for establishing a current from one terminal  9  to the other as can be appreciated by one skilled in the art. As shown in FIG. 3 b , the bimetallic disc, in its first mode, corresponds to a closed position of the switch (which, in this embodiment, is a rest mode of the switch), and has a concave shape, its outer edge  19  being directed away from contact  11  and its central dish-shaped portion flexing in a direction toward contact  11  to keep contact  13  in engagement with contact  11 . As further seen in FIG. 3 b , a step  21  is provided integrally formed with, or secured, to inner walls of housing  1 , step  21  being dimensioned with respect to outer edge  19  of bimetallic disc  15  such that a clearance C is defined between edge  19  and an engaging surface  23  of step  21  when the bimetallic element is in its first mode, which, in the shown embodiment, corresponds to a closed position of the switch. It is to be noted that, while the shown embodiment involves a switch where the first mode of the bimetallic disc corresponds to a closed position of the switch (i.e., normally closed in its rest or ambient mode), the invention includes within its scope a switch where the bimetallic element is disposed in a mode that corresponds to an open position of the switch (normally open in its rest or ambient mode), as can be appreciated by a person skilled in the art. Regardless of whether the rest or ambient mode corresponds to an open position or closed position of the switch, however, the step is preferably disposed on the contact side of the bimetallic disc to effectively isolate creep during opening of the contacts. 
     Clearance C has a dimension with is pre-selected in conformity with a curvature of disc  15 , and which is further a function of, primarily, the readily observable creeping characteristics of the disc being used between the selected threshold temperatures. What is meant by “step” in the context of the invention is any volume of material of any suitable shape which provides the desired clearance with respect to the outer edge of the bimetallic disc such that a creep thereof is isolated, as will be explained in detail further below. As shown, the “step” may be sized and shaped to extend in a plane which is substantially parallel to the plane of edge  19 , and which intersects the disc at a point within a predetermined range of 65 to 95 percent of the disc radius when the surface of the disc closest to the step is in its convex position as shown in FIG. 3 b . The above applies whether or not the switch is normally open or normally closed, that is, whether it is open or closed in its rest mode or ambient temperature. In FIG. 3 b , this plane is depicted by broken lines indicated by the letter P. Referring now to FIG. 4 a , the switch as seen in an open position shows contacts  11  and  13  as having been separated from one another for breaking the flow of current from one terminal  9  to the other. In an open position, the bimetallic disc has a convex shape, its outer edge  19  being directed toward contact  11  and its central dish-shaped portion flexing in a direction away from contact  11  to keep contact  13  separated from contact  11 . 
     As seen further in FIG. 4 b , in an open position of the bimetallic switch, the bimetallic disc has its outer edge  19  resting upon upper surface  23  of step  21 , such that there is substantially no clearance therebetween. 
     It is to be noted that, while the above example depicted in the appended drawings relates to an automatic reset switch, the invention is meant to include within its ambit switch constructions which involve other types of bimetallic switches, such as manual resets and one shots as described above. In addition, in the instant disclosure, the words “bimetallic” and “bimetal” encompass constructions where the snap action element is made of two or more types of metal for being thermally responsive to effect the desired switch over. Thus, trimetallic switches, for example, are also included within the scope of the invention. 
     In operation, the switch in its rest or ambient temperature mode is in its closed position (the bimetallic disc being in its first or closed mode) as seen in FIGS. 3 a  and  3   b  between two threshold temperatures, a high temperature or “hot snap” temperature, and a low temperature or “cold snap” temperature. As an example, these two temperatures can be, for example, 145° Celsius and 90° Celsius, respectively. When the temperature rises to close to 145° Celsius, the deformation of the bimetallic disc progresses gradually until the amount of deformation has reached the threshold value. At that time, the deformation progresses quickly. The step at which the deformation progresses gradually during the initial stage is called creep. The process at which the deformation progresses rapidly is called snap action. During the creeping stage, in the shown embodiment, outer edge  19  of disc  15  begins to deform first, tending to move in the direction of contact  11 . Without the provision of clearance C with respect to surface  23  of step  21 , a gradual deformation of outer edge  19  could cause the electrical contacts to open slightly, potentially causing arcing, and thus significantly affecting the life of the contacts. However, clearance C allows the slow deformation to take place while still allowing the contacts to remain in engagement. Since the changes in the shape of the disc occur at edge  19  first, the clearance isolates the creep area and allows only the pure snap region of the disc to be used. After the threshold temperature of 145° Celsius has been reached, this pure snap region (that is, the region which does not creep but snaps at the threshold temperature) then snaps and breaks the current between contacts  11  and  13 , thereby opening the switch (and placing the bimetallic disc in its open mode) as shown in FIGS. 4 a  and  4   b . In this mode, disc  15  is dome shaped, and its outer edge  19  rests with substantially no clearance on surface  23  of step  21 . 
     Conversely, for establishing a contact closure, the temperature must drop to below 90° Celsius. When the temperature is close to the cold snap temperature, outer edge  19  of disc  15  begins to slowly deform, that is, to exhibit creep. Any effects of such creep are removed by maintaining a minimum open contact gap O (see FIG. 4 b ) between contacts  11  and  13  in their open position so that any movement of the contacts towards one another due to creep is harmless, i.e., fails to move the contact close enough to generate any arcing. The dimension of gap O is, similar to that of clearance C, is pre-selected in conformity with a curvature of disc  15 , and is further a function of, primarily, the readily observable creeping characteristics of the disc being used between the selected threshold temperatures and at the selected voltage and amperage rating of the contacts  11  and  13 . Advantageously, clearance C has dimensions from about 0.015″ to 0.020″, (preferably about 0.020″) and the contact gap O has dimensions from about 0.010″ to 0.015″ (preferably 0.015″). These ranges are effective for substantially all of the various discs within the stated preferred temperature range of 32 to 450 degrees Fahrenheit. It is to be noted that dimensional tolerances of the individual components of the shown switch may be as high as 0.004″ or more. As can be appreciated from the above construction, and, especially, from the provision of clearance C, the construction of the switch according to the invention allows the use of bimetallic switches of a simple and cost effective construction where the effects of creep are neutralized, thereby providing a switch of simplified construction which has a longer contact life. Additionally, the provision of gap O adds to the above advantage by removing the effects of creep during contact closure, thus providing predictability of switch behavior in the vicinity of the cold snap temperature. 
     As can be readily appreciated from FIGS. 3 a ,  3   b ,  4   a  and  4   b , in this preferred embodiment of the invention, disc  15  is oriented such that a snapping direction thereof is perpendicular to a direction of current flow between the terminals  9  on the one hand and the arms  3  and  5  on the other hand, that is, the direction of current flow into and out of either of the terminals  9  as applicable. Advantageously, the bimetallic disc may be oriented in this manner such that a plane including therein the disc&#39;s outer edge  19  is orthogonal relative to the plane of the terminals  9 . This orientation advantageously enables the disc  15  to be disposed in a vertical plane in typical installations. The above allows for a smaller footprint for installation while preserving the ability to maintain large contact hold forces through the use of larger discs. A smaller horizontally placed disc may warrant the need for a moment arm system to open and close the contacts. The orientation of the disc according to this embodiment of the invention thereby advantageously provides a space-saving, simplified construction which is additionally effective for creating the desired large contact hold forces of the switch. It will be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than the preferred forms specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true spirit and scope of the invention.