Abstract:
An adjustable bimetal snap disc thermostat is disclosed which provides conventional resistance-type heaters symmetrically positioned adjacent one side of the snap disc to allow adjustment of the operating temperature of the thermostat. The heaters are supported in accurate, very close proximity to the bimetal snap disc to ensure excellent heat transfer thereto. Additionally, the volume of the chamber within which the heaters are supported is reduced by a bridge portion which also serves to further reinforce the outer housing walls. Additionally, guide surfaces for the heater terminals are provided to facilitate assembly.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates generally to bimetal snap disc thermostats and more particularly to an improved bimetal snap disc thermostat in which resistance heaters are employed to depress the ambient temperatures at which such thermostats are actuated. 
     Bimetal snap disc thermostats which provide an electrical resistance heater controlled by an external control circuit to change the operating temperature of the thermostat are known. An example of such a thermostat is illustrated and described in U.S. Pat. No. 3,248,501 (assigned to the assignee of the present invention), which patent is incorporated herein by reference. Such device includes an annular disc-shaped heater of special construction which is positioned adjacent to the snap disc. Because the disc heater is not a standard available heating device of general utility, it is relatively expensive to produce. Further, when the disc heater is installed in the thermostat, it must be subsequently connected to the terminals. Consequently, the thermostat in accordance with such patent is relatively expensive to produce and is also relatively expensive to assemble. 
     It is also known to provide strip heaters in combination with blade-type bimetal thermostats, as illustrated in U.S. Pat. No. 3,870,985. Here again, the heater is of a special construction, and therefore relatively expensive to produce. 
     More recently, however, bimetal snap disc thermostats have been developed which utilize conventional commercially available resistance type heaters. The use of such resistance type heaters is disclosed in U.S. Pat. No. 4,533,894 (assigned to the assignee of the present invention). 
     Additionally, U.S. Pat. No. 5,576,683 discloses a bimetal snap disc thermostat utilizing resistance heaters in which the resistance heaters are supported closely adjacent a bimetal snap disc by a separate thermal insulator member. In this thermostat, a thin sheet film member is required to ensure the resistance heaters are electrically insulated from the snap disc. While this thermostat provided greater temperature depression than the prior art thermostats, it required the manufacture and assembly of both the thermal insulator support as well as the sheet film insulator which resulted in increased costs. Further, the sheet film also tends to slightly thermally insulate the bimetal snap disc from the resistance heaters thus limiting the effective temperature depression that can be achieved. 
     It should also be noted that the size of air volume of the chamber within which the resistance heaters are located may adversely effect the efficiency of the resistance heaters in depressing the response temperature of the bimetal snap disc. 
     In one application, these bimetal snap action switches are utilized to control temperatures in clothes dryers. In such applications, it is increasingly desirable to provide such thermostats with the ability to offer greater and greater temperature depression capability in order to offer a wider range of drying temperatures. In previous efforts to accommodate this increased temperature depression, higher wattage resistance heaters have been required but in some cases, the increase wattage of the heaters has required the use of more costly ceramic housings as opposed to the less expensive phenolic switch cases. Because ceramic switch cases are significantly more fragile than the phenolic counterparts, it has been difficult, if not impossible, to manufacture such thermostats in an automated assembly line. This aspect also significantly increases the cost of such thermostats. 
     The bimetal snap disc thermostat of the present invention overcomes these disadvantages by providing a chamber to accommodate the resistors having a smaller volume and providing locating tabs to aid in more precisely positioning of the resistor heaters thereby enabling the elimination of the sheet film insulator between the heaters and the bimetal snap disc while still allowing the heaters to be positioned within close proximity to the bimetal snap disc. Also, the resistor heaters and their associated leads are entirely suspended in the chamber by the terminals to which the leads are secured thus providing an insulating air layer between the resistor heater body including its leads and the phenolic switch case. All of these modifications contribute to more efficient heat transfer from the heating resistors to the bimetal snap disc thus allowing greater temperature depression with lower wattage heating resistors while also enabling the use of phenolic switch case without exceeding its thermal limits. Additionally, the raised locating tabs serve an additional function of further strengthening the switch case thus reducing the possibility of damage thereto when the snap disc retainer is crimped into position. Additionally, the switch case includes guide surfaces operative to assist in assembly of the heating resistors and associated contacts during assembly. 
     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a section view of a bimetal snap disc thermostat in accordance with the present invention; 
     FIG. 2 is a bottom elevational view of the bimetal snap disc thermostat switch case shown in FIG. 1 with the heating resistors and associated contacts removed therefrom; 
     FIG. 3 is a section view of the switch case of FIG. 2, the section being taken along lines  3 — 3  thereof; and 
     FIG. 4 is a section view of the assembled bimetal snap disc thermostat of FIG. 1, the section being taken along line  4 — 4  thereof. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawing, wherein the showings are for purposes of illustrating a preferred embodiment of the invention only and not for purposes of limiting same, FIG. 1 shows a thermostat  10  having a dielectric plastic housing that includes a housing base  12  and a cover  14 . Switch terminals  16 ,  18  are attached to housing base  12  by rivets  20 ,  22 . A movable spring blade  24  carries a movable switch contact  26  and is attached to both switch terminal  16  and housing base  12  by rivet  20 . Switch terminal  18  carries a fixed switch contact  28 . 
     Housing base  12  has a central elongated sleeve  30  which is supported within housing base  12  by means of a partition  31  with an internal passage  32  receiving a reciprocating plunger  34  aligned with springblade  24 . An annular cavity  36  is defined between sleeve  30 , partition  31  and housing base peripheral wall  38 . 
     A metal disc cup  40  secured to housing base peripheral wall  38  supports a bimetal disc  42  that cooperates with reciprocating plunger  34  for opening and closing switch contacts  26 ,  28 . When a predetermined elevated temperature is reached, disc  40  snaps from the position shown in FIG. 1 to an oppositely bowed position and moves plunger  34  upwardly to bend springblade  24  and move contact  26  away from fixed contact  28 . When a predetermined lower temperature is reached, bimetal disc  40  will snap back to the bowed position shown in FIG.  1  and the spring force of switch arm  24  will move contact  26  back into engagement with contact  28 . 
     Thermostatic switches of the type described are commonly provided with internal heaters for depressing the temperatures at which the bimetal disc snaps between switch open and switch closed positions. By way of example, say that a given thermostat snaps to a switch open position at an externally sensed temperature of about 150° F. and snaps back to a switch closed position at an externally sensed temperature of about 130° F. An existing arrangement allows depression of these temperatures as much as about 30° F. by adding internal heaters to the thermostat for heating the bimetal disc. With the heaters energized, the disc will snap to a switch open position at an external temperature of about 120° F. and will snap back to a switch closed position at an external temperature of about 100° F. Temperature depression greater than about 30° F. is not possible because the internal heaters necessary to produce the required heat would also cause the thermal limits of the thermostat housing to be exceeded unless a special high temperature resistant switch case material was used such as for example a ceramic switch case in lieu of the preferred phenolic switch case. Additionally, the volume of the chamber in which the heating resistors are positioned in prior art devices of this type tends to reduce the potential temperature depression that can be achieved with a given size resistor as well as to delay the response time. 
     In the present invention, the volume of chamber  36  is reduced considerably by providing a diametrically extending bridge section  44  having a generally planar surface  44   a  integrally formed therewith, as best seen with reference to FIG.  2 . As shown, bridge section  44  is defined by a pair of chord sidewalls  46  and  48  extending along opposite sides of sleeve  30  and spaced radially outward therefrom. Bridge section  44  is integrally formed with partition  31  and serves to substantially reduce the volume of chamber  36  as compared to the volume of the heater chamber provided in prior bimetal snap action thermostats. 
     Referring now to FIGS. 1 and 4, a resistor heating assembly  50  is provided which comprises a pair of heating resistor elements  52  and  54  each of which includes a pair of leads  56 ,  58  extending outwardly from opposite ends thereof. Each of the leads  56  are connected to an electrical terminal  60  that extends upwardly from chamber  36  through slotted opening  62  provided in housing  12  and outwardly through a correspondingly aligned opening  64  in cover  14 . Similarly, each of leads  58  are connected to terminal  66  which extends upwardly from chamber  36  through slotted opening  68  provided in housing  12  and outwardly through correspondingly aligned opening  70  in cover  14 . As such, heating resistors  52  and  54  are electrically connected in parallel across terminals  60  and  66 . 
     Terminals  60  and  66  each include a tang  72 ,  74  projecting laterally outwardly therefrom which are designed to engage respective flat surfaces  76 ,  78  provided on housing  12  to limit movement thereof through slots  62  and  68 . Similarly, in order to retain terminals in assembled relationship as well as to aid in retaining cover member  14  in assembled relationship to housing  12 , the upper ends (as shown) of terminals  60  and  62  each include a staked projection  80 ,  82  which engages the outer surface of cover  14 . 
     As best seen with reference to FIG. 4, projections  72  and  74  are positioned with respect to leads  56  and  58  such that both leads  56  and  58  as well as resistors  52  and  54  are supported or suspended in spaced relationship to housing  12  and in close proximity to but spaced from bimetal disc  42 . Preferably, leads  56  and  58  will be secured to respective terminals  60  and  66  to form a heater subassembly which will then be assembled to housing base  12 . 
     Referring again to FIG. 2, the center portion of sidewalls  46  and  48  of bridge portion  44  are inclined or sloped radially outwardly toward slots  62  and  68 . Additionally, a pair of radially outer inclined wall portions  84 ,  86  and  88 ,  90  are provided adjacent opposite sides of flats  76  and  78  respectively which slope toward slots  62  and  68  and additional pairs of inclined surfaces  92 ,  94  and  96 ,  98  are provided sloping toward opposite ends of slots  62  and  68 . These inclined surfaces operate to guide respective terminals  60  and  66  into slots  62  and  68  during the assembly process. 
     Housing  12  also includes a plurality of four upstanding generally triangularly shaped post portions  100 ,  102 ,  104  and  106  which extend upwardly form the surface of bridge portion  44  and serve to reinforce the outer periphery  38  of housing  12  as well as to act as locating surfaces to assist in assuring accurate positioning of leads  56  and  58  and heating resistors  52  and  54  in spaced relationship to housing  12 . 
     Although many different materials may be used for the thermostat housing, in a preferred application, the thermostat housing will be of a phenolic plastic material having a thermal limit of about 350° F. Heaters  52 ,  54  may have lower heat output than previous arrangements. 
     Preferably resistance heaters  52  and  54  will be positioned within chamber  36  such that they are spaced from bimetal snap disc  42  a distance of about 0.062″ when snap disc is in an activated position (i.e., it has deformed such that the concave portion is facing toward metal cup  40 ). This assures excellent heat transfer to the bimetal snap disc while still assuring adequate spacing to avoid shorting of the resistor to the bimetal snap disc and avoids the need for an electrically insulating film therebetween which film will impede the rate of heat transfer. While the preferred spacing for heaters  52  and  54  is 0.062″ to maximize heat transfer efficiency, they may be positioned up to as much as 0.082″ with only a small reduction in the heating efficiency. It is also preferred that the heating resistor bodies and leads be spaced at least 0.003″ from the housing body. 
     It has been found that by reducing the volume of chamber  36  by the inclusion of bridge portion  44  and posts  100 ,  102 ,  104 ,  106  together with positioning the heating resistors in close proximity to the bimetal snap disc without incorporating an insulating film layer has enabled temperature depressions of at least 40° F. or greater to be achieved while utilizing lower wattage heating resistors than had been previously required to achieve such temperature depressions in thermostats using high temperature ceramic housings without exceeding the thermal limit of the phenolic housing. Additionally, the bridge section  44  in combination with posts  100 ,  102 ,  104 ,  106  provide greater reinforcement to peripheral wall  38  thus reducing the possibility of housing  12  being damaged during crimping of outer metal disc  40  thereto. Also, the provision of the inclined sidewalls adjacent slots  62  and  68  greatly facilitates assembly of the heater resistor terminal subassembly by serving as guides for the leading sides of terminals  60  and  62  thus facilitating automated assembly of the thermostat. 
     While it will be apparent that the preferred embodiment of the invention disclosed is well calculated to provide the advantages and features above stated, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the subjoined claims.