US2881300A - Reverse bimetal thermostat - Google Patents

Description

April 7, 1959 I A. SCHWANEKE REVERSE BIMETAL THERMOSTAT Filed March 17, 1954 2 Sheets-sh ed 1 55 5 INVENTOR.

United States Patent "ice 2,881,300 REVERSE BIMETAL THERMOSTAT Alfred E. Schwaneke, Northbrook, Ill., assignor to A-M Corporation, Chicago, 111., a corporation of IlllllOiS Application March 17, 1954, Serial No. 416,884 4 Claims. (Cl. 219-25) The present invention relates to thermostats employing bimetal blades, and more particularly to thermostats which are subjected to variable settings in household appliances which have rapid warmup and recovery cycles.

By way of example, it has been found with conventionally equipped thermostatically controlled electric irons for household use that the initial warmup objectionably overshoots the set temperature regardless of whether the setting is a high or a low one. However, in the event that the setting is low, danger of scorching the fabric being ironed is a very great one if the houswife used the iron right away. In many instances, an iron set for 425 F. has heretofore reached a temperature as high as 600 in seven minutes on the initial warmup.

In order to overcome this, it has been proposed to use a construction in which a portion of the thermostat which controls the switch is reversed with respectto the other portion as held together by a buttweld or a crush weld. Such a construction is sometimes referred to as a reversed thermostat metal in which the two pieces of thermostat metal welded together have their high and low ex pansion sides reversed.

Although this expediency has met with some success in reducing the overshoot, another problem has been incurred with these thermostats. There is a lack of uniformity between thermostats so constructed which otherwise visually appear to be exactly alike. Even though sheets from which reversed bimetal blades are made are trimmed and rolled after welding, a great deal of care and accompanying cost is necessary in all the details of the welding operation to produce a predictable weld, yet the welds vary from sheet to sheet with respect to the operational characteristics of the blades produced. More- CV61, subsequent operations in finishing to the desired thickness require such attention that reverse metal therm'os'tats are not successfully made upon a production scale basis without 100% test inspection. Inspections,

tests, and other time-consuming endeavors require expensive labor and incur great waste in discards to obtain commercial thermostats which are in any way acceptably alike in their performance throughout the range of adjustments which will be required of them.

Not only this, but not all types of thermostat metals lend themselves to conventional reverse welds, and unless the two components of the bimetal strip have similar properties such as electrical resistivity, chemical compositions and melting points, the weld is not a strong one nor one that will be capable of sustained operations under the spring tensions of the switch elements involved in the working of a thermostat. In some cases the thermostat metal has to be given a preliminary treatment in order for the two otherwise dissimilar metals to provide any kind of a bond at the joint; and, although the rolling operation after the weld very often exposes the existence of defects so that rejection can be had before a further processing of the bimetal blades, yet other defects are not dis covered with the final testing stage after assembly of the complete thermostat.

Patented Apr. 7, 1959 It is oneof the objects of the present invention to pro vide a reverse thermostat metal blade which avoids the difliculties mentioned, yet is simple to make and is uni form in its operation to control or eliminate overshoot, an overshoot of not to exceed 10 to 20 F. being desirable in some appliances.

A further object of the invention is to provide a reverse thermostat metal characterized by a rapid on and 05" cycle between narrow constant limits at high temperature settings and with less frequent on and off cycling between narrow constant limits at low temperature settings.

The invention is further characterized by a reverse thermostat metal blade which maintains its characteristics throughout its expected life.

Another object of the invention is to utilize identical ness of metal structure at the welding joint of a reverse thermost metal without need of rerolling the bimetal after welding is accomplished. I

Another object of the invention is to provide for rapid heat conductivity between the two sections of a reverse thermostat metal blade.

The invention is further characterized by a reverse thermostat metal blade construction which is easy to make, whose tolerances are simple to control, and whose operation is uniform throughout long production runs.

These being among the objects, other and further ob jects will become apparent from the drawings and the description relating thereto.

In the drawings, Fig. 1 is iron in which a preferred bodied;

Fig. 2 is a top plan view of the iron shown in Fig. 1;:

, Fig. 3 is a perspective view of a thermostat partly in section employed in the iron shown in Fig. l, in which is embodied the preferred form of the bimetal element;

Fig. 4 is an enlarged side elevational view of the bi= metal blade employed in the thermostat shown in Fig. 3', the bimetal element being shown in its cool condition;

Fig. 5 is a view similar to that in Fig. 4, showing the relative deflection of the bimetal during the initial warmup stage;

Fig. 6 is a view similar to those in Figs. 4 and 5 showing the bimetal blade in the shape assumed by it after the warmup period has been accomplished;

Fig. 7 is a section taken upon line 77 in Fig. 6;

Fig. 8 is a section taken upon line 8 -8 of Fig. 7;

Fig. 9 is a section taken upon line 99 of Fig. 7; and

Fig. 10 is a performance chart of the thermostat illus trated in Figs. 3 to 9 inclusive.

Generally speaking, for a quick understanding of the invention, it can be said that the invention finds an embodiment in transversely cutting into two pieces a bimetal blade intermediate its ends; turning the one piece over to lay it on the other piece in partial overlapping relationship at the cut edges; and, welding the two pieces together at points spaced crosswise of the blade within the overlap area. The unwelded portions of the overlap serve as plane to plane contact areas held in face-to-face relationship under pressure contact to serve as orienting guide surfaces that preserve a pre= determined relationship between the blade pieces, before, during and after the weld is accomplished. Thus, uni formity is maintained and a wide area is provided for rapid heat exchange conductivity between the two ele' ments.

More particularly, an electric iron a side elevation of an electric form of the invention is ear 10 is shown in I Fig. 1, having a heater 11 embeddel in the sole plate 12 which is energized with house current through the con venience or extension cord 13 plugged into the socket 14 at the rear end of the iron. Electrical current through the heater 11 is controlled by a thermostat 14- that it connected in series therewith and is mounted in heat exchange contact with the sole plate 12 between the sides of the U-shaped heater 11K The control shaft 15 of the thermostat extends above the top of the housing 16 where itlreceives a handle 17 within finger reach below the main handle 18 of the iron.

.1 Referring to Fig. 3, the thermostat 14 comprises a mounting bracket 20 having ears'21 on the opposite sides thereof apertured as at 22 for mounting the thermostat within the iron housing 16. At one end of the bracket 20, a sleeve rivet 23 is received which carries on it insulating,washers 19 made of ceramic which space from each other in electrically insulated relationship an upper contact carrying spring 24 stressed to spring downwardly and apertured to receive therethrough a ceramic contact 25 carried on the bottom end of the adjustment shaft 15; asecond contact carrying spring 26 disposed below the first and stressed upwardly to ride against the ceramic pin125 on the lower end of the adjustment screw 15 in all positions of its adjustment. Below these two contact carrying springs is mounted the thermostat metal blade indicated generally at 27 which carries on its outer end a metal cup 28 receiving in supported relationship a headed ceramic pin 30 which engages the upper spring 24 beyond the contact engagement of the two springs to force the upper spring 24 upwardly when the bimetal member 27 is subjected to increasing heat. The two contacts will remain in electrical contact with each other except when either the adjustment shaft member or the bimetal blade member forces its respective contact beyond the point where the other contact can follow as controlled by the other member.

\ Referring to Fig. 4, the bimetal 27 is structurally made of two layers or laminations of metal bonded together at their interface of different coetficients of heat expansione One of the layers 31, and in this connection the numeral 31a indicates the same layer, constitutes the layer of low coeflicient of expansion and preferably comprises iron having 42% nickel. The other layer 32, and 32a, constitute the layer of high coefficient of expansion, and preferably is ironhaving the content of 17% nickel and 2% chrome. The blade 27 in its initial production com prises a single stamping, preferably having parallel sides an edge lap established as at 38 at their sheared ends with one of the layers having the same 'coelficient of expansion in both pieces in contact in face-to-face relationship. In the particular embodiment shown in Fig. 4, the layer placed in face-to-face contact is the one made of iron having a 42% nickel content to provide the low coeflicient of expansion.

1 With the two sections 36 and 37 so disposed, three spaced welds are made as indicated in Fig. 7 at 40 to hold the overlapping portions together at 38, leaving two areas of the contacting faces in face-to-face contact as at 41 under pressure exerted by the welds 40. These face-to face unwelded areas held together under pressure preserve and maintain the relative planar relationship betweenthe two thermostats in their resting stage as shown in Fig. 4. Moreover, this area 41 is of substantial expanse, sufliciently so to provide for a large amount of heat. transfer conductivity between the two elements 36 and 37 in addition to the intimate relationship established at the welds 40.

6 Moreover, the pressure contact between ends as at 41 is sufliciently rugged and constant that the load imposed on the thermostat at the button 30 is sustained and carried by the thermostat with great longevity andiwih uniform results, the two sections dishing as at 42 {Fig 6) with reversed curvature to give third di-' the overlapping mension strength and the welds 40 holding the two to low temperatures controlled gether and the faces 38 supporting them before, during and after the welding in an exact predictable and controlled relationship.

It is immaterial whether the section 37 is overlapped on top of or below the section 36, the same predeterminable standards can be maintained, it being preferred in the embodiment illustrated to have the iron sections containing 42% nickel adjacent each other. Moreover, because of the fact that the contacting layers are identical metals, any welding of conventional thermostatic metals can be accomplished easily and with the comparable re sults within the scope of the present invention, the co efficients of expansion being the only variable with bimetals of different metallurgical characteristics.

However, in the particular embodiment illustrated where the thermostat is intended to be used with an electric iron, the layer of high heat expansive metal indicated at 32 is preferably on the bottom and disposed in close, if not in intimate heat exchange contact with the sole plate of the iron. Then in operation, when the heat is first turned on, heat is conducted first into the section 27 of the bimetal as shown in Fig. 5 with the other section 37 remaining substantially straight. Thus, flexure of the section 36 will operate through the straightness of the section 37 to dispose the top of the ceramic button 30 well above the point at which it normally breaks contact between the two switch contacts. In other words, the straightness or heat lag of the element 37 will cause the contacts to be separated before the normal heat, to which the section 36 is exposed, has reached the section 36 to establish its operating flexure at any given setting of the screw 15.

After the first opening of the contacts by the button 30 when the thermostat is flexed as shown in Fig. 5, further heat present in the iron is conducted to the section 37 which causes it to bend in a reversed direction to the form represented'in Fig. 6. Thereafter, the relative fiexure of both elements or sections 36 and 37 cooperate in determining the location at which the button 30 will break the contacts of the switch. In this connection, it should be noted that the location lengthwise of the bimetal blade of the line of severance 35 controls overshoot or under- The longer the element 36 is, the greater the overshoot, whereas the shorter the element 36 is, relatively speaking, undershoot conditions can be attained. However, once the location of the line of severance is determined for performance and results, uniform performtion techniques.

mounting hole is each The warmup period is indicated at 50 to a setting of and provides a desired 10 overshoot at 51. Thereafter follows a make and break heat control which varies less than 20. Successive higher settings were then made with the results as shown. In each, the overshoot was 10 or less and the thermostat held within 20 range on each setting. Thereafter, the iron was cooled off and full range setting warm up Was instituted as at 53. There was no overshoot on the warm up and no undershoot when the iron was subjected to a load and a lower reset temperature.

It will be noted that the differential between high and by the thermostat is substantially constant at all levels, indicating that the thermostat normalizes itself upon the first ofl portion of the cycle following the new setting, and thereafter performs its function with a marked evenness of cycles. A further and unexpected result with the thermostat is found at the higher temperature settings where the differential between the high and low is about the same, the cycling With a blade wide.

of the thermostat is increased in frequency, maintaining a temperature in the sole plate which is substantially constant at all temperatures. Moreover, thermostat after thermostat produced as described duplicate the characteristics shown.

Having thus described the preferred embodiment of the invention where a slight overshoot of less than F. is desired and the structure which enables mass production within the objects stated, it will be readily apparent to those skilled in the art how various changes and modifications can be made structurally within the teachings and principles of the invention disclosed, the scope of which is commensurate with the appended claims.

What is claimed is:

1. In combination with an iron equipped with a sole plate having a heater embedded therein, a thermostat connected in series with said heater and comprising a mounting bracket fixedly secured to said iron, a pair of spring blades carried at one end thereof by said bracket and extending outwardly therefrom in facing relation and being equipped at their outer ends with contact points, said spring blades being biased toward each other for engaging said contact points to establish an electric connection therebetween, one of said spring blades being provided with an opening therethrough intermediate the ends thereof, an adjustable control member mounted upon said bracket and having a portion thereof extending through said opening and into engagement with the other of said spring contacts to provide positional selectivity for the location thereof, and a bi-metallic element secured at one end to said bracket and extending in facing relation with said spring blades and being equipped at its other end with a non-conductive pin engageable with the opening-equipped spring blade to move the same away from the other spring blade for separating said contact points, said bi-metallic element comprising a pair of metal strips secured together throughout the length thereof in laminated juxtaposition and having different cocfiicients of thermal expansion, said strips being severed transversely intermediate the ends thereof to define two bi-metallic segments oriented with the severed ends thereof one above the other in overlapping relation, one of said segments being inverted with respect to the other to position the identical metal laminations thereof in adjacency, and said overlapping ends being compressed at transversely spaced points thereacross and welded together at such points to unite said segments into the aforesaid bi-metallic element, the relative effective deflection lengths of said segments as established by the extent of the relatively non-defiective overlapping areas thereof being determinative of the degree of the overshoot and undershoot afforded by said bi-metallic element in its control operation and said segments having deflection lengths that are approximately equal, the segment of said bi-metallic element having the end thereof mounted upon said bracket being oriented so that the metal lamination having the greater coefficient of thermal expansion is disposed proximate to said sole plate and heater therein.

2. In combination with an iron equipped with a sole plate having a heater embedded therein, a thermostat connected in series with said heater and comprising a mounting bracket fixedly secured to said iron, a pair of spring blades carried at one end thereof by said bracket and extending outwardly therefrom in facing relation and being equipped at their outer ends with contact points, said spring blades being biased toward each other for engaging said contact points to establish an electric connection therebetween, one of said spring blades being provided with an opening therethrough intermediate the Cir ends thereof, an adjustable control member mounted upon said bracket and having a portion thereof extending through said opening and into engagement with the other of said spring contacts to provide positional selectivity for the location thereof, and a bi-metallic element secured at one end to said bracket and extending in facing relation with said spring blades and being equipped at its other end with a non-conductive pin engageable with the opening-equipped spring blade to move the same away from the other spring blade for separating said contact points, said adjustable control member being calibrated in temperature increments whereby said himetallic element functions to separate said contact points when the temperature of said sole plate approximates that for which the control member is set, said bi-metallic element comprises a pair of metal strips secured together throughout the length thereof in laminated juxtaposition and having different co-efficients of thermal expansion, said strips being severed transversely intermediate the ends thereof to define two bi-metallic segments oriented with the severed ends thereof one above the other in overlapping relation, one of said segments being inverted with respect to the other to position the identical metal laminations thereof in adjacency, and said overlapping ends being compressed at transversely spaced points thereacross and welded together at such points to unite said segments into the aforesaid bi-metallic element.

3. In a thermostat structure characterized by being initially adjustable to provide selective degrees of overshoot and undershoot in a control operation, a bi-metallic element comprising a pair of metal strips secured together throughout the length thereof in laminated juxtaposition and having different coeflicients of thermal expansion, said strips being severed transversely intermediate the ends thereof to define two bi-metallic segments oriented with the severed ends thereof one above the other in overlapping relation, one of said segments being inverted with respect to the other to position the identical metal laminations thereof in adjacency, and said overlapping ends being compressed at transversely spaced points thereacross and welded together at such points to unite said segments into the aforesaid bi-metallic element, the relative effective deflection lengths of said segments as established by the extent of the relatively non-deflective overlapping areas thereof being determinative of the degree of overshoot and undershoot aflorded by said bi-metallic element in its control operation and said segments having deflection lengths that are approximately equal.

4. The thermostat structure of claim 3 in which one end of said bi-metallic element is fixedly mounted and the other end thereof is equipped with a non-conductive pin, and in which the segment of said bi-metallic element having the mounted end thereof is oriented so that the metal lamination thereof having the greater coetficient of thermal expansion is disposed proximate to the source of heat to be controlled by the thermostat structure.

References Cited in the file of this patent

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