US2716172A - Thermal switch with expanding cycling control - Google Patents

Description

23, 1955 E. G. FRANKLIN 2,716,172

THERMAL SWITCH WITH EXPANDING CYCLING CONTROL Filed Sept. 12, 1952 2 Sheets-Sheet 1 :28 i I36 I05 94 I 45 INVENTOR EDMOND G. FRANKLIN B ATTORNEY Aug. 23, 1955 E. G. FRANKLIN 2,716,172

THERMAL SWITCH WITH EXPANDING CYCLING CONTROL Filed Sept. 12, 1952 2 Sheets-Sheet 2 INVENTOR EDMOND G. FRANKLIN BY W ATTORNEY THERMAL SWITCH WITH EXPANDING CYCLING CONTROL Edmond G. Franklin, Minneapolis, Minn assignor to General Mills, Inc, a corporation of Deiaware Application September 12, 1952, Serial No. 309,325

19 Claims. (Cl. 200-137) The present invention relates to thermal switches and more particularly to an improved mechanism for delaying the cycling of such a switch and improving the engagement and disengagement of the switch contacts.

In many thermal switches of the prior art, the response of the thermal portion of the switch to the temperatures being controlled is so rapid that the switch contacts barely have an opportunity to complete the circuit before they are again disengaged. As a result, such switches tend to cycle very rapidly and may therefore be unsatisfactory for use in applications where slower cycling is desirable. For example, in the home appliance field there are certain standards, such as those of the National Electrical Manufacturers Association, concerning the maximum number of permitted cycles per unit of time. Furthermore, such rapid cycling and indecisive engagement and disengagement of the contacts may result in chattering of the contacts at the make or the break and thus cause arcing with resultant wear and erosion of the contact surfaces.

Proposals have been made previously for the solution of such problems, but such prior proposals have generally involved the mounting of one of the switch contacts on an auxiliary strip of bimetal to be heated and deflected by the passage of current through the contact. These devices have the difiiculty, however, that if the bimetal is made sufiiciently small in cross section to obtain rapid heating and deflection, the auxiliary bimetal will be so thin that it will not offer firm or adequate support for the contact in many cases. Such relatively small cross sections of bimetal will also yield only small deflection forces, so that particularly on engagement of the contacts there will be little increase in the contact pressure to prevent chattering.

With these problems and constructions of the prior art in view, it is one object of the present invention to provide an improved cycling delay mechanism for a thermal switch.

Another object is the provision of a cycling delay mechanism which will act rapidly and effectively to increase the contact pressure when the contacts first engage and to increase the gap when the contacts first disengage. A further object is the provision of such a cycling delay mechanism in which no auxiliary bimetallic member is required.

Still another object is the provision of a cycling delay mechanism in which use is made of the rapid expansion and contraction of a restraining strip or wire of relatively small cross section.

A still further object is the provision of an improved switch construction in which one of the contact arms includes a contact supporting portion and a base portion, the contact support being resiliently biased to a given position with respect to the base portion and then being held out of said normally biased position by means of a restraining member which, when cold, holds the contact farther away from its opposing contact than its normally biased position.

2 ,716,172 Patented Aug. 23, 1955 Other objects and advantages will be apparent from the following specification in which certain embodiments of the invention have been described with particular reference to the accompanying drawings. In these drawmgs,

Figure 1 is a side elevation, partly in section, showing a preferred embodiment of the present invention, as applied to a fiatiron or other heating plate. In this figure, the parts are shown at the instant when the contacts first engage each other during cycling.

Fig. 2 is a schematic view, similar to Figure 1, showing the position of the parts immediately after the contacts have engaged.

Fig. 3 is a view similar to Figs. 1 and 2, but with the parts in the position wherein the contacts have just barely disengaged each other.

Fig. 4 is a schematic view similar to Fig. 3 showing the action of the switch contact arm according to the invention immediately after the instant of disengagement of the contacts.

Fig. 5 is an exploded partial perspective view in brackets showing details of construction of one of the contact arms and its restraining member.

Fig. 6 is a view similar to Fig. 1 showing another embodiment of the invention.

Fig. 7 is a partial perspective view in brackets similar to Fig. 5 showing the construction of the contact arm and restraining member of the device of Fig. 6.

Fig. 8 is a chart showing operation of a device similar to that of Fig. 1 but without the cycling delay mechanism of the present invention, and

Fig. 9 is a chart showing the cycling of the specific preferred embodiment of Figs. 1 to 5.

With reference to the construction shown in Figs. 1 through 5, which constitute a preferred embodiment of the present invention, the thermal switch is designated generally at 20. The switch includes a supporting bracket 22 having a horizontal top wall 24, spaced vertical side walls 26, and horizontal bottom flanges 23. These bottom flanges may be secured by screws or bolts 30 to appropriate portions 32 of the device to be controlled. Here this device is illustrated in the form of a fiatiron soleplate 34 having a cast-in heating element 36 connected in circuit to be controlled by the switch 20 in known manner.

In the switch itself, a vertical supporting sleeve 38 depends from the top wall 24 of the supporting bracket and is secured thereto by having its upper end riveted over onto the wall as indicated at 40. Extending horizontally within bracket 24 and supported by the post or sleeve 38 are the upper and lower contact arm assemblies 42 and 44, respectively. These contact arms are insulated from the post 38 and supporting bracket 22 by suitable insulating members and inserts such as shown at 46 and 48. Part of the base portion of each contact arm extends rearwardly to provide terminal connections as indicated at 50 and 52 respectively.

The lower contact arm assembly 44 includes a base or main body portion 54 and a longitudinally overlapping extension 56 which projects beyond the portion 54. A

contact 58 is carried at the outer end of extension 5'6.

To secure the two contact arm plate portions 54 and 56 together, the plate 56 is provided at its inner end with an upturned lug or projection 60 which projects through an opening 62 in the base portion 54 of the arm to prevent relative movement at this point. The two portions 54 and 56 are further connected to each other by an insulating member 64 and stud 66 which pass therethrough and provide a means for attachment of the thermally responsive actuating portion of the switch. Thus the two switch blade portions 54 and 56 are interconnected at two longitudinally spaced points to prevent relative slippage between these portions and permit them to operate in effect as a single contact arm.

While various types of thermally responsive means can be used to engage the insulator 64 and stud 66 to actuate the lower contact arm, the invention has been illustrated in connection with means for connecting stud 66 to a portion of the fiatiron remote from the point of support of the switch. Thus, the lower end of stud .66 passes through an opening 68 in a connection plate 70 held in position against the bottom of insulator 64 by a riveted end 72 of stud 66. Connection plate 70 carries a pin or stud 74 pivoted to one end 76 of a shaft or rod 78. The other end 80 of this shaft 78 is secured by bolt 82 to a remote portion 84 of the flatiron or other heated device. If a connecting member 78 is chosen having a lower coefficient of thermal expansion than that of the heating device 34, it will be apparent that the lower contact arm assembly 44 will be deflected downwardly in response to heating and expansion of the device 34 by heating element as.

Cooperating with the lower contact 58 is an upper contact 86 having an upwardly projecting stud or shaft 88 secured thereto and connecting the contact 86 to the upper contact arm assembly 42. This upper contact arm assembly is shown in part in Fig. 5 and includes primarily a contact arm 90 and a restraining and conducting member 92. In this embodiment of the invention, contact arm 90 has a base portion 94 provided with an opening 96 through which the insulator 46 and supporting post 38 of Fig. I extend to support the arm. The base portion of the contact arm also includes an intermediate portion 98 projecting outwardly from the supporting post, and an outer contact supporting portion 190.

The respective portions are resiliently biased to predetermined initial positions with respect to each other. For this purpose the contact arm is conveniently made of resilient or spring material capable of taking a certain set or stress. In the example illustrated in Figs. 1 to 5, the intermediate portion 98 is initially biased to a deflected position with respect to the base portion 94, this displacement or bias being in a direction tending to move the outer contact supporting portion 100 away from the lower contact arm. At the same time, the outer contact support 100 is initially deformed at 104 to a pre-biased or normal position displaced from the plane of intermediate portion 98 in a direction toward the remaining contact arm of the switch.

According to a further feature of the invention, the intermediate section 93 of the contact arm 90 is provided with strengthening means, such as the parallel upwardly extending flanges 106, to maintain this intermediate portion in a substantially rigid and straight condition during operation of the switch. The base portion of the contact arm includes an opening 108 for attachment of one end of the restraining member at a point spaced substantially inwardly from the contact support ltlti. Here this opening 108 is in the intermediate portion of the base portion. The contact support also includes an opening 110 through which the vertical stud 88 of contact 86 may extend to support such contact in assembled relation.

As shown also in Fig. 5, the restraining member 92 of the present invention includes an inner end 112 having an opening 1114 designed for alignment with the opening 108 in the contact arm 94). The outer end 116 of the restraining member 92 also includes an opening 118 adapted for alignment with the opening 110 of the contact support 100. The intermediate portion of restraining member 92 is cut away as shown at 119 through a substantial length and width of the device to provide spaced strips 120 at each side of member 92 extending over a substantial longitudinal distance between the openings 114 and 118. The size of opening 119 must be great enough so that the total cross section of the remaining metal in the side strips 120 is relatively small with reference to the current to be passed through the contacts 86 and 58, and with reference to the total cross section of. the

contact arm 90. Thus, as discussed below, the passage of current through the restraining member 92 will result in substantial heating and expansion of the side strips 119 and 120 to achieve the desired objectives of the present invention.

The upright stud 88 on upper contact 36 passes through the opening 110 in the contact support 100 and through the opening 118 in the outer end 116 of restraining strip 92 and is riveted over at 122 to hold the parts in assembled relation. An intermediate insulating member and spacer 124 also extends around the stud 88 at the point of passage through opening 110 to cooperate with insulating washer 126 and support the contact 86 in completely insulated relation with respect to the supporting portion 100 and the other portions of contact arm 90. The upper end of stud 88, however, is in direct engagement with the portion 116 of restraining member 92 and is thus electrically connected thereto. At the other end of the restraining member 92, the rivet 128 passes through opening 114 in the restraining member and through opening 108 in the contact arm and provides both a mechanical and electrical connection between the parts at this point.

The length of restraining member 92 between openings 114 and 118 is so chosen that when the parts are assembled as in Fig. 1 and the restraining member is in its cold or unexpanded position, i. e., when no current passes through it, the restraining member will be connected under longitudinal tension between the contact support portion and base portion and will hold the upper end of stud 83 and the contact supporting portion 100 in a position which is upwardly displaced from the normal resiliently biased position of the portion 100. In this instance, the eifect of the restraining member is to deflect the contact supporting portion 100 upwardly away from the lower contact 58 until the portion 100 lies in substantially the same plane as the intermediate portion 98 of the contact arm. Thus the contact portion 100 is in efiect hinged at 104 by virtue of the cut-away portion 165 and is normally resiliently biased to its first position as shown in Fig. 5, but held by the restraining member 92 in its second position as shown in Fig. 1. This second position is displaced from the first position in a direction away from the other or second contact.

To adjust the operating temperature maintained by switch 20, an adjusting shaft 130 is provided. This shaft has its lower end threaded at 132, and the threaded end engages within a threaded boss 134 in the top wall 24 of the supporting bracket 22. An insulating button 136 within control shaft 130 engages the rigid intermediate portion 98 of the upper switch contact arm assembly 42, and extends between the restraining member strips 120.

The position of this insulating button 1.36 within the control shaft 130 may be calibrated by an internal calibration screw 138. A collar 140 is threaded on the end of the control shaft 130 and carries a projecting step 142 for engagement with a lug 144 struck up from the wall 24 to limit the operating range of the device. Thus rotation of the control shaft 130 will cause relative vertical movement of the insulating button 136 to determine the normal operating position of the intermediate portion 93 of the upper contact arm. This portion of the contact arm is resiliently biased upwardly by virtue of the initial resilient stress or bend at 162 (Fig. 5) so that the contact arm will at all times be resiliently urged upwardly against the insulating button 136 to assume whatever adjusted position is determined by the position of control shaft 130. This adjustment is independent of any relative movement of contact support portion 100 between its first and second positions, in this preferred form of the invention.

With the above construction of the parts in mind, the p r t n of the device may now be described. Figure is the position just at the end of the cooling portion ofthe cycle before any substantial current has had an opportunity to flow through the contacts 86 and 58. The contact support portion is thus in its second position with reference to the base portion of the contact arm. Fig. 2 illustrates in heavy lines the condition of the parts at an instant after the position of Fig. l, i. e., after the initial passage of current has caused substantial heating an dexpansion of the restraining strip portions 120. This expansion has permitted relative downward rocking of the contact supporting portion 100 and its associated contact 86 from the dotted line position of Fig. 2 (which is the same as the second position, shown in Fig. l) to the heavy line position of Fig. 2 in response to the normal resilient bias of the contact portion 190 at the line of deflection 104. Thus the heavy line position of Fig. 2 shows the contact support portion in its first position, which is relatively displaced from the second position in a direction toward the second contact and will therefore increase the contact pressure and delay the subsequent disengagement of the contacts.

Passage of current through the switch and through the associated heating element 36 will then heat the flatiron or other device 34 and cause the latter to expand with respect to the member 78. When the relative difference in expansion between the device 34 and member 78 reaches a certain point, the resulting deflection of the stud 66 and downward movement of associated lower contact portions 54 and 56 will pull the lower contact 58 downwardly until the contacts just begin to disengage as illustrated in Fig. 3. Until this instant of disengagement there has been no interruption in the current through the restraining strip 92 and thus the upper contact arm assembly has remained in the heavy line or first position of Fig. 2.

Immediately after the contacts separate as shown in Fig. 3, however, the interruption of current will termimate the heating eitect 011 the restraining member 92 and will cause its longitudinally extending strips 120 to contract rapidly. This contraction will tend to restore the upper contact arm assembly from the dotted line position of Fig. 4 to the heavy line position of the figure, in which the upper contact arm is substantially in the second position of Fig. 1. Because of this upward movement of the upper contact 86, it is clear that the gap between the contacts will be increased and that the lower contact will then have to move upwardly a greater distance before the contacts can re'engage. Thus even though the current to heating element '36 is interrupted immediately, and even though the contact 58 starts reasonably promptly to move upwardly as the soleplate cools and contracts with reference to the connecting bar '78, there will be a substantial time interval before this cooling action is sufli'cient to permit reengagement of the contacts in the original position of Fig. 1.

In other words, the construction shown in Figs. 1 to 5 provides a means for increasing the contact pressure and thus delaying disengagement of the contacts (as shown in Fig. 2) as a result of current passage on initial engagement of the contacts. The construction also provides a means for increasing the contact gap and thus increasing the time required for reengagement of the contacts after the current is interrupted by initial disengagement of such contacts. As a result, the action of the switch on engagement and disengagement of the contacts is much sharper and cleaner than would otherwise be the case, and unnecessary wear and erosion of the contacts is eliminated. Furthermore, the period of cycling is lengthened to the desired extent and the tem- 6 perature of the device 34 will, in many cases, be much more uniformly controlled.

Figs. 6 and 7 illustrate another embodiment of the invention in which the upper contact arm assembly includes two separate contact arm parts instead of a single integral contact arm strip as in Figs. 1 to 5.

The switch support 146 is similar to that of the first embodiment and is secured at 148 to one portion of the flatiron soleplate 150 or other device to be controlled. The heating element 152 is similarly connected to heat the plate 150. A depending stud 154 on the supporting bracket 146 carries upper and lower or first and second contact arm assemblies 156 and 158, respectively. These contact arm assemblies in turn carry first and second upper and lower contacts 160 and 162. The lower contact arm assembly 158 is similar to that previously described and includes an outer leaf portion 164 and an inner base portion 166 which are interconnected with each other by means of a stud 168 which also supports a connection plate 170. Stud 172 connects the connection plate to one end of the bar 174-, While the other end of the latter is connected at 178 to a remote portion of the device 150 as previously described. Diiterential expansion between the soleplate 150 and member 174 will accordingly pull the contact 162 downwardly when the soleplate is heated and permit contact 162 to move upwardly when the soleplate is cooled.

The main portions of the upper contact arm assembly 156 are shown in Fig. 7. Here the assembly includes an inner contact arm base portion 18) provided with an opening 182 for assembly with the stud 154 and associated insulating members. This base portion includes an outwardly projecting intermediate portion 184 which is resiliently pre-biased upwardly from the plane of the base portion along a line of deformation at 186. At the outer end of this intermdiate portion 184 is a resilient hinge means for the outer contact support portion 190. This hinge means includes a depending flange 138 having an opening therein for connection to the contact support portion. The outer contact arm portion 1% has a similar depending flange 192 at its inner end provided with an opening 194 through which a rivet 196 passes to connect the flanges 188 and 192. Insulating members 198 completely surround the rivet 196 and separate the flanges 188 and 192, so that the outer contact support portion 190, which supports the upper contact 16%, is completely insulated electrically from the base portion of the contact arm including the intermediate portion 184. Since the contact 160 is supported by outer contact support portion in electrically insulated relation with respect to the inner contact arm parts, it is unnecessary to insulate the contact 160 specifically from the contact supporting arm portion 190, the necessary insulation in this respect being provided at 198.

The restraining member according to this embodiment of the invention is shown at 202 and involves a single strip of relatively small cross section, one end of the strip having an opening 204 for engagement with a stud on the upper contact 160 to connect the restraining member 202 electrically and mechanically with the upper contact 160. The inner end of this restraining member 202 has an opening 207 adapted to receive a rivet 208 which will also pass through an opening 210 in the intermediate portion 184 of the contact arm. Thus the restraining strip 202 will provide an electrical connection between the contact arm base portions which are connected in circuit with the heating element 152, and the outer contact support portion 196 with its contact 160.

According to the present invention, the dimensions of the parts are so chosen that the restraining strip 202, when cold, will be under longitudinal tension and will hold the outer contact supporting portion 190 of the contact arm assembly in a second position which is displaced upwardly, i. e., away from the second or lower contact 162, with respect to the normally biased first position of the portion 190 as shown in Fig. 7. For example, one or both of the depending flanges 188 and 192 is initially formed with such an angle that the outer contact supporting portion 190 will extend downwardly below the plane of the intermediate contact arm portion 184, in the absence of restraining member 202. As shown in Fig. 6, the restraining member 202 eifectively holds the outer contact supporting portion 190 up in a more horizontal plane so that it appears in Fig. 6 substantially as an extension of the plane of the intermediate portion 184. An insulating member 212 is located between the restraining member 202 and the remaining portions of the contact arm assembly to insure that the current to the contact 160 will all pass through the restraining and conducting member 202.

For adjustment of the device of Fig. 6, a control shaft 214 is provided just as in the previous case. This control shaft is threaded into a boss 216 on the support 146 and carries an insulating member 218 which engages the upwardly biased contact arm and holds it down to the desired position. In Fig. 6, the position of the insulating button 218 is such that the intermediate contact arm portion 134- appears substantially as a coplanar extension of the part of base portion 180 which is engaged by the supporting insulators, thus eflectively straightening out the bend which normally appears at 186. This pre-bias of the portion 134 thus serves to hold the upper contact arm assembly against the insulating button 218 at all times to establish the desired operating temperature for the switch.

Operation of the device of Figs. 6 and 7 will be essentially the same as that illustrated in Figs. 1 to 5, except that the relative displacement of the contact 160 upwardly and downwardly will be a swinging movement from the region of the flanges 188 and 192 and adjusting stop 218 rather than solely a bending of the contact supporting portion 190 itself.

While both the embodiments of Figs. 1 to 5 and of Figs. 6 and 7 have provided the expanding means for cycling delay in connection with the first or upper contact arm, it will be apparent that the combination of prebiased contact arm and restraining and conducting member of small cross section could, in many cases, be associated with the lower or second contact arm, i. e., the same contact arm which is actuated by the thermally responsive portion of the switch assembly provided the thermally responsive connection to the arm is at some point other than the relatively movable contact support portion.

As pointed out above, the actual total cross section of the restraining member must, in any case, be small enough to insure substantial heating by the particular current passing through it. Also, the length of the restraining member must be great enough to provide substantial total longitudinal expansion of the restraining member and consequent lateral displacement of the contact in response to such heating. In general, some of the benefits of the present invention, such as increased contact pressure and cleaner make and break of the circuit, can be achieved even where such relative displacements are small. Preferably, however. the relative contact displacement should be of the order of at least two to four mils when used on 60 cycle A. C. at 115 volts with currents of the order of 9 to 10 amperes. Higher A. C. voltages would require still greater relative displacement to insure positive elimination of arcing.

While the specific dimensions will thus depend on the current-carrying characteristics of the material, the current to be controlled, and the thermal expansion characteristics of the restraining member, the following example illustrates one set of operating conditions and switch dimensions which will achieve the desired result.

Example 1 In a switch constructed in the form shown in Figs. 1 through 5, the current to be passed through the switch contacts and restraining member was of the order of 10 amps. In this case the restraining member was made of a nickel chromium alloy commercially available as Nichrome #5, and comprising approximately Ni and 20% Cr. The effective length of the strips of limited cross section was 0.876 inch, the thickness of each strip being 0.012 inch and the width of each strip 0.042 inch. This restraining member was connected to the contact stud above the contact support portion and spaced from such portion by an insulator whose axial thickness was 0.25 inch. The distance along the contact arm from the vertical center line of this contact stud to the hinge line between the contact support portion and intermediate portion was also 0.25 inch.

With these dimensions and currents, the relative change in vertical displacement of the upper contact resulting from interruption or reestablishment of current was sufficient to achieve the objectives of slower and more even cycling according to the invention.

According to the foregoing description, a thermal switch has been described which substantially accomplishes the objects set forth at the beginning of this specification. The desired cycling delay and clean-cut engagement and disengagement of the contacts are secured by a device which requires no auxiliary bimetallic strip. The contacts are at all times supported by contact arm members of substantial cross section and strength, and the relative expansion and contraction of the restraining and conducting member of small cross section are used merely to control the deflection of the outer contact arm portions, without having to support the contact itself.

Figs. 8 and 9 illustrate the advantages of operation of the present device. Fig. 8, for example, shows a typical chart of temperature against time during cycling of a flatiron substantially similar to that of Figs. 1 to 5, but without the cycling delay mechanism of the present invention. Here it is apparent that the frequency of engagement and disengagement of the contacts is relatively high and that the peaks 222 of the temperature curve 220, as well as the valleys 224, are far from uniform in temperature range. In contrast, the temperature curve 226 of Fig. 9 (which is typical of the embodiment of Figs. 1 to 5) shows a substantially greater time interval between successive cycles and also shows relatively uniform temperatures for the peaks 228, on the one hand, and the valleys 230, on the other.

Not only does the construction of the present invention provide the desired cycling delay to eliminate chattering of the contacts, but the particular pre-bias and pivotal deflection of the contact supporting portion of the device, particularly in the preferred embodiment of Figs. 1 to 5, provides a rocking action at the make and break which is particularly helpful in securing firm initial engagement of the contacts and clean-cut disengagement thereof at the appropriate portions of the cycle.

Since minor variations and changes in the exact details of construction will be apparent to persons skilled in this field, it is intended that this invention shall cover all such changes and modifications as fall within the spirit and scope of the attached claims.

Now, therefore, I claim:

1. A thermal switch comprising first and second contact arms carrying first and second contacts respectively, the first contact arm having a base portion and a contact support portion, means movably mounting the contact support portion with respect to the base portion for relative movement between a first position toward the second contact and a second position displaced from the first position in a direction away from the second contact, the first contact being carried by said contact support, and a longitudinally expanding and contracting restraining member connected between said contact support and said base portion and controlling the relative movement of the contact support, said restraining member having a portion of substantial length and small cross-section connected in circuit for passage of current through said portion when the contacts are engaged, said cross section being small with respect to the current passed therethrough thereby causing substantial heating and longitudinal expansion of the restraining member in response to engagement of the contacts and substantial cooling and contraction of the restraining member in response to separation of the contacts, said expansion of the restraining member acting in a direction to effect movement of the contact support from second to first position and said contraction of the restraining member acting in a direction to effect movement of the contact support from first to second position.

2. A thermal switch according to claim 1 in which the first contact is supported in electrically insulated relation to said base portion and said restraining member constitutes the sole electrical connection between the first contact and base portion.

3. A thermal switch comprising first and second contact arms carrying first and second contacts respectively, the first contact arm having a contact support portion carrying said first contact and supported for limited movement toward and away from the second contact between a first position toward said second contact and a second position displaced from the first position in a direction away from the second contact, said contact support portion being resiliently biased toward its first position, and a restraining member connected under longitudinal tension between the contact support portion and another part of the switch and normally holding the contact support in said second position against said resilient bias, a portion of said restraining member being electrically connected in circuit with one of said contacts and having a substantial length and relatively small cross section such that passage of current through said restraining member portion on engagement of the contacts expands said portion substantially and thereby permits resiliently biased movement of the contact support and first contact toward said first position, increasing the contact pressure and delaying the subsequent disengagement of the contacts.

4. A thermal switch according to claim 3 having thermally responsive control means operatively connected to one of said arms and causing engagement and disengagement of the contacts in response to predetermined operating temperatures, and manual control means operatively connected to the other contact arm for adjustment of the contact arm position to determine the operating temperature of the switch, one of said control means engaging the first contact arm at a point other than the contact support portion, and said restraining member extending generally along the contact arm and having two longitudinally extending laterally spaced portions of small cross section, between which portions said one of said control means extends, both of said spaced portions being electrically connected in circuit for heating and expansion as described.

5. A thermal switch comprising first and second contact arms carrying first and second contacts respectively, the first contact arm having a base portion by which the arm is supported, a contact support carrying said first contact and resiliently hinged to said base portion for limited movement toward and away from the second contact between a resiliently biased first position toward said second contact and a mechanically restrained second position displaced from the first position in a direction away from the second contact, and a restraining member connected between the contact support and base portion and normally holding the contact support in said restrained second position, at least one portion of said restraining member being electrically connected in circuit with one of said contacts having a substantial length and relatively small cross section such that passage of current through said restraining member portion on engagement of the contacts expands said portion substantially and thereby permits resiliently biased movement of the contact support and first contact toward said first position, increasing the contact pressure and delaying the subsequent disengagement of the contacts.

6. A thermal switch according to claim 5 having thermally responsive control means operatively connected to one of said arms and causing engagement and disengagement of the contacts in response to predetermined operating temperatures, and manual control means operatively connected to the other contact arm for independent adjustment of the contact arm position to determine the operating temperature of the switch, one of said control means engaging the base portion of the first contact arm midway between the lateral edges of the arm, and said restraining member having two longitudinally extending, laterally spaced portions of small cross section, one at each side of the control means, both of said spaced portions being electrically connected for uniform heating and expansion to move the contact support portion evenly from second to first position.

7. A thermal switch according to claim 6 in which the thermally responsive control means is connected to the second control arm and the manual control means engages the first contact arm between said portions.

8. A thermal switch comprising first and second contact arms, first and second contacts carried respectively by said arms, and thermally responsive control means operatively connected to one of said arms and causing engagement and disengagement of said contacts in response to variations from a predetermined temperature to be maintained, the first contact arm having a base portion, a contactsupporting portion connected to the base portion and resiliently biased to a given position with respect to the base portion and supporting the first contact in insulated relation with respect to the base portion, and a restraining member electrically connecting the contact and base portion, said restraining member, when cold, normally holding said contact-supporting portion out of said resiliently biased position in a direction away from the second contact, and said restraining member having a relatively small cross section with reference to the total cross section of the arm and the current passing through the contacts, passage of such current on engagement of the contacts thus causing substantial heating and expansion of the restraining member and permitting movement of the contact-supporting portion back toward its biased position thereby increasing the contact pressure and delaying disengagement of the contacts, and interruption of the current on opening of the contacts causing cooling and contraction of the restraining member and moving the contact-supporting portion away from its biased position thereby increasing the gap between the contacts and delaying reengagement of the contacts.

9. A thermal switch according to claim 8 in which said first contact arm includes a relatively straight and rigid intermediate portion between the contact-supporting portion and base portion, and manual control means operatively engaging one contact arm and establishing the normal position of said arm to determine the temperature at which the contacts open and close, one of said control means engaging the rigid intermediate portion of th first contact arm.

10. A thermal switch according to claim 9 in which said intermediate portion is resiliently biased with respect to the base portion in a direction tending to move the intermediate portion and first contact as a unit away from the second contact, said one of said control means engaging the rigid intermediate portion on the side away from the contacts and thereby limiting the resiliently biased movement of the intermediate portion to control the operating position of the contact.

11. A contact arm subassembly for a thermal switch, comprising a resilient contact arm having a base portion adapted to engage a support, said base portion including an intermediate portion extending outwardly from the base portion beyond the support, a contact support portion at the outer end of the intermediate portion, means providing a resilient hinge between the contact support portion and intermediate portion for limited relative rocking movement of the contact support between first and second positions displaced from each other in a direction generally normal to the plane of the arm, said resilient hinge means normally biasing the contact support portion toward its first position, a contact mounted on that face of the contact support portion which is opposite the direction of movement of the contact support portion from first to second position and a restraining member located beyond the opposite face of the contact support portion, said restraining member extending generally along the contact arm and connected in longitudinal tension between the contact support portion and base portion, the restraining member (when cold) normally holding the contact support portion in its second position against the bias of said resilient hinge means, means connecting a portion of said restraining member electrically in circuit with said contact, and said restraining member portion having a substantial length and a small cross section with respect to the current to be passed through it, passage of such current thereby causing substantially rapid heating and expansion of the member for resiliently biased movement of the contact portion from second to first position.

12. A contact arm subassembly according to claim 11 in which the base portion, intermediate portion and contact support portion are formed of a single integral strip of resilient material.

13. A contact arm subassembly according to claim 12 in which said resilient hinge means includes cut-out portions in the contact arm along a transverse line between the contact support portion and intermediate portion thereby forming a line of weakness at which said resilient hinge action is localized.

14. A contact arm subassembly according to claim 11 in which the contact support and base portions are separate members, and said resilient hinge means includes an angularly disposed flange at the inner end of the contact support portion and a similar flange at the outer end of the intermediate portion, said flanges extending parallel and adjacent to each other, and means fastening said flanges together.

15. A contact arm subassembly according to claim 14 in which said fastening means includes means electrically insulating said portions from each other.

16. A contact arm subassembly according to claim 11 in which said intermediate portion includes longitudinally extending deformed portions preventing substantial flexing of said intermediate portion except along a transverse hinge axis between the base portion and intermediate portion.

17. A thermal switch comprising first and second contact arms carrying first and second contacts respectively, the first contact arm having a base portion and an auxiliary portion, means movably mounting the auxiliary portion with respect to the base portion for relative movement between a first position in which the first contact is relatively nearer the second contact and a second position in which the first contact is displaced from its first position in a direction away from the second contact, and

a longitudinally expanding and contracting restraining member connected between said auxiliary portion and said base portion and controlling the relative movement of the auxiliary portion, said restraining member having a portion of substantial length and small cross-section con nected in circuit for passage of current through said lastmentioned portion when the contacts are engaged, said cross section being small with respect to the current passed therethrough thereby causing substantial heating and longitudinal expansion of the restraining member in response to engagement of the contacts and substantial cooling and contraction of the restraining member in response to sep aration of the contacts, said expansion of the restraining member acting in a direction to effect movement of the auxiliary portion from second to first position and said contraction of the restraining member acting in a direction to effect movement of the auxiliary portion from first to second position.

18. A thermal switch comprising first and second con tact arms carrying first and second contacts respectively, the first contact arm having an extension portion sup ported for limited movement toward and away from the second contact arm between first and second positions, said portion being resiliently biased toward its first position, and a restraining member connected under longitudinal tension between the extension portion and another part of the switch and normally holding said portion against said resilient bias, a portion of said restraining member being electrically connected in circuit with one of said contacts and having a substantial length and relatively small cross section such that passage of current through said restraining member portion on engagement of the contacts expands said restraining member portion substantially and thereby permits resiliently biased movement of said extension portion toward said second position, increasing the contact pressure and delaying the subsequent disengagement of the contacts.

19. A contact arm subassembly for a thermal switch comprising a flxible switch arm carrying one contact of the switch and capable of deflection to move the contact back and forth in a direction generally normal to the plane of the arm, a contact positioning portion movably mounted on the switch arm for limited movement between first and second positions which are displaced from each other in said direction generally normal to the plane of the arm, means resiliently biasing said contact positioning portion from its second towards its first position, and a restraining member connected in longitudinal tension between the contact positioning portion and another portion of the switch arm, said restraining member (when cold) having a length normally holding the contact positioning portion in its second position against the urging of said resilient biasing means, means connecting a portion of said restraining member electrically in circuit with said one contact, and said restraining member portion having a substantial length and a small cross section with respect to the current to be passed through it, passage of such current thereby causing substantially rapid heating and expansion of the member for resiliently biased movement of the contact portion from second to first position.

No references cited.

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