US3805207A

US3805207A - Thermoresponsive switch actuator

Info

Publication number: US3805207A
Application number: US00251407A
Authority: US
Inventors: C Grimshaw
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1972-05-08
Filing date: 1972-05-08
Publication date: 1974-04-16
Anticipated expiration: 1991-04-16

Abstract

A thermoresponsive switch actuator is disclosed comprising a bridge to which a switch contact may be mounted, and three mutually spaced legs extending from the bridge with one leg extending in spaced relation between the two other legs. Means are provided for mounting portions of the three legs distal the bridge with the one leg spring biased in a predetermined direction aginst the bridge, and with the two other legs spring biased against the bridge in a direction opposite the predetermined direction. Means also are provided for producing a temperature differential between the one leg and the two other legs to alter the magnitude of spring bias exerted by the three legs against the bridge and thereby cause the bridge to move.

Description

United States Patent 1191 Grimshaw Apr. 16, 1974 1 THERMORESPONSIVE SWITCH 1,215,665 2/1917 Landis ..337/141x ACTUATOR FOREIGN PATENTS 0R APPLICATIONS [751 Charles GrimshaWFulton, 677,161 8/1952 Great Britain 337/391 [73] Assignee: General Electric Company, Fort Wayne, Ind. Primary Examiner-A. T. Grimley 221 Filed: May 8, 1972 21 Appl. No.: 251,407 [57] ABSTRACT A thermoresponsive switch actuator is disclosed comprising a bridge to which a switch contact may be mounted, and three mutually spaced legs extending from the bridge with one leg extending in spaced relation between the two other legs. Means are provided I for mounting portions of the three legs distal the bridge with the one leg spring biased in a predetermined direction aginst the bridge, and with the two other legs spring biased against the bridge in a direction opposite the predetermined direction. Means also are provided for producing a temperature differential between the one leg and the two other legs to alter the magnitude of spring bias exerted by the three legs against the bridge and thereby cause the bridge to 4 Claims, 19 Drawing Figures [52] US. Cl 337/89, 337/135, 337/141, 337/345, 337/391 [51] Int. Cl. H0lh 61/00 [58] Field of Search 337/14, 89, 123, 131, 135, 337/141, 343, 365, 382, 139, 345, 391; 310/4 R, 4 A; 60/23; 318/334, 339, 471

[56] References Cited UNITED STATES PATENTS 2,412,483 12/1946 Warrington 337/139 2,429,784 10/1947 Whitted et al.... 337/345 3,497,853 2/1970 Beck 337/382 3,153,125 10/1964 Strauss et al 337/135 X move 3,090,851 5/1963 Strauss et al 337/135 3,123,695 3/1964 Strubberg 337/141 X T IIHIHHHIHI1.\ 1 lll l V i PATENTEDAPR 16 1974 SHEEI 1 BF 4 w FEGA 675? PATENTEUAPR 1619M 3.805207 SHEEY 2 OF 4 TI-IERMORESPONSIVE SWITCH ACTUATOR BACKGROUND OF THE INVENTION This invention relates to thermoresponsive switch actuators.

Most thermoresponsive switches include an actuating member which moves in response to changes in sensed temperature. This movement by the actuator operates a switch by imparting relative movement between two switch contacts. Heretofore, such actuators have generally taken the form of a pendant blade composed of two thin layers of metals having dissimilar coefficients of thermal expansion. Upon the occurrence of a change in actuator temperature one of the layers tends to expand or contract more rapidly than the other. This expansion differential causes the actuator, which is termed bimetallic in the art, to bend. By mounting one end while leaving the other end free to move a torque is developed on the distal free end which torque is used to effect movement of an adjacent switch contact.

Bimetallic actuators of the type just described have enjoyed wide usage in thermoresponsive switches of many types. They, however, have been less than ideal in several aspects. Perhaps the foremost disadvantage has been their functional response to changes in ambient temperature. For example, where switching is desired in response to flow of electric current above or below a predetermined level, and such is to be accomplished by means of passing current through a resistance heating element in thermal contact with the bimetallic switch actuator, a change in ambient temperature will alter the current level at which switch actuation occurs. Indeed, very substantial changes in ambient temperature may actually cause the bimetallic actuator to effect a switching operation without any current flow. Thus, ambient temperature compensating means have often'been required where bimetallic actuators are employed.

Another disadvantage associated with the use of bimetallic actuators has been that of their slow response time. This feature has been occasioned by the layered configuration of such actuators wherein the temperature of both the active layer, that is the layer of higher coefficient of thermal expansion, as well as the inactive layer, or that of relatively lower coefficient of thermal expansion, are altered in effecting actuation. By having to heat or cool both layers due to their high mutual thermal conductivity, more time is required in effecting a given change in actuator temperature from a given heat source than were only the temperature of the active element to be so changed.

Other disadvantageous features of bimetallic switch actuators have included those associated with fabrication, and with inherent characteristics of the metals employed such as rapidity of work hardening, spring back characteristics, and potential energy storage in hinge areas.

Accordingly, 'it is a general object of the present invention to provide a thermoresponsive switch actuator.

More specifically, it is an object of the present invention to provide a thermoresponsive switch actuator which does not require inclusion of bimetallic elements.

Another object of the invention is to provide a thermoresponsive switch actuator which is relatively insensitive to changes in ambient temperature.

Another object of the invention is to provide a thermoresponsive switch actuator having rapid response time.

Yet another object of the invention is to provide a thermoresponsive switch actuator of relatively simple and inexpensive construction.

SUMMARY OF THE INVENTION In one form of the invention a thermoresponsive switch actuator is provided comprising a bridge and at least three mutually spaced legs extending from the bridge with one leg extending in spaced relation between the two other legs. Means are provided for mounting portions of the three legs distal the bridge with the one leg spring biased in a predetermined direction against the bridge, and with the two other legs spring biased against the bridge in a direction opposite the predetermined direction. Means also are provided for producing a temperature differential between the one leg and the two other legs to alter the magnitude of spring bias exerted by the three legs against the bridge and thereby cause the bridge to move.

In another form of the invention a thermoresponsive switch actuator is provided comprising a sheet of resilient metal forming a bridge and three mutually spaced legs extending from the bridge with one of the legs extending in spaced relation between the two other legs. Means are provided for mounting portions of the two other legs distal the bridge along a plane and for mounting a portion of the one leg distal the bridge off the plane. Means also are provided for producing a temperature differential between the one leg and the two other legs thereby causing the bridge to move.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a thermoresponsive switch actuator incorporating principles of the present invention;

FIG. 2 is a side view in elevation of the thermoresponsive switch actuator shown in FIG. 1;

FIG. 3 is a plan view of another thermoresponsive switch actuator incorporating principles of the invention;

FIG. 4 is a side view in elevation of the thermoresponsive switch actuator shown in FIG. 3;

FIG. 5 is a plan view of another embodiment of the present invention;

FIG. 6 is a side view in elevation of the thermoresponsive switch actuator shown in FIG. 5;

FIG. 7 is a bottom view of yet another embodiment of the present invention;

FIG: 8 is a top view of the embodiment shown in FIG.

FIG; 9 is a side view in elevation of the thermoresponsive switch actuator shown in FIG. 7;

FIG. 10 is a cross-sectional view of a portion of the embodiment shown in FIG. 9 taken along line 10l0;

FIG. 11 is a plan view of yet another thermoresponsive switch actuator incorporating principles of the invention;

FIG. 12 is a side view in elevation of the actuator shown in FIG. 11;

FIG. 13 is a top view of a switch utilizing a thermoresponsive switch actuator incorporating principles of the present invention;

FIG. 14 is a side view in elevation of the switch shown in FIG. 13 with all switch contacts in closed position;

FIG. 15 is a fragmentary side view in elevation of the switch shown in FIG. 13 with all switch contacts in open position;

FIG. 16 is a fragmentary side view in elevation of the switch shown in FIG. 13 with one set of switch contacts in closed position and with another set of switch contacts in open position;

FIG. 17 is an end view in elevation of the switch shown in FIG. 13;

FIG. 18 is a diagrammatical illustration of idealized functional relations which may exist in practicing the present invention; and

FIG. 19 is a cross-sectional view of the thermoresponsive switch actuator shown in FIG. 1 taken along line 19-19.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now in more detail to the drawings there is shown in FIGS. 1, 2 and 19 a thermoresponsive switch actuator comprising a thin sheet of stainless steel having a bridge 1 from which three

legs

3, 4 and 5 integrally extend with leg 4 extending in spaced relation between

legs

3 and 5. Leg 4 is secured to the top of dielectric mount 7 by screw 8 while

legs

3 and 5 are secured to the bottom of mount 7 by screws 6. Two integral stiffening ribs 10 project convergently upward from the sides of leg 4. Two integrally stiffening ribs 11 project normally upwardly from the outboard sides of

legs

3 and 5 while stiffening rib 12 projects from outboard edge of bridge 1.

To leg 4 is mounted an electric heating element 15 encapsulated in a transparent dielectric tube 16 held snugly in place by convergent ribs 10. From one end of the tube extends an insulated lead wire 17 an uninsulated portion of which is sandwiched between the head of terminal screw 8 and insulating washer 19. The washer is located over an aperture in leg 4 of sufficient size to permit the screw extending from the head of screw 8 to pass in spaced relation therethrough. From the other end of the tube extends lead wire 20 which is soldered to leg 4 at solder joint 22.

Bridge 1 and

legs

3, 4 and 5 are formed from a sheet of stainless steel of some eight mils thickness. So formed, the sheet takes the general shape of the letter B. Once mounted, bridge 1 is spring biased upwardly, as viewed in FIG. 2, by leg 4 and downwardly by

legs

3 and 5. The bridge is thusly stabilized at a point between the top and bottom of mounting block 7.

In operation of the actuator is placed in a fluidic medium such as air. Terminal screws 6, which are in electrical contact with

legs

3 and 5, and terminal screw head 8 are coupled to a source of electric current causing heating element 15 to be electrically energized. As the temperature of the heating element is elevated, thermal conduction occurs between the heating element and leg 4 causing the temperature of leg 4 to become elevated above that of

legs

3 and 5. As the temperature differential between leg 4 and

legs

3 and 5 increases, leg 4 expands. This expansion results in a diminution in the magnitude of force exerted by leg 4 upwardly upon bridge 1. As a result the forces exerted by the three legs upon the bridge become unbalanced with the force exerted downwardly by

legs

3 and 5 surpassing that exerted by leg 4 upwardly upon the bridge. Thus, bridge 1 is forced downwardly through the ambient air in which it is located until equilibrium of forces is again established with the magnitude of bridge travel being proportional to the magnitude of temperature differential between leg 4 and

legs

3, 5, as will hereinafter be explained in more detail. Upon de-energization of heating element 15, leg 4 cools and contracts thereby exerting increased upwardly directed force upon bridge 1. The resulting imbalance between forces acting upon bridge 1 causes itto move back upwardly to its initial position when force equilibrium is again established. During both the upward and downward

movement stiffening ribs

10, 11 and 12 serve to inhibit bending of the actuator legs. I

In FIGS. 3 and 4 another embodiment of the invention is shown comprising a thin sheet of metal stamped into a generally E-shaped configuration including a bridge portion 30 from which juxtaposed

leg portions

31 and 32 extend. Here, the outboard edge of the bridge is arcuate rather than rectilinear as in the previously described embodiment. A switch contact 35 is shown mounted directly on the bridge. Stiffening ribs 36 and 37 are formed on the edges of legs 31 and leg 32, respectively. Legs 31 are mounted atop dielectric mount 38 while leg 32 is mounted therebeneath with terminal 39, integral with leg 32, projecting from the mount.

In the embodiment shown in FIGS. 3 and 4 the electrical resistance of leg 32 itself is utilized in providing a temperature differential, upon command, between legs 31 and leg 32. By coupling terminal 39 and contact 35 to a source of electrical current, current flows through leg 32, as schematically indicated by wave form 40, while not through legs 31 which are electrical pendant from the circuit. This embodiment thus eliminates the need, and resulting cost, of a separate heating element such as that required by the embodiment shown in FIGS. 1 and 2.

In operation, contact 35 is positioned in engagement with a switch contact extrinsic to the actuator which extrinsic contact is connected to a source of electric current. With terminal 39 likewise connected to the source, a circuit is established and current caused to flow through leg 32. With the heating of leg 32, bridge 30 is forced upwardly as viewed in FIG. 4 thereby causing contact 35 to break with the extrinsic contact. This action likewise breaks the energizing circuit terminating the flow of current through leg 32. The leg thereupon cools causing bridge 30 to return to its initial position whereupon contact 35 once again makes with the extrinsic switch contact thereby completing an energization cycle.

In FIGS. 5 and 6 principles of the invention are shown incorporated into yet another embodiment. The thermoresponsive switch actuator here comprises an E-shaped sheet of metal of homogenous composition having a bridge portion 50 from which three

leg portions

51, 52 and 53 extend.

Legs

51 and 53 are mounted atop dielectric mount 55 while leg 52 is mounted therebeneath. An encapsulated heater 56 is mounted to

legs

51 and 53 and bridge 50 which heater is adapted to be connected to a source of electric current by means of terminal screws 58. The encapsulating material preferably is Kapton, a polyimide.

Legs

51 and 53 are seen to diverge as they extend from bridge 50 which configuration inhibits actuator twisting. Stiffening ribs 59 further serve to inhibit any such twisting action as well as inhibiting any bending of the legs.

In FIGS. 710 yet another embodiment of the present invention is shown comprising a unilayer sheet of stainless steel having three spaced

leg portions

61, 62 and 63 extending from bridge portion 64. The ends of the legs distal the bridge are welded to steel support 65 with

outboard legs

61 and 63 being welded to the bottom of the support and with inboard leg 62 being welded to the top of the support. Stiffening rib 66 projects downwardly from the peripheral edge of

legs

61, 63 and portion 64. Stiffening ribs 67 likewise project downwardly from the sides of leg 62. A switch contact 68 is mounted atop bridge portion 64. A low voltage heater in the form of a thin strip of copper 70 is mounted to the unilayer sheet in thermal communication with leg 62. A strip of electrical insulation 69 of material such as Kapton is mounted therebetween. Such a heater may draw some 5 to 20 amperes of current with a potential difference of approximately one.- tenth volt placed thereacross.

In FIGS. 11 and 12 still yet another embodiment of the invention is illustrated comprising a substantially planar, unitary, U-shaped, metallic member having two leg portions 72 extending from a bridge portion 73. The distal ends of the legs are welded to the top of an L- shaped steel support 74 at W. One end of a center leg 75 is welded to the top of bridge portion 73 while its other end is welded to the bottom of support 74. Stiffening rib 76 projects upwardly from the outboard edge of bridge portion 73. Stiffening ribs 77 project upwardly from the outboard edges of leg portions 72 with portions thereof extending over the surface of support 74. Prior to assembly an electrically insulative sheath, not shown, is placed snugly about center leg 75 between its two ends and a thin insulated Nichrome wire a wire composed of a nickel-chromium-iron alloy 78 wound thereabout. Leads 78' extend therefrom which are adapted to be connected to a source of electric current. As opposed to the embodiment previously described and shown in FIGS. 7l0, that here described and shown in FIGS. 11 and 12 is particularly well suited to be energized by the application of relatively high voltage, such as in the order of 230 VAC. The resistance of the Nichrome wire 78 may be in the order of 15,000 ohms.

Output travel of the thennoresponsive switch actuators just described is believed to be directly proportional to the length of the leg or legs in thermal contact with the heater means, directly proportional with its change in length when heated, and inversely proportioned to the mounting spacing between legs. This may be better understood by reference to FIG. 18 in which a represents the length of the relatively constant temperature leg, b the length of the heatable leg, and c the spacing between the mounted portions of the legs distal their junction at bridge P where a switch contact may be mounted.

From the Law of Cosines:

Heating causes a to increase by A a, and thus, from (1) 3. 2aAa Aa 2bc(Cos A Cos[A A A]) As A a is quite small A a may be neglected. Expanding Cos (A A A), neglecting A a and multiplying both sides of (3) by 1 4. 2aAa 2110 (Cos A Cos A Cos A A Sin A Sin AA) Simplifying,

5. 2a (Aa)/2bc Cos A +Cos A Cos AA Sin A Sin AA As A A is small Cos AA approximates 1 and sin AA AA in radius. Thus, 7

6. aAa/bc Cos A Cos A l Sin A Sin A A, or,

7. aAa/b c Sin A Sin A A Where A approximates a right angle sin A approxiat 1 nd lkals armrq m ss T 8. SinAA=Aa/c Travel A'p of P is approximately 9. b sin A A or from (8),

10. A p b A a/c We thus see that bridge travel is directly proportional to the length of the unheated leg or legs and to the change in length of the heated leg or legs, and inversely proportional to the mounting spacing between the heated and unheated legs. If energy is constant, output travel is inverse proportional to acutator force. In other words, for given input energy actuators designed for substantial travel in effect switching operations operate with less force than those designed for less travel. The selected balance therebetween will ordinarily, of course, be predicated by the desired operative characteristics of the switch itself. It should, of course, be realized that these relations should only be used as general guides in designing specific embodiments.

Another aspect deserving careful consideration in practicing the invention involves that of the actuator hinge. For efficiency it is desirable to minimize energy losses in the hinge portions of the actuator during operation. Stiffening ribs thus should not be present at the hinge. Thus, it may be noted that hinge areas 79 in each of the illustrated embodiments are devoid of such ribs. In addition, the longitudinal length of the actuator legs in which hinging occurs should preferably be minimized to conserve energy. Similarly, the widths may also be reduced somewhat from that of the remainder of the legs without overly weakening the legs. Exemplary of this are the hinges shown in the embodiment illustrated in FIG. 7. As a further means of minimizing energy losses in the hinge areas, such portions may be thinned as by placing one planar surface on a mandrel and striking the opposite surface.

The three legs of the illustrated actuator are mounted to their supports in an unloaded condition. This is to say that as mounted none is under tension or compression. In operation, however, the heated leg or legs will be compressed as their expansion is resisted by the other leg or legs joined thereto by the actuator bridge. The other leg or legs conversely become tensed. In certain application however it may be desirable to mount the legs in a loaded condition. By mounting a heatable leg in a tensed condition the leg will actually release kenetic energy as it is heated. This is particularly useful where the actuator is coupled with a toggle spring to effect snap action.

As just explained, one or more legs of the described actuators may be under tension or compression during heating, cooling or when held at a constant temperature. As also previously explained, by minimizing the mass of the hinge portion of the legs, within bounds dictated by structural integrity constraints, energy losses therein may be minimized. Where a thinned, narrow hinge portion is compressed, however, risk of buckling arises. To lessen this risk during compression the hinged portion may be located over the surface of the support to which the actuator is secured. Hinge area 79 of the actuator shown in FIG. 11 exemplifies this.

The bridge and leg portions of the illustrated embodiments are preferably metallic. The selected metal or metal alloy should ideally have low flow rate and drift, low spring back, a relatively high coefficient of thermal expansion and high stability under variant environmental conditions including that of high temperature. It also should be readily machineable. Stainless steels meet these conditions quite well. Stainless steel also rapidly work hardens and can produce high strength hinges. As they may be formed quite thinly while retaining attendant strength, their surface areas may be maximized in relation to attendant volume in forming heatable legs having short time constants. That the legs and bridge portions of the actuator may be formed from a unitary sheet of metal negates need for ambient temperature compensating means. Even-variations in composition from sheet to sheet are self compensating in unitary configurations since both the active and reference elements are punched from the same sheet.

As to materials for the support to which the legs aremounted, both dielectric and conductive ones have been utilized in the various embodiments hereinbefore described. A primary advantage of the dielectric types is the electrical insulation they inherently provide between the offset legs. Their use thus negates the need for separate dielectric spacers between the unheated leg or legs and the support. Metallic supports on the other hand offer low drift and a surface to which a leg may be welded. Soft steels are particularly well suited inasmuch as they can be easily fabricated into complex shapes and also have low spring back which facilitates the fixing of the offset dimension between legs during manufacture. Output energy of the actuator as previously described, is believed to be a function of this dimension. l

The described embodiments illustrate the fact that thermoresponsive switch actuators embodying principles of the present invention may take many specific forms. Thus, many modifications may be made thereto without departure from the spirit and scope of the invention as set forth in the concluding claims. Likewise, their employment into switches may be had in innumerable ways. One of such ways, for purposes of illustration, is shown in FIGS. 13-17 which illustrates a switch control for an electric motor having start and run windings. The switch control here comprises dielectric frame member 80 upon which terminal 82 is mounted. Metallic spacer 84 overlays terminal 82 with a thin insulating strip 86 sandwiched therebetween to electrically isolate terminal 82 from the metallic spacer. Dielectric block 88 overlays spacer 84. The terminal, spacer insulating strip and dielectric block are fastened rigidly together by screws 90 and nuts 92.

One end of outer legs 94 of a three legged thermoresponsive actuator embodying principles of the present invention are fixed by such means as welding to metallic spacer 84. One end of center leg 96 of the actuating element is similarly secured to terminal 82. The unsecured ends of the three legs are integrally joined together by three legged actuator bridge portion 94' upon which is mounted double contact 98.

To the bottom of frame member is secured terminal 100 by means of screw 102 threaded through an aperture in the frame member and uponthe end of which screw is mounted motor start contact 104. Nut 106 is run up the threads of screw 102 both to secure terminal 100 rigidly to the frame member and to fix the position of motor start contact 104 relatively to the frame member.

To the top of dielectric block 88 between screws 90 is mounted terminal 108 by means of screw 110 which screw extends through

terminals

82 and 108, block 88, metallic spacer 84, insulating strip 86 and frame member 80. The aperture in terminal 82 and spacer 84 is of greater diameter than that of screw 110 whereby the screw may pass therethrough in spaced relation to the aperture defining walls and thereby prevent clectric contact from being made thereinbetween. One end of leaf spring 112 is secured as by welding to terminal 108 while to the other end is similarly secured the proximal end of pendant arm 114. To the distal end of the pendant arm is secured arm extension 116 to which is mounted motor run contact 118. One end of toggle spring 120 is held to an end of arm extension 116 by means of an extension arm tab which projects through an accommodating slot in the toggle spring. The other end of toggle spring 120 is held to toggle spring pivot l22by means of a pivot tab which similarly projects through another accommodating slot in the toggle spring. Pivot 122 is in turn rigidly secured to frame member 80 by screws 124. In the position shown in FIG. 17 toggle spring 120 spring biases pendant arm 114 and motor run contact 118 mounted thereto against double contact 98 and it in turn against motor start contact 104. The toggle spring biases the pendant arm in the opposite direction and motor run contact 118 away from double contact 98 if the arm is raised away from motor start contact 104 beyond a predetermined over center snap position as by movement of the three legged element away from the motor start contact. This is the switch condition shown in FIG. 15.

For operation, terminal 100 may be coupled to a motor start winding, terminal 108 to a motor run winding, and terminal 82 to a source of electric current through suitable power on and off switch means. Upon closure of the power switch,

contacts

98, 104 and 118 will normally be in the closed position illustrated in FIG. 17 whereby both the motor start and motor run windings will be energized. Electric current will pass through center leg 96 of the three legged actuator causing it to heat and rapidly assume temperatures elevated above those of outer legs 94. This temperature differential between the center and outer legs, coupled with the vertical offset of the end of the center leg proximal terminal 82 relative to the ends of the outer legs proximal spacer 84, cause the actuator to pivot. Bridge portion 94' accordingly moves upwardly away from frame member 80 causing double contact 98 rapidly to break from motor start contact 104. This action terminates the flow of current to the motor start windings. The relative positions now occupied by the contacts is as shown in FIG. 16.

In moving upwardly away from frame member 80 and motor start contact 104 the three legged actuator forces pendant arm 114 also upwardly insomuch as the top surface of double contact 98 mounted to the actuator engages the bottom surface of motor run contact 118. This movement is in opposition to the bias provided by spring 112. In the event the motor run winding load is within a predetermined limit the current passing through center leg 96 heats the leg sufficiently to insure that a substantial gap exists between double contact 98 and motor start contact 104. However, as seen by reference to FIG. 16, this gap will not be so great as to position pendant arm 114, forced upwardly by the movement of the three legged actuator, above an over center snap position provided by toggle spring 120. Should the motor malfunction however and the motor run winding load exceed the aforementioned predetermined level, current through center leg 86 will increase causing bridge portion 94' to move upwardly further away from frame member 80. This excessive current flow forces pendant arm 114 above the over center point at which toggle spring 120 snaps the pendant arm upwardly abruptly opening

contacts

98 and 118. In this position, which is that shown in FIG. 15, both the motor start and motor run windings are de-energized. Flow of electric current through center leg 96 accordingly terminates thereby causing it to cool and move downwardly towards frame member 80. As the three legged actuator moves downwardly, dielectric stop 123 attached thereto engages pendant arm 114 if not already having done so in arresting the snap movement thereof. Continued downward movement of the three legged element thus forces pendant arm 114 downwardly until its over center snap position is again reached beyond which point toggle spring 120 snaps the pendant arm downwardly towards frame member 80. This action, in which dielectric stop 123 acts as a trip dog, forces motor run contact 118 into engagement with double contact 98 and it in turn into engagement with motor start contact 104 as the force of the toggle spring overcomes the positional stability of the three legged element. In this manner the switch is reset and the motor start and run windings are once again energized.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an electrical switch having mounting means with a pair of oppositely spaced mounting surfaces thereon; the combination therewith comprising a thermoresponsive switch actuator having a unitary and generally planar body including a bridge portion integrally formed with three generally laterally spaced legs each having a free end portion opposite the bridge portion, contact means on the bridge portion for movement between switch energizing and de-energizing positions, one of the three legs and the other pair of the legs being respectively oppositely displaced from the general plane of the body and the free end portions of the one leg and the other pair of legs being respectively fixedly engaged with the oppositely spaced surfaces of the mounting means thereby to bias the bridge portion and contact means toward one of the switch energizing and deenergizing positions thereof, and means for establishing a temperature differential between the one leg and the other pair of legs, the one leg and the other pair of legs being relatively movable in response to the temperature differential established therebetween about the mounting means to conjointly move the bridge portion and contact means from the one of the switch energizing and de-energizing positions to the other thereof.

2. In an electrical switch as set forth in claim 1, wherein the temperature establishing means comprises heating means mounted in heat transfer relation with one of the one leg and the pair of legs.

3. In an electrical switch as set forth in claim 1, wherein at least one of the one leg and the pair of legs is connected in electrical circuit relation with the contact means.

4. In an electrical switch as set forth in claim 1, wherein the temperature establishing means comprises an electrical heating element disposed on the one leg in heat transfer relation therewith, and means for connecting at least one leg of the pair of legs in electrical circuit relation with the contact means.