US3596352A

US3596352A - Method of calibrating bimetallic elements in a thermal overload switch

Info

Publication number: US3596352A
Application number: US721755A
Authority: US
Inventors: Eduard W Isler
Original assignee: AO Smith Corp
Current assignee: AO Smith Corp; Joslyn Manufacturing and Supply Co; Joslyn Clark Controls Inc
Priority date: 1968-04-16
Filing date: 1968-04-16
Publication date: 1971-08-03
Anticipated expiration: 1988-08-03

Abstract

This disclosure relates to a thermal bimetal operator for a multiple line control switch unit and to the method of construction. A switch housing is provided with a series of small cavities in one end wall. A bimetal strip includes an L-shaped tab which projects into the recess. A thermoset resin is disposed within the respective cavities and the bimetal strips are resiliently clamped with the outer end of the bimetal element disposed outwardly of a set position. A U-shaped trip bar is pivotally mounted within the housing with the base engaging the free ends of the bimetal strips and with an outer arm aligned with a switch blade. The switch is a snap action overcenter variety having the compensating switch arm or blade projecting laterally parallel to the trip bar and interengaged by the depending arm of the trip bar. The switch includes an adjustment screw connected to a pivot support to control the snap action position. The assembly is placed in a fixture having a support pin extending through a housing opening to support the outer arm and an adjustable tool spring loaded rod extending through an opposite housing opening into engagement with the switch blade to position the components including the bimetal strips. the fixture and assembly are placed in a curing oven which is raised to the curing temperature. Prior to the setting of the resin and after the operating temperature is established, the switch adjustment is made. The curing step is then completed.

Description

United States Patent [72] Inventor EduardW.Isler Cleveland, Ohio [21] Appl, No. 721,755 [22] Filed Apr. 16, 1968 [45] Patented Aug. 3, 1971 [73] Assignee A.O.SmithCorporation Milwaukee, Wis.

[54] METHOD OF CALIBRATING BIMETALLIC ELEMENTS IN A THERMAL OVERLOAD SWITCH 3 Claims, 7 Drawing Figs.

[52] US. Cl .1 29/622, 29/407, 29/593, 29/622, 73/1, 200/166, 200/1,

[51] Int. Cl 1101b 11/00,

H0lh 1l/02,l-I01h 65/00 [50] Field of Search 29/407,

Primary Examiner-John F. Campbell Assistant Examiner-Robert W. Church Attorney-Andrus, Sceales, Starke and Sawall ABSTRACT: This disclosure relates to a thermal bimetal operator for a multiple line control switch unit and to the method of construction.

A switch housing is provided with a series of small cavities in one end wall. A bimetal strip includes an L-shaped tab which projects into the recess. A thermoset resin is disposed within the respective cavities and the bimetal strips are resiliently clamped with the outer end of the bimetal element disposed outwardly of a set position. A U-shaped trip bar is pivotally mounted within the housing with the base engaging the free ends of the bimetal strips and with an outer arm aligned with a switch blade. The switch is a snap action overeenter variety having the compensating switch arm or blade projecting laterally parallel to the trip bar and interengaged by the depending arm of the trip bar. The switch includes an adjustment screw connected to a pivot support to control the snap action position. The assembly is placed in a fixture having a support pin extending through a housing opening to support the outer arm and an adjustable tool spring loaded rod extending through an opposite housing opening into engagement with the switch blade to position the components including the bimetal strips. the fixture and assembly are placed in a curingoven which is raised to the curing temperature. Prior to the setting of the resin and after the operating temperature is established, the switch adjustment is made. The curing step is 5 6 References Cited UNITED STATES PATENTS 2,962,804 12/1960 Nelsen 29/622 2,745,924 8/1958 Coates..... 200/33 3,057,047 10/1962 Zimmer 29/622 3,344,250 9/1967 Duval 337/89 3,230,607 1/1966 Gelzer 29/622X thencompleted.

PATENTEMUG 3197: 3,596 352 sum 1 OF 2 gal ward 15/47 METHOD OF "CALIBRATING BlIMETALLIC ELEMENTS IN A THERMAL OVERLOAD SWITCH This invention relates to a thermal bimetal operator and particularly to such an operator for a multiple line control switch unit wherein a plurality of separate bimetal means are mounted in spaced relation and coupled to move a common actuator.

In electrical control circuits, thermal responsive controls may be connected in the lines to open the circuit in the event of abnormal current conditions and the like. Such thermal overload controls may take a variety of different forms including bimetal elements forming a current-conducting contact of a switch or heated from a separate heating element connected in the circuit to actuate an isolated switch. The heating of a bimetal element is directly proportional to the amplitude or level of the current. If the current rises above a selected level, the heating effect on the bimetal element, with the well known differences of thermal expansion and contraction of the individual elements, is such as to actuate the switch.

A highly satisfactory thermally activated switching system is disclosed in the copending application of Eduard W. Isler entitled Electrical Contactor Assembly, which was filed on Sept. 25, 1967, wherein separate heater means are provided in each line for actuating a common thermal overload switch. The above application is particularly directed to a three-phase alternating current motor control having a main contactor to which an overload switch unit is secured. A T-shaped heating unit has one terminal connected to the overload unit and the opposite contact connected directly to the electromagnetic main switch.

In bimetal switching elements and the like, it is often necessary to calibrate the movement of the bimetal elements to a high degree of accuracy because the deflection versus temperature characteristic within the operating range of the bimetal is very flat. This becomes particularly important where a plurality of separately responsive bimetal elements are provided and interconnected to operate a switch means through a common actuator. To provide proper actuation, each of the bimetal members should engage the common actuator with an equal force for a given condition. Generally, accurate calibration has been obtained in the past by providing calibration pins in the common actuator and selectively fixed therein to provide the necessary projection toward the bimetal element to provide equal force balance. Although satisfactory switch assemblies have been provided by this method, the procedure is time consuming and therefore costly and cannot practically be adapted to an automatic or semiautomatic production line method.

In accordance with the present invention, a switch assembly includes a common actuator movably mounted in the path of the terminal ends of a plurality of bimetal elements. The bimetal elements are correspondingly supported at the opposite end by a potted construction, preferably a suitable thermoset resin in such a manner as to insure equal force engagement of each bimetal element with the common actuator for any given temperature condition. A completely isolated electrical switch mechanism is thereby accurately actuated in response to a given temperature condition applied to any one or all of the bimetal elements.

In a particularly novel method of construction, the three individual bimetal elements are secured to the support with the support ends embedded within a resin in the unset or liquid state. The common actuator is mounted to the assembly in common engagement with the bimetal elements and with the switch mechanism. The total assembly is then mounted within a curing oven which is set at essentially the effective operating temperature of the assembly in normal operation. The common actuator and bimetal elements are then moved to a set condition which just actuates the switch mechanism and constitutes the nominal setting of the assembly. The physical interengagement of the trip lever and the switch mechanism is maintained with the three metal elements as a backup such as to maintain the desired equal force engagement: The resin is set or cured such as to subsequently hold the mechanism in the nominal setting with equal force engagement between the elements and the common actuator.

The switch is preferably of a snap action overcenter variety having a compensating switch arm projecting laterally parallel to the trip bar and interconnected to a depending arm of the trip bar.

In a preferred construction and novel method of assembly a switch housing is provided with a series of small cavities to each of which one end of a corresponding bimetal element is secured. A thermoset resin in a plastic state is disposed within the respective cavities and the one end of each bimetal element is resiliently clamped within the corresponding recess by a resilient clip with the outer end of the bimetal element disposed outwardly of the set position. A trip bar is pivotally mounted within the housing with a common portion engaged by each of the bimetal elements and a spaced portion engaging a switch am. An access opening is provided in the housing to permit entrance of a suitable tool for movement of the trip bar to move the bar and the bimetal elements and thereby establish equal force engagement for any given set condition.

The assembly is placed in a fixture having an adjustable tool aligned with the opening and the assembly and fixture are placed in a curing oven which is raised to the curing temperature. After the setting of the resin and after the operating temperature is established, the switch adjustment is made. The curing step is then completed. Establishing a curing temperature and affecting the calibration in the same temperature condition has been found to provide very accurate calibration of the bimetal elements.

The present invention thus provides a very simple, inexpensive and efficient means for accurate calibration of a plurality of bimetal or similar thermostatic operating elements for mov ing a common actuator member.

The drawings furnished herewith illustrate a preferred construction of the present invention in which the above advantages and features are clearly disclosed as well as others which will be clear from the description of such embodiment.

In the drawings:

FIG. l is a plan view showing an overload relay connected to a main electromagnetic relay;

FIG. 2 is an exploded pictorial view of the thermal switch shown in FIG. 1;

FIG. 3 is an enlarged fragmentary section through the thermal switch shown in FIGS. 1 and 2;

FIG. 4 is an enlarged fragmentary view more clearly illustrating the bimetal element mounting;

FIG. 5 is a sectional view through a switch mechanism actuated by the bimetal elements;

FIG. 6 is a view similar to FIG. 4 showing a step in the method of assembly; and

FIG. 7 is a side elevational view partially in section to show a fixture for proper positioning of the bimetal elements and the common actuator.

Referring to the drawings, and particularly to FIG. 1, the present invention is shown applied to an AC contactor 1 which is adapted to be connected in a three-phase motor starting circuit, not shown. An integrated thermal overload switch unit 2 is connected to one side of the AC contactor 1 and is constructed to respond to load current to prevent damaging of the load or the motor components. The AC contactor may be of any desirable construction. For example, as referred to in applicant's previously identified copending application, the main contactor ll includes an outer insulating housing having three laterally displaced contact chambers 3 with line and load terminals 4 and 5, respectively, secured to the opposite ends of the chambers. The line terminals 4 are exposed to the top and one side of the housing and are interconnected to the load terminals 5 similarly mounted to the opposite side of the housing by a bridging contactor assembly 6 which is slidably mounted within the housing. The assembly 6 includes bridging contacts, not shown, which are actuated by a suitable electromagnetic means or the like. As the main contactor 1 may take any variety of forms and be of any suitable construction, no further illustration or description thereof is given.

The three corresponding terminals 4 on the line side of the contactor 1 are suitably connected to the incoming power lines of a three-phase system whereas the load side of the contactor are interconnected through the thermal overload switching device 2.

Generally, the thermal overload device 2 includes a twopiece rectangular housing of a relative deep switch portion and a shallow cover 8. The housing is formed of a suitable insulating material and is mounted in abutting relationship to one end face of the contactor housing of the main contactor 1. A pair of dovetailed projections and mating grooves 9 interconnect the upper end of the device 2 to contactor 1. The housirg of device 2 and contactor 1 are suitably secured to a common mounting plate 10 to rigidly interlock switch device 2 to contactor 1.

The housing is divided into three heating chambers 11 aligned one each with the adjacent terminals of the main contactor l. The dividing walls may be provided with forward projecting bifurcated walls 12 mating with corresponding vertical dividing walls 13 of the main contactor 1 to properly locate and more firmly interconnect the housings.

The thermal overload housing is provided with three load terminals 14 on the outer wall opposite that abutting the contactor housing with each of the terminals 14 aligned with one of the chambers 11 and the corresponding terminals 5 of the contactor 1. Contact terminals are thereby provided to the opposite side of each of the heater chambers 11 and support a T- shaped heating element 15 within the corresponding chamber. The heating elements 15 are constructed in accordance with the teaching of applicant's previously identified copending application and are therefore only briefly described herein.

Each T-shaped heating element 15 includes a stern including spaced

conductors

16 and 17 connected at the inner end and laterally bent in opposite directions at the outer end to form contacts 18 and 19 overlying terminals 5 and 14, respectively. The heater stem is located in the heating chamber 11 with the cross arm portion of contacts 18 and 19 bridging the gap between the overload device terminal and the main contactor terminal. The current flow in the circuit in each of the lines is through the corresponding heater element 15 to establish a temperature within the related chamber 11 in accordance with the magnitude of the current flow.

The housing 9 abutting the main contactor 1 is a relatively shallow wall member having three shallow recesses with identical bimetal elements 21 mounted in laterally spaced relation therein in accordance with the spacing of the three chambers 11. Each bimetal element 21 responds to the tem' perature condition in the related chamber 11 to actuate a common switch mechanism 22 housed in the lower portion of housing 3, in the event an abnormal current overload is present for a given period.

Each of the bimetal elements 21 is similarly constructed and a single unit is described. The element 21 includes a pair of flat metal strips 23 and 24 bonded to each other and having selected different thermal coefficients of expansion and contraction. The element 21 is secured immediately adjacent its outer end within a cavity 25 in the housing. In accordance with the illustrated embodiment of the present invention, an L-shaped mounting strip 26 is secured to the element 21 and defines an inwardly projecting tab 27 which extends into cavity 25 and abuts the back wall. The cavity 25 is filled with a suitable adhesive, such as a thcrmosetting resin 28 to rigidly fix the upper end of the bimetal element 21 to the housing.

The inner ends of the three bimetal elements 21 similarly engage a pivotally mounted trip member or bar 29 for actuating the switch mechanism 22. The trip bar 29 is generally a U- shaped member formed of Bakelite or any other suitable light, insulating material with upstanding arms 30 disposed one each to the opposite side of the three chambers 11. The upper ends of the arms 30 are provided with V-shaped pivot notches 31 which are supported on similar V-shaped horizontal knife edges 32 integrally formed in the housing. The trip bar 29 includes the base portion 33 which spans the lower ends of the three bimetal elements 21 and is provided with bearing and wear-resistant pins 34 aligned one with each of the elements 21. In the prior art, such pins are driven into a trip bar to provide a predetermined force balanced engagement with the bimetal elements. In the present invention, they merely provide wear surfaces.

The trip bar 29 further includes a depending arm 35 extending downwardly from the base portion 29 in the opposite direction from arms 30 and into alignment with compensating cantilevered operating arm 36 of the switch mechanism 22.

Referring particularly to FIG. 5, the illustrated switch mechanism 22 is a spring-loaded, overcenter, snap action switch having a main pivot support bracket 37 secured within the housing. The bracket 37 is U-shaped and opens inwardly with the web attached by a screw 38 to the front wall of the housing and opening rearwardly within the housing. An overcenter spring plate or blade 39 is generally U-shaped with the ends of the legs pivotally mounted in the outer side edge of the bracket 37 as at 40. A coil spring 41 is secured at one end to the base of the overcenter spring blade 39 and at the opposite end to a pin 42 secured between the arms of the bracket 37.

The overcenter spring blade 39 has two alternate stable positions, depending upon the position of the blade with respect to the fixed pin 42. The overcenter blade 39 is positioned in one position by the trip bar 29 and arm 36 and returned by a resetting lever 43, as hereinafter more fully described.

Generally, the compensating cantilevered arm 36 extends laterally across the housing from a pivot support connection to bracket 37, with the outer end aligned with the depending arm 35 of the trip bar 29. The cantilevered arm 36 includes a generally U-shaped portion 44 telescoped into the bracket 37 in the end opposite the overcenter blade 39. The U-shaped portion includes apertured side arms and aligned with guide slots 45 in the U-shaped portions and includes oppositely extending tabs or arms 47 passing through the apertures and into the guide slots 45. A calibration and adjusting screw 48 extends through the front wall of the overload housing 7 and threads into the nut 46 with a coil spring 49 encircling the screw. The tightening and loosening of the screw 48 provides corresponding positioning of the pivot tabs 47 within the slot 45 and therefore varies the position of the cantilevered pivot support with respect to the overcenter blade 39.

An operating hook or lever 50 is integrally formed with the one side of the U-shaped portion of cantilevered arm 36 and terminates in a bearing tip engaging the overcenter blade 39 immediately outwardly of its pivot connection 40 to the bracket 37.

In operation, the overcenter blade has one stable position shown in FIG. 5. The pivoting of the cantilevered arm 36 and the integrally connected lever 50 cause the positive displacement of the overcenter blade 39 and the attached spring 41 to the opposite side of center with respect to pin 42.

The switch mechanism 22 further includes a blade contact 51 secured as by a rivet to the one side of the overcenter blade 39 and projects outwardly along the blade and beyond the outermost end of it. Contact buttons are secured to the opposite faces of blade 51 and selectively make contact with normally closed contact 52 and alternatively a normally open contact 53. A backing plate or strip 54 is secured to the back side of the contact strip 51 and is shown coextensive therewith to limit the backward deflection of the contact strip with the switch mechanism 22 in the normally closed contact position shown in HO. 5 of the drawings. The backing strip 54 is provided with a through contact button at its outer end in alignment with the contact buttons of contacts 51 and 53. Contact 51 is connected in circuit through the bracket 37 and a contact terminal 55 which is welded or otherwise secured to one side of bracket 37 and extends upwardly adjacent the front wall of the housing to the exterior, as shown in FIG. 3. The

normally closed contact 52 is a contact strip having offset and parallel legs such that it extends laterally of the housing and then forwardly to the rear wall and along the rear wall to a terminal 56 which extends upwardly to the exterior, as shown in FIG. 1.

The normally open contact 53 is a generally flat strip secured to the opposite front of the housing to a contact terminal 56a, as viewed in FIG. 5, and extends laterally inwardly into alignment with the contact 51.

When the switch mechanism is actuated to the alternate position, the contact 51 is connected to contact 53 and provides a current path for operating of a trip indicator or the like, not shown. The switch mechanism 22 and particularly contact 51 is adapted to be reset by reset lever 43 which is shown as a small plastic striplike member slidably mounted adjacent the front wall of the housing adjacent contact 53 and suitably spring loaded to continuously urge the reset lever 43 outwardly. An offset cam portion 57 extends between the front wall of the housing and the normally open contact 53. Depressing of the reset lever 43 moves the cam portion 57 behind the normally open contact 53 and pivots it outwardly around its fixed mounting to the contact terminal 56 and positively pivots the contact 51 to the reset position.

In normal operation, the switch mechanism 22 is in the normally closed position with the cantilevered arm 36 bearing on the depending arm of the trip bar 29. The trip bar 29 in turn engages the bimetal elements 21, which deflect from their normal position in accordance with the temperature within the associated chamber. If for any reason, the line current should rise above a normal level, the increased temperature will result in deflection of one or more of the bimetal elements 21 beyond the trip position causing the movement of the overcenter blade contact 51 to open the normally closed contact 52 with a snap action. Thus, once the cantilevered arm 36 moves overeenter blade contact 51 beyond the centered position, the contact 51 moves to the alternate position under the force of spring 41 with a snap action movement.

In order to provide accurate calibration of the three bimetal elements 21, the overload device is constructed in accordance with the particularly novel aspect of the present invention in the following manner.

The shallow portion of housing 8 is formed with the clamping recesses or cavities 25. The bimetal elements 21 are secured with the L-shaped mounting tabs 26 disposed within the proper recesses which are filled with a suitable thermosetting resin 28 in a liquid or uncured state. The bimetal elements 211 are clamped within the recess and the uncured resin by suitable spring clips 53 with the elements angularly disposed from the normal operating plane, as shown in FIG. 6. The clipped assembly is disposed on a jig 59 with the bimetal elements 21 facing upwardly and extending at an angle from the recesses 25, as shown in FIG. 7, The jig 59 includes a small upwardly projecting support pin 60 which projects into the housing ti through a suitable opening 61 in alignment with the arm 35 of trip bar 29. The trip bar 29 is mounted on top ofthe housing 53 with the three wear pins 34 engaging the corresponding ends of the bimetal elements 2i and the switch arm 35 resting on the pin 60. The outer deeper portion 7 of switch housing is assembled with the switch mechanism 21 and then secured over the shallow housing 8 as by the attachment screws. The outer portion of housing 8 includes an adjustment opening 62 aligned with the pin 60 of the jig, and therefore also in alignment with the outer end of the cantilevered arm 36.

A calibration adjustment rod or tool 63 is shown mounted on a post 66 as a part of jig 59 for selecting alignment with the opening or aperture 62. The illustrated tool 63 is a depending rod slidably secured within the outer end of an overhanging arm 65 and having a knob 66 secured to the upper end. A

' spring 67 encircles the tool rod 63 between the underside of arm 65 and a small collar 68 on the intermediate portion of the rod. The tool 63 is moved downwardly by the force of the spring 67 and is normally disposed below the upper end of the pin 60.

The total assembly is now disposed within an oven 69 with the tool 63 disposed to one side of the switch unit. The oven is raised to the normal operating temperature of the bimetal elements 21. The bimetal elements 21 will then be in the normal deflected state, e.g., in an operating state such that the switch mechanism should be tripped. In assembly, the rod 63 is then pulled upwardly against the force of spring 67, aligned with opening 62'and released to move downwardly toward pin 60. The arm 35 of the trip bar 29 and the aligned arm 36 are clamped downwardly onto the support pin 60. The arm 35 of the trip bar 29 and the aligned arm 36 are clamped downwardly onto the support pin 60. The switch arm 36 is brought into physical contact with the trip lever arm 35 which is backed up by the three bimetal elements 21. The elements '21 are pivoted from the angular position of FIG. 6 by the force of the tool 63 to establish equal force engagement between the elements 21 and the bar 29. This establishes the nominal setting of the overload relay thermal switch unit. The system is held in this position within the oven 69 until the resin 28 is cured to provide a firm physical support of the trip bar 29 and the bimetal elements in the calibrated position. The calibration and adjusting screw 48 is then positioned to establish the factory calibration and a limit knob 70 may be placed on the screw to permit field adjustment.

It is found that by providing the curing and calibration under a common temperature condition, and holding of the bimetal elements in the calibrated position during curing, the accuracy of calibration is directly related to the tolerance within which the oven temperature is maintained. The curing oven temperature can, for example, be readily maintained within plus or minus one degree Centigrade for a curing temperature of C.

In particular, the curing of the resin at the normal switch operating temperature reduces the curing time and more importantly automatically compensates for the bimetal parameters or manufacturing tolerances such as thickness, deflection characteristic and width at the working or operating temperature. The latter feature does therefore insure essentially equal force engagement of each bimetal element at the preselected operating temperature for proper actuation of the electrically isolated switch unit. Further, by calibrating or adjusting the switch unit and particularly the operating arm with respect to its engagement with the trip bar in the same temperature condition, it provides accurate calibration of the nominal setting of the switch unit.

Iclaim:

I. The method of mounting a plurality of bimetal temperature-sensitive strips to a support in predetermined engagement with an actuator member and having a switch assembly with means to adjust the actuating position with respect to the position of the actuator member for calibrating of the switch assembly and securing a fixed end of each of said strips to the support, comprising resiliently securing the fixed end of each strip to the support with each strip angularly displaced from its operating position and with the fixed end embedded within uncured thermosetting resin;

mounting said actuator member in simultaneous engagement with the free ends of the strips,

moving said actuator member to a selected position and thereby correspondingly pivoting said strips about said fixed ends, and

heating said assembly to cure said thermosetting resin at a selected operating temperature of the switch assembly and with said actuator member positively held in said selected position, and

positioning said switch assembly with respect to the actua tor member during said heating step.

2. The method of claim 1, wherein said bimetal strips are flat strips and including forming a plurality of aligned recesses in the support, one

for each of the flat strips,

filling the recesses with a thermosetting resin,

resiliently mounting one end of a different bimetal strip in each of said recesses with a resilient clamp to dispose the strips extending angularly outwardly in an essential common plane, said clamp providing for pivotal movement of the strips about said one end,

said actuator member including a U-shaped member formed of a rigid insulating material and having a pair of side legs pivotally supported at the free ends and joined by a flat base portion and a depending arm and having a switch assembly including a cantilevered switch arm having a free end aligned with said arm and a pivot end,

mounting of the actuator member with the flat base portion resting on the outer ends of the bimetal strips to form a subassembly,

pivoting the base portion and bimetal strips about said one end and against the force of said clamp to a selected position within said recesses to establish a nominal setting of said actuator member and said bimetal strips, and curing said resin with said assembly maintained at the selected operating temperature and in said selected posi tion. 3. In the method of mounting a plurality of bimetal strip elements for actuating a common trip bar coupled to a switch unit at a selected operating temperature comprising forming a support plate having a plurality of aligned recesses,

filling the recesses with a thermosetting resin,

resiliently securing a corresponding fixed end of a different bimetal element in each of said recesses with the elements extending outwardly in a generally common plane,

mounting of the trip bar on the outer ends of the bimetal elements,

mounting said switch unit with a switch arm engaging said trip bar to form a subassembly,

heating said subassembly to a selected operating temperature,

moving said trip bar against said elements to position said trip bar and said bimetal strip elements and said switch arm about the fixed ends of the elements and locate the fixed ends within said recesses to establish a nominal setting of said trip bar,

curing said resin with said assembly maintained at the selected operating temperature and in said selected position, and

calibrating said switch unit by positioning of the switch arm relative to said trip bar with said assembly maintained at a selected operating temperature and in said selected positron.

Claims

1. The method of mounting a plurality of bimetal temperaturesensitive strips to a support in predetermined engagement with an actuator member and having a switch assembly with means to adjust the actuating position with respect to the position of the actuator member for calibrating of the switch assembly and securing a fixed end of each of said strips to the support, comprising resiliently securing the fixed end of each strip to the support with each strip angularly displaced from its operating position and with the fixed end embedded within uncured thermosetting resin; mounting said actuator member in simultaneous engagement with the free ends of the strips, moving said actuator member to a selected position and thereby correspondingly pivoting said strips about said fixed ends, and heating said assembly to cure said thermosetting resin at a selected operating temperature of the switch assembly and with said actuator member positively held in said selected position, and positioning said switch assembly with respect to the actuator member during said heating step.

2. The method of claim 1, wherein said bimetal strips are flat strips and including forming a plurality of aligned recesses in the support, one for each of the flat strips, filling the recesses with a thermosetting resin, resiliently mounting one end of a different bimetal strip in each of said recesses with a resilient clamp to dispose the strips extending angularly outwardly in an essential common plane, said clamp providing for pivotal movement of the strips about said one end, said actuator member including a U-shaped member formed of a rigid insulating material and having a pair of side legs pivotally supported at the free ends and joined by a flat base portion and a depending arm and having a switch assembly including a cantilevered switch arm having a free end aligned with said arm and a pivot end, mounting of the actuator member with the flat base portion resting on the outer ends of the bimetal strips to form a subassembly, pivoting the base portion and bimetal strips about said one end and agaInst the force of said clamp to a selected position within said recesses to establish a nominal setting of said actuator member and said bimetal strips, and curing said resin with said assembly maintained at the selected operating temperature and in said selected position. , 3. In the method of mounting a plurality of bimetal strip elements for actuating a common trip bar coupled to a switch unit at a selected operating temperature comprising forming a support plate having a plurality of aligned recesses, filling the recesses with a thermosetting resin, resiliently securing a corresponding fixed end of a different bimetal element in each of said recesses with the elements extending outwardly in a generally common plane, mounting of the trip bar on the outer ends of the bimetal elements, mounting said switch unit with a switch arm engaging said trip bar to form a subassembly, heating said subassembly to a selected operating temperature, moving said trip bar against said elements to position said trip bar and said bimetal strip elements and said switch arm about the fixed ends of the elements and locate the fixed ends within said recesses to establish a nominal setting of said trip bar, curing said resin with said assembly maintained at the selected operating temperature and in said selected position, and calibrating said switch unit by positioning of the switch arm relative to said trip bar with said assembly maintained at a selected operating temperature and in said selected position.