US2347014A

US2347014A - Thermal switching device

Info

Publication number: US2347014A
Application number: US226640A
Authority: US
Inventors: Hans H Willmann
Original assignee: Westinghouse Electric and Manufacturing Co
Current assignee: CBS Corp
Priority date: 1937-09-24
Filing date: 1938-08-25
Publication date: 1944-04-18
Anticipated expiration: 1961-04-18

Description

THERMAL SWITCHING DEVI CE Filed Aug. 25, 1938 F 3- LInsulation v y g 18 20 Fig' 4' 19 1Q 16 j 0 0 l Fig 8.' Fig. .9.

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Patented-Apr. 18,

UNITED STATES 'PATENT oF-Fics THERMAL swrronmo Davies Application August 25, 1938, Serial No. 226,840

In Germany September 24, 1937 v Y 1d Claims. The invention relates to thermally responsive control devices in generaland more particularly to thermal switching devices having bimetal control elements.

Thermal switching devices of the type having I conducts heat from the resistance tothe bimetal a bimetal strip supported between two bearingf points have not proved entirely satisfactory due to the fact that the bimetal strip falls out of its bearing when the strip flexes more than a limited amount and also when the adjustable bearing is moved too great an amount. Such arrangements also require a very accurate maintenance of the length of the bimetal strips and of a thermal switching device having an improved bearing means for supporting the bimetal strip which prevents the strip from falling out of its bearings.

Another object of the invention is the provision of a thermal switching device having an improved bearing means for the bimetal element which permits maximum flexing and adjustment of the element. and at the same time prevents the bimetal element from falling out of its bearings.

1n accordance with the above objects of the invention, the bimetal strip eitheralone or with4 a separate heating strip therefor, is supported at its ends between two bearing means, one of which has a resilient construction to produce a resilient force on the end of the strip in the direction of its longitudinal axis. The other bearing is adjustable in a direction perpendicular to the longitudinal axis of the bimetal strip to adjust the control characteristic of the device.

While there are few difficulties in constructin thermal switching devices which operate reliably in the case of relatively large currents. rather substantial diiculties arise at lower currents.,

since the amount of heat available for heatini the bimetal element is relatively small. Because the amount of heat available is small. it has been the usual practice to heat the bimetal element indirectly by a heater strip or resistance which is electrically insulated from the bimetal strip by a sheet of mica. The mica sheet. however, is not a very good heat conductor and it does not permit sufficient heat to reach the bimetal element in the case of either high or low currents.

Another object of the 'invention is the provision of c thermal switching device having a therefor, with anl improved means interposed between' the bimetal strip and the resistance which electrically insulates the heating resistance, and which at the same time effectivehY element.

` According to the preferred embodiment of the invention the above object is carried out by providing an aluminum strip having oxidized l surfaces. between the heating resistance and the -bimetal strip. The aluminum strip serves to eifectively conduct heat irom the resistance to the bimetal strip. The oxidized surfaces are heat conducting, and also have a high dielectric strength so that they serve to electrically insulate the heating resistance from the aluminum strip and from the bimetal strip. The time constents or control characteristics of the device may be readily varied by providing a plinality N of zum aluminum strips, in neu of one thick strip, between the heating resistance and the bimetal strip.

The novel features that are considered characteristic of the invention are set forth in particular in the appended claims. The invention itself, however. both as to structure and operation together with additional objects and ad` u switching device of a type that has heretofore been yknown and used.

Fig. 2 is a side elevational view of a thermal switching device constructed in accordance with the present invention.V

Fig. 3 is an end elevational view of the resilient bearing means shown in Fig. 2 which sup ports one end of the bimetal strip.

rig. 4 is a. p1an--view or the bimetai strip ofy the device shown in Fig. 2.

4s Fig. 5 is an enlarged fragmentaryv sectional 60 the device shown in Fig. 5.

Fig. 8 is a plan view of the electrical heating resistance of the device shown in Fig. 5.

Fig. 9 is a plan view of the heat insulating strip used in the device shown in Fig. 5.

bimetal -strip and an electrical heating resistance u Fig. 10 is a plan view of the supporting strap for engaging the free end oi the heat insulating strap, and

Fig. .il is a fragmentary sectional view similar to lliig. showing a slightly modied form oi the device.

Reierring to Fig. l of the drawing which shows one type oi thermal switching device, i is a bime tal strip which is supported at its ends in the indentation 2 of a rigid bearing bracket it, and in the indentation l of an adjustable bearing E. The bearing it is guided by a bracket d and is adjustable vertically by means of a screw l. It is apparent that in the above construction when the bimetal strip flexes more than a limited amount, orwhen the adjustable bearing il is moved too far either upwardly or downwardly, the bimetal strip may fall out of its bearings.

The above described fault is avoided by the construction oi the present invention illustrated in Figs. 2, 3, d and 5. Referring to these figures, d is a base of insulating material on which is mounted a pair of rigid bearing brackets d and it. An insulating bridge il connects the upper ends of the brackets 9 and l@ and serves to maintain them at a constant distance from each other.

An adjustable bearing i2 is threaded on an adjusting screw i8 which is vertically mounted on the 'bracket it. The bracket iB serves to guide vthe bearing i2 as it is moved vertically by theadjusting screw it.

A bimetal strip id is supported at one end in a notch i5 formed in the adjustable bearing l2. 'I'he other end of the bimetal strip is notched at i (Fig. e) and. this end is supported by a resilientbearing il comprising 'a spring clip which has its lower end rigidly secured to the foot of the bracket t.

.".lhe spring clip il hasa reduced upper end I d (Fig. 3) forming a pair of shoulders it for supporting the bifurcate ends 2@ of the bimetal strip iii. The reduced portion it of the spring clip engages in the notch ld of the bimetal strip, and the upper end of the spring clip is bent inwardly at 2i to prevent upward displacement of the endof the bimetal strip it. The shoulders is prevent downward displacement of the strip i li.

The resilient spring clip il produces a resilient vforce on the end of the bimetal strip le in the direction ofthe longitudinal axis of the strip and thus prevents the bimetal strip from falling out of its bearings even when the strip I4 flexes its maximum amount, or when the bearing ir is moved upwardly or downwardly to the`limits of its movement.

A pair of resilient or exible conducting terminal straps 22 andY 23 serve to connectthe ends of the bimetal strip il in an electrical circuit.

The above described device may be used to conasesora to lower magnitude currents of predetermined value and duration. Referring to Figs. 5 through l0, the bimetal strip it is supported between two bearing points in the manner previously described, by thespring clip il and the adjustable bearing i2. The heating of the bi'metal strip Iii is effected by means of a heating resistance strip 25. The strip 25 consists of any suitable resistance material such as a nickel-chromium or other altrol circuit breakers, switchesi or any pair vof electrical contacts in a. manner well known in ,i the art. When the blmetal strip is heated a precontrols.

According to one embodiment of the invention,

l the bimetal strip I4 is preferably heated indirectly by a. vseparate heating resistance in order that it may be heated a suiilcient amount in response bimetal strip.

heating resistance from the bimetal strip. ilc-v cording to thel preferred embodiment of the inverition, kthe strip 26 is composed of aluminum,

either in pure or alloy form, which is a good heat conducting material. The surfaces of the aluminum strip 2d are oxidized galvanically, or in any` other suitable manner. The oxidized surfaces 0f the aluminum strip have a high dielectric strength and serve to electrically insulate the heating resistance 25 from the aluminum strip 25 and from the bimetal strip lll. The oxidized surfaces of the aluminum strip while having a high dielectric strength are also good heat conductors so that an efficient heat transfer from the heating resistance 25 to the bimetal strip id is attained` even4 on small currents. The strip 26 may be composed of other materials providing the material is a good heat conductor and has a very thin coating of an electrical insulator. Alloys ci aluminum and magnesium with an oxide coating may be used. While oxidized aluminum has the advantage that it is a very good insulator against substantial voltages, other materials such as copper with an oxide coating on each side may be used where it is only necessary to insulate against lower voltages.

The heating resistance 25 is preferably covered on its side opposite the bimetal strip ld, by a strip 2l of suitable heat insulating material such as hard paper.

If desired, a plurality of thin aluminum strips (Figs. ll), in lieu of the one strip, may be interposed between the heating resistance 25 and the bimetal strip Hi, for the purpose of introduc` ing or varying the time constant of the` device. By using a plurality of thin strips of aluminum in place of a single thicker strip. the flexing of the thermally responsive control unit is not interfered with. The time constant of the device may be varied byusing strips 26, or composite strips, o! diierent thickness between the heating strip and the bimetal strip.

The bimetal strip I4, the aluminum strip 26, the heating resistance 25, and the heat insulating strip 21 are united together by means of a pair of

rivets

28 and 29 to form a single unit which is supported between the two bearings I2 and I'I. To avoid any hindrance of"flexing movement of the bimeta1 strip I4, the aluminum strip 26 is secured only at one end by the rivet 29. its free end terminating short of the secured ends of the other strips. Similarly, the heat insulating strip is secured at one end only by the rivet 28, its other end terminating short of the secured ends of the other strips. The heating resistance strip 25 is resilient due to its zigzag form and may theretore besecured at each end'by the

rivets

28 and 29 without interfering with the bending of' the I'he rivet 2Q is enclosed in an insulating bush- A spacer element 33 of the same thickness as the strip or composite strips 26 is interposed between the left-hand ends wof the heater strip 25 and the bimetal strip I4 beyond the free end of the strip or strips 26.

'Ihe

conductors

22 and 23 constitute the terminals of the device and serve to electrically connect the heater strip 25 in the electric circuit. It will be noted that the'hea'ter strip 25 is electrica-lly insulated from the bimetal strip Il by the oxidized surfaces of the aluminum strip 2S, and bythe insulating bushing 30. The terminal conductor 23 which is connected to the right-hand end of the heater strip 25 need not be secured directly to the bracket i0. If, however, the terminal conductor is secured to the supporting bracket Ill, it is electrically insulated therefrom in any suitablel manner, as by .an insulating washer and bushing (not shown). However, the bearing block i2 may be composed of insulating material in which case no insulation is necessary between the terminal 23 and the bracket 5.

While the invention has been disclosed in accordance with the provisions of the patent statutes, it is to be understood that various changes in the structural details of the device may be made without departing from the spirit of the invention.

The invention claimed is ras follows:

1. In a thermal control device, a thermally reresistance disposed adjacent said element for heating said element, and a metallic member of A y good heat conducting material interposed between the heating resistance and the control element forA conducting heat ,from said resistance to said element, said conducting member having a surface which conducts heat from said resistance to said element and which also electrically insulates said element from said heating resistance, said surface on opposite sides of said metallic member being disposed in contact with said heating resistance and said thermally responsive control element.

2. In a thermal control device, av control element of bimetallic material, an electrical heating resistance disposed adjacent said element for heating said element, and a metal member of i good heat conducting material interposed between face, interposed between said element and said 7| i sponsive control element, an electrical heating nesting resistance ma in consist with sich for conducting heat from said resistance to said element. e

4. In a thermal control device, a thermal element, an electrical heating resistance disposed adjacent said element for heating said element, and an aluminum strip interposed between said elementand said heating resistance for conducting heat from said resistance to said element, said strip having oxidized surfaces in contact with said heating resistance and said thermal element.

5. In a thermal control device, a control element of bimetallic material, an electrical heating resistance disposed adjacent said element for.

heating said element, and an aluminum strip interposed between said element and said resistance, said strip having oxidized surfaces in con'tact with said bimetalllc element and said electrical heating resistance which serve to electrically insulate said resistance from said strip and said element and whichalso conduct heat from said resistance to said element.

i 6. In a thermal switching device, a thermal trip element comprising a strip of bimetallic material, an electrical heating resistance disposed adjacent and parallel to said bimetal strip, and an aluminum strip having an oxidized surface interposed between said bimetal st rip and said heating resistance and having its surface in contact with each for conducting heat from said resistance to said bimetal strip, said oxidined surface serving to electricallyy insulate said resistance from said aluminum strip and said bimetal strip and also serving to conduct heat from said resistance to said bimetal strip.

7. In a thermal control device, a thermal element, an electrical heating resistance disposed adjacent to said element ier heating said element, and a composite member comprising a plurality of thin metallic conducting strips interposed between said heating resistance and said element for conducting heat from said resistance to said element, and at least one of the surfaces of at least one of the strips of said composite member havlnga thin coating to electrically insulate said heating resistance from said strips and said element andA to conduct heat from said heating resistance to said element.

i 8. In athermal switching device, a bimetallic strip, an electricaloheating resistance disposed adjacent and parallel to said bimetal strip for heating" said bimetal strip, andan aluminum strip having oxidized surfaces, interposed between said bimetal strip and said heating resistance, and means securing said bimetal strip, said aluminum strip and said heating resistancetogether and in contact at one end.

9. In a thermal switching device, a bimetallic strip, an electrical heating resistance disposed adjacent and parallelto said bimetal strip for heating said bimetal strip, and a strip of conducting material having an insulating surface, interposed betweenrsaid bimetal strip and said heating resistance, means for rigidly mechanically connecting said bimetal strip and said conducting strip in contact together at one end, and the other end of said conducting strip being movable relative to the other end of said bimetal' strip.

l0. In a thermal switching device, a bimetallic strip, an electrical heating, resistance disposed parallel to and adjacent said bimetal strip for heating said bimetal strip, a strip of conducting material having a verythin electrically insulating surface, interposed between said heating resistance and said bimetal strip and disposed with as insulaires starac@ is contact with esca for conducting heat trom said heating resistance to said bimetal strip, and a strip of heat insulating material on the side of said heating resistance opposite the biinetal strip. y

li.. In a thermal switching device, a, bimetallic strip, an. electrical heating resistance disposed parallel to and adjacent said bimetal strip for heating saidloirnetal strip, an aluminum strip having oxidized surfaces, interposed between said heating resistance and said bimetal'strip or'conducting heat from said heating resistance to-said hirnetal strip, a strip of heat insulating inaterial on the side of said/heating resistance oppo site the bimetal strip, means rigidly mechanically connecting said bimetal strip, said heating resistance and said heat insulating strip together at one end, and means rigidly mechanically connecting only the other ends of said bimetai strip and said heating resistance together with oneend of said aluminum strip, said last mentioned mechanical connection including a clamping strap for engaging the free end of said heat insulating strip to hold said end of said strip in position' against said heating resistance.

l2. in a thermal switching device, a. strip ci bimetallic material, an electrical heating resists ance for heating said strip, an aluminum strip having oxidized surfaces, interposed between said resistance and said bimetal strip for conducting heat from said resistance to said bimetal strip, means mechanically securing said elements togather to form a unit and a pair of spaced bearing means engaging the ends of said unit for supporting said unit, at least one of said bearing meansbeing resiliently mounted to produce a' reaillent force on the end of said unit acting in the direction of the longitudinal axis of the unit, and means for adjusting the position of one of said bearing means in a direction substantially perpendicular to the longitudinal axis of the bimetallic strip. Y

i3. in a thermal control device, a strip of bimetallic material, a pair of spaced bearing means cngagingthe ends of said strip for supporting said strip, atleast one of said bearing means having a resilient construction for producing a reaillent iorce on the end of said strip in the direcl @essere tion of theflonaitudinal axis ci said strip to prec vent said clrnetal strip from falling out of its bearinss, said force being insumcient to produce any substantial exing of said bimetal strip and offering no substantial resistance to the bending oi said strip when the strip is heated.

la. In a thermal control device, a'strip of bi metallic material, supporting means for said strip comprising a rigid bearing having a notch in which one end of said strip is engaged, and a y resilient hearing means engaging the other end of ,metallic material having a, notch in one end the strip for varying the trip characteristic of the device, and the other of said bearing means having a resilient construction for producing a resillent force on the end of said strip in the direction of the longitudinal axis or said strip toward said adjustable bearing means.

i6. In a thermal switching device, a strip of bithereo, a bearing engaging the end o said strip opposite the notched end and a resilient bearing engaging the notched end of Said strip for producing a resilient force on the end of said strip in the direction of the longitudinal axis of the strip,

to prevent said bimetal strip from falling out of its bearings, said force being lnsumcient to produce any substantial flexing of said bimetai strip and offering no substantial resistance to the bending of said strip when the strip is heated, 'said resilient bearing comprising a, spring clip'having a reduced end portion engaged in said notch and an inwardly bent end on the reduced portion for preventing lateral displacement of the notchedy end o -said strip.

HANS H. WIIlrii/ANN.