US2825785A - Thermally controlled electric switches - Google Patents

Description

March 4, 1958 E. H. TAYLOR 2,825,785

THERMAL-LY CONTROLLED ELECTRIC SWITCHES Filed April 2, I954 2 Sheets-Sheet 1 F/ G. 3 g 6 g 7 wins Inventor A Horn e y March 4, 1958 E. H. TAYLOR 2,825,785

THERMALLY CONTROLLED ELECTRIC SWITCHES Filed April 2, 1954 2 Sheets-Sheet 2 Inventor M A a. M

A llorney United States Patent Ofifice 2,825,785 THERMALLY CONTROLLED ELECTRIC SWITCHES Eric H. Taylor, Ryecroft, The Park, Buxton, England Application April 2, 1954, Serial No. 420,645 15 Claims. (Cl. 200-113) This invention relates to thermally controlled electric switches, and more particularly to such a switch which may be used as an overload circuit breaker in circuits where only a comparatively small operating current is available, such as for instance one ampere.

The present invention is concerned with an electric switch of the type disclosed in my United States Patent 2,503,008. In this patent there is described and claimed a thermally controlled electric switch comprising a fixed contact and a movable contact, a bimetallic member controlling movement of said movable contact with respect to said fixed contact, a diiferentially stressed switch actuating plate and means for stressing said plate across its width so as to buckle it in two directions at right angles, said plate being mounted for movement between two stable positions through an unstable position, said bimetallic member being operatively associated with said plate, and said fixed contact constituting an abutment against which said bimetallic member reacts and causes said plate to move in a switch operating direction when a predetermined temperature change occurs, said movable contact being urged against said fixed contact in a direction opposite to that in which it is ultimately moved by the movement of said plate through its unstable position.

The term a differentially stressed system as used in my earlier patent means a flexible strip or plate held in compression by stresses applied across its width so that the strip or plate will be caused to buckle out of its original plane in two directions, e. g. across its breadth and along its length, thus forming a system which has a stable position on each side of an unstable position through which it passes when moving from one stable position to the other. As used herein the term differentially stressed system means an elongated strip like member having the above mentioned characteristics.

In my prior patent a preferred form of switch is described which includes a bimetallic member generally in the form of a U, with a third leg projecting between the legs of the U, said third leg carrying the movable contact. Electric current in this form passes through both legs of the U and along the third leg to said contact. While such an arrangement has been satisfactory as a thermostat and/ or as an overload circuit breaker of currents more than 5 amps and other uses, it has been found that it cannot be conveniently applied as a circuit breaker in circuits carrying currents of lower value, and it is therefore an object of the present invention to provide a thermally controlled switch which, while operating in similar manner so that of switches according to U. S. Patent No. 2,503,008, is yet improved in that inter alia it will operate on smaller currents.

According to the invention, therefore, there is provided a thermally controlled electric switch, comprising a fixed contact and a movable contact, a movable contact member controlling movement of said movable contact with respect to said fixed contact, an elongated bimetallic actuating strip carrying said contact member at one r 2,825,785 Patented Mar. 4, 1958 end thereof and means for supporting said actuating strip at at least two spaced points along its length which serve also for ditferentially stressing said strip so as to buckle it between said spaced points in two directions at right angles, said strip being movable between two stable positions through an unstable position, said fixed contact constituting an abutment against which said contact member reacts and causes said elongated strip to move in a switch operating direction when a predetermined temperature change occurs, said movable contact being urged against said fixed contact in a direction opposite to that in which it is ultimately moved by the movement of said strip through its unstable position.

Switches according to the present invention therefore have the advantages of those of my Patent No. 2,503,008 in that the contact pressure increases as the switch is about to open, and thereby arcing and chattering at the contacts is avoided. The use of an elongated striplike bimetallic member gives however, added sensitivity so that switches according to the present invention may be used in low current circuits.

The said strip-like bimetallic member may take various forms, e. g. a circular arc, or two sides of a triangle, or three sides of a rectangle. This member is stressed so that its unstable position corresponds to its unstressed plane condition while in each of the two stable positions the strip in cross-section is inclined to the plane it occupied when unstressed. For example, in a preferred form the strip is semi-circular and when stressed tends to the shape of an Eton collar, movement from one stable position to the other corresponding to the collar being turned inside-out.

The contact member normally is in the form of an arm with one end secured to or integral with the bimetallic member, and is so related to said member that the movable contact carried by said contact member, normally at its other end is moved substantially normal to the plane of the unstressed strip whenever said strip moves from one to the other of its stable positions. Where the strip is inthe form of a circular are, for example, the contact member may be an arm extending along a radius of such arc.

Whilst in a preferred embodiment of the invention the strip is of a semi-circular form, it may be shaped to constitute the greater part of a closed figure, e. g. of a circle or triangle and then the straight line joining the movable contact to the junction of the contact member and the strip should project into or cross the enclosed area of such figures. The required stress may be produced in the strip by providing a fixed support at one end and a second support, spaced from the fixedly secured end and so positioned that it is outside the closed figure and compresses the strip towards the inside of such figure. Such second support however, may allow angular movement of the strip and for this purpose conveniently may include an aperture to receive a projection on the outside edge of the strip. Variation of stress in the strip may be provided for by making such second support adjustable.

The adjustment of said second support may be accomplished in a number of ways, thus in a simple form a plain upright member may be used, and may be bent to various angles to the vertical to give various degrees of stress in the strip. It is preferred however, to provide some more exact means of adjustment, and in one preferred form a plain upright may be carried on an intermediate portion of an arm movable angularly in a horizontal plane about one of its ends, and its other end may be connected to one end of a spring whose stress may be varied e. g..by providing a tension spring with a plurality of alternative anchorages for its other end.

For many purposes adjustment of the stress .-need not be allowed for, and then for example the strip may be carried on fixed supports riveted to a base plate in positions predetermined to give a desired stress. If considerable resilience is required in one of the supports it naay comprise an upright strip double back upon itself 111 a narrow U or hairpin shape.

Normally the contact member is made of similar bimetallic material to that of the strip-like member.

In order that the invention may be well understood, preferred embodiments thereof will now be described, by way of example only, referring to the accompanying drawings in which:

Figure 1 is a plan view of a switch embodying the invention;

Figure 2 is a cross-section on line IIII of Fig. 1;

Figure 2A is a detail view of a modified form of the switch shown in Figure 2;

Figures 3 and 4 show detail modifications;

F gure 5 is a plan view of a further form of switch according to the invention;

igures 6 and 7 are perspective views of further forms of switch according to the invention;

Figure 8 is a perspective view of a yet further form of switch; and

Figure 9 is a detail view of a modification of the switch of Figure 8.

Referring first to Figures 1 and 2 of the drawings, a thermally controlled switch includes a semi-circular strip 3. of bimetal construction, one end of which is fixedly secured to a support 2 mounted on a suitable base-plate 3 of lnsulating material, said support 2 comprising a rivet extending through the base-plate 3 and the strip 1 w th a spacer washer 4 between the strip 1 and baseplate 3. At the other end of the strip a projection 5 on the outside edge of the strip 1 is received in an aperture or a depression in a second support 6 upstanding from the base-plate 3, so that the strip 1 is supported parallel to and spaced from the base-plate. Said second snpport 6 is conveniently an angle-bracket of metal strip riveted to the base-plate, and the projection 5 and coopcrat ng aperture or depression are arranged not to prevent angular movement of the bimetallic strip 1 relative to the support. It will be seen best from Fig. 2, that the support 6 comprises a strip bent back upon itself in a hairpin shape, this providing a more resilient support for the strip 1. The base-plate 1 is notched as shown in Figs. 1 and 2 to receive the bent portion of the support 6 so that the latter does not project unduly far above the base-plate 3, although where this consideration is not material, or it is preferred not to notch the baseplate, the bend may be at the top of the support, and this form is illustrated in Fig. 3.

In either case the support 6 is fixedly secured to the base-plate 3 by a rivet 7, an end portion 6a of the support 6 being bent horizontally for this purpose. The stress applied to the strip may be varied by bending the support 6 relative to its fixed end portion 6a.

The support 6 is slightly closer to the rivet 2 than the spacing of the projection 5 at the riveted end of the bimetallic strip 1 when unstressed, and thus the strip is stressed and distorted out of its original plane by pressure of support 6 on projection 5, said strip becoming a differentially stressed system in which the unstable position corresponds to the original plane of the strip, and in each stable position its surfaces have the form of sections of the surface of a cone i. e. similar to an Eton collar.

At the same end of the strip 1 as is said projection 5, a straight arm 8 integral with the semi-circular strip 1 extends from the strips inner edge along a radius thereof, and at its end, slightly past the centre of said semi-circle, carries a contact 9 facing the base-plate. A further contact 10 is secured to the base-plate in register with the contact 9 on the arm and normally said two contacts 4 9, '10 are touching, the movable contact9 being urged against the fixed contact 10 by the force arising from the stressing of the strip 1. This force may be varied by moving the support 6, either bodily or by bending.

The fixed contact 10 is the head of a rivet extending through the base-plate 3, and such rivet serves also to hold a connecting tag 11 against the underside of the base-plate. A similar tag 12 is held by the rivet 2 and the two tags 11, 12 provide connecting means whereby the switch can be connected in an external circuit. The tags 11, 12 extend past the rivets, 2, 10 and have small projecting tabs 13 at their ends bent upwardly into small locating holes in the base-plate 3, so that said tags are restrained from moving angularly on their rivets to touch one another and thus short-circuit the switch.

It will be understood that the bi-metal strip 1 is of laminated form consisting of two component strips of identical form secured together in superposed relation, the component strips being of different metals having diiferent heat-expansion properties. Since the bi-metal strip is made in semi-circular form, it may besaid to be bowed in the plane of lamination.

The semi-circular strip of bimetal is mounted with its high-expansion side remote from the baseplate 3 and the fixed contact 10 and thus upon rise of temperature extra stress will be produced and will be relieved ultimately by the strip moving to its other stable position, with the high-expansion face on the outside of the conical curvature. The contact-carrying arm will, however, as the temperature rises, e. g. upon passing an electric current through the strip and arm, tend to bend to urge the movable contact downwards, but as such movement is prevented by the fixed contact, the contact pressure will be increased and the reaction from such pressure will tend to urge the strip to its other stable position.

A stop member 14 of substantially Zshape is riveted to the base-plate 3 and overhangs the movable contact 9, so as to restrict movement of said movable contact away from the fixed contact. When heating of the strip 1 causes such movement of the movable contact, the latter comes to rest against the stop member 14, and when the strip cools the stop member acts as an abutment against which the movable contact may cause or assist return of the stressed system, i. e. the strip, to its original position. If desired the stop member may also carry a contact 14a so that the switch performs a change-over instead of merely a break operation as the strip is heated as shown in Figure 2A.

An alternative mounting for one end of the strip 1, to replace the support 6, is shown in Fig. 4, and comprises merely a stud 15 over which the strip may be pressed, an aperture being provided in the strip to receive said stud. The said stud has a wider base portion 15a to bear against the base-plate 3, and a narrow por tion which in use extends through the base-plate, its end being threaded to receive a securing nut 16. A groove 17 is provided in the stud to locate the strip 1 and the aperture in the strip is of greater diameter than the stud so that only one edge of this aperture, normally the edge nearer the arm 8, is in contact with the stud and the strip may thus rock from one stable position to the other without restraint by the stud. With this form of support, no projection 5 (Fig. 1) is needed on the strip 1.

In Figure 5, there is illustrated a modified form of switch which, while in many respects similar to the switch of Figs. 1 and 2, has an arrangement to facilitate adjust ment of the stress of the strip.

In this form an upright 18 receives a projection 19 from a bimetallic strip 26, but the upright 18 is integral with a horizontal arm 21 which is angularly movable over the face of the base-plate. The said upright 18 extends from approximately the mid-point of the arm 21, and the latter has one end pivotally secured to the base-plate by a rivet 22 while its other end is associated with a tension spring 23. Said spring has hooked ends one of which is engaged with the arm 21, being located in a notch 21a therein. The other end of said spring may be placed in any desired one of a series of notches 24 in the far side of the base-plate, the notches 24 being of different depths whereby a plurality of alternative anchorages are provided and the stress applied to the strip through the arm 21 and upright 13 may be varied according to which is chosen.

Various changes and modifications may be made in the details described without departure from the scope of the invention.

If desired, the support 6 of Figs. 2 and 3 may be employed in conjunction with the spring biased arm 21 of Fig. 5 and this combination is illustrated in Figs. 6 and 7, respectively.

In Fig. 8 there is shown a form of the invention in which the strip 1 comprises nearly a complete circle. One end of the strip is rigidly mounted on the support 2 and a second support 24 positioned outside the circle of the strip engages the other end thereof. The support 24 is mounted on the pivoted arm 21 which is biased by the spring 23 so as to urge the support 24 in the direction of the fixed support 2 and thus stress the strip 1 in the required manner. The straight arm 8 extends radially inwardly of the strip and carries a contact 9 for engagement with the contact 10.

In Fig. 9 the strip 1 is in the shape of a triangle adapted for mounting in the manner shown in Fig. 8. With the strips shown in Figs. 8 and 9 it is found desirable to provide a third support comprising a projection 25 disposed at the outside edge of the strip for engagement with a fixed aperture 26.

I claim:

1. A thermally controlled electric switch, comprising a fixed contact and a movable contact, a movable contact arm controlling movement of said movable contact with respect to said fixed contact, an elongated bimetallic actuating strip having a bowed form in its plane of lamination and carrying said contact arm at one end thereof and extending laterally therefrom, means mounting said one end of said strip to permit tilting of the end portion of the strip about its longitudinal axis, means rigidly supporting the other end of said actuating strip in a fixed plane, means for subjecting said strip to compression between two spaced points along its length so as to buckle it between said spaced points in two directions at right angles, said strip being movable between two stable positions through an unstable position and causing said contact arm to tilt to opposite sides of said plane of lamination, said fixed contact constituting an abutment against which said contact arm reacts and causes said elongated strip to move in a switch operating direction when a predetermined temperature change occurs, said movable contact being urged against said fixed contact in a direction opposite to that in which it is ultimately moved by the movement of said strip through its unstable position.

2, A thermally controlled electric switch, comprising a fixed contact and a movable contact, an elongated bimetallic actuating strip of a shape in its plane of lamination to constitute the greater part of a closed figure, means rigidly clamping one end of said strip in a fixed plane, means at at least two spaced points along said elongated strip for supporting and for differentially stressing said strip so as to buckle it between said two points in two directions at right angles said strip and being movable between tWo stable positions through an unstable position, and a contact arm connected to the free end of said strip and extending laterally therefrom, said contact arm carrying said movable contact, the point of connection of said contact arm to said strip and said movable contact being relatively so positioned that a straight line joining them is directed across the enclosed area of said figure, said fixed contact constituting an abutment against which said contact arm reacts and causes said elongated strip to move in a switch operating direction when a.

6 predetermined temperature change occurs, said movable contact being urged against said fixed contact in a direction opposite to that in which it is ultimately moved by the movement of said strip through its unstable position.

3. A thermally controlled electric switch as set forth in claim 1 in which said actuating strip is in the form of a semi-circle, said contact arm extending along a radius thereof.

4. A thermally controlled electric switch, comprising a fixed contact and a movable contact, a movable contact arm controlling movement of said movable contact with respect to said fixed contact, an elongated bimetallic switch actuating strip having a bowed form in its plane of lamination, a fixed support rigidly mounting one end of said strip, an abutment spaced from said support, means providing a freely pivotable contact between the other end of said strip and said abutment, said contact arm being connected to said strip adjacent its pivoted end and extending laterally therefrom, means subjecting said strip to compression between said abutment and said fixed support so as to buckle said strip therebetween in two directions at right angles, said strip then being movable between two stable positions through an unstable position, said fixed contact constituting an abutment against which said contact member reacts and causes said elongated strip to move in a switch operating direction when a predetermined temperature change occurs, said movable contact being urged against said fixed contact in a direction opposite to that in which it is ultimately moved by the movement of said strip through its unstable position.

5. A thermally controlled electric switch, comprising a fixed contact and a movable contact, an elongated bimetallic switch actuating strip of a shape to constitute the greater part of a closed figure, two spaced supports for carrying said strip said supports being arranged so as to compress the ends of said strip towards one another thereby buckling said strip therebetween in two directions at right angles, said strip then being movable between two stable positions through an unstable position and a contact arm connected to one end of said strip adjacent one of said supports, said contact arm carrying said movable contact, the point of connection of said contact arm to said strip and said movable contact being relatively so positioned that a straight line joining them is directed across the enclosed area of said figure, said one support embodying means permitting the end portion of said strip carrying said arm to tilt about its longitudinal axis, said fixed contact constituting an abutment against which said contact arm reacts and causes said elongated strip to move in a switch operating direction when a predetermined temperature change occurs, said movable contact being urged against said fixed contact in a direction opposite to that in which it is ultimately moved by the movement of said strip through its unstable position.

6. A thermally controlled electric switch, comprising a fixed contact and a movable contact, an elongated bimetallic switch actuating strip of a shape to constitute the greater part of a closed figure, a fixed support for said strip adjacent one end thereof and a second support for said strip spaced from said fixed support and positioned outside said closed figure, said second support providing a pivoted connection with said strip allowing the adjacent portion of the strip to tilt about its longitudinal axis, said supports serving to compress the ends of said strip towards one another thereby buckling said strip therebetween in two directions at right angles, said strip then being movable between two stable positions through an unstable position, and a contact arm connected to one end of said strip adjacent said second support, said contact arm carrying said movable contact, the point of connection of said contact arm to said strip and said movable contact being relatively so positioned that a straight line joining them is directed across the enclosa area of said figure, said fixed contact constituting an abutment against which said contact arm reacts and causes said elongated strip to move in a switch operating direction when a predetermined temperature change occurs, said movable contact being urged agaihst said fixed contact in a direction opposite to that in which it is ultimately moved by the movement of said strip through its unstable position.

7. A thermally controlled electric switch as set forth in claim 6 in which said second support is adjustable whereby the stress in said strip may be varied.

8. A thermally controlled electric switch, comprising a fixed contact and a movable contact, a semi-circular bimetallic switch actuating strip, an aperture in said strip adjacent one end thereof, a first support for said strip, a strip supporting stud spaced from said first support, said stud being received in said aperture defined in said strip and being of less diameter than said aperture so that only one edge of said aperture is in contact with said stud and a contact member connected to said strip and controlling movement of said movable contact with respect to said fixed contact, said strip being supported between said first support and said stud and differentially stressed so that it is buckled therebetween in two directions at right angles, said strip then being movable between two stable positions through an unstable position, said fixed contact constituting an abutment against which said contact member reacts and causes said elongated strip to move in a switch operating direction when a predetermined temperature change occurs, said movable contact being urged against said fixed contact in a direction opposite to that in which it is ultimately moved by the movement of said strip through its unstable position.

9. A thermally controlled electric switch, comprising a fixed contact and a movable contact, a semi-circular switch actuating elongated bimetallic strip, a fixed support and a resilient U-shaped support spaced from said fixed support, said resilient support being positioned outside said semi-circular strip, said supports serving to carry said strip and to differentially stress it so as to compress the ends thereof towards one another thereby buckling said strip therebetween in two directions at right angles, said strip then being movable between two stable positions through an unstable position and a con tact member connected to said strip and controlling movement of said movable contact with respect to said fixed contact, said fixed contact constituting an abutment against which said contact member reacts and causes said strip to move in a switch operating direction when a predetermined temperature change occurs, said movable contact being urged against said fixed contact in a direction opposite to that in which it is ultimately moved by the movement of said strip through its unstable position.

10. A thermally controlled electric switch as set forth in claim 8 in which said contact member is a radial arm. integral with said strip, and carrying said movable contact and said stud is so shaped as to define an annular groove in the outer surface thereof which serves to locate said strip on said stud.

11. A thermally controlled electric switch as set forth in claim 9 in which said strip includes a projection, one arm of said U-shaped support including means for receiving said projection, and in which said contact memher is a radial arm, integral with said strip and carrying said movable contact.

12. A thermally controlled electric switch, comprising a fixed contact and a movable contact, a movable contact member controlling movement of said movable contact with respect to said fixed contact, a semi-circular elongated bimetallic switch actuating strip, a fixed support andan adjustable support for said strip, and a base-plate, said strip being carried by said two supports and differentially stressed so as to be buckled therebetween in two directions at right angles, said strip then being movable between two stable positions through an unstable position, said contact member being connected to one end of said strip, said adjustable support comprising a lever arm pivotally secured at one end to said base plate, an upright carried on said arm, and a spring secured at one end to the free end of said arm and adjustably secured at its other end to said base plate, said upright serving to support said strip, the tension therein being adjustable upon adjustment of said spring, said fixed contact constituting an abutment against which said contact member reacts and causes said strip to move in a switch operating direction when a predetermined temperature change occurs, said movable contact being urged against said fixed contact in a direction opposite to that in which it is ultimately moved by the movement of said strip through its unstable position.

13. A thermally controlled electric switch as set forth in claim 12 in which said contact arm is a radial arm integral with said strip and carrying said movable contact, the end of said spring remote from said lever arm is retained in one of a plurality of alternative anchorages whereby the stress applied to said strip may be varied according to which anchorage is utilised, and in which said strip includes a radially extending projection at one end thereof, said upright including means for receiving said projection and thus supporting said strip.

14. A thermally controlled electric switch, comprising a fixed contact and a movable contact, a movable contact arm controlling movement of said movable contact with respect to said fixed contact, an elongated bimetallic switch actuating strip having a bowed form in its plane of lamination, a fixed support rigidly mounting one end of said strip, an abutment spaced from said support, means providing a freely pivotable contact between the other end of said strip and said abutment, said Contact arm being connected to said strip at a point spaced from its rigidly mounted end and extending laterally from said strip, said fixed support and said abutment subjecting said strip to compression therebetween so as to buckle said strip therebetween in two directions at right angles, said strip then being movable between two stable positions through an unstable position, said fixed contact constituting an abutment against which said contact member reacts and causes said elongated strip to move in a switch operating direction when a predetermined temperature change occurs, said movable contact being urged against said fixed contact in a direction opposite to that in which it is ultimately moved by the movement of said strip through its unstable position.

15. A thermally controlled electric switch, comprising a fixed contact and a movable contact, an elongated bimetallic switch actuating strip having a bowed form in its plane of lamination, a first support rigidly supporting one end portion of said strip, a second support for said strip spaced from said first support and providing a pivoted connection with the other end portion of said strip allowing the adjacent portion of the strip to tilt about its longitudinal axis, said supports serving to compress the ends of said strip towards one another thereby buckling said strip between said supports in two directions at right angles, said strip then being movable between two stable positions through an unstable position, and a movable contact arm carried by said strip at a point spaced from its rigidly supported end and extending laterally from said strip, said contact arm carrying said movable contact at its outer end, said fixed contact constituting an abutment against which said contact arm reacts and causes said elongated strip to move in a switch operating direction when a predetermined temperature change occurs, said movable contact being urged against said fixed contact in a direction opposite to that in which 10 it is ultimately moved by the movement of said strip 2,3 4,347 Fetter Dec. 8, 1942 through its unstable position. 2,462,244 Wise Feb. 22, 1949 2,503,008 Taylor Apr. 4, 1950 References Cited in the file of this patent 5,62 ,819 Spina et a1. Jan. 6, 1953 p 5 ,630,504 Burch et a1 Mar. 3, 1953 1775 296 Z F STARS PATENTi 70 1929 2,777,032 Burch Jan. 8, 1957 a ge ug- 1,988,345 Vaughn Jan. 15, 1935 FOREIGN PATENTS 2,041,775 Mottlau May 26, 1936 657,434 Great Britain Sept. 19, 1951

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