US3229064A

US3229064A - Dual bimetal switches for electrical heaters

Info

Publication number: US3229064A
Application number: US164958A
Authority: US
Inventors: Laing Nikolaus
Original assignee: LAING VORTEX Inc
Current assignee: LAING VORTEX Inc
Priority date: 1961-01-07
Filing date: 1962-01-08
Publication date: 1966-01-11
Anticipated expiration: 1983-01-11
Also published as: DE1413910A1; GB996882A

Description

N. LAING 3,229,064

DUAL BIME'IAL SWITCHES FOR ELECTRICAL HEATERS Jan. 11, 1966 4 Sheets-Sheet 1 Filed Jan. 8, 1962 Jan. 11, 1966 N. LAlNG 3,229,064

DUAL BIMETAL SWITCHES FOR ELECTRICAL HEATERS Filed Jan. 8, 1962 4 Sheets-Sheet 2 if j? p/ Jan. 11, 1966 N. LAING 3,229,064

EATERS Filed Jan. 8, 1962 4 Sheets-Sheet 5 N. LAING 3,

DUAL BIMETAL SWITCHES FOR ELECTRICAL HEATERS Jan. 11, 1966 4 Sheets-Sheet 4.

Filed Jan. 8, 1962 LVEUIOQ W mam/5 vs United States Patent J s 3 Claims. (Cl. 200-122) This invention relates to thermal switches.

Cheap thermal switches sometimes fail to operate: in some applications failure to operate could have disastrous consequences. Now to improve. reliability a precision-built thermal switch can be used, but in applications where cheapness is a paramount consideration this again has its disadvantages.

The object of the invention is to provide a thermal switch which while it can be made with little more expense than cheap switches commonly in use, gives a greatly improved reliability.

The invention accordingly provides a thermal switch comprising at least two bimetallic strip portions, a set of contacts associated with each such portion which sets of contacts are connected or adapted to be connected in series.

The bimetallic strip portions may be provided by one single bimetallic strip encastered at a middle section thereof. Alternatively the bimetallic strip portions may be separate and spaced apart so as to be sensitive to adjacent environments. In this case each bimetallic strip portion may be set to operate when the temperature reaches a given value in its environment; however if one fails to operate at the given value and the temperature of its environment continues to change, the other will be affected before any danger arises.

It will be seen that if (say) the chance of one bimetallic strip portion failing to operate is 10 the chance of both failing is 10'". Even greater security can be obtained with additional bimetallic strip portions and series-connected sets of contacts, but for most purposes two strip portions and contact sets will suffice.

Various embodiments of the invention will now be described by way of example with reference to the accon1- panying drawings, in which:

FIGS. 1, 2 and 3 are respectively a sectional plan and two other sectional views of a thermal cut-out, the section planes of FIGS. 2 and 3 being shown by the lines 2a2a and Zia-3a respectively in FIG. 1;

FIG. 4 is a perspective view of a heater having a further form of thermal cut-out associated therewith;

FIG. 5 is a side elevation of part of the FIG. 4 heater and cut-out;

FIG. 6 is a partial perspective view of a further heater and cut-out, and

FIGS. 7, 8 and 9 are views of yet another heater and cut-out, FIG. 7 being a longitudinal vertical section, FIG. 8 a partial side elevation of one end portion of the heater and cut-out with parts removed, and FIG. 9 an exploded partial perspective of that end portion, shown on a greater scale than FIGS. 7 and 8.

Referring to FIGS. 1 to 3 of the drawing, the thermal cut-out there shown comprises an insulating rectangular base 1 with an integral flange 2 along one long side. A single bimetallic strip 3 is clamped against a spacer 4 located at the mid-point of the base 1 by means of two rivets 5 which pass through the strip 3 and spacer. The bimetallic strip 3 is thus divided into two equal strip portions 3a, 1% which are supported as cantilevers and are capable of acting independently. Adjusting screws 6a, 6b extend through threaded inserts 7a, 7b set in the base 1 3,229,064 Patented Jan. 11, 1966 equidistantly on either side and press against the strip portions 3a, 3b respectively. The ends of the bimetallic strip 3 are turned up away from the base 1 to form flanges 8a, 8b: the height of the flanges increases going towards the flange 2 on the base 1.

A single leaf spring 10 having the form of a bent rectangular strip is clamped over its mid-portion to a thickened middle portion 11 of the flange 2 on the base 1 by two rivets 12: the leaf spring is thus divided into two equal and similar portions 10a, 10b which are supported as cantilevers and are capable of acting independently. The ends 13a, 13b of the leaf spring 10 extend beyond the base 1: adjacent the ends 13a, 13b strip spring portions 10a, 10b carry moving contacts 14a, 14b cooperating with fixed contacts 15a, 15!) mounted on the flange 2 of the base 1 near the ends thereof.

In use of the cut-out, a circuit is completed through a current source 16, a utilization device 17 such as a heater, the contacts 15a, 15b and the leaf spring 10, the contact sets 14a, 15a and 14b, 1519 being series-connected.

In the normal functioning of the cut-out, the portions of the leaf spring 10 adjacent its ends 13a, 13b contact the abutments formed by the adjacent ends of the flanges 8a, 8b: the spring portions 10a, lllb are thus kept stressed and the contacts of each contact set 14a, 15a and 14b, 15b are maintained in contact-making relation. When the environment heats up beyond a predetermined temperature, the bimetallic strip portions 3a, 3b will flex towards the base 1 whereby the ends of the flanges 8a, 8b will clear the leaf spring portions 10a, 1012, which, are no longer constrained, revert to an unstressed position wherein the contacts of the contact sets 14a, 15a, 14b, 1512 will separate to break the circuit. It will be seen that so long as contact is broken at one or other of the contact sets 14a, 15a; 14b, 1512 the utilization device 17 will be on open circuit.

Unce contact is broken at either contact set 14a, 15a; 14b, 15b it will remain broken, even if the temperature drops again, until the ends 13a, 13b of the leaf spring are pushed towards the fixed contacts 15a, 15b, e.g. by manually controlled push rods (not shown): the end portions of the leaf spring then ride over the sloping edge of the flanges 8a, 8b on the bimetallic strip 3, thereby camming the ends of the strip resiliently towards the base 1, until the initial position is once again reached when the flanges 8a, Sb snap upwardly once more to hold the leaf spring portions 10a, ltlb with the movable contacts 14a, 14b against the fixed contacts 15a, 15b.

FIG. 4 shows a heater in the form of an insulating base 20 and a horizontal rectangular support strip 21 carrying insulating strips 22 along either long edge, these insulating strips being notched at regular intervals as shown at 23 to receive the convolutions of a helically coiled resistor strip 24; the resistor strip 24 is more or less rigid and has its ends 25, 26 fixed to a base 20. Thus the resistor strip helix mounts the support and

insulating strips

21, 22 by reason of the engagement of the latter with the convolutions of the helix, the

strips

21, 22 helping to rigidity the helix for the same reason.

A bimetallic strip 27 overlies the support strip 21 and is secured thereto by a pair of rivets 28 passing through a central portion 29 of both strips: at either side of the central portion 29 the bimetallic strip 27 is cranked away from the support strip as indicated at 30. The bimetallic strip 27 is thus divided symmetrically into two portions 27a, 27b supported cantilever-fashion and capable of functioning separately.

The ends 31a, 31b of the bimetallic strip 27 are bent downwardly and inwardly and in the normal position of the arrangement (shown in FIG. 4 and in full lines in FIG. 5) engage the free ends of leaf springs 32a,

3% the other ends of which are clamped against the ends of the base by screws 33. The leaf springs 32a, 32b carry moving contacts 34 cooperating with fixed contacts 35a, 35b secured on conductive brackets 36a, 36b fixed to the base 20, the same screws 37a, 37b as secure the ends 25, 26 of the helix serving also to secure the brackets. The engagement of the bentdown ends 31a, 31b of the bimetallic strip 27 stress the leaf springs 32a, 32b and hold the contacts of the

contact sets

34, 35 in contacting relationship. When the temperature rises the bimetallic strip portions 27a, 27b flex away from the support strip 21 until one or other of the ends 31a, 31b of the bimetallic strip clears the end of the corresponding leaf spring 32a, 32b and thereby releases it: the leaf spring then springs away from the bimetallic strip and breaks contact between the contacts of the corresponding contact set. This operated position of the bimetallic strip portion 27a and leaf spring 32a is illustrated in dotted lines in FIG. 5. The leaf springs 32a, 32b and the ends 31a, 31b of the bimetallic strip 27 are shaped so that by pressing the spring inwardly from its released position its free end cooperates with the end of the bimetallic strip to push up the latter by carnining action until the end of the strip is able to drop down behind the spring to retain it in the normal position previously described.

An electric power source 38 is connected across the leaf springs 32a, 32b so that in the normal position of the apparatus current is transmitted to the resistor strip 24 through the leaf springs 32a, 3212, the contact set 34, 35 and the bracket 36a, 36b; separation of the contacts of either contact set suflices to open-circuit the helix.

It is assumed that the bimetallic strip 27 is of high resistance; alternatively its ends 31a, 31b can be insulated or it can be formed in two parts.

FIG. 6 shows a further form of heater comprising a resistor strip Wound into a helix: the strip is here designated 40 and supported on a pair of more or less

parallel support strips

41, 42 having insulating edge portions notched at 43 to receive the convolutions of the helix. Unlike the previous embodiment the

support strips

41, 42 are mounted between plate-like end members 44 which are secured to a base (not shown).

The upper support strip 41 mounts at each end a bimetallic strip 45 extending generally parallel thereto: one end portion of each bimetallic strip 45 is riveted to the support strip 41 at 4-6 and the strip 45 is cranked as shown at 47 so that the remainder thereof lies spaced from the support strip. The free end of the bimetallic strip 45 extends through a window 48 in the adjacent end member 44 and is turned down to form a sloping flange 49.

A U-shaped spring 50 has the extremity of one arm secured to each end member 44- at 51, while the extremity 52 of the other arm 53 is outwardly bent to cooperate with the abutment provided by the larger end of the flange 49 on the corresponding bimetallic strip 4-5. The spring 50 is mounted to flex in a plane parallel to the end member 44, while the strip 45 is arranged to move in a plane perpendicular thereto: in the normal positions shown the flange 49 keeps the spring 50 stressed whereby to maintain contact between a movable contact 55 on the spring arm 53 and a fixed contact 56 mounted on the end member 44.

A leaf spring 57 having one end encastered in the end member 44 extends into the window 48 and with its free end 58 contacts the top of the bimetallic strip 45 to urge it downwardly. An adjusting screw 59 allows the spring pressure on the strip 45 to be adjusted.

The springs 50 at the two ends of FIG. 6 apparatus are connected to corresponding ends of the helix 40, and the fixed contacts 56 are connected across an electrical power source (not shown). When the temperature rises above a pre-set value the strips 45 flex upward so that the ends of the flanges 49 clear the extremities 52 of the springs 50 and allow the latter to spring open, thus break- 4 ing contact between the contacts of the two sets thereof 55, 56.

When the cut-out has operated it can be returned to its normal condition by manually or otherwise moving the spring arm 55 towards the other arm. The extremity 52 rides up the sloping flange 49 on the bimetallic strip 45, thereby displacing it upwards, until the extremity has passed beyond the flange whereupon the latter snaps down to hold it in the position illustrated ready to operate again.

FIGS. 7, 8 and 9 show an assembly including a heater similar to that in FIG. 6; similar parts are given the numerals used in that figure and will not need further description.

The end members 44 in the present embodiment are substantially rectangular, and are recessed from the top at 69 to provide a seat 61 for the corresponding extremity 62 of the upper support strip 41 which in this instance is of dished section. An insulating spacer 63 is received in the recess 66 on top of the extremity 62 and is shaped to mate therewith. A spigot 64 upstanding centrally from the bottom of the recess 66 extends through a mating hole 65 in the extremity 62 and into a recess (not shown) in the bottom of the spacer 63. The spacer 63 carries an integral upstanding spigot 66 aligned with the spigot 64 which in turn extends through a hole 6'7 in a bimetallic strip 6% and into an insulating retainer element 69. Aligned central vertical bores in the element 6 spigot 66 and spacer 63, spigot 64 and end member 44 terminating in a slot 79 receive a clamping bolt 71 which is tightened into a nut 72 located in the slot 70, whereby to hold the assembly rigidly in position.

The bimetallic strip 68 has an integral terminal lug 74 extending to the side of the end member 44 away from the helix 40. The part of the strip 68 on the side of the helix 40 is formed with a U-shaped cut-out 75 whereby to define an internal tongue 76. The free end of the tongue carries a moving contact 77 which cooperates with a fixed contact '78 on the support strip 41 connected to one end of the helix. In the normal (FIG. 7) position of the bimetallic strip the tongue 76 presses the contact 77 firmly against the contact 78 by reason of the springy nature of the strip.

The terminals '74 at opposite ends of the apparatus are connected to an electric power source (not shown). The helix 4% is thus connected through the bimetallic strips 63 and the two contact sets 77, 78. When the temperature rises above a preset value the tongue 76 of each bimetallic strip snaps up to the position shown in FIG. 8, thereby open-circuiting the helix 40. When the temperature drops again the contacts 77, '78 close automatically.

The heater and cut-out assemblies of FIGS. 4 to 9 are particularly suitable for use with cross-low blowers (see for example British Patent Specification 876,611) in which case the blower rotor and helix are equal in length and arranged opposite one another. In such a combination heat dissipation from the helix is dependent on the flow therethrough of air from the blower: it is usually considered best for the heater to be disconnected as soon as any part approaches the temperature when it would glow red but the apparatus will only be dangerous at a somewhat higher temperature. Each bimetallic strip portion will normally operate when the temperature at the adjacent end of the helix reaches the preset value. In the worst condition possible flow adjacent one end will be blocked while flow at the other end is normal, and the bimetallic strip portion at the first end will fail. Only slight further heating at the end will produce a rise in temperature at the other end that will operate the bimetallic strip portion thereof.

1 claim:

1. Apparatus comprising an electric heating element extending over an area and thermal protective switch means therefor to interrupt flow of current through the element on overheating thereof; the thermal protective means comprising two pairs of Contacts series connected with each other through the heater element, and a first bimetallic strip and a second bimetallic strip located adjacent said heater element and spaced from one another over said area, each bimetallic strip controlling one pair of contacts and operative to open a pair of contacts when the temperature of the respective strip rises above a preset value.

2. Apparatus as claimed in claim 1, wherein the heating element surrounds the major part of each bimetallic strip.

3. Apparatus as claimed in claim 1, wherein the heating element includes a resistance element and support means therefor comprising first and second brackets, said pairs of contacts being located one at the first bracket and the other at the second bracket, one bimetallic strip being mounted adjacent the first bracket. with a free end terminating at this point and controlling the pair of contacts at that bracket, and the other bimetallic strip being mounted adjacent the second bracket with a free 6 end terminating at this point and controlling the pair of contacts at the second bracket.

References Cited by the Examiner UNlTED STATES PATENTS 831,847 9/1906 Erickson 200-122 1,323,477 1/1920 Baker 200122 1,435,580 3/1924 Zimmerman 200-116 1,550,641 8/1925 Whittingham 200122 1,678,401 7/1928 Luttrell 200-122 1, 48,938 2/1934 Lawton 200-122 2,016,244 10/1935 Gregory 200-l22 2,414,531 1/ 1947 Johns 200122 2,478,541 8/1949 McCracken 200-122 2,528,591 11/ 1950 Getchell 200-122 FOREIGN PATENTS 418,810 10/1934 Great Britain.

BERNARD A. GLLHEANY, Primary Examiner.

Claims

1. APPARATUS COMPRISING AN ELECTRIC HEATING ELEMENT EXTENDING OVER AN AREA AND THERMAL PROTECTIVE SWITCH MEANS THEREFOR TO INTERRUPT FLOW OF CURRENT THROUGH THE ELEMENT ON OVERHEATING THEREOF; THE THERMAL PROTECTIVE MEANS COMPRISING TWO PAIRS OF CONTACTS SERIES CONNECTED WITH EACH OTHER THROUGH THE HEATER ELEMENT, AND A FIRST BIMETALLIC STRIP AND A SECOND BIMETALLIC SRTIP LOCATED ADJACENT SAID HEATER ELEMENT AND SPACED FROM ONE ANOTHER OVER SAID AREA, EACH BIMETALLIC STRIP CONTROLLING ONE PAIR OF CONTACTS AND OPERATIVE TO OPEN A PAIR OF CONTACTS WHEN THE TEMPERATURE OF THE RESPECTIVE STRIP RISES ABOVE A PRESET VALUE.