US2688063A - Snap acting thermal switch - Google Patents

Description

Aug. 19154 J. R. GOMERSALL SNAP ACTING THERMAL SWITCH 2 Sheets-Sheet 1 Filed Dec. 13, 1949 R m W/ mm Ur m .4 1R. w M m. MM y 2 I 3 N W 4 I m I 2. m (K 27 BI ME 77?].

Aug. 31, 1954 J. R. GOMERSALL 2,538,063

SNAP ACTING THERMAL SWITCH Filed Dec. 13, 1949 2 Sheets-Sheet 2 IN V EN TOR. JOHN R GOMERSALL BY 0m 10- Arrk I Patented Aug. 31, 1954 SNAP ACTING THERMAL SWITCH John R. Gomersall, Elgin, IlL, assignor to McGraw Electric Company, Elgin, 111., a corporation of Delaware Application December 13, 1949, Serial No. 132,677

4 Claims. 1

My present invention relates to electric switches and particularly to thermally-responsive, snapacting, electric switches. Among the shortcomings that may appear in thermal snap switches are:

A. A tendency to dead-break, that is, to open its contacts slowly. This action is often ascribed to friction in the snap mechanism.

B. Low contact pressure before snap. This situation usually must be present to permit deadbreak. It may increase the electric resistance of the point of contact and it also makes the contacts likely to chatter if subjected to vibration. This diificulty is especially troublesome in a thermally-actuated switch because such a switch may be controlled by a slowly changing temperature, and so may be held at the low-contactpressure position for long periods of time, and may heat and cool slowly and alternately while on the verge of snapping open.

C. Excessive heating and welding of contacts. Severe cases of this trouble result from low contact pressure and from dead-break. They may also result from bouncing of the contacts during the closing operation, a disturbance that may be aggravated by the small contact travel often required for achieving a small temperature difference between on and off operations. Heating and welding are aggravated by large electric currents and high voltage.

D. Inaccurate operation, that is, failure to operate repeatedly at the same temperature. This trouble may result directly from friction but the worst effects often result from welding of the contacts, and so the trouble may be worse when the switch controls a heavy load.

It has been thought that some slight welding is necessary for establishing a low-resistance engagement between the switch contacts, and that Welding is even desirable for arresting bounce, for alleviating the efiect of low contact pressure and for reducing the tendency to dead-break; But the mechanical strengths of such welds vary from one operation to the next, so that the varying forces required for breaking them introduces another source of inaccurate operation. Thus even these slight welds can not be accepted as a cure for other troubles but must be kept small, even under heavy loads, if accuracy is to be achieved. Another source of inaccuracy may be a poor thermal response of the bimetal or other thermal actuator of the switch to the object, the temperature of which is to be controlled. Contributing factors include lack of good thermal coupling to the object whose temperature should control the switch and too much thermal coupling to ambient conditions or to other objects. Many remedies for these difiiculties have been proposed, but prior attempts have resulted in bulky, elabcrate and expensive mechanisms.

It is an object of the present invention to provide a thermally-actuated switch for controlling an electric water heater. :Such a switch must control a heavy load, as for example 10 or 25 amperes at 230 volts. It must respond to temperatures that change slowly, as for example 1 Fahrenheit in several hours, and that may alternately rise and fall slightly and slowly. It must be dependable in operation and must open and close the circuit accurately at the temperatures called for by the function it is to perform.

Another object of'my invention is to provide a snap-acting thermal switch that is simple in construction and dependable in operation.

Another object is to provide a snap-acting thermal switch having a minimum number of parts which are easily and quickly assembled and disassembled.

Another object is to provide a novel form of magnetic thermal switch.

Still another object is the provision of an external thermostat having good thermal coupling to the contents of a tank or the like.

Other objects and advantages of my invention will be apparent from the following description of one form of device embodying my invention, and illustrating the manner in which it may bev put into practice. In the drawings:

Figure 1 is a top plan view of my snap-acting thermal switch with a part of the top cover cut away;

Fig. 2 is a vertical longitudinal section therethrough, taken along the broken line 22 of Fig. 1;

Fig. 3 is a horizontal longitudinal section therethrough taken on the line 33 of Fig. 2; and,

Figs. 4, 5 and 6 show certain of the parts of the device in different operated positions for facilitating the description.

In Figs. 1, 2 and 3 I have shown a device embodying my invention and comprising a base ll of substantially rectangular shape, which in this specific construction is made of cast aluminum and which has a cover 53 screwed thereto as by a plurality of screws [5. The cover 13 may be made of any suitable material and is substantially of inverted cup shape as shown more particularly in Fig. 2 of the drawings. I have here shown a clear, plastic transparent material such as methyl methacrylate.

The base member I is provided with two vertical longitudinally-extending ribs 2! and 23, each of which is provided with a plurality of spaced enlargements 25, 21 and 29, for receiving screws. I provide a plate 3! made of any suitable electricinsulating material supported on enlargements 25 and 29 by screws 33. Plate 3! is provided on its upper surface with a pair of contact terminals 35 and 37 which by means of L-shape brackets 39 hold adjustable contact screws 4! and 43 respectively. A third contact terminal 45 is also mounted on plate 3! and is connected by a flexible lead 4'! to a contact piece 49, which is mounted on a thin magnetic bar or armature which magnetic bar is pivotally supported on a screw 53 on plate 3!.

I provide also a pair of bimetal bars 55 and 5! (Fig. 3), one end of each of which is supported in cantilever fashion on bar 58 pivoted at 67. The other end of each of said bimetal bars is provided with an upwardly-extending small rod 6!, which rods are adapted to fit loosely in elongated slots 63 provided in a horizontal bar 65, which bar is pivotally supported by a short shaft also at 6'5, for swinging movement between stops 62 and 64. I provide also a relatively large and strong permanent magnet 69 which is carried by pivoted bar 65 and is provided with a pair of pole pieces 1! and 13 (Fig. l) spaced apart to form an air gap 15. Each bimetal bar has its high expansion portion on the upper side as seen in Figs. 1 and 3 so that magnet 69 moves down as the temperature rises.

Means for adjusting the position of the bar 65 and the movable permanent magnet 69 comprises a short shaft 1! which is held in a tubular support 19, which tubular support is secured in electric-insulating plate 3! at its right hand end. Means for frictionally holding member T! in tubular member '!9 comprises a spring 8! and a set screw 83, which set screw has screw-threaded engagement with member 1! and extends through a horizontal slot in member 19. The adjusting means includes also a cam member 85 fixed on the lower end of member H, and an adjusting screw 8'? in pivoted bar 68, which supports bimetal bars 55 and 51. A coil tension spring 89 holds adjusting screw 8'! in engagement with cam 85.

The bimetal bars 55 and 5! are substantially encompassed, as though in an oven, by the cast aluminum base I! and longitudinal ribs 2! and 23 so as to have good exposure to the temperature thereof and to be substantially unaffected by other temperatures. good thermal coupling of bimetals 55 and 51 to a tank or the like on which base i I is mounted and provides also thermal shielding or decoupling from other objects.

Means for mounting the switch against a hot water tank or the like comprises a pair of machine screws 95 and 9'! which fit into openings in the intermediate enlargements 21. While I may make the outer or lower surface of plate I fiat or other suitable shape, I may also interpose an intermediate member between plate, and the outer circular surface of a domestic hot water tank in order to provide a relatively large area of contact with the hot water tank.

In operation, the bimetal bars 55 and 5'! move the permanent magnet (it slowly, and the magnet, as it moves, induces armature 5! to move with a snap from one extreme position to the other. Thus, in the position shown in Fig. l, magnet 69 induces magnetization of the bar 5,! with the This construction provides polarity indicated so that the upper end of bar 5! as seen in Fig. 4 is attracted toward the right hand pole piece of magnet 69 (the pole piece near it), and repelled by the left one, so that it lies against contact l! which serves as a stop. for it. The mutual magnetic force between magnet 69 and armature 5! also urges the magnet toward the left. This motion is limited by stop 6% (shown in Figs. 1 and 3) which with stop l! prevents magnet 69 and armature 5! from touching each other. As the bimetal bars 55 and 5? cool they first develop a force sufficient to overcome the magnetic force holding the magnet against stop 6d. Then as they cool further and move magnet '69 toward the right as indicated by arrow 99 in Fig. 5', contact A! prevents bar 5! from following it. As the right hand pole piece (as seen in Fig. 5) of magnet 09 recedes from bar 5! and the left one approaches, the net magnetizing effect exerted on bar 5! by magnet 59 reduces to zero and then begins to reverse. When this reversed magnetizing effect becomes large enough to exceed the coercive force of the bar 53, the magnetism in bar 5! reverses in polarity, and this reversal causes the bar 5! to be attracted toward the left so that it turns on its pivot 53. As the armature 5! begins to move, the attractive force increases rapidly so that the driving force increases. Consequently the armature 5% moves with an abrupt, or snap, motion from the contact 4! over against the contact 53 as shown in Fig. 6. Stop 52 (Figs. 1 and 3) prevents magnet 59 from moving far enough to the right (in Fig. 6) to engage armature 5! and move it away from contact 42.

As shown, the switch mechanism is so designed that the magnet 69 is moved between stops 62 and 54 by the bimetals 55 and El, and the movement of the armature 5! is limited by the ad" justable contacts 4! and 53. These contacts may be adjusted to vary the temperature difierential of the switch and to place the end of the armature in a correct position in the magnetic field.

Magnet 69 is composed of Alnico II, having a coercive force of 550 oersteds. The external energy product of the magnet is 1.64 and the air gap between the south and the north pole faces is approximately The composition of armature 5! is high carbon steel having a coercive force of about 45 oersteds and an external energy product of about .22. The physical dimensions of the armature are wide and may be .015 to .020" thick and have a length of about 3.

I prefer that armature 5! be hardened because it operates better, providing a clean break under less favorable operating conditions, and executing a more vigorous snap action. I believe that there are two reasons for the better performance of the hardened armature. First, the larger coercive force provides a steeper magnetizing curve, so that the flux reverses more rapidly, that is, more nearly trigger-like. Second, the larger coercive force requires that the magnet 69 be moved farther beyond center so that the armature 5! is closer to a pole of the magnet 59 and so experiences a high magnetizing effect and a larger attractive force. While the coercive force of hardened high-carbon steel is large compared to that of annealed carbon steel it is low compared to that of Alnico or the like composing the magnet 69. The armature 5 must be short enough compared to the size of magnet 68 that the magnetism of armature 5! will not fail to reverse.

In order to make a properly operative thermal switch having minimum dimensions, and to provide suflicient power, I provide two bimetal bars 55 and 51 so that a temperature differential on the order of 7 F. or less will be efiective to cause operation of the switch. In choosing a bimetal for a particular job all pertinent facts must be taken into consideration, such as the space available, temperature diiferential, force exerted and motion effected per degree of change of temperature and many other physical characteristics. While a single bimetal bar may be used, the use of two bars permits a more compact and cheaper construction. In a model built substantially in accordance with the dimensions shown, the temperature differential may be adjusted from a minimum of 7 F. to 10 F. and up, and no creeping whatever has been experienced.

The fact that the magnet 69 has a much greater mass, and therefore a much greater moment of inertia than the armature 5|, imposes greater accelerations on the armature, improves its snap action and makes it less susceptible to chattering when subject to vibration. Since the movement of armature 5| is controlled solely by the magnet 59 and stops 4| and 43, pivot 53 is subjected to only slight mechanical forces, a situation that favors free motion and substantial elimination of frictional restraint.

The thermal switch of my present invention is simple, rugged, compact and inexpensive. It has few parts, may be easily taken apart, reassembled and adjusted. It closes the circuit with a firm, low-resistance and stable engagement of its contacts. It is noticeably free of dead-break, sizzle, chatter, bounce, sticking, and excessive heating and welding of the contacts. And it is accurate and dependable in operation even when controlling heavy electric loads on small temperature difierentials.

Various modifications may be made in the device embodying my invention without departing from the spirit and scope thereof and I desire that all such modifications coming clearly within the scope of the appended claims shall be considered a part of my invention.

I claim:

1. In combination in a snap-acting switch, a pivoted arm, a permanent magnet mounted thereon for swinging movement therewith, said magnet having two opposed poles, lying at different angular positions about the axis of the pivot of said arm, a bimetal bar lying alongside said arm and extending in the same direction, said bimetal bar having a. cantilever mounting at one of its ends near said pivot and a driving engagement with said arm and magnet at its other end, and a magnetizable armature having a portion between said poles and movable therebetween, back and forth approximately in the direction of a line between said poles, and means for limiting the movements of said armature.

2. The combination of the immediately preceding claim wherein there is included a pivoted bracket on which said bimetal bar has its cantilever mounting, and adjusting means for setting the angular position of said bracket and for holding it during the thermal operation of said armature by said bimetal and magnet.

8. In combination in a snap-acting switch, an arm pivoted at one end, a permanent magnet mounted on the other end for swinging movement therewith, said magnet having two opposed poles and a recess therebetween, said recess facing the pivoted end of the arm with said poles being spaced apart in the direction of swinging movement of said magnet and arm, a magnetizable strip lying alongside said arm, having one end between said poles and the other end pivoted about an axis near, and substantially parallel to, the pivot axis of said arm, means independent of said magnet and arm for limiting the swinging movement of said magnetizable strip, a thermostatic bimetal bar lying alongside said arm and magnetizable strip, having at one end a cantilever mounting near said pivot, and at the other end a driving engagement with said magnetsupporting arm.

4. The combination of claim 1 wherein said magnetizable armature has a coercive force at least approximately as great as that of hardened high carbon steel, and said permanent magnet has a coercive force much greater than that of said armature.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,958,482 Leins May 15, 1934 1,996,907 Drake Apr. 9, 1935 1,997,604 Stewart Apr. 16, 1935 2,057,605 Blosser Oct. 13, 1936 2,062,674 Pirwitz Dec. 1, 1936 2,240,847 Hildebrecht May 6, 1941 2,397,116 Armstrong Mar. 26, 1946 2,518,480 Lilia Apr. 15, 1950

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