US2605372A - Thermostatic switch - Google Patents

Description

y 1952 D. c. SPOONER, JR, ETAL 2,605,372

THERMOSTATIC SWITCH Filed April 25, 1951 Inventor-s: I David C. SpooheT'JT Robert G. Holmes,

Their- Attorneg.

. cuit is well known in the art.

Patented July 29, 1952 I 2,605,372 THERMOSTATIG SWITCH David C. Spooner, Jr., Westport, and Robert G.

Holmes, Bridgeport, Conn., assignors to General Electric Company, a corporation oi New York Application April25, 1951, Serial .N 0. 222,794

3 Claims.

This invention relates to thermostatic switches and, more particularly, to those switches having a movable contact actuated by a temperaturesensitive bimetallic element.

The'type switch which uses a contactmounted on a bimetallic element movable with respect to a fixed contact to make and break an electric cir- This type switch has found wide application for energizingand deenergizing heating circuits, such as those used in an electric blanket, where a narrow cyclical fluctuation above and below the mean heat desired is permissible. However, a serious drawback to such use, particularly in applications where satisfactory service is required over agreat period of time, is that dirt particles often lodge on one contact or the other and interfere with conduction between the two. If asufficient film forms, the operation of the circuit and its resultant output are seriously impaired. Another problem is that the contacts often become pitted in use, again causing poor conduction and faulty circuit operation. One of the best solutions to these problems is to provide a wiping action between the movable contact and the stationary contact.

An object of this invention, therefore, is to provide anew and improved meansfor establishing a wiping action between two contacts of a thermostatic switch of the type mentioned.

A further object of this invention is to provide a thermostatic switch of the type mentioned having a wiping action between its two contacts, which is mechanically sturdy, simple in construction, and inexpensive to manufacture.

Incarrying out a preferred embodiment of this 4 invention, a pair of relatively movable cooperating contacts are mounted respectively on the movable portions of two temperature-responsive members with the path of heat-responsive motion of one of the contacts being substantially transverse to the corresponding motion of the other contact. In such a construction, the transversely moving contact provides a wiping action, while the cooperating contact is responsive to the condition being controlled.

For a better and more complete understand ing of this invention, reference should now be had to the following specification and to the accompanying drawing which is a perspective view of a preferred embodiment of the invention.

Referring to the drawing, a circuit making and breaking contact I is mounted on the principal surface of a temperature-sensitive member, for example, bimetallic element or strip 2. Preferably, contact I moves between its two operating positions with a snap action and for that purpose a magnetic armature 3 is mounted on strip 2. The bimetallic strip 2 is firmly secured to an insulating block or support '4. Also secured to block 4, preferably with its principal plane sub- (Cl. 2D0122) stantially at right angles to that of strip 2, is a second temperature-sensitive bimetallic element or strip 5. A contact 6 is mounted on strip 5 to cooperate with contact I, the two contacts being engaged at the cold temperature position of the bimetallicstrips. Contact 6 is so mounted that its direction of motion upon heating of strip 5 is transverse to the direction of motion of'contact I and contact 6 is, therefore, referred to hereinafter as the transversely moving contact. Also mounted on strip 5 is a permanent magnet I positioned to cooperate with armature 3 for the above-mentioned purpose. To provide actuation, means are included for heating the bimetallic strips. By way of example, the drawing illustrates a heater 8 for warming strip I and a heater 9 for strip 5. Heaters 8 and 9 may be connected in any suitable manner to a power source, for example, in series through strip 2, contacts I and 6, strip 5, and through a connected load (not shown).

With a suitable voltage applied to the incoming line, and contacts I and 6 closed, a current flows in series through he'aterB, strip 2, contacts I and 6, strip 5 and heater 9. As strip 2 heats up, contact I tends to move away from contact 6. However, magnet I, acting on armature 3, retards the bending of strip 2 until sufiicient force builds up therein to suddenly pull contact I free. This is commonly referred to as snap action.

However, as strip 5 heats up, retarded only by friction between the contacts and between the magnet and armature. Thus, strip 5 tends to move contact 6 in a plane transverse to the ultimate direction of motion of contact I. This transverse action or force results in the desired rubbing or wiping action between contacts I and 6.

As soon as contact I snaps free, the circuit is interrupted and the bimetallic strips begin to cool. When strip 2 cools sufficiently, the contacts once again touch and the cycle starts to repeat itself.

In order that the snap action of contact I opens the circuit, rather than any transverse movement of contact 6, it is necessary that the transverse distance traveled by contact 6 be insufiicient to clear contact I. One manner of accomplishing this would be to have strip 5 bend only a slight amount during the period of heating necessary to'cause the snap action of strip 2 so that contact 6, although tending to move, is always in a contact-making position. This desirable characteristic of short travel of strip '5 as compared to strip 2 can be accomplished obviously in several ways. One manner is to design bimetallic element 5 with greater thermal capacity than bimetallic'element 2. By making the strips of material having different thermal its bending is! masses, the thermal mass of strip being greater thanv that of strip 2, strip 5 is slower to bend during both the heating and the cooling portions of the cycle. Another way of constructing strip 5 would be to employ component strips of the bimetal having less diiierence in their coeflicients of expansion than the corresponding dissimilar component strips of bimetal 2. A further method of slowing the bending of strip 5 is to cover it with a layer of thermal insulating material. The layer of insulating material slows the heating of the strip 5 and thereby retards its motion. Still other manners in which transversely moving contact 5 can always be kept in a contact-making position with circuit breaking contact I are limiting the motion of strip 5 with mechanical stops, or providing contact 5 with an extra large contact area.

Although the invention has been described with a separate heater for each bimetallic strip, and a series connection between them, the heaters could be connected in parallel or a single heater could be used to heat both strips. Similarly, either one or both heaters could be in series with the load circuit controlled by the contacts, or could be in parallel with the load circuit through the contacts.

Moreover, bimetallic strip 5 could be mounted not only with its principal plane perpendicular to th principal plane of strip 2, but also with its greatest length perpendicular to the greatest length of strip 2, the wiping action between contacts l and 6 being accomplished by placing the transversely moving contact 6 on the edge of the end of strip 5.

If one of the materials comprising strip 2 is magnetic, armature 3 is unnecessary and can be omitted from the construction of the device. In fact, although the method described is preferred, other methods well known in the art can be employed to produce the desired snap action. The snap action means may even be eliminated, if such action is not necessary for the intended application of the thermostatic switch.

Excellent results have been obtained with a thermostatic switch constructed according to this invention in which the transversely moving bimetal did not produce sufficient force to cause observable transverse motion, but merely produced a tendency to wiping action. The contaots, nevertheless, maintained excellent condition and the cycling was very uniform. This is explained by the fact that microscopically all contact surfaces, no matter how finely polished,

actually constitute a series of peaks and valleys.

Pressures between the sides of these peaks on the contacts occur as the transverse bimetal heats up even though the transverse bimetal does not actually move. These pressures between the peaks produce results similar to those obtained when the transverse bimetal and the contact mounted thereon have observable motion.

While what has been described is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In a thermostatic switch, first and second temperature-sensitive bimetallic elements, each having a fixed portion and a movable portion responsive to temperature change, means supporting said bimetallic elements with the principal planes thereof perpendicular and the movable portions thereof adjacent, a first contact mounted on a principal surface of said first bimetallic element and a second contact mounted on an edge of said second bimetallic element, said contacts being engaged at one temperature of said bimetallic elements and separated at a second temperature, said second contact moving in a plane transverse to the direction of motion of said first contact upon temperature-responsive motion of said bimetallic elements, said second bimetallic element providing a temperature-responsive contact motion in a range to retain said second contact in a contact making position.

2. In a thermostatic switch, first and second temperature-sensitive bimetallic elements, each having a fixed portion and a movable portion responsive to temperature change, means supporting said bimetallic elements with the principal planes thereof perpendicular and the movable portions thereof adjacent, a first contact mounted on a principal surface of said first bimetallic element and a second contact mounted on an edge of said second bimetallic element, said contacts being engaged at one temperature of said bimetallic elements and separated at a second temperature, said second contact moving in a plane transverse to the direction of motion of said first contact upon temperature-responsive motion of said bimetallic elements, said second bimetallic element providing a temperature-re sponsive contact motion in a range to retain said second contact in a contact making position with said first contact, a magnet mounted on said edge of said second bimetallic element, a magnetic armature mounted on said principal surface of said first bimetallic element in cooperating position with said magnet to effect a snap action in the motion of said first bimetallic element, and heating elements for heating said bimetallic elements in response to said contacts being engaged.

3. In a thermostatic switch, a first thermostatic member having a portion movable in a predetermined direction, a second thermostatic member having a portion movable in a direction substantially transverse to said predetermined direction, a pair of contacts movable into and out of engagement, one of said contacts being mounted on the movable portion of said first thermostatic member and the other of said contacts being mounted on the movable portion of said second thermostatic member, and heating means associated with said thermostatic members to cause deflection of said contacts in said predetermined direction between closed and open positions and to provide force tending to move said contacts transversely of each other when said contacts are closed.

The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Thomas Jan. 5, 1932 Number

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