US2233884A - Thermal switch - Google Patents

Description

March 4, 1941.

E. W. GENT THERMAL SWITCH Filed March 4, 1957 INVENTOR E. M. GENT @QQM A TmRNEY Patented Mar. 4, 194i UNITED STATES PATENT, OFFICE Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York I Application March 4, 1937, Serial No. 128,966

1. Claim.

This invention relates to thermal switches of the type in which the operation of a bimetallic strip is effective to close electrical contacts for controlling the operation of -a heating element as used for example in a quartz crystal oscillator housing for compensating for atmospheric temperature variations.

In such thermal switches, however, it has been found that the distance or air gap variations between the electrical contact carried by the bimetallic strip and its cooperating contact carried by the support due to the expansion of the latter when submitted to a difierence in temperature greatly ail'ect the operation of the bimetallic strip and therefore that of the heating element controlled thereby.

The object of the invention is to provide a thermal switch of the type above referred to which will be simple, cheap to manufacture and which will operate in a positive and accurate manner.

A feature of the invention resides in'the provision of a bimetallic strip which is formed by welding a leaf of a material possessing a maximum coeflicient of expansion to a leaf of a material having substantially zero coemcient of expansion whereby maximum operating amplitude and sensitiveness of the bimetallic strip thus formed is obtained.

Another feature resides in the provision of a support which is made of a material having substantially zero coeflicient of expansion whereby the variations in the temperature of this support do not substantially aflect the operation of its cooperating bimetallic strip.

' Still another feature resides in the provision of means formed with the support interlocking with means formed with the strip for holding the latter and a contact carried thereby in. operatlve relation with an associated contacting member carried by the support.

Other novel features and advantages of the invention will appear by the following description and by the claim appended thereto, reference being bad to the accompanying drawing in which Fig. 1 is a diagrammatic view of the operating circuit of the thermal switch of the invention;

Fig. 2 is a side elevation view showing the easing and a number of operating parts with portions cut away;

Fig. 3 is a front elevation view showing the casing in section and a number of operating parts with portions cut away; and

Fig. 4 is a top assembly view showing the casing in section.

In the drawing, HI indicates a base of insulating material formed with a shoulder portion ll provided for mounting a metallic plate I! in position perpendicular to the base I, this plate being held securely against the shoulder portion H by screws l3 and I4 shown in Figs. 2, 3 and 4. A support l5 formed with an arcuated shape in the plane of its thickness so as to obtain maximum rigidity for a minimum thickness in such a plane is made of a material having substantially zero coefllcient of expansion such as a nickel steel alloy. This support is insulatively mounted on plate l2 and is secured thereon by screws [8 and I1. Support I5 is provided at its ends across its thickness with hooked portions l8 and I9 shown in Fig. 3 for engaging the V- shaped recesses l8 and I! at the ends in a manner to position the width of the strip at right angles to the plane of support I5 while the eileotive bimetallic strip of a length of the strip 20, that is, the length directly across the hooks I8 and I9 oi the support i5 is slightly longer so as to cause this strip to normally flex in the position as shown in Fig. 3, this strip being made of a brass leaf and a leaf of nickel steel having substantially zero coemcient of expansion.

An adjustable contact in the form of a screw 2! engages an insulating bushing 22 in the support l5, this contact being normally engaged by contact 23 carried by the metallic strip for completing an energizing circuit to a heater element which will be hereinafter described in detail. On the metallic plate [2 and adjacent the bimetallic strip 20 is mounted the heater element consisting of an insulating strip 24 serving as a core for the resistor wire 25 coiled thereon, the strip 24 being held securely at each end on the plate l2 by two sets of screws 26-21 and 2829 shown in Fig. 3.

To the opposite side of plate l2 are mounted the electrodes 3! and 28 and a quartz crystal plate 38 shown in Figs. 2 and 4. The electrodes 31 and 38 and the plate 39 are held firmly against the metallic plate 12 by a clamping spring ll having its lower end portion engaged in a slot ii in the base to and its upper disposed end portion engaging in a groove in the end of an insulating stud secured in the metallic plate l2.

The adjustable contact 2! is electrically connected to a plug 3 by a wire 36 shown in Figs. 2, 3 and 4, this plug having one end embedded in the base It. The resistor wire 25 at one end is electrically connected to the bimetallic strip II and therefore to the support I! by a wire 3| while the other end of resistor wire I! is connected to a terminal Si by a wire 32. The terminal II is secured to the electrode 31 by a screw 56 and one end of the resistor wire 25 and the electrode 31 connected to a plug II by a wire 5|, while the electrode ll is connected to a plug 52 by a wire I, the plugs III and I! having each end embedded in the base I. in position to form, in cooperation with plug 35, a triangular formation for engaging a group of similarly disposed connecting jacks not shown.

The base I is formed with a peripheral shoulder portion 54 against which the edge of a casing 55 abuts to form therewith a dustproof closure between the base III and the casing 55, this casing being securely held on the base II by screws 56 and 51 shown in Figs. 3 and 4.

In the operation or the thermal switch of the invention and supposing the bimetallic strip in the position shown in Fig. 3, that is, with contacts 2| and 23 closed which occurs upon a drop in the temperature, the current will flow from the positive side of battery '0 through wire 36, the contacts 2| and 23, the bimetallic bar II, the wire ill. the resistor wire I! to the negative side of battery i0 through conductors "and II, thus energizing the heater element. The heat generated by said element heats the bimetallic bar 2. which upon reaching a suiiiciently high temperature is caused, through the elongation of the brass element in the bimetallic strip, to move in the position shown in dotted lines in Fig. 3, for breaking the heater element circuit and thereby maintaining the temperature in the housing 55 within a few degrees of temperature difl'erence, which may be controlled by the ad- Justment of the contact 2|. .By the use 0! a material having a substantially zero coeiflcient of expansion for the support I and for one o! the elements of the bimetallic strip 20 added to the arcuated shape of the support I! and its consequent great rigidity, a thermostat device is obtained which operates with a high degree of accuracy and thereby makes possible to maintain the operating temperature of the quartz crystal plate substantially constant.

What is claimed is:

In a thermal switch, a base, a plate secured to said base perpendicularly thereof, an oblongshaped bimetallic strip, a plate-like arcuate mounting of unit structure for said strip having a reenforced portion midway betweenits ends and means formed at one 0! its ends for securing it to said plate in a plane parallel thereto, and projections at the ends of said mounting converging towards each other to form hook portions, and notches formed at each end of said strip for engaging said hook. portions so as to position the plane of said mounting at right angles to the width of said strip.

EDGAR W. GENT.

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