US2371018A - Thermal relay - Google Patents

Description

Patented Mar. 6, 1945 THERMAL RELAY Harry E. Ashworth, Penn Township, Allegheny County, and Selden B. Aylsworth, Edgewood, Pa... assignors to The Union Switch & Signal Company, Swissvale, Pa., a corporation of Pennsylvania Application April 16, 1943, Serial No. 483,260

8 Claims.

Our invention relates to thermal relays, and particularly to thermal relays which employ a bimetallic strip heated by a heater element for actuating contacts at the expiration of a predetermined time interval after the application of power to the relay.

The principal object of our invention is to improve the operating characteristics of a relay of the type described by providing more nearly uniform performance under varying conditions of ambient temperature.

Relays embodying our invention are an improvement upon the relay described and claimed in Letters Patent of the United States No. 2,114,895, granted to Harry E. Ashworth on April 19, 1938.

According to our invention, the relay comprises a first bimetallic strip supported at one end through the medium of an oppositely disposed bimetallic strip which compensates insofar as possible for the effects of ambient temperature on said first strip. A metal bracket is secured to said first strip adjacent to its free end and is provided with an offset portion carrying a heating element. The offset portion is so disposed that the heat generated by said heating element will be transferred to said first strip both by conduction through said bracket and by radiation across the intervening space to cause said first strip to deflect, and that as said first strip deflects said heater element will move closer to said first strip and will thus increase the heat transfer in a manner to compensate for any tendency of the strip to decrease its rate of bending as the amount of deflection increases to thereby provide a nearly linear relationship between distance moved and time required to move such distance. This compensation is effective throughout the range of ambient temperatures to which thermal relays for use in railway signaling systems are generally subjected. To further compensate for the efiects of changes in ambient temperature on the characteristics of the relay the heating element is covered by means which regulates the rate of heat transfer from the heating element to said first strip to the proper value to obtain a nearly uniform operation which is independent of variation in ambient temperature.

Other objects and characteristic features of our invention will become apparent as the description proceeds.

We shall describe one form of relay embodying our invention, and shall then point out the novel features thereof in claims.

In the accompanying drawing, Fig. 1 is a front elevational view showing one form of relay embodying our invention, the parts being in their normal positions. Fig. 2 is a view similar to Fig. 1 but showing the parts in their operated positions. Figs. 3 and 4 are side elevational and bottom views of the thermal element TE of the relay shown in Fig. 1. Fig. 5 is an enlarged side view of a portion of the thermal element TE of the relay shown in the preceding views. Fig. 6 is a sectional view taken on the line VI-VI of Fig. 5. Fig. '7 is a sectional view taken on the line VII-VII of Fig. 5.

Similar reference characters refer to similar parts in each of the several views.

Referring first to Figs. 1, 3 and 4-, the relay comprises a top plate I of suitable insulating material, such as Bakelite, which top plate supports all of the operating parts of the relay. Secured to the top plate I in spaced relation are three terminal posts 2, 3 and 4 which support respectively a fixed front contact 5, a thermal element TE, and a fixed back contact 6. The front contact 5 and back contact 6 are of well known constructions which are in widespread commercial use, and inasmuch as they 'form no part of our present invention it is deemed unnecessary to describe them in detail herein. It should be pointed out, however, that these contacts are each provided with suitable means for adjusting their positions relative to the thermal element TE to enable the desired time calibration to be obtained.

The thermal element TE comprises a main bi metallic strip secured at its upper end to an oppositely disposed auxiliary bimetallic strip 8, which latter, in turn, is rigidly fastened to a rigid strip 9 secured within a transverse slot It! provided in the head of the terminal post 3. The main strip 1 is provided at its lower end with an insulating block II carrying a low resistance contact member l2 for cooperation with the back contact 6, and with a metal bracket 13 carrying a low resistance contact member I4 for cooperation with the front contact 5. Contact member 22 is provided with a flexible connector 15 which is adapted to be secured to a suitable terminal post (not shown) mounted on the top plate I. Contact member I4 is electrically connected to the strip I through the bracket l3 so that electrical connection with this contact member can be made at terminal post 3.

The bracket I3 is provided with an offset portion I312 which carries a heating element 16. The heating element is insulated from the offset portion l3a by suitable insulating means here shown as a layer 2| of asbestos paper, and has its terminals I611 and "lb connected to flexible connections I! and I8 by means of rivets l9 and 20 mounted in the insulating block ll adjacent the opposite sides thereof. The flexible connectors I1 and I8 are intended to be connected at their free ends to suitable terminal posts (not shown) mounted on the top plate I.

When the parts are at, ambient temperature, the contact member l2 engages the contact member 6 to close the contact 12-6, and the cont-act member i4 is spaced from the contact member so that the contact l4-5 is open. When heating'element I6 is energized the heat is transferred to strip 1 by conduction through the bracket I3 and also by radiation, and this heat causes the strip 1 to deflect in the manner shown in Fig. 2, thereby opening contact l2-6 and closingcontact M5. The opening of contact |26 will occur at the expiration of a very short time interval after the heating element 16 becomes energized, but due to the comparatively slow rate of heat transfer from the heating element to the strip 1, contact l4-5 will not become closed until the, expiration of a relatively long time interval after heating element is becomes energized.

Thermal relays of the type dscribed are frequently employed in railway signaling systems to provide a predetermined time interval between the functioning of related circuits of the system, and when so used it is essential that the time interval be as nearly as possible independent of variations in ambient temperature. Where the variations in ambient temperature are relatively small the necessary compensation can be obtained by means of the auxiliary bimetallic strip 8, which, being oppositely disposed with respect to the main strip I, deflects in the opposite direction to the strip I in response to changes in ambient temperature, and which is of such length that the deflection of this strip for small variations in ambient temperature will just offset the deflection of the main strip 1 so that the contact end of the strip 1 will remain in a substantially fixed position with respect to the associated fixed front and back contact members 5 and 6. We have found, however, that at the extremes of ambient temperature at which relays of the type described must at times operate, the compensation that can be obtained by the auxiliary strip 8 is not complete and that as a result, if no other compensation is provided, there will be a considerable variation at these extremes between the time interval for any particular setting and voltage and that obtained for the same setting and-voltage at normal ambient temperatures. That is to say, any particular relay will vary considerably from its desired time interval with variations in ambient temperature in spite of the thermal compensation provided by the auxiliary strip 8.

This variation is undesirable, and appears to be due at least in part to the fact that the rate of bending of a bimetallic strip, when actuated by heat, decreases as the amount of deflection increases, the action being such that the strip appears to increase in stiffness with increases in deflection. This results in a non-linear relation between the distance moved by the free end of the strip and the time required for the strip to move such distance, and in accordancewith our present invention we overcome this nonlinear relationship by so disposing the offset portion of the bracket I3 that as the strip 1 deflects due to the heat generated in the heating element IS the heating element will move closer to the strip 1 as shown in Fig. 2, and will thus increase the rate of heat transfer in a manner to compensate for the tendency of the strip to decrease its rate of bending with increases in deflection. Furthermore, we also provide means for conserving the heat generated by the heating element and for'causing it to pass to the bi- JBtaHlC strip 1 at the correct rate to maintain a nearly uniform rate of bending regardless of the ambient temperature. This latter means comprises a thin piece 24 of mica disposed on the side of the heating element l6 farthest from the bimetallic strip, a turn and a half of asbestos paper 22 surrounding the element and said sheet of mica and lapped on the same side of the element as the mica, and a copper band 23 which surrounds the asbestos and serves as a means to clamp both the mica and the asbestos in place. These coverings act to prevent excessive radiation of heat from the heating element at the side farthest from the bimetallic element, insofar as possible, and at the same time store heat in just suificient quantity to cause the relay to operate as desired. The whole efiect is to cause the relay to operate at wide extremes in temperature with an exceedingly small percentage of variation in time. In actual tests, the variation in time with the covering is about one-half that obtained without the coverings.

Although we have herein shown and described only one form of relay embodying our invention. it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of our invention.

Having thus described our invention, what we claim is:

1. In a thermal relay, in combination, a thermal element comprising a bimetallic strip supported at one end and provided at the other end with means for operating contacts at the expiration of a time interval in response to heating of said strip, and means for heating said strip secured to said strip by means which causes the heat supplied to said strip to increase as the deflection of said strip resulting from said heat increases.

2. In a thermal relay, in combination, a thermal element including a first bimetallic strip supported at one end by an oppositely disposed bimetallic strip which at least partly compensates for changes in ambient temperature, and a heating element for heating said first strip mounted on a bracket secured to the free end of said strip, said bracket being so constructed that as said strip deflects due to the heat supplied thereto by said heating element the distance between said heating element and said strip will decrease to further compensate for changes in ambient temperature.

3. In a thermal relay, in combination, a thermal element including a bimetallic strip supported at one end, a bracket secured to said strip and provided with an ofisetportion which moves closer to said strip in response to deflection of said strip due to heating of the strip, a thermal element mounted on but insulated from said offset portion, and means for conserving the heat generated by said heating element and for causing it to pass to the bimetallic strip at the correct rate to maintain a nearly uniform rate of bending regardless of variations in ambient temperature, said means comprising a piece of mica disposed at the side of said heating element farthest from said strip, a turn and a half of asbestos paper surrounding said element and said sheet and lapped on the outer side of said mica, and a metal band surrounding the asbestos and serving as a means to clamp both said mica and said paper in place.

4. In a thermal relay, in combination, a thermal element including a first bimetallic strip supported at one end by an oppositely disposed bimetallic strip which at least partly compensates for changes in ambient temperature, a heating element for heating said first strip mounted on a bracket secured to the free end of said strip, said bracket being so constructed that sheet and lapped on the outer side of said mica,

and a metal band surrounding the asbestos and serving as a means to clamp both said mica and said paper in place.

5. In a thermal relay, in combination, a thermal element including a bimetallic strip supported at one end, a bracket secured to said strip and provided with an offset portion which moves closer to said strip in response to deflection of said strip due to heating of the strip, a thermal element mounted on. but insulated from said offset portion, and means for conserving the heat generated by said heating element and for causing it to pass to the bimetallic strip at the correct rate to maintain a nearly uniform rate of bending regardless of variations in ambient temperature, said means comprising an insulating covering surrounding said heating element.

6. In a thermal relay, in combination, a thermal element including a bimetallic strip supported at one end, a bracket secured to said strip and provided with an offset portion which moves closer to said strip in response to deflection of said strip due to heating of the strip, a thermal element mounted on but insulated from said offset portion, and means for conserving the heat generated by said heating element and for causing it to pass to the bimetallic strip at the correct rate to maintain a nearly uniform rate of bending regardless of variations in ambient temperature, said means comprising an asymmetrical insulating covering surrounding said heating element.

'7. In a thermal relay, in combination, a thermal element including a, bimetallic strip supported at one end, a, bracket secured to said strip and provided with an offset portion which moves closer to said strip in response to deflection of said strip due to heating of the strip, a thermal element mounted on but insulated from said offset portion, and means for conserving the heat generated by said heating element and for causing it to pass to the bimetallic strip at the correct rate to maintain a nearly uniform rate of bending regardless of variations in ambient temperature, said means comprising an asymmetrical insulating covering surrounding said heating element and disposed with the maximum insulation at the side opposite said bimetal strip.

8. Ina thermal relay, in combination, a thermal element including a bimetallic strip supported at one end, a bracket secured to said strip and provided with an offset portion which moves closer to said strip in response to deflection of said strip due to heating of the strip, a thermal element mounted on but insulated from said oilset portion, and heat insulating means associated with said heating element for conserving the heat generated by said heating element and for causing it to pass to said bimetallic strip at the correct rate to maintain a nearly uniform rate of bending regardless of variations in ambient temperature.

, HARRY E. ASHWORTH.

SELDEN B. AYLSWORTH.

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