US2769060A

US2769060A - Thermal switch with temperature differential cycling delay

Info

Publication number: US2769060A
Application number: US426746A
Authority: US
Inventors: Edmond G Franklin
Original assignee: General Mills Inc
Current assignee: General Mills Inc
Priority date: 1954-04-30
Filing date: 1954-04-30
Publication date: 1956-10-30
Anticipated expiration: 1973-10-30

Description

2 Sheets-Sheet 1 JZ ZZJVZWR EDMOND G. FRANKUN E. G. FRANKLIN THERMAL. SWITCH WITH TEMPERATURE DIFFERENTIAL CYCLING DELAY Oct. 30, 1956 Filed April 30, 1954 E. G. FRANKLIN THERMAL SWITCH WITH TEMPERATURE DIFFERENTIAL CYCLING DELAY 2 Sheets-Sheet 2 Filed April 50, 1954 [ZZYEJYZ'QR EDMOND G. FRANKLIN AYJ'Q ZJKEY Unite States Patent THERMAL SWITCH WITH TEMPERATURE DIFFERENTIAL CYCLING DELAY Edmond G. Franklin, Minneapolis, Minn., assignor to General Mills, Inc., a corporation of Delaware Application April 30, 1954, Serial No. 426,746

7 Claims. (Cl. 200-138) The present invention relates to thermal switches, and more particularly to an improved thermal cycling delay mechanism for such a switch.

Thermal switches are well known in which the frequency of cycling is substantially greater than is desired for particular applications. For example, in the home appliance field, there are certain standards such as those of the National Electrical Manufacturers Association which limit the maximum number of cycles per unit time. In any case, a relatively high cycling frequency in a thermal switch is conducive to substantial wear and erosion of the contacts, which not only shortens the effective life of the switch, but may contribute substantial errors in the operating temperatures maintained by the switch.

To remedy these difiicnlties, some devices have been proposed in the past in which one switch contact was supported by an auxiliary bimetallic strip having a cross section small enough to be heated by the current passing through the contact. The heating of such a bimetallic strip would then cause deflection of the strip to increase the contact pressure at the make and to increase the contact gap on cooling of the bimetal after the break. In these cases, however, to achieve the necessary heating, the cross section of the auxiliary bimetallic strip had to be so small that the strip offered relatively little support for the contact member.

With these problems of the prior art in view, it is one object of the present invention to provide a cycling delay mechanism for the thermal switch in which one contact is supported by an auxiliary thermally responsive strip of substantial cross section.

Another obiect is the provision of such a switch in which a definite temperature differential is maintained between the thermally responsive supporting member for one contact and the contact supporting arm of the other contact.

A further object is a cycling delay arrangement for a thermal switch in which heat is conducted from one contact arm to the other upon engagement of the contacts, and in which this transfer of heat causes deflection of the other contact in a direction toward the hotter contact.

Other objects and advantages of the invention will be apparent from the following specification in which a preferred embodiment is described.

In the drawings which accompany this application and in which like reference characters indicate like parts,

Figure l is a side view with certain portions in section and other portions broken away, illustrating one form of thermal switch embodying the present invention. In this figure the parts are shown in the position occupied during the cooling portion of the cycle, just before the contacts re-engage each other;

Fig. 2 is a schematic view similar to Fig. 1 showing the parts at the instant when the contacts first re-engage;

Fig. 3 is a view similar to Fig. 1 illustrating the position of the contacts shortly after the engagement illustrated in 2,769,060 Patented Oct. 30, 1956 Fig. 2 has resulted in substantial heat transfer from one contact to the other;

Figure 4 illustrates the position of the parts at the end of the heating cycle just as the contacts disengage each other; and

Fig. 5 is a partial top view of the device of Fig. 1.

In the device shown in Figs. 1 through 5, the thermal switch is indicated generally at 20. For convenience in illustration, the switch has been shown in combination with a plate 22 having an electrical heating element 24 associated therewith. The switch itself includes a main supporting bracket 26 with horizontal flanges 28 at its lower edges. These horizontal flanges are secured to appropniate portions of the heated plate 22 by bolts 30. This bracket preferably should be a poor heat conductor such as stainless steel.

Bracket 26 includes a horizontal top wall portion 32 on which a depending stud or post 34 is carried. Post 34 in turn supports the various switch elements. Immediately below the top 32 of bracket 26, post 34 carries an insulating block 36. Beneath this block is secured the base portion 38 of an upper contact arm 40. Next below the contact arm portion 38 is another insulating member 42, and below that is secured the base portion 44 of a shielding or heat insulating member 46. Another insulating block 48 spaces the base portion 50 of a lower contact arm 52 below the heat shield. An insulator 54 and retaining plate 56 are secured to the bottom of post 34 to complete the assembly.

The base portion 38 of the upper cont-act arm includes a terminal extension 58, while the base portion 50 of the lower contact arm includes a similar terminal 60.

Wires

62 and 64 are connected to the

respective terminals

58 and 60 and serve to connect the heating element 24 in circuit with the switch in known manner.

The upper Wall 32 of supporting bracket 26 is provided with a threaded boss 66 which receives the threaded end 68 of a manually adjustable control member 70. An insulating button 72 at the lower end of control shaft engages the outer end 74 of flexible contact arm 40. This arm is normally resiliently bias-ed upwardly so that end portion 74 is held against insulating button 72 at all times. Thus, rotation of shaft 70 changes the vertical position of the insulating button and predetermines the operating position of the upper contact arm 40.

The upper contact 76 is supported from contact arm 40 by means of an auxiliary thermally responsive member. For purposes of illustration l have shown a bimetallic strip 78. This strip extends generally horizontally parallel to and slightly below the outer end 74 of the contact arm. At its inner end, the bimetallic strip 78 is offset upwardly at 8-0 and has its end portion 82 riveted at 84 to the contact arm. This point of attachment is so chosen that flexing of the contact arm 40 in response to adjustment of shaft 70 will cause corresponding changes in the initial position of contact 76. The relative orientation of the high and low expansion sides of the bimetallic strip 78 is such that upon heating of the strip the upper contact 76 will be deflected downwardly toward its opposing contact 86.

This lower contact 86 is carried at the outer end 88 of the lower contact arm 52. The lower contact arm 52 is resiliently biased upwardly so that contact 86 is normally urged up toward its cooperating contact 76. The actual position of this lower contact is controlled in response to changes in the thermal condition to be controlled. For

this purpose, and solely by way of illustration, a connection is provided by a rivet 90 between the lower contact arm and a bracket member 94. This bracket member 94 has a horizontal flange 92 at its upper end which is electrically insulated from the contact arm 52 by insulating members 96. The parts are held firmly in assembled relation by the above-mentioned rivet.

The lower end of bracket 94 includes a horizontal flange 98 which carries a connecting pin or rivet 100 for attachment of one end 192 of a thermal control member 104. The other end of member 104 is connected at 106 to a remote portion of the heating plate 22 by means of a bolt 110 threaded into a projection 108 on the plate.

The material from which the member 104 is made preferably has a substantially smaller coefficient of thermal expansion than the heating plate 22 itself. Thus the relative difference in expansion between the plate 22 and member 104 over the distance between switch securing bolts 36 and the control retaining bolt 110 provides a thermally responsive unit which effectively raises and lowers the lower contact arm 52. Since the lower contact 86 is normally biased upwardly, it will be apparent that the linkage will involve no lost motion and that the control member 104 will at all times be under tension to hold the contact arm 52 downwardly to a greater or lesser extent against the upward bias of arm 52.

Instead of the difference in expansion between member 104 and plate 22, other thermal controls of known type may be used to actuate the lower contact arm 88 of the switch. For example, a bimetallic strip responsive to temperature of plate 22 could be connected in known manner to move the lower contact arm and operate the switch in response to desired predetermined plate temperatures.

The heat insulating shield 46 described above projects outwardly to a point 112 substantially close to the coopcrating

contacts

76 and 86. This shielding member is wider than the auxiliary bimetallic strip 78 of the upper contact. As shown in the drawing, the shield member 46 lies between this bimetal strip and the lower contact arm 52 throughout a major area of the bimetal. Thus the bimetallic strip 78 is effectively shielded against transfer of heat by radiation from the lower contact arm 52 or from the heating plate 22 and associated heating element 24. End 112 of the shield may serve as a limiting stop for the upper contact, as shown in Figs. 3 and 4.

On the otherhand, the lower contact arm 52 and its contact 86 are not shielded in this respect and are so located thatthey will normally have a substantially higher temperature than the shielded bimetallic strip portion 78, whenever

contacts

76 and 86 are separated.

According to the

present invention contacts

76 and 86 are made of silver or some alloyiof very high heat-conducting properties. Thus the engagement of thesev contacts not only completes the electrical circuit between the

terminals

58 and 60, but also provides a good heatconducting path from the lower contact arm'at 88 to the bimetallic supporting strip 78.

. With these details of construction in mind, the operation of the switch shown in the drawings will be readily understood. Fig. 1 illustrates the position of the parts during the cooling portion of the cycle while the contacts are disengaged and the lower contact is gradually moving up toward the upper contact to re-establish the heating cycle. During this part of the cycle the lower contact 86 and contact arm 88 will reach a substantially higher equilibrium temperature than the upper contact supporting portion 78, both because of the relative nearness of the 'lower arm to the heating plate 22 and also because of the effect of the intervening heat shield 46. Because of the shielding effect, the bimetallic supporting member 78 will occupy a normal position which is substantially undefiected.

7 On further upward movement of the lower contact, as illustrated in Fig. 2, the

contacts

76 and 86 will re-engage each other to establish a further heating cycle. Fig. 2 illustrates the position of these bars just at the instant of re-engagement, before there is any substantial heat trans fer from the lower contact arm to the upper one. Fig. 3 illustrates the condition of the parts a moment later, after the relative difference in temperature of the lower and upper contact arms has been equalized by rapid conduc tion through the contacts themselves. The resulting increase of temperature of the upper contact arm causes it to be deflected downwardly from the position of Fig. 2 to the position of Fig. 3.

This downward deflection has been exaggerated in the drawing for clearness, although in a practical case, the amount of such deflection will depend to some extent upon the relative rigidity of the lower contact arm at 88. If this lower contact arm is relatively rigid, there will be little deflection, but merely an increase in contact ressure by the upper arm. Whichever situation results, however, it will be necessary for the lower contact to move downwardly a greater distance than would otherwise be the case, in order to separate the contacts once more. Thus the heating portion of the cycle is effectively lengthened.

It should be noted at this point that the relative crosssection of the bimetallic supporting strip 78 may be great enough so that no substantial heating and deflection of the bimetal will occur solely by passage of current to the contact. In such a case the primary cause of deflection is the increase and decrease of temperature in the bimetallic strip resulting from engagement and disengage ment of the contacts and the substantial differences in temperature at which the respective contact arms are maintained. In the preferred form of the invention, however, both effects are used cumulatively. The deflection of the contact support, or the change in contact pressure, is then due both to the heating of the thermal member by current passing to the contact and to the conduction of heat from the other contact. Thus the cross-section of the thermally responsive cycling delay member can be great enough to provide a firm support for the contact, rather than the weak and flexible support involved in some prior art constructions where the deflection of the contact is achieved solely by heating a bimetal through passage of current to the contact.

As the heating cycle continues, the lower contact will gradually move downwardly until it reaches the position of Fig. 4 at which the contacts have just begun to disengage each other. As soon as the contacts disengage, the heat transfer path from one to the other is interrupted, and the respective contact arms are free to resume their different equilibrium temperatures as before. Thus there is no further heat transfer from the lower contact arm to the upper one and the upper contact arm will begin to cool and be deflected back upwardly toward the position of Figs. 1 and 2. This upward movement increases the contact separation and thus increases the time required before cooling of the plate 22 can result in sufficient movement of contact 86 to re-establish the circuit.

The foregoing cycle will be repeated at each make and break of the contacts and will thus achieve the desired cycling delay, primarily as a result of maintenance of a substantial temperature differential between the Zones in which the upper and lower contact arms are located and the conduction of heat directly from one contact arm to the other as the contacts engage. The relative temperature differential is enhanced by the particular location and arrangement of the shielding member 46.

According to the foregoing description a thermal switch has been provided which substantially accomplishes the objects set forth at the beginning of this application. It should be noted that a thermal differential cycling delay of the type described herein is most effective in applications where a relatively great temperature differential can be readily obtained between the region of the lower contact arm and the area in which the upper contact arm is located. Also, while the thermally responsive contact support has been shown in the form of a bimetal strip, there may be situations in which a thermally responsive expanding and contracting member may offer added advantages. Such members are shown, for example, in my earlier copending application Serial No. 309,325, filed SeptemberlZ; 1952, now Patent No. 2,716,172, and in the copending application of Franklin and Tsai,

Serial No. 309,326, filed September 12, 1952, now Patent No. 2,716,174. The invention also contemplates the alternate possibility of rearrangement of parts for removal of heat from the thermal portion by conduction through the contacts to achieve the desired results.

Since minor variations and changes in the exact details of construction will be apparent to persons skilled in this field, it is intended that this invention shall cover all such changes and modifications as fall within the spirit and scope of the attached claims.

I claim as my invention:

1. A thermal switch comprising a first contact arm and a first contact carried by said arm, first thermally responsive means on the first arm having direct thermal engagement with the first contact and at least partially controlling the relative position of the first contact, a second contact, second thermally responsive means for causing relative engagement and disengagement of the contacts in response to predetermined changes in temperature of said second means, and means normally maintaining a substantial temperature differential between the second contact on the one hand, and the first contact and first thermally responsive means on the other hand when the contacts are disengaged, said contacts having a relatively high thermal conductivity such that engagement of the contacts causes heat transfer by conduction through the contacts thereby changing the temperature of the first thermally responsive means and urging the first contact toward the second in response to said temperature change by conduction.

2. A thermal switch according to claim 1 in which the first thermally responsive means is in electrical circuit with one contact and has a cross section such that passage of current through the contacts when they engage each other causes surficient heating of the first thermally responsive means to increase the urging of the first contact toward the second in response to engagement of the contacts.

3. A thermal switch comprising first and second movable contact arms, a bimetallic strip secured to the first arm and supporting a first contact, and a second contact secured to the second arm, each of said first and second contacts being of high thermal conductivity and in direct heat conducting engagement with the bimetallic strip and second contact arm respectively, and means maintaining a substantial temperature differential between the second contact arm and the bimetallic strip when the contacts are disengaged, the relatively high thermal conductivity of said contacts causing heat transfer by conduction through the contacts when they engage each other and thereby changing the temperature of the bimetallic strip, said strip being oriented and connected to urge the first contact toward the second in response to said temperature change by conduction.

4. A thermal switch according to claim 3 in which said temperature differential maintaining means includes a heat insulating shield between the bimetallic strip and second contact arm.

5. A thermal switch for use in combination with a heated member and a heating element for said member, said switch comprising first and second movable contact arms for connection to control said heating element, a

bimetallic strip secured to the first contact arm and extending generally parallel thereto, a first contact sup ported by said strip, and a second contact supported by said second contact arm, supporting means for said contact arms adapted to support the second contact arm in a first temperature and thereby maintain said arm at a given ambient temperature, said supporting means also being adapted to carry the first contact arm and bimetallic strip in a second zone of substantially diiferent temperature, thereby maintaining the bimetallic strip at a substantially different ambient temperature from the second contact arm, said contacts having a relatively high thermal conductivity such that engagement thereof causes substantial heat transfer between the second contact arm and the bimetallic strip, thereby deflecting the latter in a direction urging the first contact toward the second, and thermally responsive means operative y connected to one of said contact arms and moving the contacts into and out of engagement in response to predetermined variations in the temperature of the heated member.

6. A thermal switch for use in combination with a heated member and a heating element for said member, said switch comprising first and second movable contact arms for connection to control said heating element, a bimetallic strip secured to the first contact arm and extending generally parallel thereto, a first contact supported by said strip, and a second contact supported by said second contact arm, supporting means for said contact arms adapted to support the second contact arm relatively close to the heated member and thereby maintain said arm at a high ambient temperature, said supporting means also being adapted to carry the first contact arm and bimetallic strip relatively farther from the heated member, a heat-insulating shield carried by said supporting means and located between the bimetallic strip and second contact arm, thereby maintaining the bimetallic strip at a substantially lower ambient temperature than the second contact arm, said contacts having a relatively high thermal conductivity such that engagement thereof causes substantial heat transfer from the second contact arm to the bimetallic strip, thereby heating and deflecting the latter in a direction urging the first contact toward the second, and thermally responsive means operatively connected to one of said contact arms and moving the contacts into and out of engagement in response to predetermined variations in the temperature of the heated member.

7. A thermal switch according to claim 6 in which the thermally responsive means is operatively connected to the second contact arm, and manually adjustable control means operatively connected to the first contact arm, adjustment of said control means changing the position of the first contact arm and thereby determining the operating temperature of the switch.

References Cited in the file of this patent UNITED STATES PATENTS 1,959,205 Hanel May 15, 1934 2,208,432 Samuels July 16, 1940 2,667,565 Wallower Jan. 26, 1954