US1818191A - Electric heating appliance - Google Patents

Description

Aug. 11, 1931. J. A. BOYER ELECTRIC HEATING APPLIANCE Filed Aug. 19, 1929 m t M \H W m m. w

employing non -metal Patented Aug. 11, 1931 STATE ammonia-.1) i

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NEW YORK, A CORPORATION OF NEW YORK msmm HEATING APPLIANCE Application fled August 19, 1929. Serial No. 886,998.

This invention relates toelectric heating appliances, and more especially to contact terminals for electric eating appliances employin rigid resistors as the heating elements. he invention relates especially to a contact terminal of an aluminum alloy vparticularly adapted for this purpose.

In the drawings,-

Figure 1 is a section taken partly in elevation of an electric heating device embodyin my invention;

igpres 2 and 3 are detail elevations showing t e construction of one of the contact terminals;

Fi re 4 is a sectional view taken on the line V-IV of Figure 1; and

Figures 5 and 6 are fragmentary elevations showlng forms of contact terminals.

Heating appliances, such as domestic electric room heaters, hot plates, stoves, etc., are often made with rigid non-metallic resistors as the heating elements. These non-metallic resistors are usually made of or stalline refractory conducting material ormed into rods or bars. As an example of such type of resistors may be mentioned the resistors sold under the trade name of Globar which are bars consisting principally of recrystallized silicon carbide. I

These non-metallic resistor elements are highly refractory and may be operated at a high temperature and have a long life. The surfaces of such resistors are somewhat rough and present hig spots or projecting crystals as contrasted with a metallic surface which may be finished to a smooth condition.

one of the great practical difiiculties in ie rigid resistors of this type has been the provision of suitable contacting meansbetween theresistor and the metallic terminalmembers; A metallicjcontact'held against the surface'of the resistor bears only on the high p'ointsof-the somewhat rough resistor surface, and the'current passing from the;

resistor is' therefore conoentratedatrelativesmall contact areas or points. f The result y has been a tendency andloc'al overheating. 1

Many attempts havebeen made solvethe ble,

= terminals haveproved etal to the body of the difliculty of providing satisfactory contacts. In general these have comprised principally mechanical designs for the metal contact member with the incorporation of a piece of heat-resisting metal at the junction of the resistor and contact terminal, or the ends of the rods have been coated with a film of' metal by spraying or plating.

Such mechanical devices have included metal caps for the rods, cumbersome split rings and side contacts, wrapping the ends of the resistor elements with heat-resistant wire, and similar mechanical, devices. In such previous attempts principal reliance has been upon a metal which is particularly heat resistant, such as the nickel-chromium alloys, iron, chromium alloys, nickel, and nickel al-' loys. These alloys are hard as well as heat resistant, and in fact, hardness has been considered a desirable property in most of these metals.

Thin metal films ap lied to the ends'of the rod are not durable ecause their intimate union with the resistor causes them to assume the same temperature as the resistor and because they are too thin to form a cushion and thus eliminate arcing and local overheating.

Contacts of soft, relatively low melting oint metal, such as aluminum, have also can suggested, and are described and claimed in the copending application of Harold N. Shaw, filed of even date therewith. Metallic aluminum is soft and readily fusithe high points of the resistor surface tend to embed themselves in the aluminum andgive a greater area of contact than I with a hard metal.

so that when used asthe contact pieceor projecting crystal points Thealuminum melts at approximately 660 centigrade and becomes soft 1 at 4 even lower While the .aluminum contact satisfactory where the terminals are-exposed -and can readily'radiate temperatures.

oints o-f the r surface may embed emselves in the metal, and

' or conduct'away' the heat; they are not eninvention rei;asses h Class of; .tei-minals," namely-,' those-fin which the high" .especially to terminals having greater heat ances, the contact terminals are so exposed that the metallic aluminum may be used as a contact material. However, in other appliances the terminals are enclosed or partially enclosed or shielded so much hotter than open or exposed terminals. In such cases contacting members of the same general nature as aluminum but of a higher softening point are required.

I have found that certain of the aluminum alloys have a higher melting point as well as a softening range prior to complete fusion, and that these alloys provide durable contacting members for rigid non-metallic resistors, particularly for intermittent service and in devices which prevent rapid dissipation of heat from the terminal members. Most domestic appliances are subjected to intermittent service inwhich the current that they become is turned on to the resistors for a short time and then the current shut off, so that the devicesare subjected to continued cycles of heating and cooling. This makes the termi- 'nal conditions severe, tending'to cause arcing andlocal overheating.

As examples of such alloys are the aluminum base alloys containing a metal of the iron group, more particularly. the iron family comprising the metals iron, nickel and cobalt, preferably up to about 40% of nickel or up to about 30% of iron. Greater amounts of nickel and iron may be alloyed with the aluminum in certain cases where plasticity of the alloy under heat is not re quired, but in cases where the successful op eration of the contact depends upon the high points of the resistor surface becoming em bedded in the contacting member to prevent excessive heating, it is desirable that the aboye amounts of nickel and iron should not be greatly exceeded. This is because of the fact that with these figures as the upper limits for the nickel or iron, aluminum exists in the resulting alloy in a free-as well as in a. combined state. It is a well recognized fact that when aluminum is alloyedwith a second metal, thealuminum may crystallize as substant ally pure metal microscopically'intermingled with the other constituents of the alloy, or it may be'combined as an interme'tallic compound or separate as a phase having none of the characteristics of the pure metal. The intermetallic compounds usually have both physical and chemical properties which are entirely different from those of the component metals, so that as the percentage composition of the compornd is approached or exceeded, the properties of the aluminum disappear. In the alloys havin a composition within the limits specified-a ove, a portion of the alupressure. Thus, aluminum-nickel the alloy, the alloys of aluminum with the metals of the iron family do not possess a low incipient melting point, and the property of softening at a comparatively low temperature is lost.

Alloys consisting of nickel and aluminum and containing up to about 42% of nickel have an incipient melting or softening temperature of approximately 620 centigrade. An alloy containing 40% nickel has a very small percentage of molten metal at this tem perature and is not completely melted until it reaches approximately l100 centigrade. By a variation of composition in either direction the amount of the low melting constituent, aluminum, may be varied, thus changing the effectual melting point, or the temperature at which the alloy becomes too soft to retain its shape under the contact alloys containing up to about 42% nickel retain to a degree the desirable characteristic, plasticity, of pure aluminum while hot, even though they may be brittle at ordinary room temperature. Moreover, these alloys have a so lidification range of several hundred degrees centigrade.

A similar relationship obtains with alloys of aluminum and iron. Free aluminum vanish'es as an alloy constituent at approximately 40% iron, but as this composition is approached the alloys become so brittle that the contacts are apt to crack in service. I

have found that an aluminum-iron alloy containing 30% iron has an incipient melting point of approximately 048 Centigrade, and is not completely molten until approximately l100 centigrade, thus giving a plastic range of approximately 450 centigrade. \Vhile these aluminum-nickel and, aluminum-iron alloys are somewhat harder than pure aluminum, they are nevertheless softer at the operating temperature than the iron-chromium and iron-nickel alloys commonly employed as contact terminals and allow the projecting crystals points of the resistors to press into the metal. Moreover, the lower end of their plastic range is reached at a relatively low temperature and one considerably below their temperature of complete melting. so that if the current becomes concentrated at the point of a projecting crystal so that overheating results, the metal at this point hecomes plastic and allows the point to become ing the local overheating. This capacity of l 7 much superior to those of the commonly used materials, such as the chrome-iron, chromenickel steels which have heat resistant but hard surfaces, and are found to last from ten to fifty times as long on a continuous intermittent test which comprises subjecting the resistor and its contacts to alternate fifteen minute periods of current turned on and oil". The contact terminals of the aluminum alloys as herein described are an improvement upon and are superior to a plain aluminum ctmtact, particularly when subjected to severe usage or when employed on enclosed terminals or other terminals which become more highly heatedthan the ordinary exposed terminals.

Referring to the illustrated embodiment of the invention, reference'numeral 1 indicates a domestic hot plate having non-metallic rigid silicon carbide resistors 2. The resistors are held at each endby contact terminals 3. As'shown in the drawings, these cont-act term nals are partially enclosed or shielded by the surrounding parts of the hot plate so that they do not have a good opportunity to radiate heat.

Each contact terminal comprises an alloy steel spring 4 having upper ends-bent over to form inwardly directed flanges 5 to hold a contact piece 6 of an aluminum-nickel or aluminum-iron alloy. This piece 6-is preferably hollowed out somewhat as indicated at 7 to provide a seat for the end of the resistor rod 2. The end of this resistor rod is,

'of course, somewhat rough 'because of its crystalline nature. however, permits the roughend of the rod to become embedded in it and furnish a good contact even under the severe conditions of intermittent use to which such devices are subjected.

In Figure 5 there is illustrated another form of contact; namely, a cone-shaped'piece 10 of aluminum alloy yieldingly pressed by spring 11 against the end of a resistor'rod 12.

In Figure 6 is illustrated still another ,modiiication of the invention in which a cup shaped piece 20 of aluminum alloy is pressed by spring 21 againstthe'endof a resistor rod While I have specificallyillustrated and described the preferred embodiments of my invention, it is to be understood that the pres-v ent invention is not so limited but may be otherwise embodied within the scope of the following claims.

The aluminum alloy,

a 191 3 f v I claim:' I 1. In an electric heating device, the combination of a rigid resistor, and terminal members having contact surfacesof an alloy of aluminum and a metal of the iron family and in which some of the aluminum is present in a free and uncombined state.

2. In an electric heating device,'the combination of a rigid resistor, and terminal members having contact surfaces of an alu- 5 minum-nickel alloy, the alloy containing some aluminum in a free and uncombined state.

3. In an electric heating device, the combination of a rigid resistor, and terminal members having contact surfaces of an aluminum-nickel alloy containing up to about 12% nickel.

4. In ,an electric heating device, the combination of a rigid resistor,and terminal members having contact surfaces of an alu- 35 minum-nickel alloy containing approximately 40% nickel.

5. In an electric heating device, the combination of a rigid resistor, and terminal members having contact surfaces of an aluminum base alloy containing aluminum in a free state having a temperature interval be tween its temperature of incipient melting and its temperature of complete melting o more than 400 C. v

6. In an'electric heating device, the combination of a rigid resistor, and terminal members having contact surfaces of an aluminum base alloy containing some aluminum in the free state and having an incipient melting point substantially the same as the melting point of pure aluminum and a temperature' of complete melting at least 300 C. above the melting point of pure aluminum.

7. In an electric heating device, the com bination of a rigid resistor and terminal members having contact surfaces of an alloy of. aluminum and an iron family metal containing some pf the aluminum in a free state which softens at a temperature several hundred degrees below its fusion point so that projections on the risistor where local overheating occurs may become embedded in the metal. b

8. In an electric heating device, .the combination of a rigid resistor, terminal members having contact surfaces of an alloy ofv aluminum and a metal of the iron family and in which some of the aluminum is present in a free and uncombined state, and means for 'yieldingly holding the terminal members against the resistor.

9. In an electric heating device, the combination of a rigid non-metallic resistor and terminal members having contact surfaces of an aluminum iron alloy having not more than 40% of iron.

i .In testimony whereof I have hereunto set m hand.

y w A. BOYER- 13o

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