US1959776A - Electric heating unit - Google Patents

Description

May 22, 1934. A T 1,959,776

ELECTRIC HEATING UNIT Filed Dec. 3, 1929 V mm Inventor". Charles C. Abbott, by

His Attorheg.

- Patented May 22, 1934 1,959,776

UNITED STATES PATENT OFFICE ELECTRIC HEATING UNIT Charles 0. Abbott, Pittsfleld, Mass., assignor to General Electric Company, a corporation New York Application December 3, 1929, Serial No. 411,372

" 4 Claims. (01. 201-67) My invention'relates to electric heating units, plates the provision of a method for making an more particularly to electric heating units of the electric heater of the sheath core wire type sheath wire type in which the resistance elewhereby it is possible to form a continuous elecment is embedded in a compacted mass of intric heating unit having any desired length withsulating material enclosed by a metallic sheath, in very wide limits. 60

and has for its object the provision of an im- In carrying my invention into effect in one proved electric heating unit of this character, form thereof, I provide a heating element of the and a simple and eflicient method of making it. core wire type by winding a plurality of elec- Electric heating units of thesheath wire type trical resistance conductors together in twisted are now in common use in electric heaters and form. During the swaging operation, the twist es the like. Electric heaters of this type may-have in the conductors will straighten out and thereeither a helical core of resistance wire or a by prevent an undue strain in the conductors. straight core depending upon the service'to I have also provided suitable means for mainwhich the heater is to be subjected. The re-. taining the elongated core wire resistance elesistance element or core generally is mounted ment central in the metallic sheath while the 70 centrally in the metallic sheath and is completesheath is being loaded with the insulating maly embedded in a powdered heat refractory interial and during the initial swaging operation. sulating material, such as magnesium oxide, For this purpose, I indent or otherwise shape the compacted around the resistance element in metallic sheath so as to provide flattened porsome manner as for instance by reducing the tions at a plurality of points. Preferably, the 75 diameter of the sheath. flattened portions will be positioned in the sheath Often-times, particularly in relatively long in pairs; the portions forming each pair being heating units, it is desirable to use the straight positioned substantially at right angles to each core rather than the helical core as the resistance other. It will be understood that the sheath at element because of the specific resistance rethese flattened portions will be just large enough quired. However, it has been very diflicult hereto receive the resistance element andthus, the tofore to make a relatively long continuous element will be secured in its central position heating unit of this type with a straight core at a plurality of points spaced throughout the resistance element because during the swaging length of the sheath. Afterthe sheath has been 30 operation there was a considerable tendency for loaded with the magnesium oxide but before the straight core wire to break up into comswaging,Icompact the insulating material which paratively short lengths and thereby. open ciris located between each pair of flattened porcuit the heating unit. tions so as to further insure the central location This results from the fact that the metallic of the heating element. p

5 sheath stretches faster than the core wire re- I have further provided a simple and reliable sistance element and consequently, strains which connection between the terminal and the heatoften are sufliciently great .to cause rupture, are ing element whereby the unit may be swaged set up in thecore wire. Even though the core to any desired extent without injury either to wire was not ruptured during the swaging opthe heating element or to the terminal. In car- 40 eration, it was found that in the finished heater rying out this part of my invention, I have pro- 95 the core conductor often had a very irregular vided a recess in the terminal intowhich an end cross section, certain portions of the wire being portion of the twisted element is inserted. Then reduced nearly to the breaking point thereby the diameter of the terminal is reduced so that causing bad spots. The heating element in servthe metal of the terminal is forced into the spaces ice frequently separated at these reduced porbetween the turns of the twisted element. This mo tions because of the alternate extreme heating terminal connection is claimed specifically in my and cooling. copending application, Serial No. 594,276, filed In an endeavor to overcome this difiiculty, it February 20, 1932, which is a division of this has been customary to make the heating unit application.

in several portions and join these portions so as For a more complete understanding of my in- 105 to provide a. unit having the requisite resistance vention reference should be had to the accomand length. This practice proved to be more or panying drawing in which Fig.1 is a sectional less unsatisfactory because oftentimes the joints view illustrating an electric heating unit connecessarily had to be located in very hot zones. structed in accordance with my invention before In one-of its aspects, my invention contemthe indenting operation; Fig. 2 is a fragmentary 1101 its terminal; Fig. 5 is an elevation of the terminal partly in section illustrating the terminal prepared for connection with the heating element; Fig. 6 'is a sectional view showing the assembled heating unit before the swaging operation; Fig. 7 is anelevation of one of the loading washers used in the insulating loading operation; and Fig. 8 is a partial elevation of the-finished electric heating unit constructed in accordance with my invention.

Referring to the drawing, in one form of my invention the elongated heating unit 10 comprises a straight core wire resistance heating element 11 formed from a pair of twisted resistance conductors 11a, 11b, extending centrally of a metallic sheath 12. The resistance element is made from a suitable high resistance material having considerable resiliency, such as nickel-chromium alloy. The heating elementll is embedded in a compacted mass of powdered heat refractory insulating material 13 such as magnesium oxide.

Metallic terminals 14 and 15 are provided for the heating element 11. Each terminal is provided with a stepped longitudinal recess 16 for receiving a correspondingly stepped portion 1'7 of the twistedheating element. The stepped terminal portion of the heating element is formed by a reduced twisted portion 17a of the end portion of the heating element and by the end portion 17b of one of the conductors which projects beyond the twisted portion, preferably in the axis of the element. It will be observed that when the terminal portion 1.7 is inserted in the stepped bore of the terminal, the reduced twisted portion 170. will be received in the larger terminal recess portion 16a and the projecting portion 17b will be received in the smaller recess portion 16b. Each terminal is compressed on its heating element terminal portion so that the metal of the terminal is forced into the spaces or grooves between the turns of theltwisted core heating element. A very 'good electrical and mechanical connection is thus provided between the terminals and the heating element. It will be observed that before a terminal could beremoved or pulled from the heating element, it would be necessary to strip the inner threads which are formed in my invention, the electrical resistance elements 11a and 11b are loosely twisted together with a relatively short pitch to form the resistance heating element 11. While I have. shown but two resistance strands wound about each other to form the twisted heating element, it is to be understood that several, as for instance three or four may be .used if desired.

The terminals 14 and 15 are then attached to their respective end portions of the heating element 11. The heating element 11 is prepared for connection with the terminals 14 and 15 by providing the stepped end terminal portions 17. The projecting portion 17b may be formed by cutting oif or otherwise detaching a portion of one of the conductors (11a, 111)) so as to leave the single strand. After this, the twisted end portion within the sheath. It will be understood that the heating element will have a length sufficient to the larger terminal recess portion 1611. It will be understood that the projecting portion 17b will be bent or twisted so as to lie in the axis of the heating element and further that the portion 16b of the terminal recess will have a diameter approximately equal to that of the projecting strand 17b so that it will just receive this portion. The terminal portion 17 is then inserted in the recess 16 of the terminal, after which the terminal is swaged or otherwise compressed, preferably to a square cross-section, so that its metal is forced into the grooves or spaces between the twisted conductors of the terminal portions 17a of the heating element.

This terminal arrangement is a very important feature of my invention. It will be understood that if the twisted core wire heating element were left full size to the end rather-than having the stepped formation as shown, and the terminal were provided with a straight bore to receive it, the terminal would be so weak at the end of the core wire that it would practically draw in two at this point during the subsequent swaging of the assembled heating unit.

As has been pointed out, I provide the metallic sheath 12 with a plurality of flattened portions 12a positioned so as to maintain the heating element central during the insulating material loading operation and during the initial swaging operations. The sheath at these flattened portions will be sufliciently wide to receive the heating element but ordinarily will not be large enough for the terminals to pass through. Consequently it is generally necessary to assemble the heating elementwith the sheath before the indenting operation.

The metallic sheath 12 is prepared for the.

permit theterminals to project fromthe ends of the sheath. The heating element may be supported and tensioned in the sheath by placing washers 21in the counterbores 20 and which fit onthe terminal members so as to hold them centrally of the sheath. Suitable sleeves or collars 22 are then provided on the terminal members and are secured by means of nuts 23 which are tube and to prevent its kinking during the indenting operation.

After the heating element has been assembled with the sheath as shown in Fig. 1, the sheath is ready for the indenting operation. As shown (Figs. 2 and 3), the indents are placed in the 14 metallic sheath so as to provide the flattened portions 12a in pairs with the axes of the adjacent portions positioned substantially at right angles to eachother. It will be observed that by reason of this arrangement, the reduced portions of each pair taken together form a squareshaped retaining section around the resistance element (Fig. 3). It will be understood that a number of these pairs will be spaced at intervals along the length of the sheath.

It will be observed by reference to Fig, 3 that the passageway through the sheath at these flattened portions is sufficiently large to receive the heating element 11 and by reason of the fact that the adjacent flattened portions are arranged substantially at right angles to each other, the heating element will be secured centrally of the sheath at a plurality of points spaced at intervals throughout its length. .However, it is very important that the reduced sections of the sheath be sufficiently large to prevent gripping of the resistance element at any point. Thus, generally there will be some clearance between the walls of the sheath and the element. It is preferable, therefore, to form each pair of flattened portions in the sheath at substantially 45 to the adjacent pairs. In other words one pair of indents will be formed with their axes substantially at 90 to each other. Then the sheath will be rotated aboutits longitudinal axis through 45 and'another set of indents will be formed with their axes substantially, at 90 to each other. In effect,

therefore, the four indents of adjacent pairs taken together will form an octagonal shaped retaining section. This arrangement precludes any possibility of the element moving from its central position beyond the limits defined by the clearance between the resistance element and the side portions of the two square retaining sec-' tions formed by the adjacent pairs. It will be understood, of course, that if the axes of the flattened portions of each pair were not'thus rotated relatively to the axes of the corresponding flattened portions of the adjacent pairs, the limits of the clearance would be defined by'the distance between the surface of the resistance element and the corner portionsof the square retaining sections.

It will be understood that the flattened portions will be spacedso that the heating element will be properly centered within the sheath but will not be positioned so close to each other that the passageway through the sheath will be restricted. Moreover, it will be understood that the indents will be placed in the sheath not only to maintain the heating element 11 central but also to maintain the terminal members 14 and 15 centrally of the sheath (Fig. 6).

removed and the interior of the sheath is thoroughly cleaned so that any scale which might have become loosened by the indenting process is removed. The interior may thus be cleaned by blowing dry air through the sheath.

The sheath is now ready for the loading operation. The sheath is closed at one end, for example at the left hand end, as viewed in Fig. 6, by a pair of solid washers 21 which fit on the terminal 14 so as to hold the terminal centrally of the tube. These washers, as show'n,are received in the left hand counterbore 20. The sleeve 22 is then positioned on the projecting end portion of the terminal 14 and is secured by means of the nut 23. The terminal 15 is secured in a similar manner centrally of the sheath by means of a triangular shaped washer 26 which is positioned in the counterbore 20 at the right hand end of the sheath. The resistance conductor 11 is placed under a slight tension by turning the nuts 23 inwardly on the terminals.

The sheath is now filled through the openings provided by the triangular washer 26 with the heat refractory powdered insulating material 13. To accomplish this, the sheath is held in an upright position in a special vibrating machine (not shown) with the end provided with the triangular washer uppermost. As the sheath is filled with the insulating material through the openings provided by the triangular washer, it is vibrated to shake the insulating material together and distribute it uniformly around the terminals and the resistance element; It will be understood that the powdered insulating material will readily pass through the flattened portions 12a to the lower portions of the upright sheath. When the sheath has been filled, a solid washer 27 which is similar to the washers 21 is inserted in place around the terminal 15 to close the sheath. The sleeve 22 and the nut 23 are then replaced and the core Wire resistance element 11 is placed under a slight strain by turning the nut 23 inwardly on the terminal 15. The ends of the sheath are then turned over so as to secure the washers in the ends of the sheath.

After the sheath has thus been loaded with the powdered insulating material, I compact the magnesium oxide in these portions of the sheath between the flattened portions 12a around the be accomplished by striking the metallic sheath between each pair of flattened portions with a ratherheavy impacting blow, (as by hitting the sheath with a short split dye in a press. It has been found that this operation so compacts the magnesium oxide around the core wire that it will be held perfectly central within the sheath during the swaging operations. After the insulating material has thus been compacted, the nuts 23 and the sleeves 22 are again removed.

The entire unit is now passed through a swaging or rolling machine for the purpose of compacting the insulating material within the sheath to a hard dense mass, the swaging operation reducing the diameter and increasing the length of the unit. Several passes through the swaging machine generally will remove the flattened portions of the sheath bulging the flattened sides outward somewhat, and it has been found that the sheath will assume a complete cylindrical shape throughout its length. The sheath will lengthen somewhat faster than thecore wire during the first two or three passes through the machine. In otherwords, the projecting terminals will be drawn within the sheath for a short distance. However, the insulating material will generally be sufficiently compacted by the first several passes to secure the terminals against further inward movement. The twisted core wire resistanceduring the swaging operation will be lengthened so that the original relatively short pitch of the wire will be greatly increased. Thus in one specific heating unit, the pitch which was approximately before swaging was increased by the swaging operation to approximately H5. Moreover the terminals 14 and 15 are gradually r ed ucedin diameter by the swaging, the recessed ends contracting around having a final length of approximately 232" which were swaged from an original length of 112". The diameter of this heating unit was reduced by swaging from approximately before swaging to .318" after swaging. It is to be understood, however, that units having much greater lengths may be constructed by my method.

After the unit has been swaged to the desired extent, the Washers at each end are cut off or otherwise removed and the ends of the unit are squared off (Fig. 8). It will be understood that the terminals 14 and 15 will be held firmly in stood that the ends of the units may then be provided with any suitable terminal members such as those shown in the completed heater of Fig. 8.

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

1. The combination in an electric heating unit provided with a metallic sheath and powdered insulating material compacted in said sheath by reducing and elongating the sheath, of a resistance element embedded in said insulating material having substantially the relatively low resistance characteristics of a straight uncoiled resistance element, said resistance element being formed from a plurality of initially straight resistance conductors twisted together in electrical engagement with each other throughoutsubstantially their entire lengths and being thereby longitudinally yieldable without deterimental effects on said resistance conductors when said sheath is reduced and elongated to compact said insulating material. i

2. The method of making an electric heating unit having substantially the low resistance characteristics per unit length of a unit provided with a straight resistance element which comprises twisting a plurality of initially straight elongated resistance conductors together in electrical engagement with each other substantially throughout their entire lengths and with a relatively short pitch to form a resistance element, mounting said twisted conductors lengthwise in an elongated metallic sheath, filling said sheath with powdered insulating material and compacting said insulating material by elongating and reducing said sheath, the pitch of said twisted conductors increasing materially during said,elonresistance element embedded in said insulating material having substantially the relatively low resistance characteristics of a straight uncoiled resistance element, and terminal members secured to the end portions of said element, said resistance element comprising a plurality of initially straight resistance conductors twisted together in electrical engagement with each other substantially throughout their entire lengths and being thereby longitudinally yieldable to provide for the reducing and elongation of said sheath sufiicient to compact said insulating material without the detrimental eifects on said resistance conductors due to excessive strains such as would be produced in a straight uncoiled resistance element having the same relatively great length.

4. The method of forming an electric heating unit which comprises winding a plurality of resistance elements together to form 'a twister resistance heating element, mounting said resistance heating element lengthwise within a metallic sheath, filling said sheath with powdered insulating material,'swaging said sheath so as to compact said insulating material to a hard dense mass around said heating element with the end portions of said heating element released so as-toprevent excessive strains in said resistance element during said swaging process.

CHARLES C. ABBOTT.

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