US3007235A - Method of making sheathed electric heating units - Google Patents

Description

Nov. 7, 1961 c. H. YOHE 3,007,235

METHOD OF MAKING SHEATHED ELECTRIC HEATING UNITS Filed Sept. 2, 1958 INVENTOR. CHARLES H. YOHE Q LMA/ ATTORNEY 3,007,235 METHOD OF MAG SI-EATHED ELECTRIC HEATING UNITS Charles H. Yohe, Glen Ellyn, llll., assignor to General Electric Company, a corporation of New York Filed Sept. 2, 1958, Ser. No. 758,241 3 Claims. (Cl. 29-15564) The present invention relates to methods of making sheathed electric heating units, and, more particularly, to a method for making a plurality of such units from a single assembly.

An electric heating unit of the sheathed type usually comprises an electric resistance conductor or element having a helical form, an enclosing tubular metal sheath, and a body of compressible heat-conducting and electrical-insulating material embedding the resistance conductor and holding it in a substantially central location within the sheath. One of the methods presently employed in manufacturing such electric heating units is disclosed in U.S. Patent No. 2,677,172, granted on May 4, 1954, to Sterling A. Oakley. In that patent, the method disclosed is that of compacting and compressing the heat-conducting and electrical-insulating material around and throughout the electric resistance conductor by means of cold-rolling the assembly through a series of angularly off-set rolls which have grooves designed to grip and compress the metallic sheath around the enclosed materials. According to that disclosed method of making sheathed electric heating units, the unit is produced by winding the resistance conductor on a mandrel to produce a helix, providing the helix with electrical terminals, centrally locating the helix within a tubular sheath, closing off the bottom end of the sheath with a combustible washer, and then filling the sheath in an upright position with a heat-conducting and electricalinsulating material such as magnesium oxide in a granulated form. The insulating material, which is tamped down firmly during the loading operation, fills the space between the helix and the walls of the tubular sheath and also fills up the void in the interior of the helix. Subsequently, the assembly is subjected to a plurality of successive, gradual, cold-rolling passes which act to compress and elongate the sheath and to compact the insulating material in and around the centrally located helix.

it can readily be seen that each electric heating unit which is produced by this method has to be produced as an individual unit. Correct allowance must be made when cutting the helical winding and the tubular sheath to length prior to the subsequent rolling and elongating operations, in order that the finished electric heating unit will be of proper length. Likewise the necessity of fitting the helix with the electrical terminals prior to insertation of the helix into the sheath and subsequent cold-rolling necessitates that the terminals be afiixed by resistance welding or some other method to securely fasten the terminals to the helix. Such a method of ailixing the terminals to the helix is disclosed in US. Patent No. 2,546,315 granted on March 27, 1951, to S. A. Oakley. As pointed out in that patent, one of the primary considerations which led to the introduction of resistance welding the terminals to the resistance conductor helix was the tendency of the conductor to pull away from the terminal during the operations of loading the sheath with the insulating material and the subsequent operation of elongating the sheath to comp-act the insulator. This tendency is accentuated where the helix is composed of a fine wire resistance conductor.

These and other problems led the applicant to strive to improve the manufacturing efliciency of producing such sheathed electric heating units by fabricating a single elongated sheathed electric heating assembly and producice Patented Nov. 7, 1961 ing a plurality of electric heating units of the proper length by severing them from the assembly. In order to accomplish the foregoing, a suitable method of attaching the terminals to the severed portions had to be devised, since it would no longer be possible to attach terminals to the resistance conductor prior to the loading and cold-rolling operations. Likewise, the presence of the compacted insulating material inside the helix made any method of terminal attachment impracticable, since the highly compacted insulating material had to be gouged out from the interior of the helix for a substantial distance in order to expose a sufficient length of the resistance wire helix to insure electrical connection with the terminal. Because of the fineness of the resistance wire and small inside diameter of the helix, it would be exceedingly difficult to avoid damage to the helix.

Accordingly, it is an object of the present invention to provide a method for manufacturing a plurality of terminal-fitted, sheathed electric heating uni-ts from a single elongated assembly.

Another object of the invention is to provide an improved method for manufacturing such an elongated assembly which will be free to receive an electricterminal at any point of severance along the winding.

The invention, in one form, is practiced by winding the resistance conductor helix in such a manner that the turns of the helix are in turn-to-turn relationship to one another so as to preclude the entrance of any substantial quantity of the heat-conducting and electrical-insulating material into the void in the interior of the helix during the loading operation. The insulating material is tamped between the interior walls of the tubular sheath and the exterior of the helix and is subjected to a series of coldrolling passes which compact the insulating material densely around the exterior of the helix. Due to the manner of winding the helix, not more than a very small quantity of the insulating material will enter into the interior of the helix during these cold-rolling operations. Thus when the desired length of electric heating unit is severed from the elongated assembly, the interior of the helix will be free to receive a terminal pin as will be described.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. My invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which:

FIG. 1 is a side elevational view of the loaded sheath in its assembled state subsequent to cold-rolling and annealing.

FIG. 2 is a fragmentary perspective view of the assembly following the loading operation, and undergoing coldrolling.

FIG. 3 is an enlarged transverse sectional view taken along the line 33 in FIG. 2, looking in the direction of the arrows.

FIG. 4 is a view taken along the line 44 in FIG. 2, looking in the direction of the arrows.

FIG. 5 is a view taken along the line 5-5 in FIG. 2, looking in the direction of the arrows.

FIG. 6 is a view taken along the line 6-6 in FIG. 2, looking in the direction of the arrows.

FIG. 7 is a side elevational view, partly in section, of an electric heating unit, as severed from the elongated assembly.

Referring now to the drawing, in FIG. 1 there is illustrated the assembly as it appears subsequent to the coldrolling operation. The assembly may be, for example, of a length of 10 or 12 feet. The final electric heating units which will be produced from this assembly, may, on the other hand, be only a few inches long. The assembly, which is shown generally at 1, comprises an outer metallic tubular sheath 2 that may be formed of a suitable nickel-chromium-iron alloy which has a substantially circular cross-section, and an electrical resistance conductor or element 3 that may be formed of a suitable nickelchromium alloy and located substantially centrally within the sheath 2 and surrounded by a body of heat-conducting and electrical-insulating material 4 such as magnesium oxide. The insulating material 4- is in finely divided or granulated form and holds the resistance conductor 3 in a substantially coaxial relationship with the sheath. The resistance conductor 3 comprises a helix which is formed by winding the resistance wire on a suitable mandrel to establish the ultimately desired resistance per linear inch of helix. Pursuant to this invention, the helix is initially wound in tight turn-to-turn relationship and is of such length relative to the sheath that when it is inserted into the sheath and is stretched as a result of the elongation of the sheath as presently described, the resulting spacing between the turns will not be sufficient to admit any substantial quantity of the insulation material into the interior of the helix during any of the manufacturing operations. In the preliminary assembly In, FIG. 2, the helix is fitted with terminal members 5, preferably pursuant to the above-noted Patent 2,546,315. The terminai members act both to close off the ends of the helix to prevent the entrance of the insulating material therethrough and to provide a support to hold the helix coaxial with the sheath during the loading operation. A combustible washer (not shown) is provided at the lower end of the assembly in, pursuant to Oakley et al. US. Patent 2,483,839, granted October 4, 1949, and the insulating material is then loaded thereinto while it is in an upright position in a suitable loading machine such as is shown in US. Patent 2,316,659, granted on April 13, 1943, to l. L. Andrews. The material is tamped down firmly during the loading operations so that the insulating material lies between the interior walls of the tubular sheath and the exterior of the helix.

The powdered magnesium oxide insulating material is usually a mixture of grains of several sizes, as follows:

The remaining four percent will pass through the final screen. It is probable that at least some of this latter insulating material will also pass between the turns of the helix and accumulate at the bottom of the helix during the loading operation because of the vibration or reciprocation of certain centering devices employed in the said Andrews loading machine. However, the quantity is so small that it can have no effect on the later operations.

After the loading has taken place, another combustible washer (not shown) is provided at the top of the elongated winding and the assembly is then subjected to a series of cold-rolling operations which are substantially as disclosed in the aforementioned U.S. Patent No. 2,677,- 172. In brief, according to the method described in the patent, a series of matching rolls are arranged in vertical sequence. Individual drives are provided for rotating each succeeding set of rolls, which lies over the adjacent set of rolls, at an increased rate of speed in order to adjust for the elongation of the assembly as it is passed between the sets of rolls. Such an arrangement may be seen by referring to FIG. 2 which shows the assembly 1a in vertical position passing through the first three sets of rolls, numbered 8a, 8b, 9a, 917, 10a, and 1%.

As the assembly commences the cold-rolling process, a cross section through the winding, seen in FIG. 3, illustrates the absence of insulating material inside the helix. Each set of rolls, for example, the set of rolls shown as 8a and 81) have complementary grooves 11a and 11b which serve to provide a gripping action and to propel the winding upward through the rolls. The grooves are, in the initial stages, semi-elliptical in cross-section, in order to deform the tubular sheath inwards around the helix and thus to compress and compact the insulating material, as may be seen in FIG. 4.

The succeeding set of rolls 9a and 9b, which are disposed adjacent and vertically above rolls 8a and 8b, are positioned at right angles to the lower rolls, in order to compress the tubular sheath so as to distort it at right angles to the previous distortion. Also the ellipse formed by the grooves 112a and 12b in the set of rolls shown as 9:: and 9b is of smaller cross-section than the preceding elliptical cross-section. The arrangement of the view looking downward on rolls 9a and 912 may be seen in FIG. 5.

Subsequent passes, the next of which may be seen in FIG. 6, are through matching sets of rolls forming ellipses of decreasing cross-sections, each set of rolls running at a greater speed than the preceding set. The final rolls (not shown) have grooves which form circular crosssections in order to produce a final, densely compacted assembly having a circular cross-section. As may be seen in PlGS. 3, 4, 5, and 6, the insulating material is compressed between the exterior of the helix and the inner walls of the tubular sheath. During the rolling operation, the helix is stretched consistent with the elongation of the sheath, and although the individual turns of the helix will then be separated the one from the other as shown in FIG. 1, only a small amount, if any, of the insulating material enters the interior of the helix, by virtue of the initially closely adjacent turns of the helix which were provided for this purpose. At the conclusion of the coldrolling operation, the insulating material about the outside of the helix is quite hard and dense. The elongated winding is next subjected to an annealing operation to remove the hardening effects of cold-rolling; during the annealing, the combustible washers burn themselves out.

At this stage of the operation, represented by FIG. 1, the elongated assembly is ready to be divided into shorter lengths or segments for making electric heating units. At FIG. 7 may be seen such a segment which, in practice, may be cut out of any portion of the elongated assembly such as, for example, through the two points indicated at 77 in FIG. 1. It is to be understood that such severing occurs after the cold-rolling and annealing have been completed.

Reference to FIG. 7 shows the segment, which has een cut out of the elongated winding, during two stages of affixing the terminals to the ends of the segment. At the lower end of the drawing is shown one end of the segment after a reaming operation has been performed in order to remove the insulating material from between the sheath and the helix and to clip off a. portion of the helix so as to produce a void space 13 which will subsequently receive a final heat-resistant seal of some material such as glass or plastic. An electrical terminal pin 14 having external threads adapting it to be screwed into the helix, is shown in position for insertion into the end of the helix. The pin is thereby affixed to the helix on the interior surface thereof in intimate relation so as to provide both a positive mechanical and a good electrical connection. Alternatively, the connection may be accomplished by force fit, such as by driving a slightly oversize, smoothsurfaced terminal pin, such as the upper terminal pin 14a, FIG. 7, into the helix. Both of these terminal pin constructions are common in the art.

This is obviously a much more satisfactory operation than would be required if it were necessary to remove compacted magnesium oxide from the interior of the helix to accommodate the rather substantial penetration of the terminal pin thereinto. In addition to the inherent difiiculty of removing the compacted material from a central area which may be only 0.09" in diameter, it is obviously necessary that the resistance wire itself-which may be #27 gauge (0.0142" in diarneter)be undamaged during the process.

The top portion of FIG. 7 shows an electric heating unit as it appears in final form with the terminal pin inserted in the helix and the final terminal seal 15 applied. The terminal seal 15 may be applied in the manner described in Patent No. 1,992,787, granted on February 26, 1935, to Robert I. Sutton. Depending on the ultimate use of the heating element, the seal may also be of the various plastic materials which are now available for such use.

Thus it can be seen that the invention, as disclosed, provides -a method for producing a plurality of segments which are easily adapted to receive terminal connections. Whereas previous methods compacted the insulating material both inside and outside of the helix, in the present method, the interior of the helix is clear to receive a terminal pin which can make a good mechanical and electrical connection. There is no necessity for reaming out the interior of the helix and possibly damaging the resistance conductor winding. The terminal pin may be attached by a simple mechanical connection rather than by rested-ing to resistance welding.

Also it can be seen that by producing an elongated winding, substantial savings can be effected in the manufacturing operation by reducing handling costs, since the winding is produced as a single unit during loading, rolling, and annealing. Previous methods necessitated the handling of many smaller units and entailed greater expense than the disclosed method due to the cost of handling these small units.

An additional saving is accomplished since no allowance need be made .for the elongation of the element in passing through the series of rolls. As previously mentioned, such allowance had .to be carefully calculated in order to produce a resultant unit of the proper length. Some waste was therefore encountered with units which sometimes did not achieve the proper length. In the disclosed method, little regard need be paid to the resultant length of the elongated assembly, since the segments are cut from this winding to the proper length, after the cold-rolling has taken place.

Another advantage of the invention results from the fact that fora given unit of length of the finished heating unit, the closer spacing of the turns of the resistance helix requires a greater overall length of resistance wire as compared with that of presently known units. For the same resistance per unit of length, therefore, the resistance wire of the present invention will be of heavier gauge, resulting in a lower wire temperature during use, and longer life.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications which fall within the scope of this invention.

I claim:

1. The method of making an elongated heating unit assembly suitable for division into shorter lengths to which electrical connection terminals may be applied, which includes the steps of providing a quantity of finely divided granular insulation material in which the majority of the grains exceed a certain size, providing a resistance wire helix in which the spacing between each turn and its next succeeding turn is less than the size of the said majority of insulation material grains, confining said helix axially within a straight length of tubular metallic sheath in spaced relation to the inner wall thereof, filling only the space between said helix and the said sheath with said insulation material, confining the insulation material against escape from said sheath, and reducing the diameter of said sheath along the length thereof to comp-act the insulation material into -a hard, dense mass between the helix and said sheath while maintaining the spacing between the turns of the helix too small to pass the said majority of the granular material and thereby keeping interior of said helix substantially free of insulation material, whereby at any intermediate length severed from the assembly, the said helix will freely admit terminal members interiorly thereof in electrical connection therewith.

2. The method of making an elongated heating unit assembly suitable for division into shorter lengths to which electrical connection terminals may be applied,

which includes the steps of providing a quantity of finely divided granular insulation material in which the majority of the grains exceed a certain size, providing a resistance wire helix in which each turn and its next succeeding. turn are in tight turn-to-turn relation, confining said helix axially within a straight length of tubular metallic sheath in spaced relation to the inner wall thereof, filling only the space between said helix and the said sheath with said insulation material, confining the insulation material against escape from said sheath, and elongating the sheath and therein contained helix to reduce the diameter of said sheath to compact the insulation material into a hard, dense mass between the helix and said sheath while establishing a spacing between the turns of the helix insufiicient to pass the said majority of the granular insulation material therethrough, whereby the said helix will be free of compacted insulation material and thereby accommodate terminal members interiorly thereof in electrical connection therewith.

3. The method of making a plurality of terminal-fitted electric heating units from a single, elongated assembly, which includes the steps of providing an electrical resistance conductor wound in the form of a helix having a multiplicity of turns of substantially uniform diameter in which each turn is closely adjacent the next succeeding turn, securing a conductive terminal within each end of the helix in electrical connection therewith, disposing said helix axially within a straight length of tubular metallic sheath in spaced relation to the inner wall thereof with the said terminals projecting axially therefrom, filling the space between said helix and said wall within finely divided granular insulation material in which the majority of the grains are too large to pass into the center space of the helix, confining said insulation material against escape from said sheath, compacting said insulation material in said space into a hard, dense, mass while maintaining the spacing of the coils of the helix sufficient to keep the interior of the helix substantially free of insulation material, dividing the thus-prepared assembly into desired shorter lengths, and inserting conductive terminal members into the open ends of the helix as needed to provide each of the shorter lengths with an electrical terminal at each end thereof.

References Cited in the file of this patent UNITED STATES PATENTS 1,523,434 Lightfoot et al. Jan. 20, 1925 2,093,810 Karmazin Sept. 21, 1937 2,166,109 Karmazin July 18, 1939 2,546,315 Oakley .Mar. 27, 1951 2,629,922 Finch Mar. 3, 1953 2,677,172 Oakley May 4, 1954

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