US 3513540 A
Abstract available in
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Description (OCR text may contain errors)
May 26, 1970 A. c. BoGGs 3,513,540
TERMINAL CLOSURE FOR METAL SHEATHED, ELECTRIC RESISTANCE HEATING ELEMENTS Filed Sept. 6, 1967 2 Sheets-Sheet 1 Iwsl.
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Bosse /l/w MM ALBEN C.
A NORA/@f5 United States Patent O 3,513,540 TERMINAL CLOSURE FOR METAL SHEATHED, ELECTRIC RESISTANCE HEATING ELEMENTS Alben C. Boggs, Pittsburgh, Pa., assignor to Edwin L.
Wiegand Company, Pittsburgh, Pa.
Filed Sept. 6, 1967, Ser. No. 665,830
Int. Cl. Hb 3/00 U.S. Cl. 29-611 3 Claims ABSTRACT 0F THE DISCLOSURE The method of making a hermetically sealed closure at the open end Iof a tubular, metallic sheathed, electric resistance heating element, and the article produced by such method, which consists of disposing a quantity of flowable, heat-expandable dielectric material at the element open end and between the latter and a closure member, compressing the dielectric material by urging the closure member toward the element end, hermetically securing the closure member to the element sheath to entrap the dielectric material, and applying heat to the assembly to expand the dielectric material in situ.
PREAMBLE The terminal ends 'of metal sheathed, electric resistance heating elements, especially those which are hermetically sealed and operate at relatively high temperatures, have presented a manufacturing problem. Not only is the manufacture of such terminal ends expensive, in many cases an electrical breakdown occurs internally of and at the terminal end which renders the element useless. Unfortunately, such breakdowns occur after completion of the element and thus it is a iinished element which must be scrapped.
It has long been known that highly compacted, granular dielectric material provides a highly eiiicient barrier to high voltage. Heretofore, however, it has been impossible to utilize such material at the terminal end of a hermetically sealed heating element since there was no known way to obtain the necessary degree of compaction.
The present invention, by use of a known material in a novel manner, provides an hermetically sealed, metal sheathed, electric resistance heating element having superior resistance to electrical breakdown. This and other advantages will readily become apparent from a study of the following description and from the appeneded drawings.
OUTLINE OF DRAWINGS In the drawings accompanying this specification and forming a part of this application there is shown, for purpose of illustration, embodiments which the invention may assume, and in these drawings:
FIG. 1 is a broken, side elevational view of the terminal end of an electric resistance heating element embodying the invention,
FIG. 2 is a view in longitudinal section of the heater end seen in FIG. l,
FIGS. 3, 4 and 5 are sectional views showing various parts at successive stages of manufacture, and
FIG. 6 is a View similar to FIG. 2 but of a slightly different embodiment.
With reference to FIGS. 1 and 2, the heating element fragmentarily shown therein comprises the usual tubular metallic sheath 10 in which a coiled resistor conductor 11 is generally concentrically disposed. Resistor conductor 11 terminates short of the end of the sheath 10` and a terminal conductor member, or pin 12, is mechanically and electrically secured to the resistor 11 and protrudes axially outwardly of the sheath. The resistor 11 and the ICC terminal conductor 12 are maintained in spaced relation to the inner wall of the sheath by the usual highly compacted, electric-insulating, heat-conductive refractory material 13, such as granular MgO, in which they are embedded. In the present embodiment, the refractory material 13 preferably terminates short of the end 'of the sheath 10 to provide a pocket 14 for a purpose to appear.
Closing the terminal end of the heating element is a closure member assembly 15 (see also FIG. 4) which comprises an elongated, cylindrical ceramic body 16 apertured at 17 to closely receive the terminal conductor 12. One end of the body is reduced in diameter at 18 to iit within the sheath pocket 14 and a metal sleeve 19 is suitably bonded to the body 16 and has a portion 19.1 preferably overhanging the body end 18 and Iof a size to slidably receive the open end of the sheath 10. Suitably bonded to the other end of body 16 is a reducing sleeve 20 having a first portion 20.1 spaced from the sleeve 19 in a direction axially of the body 16 to insure adequate dielectric clearance therebetween, and, also having a reduced diameter portion 20.2 of a size to closely receive the terminal conductor 12.
With particular reference to FIG. 2, the sleeve end 19.1 is welded, brazed, soldered or otherwise hermetically sealed to the sheath 10 at 21 while the sleeve end 20.2 is similarly secured to the protruding terminal conductor 12 at 22. To facilitate making an electrical connection, (not shown) to the terminal conductor, the sleeve end 20.2 may be externally threaded as shown.
Disposed in the previously mentioned sheath pocket 14 and filling the space dened by the latter and the body end 18 s a compacted mass of dielectric, heat-resistant material 23 which prevents an electrical breakdown, or short circuit, between the terminal conductor 12 and the sheath 10. At the present time and because of its peculiar expansion properties later to be disclosed, material 23 is tinely divided vermiculite.
In the manufacture of the assembly seen in FIGS. l and 2, the electric heating element, less the closure member assembly 15, will be disposed with its terminal end in upright relation as shown in FIG. 3. It will be understood that the refractory material 13 will have previously been highly compacted in the usual manner in the sheath to embed the resistor conductor 11 and the terminal conductor 12 therein. A quantity of finely divided vermiculite in concentrate (or unexpanded) state, as marketed by the W. R. Grace & Co. of Chicago, Ill., and described in their brochure G-201 revised 1961 and 1963, is then disposed in the sheath pocket 14 as illustrated in FIG. 4. The closure member assembly 15 will then be slid over the terminal conductor 12 with the body portion 18 in the sheath pocket and resting upon the mass of vermiculite 23 and with the closure member sleeve 19 about the element sheath 10 as shown in FIG. 5.
The closure member 15 and element will then be forced together, as by application of weight to the closure member, to compress the vermiculite mass 23 and cause it to lill the interstices between the closure member and the adjoining end of the element assembly. With the parts thus assembled, the sleeve end 19 is fused to the sheath by the weld or solder 21 to form a hermetically sealed juncture therebetween.
With the closure member thus secured to the sheath, the force or weight urging the closure member toward the element end may then be removed and the assembly will next be disposed in an oven and subjected to suicient heat for a sufficient length of time to drive off any moisture trapped within the element sheath and to cause expansion or exfoliation of the entrapped vermiculite mass 23. Expansion of the mass 23, since it is in a conlined space, will have the effect of greatly increasing the compression thereon and will insure that no voids exist across which'an `electric current might jump between the sheath and the terminal conductor 12 or between the latter and the sleeve 19'.
After the moisture has been driven from the heater and the mass of vermiculite has been expanded, the assembly will be removed from the oven and preferably permitted to cool in a moisture-free atmosphere. Upon cooling, the sleeve 20 will be fused to the terminal conductor 12 by weld or the like 22 to form a hermetically sealed juncture therebetween.
While only one element terminal end has been disclosed, it will be understood that the usual element has two ends, each of which will preferably be sealed in the manner hereinbefore disclosed.
The embodiment of FIG. 6 is similar to that heretofore disclosed; accordingly, corresponding parts are identified by the same reference characters but with the suix a added. In this embodiment, the pocket at the open end of the element sheath has been omitted; that is, the compacted refractory material 13a extends all the way to the end of the element sheath 16a. Moreover, the reduced diameter end of the closure member ceramic body 16 is omitted and such body end is merely disposed adjacent the sheath end with the mass of vermiculite intervening.
During assembly 0f the embodiment seen in FIG. 6, since there is no pocket at the sheath end in which the mass 0f verrniculite may be disposed prior to assembly of the closure member with the element, the vermiculite may merely be deposited in a mass atthe end of the element and about the terminal conductor. Alternatively, during assembly of this embodiment, the closure member may lbe inverted from the position seen in FIG. 4, and the vermiculite may be deposited in the closure member end within the pocket provided by the member sleeve 19a and the adjoining end of the member ceramic body 16a prior to assembly of the closure member with the element end. Thereafter, assembly of this embodiment may proceed in the manner previously outlined.
While manufacture of a hermetically sealed heating has been disclosed, it will be understood that the invention may find application wherev a hermetically sealed heating element closure is not required.
In view of the foregoing it will be apparent to those skilled in the art that I have accomplished at least the principal object of my invention and it will also be apparent to those skilled in the art that the embodiments herein described may be variously changed and modified, without departing from the spirit of the invention, and that the invention is capable of uses and has advantages not herein specifically described; hence i't will be appreciated that the herein disclosed embodiments are illustrative only, and that my invention is not limited thereto.
1. The method of making a closure at the open end of a tubular, metallic sheathed, electric resistance heating element having a terminal conductor em-bedded in compacted, electric-insulating, heat-conducting refractory material within the sheath and protruding from an open end thereof, which method comprises entrapping a quantity of granulated vermiculite surrounding said terminal conductor in a space formed by said open end and a closure rnember tting the same, forcing the closure member and the sheath together to decrease the size of the space and compress the vermiculite therewithin, and applying heat to said heating element to exfoliate the vermiculite.
2. The method of -claim 1 and further including the step of securing said closure member to said heating element in hermetically sealed manner.
3. The-method of claim 1 wherein said closure member includes a metal sleeve having one end closely fitting around the sheath open end, a ceramic sleeve having one end closely tting within the other end of said metal sleeve and hermetically sealed thereto, a tubular metallic terminal having one end closely fitting around the other end of said ceramic sleeve and hermetically sealed thereto, said space being formed 'by said open sheath end, the interior of said metal sleeve and the adjoining end of said ceramic sleeve With said terminal conductor loosely fitting through said space and through said ceramic sleeve and said tubular metallic terminal, said method including the further steps of fusing said metal sleeve to the exterior of said sheath to form a hermetic seal therebetween after said vermiculite has been compressed, thereafter applying heat to said heating element to first remove moisture entrapped within said heating element and to then cause said exfoliation of said vermiculite, said moisture being vented through space between said terminal conductor and said metal sleeve, said ceramic sleeve and outwardly through said tubular metallic terminal, and thereafter fusing said terminal conductor to said tubular metallic terminal effect a hermetic seal therebetween.
References Cited UNITED STATES PATENTS 1,735,831 11/1929 Lightfoot. 2,462,255 2/ 1949 Charman et al. 3,122,718 2/1964 Boggs 338-273 3,387,363 `6/1968 Boggs 29-619 FOREIGN PATENTS 495,212 11/ 1938 Great Britain.
CHARLIE T. MOON, Primary Examiner V. A. DIPALMA, Assistant Examiner U.S. Cl. X.R.