USRE25994E - Hermetically sealed, sheathed electric heating elements - Google Patents

Description

J7@ j fg/? A. C. BOGGS Original Filed Aug. 16, 1961 HERMETICALLY SEALED, SHEATHED ELECTRIC HEATING ELEMENTS P/v'af' rl" /j/ March 29, 1966 United States Patent O 25,994 HERMETICALLY SEALED, SHEATHED ELECTRIC HEATING ELEMENTS Alben C. Boggs, Pittsburgh, Pa., assignor to Edwin L. Wiegand Company, Pittsburgh, Pa.

Original No. 3,122,718, dated Feb. 25, 1964, Ser. No. 131,851, Aug. 16, 1961. Application for reissue Dec. 23, 1964, Ser. No. 441,941

Claims. (Cl. SSS- 274) Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specilication; matter printed in italics indicates the additions made by reissue.

The present invention relates to electric heating, more particularly to hermetically sealed, sheathed electric re sistance heating elements and methods of making the same, and the principal object of the invention is to provide new and improved methods and articles of the character described.

When a metallic, tubular sheathed electric resistance heating element is hermetically sealed and then operated at elevated temperatures, a perplexing phenomenon has been observed. Initially, breakdown voltage, i.e., that voltage which will jump from the normal current carrying internal parts of the element to the external sheath, will be satisfactorily high indicating that internal di electric clearances are adequate; however, after the unit has been operated for some time at elevated temperatures, its breakdown voltage drops to such a low level that the element will short out `and thus fail at normal operating voltages. Study of elements which have failed as above described indicates that the air gap at the terminal end of the element for some reason becomes inadequate after a short period of operation although it is quite adequate initially and quite adequate for elements which are not hermetically sealed.

The present invention, by means of a novel structure and a novel method, provides a hermetically sealed, tubular sheathed electric resistance heating element whose breakdown voltage remains sufciently high `to insure against premature element failure. This is accomplished in a relatively simple manner and at little increase in manufacturing cost. These and other advantages will readily become apparent from a study of the following description and from the appended drawing.

In the drawing accompanying this specication and forming a part of this application there is shown, for purpose of illustration. an embodiment which the invention may assume, and in this drawing:

FIGURE l is a fragmentary, broken View in longitudinal section of a heating element embodying the present invention,

FIGURES 2, 3 and 4 illustrate successive stages in the manufacture of the element seen in FIGURE l,

FIGURE 5 is a sectional view of certain structure seen in FIGURE l, and

FIGURE 6 is a View similar to FIGURE l but of prior art construction.

With reference to FIGURES l and 2, there is fragmentarily illustrated one end of an electric resistance heating element of the well-known type having a tubular metallic sheath 10 enclosing a coiled resistor conductor l1 which terminates short of the sheath end. A terminal conductor pin 12 is connected to the end of the resistor conductor 11 and projects axially outwardly beyond `the end of the sheath 10 for connection to a source of electrical energy. Sheath 10 is lilled with highly compacted, electric-insulating, heatconductive material 13 which maintains the resistor conductor and the terminal pin centered within the sheath. For a purpose to appear,

Reissued Mar. 29, 1966 ICC material 13 preferably terminates short of the end of the sheath to form a recess 14 thcreat.

Substantially filling the sheath recess 14 is a body of cement-like, dielectric material 15 in which is embedded one end of a dense, dielectric ceramic sleeve 16 through which passes the terminal pin 12. Sleeve 16 terminates short of the free end of the terminal pin for a reason later to appear.

The means for hermetically sealing the heating element thus far described comprises an assembly 17 best seen in FIGURE 5. Such assembly includes an enlarged, dielectric ceramic sleeve 1S having at one end a metal collar 19 and at the other end a metal ferrule 20. Collar l? and ferrule 20 are hermetically sealed to the exterior of sleeve 18 in axially spaced relation and each projects beyond adjoining sleeve ends. The projecting portion of collar 19 is of a size to closely fit over the exterior of the sheath 10 while the projecting portion of ferrulc 2l) is necked down at 21 to pass the terminal pin 12. Feriule portion 21 may be externally threaded, as shown. to facilitate its connection to a source of electrical energy.

Assembly 17 is secured to the sheath 1! of the heating element by sliding it over the terminal pin 12 until the end of the sheath enters the collar 19 and abuts the sleeve 18. Collar 19 may then be hermetically sealed to the sheath by means of a silver soldered joint 22 or the like. Completing the assembly, a silver soldered joint 23 secures the ferrule 20 to the terminal pin 12.

In assembling the heating element illustrated in FIC-- URE l, the recess 14 of the unsettled element as seen in FIGURE 2 is first substantially llcd with the material 15 in a plastic state as shown in FIGURE 3. The sleeve 16 is then slid over the terminal pin 12 and forced into the still plastic material 1S in the sheath recess 14. The sleeve end will be deeply embedded in material 1S. the latter being compressed and extruded about the exterior of the sleeve and into its interior substantially as shown to preclude the presence of any air gaps across which electrical current might jump. Thereafter, and preferably following hardening of the material 1S to a rock-like mass, the assembly 17 will be slid over the terminal pin and silver soldered or the like to thc element shealh 10 and to the terminal pin 12 previously described.

While but one terminal end of the healing element has been shown and described, it will be understood that its other terminal end will be similarly constructed. More over, it is also to be understood that the invention is not limited to elements having terminals at opposite ends thereof but may also he employed with elements having their terminals in adjoining relation.

The advantage of the present invention will become apparent from a study of the prior art construction seen in FIGURE 6 wherein similar parts are ideutihed with the same reference characters as before but with the suffix a added. In this prior art construction, current will jump the relatively small air gap between the terminal pin 12a and the extreme terminal end of the sheath 10a under the conditions previously outlined. However, with the construction shown in FIGURE l. the addition of the sleeve 16 will force the current to travel practically the entire length of such sleeve in order to bridge the gap between the terminal pin and the extreme end of the sheath. In practice, the normal voltages impressed across the heater terminals will be unable to jump such a tremendously increased air gap and thus electrical breakdowns which previously plagued sealed heating elements is ellcctively prevented.

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 embodiment 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 it will be appreciated that the herein disclosed embodiment is illustrative only, and that my invention is not limited thereto.

l claim:

1. An electric resistance heating element comprising a tubular metallic sheath and a resistor conductor in radially spaced relation with said conductor disposed within said sheath in axially spaced relation from one end thereof` a terminal conductor pin secured to said resistor conductor within said sheath and projecting outwardly of said one sheath end, compacted` electric-insulating, heat conductive material within said sheath embedding said resistor conductor and terminal [conductors] conn'mtor pin and such material terminating short of said one sheath end to form a recess thereat, a metallic sleeve having a iinidtight fused connection with said one sheath end and projecting axially therebyond, a metallic cap haring a fluid-tight fused connection with the outwardly projecting portion of said terminal pin and disposed in axially spaced relation with the metallic sleeve aforesaid. [and ceramic sleeve means through which said termnnl pin extends and having a iirst portion extending between and hermetically sealed to said metallic sleeve and said metallic cap and a second portion extending into said sheath end recess. said ceramic sleeve means coniprising] a first ceramic sleeve extending over said terminal pin between and hermetic-filly scaled to said metallic sleeve and said metallic cap, and a second ceramic sleeve extending over said terminal pin and projecting from the recess at said one sheath end axially beyond the latter.

2. The construction of claim 1 wherein said first and second ceramic sleeves are in concentric[, superimposed] relation with thc rst (about thc second.

3. The construction of claim 2 wherein cement-like dielectric material is hardened in situ in said sheath end recess and embeds the adjoining portion of said second ceramic sleeve.

4. The method of forming a hermetically sealed closure at the terminal end of la tubular, metallic sheathed, embedded electric resistance heating element having at such end an axially facing recess through which a terminal conductor pin protrudes beyond the sheath end, which method comprises disposing a plastic, cement-like dielectric material in said recess which subsequently hardens in situ, sliding over said terminal conductor pin and into said recess a lirst ceramic sleeve having a length less than that of the protruding portion of said pin and greater than that of said recess to embed such sleeve in said plastic material, sliding over said terminal conductor pin and said first ceramic sleeve a second ceramic sleeve having a metal collar and a metal ferrule hermetieally sealed to respective sleeve ends in axially spaced relation with each other so that the element sheath tits within said collar and said terminal pin fits within said ferrule, and forming a fluid-tight juncture between the element sheath `and said collar and between said terminal conductor pin and said ferrule.

5. The method of forming a hcrmctically sealed closure at the terminal end of a tubular, metallic sheathed, embedded resistance heating element having al snc/1 end nn axially facing recess through which cz terminal condnctor pin prorrudcs beyond the sheath end, 'which method comprises disposing a plastic, cement-like dielectric nmtcrial in said recess which subsequently hardens in sitzt, sliding over Said terminal conductor pin ceramic sleeve means having a reduced end tting within .mid lrllcftth rcccirs cmd cxtruding said plastic cement-like matcrial about its peripheral end surfaces and into bonding relation therewith and with thc adjacent inner surface of said sheath, said sleeve means having a metal collar at one cfzd and a metal fcrrnle at its other end, both collar and ferm/e being hermctically sealed to Said .riceve means, said collar slidably fitting over said sheath cna' and raid fcrrlllc slidably jtting over said terminal conductor pin, and for/ning n fluid-light juncture between the clement sheath and said collar and bctufccn said terminal conductor pin and said fcrrnle.

References Cited by the Examiner The following references, cited by the Examiners, are of record in the patented file of this patent or the original patent.

UNITED STATES PATENTS 2,272,282 2/1942 Wiegand 338-238 2,455,102 11/1948 Temple 338-242 2,480,903 9/1949 Charbonneau 338-274 X 2,740,874 4/1956 Kelty et ai. 33e-274 X 2,767,288 10/1956 Lennox 33e- 23s 3,134,956 5/1964 Boggs 33e-773 FOREIGN PATENTS 755,741 9/1933 France.

RICHARD M. WOOD, Primary Examiner.

V. Y. MAYEWSKY, Assistant Examiner.

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