US2836806A

US2836806A - Conductive pad for electrode joint

Info

Publication number: US2836806A
Application number: US512611A
Authority: US
Inventors: Robert C Stroup
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1955-06-02
Filing date: 1955-06-02
Publication date: 1958-05-27
Anticipated expiration: 1975-05-27
Also published as: DE1023534B

Description

May 27, 1958 R. c. STROUP 2,336,806

CONDUCTIVE PAD FOR ELECTRODE JOINT Filed June 2, 1955 2 Sheets-Sheet 1 Nipple Electrically Conducfive Pad raphite "3 I" Aluminum Foil Hi H HIII INVENTOR TTORNEY May 27, 1958 R. c. STROUP 2,836,806

CONDUCTIVE PAD FOR ELECTRODE JOINT Filed June 2. 1955 2 Sheets-Sheet 2 d Nipple Graphite Elecfrodes INVENTOR ROBERT C. STROUP B ATTORNEY United States Patent Ofitice 2,836,806 CONDUCTIV E PAD FOR ELECTRODE JOINT Robert C. Stroup, Lewiston,

N. Y., assignor to Union Carbide Corporation,

This invention relates to an improvement in the joint between sections of an electric-furnace electrode and has for an object to provide a more conductive joint at operating temperatures than has heretofore been posisble. With such joints having a tapered nipple threaded into each electrode section, another object is to reduce the danger of an electrode section rupturing due to the absence of adequate thread clearance space in one such section. A further object is to provide more nearly equal conductivity between a tapered nipple and each connected section.

The prior art has appreciated the need for an electrically conductive pad at each end of the usual nipple in the joint between sections of an arc-furnace electrode. Without such a pad there is excessive current density and heating in the socket wall at its base and more rapid oxidation on the outer surface of the electrode sections radially outside such nipple end portions. A variety of materials have been proposed forsuch pads. One suggestion was metal wool but the electrode sections and their joints are subjected to a higher temperature than that at which the usual metals melt.

When the nipple is either straight or tapered the foregoing advantage accrues but with the tapered nipple other and more difiicult problems arise. It is diflicult to insert the tapered nipple into the first and usually the lower electrode section just the correct amount and no more. If the tapered nipple is inserted too far into that first section it will be so tight that the usual threadclearance spaces between the nipple and socket threads disappear. One danger from that happening is the greater likelihood of rupture occurring in either the nippie or electrode section due to thermal stresses and vibrations of the electrode in a furnace during use. Another danger is that when the last section to be connected has been screwed onto the nipple until the end faces of the sections are in abutting contact, the thread clearance spaces in such last section are likely to be too large resulting in less contact area in the last section than in the first with unequal electrical resistance and unequal heating of the two electrode sections during use. If the tapered nipple is screwed into the first section to be connected less than it should be the danger of inadequate thread clearance space increases in the last section in bringing the end faces of the sections into contact.

According to this invention lower electrical resistance is obtained with either a straight or tapered nipple by insertion of an electrically conductive pad of compressible graphite between each end of the nipple and the base of the socket into which the nipple fits in each of the connected sections. In an electrode joint having a tapered nipple, according to the present invention it has been discovered that the use of a compressible graphite pad in the bottom of the socket of the lower electrode section into which a tapered nipple is first inserted provides a convenient means to insure such nipple not be- 1 2,836,806 Patented May 27, 1958 ing screwed in too tightly with loss of the desired thread clearance spaces. With the pad and dimensions of the nipple and socket carefully made, each pad should be of the same size, and it is only necessary to apply a predetermined torque to insure the compression of the pads to the desired amount for having the thread-clearance spaces equal on each side of the nipple.

The high-temperature conductive materials for the maximum amount of compression are skeletal and natural graphite. Pads containing such materials are compressible up to about 50% of their original thickness. Such a degree of compressibility is desirable to produce centering of the nipple. For much less compressibility ordinary graphite is suitable, or a combination of the compressible graphites with other materials may be used. A pure skeletal or natural graphite is not necessary so long as a pad possesses a compressibility commensurate with variations in dimensions encountered, or, in the case of the tapered connection, is sufilcicntly compressible to produce the centering of the nipple.

Referring to the drawings:

Fig. 1 is a view through an electrode joint embodying this invention, parts of each electrode section and part of the nipple being broken away and shown in section;

Fig. 2 is a perspective partly in section of the conductive pad of the present invention;

Fig. 3 is a partial longitudinal section through an electrode joint having a tapered nipple and embodying the present invention.

As shown in Fig. l arc-furnace electrode sections 10 and 11 are connected by a straightthreaded nipple 12 in order that their end faces 13 may abut one another and conduct current between the sections through these end faces as well as through the nipple 12. It has long been known that it is not commercially possible to have good conductive contact between both end faces 13 and also between each end of the nipple 12 and the adjacent base 14 of the socket in which the nipple is received. In order to reduce the current density in the socket side walls a variety of suggestions have been made for a conductive pad between the ends of the nipple and the base of the nipple socket. Under this invention a conductive pad 15 is provided which is compressible. In the installations where the maximum compressibility of such a pad 15 is required it has been found that either natural or skeletal graphite 16 is preferable. Skeletal graphite is one formed by the decomposition of silica carbide at elevated temperatures and these two materials are the only ones found suitable where the pad must be subjected to a compression reducing it to only half its original thickness. Such a degree of compressibility is encountered where no close tolerances are possible in the formation of the socket. The pad is usually made of slightly smaller diameter than the end of the nipple and expands slightly in a radial direction when compressive forces are applied to it. However, it does not necessarily have to expand radially since some compressible graphites when compressed longitudinally become more dense rather than expanding laterally. The

compressible graphite 16 being deformable and difiicult to handle with cleanliness, is provided with a wrapper 17 of a material which is capable of being destroyed under the high temperatures to which the electrode joint is subjected in use. For example an aluminum foil is a suitable wrapper which is conductive at the lower temperatures and which melts at the higher temperatures and substantially before reaching such maximum temperatures. Other materials found suitable include kraft paper and a wide variety of decomposable plastic insulating sheets such as polyethylene and others. These dielectric wrappers while not conductive at low temperatures nevertheless are destroyed or consumed or decomposed at elevated temperatures and the thickness of the wrapper is-small compared to the thickness of the pad. Ordi-- sadly it has been found that on destruction of the wrapper good contact for the conduction of current through the pad is obtainable. However, where close tolerances are obtainable it is sometimes desirable to have the pad expand slightly laterally as well as be compressed axially especially after destruction of the wrapper and in such case a conductive wrapper of a mixture of graphite with a small amount of oxidized graphite will be found to expand slightly as well as be compressed slightly. To maintain substantially uniform distribution of the compressible graphite and to stiffen the pad, compartmentalizing means such as staples 19 (Fig. 2 maybe used.

Referrring to Fig. 3 electrode sections a and 11a, preferably of graphite, are connected by a tapered nipple 12a until their end faces 13a meet and form a currentconducting contact that will pass a large portion of the current. Between the base of each section recess 14a and an adjacent end of the nipple is placed a compressible graphite pad 15a which. corresponds to the pad 15 of Fig. 2. Adjacent each end of the nipple are provided radiai or diametral recessesZt for receiving a preformed stick or cartridge of pitch or other thermal setting cementitious material as described in the prior application of Johnson et 211., Serial No. 461,714, filed October 12, 1954, for Pitch Cartridge for Electrode Joint, now Patent No. 2,735,705.

Pad 15a and nipple 12a are first inserted into either electrode section. of the nipple recess in the heated section and the nipple end face it would be difiicult to determine when the nipple had been threaded into its section the correct amount. If the nipple is threaded into the face section too far the usual thread clearance spaces shown in the drawing will be filled up, with the result that when the next section is screwed on to the nipple and the end faces 13a brought into contact the thread clearance spaces in the later added section will be larger than those in the first section. This will entail the disadvantage of having the resistance between section and the nipple of different value from thatbetween the nipple and the other section, whereas it is desirable that the conductivity between the nipple and each section be substantially the same. A usual process for insertion of the pads andnipple insures equal thread-clearance spaces on each side of the nipple. After inserting the pad 15a in the first section to receive the nipple 12a the nipple is usually tightened to slightly compress said pad under hand pressure. pad is then inserted in the opposite section and the nipple inserted in said second section far enough to produce a r' slightly greater compression on the second than on the first pad. Additional relativeturning takes place between the sections. This causes the first pad to be compressed, then the second, then again the first, etc. alternately further compressing each pad by small increments until the end faces 13a are brought into good conductive contact and the thread spaces on each side of the nipple are thus automatically equalized with such spaces on the opposite taper of the nipple without the necessity for care fully measuring the torque applied for compressingeach pad. In this way the thread clearance spaces areequalized for each taper of the nipple.

Among the advantages of this invention may be men- Without a pad between the bottom The opposite tioned the ability of the present pad to reduce the current density in the side wall portion of the nipple socket adjacent its base with reduction of oxidation of the outer surface of the socket walls at a position radially outside the ends of the nipple. Unlike any prior art compressible pad, that of the present invention is able to enhance conductivity of the joint at the high operating temperature at which ordinary metals melt. In a tapered nipple type joint the use of the pad in at least the first section into which the nipple is threaded enables an operator to be more certain the nipple is threaded into that socket just the right amount. This equalization of thread clearances between the two' sockets of the joint produces an assembly less prone to fail through socket cracks since there is clearance in both sockets to absorb any strains developed as opposed to the normal assembly with no pads where there is no clearance in one socket.

I claim:

1. In a joint between sections of an electric-furnace electrode having a threaded nipple received Within sockets in adjacent ends of the connected sections, the improvement comprising a pad of compressible graphite between an end of said nipple and the bottom of the socket in which said end of the nipple is' received, said pad having an area approximately as large as said nipple end, whereby said pad is electrically conductive at temperatures above the melting point of steel wool and affords improved electrical conductivity in said joint under hightemperature operating conditions, said electrode being graphite and said pad being at least one of skeletal and natural graphite capable of being compressed to about half its formerthickness.

2.: A compressible, graphite conductive pad for in sertion" between the endof a threaded nipple and the base of the nipple socket in furnace electrode section;

saidpadbeing of at least one of skeletal and natural graphite and which is capable of being compressed to half its thickness.

3. A pad according to claim 2 in which a wrapper" is provided that is capable of melting or or decomposing at elevated temperatures.

4. A- pad according to'claim 3 in which the graphite is A at least slightly expansible in a direction normal to its thickness dimension.

5. A compressible pad of cylindrical shape comprising a graphitic core; a wrapper capable of being consumed at high temperatures andenclosing said graphitie core, and means for retaining said core and wrapper ir'i assembled relationfwh ereby the application of compressive forces to said pad may be substantially uniformly dis-- tributed. a

6. In a pad constructed according to claim 5, said core consisting of at least one carbonaceous material-from the group-consisting of skeletal and natural graphite.

7. In a pad according to claim 5, said means beings plurality of staples.

lieferences Cited in the file or this patent Iohnsonet al Feb. 21, 1956