Patent GB-883759-A (GB)
Title: Improvements in or relating to screw joints between graphite or other carbon electrodes
IPC: H05B7/14
Filed: 1958-04-11
Abstract: <PICT:0883759/III/1> <PICT:0883759/III/2> Two carbon electrodes 5, 8 for electric furnaces, e.g. for steel production are joined in abutment by a double-conical nipple plug 2 having screw threads engaging corresponding threads of conical sockets one formed in each electrode, and the joint is arranged so that when assembled for use the thread flanks 4 that on each one of the two nipple cones face away from the abutment surface 7 of the electrodes are spaced from the adjacent socket-thread flanks by a distance approximately equal to the corresponding distance between the thread flanks 4 of the other nipple cone and socket, so that when the assembled electrode is in use and the nipple plug 2 is hotter than the sockets, the nipple plug can expand freely in the sockets.  The thread flanks 3 on both cone parts of the nipple facing toward the abutment surface 7 of the electrode are in full contact with the threads of the sockets.  In one embodiment the equal spacing of the thread flanks 4 on both cones of the nipple from the socket threads is ensured by inserting spacer members of thermally unstable material, e.g. paper, synthetic plastic or metals of low M.P., e.g. lead, tin, zinc or aluminium, on the turns of the socket or nipple portions first to be joined together and preferably on the turns of both sockets and conical parts of the nipple.  The inserts preferably have on both sides hard granules which, before carbonizing or deformation of the inserts during heating of the electrode, prevent loosening of the screw-threads. Alternatively the desired spacing between the thread flanks 4 and the socket is achieved by disposing a spacer member of thermally unstable material on the bottom of the socket of the electrode to be first joined with the nipple so as to be in pressure engagement with the end of the nipple to limit the screwing-in of the nipple when it is tightened.  This spacer member has a coefficient of expansion at most equal to that of the electrodes, is plastically deformable above 200 DEG  C. and may be a hard pitch which at operating temperature carbonizes to cake and forms an electric connection between the socket and nipple.  The spacer member may consist partly of a carbide-forming metal, or may be of the same material as the nipple and form an integral projection of the nipple.  In another embodiment, Fig. 8, the nipple 2 is provided at its greatest diameter with a protruding annular shoulder 31 which is tightened against an annular recess in the electrode 5 in which the nipple is first inserted in order to obtain the spacing of the thread flanks 4 from the socket. This spacing may alternately be achieved by first tightening the nipple in the electrode 5, then loosening the nipple slightly to obtain the spacing of the thread flanks 4 and securing the nipple in that position by a transverse pin.  The nipple plug may have conical threads shaped to provide in the tightened joint a radial spacing 6 between the crest of the nipple and the bottom groove of the socket thread at least 0,05% of the outer nipple diameter. The nipple thread flanks 3 facing toward the electrode abutment 7 are inclined preferably at less than 30 degrees to the electrode front face.  The ratio of the coefficients of thermal expansion of the nipple in the longitudinal and transverse directions thereof is at most equal to the tangent of the angle of inclination between the thread flanks and the electrode front face.