Method and apparatus for joining new electrode to consumed electrode of electric furnace

Abstract: A new electrode is joined to a consumed electrode of the electric furnace according to a method comprising the steps of lifting and transporting the new electrode suspended from a joining apparatus by a crane, aligning both electrodes by throwing a laser beam from a projector disposed to the crane on a positioning marker disposed on a deck plate provided in the vicinity of the electric furnace, screwing a screw thread of the new electrode into a threaded hole of the consumed electrode by actuating an electric motor disposed in the joining apparatus, and tightening both electrodes at desired tightening torque by operating the motor in a stalled state with a small electric current of low frequency.

Description: This invention relates to a method and an apparatus used for joining a new electrode to an old and consumed electrode fitted to an electric furnace, and in particular to a method for aligning both axes of the new and the consumed electrodes, a method for controlling an electric motor in order to tighten both the electrodes with each other, and an apparatus for lifting the new electrode and rotating the new electrode so as to screw and tighten both the electrodes with each other proceeds and from US-A-4,400,815. In an electric furnace used for manufacturing, for example, steel, ferro alloy and so on,by generating an arc between electrodes and materials charged in a hearth, the electrodes are consumed in the continuation of the operation. Therefore, in an electric furnace of this kind, the electrodes supported by electrode holders of the electric furnace are so designed as to be fed into the furnace according to the consumption of the electrodes through electrode holes disposed in a furnace cover, and are extended by adding new electrodes to the old and consumed electrodes. Heretofore, on the occasion of joining a new electrode to an old and consumed electrode, a new electrode 100 to be added to a consumed electrode 106 is suspended by a crane 102 through a joining apparatus 103 and moved to a position just above the consumed electrode 106 of an electric furnace 104 as shown in Figure 11. Subsequently, the new electrode 100 is joined to the old and consumed electrode 106 fitted to the electric furnace 104 through a holder arm 105 by rotating the suspended new electrode 100 with an electric motor disposed in the joining apparatus 103 at the same time as lowering the new electrode 100 to screw a screw thread 108 formed on its lower end into a threaded hole 110 formed in the upper end of the consumed electrode 106 of the electric furnace 104. However, in such a method for joining electrodes 100 and 106, it is difficult to align the axis of the new electrode 100 suspended from the crane 102 with the axis of the consumed electrode 106 of the electric furnace 104 accurately, and there is a problem that much labor and time is required to align the axes of both the electrodes 100 and 106. Additionally, at the time of screwing the screw thread 108 of the new electrode 100 into the threaded hole 110 of the consumed electrode 106 by rotating the new electrode 100 through the electric motor of the joining apparatus 103, it is difficult to make the final tightening torque applied on the threaded portions 108 and 109 repeatedly uniform because of subtle differences in the size of the threaded portions 108 and 109 caused by variance in manufacturing accuracy. Accordingly, there is the possibility that the threaded portions 108 and 110 are tightened insufficiently or excessively depending on circumstances. Namely, at the time of screwing the screw thread 108 of the new electrode 100 into the threaded hole 110 of the consumed electrode 106 to tighten both the electrodes 100 and 106 by rotating the new electrode 100 through the electric motor of the joining apparatus 103, the electric motor is put into an overload state when both of the electrodes 100 and 106 are tightened to some degree, and is brought to a standstill, that is the motor falls into a stalling state. Stalling operation means to work the motor insuch an overloaded state, with an electric current flowing in the motor being held constant and torque output from the motor showing fixed value in this state. Generally, if the motor is connected to the source of a commercial electric power supply and operated in the stalling state, an overcurrent flows in the motor, the motor overheats and is burned out before long. Therefore, in the conventional method the electric motor is prevented from overheating by cutting off the power supply at the same time as the motor falls into the stalling state. In such a method, however, it is impossible to apply the tightening torque required for tightening both the electrodes 100 and 106 to the threaded portions 108 and 110. Consequently, there is another problem in that the threaded portions 108 and 110 generate heat owing to the increase of electric resistance therebetween if the threaded portions 108 and 110 are tightened insufficiently, and threads of the threaded postions 108 and 110 may be broken by excessive tightening, thereby causing disconnection of the electrodes and deterioration of the lifetime of the electrodes. Furthermore, although the new electrode 100 to be added to the consumed electrode 106 is generally packed horizontally and stored in a state in which it is laid down on the floor as shown in Figure 12, a connector 103a of the conventional joining apparatus 103 can only be connected with the new electrode 100 in a standing state. Therefore, it is necessary to set up the electrode 100 laid on the floor and set it in the standing position in order to connect the new electrode 100 to the connector 103a of the joining apparatus 103. Accordingly, a problem arises in that it requires much time and hard labor to set the electrode 100 to the joining apparatus 103. DE-A-3445830 discloses the use of lasers to locate accurately a part which is carried by a crane. In US-A-4,400,815 an electrode aligning apparatus is disclosed by Dunn et al which is provided with a trolley having an electrode rotating and supporting means and movable in two directions in a rectilinear pattern on guide rails provided in surrounding and overlying relation to the electric furnace, limit switches actuated by an -abutment disposed on a trolley frame, and stops to abut with a shock absorber pad of the trolley. Accordingly, although it is possible to align the replacement (new) electrode with the working (consumed) electrode accurately, unsolved problems remain in that it is not possible to tighten the new electrode to the consumed electrode accurately to the desired torque, and much time and hard labor is necessary to set the new electrode to the apparatus. Furthermore, there is another problem in that scaling up of the apparatus cannot be avoided in the design employed. The present invention is made in view of the afore-mentioned problems of the prior art and an aim of the invention is to provide a method for forming a new electrode to a consumed electrode of an electric furnace in which it is possible to align the axes of both electrodes very easily and possible to tighten both electrodes to each other at a predetermined suitable tightening torque; and an apparatus for carrying out the method of the present invention by which it is possible to lift the new electrode very easily and join the new electrode to the consumed electrode without hindrance using a carrier means generally installed in a workshop, such as an overhead travelling crane, even if the new electrode is stored in a state of being laid on the floor in the workshop. Accordingly, the present invention provides a method for joining a new electrode to a consumed electrode which is fitted vertically in an electric furnace, comprising the steps of lifting the new electrode by a carrier means and transporting the new electrode to a position just above the consumed electrode, and rotating the new electrode about an axis thereof and joining the new electrode to the consumed electrode through a screw pair formed on ends of said both electrodes, characterised in that said new electrode is suspended from the carrier means by a joining apparatus which is provided with an electric motor for rotating the new electrode about the axis, said electric motor is operated such that the stalling torque of the electric motor corresponds to a predetermined tightening torque, said stalling torque being set by controlling strength and frequency of an electric current flowing in the electric motor, and said new electrode is tightened to the consumed electrode at the predetermined tightening torque applied by the electric motor in the stalling state. Namely, a stalling current which is an electric current flowing in the electric motor in the stalling state decreases in proportion to the decrease of voltage and frequency of the current supplied to the electric motor. Therefore, it is possible to lower the stalling current so as not to overheat the motor by controlling the voltage and the frequency of the electric current to be supplied to the motor. Preferably, both electrodes are aligned with each other by projecting a laser beam on a positioning marker disposed on a deck plate of the electric furnace, said laser beam being generated by a laser beam projector disposed to the carrier means. Therefore, it is possible to transport the extensional new electrode into the objective position speedily and possible to stop the electrode at the objective position very exactly by moving the crane so that the laser beam from the projector shines on the positioning marker as checking a spot struck by the laser beam. Preferably, said electric motor is operated with a lower electric current having lower frequency in the transition between a screwing stage and a tightening stage of the electrodes and in the tightening stage of the electrodes as compared with an early stage in the screwing stage of the electrodes. More preferably, the rotational speed of said new electrode is not more than 20 rpm in the transition from the screwing stage to the tightening stage of the electrodes. Using this method of the present invention, it is possible to obtain proper tightening torque, prevent the electrodes from becoming disconnected during use and extend the lifetime of the electrodes. The present invention also extends to an apparatus for joining a new electrode to a consumed electrode which is fitted by a holder arm vertically in an electric furnace, comprising: an electric motor for generating a rotational driving power for rotating a vertically suspended new electrode about its axis; a transmission means, an upper part of which is connected with said electric motor, for transmitting the rotational driving power of the electric motor to a connector means which is disposed on a lower part of said transmission means and is for connecting the new electrode with said transmission means, said transmission means including a rotatable joint means such that in use a lower portion of said transmission means together with said connector means can be moved from horizontal to vertical positions; and a telescopically retractable cylindrical hood for covering said new electrode which is suspended from said transmission means through the connector means. Preferably, said cylindrical hood is further provided with a projection at the lower end thereof which is engagable in a groove formed between a first vertical rib and second vertical rib which is shorter than said first vertical rib, disposed on a holder arm which supports the consumed electrode of the electric furnace when said both electrodes are aligned with each other. In the joining apparatus according to this invention, the lower side portion of the transmission means and the connector means are so designed, by provision of the rotatable joint means, as to be rotatable from the vertical position into the horizontal or substantially horizontal position. Accordingly, it is possible to connect the new electrode to the transmission means through the connector means without hindrance even if the new electrode is stored in the state of being laid on the floor. Subsequently, the new electrode is set up vertically from the lying position by lifting the new electrode with a carrier means such as a crane using the joining apparatus according to this invention. The new electrode suspended vertically is transported to the position just above the consumed electrode of the electric furnace supported by a holder arm, and the new electrode connected to the transmission means through the connector means is joined to the consumed electrode of the electric furnace by transmitting a driving force to the new electrode from the drive means through the transmission means and rotating the new electrode. According to the joining apparatus of this invention, it is possible to attach a new electrode left in the state of being laid on the floor to the joining apparatus very easily, whereby the attachment of the new electrode to the joining apparatus is remarkably simplified. Furthermore, it is possible to decrease the work of the operators because it is unnecessary to raise the electrode in the standing position. Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:- Figure 1 is a schematic representation wholly illustrating the method and the apparatus according to an embodiment of this invention; Figures 2A and 2B are enlarged vertical sectional views illustrating the joining apparatus shown in Figure 1; Figure 3 is a horizontal sectional view illustrating the connecting structure between the geared motor and the transmission shaft shown in Figures 2A and 2B; Figures 4A and 4B are enlarged details illustrating the lower portion of the transmission shaft shown in Figures 2A and 2B; Figures 5A and 5B are detail views illustrating the neighboring structure of the rotatable shaft of the lower portion of the transmission shaft shown in Figures 4A and 4B; Figure 6 is a schematic representation wholly illustrating the procedure for positioning the electrode in the method according to the embodiment of this invention; Figure 7 is a schematic plan view illustrating the procedure for positioning the electrode shown in Figure 6; Figure 8 is an enlarged schematic representation illustrating the procedure for positioning the electrode shown in Figure 6; Figure 9 is a graph illustrating the relationship between progress of the joining and ensuing fluctuation of the electric current flowing in the electric motor in the method according to the embodiment of this invention; Figure 10 is a graph illustrating the effect of the electric current and its frequency in the electric motor in a stalled state on the tightening torque applied to the electrodes in the method according to the embodiment of this invention; and Figure 11 and Figure 12 are schematic representations of the conventional procedure for joining electrodes of an electric furnace. An embodiment of the method and the apparatus according to this invention will be described below on the basis of Figure 1 to Figure 10. Figure 1 shows an electric furnace (electric arc furnace), apparatuses used for joining a new electrode to a consumed electrode fitted to the electric furnace, and the procedure for joining both the electrodes. In Figure 1, numeral 10 denotes the electric furnace, which is provided with three electrodes 12a, 12b and 12c made from graphite. Each of the electrodes 12a, 12b and 12c is held in a holder hole 20 formed in one end of a holder arm 18 as shown in Figure 2. Each of the electrodes 12a, 12b and 12c fitted to the electric furnace 10 is formed with a threaded hole 22 in the upper end thereof as Shown in Figure 2, and so designed as to be joined with a new electrode 12d by screwing a nipple 24 previously fitted to the new electrode 12d into the threaded hole 22 thereof. The nipple 24 is made from graphite of the same quality as the electrodes 12a-12d, formed with an external screw thread on the outer periphery thereof, and so designed that a half length of the nipple 24 may be screwed into a threaded hole 26 of the new electrode 12d and the other half length of the nipple 24 may be screwed into the threaded hole 22 of the electrode 12a, 12b or 12c of the electric furnace 10. Numeral 14 denotes a joining apparatus used for adding the new electrode 12d to the electrode 12a, 12b or 12c of the electric furnace 10, and numeral 16 denotes a crane used for lifting and transporting the new electrode 12d as a carrier means in Figure 1. The crane 16 is positioned by a laser beam projector 15 so as to send the laser beam directly therebelow as shown in Figures 6 and 8. A deck plate 11 (stationary member) disposed fixedly in the vicinity of the electric furnace 10 is provided with positioning markers 13A 1 , 13A 2 , 13B 1 , 13B 2 , 13C 1 and 13C 2 on the upper face thereof as shown in Figures 6 to 8. Among them, the positioning marker 13A 1 and 13A 2 corresponds to the electrode 12a, and the positioning markers 13B 1 , 13B 2 and 13C 1 , 13C 2 correspond to the electrodes 12b and 12c, respectively. The positioning markers 13A 2 , 13B 2 and 13C 2 of the aforementioned markers are so located in advance that a center line (axis) of the electrode 12d held by the joining apparatus 14 may coincide exactly with center lines of the electrodes 12a, 12b and 12c of the electric furnace 10, respectively when the laser beam from the projector 15 shines on the respective positioning markers. The joining apparatus 14 is provided with a geared electric motor 38 housed in a cylindrical casing 32 for rotationally driving the new electrode 12d and a transmission shaft (transmission means) 40 connected to the motor 38 for transmitting driving force of the motor 38 to the electrode 12d as shown in Figures 2A and 2B. The transmission shaft 40 is provided with a metallic nipple 42 as a connector means on the lower side thereof, and connected with the new electrode 12d by screwing the metallic nipple 42 into the threaded hole 22 of the electrode 12d, which is suspended by the joining apparatus 14 through the transmission shaft 40. The transmission shaft 40 is provided with a threaded shaft 44 at the upper end thereof, and the treaded shaft 44 is screwed into a nut member 46 secured in the cylindrical casing 32 to form a screw pair. The threaded shaft 44 is formed with a square head 47 at the upper end thereof, which is fitted in a square tubular section 48, which shaft 44 rotates together with the motor 38 so as to move in the axial direction and not to rotate with the square tubular section 48 as also shown in Figure 3. Namely, the new electrode 12d connected to the transmission shaft 40 through the metallic nipple 42 is so structured as to he lowered through the screw pair at the same time being rotated by actuating the motor 38, thereby screwing the nipple 24 of the new electrode 12d into the threaded hole 22 of the electrode 12a, 12b or 12c. The cylindrical casing 32 which houses the geared motor 38 is provided with a retractable hood having an outer cylinder 28 and an inner cylinder 30 for covering the transmission shaft 40 and the new electrode 12d suspended by the shaft 40. The outer and inner cylinders 28 and 30 are so structured asto move up and down by winding up and down a wire 36, connected through geared motors 34 disposed on the upper side of the cylindrical casing 28. Namely, the electrode 12d is exposed when the outer and inner cylinders 28 and 30 are raised by winding up the wire 36 as shown in Figure 2B, and covered by the cylinders 28 and 30 extended telescopically by winding down the wire 36 as shown in Figure 2A. Furthermore, the outer cylinder 28 of the retractable hood is provided with a projection 50 protruding downwardly from the lower end thereof, and the holder arm 18 of the electric furnace 10 is provided with a groove 52 between a pair of vertical ribs 51 and 53 formed on the outer periphery there-of. The projection 50 is designed to locate in the groove 52 and to be engaged therewithin when the axis of the new electrode 12d coincides with the axis of the consumed electrode 12a, 12b or 12c of the electric furnace 10 as also shown in Figure 8. The vertical rib 53 is formed so as to be shorter than the rib 51 thereby opening the grocve 52 rightwardly on the upper side of the groove 52, whereby it is possible to simplify the engagement of the projection 50 of the joining apparatus 14 with the groove 52 of the holder arm 18 by lowering the joining apparatus 14 after the projection 50 comes in contact with the upper part of the longer vertical rib 51 as shown in Figure 8. In addition to above, the groove 52 formed between the vertical ribs 51 and 53 also serves to hold the joining apparatus 14 such that it does not rotate together with the electrode 12d at the time of actuating the electric motor 38. The transmission shaft 40 consists of an upper portion 60, a lower portion 62 and a joint 64 connecting the upper and the lower portions 60 and 62 as shown in Figures 2A and 2B, and the lower portion62 is, as shown in Figures 4A and 4B, further divided into a first portion 66 on the upper side and a second portion 68 on the lower side. The second portion 68 is provided with the metallic nipple 42 on the lower end, and provided with a handle 70 on the upper end. The first and the second portions 66 and 68 are connected with each other through clutch members 72 and 74, and a rotatable shaft 76 disposed on the clutch member 72 by which a rotatable joint means is formed. The clutch member 72 has a block-like shape with a stepped hole 78 in the center thereof, and the stepped hole 78 is fitted with the second portion 68 as also shown in Figures 5A and 5B. The stepped hole 78 is provided with a larger hole part 80 on the upper side thereof in Figure 4B (on the right side in Figures 4A and 5A), and the larger hole part 80 houses a flange 82 of the second portion 68. Namely, the second portion 68 is supported so as not to fall out of the clutch member 72 by the flange 82 in contact with a stepped face 84 of the stepped hole 78. The second portion 68 is formed with a leveled part 86 having a square section, and another clutch member 74 is fitted on the leveled part 86 so as to move in the axial direction and not to rotate against the second portion 68 by means of a square hole 88 of the clutch member 74. The clutch member 74 is provided with a pair of teeth 90 projecting downwardly in Figure 4B, and the clutch member 72 is provided with a pair of teeth 92 projecting upwardly in Figure 4B. The teeth 90 are so structured as to be engaged with or disengaged from the teeth 92 by moving the clutch member 74, along the leveled part 86 of the second portion 68 in the axial direction. The rotatable shaft 76 provided on the clutch member 72 is held in the horizontal position as shown in Figure 4B so that the second portion 68 may be rotate about the horizontal axis in its whole body. Additionally, it is preferable to engage the teeth 90 and 92 of the clutch members 72 and 74 with some play in the rotational direction. Next, an explanation will be given of the procedure for joining the new electrode 12d to the consumed electrode 12a of the electric furnace 10. In general, the new electrode 12d to be added to the consumed electrode 12a (12b or 12c) of the electric furnace 10 is stored in the lying state on the floor as shown on the right side in Figure 1. The new electrode 12d is set to the joining apparatus 14 according to the following procedure. In the first place, the joining apparatus 14 is moved horizontally by the crane 16 to the position where the electrode 12d is stored, and then lowered. After the joining apparatus 14 is lowered to a suitable position, the electrode 12d is set to the joining apparatus according to the procedure illustrated in Figures 1A and 4A. Namely, the clutch member 72 of the transmission shaft 40 provided in the joining apparatus 14 is moved toward the handle 70 along the leveled part 86 of the second portion 68, thereby disengaging the clutch member 74 from the clutch member 72 (Figure 4A). Subsequently, the whole of the second portion 68 is turned about the rotatable shaft 76 to make it level, and the metallic nipple 42 is screwed into the threaded hole 22 of the electrode 12d by rotating the second portion 68 in its whole body through the handle 70. After screwing the metallic nipple 42 into the threaded hole 22 sufficiently, the clutch member 74 is engaged with the clutch member 72 by returning the clutch member 74 to the original position. The setting procedure of the electrode 12d with the joining apparatus 14 is completed as described-above, and then by lifting the joining apparatus 14 through the crane 16, the electrode 12d is set up to the standing position from the lying position and at the same time the second portion 68 of the transmission shaft 40 turns about the rotatable shaft 76 and is lifted upwardly in a state of being suspended by the joining apparatus 14. In the second place, the electrode 12d is transported by the crane 16 along a line 17 shown with a chain line in Figure 7. In a case in which the new electrode 12d is joined to, for example, the consumed electrode 12a of the electric furnace 10, the crane 16 is moved to the position where the laser beam projector 15 disposed on the crane 16 shines on the positioning marker 13A 1 disposed on the stationary deck plate 11 provided in the vicinity of the electric furnace 10, and is then moved to the position where the projector 15 throws exactly the laser beam on the positioning marker 13A 2 . The crane 16 is then stopped at this position and holds the electrode 12d at the proper level as also shown in Figure 6 and Figure 8. In this state, the center lines of the new and the consumed electrodes 12d and 12a coincide with each other and the projection 50 of the joining apparatus 14 comes in contact with the longer vertical rib 51 formed on the outer periphery of the holder arm 18 as shown with a broken line in Figure 8. At the same time as lowering the electrode 12d together with the joining apparatus 14 to some degree, the electrode 12d is driven rotationally by actuating the electric motor 38 of the joining apparatus 14. Whereby, the nipple 24 fitted to the lower end of the new electrode 12d is screwed into the threaded hole 22 of the consumed electrode 12a and both the electrodes 12a and 12d are joined with each other. In this time, the projection 50 of the joining apparatus 14 enters into the groove 52 between the longer and the shorter vertical ribs 51 and 53 formed on the outer periphery of the holder arm 18 and acts as a catch to the counteraction caused by the rotation of the electrode 12d. The electric motor 38 is operated at high speed at the beginning, and switched to a low speed operation when the nipple 24 and the threaded hole 22 are engaged loosely with each other as shown in Figure 9. The low speed operation of the motor 38 is then continued. When the nipple 24 is tightened sufficiently in the threaded hole 22 after a certain time, the load applied on the motor 38 increases abruptly and the motor 38 is put into the overload state and falls into the stalled state. Figure 9 illustrates fluctuation of the electric current flowing in the motor 38, the electric current does not flow so much since the load applied on the motor 38 is not substantial in the high speed operation at the beginning of the screwing, the load does not increase so much and there is merely some increase of the current also in the low speed operation at the next step as shown in Figure 9. In the high load (overload) state in which the nipple 24 is screwed and tightened sufficiently into the threaded hole 22, the electric current increases abruptly, and the current curve shows a constant value after the sharp rise. That state is a stalling operation, and a large current flows in the motor 38. If the motor 38 in the stalled state is actuated by directly connecting the motor 38 to a commercial power source (220V, 60Hz), torque output from the motor 38 increases excessively and the motor 38 overheats and is damaged owing to the overcurrent flowing in the motor 38 as shown with a two-dot chain line in Figure 9. It has been confirmed that the output torque (stalling torque) decreases with a decrease in stalling current (an electric current flowing in the motor in the stalled state), the stalling current decreases with a decrease in the voltage, and the output torque obtained by the same stalling current changes (increases) by lowering the frequency of the current. Therefore, in the method according to this invention, the torque and the electric current flowing in the motor 38 in the stalled state are controlled properly by regulating the voltage and the frequency of the electric current supplied to the motor 38 through an invertor. Figure 10 is a graph showing an effect of the stalling current and its frequency on the output torque (tightening torque). In Figure 10, the change in the stalling current under the same frequency is based on the change in the voltage. Namely, the stalling current increases with an increase in the voltage and decreases with a decrease in the voltage. In this embodiment, a 7.5KW motor (reduction gear ratio: 1/59) is used as the motor 38 and torque of 250 kgf-m is required to tighten both the electrodes 12a and 12d. The tightening torque of 250 kgf-m can be obtained under the frequency of 10 to 30 Hz, but the stalling current changes according to the frequency. Therefore, the motor 38 is supplied with electric power of 15Hz frequency in order to decrease the stalling current sufficiently so as to prevent the motor 38 from overheating, and the voltage is controlled so as to decrease the stalling current to 72A approximately in this embodiment. The tightening torque becomes constant at 250 kgf-m by operating the motor 38 in the stalled state under such a condition, whereby the nipple 24 of the new electrode 12d is screwed into the threaded hole 22 of the consumed electrode 12a and both the electrodes 12d and 12a are connected with each other. It is preferable to lower the rotational speed of the electrode 12d at the time of screwing using the motor 38 because the degree of the tightening varies widely under the influence of the force of rotational inertia if the electrode 12d is rotated at a high rotational speed. It is desirable that the rotational speed is 20 rpm or less. In a preferred embodiment it is possible to stop the crane 16 at a desired position exactly according to a signal from a photo sensor disposed on the position corresponding to the positioning marker. As described above, according to this invention, it is possible to set the new electrode in the lying state on the floor very easily and possible to simplify the lifting work of the new electrode using the joining apparatus. Furthermore, it is possible to transport the new electrode into the position just above the consumed electrode of the electric furnace speedily and exactly by moving the crane so as to throw the laser beam on the positioning marker. In addition to the above, it is possible to join both electrodes at proper tightening torque without overheating the motor by controlling the motor in the stalled state so as to decrease the electric current and its frequency.

Claims: A method for joining a new electrode to a consumed electrode which is fitted vertically in an electric furnace, comprising the steps of: lifting the new electrode (12d) by a carrier means (16) and transporting the new electrode (12d) to a position just above the consumed electrode (12a), and rotating the new electrode (12d) about an axis thereof and joining the new electrode (12d) to the consumed electrode (12a) through a screw pair formed on ends of said both electrodes (12d,12a); characterised in that said new electrode (12d) is suspended from the carrier means (16) by a joining apparatus (14) which is provided with an electric motor (38) for rotating the new electrode (12d) about the axis, said electric motor (38) is operated such that the stalling torque of the electric motor (38) corresponds to a predetermined tightening torque, said stalling torque being set by controlling strength and frequency of an electric current flowing in the electric motor (38), and said new electrode (12d) is tightened to the consumed electrode (12a) at the predetermined tightening torque applied by the electric motor (38) in the stalling state. A method for joining a new electrode to a consumed electrode as defined in claim 1, wherein said both electrodes (12a,12d) are aligned with each other by projecting a laser beam on a positioning marker (13A1,13A2) disposed on a deck plate (11) of the electric furnace (10), said laser beam being generated by a laser beam projector (15) disposed to the carrier means (16). A method for joining a new electrode to a consumed electrode as defined in claim 1, wherein said electric motor (38) is operated with a lower electric current having lower frequency in the transition between a screwing stage and a tightening stage of the electrodes (12a,12d) and in the tightening stage of the electrodes (12a,12d) as compared with an early stage in the screwing stage of the electrodes (12a,12d). A method for joining a new electrode to a consumed electrode as defined in claim 3, wherein the rotational speed of said new electrode (12d) is not more than 20 rpm in the transition from the screwing stage to the tightening stage of the electrodes (12a,12d). An apparatus for joining a new electrode to a consumed electrode which is fitted by a holder arm vertically in an electric furnace according to the method of any one of claims 1 to 4, comprising: an electric motor (38) for generating a rotational driving power for rotating a vertically suspended new electrode (12d) about its axis; a transmission means (40), an upper part of which is connected with said electric motor (38), for transmitting the rotational driving power of the electric motor (38) to a connector means (42) which is disposed on a lower part of said transmission means (40) and is for connecting the new electrode (12d) with said transmission means (40), said transmission means (40) including a rotatable joint means (72,74,76) such that in use a lower portion of said transmission means (40) together with said connector means (42) can be moved from horizontal to vertical positions; and a telescopically retractable cylindrical hood (28,30,32) for covering said new electrode (12d) which is suspended from said transmission means (40) through the connector means (42). A joining apparatus as defined in claim 5, wherein said cylindrical hood (28,30,32) is further provided with a projection (50) at the lower end thereof which is engagable in a groove (52) formed between a first vertical rib (51) and a second vertical rib (53) which is shorter than said first vertical rib (51), disposed on a holder arm (18) which supports the consumed electrode (12a) of the electric furnace (10) when said both electrodes (12a,12d) are aligned with each other.

Inventors: AMANO HAJIME, IKITSU YASUAKI, KAMEI NOBUYUKI, OKAMOTO TETSUO, AMANO, HAJIME, IKITSU, YASUAKI, KAMEI, NOBUYUKI, OKAMOTO, TETSUO
Applicants: DAIDO STEEL CO LTD, DAIDO TOKUSHUKO KABUSHIKI KAISHA
Application year: 1993
Main classification: H05B7
IPC codes: H05B7, B23P19, B66C13