High frequency bolt heater

Abstract: A high frequency bolt heater which can heat up the inside of a narrow hole in a speed much faster than that of a conventional bolt heater employing a resistance wire is provided. The high frequency bolt heater comprising an induction heating coil (1) made of a copper tube turned around in a hairpin fashion and whose shape is rendered approximately circular in section by pinching a magnet (2) between reciprocating paths thereof is inserted in a hole bored in the axial direction of a metallic bolt. A length of the coil is approximately equalized with that of the bored hole and a portion where the magnet is omitted is provided at a region which corresponds to a screw portion of the bolt for mounting a nut (15). An adjustable stopper (3) made of a heat resistant electrical insulator for setting a necessary length of the induction heating coil (1) to be inserted is provided and a heat resistant insulator (8) is applied on the surface of the induction heating coil (1).

Description: The present invention relates to a high frequency bolt heater according to the preamble of claim 1 (see for example DE-B-1050 935) used for fastening and loosening bolts of a steam turbine casing, etc., applicable for a heating system for heating large size bolts of other various machines and for heating a narrow and long hole from the inside thereof, and usable where heating cannot be done from the outside such as a heat treatment within a narrow hole, heating of a boiler tube, heating of an inner tube of a double tube and improvement of stress of welded section of a narrow hole. Description of the Related Art: A bolt heater is used for fastening or loosening a large size bolt which is used for a steam turbine casing for example and which cannot be fastened by a wrench or spanner. A case when loosening a bolt will be explained referring Figs. 5 and 6. Fig. 5 shows a state where a pair of flanges (C) is strongly fastened by a bolt (a) and nuts (b). When a bolt heater (d) is inserted to a center hole (f) bored at the center of the bolt (a) in the longitudinal direction thereof and the bolt (a) is heated by the bolt heater (d), the bolt (a) thermally expands in the axial direction by a length (l) as shown in Fig. 6. Thereby, the nut (b) can be rotated easily and be loosened. In this case, it is necessary to heat only an parallel portion (e) of the bolt (a) and not to heat screw portions of the bolt and nut as much as possible. In a case when the bolt is fastened, the bolt (a) is similarly heated up to thermally expand in the axial direction thereof. When the bolt (a) is expanded, the nut (b) is rotated in the direction of fastening the bolt (a). Then the bolt (a) is cooled down to obtain a predetermined fastening force. Conventionally, a bolt heater by means of a resistance wire heating element is inserted to a center hole of such bolts for lateral joint faces or the like of a steam turbine casing for land or for ship when heating them to fasten or loosen. Referring now Fig. 4, one example of a prior art heater by means of a resistance wire will be explained. In the figure, resistance wire heating elements 21 are embedded in a heat resistant insulator 22. A jacket 23 is made of a heat resistant metal such as SUS. A handle 27 is attached to a terminal box 24. Terminals 25 and 25' connect the resistance wire heating elements 21 with a cable 26. The reference numerals 28 and 28' indicate cross sections of an object to be heated such as a bolt. In Fig. 4, when power is supplied by connecting the cable 26 to a power supply which is not shown, the resistance wire heating elements 21 generate heat and the jacket 23 is heated up through the intermediary of the heat resistant insulator 22. However, because the resistance wire heating elements 21 are embedded in the heat resistant insulator 22 within a narrow stainless tube in the conventional bolt heater by means of the resistance wire heating element 21 shown in Fig. 4, a quantity of heat input is limited from its structure and because the heating is carried out indirectly, it is inefficient. Due to that, it takes a long time to fasten or loosen the bolts and has been a bottleneck in assembling a turbine. Further, because it takes a long time for heating and the resistance wire heating element 21 is exposed in a high temperature state for a long time, it is deteriorated appreciably. Still more, because the nut portion and flanges of the casing are also heated up and thermally expand due to the long heating, an error is brought about in a necessary elongation length of the bolt, thereby complicating its management. Because the temperature does not rise in a short time, not only a number of heaters are required (also for bolts whose length differ), but also there is a danger that an operator may touch the hot nuts and casing flanges. Accordingly, it is an object of the present invention to eliminate the aforementioned problems by proving a high frequency bolt heater which can heat up the inside of a narrow hole in a speed much faster than that of the prior art bolt heater using the resistance wire heating element. SUMMARY OF THE INVENTION According to the present invention as defined in claim 1, a high frequency bolt heater comprising an induction heating coil is inserted to a hole bored in the axial direction of a metallic bolt. The coil is made approximately circular in section by pinching a magnet between reciprocating paths of a copper tube turned around in a hairpin fashion and a length of the coil is approximately equalized with that of the bored hole. The coil has a portion where the magnet is omitted at a region which corresponds to a screw portion of the bolt for mounting a nut. The high frequency bolt heater of the present invention is provided with an adjustable stopper made from a heat resistant electrical insulator for setting a necessary length of the coil to be inserted on the induction heating coil. Further, the high frequency bolt heater of the present invention uses a flexible member for a connection between the induction heating coil and a high frequency transformer. As described above, the magnet provided between the reciprocating paths of the coil turned around in a hairpin fashion in the axial direction thereof is omitted at the portion which corresponds to the screw portion of the nut and bolt. Because a pitch of the reciprocating paths of the coil where the magnet is omitted is rendered narrow, magnetic flux which penetrates through an object to be heated is canceled out and it is almost impossible to heat in this portion. Therefore, a range in which the magnet is provided is an effective heating zone and the inside of the narrow hole can be heated up rapidly and efficiently. The length of the zone can be changed by adjusting the length of the magnet pinched between the coils, so that this high frequency bolt heater can be applied even for a bolt having a different length. The above and other related objects and features of the present invention will be apparent from a reading of the following description of the disclosure found in the accompanying drawings and the novelty thereof pointed out in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side view of an embodiment of a high frequency bolt heater of the present invention; Figs. 2a and 2b are a front view of the heater and a section view thereof, respectively; Fig. 3 is a side section view showing a state in which the present invention is applied to a steam turbine; Fig. 4 is a section view of a prior art resistance wire heater; Fig. 5 is a side section view of a bolt into which the prior art bolt heater is inserted; and Fig. 6 is a side section view of the bolt shown in Fig. 5 showing a state when it has thermally expanded. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawings wherein like reference numerals refer to like parts throughout several views, a preferred embodiment of the present invention will be explained. Figs. 1 through 3 show the embodiment of the present invention, wherein Fig. 1 is a side view of a high frequency bolt heater of the present invention, Fig. 2a is a front view thereof, Fig. 2b is a section view thereof and Fig. 3 shows a case when the present invention is applied to a bolt for fastening lateral joint faces of a steam turbine casing. In the high frequency bolt heater 10 shown in Figs. 1 and 2, a coil 1 made from a copper tube turned around in a hairpin fashion is rendered approximately circular in section by removably pinching a magnet 2 between reciprocating paths of the coil 1. A length of the coil 1 is equalized to that of a hole bored in a metallic bolt 12 into which the bolt heater 10 is inserted. A magnet omitted section 9 where the magnet 2 is omitted is provided at a region which corresponds to a screw portion for mounting a nut 15. A stopper 3 made of a heat resistant electrical insulator is provided on the coil 1 to set a length of the high frequency bolt heater 10 to be inserted to the metallic bolt 12 and its fixed position to the coil 1 can be changed by loosening a screw thereof. The surface of the high frequency bolt heater 10 is covered by a heat resistant insulator 8. The high frequency bolt heater 10 is also provided with a handle 4, an inlet port 5 for supplying water to the coil 1, an exhaust port 6 for draining water from the coil 1 and a power feeding section 7 for feeding power to the coil 1. Now the high frequency bolt heater 10 will be explained in more detail. The coil 1 is made of the copper tube, an electrically good conductor, which is formed into a semicircular tube and a water channel is provided therein. The surface of the coil 1 covered by the heat resistant insulator 8 is insulated by thermal spray coating an alumina insulator or by winding a glass tape for example. The outer diameter of the coil 1 is made smaller than the diameter of the hole of the bolt by 1 to 2 mm considering a heating efficiency. In operation, the coil 1 having a construction as exemplified in Fig. 2 is inserted to the center hole of the bolt 12 as shown in Fig. 3. At this time, the length of the coil 1 is set beforehand by means of the adjustable stopper 3 provided on the coil 1. A region to be heated may be selected by loosening the screw provided on the stopper 3 and fastening it after moving the coil 1 in the axial direction. A current flows from a power source which is not shown to the power feeding section 13 and to the coil 1 through the intermediary of a high frequency transformer 14. A heat is generated from the inner surface of the hole of the bolt 12 and the bolt 12 elongates due to its thermal expansion in a few minutes. In Fig. 3, the high frequency bolt heater 10 is inserted to the narrow hole of the metallic bolt 12 securing lateral joint flanges 11 of the casing until to a position of the stopper 3 and the power feeding section 13 is connected to the high frequency transformer 14. A spherical washer 16 is disposed between the nut 15 screwed on the bolt 12 and the lateral joint flange 11. The reference numeral 17 denotes a dial gauge. A flexible member such as a flexible cable is used for a member 18 connecting the coil 1 and the high frequency transformer 14. Then the inventors measured temperatures at three places of the surface of the outer surface of the bolt 12 at the longitudinal middle portion, the surface of the nut 15 and the casing flange 11 by attaching thermoelectric thermometers and by recording by a temperature recorder from the beginning of heating to the completion of cooling. The elongation of the bolt 12 was measured by using the dial gauge 17 while contacting it with the head of the nut 15 and securing it at a position which would not be influenced by an elongation of the casing. Tables 1, 2 and 3 show the results of the measurement implemented in the manner as described above. Tested Bolts & Nuts Bolt No. Size (dia. x length) Type Necessary Elongation (mm) 1 101 x 1275 Double-end nut & bolt 1.7 2 108 x 1315 Reamer double-end nut & bolt 1.7 3 101 x 925 Stud bolt with bottom 1.1 Measured Result Bolt No. Heating time (sec) Temp. of bolt (°C) Temp. of nut (°C) Temp. of flange (°C) Length elongated (mm) 1 540 204 normal temp. 39 2.73 2 450 258 normal temp. 39 1.74 3 360 180 normal temp. 18 1.76 Electrical Condition Bolt No. Output (kw) 1 20 2 20 3 20 As described above, according to the present invention, the magnet provided in the axial direction between the reciprocating paths of the coil wound around in the hairpin fashion is omitted at the portion which corresponds to the screw portion of the nut and bolt. Because a pitch of the reciprocating paths of the coil where the magnet is omitted is narrow, magnetic flux which penetrates through an object to be heated is canceled out and it is almost impossible to heat in this portion. Therefore, the range in which the magnet is provided is an effective heating zone. The length of the zone can be changed by adjusting the length of the magnet pinched between the coils, so that this high frequency bolt heater can be applied for a bolt having a different length. Accordingly, the present invention allows to heat up the inside of a narrow hole rapidly and efficiently by flowing a current through the high frequency bolt heater. Thereby, it allows to heat up bolts in a speed faster than that of the prior art bolt heater employing the resistance wire by about 10 times, so that a manday for assembling steam turbine and the like may be considerably reduced. Further, because the screw portion for mounting the nut and the flange portion of the casing are not directly heated up, the elongation of the bolt may be accurately measured. Still more, because the flexible member is used for the connection between the induction heating coil and the high frequency transformer in the present invention, the connection and operation of the both can be readily implemented. The heating can be suppressed and the problem of deterioration of the heater due to the repetitive heating may be eliminated by providing the water channel in the coil. While the described embodiment represents the preferred form of the present invention, it is to be understood that modifications will occur to those skilled in that art without departing from the spirit of the invention. The scope of the invention is therefore to be determined solely by the appended claims.

Claims: A high frequency bolt heater, including an induction heating coil (1) to be inserted in a hole bored in the axial direction of a metallic bolt, a shape of said coil is rendered approximately circular in section, characterized by pinching a magnet (2) between reciprocating paths of a copper tube turned around in a hairpin fashion and a length of said coil is approximately equalized with that of the bored hole, and said coil has a portion where said magnet is omitted at a region which corresponds to a screw portion of said bolt for mounting a nut. The high frequency bolt heater according to Claim 1, characterized in that an adjustable stopper (3) made of a heat resistant electrical insulator for setting a necessary length of said coil (1) to be inserted is provided on said induction heating coil (1). The high frequency bolt heater according to Claim 1, characterized in that a heat resistant insulator (8) is applied on the surface of said induction heating coil (1). The high frequency bolt heater according to Claim 1, characterized in that a flexible member (18) is used for a connection between said induction heating coil (1) and a high frequency transformer (14).

Inventors: MAENOSONO TSUKASA, MATSUURA ICHIRO, MORITA YOJI, SHIMAMOTO TAKIJIRO, SHIMASAKI ICHIRO, MAENOSONO, TSUKASA, MATSUURA, ICHIRO, MORITA, YOJI, SHIMAMOTO, TAKIJIRO, SHIMASAKI, ICHIRO, MAENOSONO, TSUKASA, C/O EQUIPMENT DIV., Matsuura, Ichiro, c/o Nagasaki Shipyard &amp; Machin., Morita, Yoji, c/o Nagasaki Shipyard &amp; Machinery, Shimamoto, Takijiro, c/o Nagasaki R &amp; D Center, Shimasaki, Ichiro, c/o Nagasaki Shipyard &amp; Machin.
Applicants: DAI ICHI HIGH FREQUENCY CO LTD, MITSUBISHI HEAVY IND LTD, DAI-ICHI HIGH FREQUENCY CO., LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1993
Main classification: B23P19
IPC codes: H05B6, B23P19