CA1138535A - Bolt heater - Google Patents

Abstract

46,477 BOLT HEATER

ABSTRACT OF THE DISCLOSURE
Heating apparatus for axially expanding a connect-ing bolt prior to tightening to provide a predetermined de-gree of compression. The connecting bolt is provided with a central bore for receiving an electrical conductor. A low resistance conductor is disposed in electrical contact with the bolt at a suitable position along the inside diameter surface of the bore. Alternating current is transmitted from a power source through the conductor to its point of contact with the bolt. In response to the flow of alternating current through the conductor, an electromagnetic field is established around the conductor and on the surface of the central bore. This field confines the current flow within a limited cylindrical interior region of the bolt as it is conducted from the conductor to the top of the bolt. The current flow in the bolt material is equal and opposite in direction at any moment to that in the central conductor.
The current penetrates the bolt to a radial depth which is a function of the magnetic saturation characteristic and resistivity of the bolt material, and of the current magni-tude. The bolt is heated by the effective resistive loss of the current flow within the iron material of the bolt.
Since the heat is generated directly within the iron material of the bolt, direct heat transfer from the central conductor to the bolt is only incidental and is not essential to successful heating by the method and structure of the inven-tion. Furthermore, surface heat transfer limitations asso-ciated with conventional resistance heater arrangements are 46,477 avoided.

Description

BACKGROUND OF THE INVENTION
Fleld oP the Inventlon:
The present inventlon relates to method and appar-atus for heating a metallic workpiece, and in partlcular, to method and apparatus for heating a connecting bolt of rela-tively large size of the type suitable for use in ~oining the rotor shaft of a hydrogenerator to the shaft of a hydraulic turbine.
Description of the Prior Art:
In conventional practice, the rotor of a hydrogen-erator is turned by means of a hydraulic turbine. The hydraulic turbine and the rotor of the hydrogenerator each have a shaft whlch is furnished with a coupling member or flange for forming a strong mechanical union between the two shafts so that torque may be transmitted from the hydraulic turbine to the dynamoelectric machine rotor. The connecting bolts whlch are used to secure the matlng ~langes or coupling members are unusually large wlth diameters which may exceed 8-1/2 lnches.
It is common practice to thermally "stretch" such large connecting bolts prior to tightening nuts which are located in t~readed engagement on at least one end of the connecting bolts to obtain a predetermined level of flange compresslon or bolt tension. In order that such bolts may be suitably heated to provide the required thermal expansion, they may be provided wlth an internal bore extending either a material portion of the way through or extending completely from end-to-end~ whereby a suitable heating tool may be 3 inserted therein.

'`'' " ~

. _ . ._ .. ..

~ ~ 46,477 One heating tool which has been used suecessfully on connecting bolts of relatively smaller diameters ls the reslstance heater which depends on direct heat transfer from the resistance element to the inside diameter bore surface of the bolt, for example as disclosed by Clements in British Patent Specification 541,584. Because the amount o~ heat transfer is limited by the surface transfer of heat from the heater through the surrounding bore, the apparatus as dis-clo~ed by Clements is not appropriate for use on unusually large connecting bolts.
A double bolting arrangement is disclosed by Miller in U.S. Patent 2,359,046 which utilizes a higher proportion of the bolt area to avoid the troubles of heating due to surface heat transfer limitations. However there is no teaching by Miller as to the type of bolt heater arrange-ment to be used except that it be suitable.
A further example of the use of a resistance heat-ing element to cause thermal expansion of a connecting bolt is disclosed by Hodgkinson in U.S. Patent -3,G8~,33~. Accord-ing to the teachings of Hodgkinson, an electrical heatingelement is axially disposed within a connecting bolt for heatlng and axially extending or stretching the bolt where-upon securing means such as nuts may be subsequently tightened.
However, Hodgkinson does not indicate a specific heating arrangement and only indicates that any suitable heating element may be used to good advantage.
As the size of the connecting bolts increases, the problem oX heating the bolts to obtain sufficient thermal stretching becomes more difficult, especially in the use of conventional resistance heating elements which require bores ~ 46,477 of larger diameters in order that the required amount of heat may be applied in a suitably short time. As the bore for receivlng the heating element increases in size to accommodate larger heating elements, the useful bolt cross-sectional area is reduced and the effective strength of the connecting bolt is therefore diminished correspondingly.
Therefore, it ls desirable to provide apparatus and procedure for expanding a bolt of relatively large slze to permit lt to be tightened properly, particularly where lt is impractical or impossible due to associated surface a~d structural heat transfer limitations to apply sufficient heat to the bolt by conventional resistance heating element means to cause it to expand before the surrounding coupling or supporting structure is also expanded. If the heat which is delivered to the interior of the bolt reaches the sur-rounding coupling ma~erial, it either limits the bolt tem-perature or heats the coupling sufficiently so that the bolt cannot be tightened to useful stress levels.
SUMMARY OF THE INVENTION
It is an ob~ect of the present invention to provide method and apparatus for thermally expanding an elongated workpiece, such as a large connecting bolt, particularly where it is impractical or impossible due to surface and material heat transfer l$mitations.
According to one aspect of the present disclosure, the invention may be practiced in a dynamoelectric machine in which a first shaft having a coupling member or flange is ~oined to a second shaft, for example, from a hydraulic turbine, also having a coupling member or flange. The coupling members of the shafts are ~oined together by means 46,477 of connecting bolts which extend through openings in the coupling members. Each connecting bolt is provided with a bore which extends concentrically along its longitudinal axis. A low reslstance conductor is disposed within the central bore of the connecting bolt for establishing an electromagnetic field around the connecting bolt. In response to the flow of alternating current through the conductor, an electromagnetic field is established within a limited cylindrical interior region of the bolt. Heat is generated by the effective resistive loss incidental to the flow of alternating current through a substantlally cylin-drical interior region of the bolt whose inside diameter is the central bore and whose radial thickness is the depth of penetration of the current.
The conductor is electrically insulated by suit-able means such as a small air gap or by suitable dielectric insulation material from the central bore of the bolt except where electrical contact is made with the inside diameter surface portion of the connecting bolt at the position where it is desirable to limit the extent of the heating. In a preferred ~mbodiment, heating below an inner portion of a tightening nut is limited to heat conducted ~n the bolt from the heated port~on above the point of electrical contact.
This reduces the radial expansion of the bolt in the area o~
the tightening nut so the nut may be easily tightened with-out radial interference ln the threaded area.
Alternating current is conducted from an alternating current 3 single phase power source ha~ing first and second power output terminals from the first power output terminal through the conductor~ and from the conductor to the inner 46,477 surface bore of the bolt at a suitable contact position, thence returning on the inner surface of the bolt in close proximity to the conductor to a second power output terminal.
The foregoing and other ob~ects, advantages and features of this invention will hereinafter appear, and for purposes of illustration, but not of limitation, an exemplary embodiment of the sub~ect invention is shown in the appended drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a simplified vertical sectional view, with some parts in elevation, of a vertical shaft of a hydrogenerator which is ~oined to the vertical shaft of a hydraulic turbine assembly by connecting bolts which have been thermally expanded acco~ding to the teachings of the present invention;
Figure 2 is an elevation view in section which illustrates the details of the heating apparatus as disclosed by the present invention;
Flgure 3 is a graphical representation of temper-ature as a function time and the radial distance from thecenter of a typical connecting bolt heated according to the teachings of the present invention;
Figure 4 is an enlarged view, in section, of a portion of the apparatus shown in Figure 2; and, Figure 5 is a view similar to Figure 4 which illustrates a preferred arrangement of the conductor with respect to the bore of the connecting bolt of Figure 4.

DESCPIPTION OF THE PREFERRED EMBODIMENT
The inYention as shown in the drawing is embodied in the coupling apparatus of a large hydrogenerator-'-` 11~8535 46,477 hydraulic turbine assembly.
Referring to Figure 1, there is shown a shaft 10 of a large hydrogenerator (not shown) which has a rotor assembly (not shown) which is mounted in operating position for concurrent rotation with a shaft 12 of a large hydraulic turbine assembly (not shown).
The generator shaft 10 has a shaft coupling member or flange 14 and the hydraulic turbine shaft 12 has a shaft coupling member or flange 16 which is mechanically ~oined to the flange 14 in a mechanical coupling assembly 18.
The mechanical coupling assembly 18 comprises the mating flanges 14 and 16 which are disposed in abutting, surface-to-surface engagement and are held under compression by a plurality of connecting bolts 20. In order that torque may be transmitted efficiently from the hydraulic turbine to the rotor of the generator, the hydraulic shaft 12 and the generator shaft 10 must be tightly ~oined together with the coupling members 14 and 16 ~oined in a tight compressive union. This compressive union is establisned according to the teachings of the present invention by tightenlng nuts 22 on the connecting bolts 20. The bolts are tightened by heating them to cause them to stretch and then torquing the nuts 22 which results in a shrlnk fit as the bolts cool to coupling temperature.
Referring now to Figure 2 of the drawing, a pre-ferred arrangement of the heating apparatus for thermally expanding the connecting bolts 20 is shown. The connecting bolt 20 1s shown disposed within a bore 24 which extends through e~ch of the coupling members 14 and 16. The bores or open~ngs 24 are aligned to receive the connecting bolt 46~477 20. Nuts 22 are disposed in threaded engagement with threaded end portions 26 and 28, respectively, of the connect-lng bolt 20.
A conductor 30 is disposed within a central bore 32 of the connecting bolt 20. The conductor is preferably a low resistance copper rod having an electrical impedance to the flow of alternating current which is substantially less than the electrical impedance of a conventional resistive conductor Or comparable physical size. A relatively low electrical impedance is necessary to prevent the conductor from becomlng too hot or melting during operatlon at high current density levels. The bolt is heated substantially by the flow of current through a limited cylindrical interior region defined by flux penetration within the bolt ln response to the electromagnetic fleld established by the flow of alternating current through the conductor 30.
The conductor 30 is provided with a pair of elec-trical terminal portions, one terminal portion being an end portion 33 which pro~ects axially from the inner bore 32 and the other terminal portlon being a threaded end portion 34 ~-hich engages a mating threaded area 36 of the bore 32.
Alternating current is conducted through a short electrode conducting terminal 4~ of an alternating current power source 42, which may be a brazing transformer assembly. A
second electrode terminal portion 44 of the AC power source 42 is electrically connected to an end portion 45 of the connecting bolt 20 through a circular contact plate 46 to complete the electrical circuit. It is deslrable and prefer-able that the electrical contact for the second power terminal be made uniformly around the bore of the bolt. The contact ~38535 46,477 plate 46 is preferably low resistance copper. Reasonably uniform pressure over t~e contact area should be applied.
Non-uniform heating in the contact plate 46 may cause some permanent distortion if the pressure is initially close to the bending limits of the copper. According to this arrange-ment, an alternating current power source such as the brazing transformer assembly 42 may be physically placed in close proximity to the end portion 45 of the bolt so that the length of the conducting leads 40, 44 from the brazing transformer to the bolt ~s minimized, thereby maximizing the power transfer from the brazing transformer for any voltage setting to the inner surface of the bolt.
In operation, alternating current is conducted from the alternating current power source 42 through the conductor 30 returning through an inner cylindrical region (the radlal extent being indicated generally by the dashed lines 48) of bolt 20 to terminal 44. The alternatlng current is conducted substantially from the first power output terminal 40 through the terminal portion 33 of the conductor 30 and from the threaded portion 36 of the conductor 30 through the thin cylindrical region 48 of the bolt 20, with the alternating current being returned to the power source 42 through the contact plate 46 to the electrode terminal portion 44 of the power source 42.
Any desired form of physical contact between the conductor rod 30 and the connecting bolt 20 can be used, but good electrical contact must be established. In a preferred embodiment of the invention, the bore 32 is provided with threads and grooves in a limited, predetermined zone (as indicated by the da~hed lines at 36) and the conductor rod _9_ ~138535 46,477 30 is also provided with threads and grooves which cooperate to provide threaded engagement of the bore 32 with the conductor rod 30. An improved electrical connection between the threads on conductor rod 30 and the conductor bolt 20 is achieved by pro~iding a compressive union between the threaded portions of the conductor and the bore of the connecting bolt. A retalning bolt 50 is disposed in threaded engagement with a cooperating threaded annulus 52 which is concentric with the bore 32. The retaining bolt 50 is simply advanced and tightened against the threaded end portion 34 of the conductor rod 30 to force the threads of the conductor rod 30 into firm engagement with the cooperating threads of the bore 32.
The conductor rod 30 must be insulated or otherwise electrically isolated from the bolt 20 except in the desired zone of electrical contact 36. Accord~ngl~, a layer 51 of dielectric insulatlng material is shownfidis'posed around the conductor 30 to prevent electrical contact with the bolt 20.
According to another embodiment, as shown in Figure ~, the conductor 30 is separated from the bolt 20 by a small dielec-trlc air space 53. Either arrangement may be used to good advantage.
By utilizing short and closely coupled leads (the electrode terminal portions 40 and 44), the external induc-tance and resistance is small, thus keeping the external voltage drop to negligible values. This configuration is made possible by utilizing the pro~ecting portion 45 of the connectlng bolt 20 in combination with the contact plate 46 as a return electrode f3r alternating current conducted by the conductor 30. The alternating current power source 42 ~1 ~ 5 ~ 46,477 preferably is supported in close proximity to the contact plate ~ of the connecting bolt 20 by means of a lifting device 56 which may be supported by any convenient ad~ustable attachment.
The skin effect phenomenon is utilized in the present invention to confine the flow of alternating current from the conductor through the bolt to the substant~ally cylindrical region 48 wholly within the bolt so that it may be heated and expanded before heat is transferred to sur-rounding structure.
The distribution of induced current in a workpiecesuch as the connecting bolt 20 ls maximum on the inside diameter surface of the bore 32 and decreases rapidly within the bolt; the effective penetration of current increases with a decrease in frequency and increase of iron temperature or resistance. The distr~bution of induced current is influenced also by the magnetic and electrical character-istics of the part being heated; and since these properties change with temperature, the current distribution will change as the work is heated. For example~ in a recent application of this heating apparatus, alternating current wa~ passed through a large 8-l/2 diameter, l,000 l~. con-necting bolt 20 at approximately 15,000 amps at 8 ~olts RMS
and 60 Hertz. As the inner æurface of the bolt approached 300C, the current flow through the conductor 30 reduced to approximately 8,700 amps due to the increased effective resistance of the steel material comprising the bolt. The depth of e~ective flux penetration in this case was approxi-mately 4/lO o~ an inch.

Because the heat conducts radially outward as soon ~6,477 as the inner surface is heated, the actual temperature profile at any moment is determined by the duration of heating and the power density as well as by the frequency, as illustrated in Figure 3 of the drawing.
In the arrangement shown in Figure 2, the heating of the bolt 20 takes place somewhat non-uni~ormly in a radial direction from the surface of the bore 32. The concentration of the heating due to eddy currents in the cylindrical region 4g near the inside surface of the bore 32 is called the "skin effect". The lines of magnetic flux surrounding the curre~t carrying conductor 30 flow circumferentially with respect to the conductor 30.
Before the M ux reaches interior regions 60 of the bolt 20, there is sufficient flux to induce a voltage within the iron cancelling the voltage available at the bore surface.
This means that substantially all of curren~ flow will take place with the cylindrical region 4g in the surface layers of the bolt 20 which are immediately adjacent ~he inside diameter of the bore 32 and consequently the interior regions of the bolt will reach the desired tem-perature for thermal expansion before any substa~tial amount of heat is conducted to s~rrounding structure.
Most of the heat generated by the current flow is concentrated in a relatively small c~lindrical volume of bolt material which immediately surrounds the con-ductor 30. The temperature at the inner surface of the bore 32 begins to rise very rapidly, and as the tempera-ture rises, the hea~ generated is reduced and at the same time heat flows radially outward through the bolt. Th~
temperature~tIme profile of the bore surface 32 is illustrated by curve 70 in ~igure 3 ,~

~3ss3s 46,477 of the drawing. In Figure 3, To represents the desired tem-perature at the bore of the connecting bolt, and R represents the radlal distance from the bore of the connecting bolt.
The lnitial rate of rise is rapidly reduced as the percentage of generatedl~K~ is conducted into the colder bolt material incrc~s. Curves 72-80 illustrate temperature distributions for selected intervals after the heating process has started.
The radial thermal gradient assuming negligible axial flow is .9C per watt per sq. inch per inch of travel for low carbon steel. The numbers in parentheses associated with the curves 72-80 represent the time in minutes elapsed after the flow of current through the bolt 20 has begun. It should be noted that as the heat moves radially outward, the volume of steel and area for heat conduction in the bolt 20 increases with the radius, thus reducing the gradlent as well as increasing the amount of heat stored for a given temperature change. This storage of heat in the cylindrical region 48 of the bolt 20 immediately surrounding the conductor 30 permits the bolt 20 to expand axially before the heat is conducted to ad~acent supporting structure.
It has been determined that the rate of travel of the initial heat is for practical purposes approximately 1 inch per minute for low and medium carbon steels and is substantially independent of the rate of heating or of the thermal gradient. Therefore, to heat the bolt 20 before the heat has had time to be conducted into the surrounding structural members (such as the coupling members 14 and 16), the heating must be done in a time in minutes which is approximately numerically equal to the radial thickness of 3~ the bolt. Of course, if the bolt 20 is thermally insulated 3 ~ ~
46,477 from the surrounding structural supporting members, then the heating can be applied much more slowly with more uniform temperatures.
The bolt 20 will also expand radially in diameter in the same manner as it expands axially in length in response to the heat generated. The radial expansion can be compen-sated for by proper selection of tolerances of the thread diameter and of the thread clearance of the circular nuts 22 to permit limited radial expansion without binding between the cooperating threads of the circular nut and of the threaded end portions 26, 28 of the connecting bolt 20 when the bolt is heated and the nut requires tightening. The problem is not one of thread engagement but rather that the bolt expansion will expand into the nut so tightly that it cannot be turned. Hence the terminal end 34 of the conductor 30 should be displaced a short distance from the top end of the bottom nut to assure that the heat in the bolt portion engaging the bottom nut is limited to a modest amount.
In some cases involving extremely high current densities, it may be desirable and necessary to include means for circulating a cooling fluid through a central bore 7~
82 (Figures 2, ~) which extends axially through the conductor 30. In Figure 2 of the drawing, the central bore 82 is shown connected in fluid communication with a conduit 84 which supplies a cooling fluid such as water from a suitable supply (not shown). The same cooling water may be used to cool the transformer of the alternating current power source 42, for example by means of a conduit (not shown) which is connected in series fluid communication with the conduit 82 and the water supply. The cooling fluid may be discharged ~138S~ 46,477 through any convenient means such as a conduit 88.
The alternating current power source 42 is preferablya brazing transformer which may have any suitable regulation factor, for example approximately 10%, but any regulation in the power supply cables must be added. The regulation of the auxiliary power source may range from 0 to 50% depending upon the length of the power supply cables ~t~r~nY~
Suitable voltage taps on the brazing transformer should be provided to obtain the desired voltage.
The power factor of current flowing in magnetic steel assuming the physical form of the connecting bolts 20 is between 86 and 88% so that the heating effect provided by this apparatus is quite efficient. The ma~or portion of the heat is created in the steel itself and no significant surface heat transfer coefficients are involved as the heat ls generated within the interior of the bolt. Thus, it is apparent that the apparatus and method of the present invention permits heating of extremely large bolts at many times the limiting rate previously provided by conventlonal methods.
Although a preferred arrangement has been disclosed in association with a dynamoelectric machine, the structure and method as defined by the appended claims may be employed to good advantage to heat other metallic ob~ects regardless of size or structural association.

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of heating a metallic workpiece, said method comprising:
conducting alternating current through a low resistance conductor disposed within a spatial region within said work-piece;
causing alternating current to flow through said workpiece in the vicinity of said spatial region;
whereby the magnetic field produced by the alter-nating current in said conductor causes the current within said workpiece to concentrate in the surface of said workpiece defining said spatial region thereby heating said workpiece.

2. The method of claim 1 wherein said conductor and said workpiece are connected in electrical series to a single source of alternating current.

3. A method of heating a bolt, said method com-prising:
providing said bolt with a bore;
inserting a conductor having a first terminal portion and a second terminal portion into said bore;
joining said first terminal portion in series electrical contact with a source of alternating current;
joining said second terminal portion in series electrical contact with the bore of said bolt;
joining said bolt in series electrical contact with said source of alternating current; and conducting alternating current from said alternating current power source through said conductor and said bolt, said current being conducted substantially from a first power output terminal of said source through said conductor and from said conductor through said bolt, said alternating current being returned to a second power output terminal of said power source through said bolt.

4. The method as defined in claim 3, including the step of cooling said conductor as alternating current is conducted through said conductor and said bolt.

5. The method as defined in claim 4 wherein said conductor is a hollow rod and the step of cooling said conductor is performed by circulating a cooling fluid through said hollow rod.