(induction heating fundamentals)

The coil used for heating the inner surface of a rocker arm which required hardening the coil comprises two turns positioned so that they lie adjacent to the surface requiring heating.

 A good many applications of induction heating require the use of coils which might have two or more windings spaced widely apart to suit surfaces of varying size.  In this case it is important that the paths of the current flow in the same direction.  Assuming that a double-turn coil is desired, like that shown at A in Fig. 63, the winding arrangement should be such that the current flows in the same direction, as indicated by the arrows.  If a coil were made such as that as shown at B, with the current flowing in one direction in one coil and opposite in the other, the magnetic flux of one would have a tendency to cancel that of the other, so that practically no heating would result.  This is an important consideration in the construction of coils of this type, particularly where multiturn and series type heating coils are needed.  

 A multiturn coil of the series type arranged for hardening two surfaces is illustrated in Fig. 64.  While applications of this type can be provided for as shown, there are bound to be some losses due to the length of the leads between each coil section and, before the adoption of multiple heating as illustrated, the advisability of hardening the surfaces separately should be considered.  In any case, a coil of this type should be so arranged that the leads are close together and never as indicated by the dotted lines, for the inductance resulting from the jumper and the input leads which would cause heat losses.

Fig. 63 - When making series-type coils, particularly those of the flat type, it is essential that all turns be made to carry the current in the same direction, as shown at the left.

Fig. 64 - In constructing a series-type coil as shown here, it is important that the leads be kept close together in order to assure maximum heating to the areas on which heat is required.

Fig. 65 - A series-type coil assembly, comprising four multiturn coils, arranged for internal heating.

Multiturn copper tube coils of the series type can be made of a single piece of tubing, as has been shown, whereas another practical design is that shown in Fig. 65.  Here the coils are made separately and joined together by bus bar jumpers, or connectors.  The hose shown at the top is used for providing a continuous flow of water through all four coils.  The two supporting members, through which the copper tubing is assembled, are made of asbestos board.  

In Fig. 66 is illustrated a series-type internal-heating coil showing another means of making a connection.  Each coil is made separately and then connected to a jumper plate A, through

Fig. 66 - Double-type internal-heating coils arranged with a bus-bar connector and a hose for the continuous passage of water.

which the high frequency current passes from one coil to another.  The ends of the coil tubing are then connected with a section of hose, to provide continuous passage of cooling water.  The other two ends of the coils are connected to the output leads of the high-frequency generator.  This design can be used for a greater number of coils, which may have to be joined together and, where operating conditions permit, operated in series.

A multiturn coil used for brazing together two steel sections in which heat is desired from the inside surface is illustrated in Fig. 67.  The part has been cut in half  to show the relative location of the coil.  When long leads are required, as is true in this case, it is necessary to keep the leads close together, particularly where they enter the bottom opening, like that shown. It must be remembered

Fig. 67 - A series-type internal-heating coil with two separate coil units, which is used for the brazing of two steel cases simultaneously.

that if the leads lie adjacent to a metallic surface, thy will generate heat where it may not be wanted and dissipate some of the energy needed for a particular portion.  In this case, the leads are mounted in insulating blocks attached together but separated by mica insulation.  for the operation shown, the two pieces are handled in one setting and the two coils are series connected.  All lead connections are made on the underneath side of the table

 The coils shown in Fig. 68 is an excellent example of a solid-type inductor for heating two parts at one time.  The coil is made from a plate of copper 3/8 in. thick and bored out to provide suitable clearance around the two work pieces to be heated.  The copper tube, used for cooling, may be seen around the outside edge of the coil.  The coil is supported by two angle plates connected to the front panel of the worktable.  Since the panel is made of insulating material, the brackets have no means of shorting the high-frequency current.  When making supports for coils it is important to provide proper insulation.  The coil is used for brazing two steel tubular parts together, like those shown at the right.  

A series-type coil made from several copper bushings attached together  around the upper and lower portions of the bushing are brazed the copper tubes used for cooling.

Fig. 68 - Tandem design heating coil of the single-turn type used for the brazing of steel tube assemblies.

Saw cuts are provided between the bushings in order to provide for a continuous path of high frequency current.  The operation in this case is the brazing of a steel tube to a cap.  The coil and fixture are arranged so that eight parts are completed simultaneously.  A coil of this type is relatively easy to build and may be employed in a variety of operations requiring multi-setups.

 The same general principle is applied to series - type coils made from a flat copper plate, like the one illustrated in Fig. 70.  Here the cooling tube is brazed on the underneath side of the plate and shaped to conform to the coil openings.  The tubing connections are brought out at one side, to provide a suitable connection to the output leads of the generator.  The coil is provided with small end plates, as at A, to which supporting braces can be attached, to provide for rigid mounting.  

Fig. 70 - A heating coil made from a flat copper plate and provided with a cooling tube, placed on the underneath side, is illustrated above.

 High-frequency current will circulate around the surface of the metal part even though this is not completely surrounded by a coil.  Naturally there are limits in the extent to which this principle can be applied, but for average small parts, usually requiring high-frequency heating, it is possible to use two parallel inductors and to pass the work underneath, or arrange it in approximate relation to the inductors, so that heat will be absorbed around its entire outer surface.  This principle is illustrated in Fig.76.  Here, at A may be seen the relation of two parallel inductors used for soldering a cover plate to the body of a round condenser can.  In this case the heat is

Fig. 75 - Another form of series type internal heating coil.  Copper tubing is used for connecting both sections, as well as to provide for the flow of cooling water.

Fig. 76 - An example of parallel-type inductor, comprising two bars, which is used for the soldering of condenser ans.

 concentrated to the edges only.  For the example shown at B, the inductor bars are located directly above the joints to be soldered, which in some cases will be found preferable.  The principle of heating is shown below.  It will be seen that the high-frequency current circulates through the bars of the coil which, in turn, is induced into the work located underneath in the opposite direction.  In handling operations of this kind, the work can either be placed in a fixture that provides correct relation to the inductors, or conveyor-fed progressively under the inductors.

Another form of inductor, comprising two bars, is illustrated in Fig. 77.  Here the longer bar A is adjustable by means of the jumpers B.  As will be seen, the bars are provided with cooling

Fig. 77 - A two-bar parallel inductor of the adjustable type is shown above.

tubes and have hose connections at their ends for the continuous passage of cooling water.  

The coil shown in fig. 78 also is of the two-bar type and is arranged so that work can be passed through the opening, as illustrated at A.  The part represents the end of a drawn-steel shell which requires annealing.  The operation is performed by feeding and rotating the work through the bars, starting at one end and leaving at the other.  In mounting coils of this type, it is necessary to provide suitable supports, such as stand-off insulators, in order that rigid mounting can be obtained.  

In Fig. 79 is illustrated a two-bar inductor of the parallel type used for the heating of a long steel bar requiring hardening at one edge only.  In this example the inductors are cut out to conform to the shape of the part, as shown in the cross-sectional view at A.  Holes are drilled lengthwise through the inductors for the passage of cooling water.  At one end a jumper is provided, whereas at the other end of each bar the terminals connected with the generator are brought out.  Coils of this type can be insulated by means of mica, in which case it is possible to provide a means for clamping them firmly together.  The sectional view taken through the inductors shows the work in the heating position.  Only the edge of the bar is heated as it is progressively fed through the inductors.  A spray

Fig. 78 -  A two-bar-type inductor arranged so that the work can be rolled between the plates as may be required for annealing the ends of shells or tubes.

quench unit, not shown, is located at the left of the inductor, in order to complete the hardening cycle. In making heating coils for continuous feed operations, it is often desirable to use copper tubing

Fig. 79 - A solid-type parallel inductor used for the transfer of heat to the edge of a long shear blade, requiring progressive hardening.  

made into the form of a hairpin, under which the parts to be heated are fed.  With such a coil it is often necessary to  bend the end of the coil upward, as illustrated in Fig. 80.  If the coil remains on a true horizontal plane, as illustrated at A, there may be a likelihood of excess heating  on the edge of the work, as at B, especially is sharp corners are encountered.  Usually the thinner the work, the more necessary it is to provide this bend at the end of the coil.  

Fig. 80 - When using hairpin-type coils for heating the edge of narrow strips, it is advisable to turn the end of the coil upward to avoid overheating on the corners of the work.