Induction Heating Consultant

Induction Heating

Coil Design And Fabrication Tech Note

Click to view PDFby STANLEY ZINN and S. L. SEMIATIN
as featured in Heat Treating Magazine, June 1988

Part 1 • Basic Designs and Modifications

Part 2 • Speciality Coils

Part 3 • Fabrication Principles

Part 2 - Speciality Coils

S. Zinn is an independent consultant on induction heating; (585) 427-7840. S.L. Semiatin is a project manager in the Center for Materials Fabrication at Battelle Columbus Division, (614) 424-7742. This article is excerpted from the book "Elements of Induction Heating.' published by Electric Power Research Institute (EPR) and distributed by ASM International. (516) 338-5151 and used with permission of EPRI

Coil designs are based on the heating pattern requirements of the application, the frequency, and the power density requirements. In addition, the material handling techniques to be used for production determine, to a large extent, the coil to be used. If a part is to be inserted in a coil, moved on a conveyor, or pushed end to end, or if the coil/heat station combination is to move onto the part, the coil design must take the appropriate handling requirements into consideration. Accordingly, a variety of specialty coil designs have evolved for specific applications.

Master Work Coils And Coil Insert
When production requirements necessitate small batches (as in job shop applications) and a single-turn coil can be used, master work coils provide a simple, rapid means of changing coil diameters or shapes to match a variety of parts. In its basic form, a master work coil consists of copper tubing that provides both an electrical connection to the power supply and a water-cooled contact surface for connection to a coil insert (N. B. Stevens and P.R. Capalongo, "Inductor for High-Frequency Induction Heating," U.S. Patent 2,456,091, December 14, 1948). A typical design, shown in Fig. 1, consists of a copper tube that is bent into the form of a single-turn coil and soldered to a copper band that conforms to the slope of the coil insert and is recessed. Holes in the inserts that match tapped holes in the master coil securely clamp the inserts to the master coil, providing good transfer of electrical energy and heat removal. Inserts are machined from copper with a thickness that matches the required heating pattern, and should be somewhat greater in thickness than the depth of the recess for easy removal. Special coil shapes are easily configured. It is important to note that, because of the less-than-optimal cooling technique, coil inserts are particularly well adapted to processes requiring short heating times or those in which they are also cooled by the quenching medium.

In machining of coil inserts, care must be taken to relieve sharp corners, unless it is desired to have a deeper heating pattern in these locations. Fig. 2 shows the effect of sharp corners on a closely coupled part. Flux from both inductor sides couples to the corner, which, due to a lack of mass, tends to overheat relative to the rest of the pattern. Decoupling of the coil from these locations provides the desired pattern but tends to reduce over-all efficiency, thus slowing the heating rate and resulting in a deeper case. Relieving or decoupling of only the corners is a better alternative, particularly when a solid, inductor is used, and the relief can be machined as required.

Figure 3 Figure 4 Figure 5
Fig. 3: Inductor/quench designs for induction scanning: (a) separate coil and quench; and (b) two-chamber, integral coil and quench (from F.H. Reinke and W. H. Gowan, Heat Treatment of Metals, Vol. 5, No. 2, 1978, p. 39) Fig. 4: Diagram (a) and schematic illustration (b) of a split inductor used for heating crankshaft journals (from M.G. Lozinski, Industrial Applications of Induction Heating, Pergamon Press, London, 1969) Fig. 5: Design of metal and ceramic pins for fixing the position of a split inductor on a crankshaft journal (from M.G. Lozinski, In-dustrial Applications of Induction Heating, Pergamon Press, London, 1969)

Coils For Induction Scanners
Coils for progressive hardening (scanning) are built using two techniques. The simpler of the two employs a simple single-turn or multiturn coil with a separate quench ring that can be mounted on the scanner (Fig. 3a). For larger production runs, a double chamber coil that incorporates both coil cooling and quenching capabilities is often the preferred choice. The scanning inductor shown in Fig. 3b is typical of the latter type of design. Cooling water flows through the upper, or inductor, chamber to keep the copper resistivity low. The quenchant is sprayed from perforations in the beveled face onto the workpiece as it exits from the inductor. The beveled face normally is at an angle of 300 to the vertical, so that there is some soaking time between the end of induction heating and the quenching operation. This delay time helps to increase uniformity. Proper choice of the spray direction also reduces the amount of fluid runback on the shaft, which could cause variation in bar temperature and result in uneven hardness. Well-directed quench spray holes are required inasmuch as "barber poling" can occur due to erratic or misdirected quenchant that precools the part ahead of the main quench stream.

Split Coils
Split coils are generally utilized as a last resort for applications in which it is difficult to provide a high enough power density to the area to be heated without very close coupling, and where part insertion or removal would then become impossible. One such situation is the hardening of journals and shoulders in crank-shafts. In this case, the split-coil design would also include the ability to quench through the face of the inductor. Typical methods of hinging split inductors are shown in Fig. 4.

It should be noted that with a split inductor, good surface-to-surface contact must be made between the faces of the hinged and fixed portions of the coil. Generally, these surfaces are faced with silver or special alloy contacts that are matched to provide good surface contact. Clamps are used to ensure closure during heating. High currents at high frequency pass through this interface, and the life of the contact is generally limited due to both wear and arcing.

Coolant for the coil chamber of a split inductor is carried by flexible hoses that bypass the hinge so that excessive heating does not occur in the movable section during the cycle. The quench chamber is fed by a separate hose arrangement. The face of the quench chamber is closest to the work during heating, and therefore carries most of the current. Accordingly, it must be sufficiently thick to preclude either melting or distortion during the heating cycle.

With split coils it is also frequently necessary to provide some means of locating the part in the coil to maintain the proper coupling distance. Ceramic pins or buttons are frequently secured to the face of the inductor. These pins contact the part during the heating cycle and establish rigid relative positioning between part and coil. However, they are subject to thermal shock during the heating and quenching cycles and suffer mechanical abuse as well. Therefore, they should be designed for simple replacement as required. Fig. 5 depicts an arrangement for the use of either ceramic or metal pins that compensates for these problems. Here, the ceramic pin is approximately 0.25 inch (0.64 cm) in diameter and 0.5 inch (1.3 cm) long with a 0.27-inch (0.69) head diameter. The rubber packing absorbs the clamping stress. A threaded tube passes through the chamber, and a screw presses the pins against the shaft. In Fig. 5b, a 0.125 inch (0.32 cm) nichrome pin is used with a ceramic tube as an insulator. Being in compression, the tube undergoes comparatively high loads without breaking. The metal pin provides longer life in these conditions than the ceramic pin.

Butterfly Coils
One of the most difficult heating challenges is the creation of an even heating pattern at the end of a bar or shaft. Patterns developed with a pancake inductor produce a dead spot at the center, due to field cancellation in this area.

The butterfly coil (Fig. 6), so named because of its appearance, utilizes two specially formed pancake coils. The current paths of the adjacent sides are aligned so that they are additive. The "wings" of the butterfly may be bent up to decouple their fields from the shaft, or, if heat is required in this location, they may be coupled with the shaft itself. In winding this coil, it is important that all center turns be wound in the same direction so that they are additive. Further, only these turns should couple directly with the part to produce the desired pattern.

Split-Return Inductors
If a narrow band of heat is required and heating must be accomplished from one surface only, the split-return inductor offers distinct advantages (Fig. 7). With this design, the center runner of the work coil carries twice the current of each of the return legs. The pattern on the workpiece, being a mirror image of the coil, produces four times as much heat under the center leg as in each of the return loops. With proper balancing, the high-heat path can then be extremely narrow, while the heat produced in each of the return legs is insufficient to affect the remainder of the part.

Tapped Coils
Induction coils can be provided with taps to allow for differences in heated length. A typical application is a forging coil for heating "off the end" of a bar, in which provision must be made to adjust the length being heated. Taps are brazed to the work coil at locations where a water-cooled strap can be moved from tap to tap. The active portion of the coil is then between the power-supply connection and the tap. Water cooling, however, should be maintained through all portions of the coil, both active and inactive.

Transverse-Flux Coils
In heating of parts that have a long longitudinal axis and a thin cross-section, a circular coil wrapped around the workpiece produces a heating pattern (Fig. 8) that, due to coupling distances, is effective only at the edges. In transverse-flux heating, however, the coil is designed to set up a flux field that is perpendicular to the sheet or similar part. In this way, the path of the eddy currents is changed so that it is parallel to the major axis of the work. For example, in the manufacture of items such as hacksaw blades, the steel moves between the turns of the coil and the eddy current path is a circular one across the flat of the blade. For heating of wide sheet materials, specially designed transverse-flux inductors have, in recent years, also become available.

Conveyor/Channel Coils
Often when power densities are low and heating cycles not extremely short, parts can be processed by use of a turntable or conveyor in a continuous or indexing mode. The coil must then be designed to permit easy entry and exit of the part. The simplest conveyor or channel coil used in these situations is a modification of the hairpin inductor (Fig. 9). With the indexing technique, in which the part is at rest in the coil during the heating cycle, the ends of the hairpin can be decoupled to prevent overheating of the ends. These raised portions or bridges also facilitate passage of the part through the coil. When a wide heating zone is to be produced on the part, coupling over a greater area can be accomplished through the addition of a liner to the coil turn (Fig. 10), or more ampere turns can also be produced with a multiturn channel inductor (Fig. 11). Channel-coil liners may also be configured to produce specialized heating patterns where greater heat densities are required in specific areas (Fig. 12).

During design of heating operations using channel coils, there is a "fill factor" that must be considered from an efficiency standpoint. The unused portions of the coil appear as lead losses. Therefore, parts must be as close as possible to each other, without touching, to utilize the full capabilities of the inductor. Another important consideration in the use of a channel coil is the fact that those areas of the workpiece closest to the coil receive the greatest portion of the flux and therefore heat the fastest (Fig. 13). If conduction through the part is slow, the part should be rotated while passing through the coil. Sufficient time (in an indexing conveyor or turntable) or speed variation (in a continuous-motion device) must be provided to allow heat uniformity to occur in part areas farthest from the coil turns.

Stanley Zinn

Stanley Zinn • Induction Consultants • Tel: 585-737-8824
15307 Strathearn Drive, Unit 11202, Delray Beach FL 33446 •
email address

Stanley Zinn
Figure 1
Fig. 1: Schematic illustration showing the design of a master coil with changeable inserts (from M.G. Lozinski, Industrial Applications of Induction Heating, Pergamon Press, London,1969}
Fig. 2: Inductor with a relief designed for the hardening of the lateral surface of a template (from M.G. Lozinski, Industrial Applications of Induction Heating, Pergamon Press, London, 1969)
Figure 6
Fig. 6: Schematic illustration of a butterfly coil: (a) coil construction (arrows indicate reinforcing type of curent flow in coil); and (b) coupling between the turns of the coil and the end of a bar to produce a uniform heating pattern
Figure 7
Fig. 7: Two types of split-return coils (from C. A. Tudbury, Basics of Induction Heating, Vol. 1, John F. Rider, Inc., New York, 1960)
View LARGER Image
Figure 8
Fig. 8: Illustration (a) of one type of trans-verse coil for heating a thin section; sketch in (b) indicates the current path in the work-piece (from F. W. Curtis, High Frequency Induction Heating, McGraw-Hill, New York, 1950)
Figure 9
Fig. 9: Typical channel coil used to heat the edges of discrete lengths of rectangular bar stock; end of coil is decoupled by bending to prevent overheating of ends (from F. W. Curtis, High Frequency Induction Heating, McGraw-Hill, New York, 1950)
Figure 10
Fig. 10: Use of a liner on a single-turn channel coil to provide a wider heating pattern on the workpiece (from F. W. Curtis, High Frequency induction Heating, McGraw-Hill, New York, 1950)
Figure 11
Fig. 11: Multiturn channel coil used to increase the ampere turns coupled to an induction heated workpiece (source: Lindberg Cycle-Dyne Inc.)
Figure 12
Fig. 12: Multiturn channel coil with a liner added to control the heating pattern (from F.W. Curtis, High Frequency Induction Heating, McGraw-Hill, New York, 1950
Figure 13
Fig. 13: Development of the heating pattern in parts moved through a channel coil.
Figure 2