DESIGNING FOR INDUCTION HEATING

High frequency induction heating equipment for hardening, brazing, and soldering is having a marked effect on manufacturing methods and as times goes on will have a decided influence on the constructional  design of many products.  While in many certain cases design changes will be limited, there are examples of certain modifications which will make the adoption of induction heating more practical.  These changes usually are not drastic, but more on the order of improvements to suit the characteristics of this method of heating.

All in all, it is largely a case of acquiring the technique of induction heating requirements, and then considering these needs to make its use successful.  A sound general knowledge of what induction heating equipment can and cannot do, though relatively simple, is basic to its practical application.

In checking case where induction heating equipment has not produced exactly what was wanted or expected, the cause usually could be traced to some design feature or details which  could easily have been modified when the design was started.  Thus it is evident that knowing from the beginning what is required to make induction heating successful may help to develop its full possibilities.

Induction heating equipment provides the means of heating parts locally and heating them fast, whether in hardening or brazing.  The only difficulty, generally speaking, lies in making a heating coil or inductor to surround or fit the surface of the part requiring heat, and to have a high frequency generator with sufficient output power to heat the surface or surfaces adequately.  With this, of course, a comprehensive knowledge of the formation of the heat pattern on the adjacent surfaces is advisable.

Induction heating coils can be multiturn or single turn.  From these two type any number of shapes and styles can be arranged to heat outside, inside, flat and irregular surfaces.  The magnetic lines of force surrounding a coil theoretically are equal in all directions from the center line of the coil.  Their density varies inversely as the distance between the part to be heated and the coil is increased.  The nearer the work to the coil, the faster will heat be generated to the work's surface and vice versa.

When induction heating the surface of a shaft adjacent to a shoulder, it is difficult to obtain  heat in the fillet or corner, since some of the flux energy from the end of the induction coil will flow into the face of the flange.  This condition results in a "cold spot" at the fillet, which can be heated only by conduction and which, of course, would require added time in the heating cycle.  This added time might also result in excessive heating at other portions of the work, so that a modification of the design might be the solution.  By adding a relief, the condition is improved, or if a slight shoulder is desired, such as may be needed for needle bearings.  In this case the hat can be obtained in the sharp corner, the shoulder being comparatively light.

Another consideration in induction heating is that thin surfaces are liable to become overheated in relation to adjoining heavier sections.  Where possible, therefore, uniform areas  should be provided in the design of a part, to make heating more even.  The end of the shaft for example, requiring hardening on the internal hole, would result in the generation of excessive heat through the thin section before the remainder of the hole reached the proper quenching temperature.  By adding more metal between the hole and the bottom of the keyway, a more uniform heat will result and the danger of burning is overcome.

Sometimes this condition can be corrected by applying a copper plate in the keyway during the heating operation, so that the high frequency current does not have to crowd through the thin area of the work only; but usually a modified design is to be preferred.  Sometimes other forms of copper shield are used to cut off heat at limited areas, but these usually are applied on existing parts which were not designed for induction heating.  Sharp corners, too, are liable to result in some overheating, sometimes referred to as "edge effect".   Here again, by modification in design and a knowledge of induction heating requirements, such difficulties can be easily overcome.

Since localized surfaces can be inductively hardened without heating adjoining areas, the application of induction heating is going to result in new designing techniques.  In some cases improved products will be made available, in others it will be possible to obtain results that ordinarily would be impossible or too difficult to consider by other means.  Manufacturing processes will also be simplified.  For example, carburizing by substituting higher carbon steels will be eliminated, while parts from two or more sections will be fabricated instead of a one-piece design which might necessitate difficult machining setups.

In the design of a machine shaft to operate in anti-friction bearings, where space was a limiting factor, a change from ball bearings to needle bearings was desired.  The former design could not be used, due to a parallel shaft operating on close centers.  Substituting the design it was possible to harden the bearing surfaces selectively, using the heating coil and quench ring.  The shaft, made of 0.50 carbon steel, is hardened to 62 to 64 Rockwell C, suitable for a needle bearing race, and since heating is localized the usual straightening and cleaning operations ordinarily required have been eliminated.  This same means of induction hardening can be applied to races for ball bearings thus eliminating difficulties that might arise through heating of such a part.

An example of part in which the inner ball race is made integral with one end a single turn induction coil is arranged to surround the groove.  With this coil energized, heating of the part will be localized to the ball race.  The depth of penetration and the area heated can be controlled by the time cycle.  After quench, the ball race will be exceptionally hard and, since heating was localized to so small an area, the balance of the part remains metallurgically unchanged, and warping or deformation cannot take place.

Through induction heat it is possible to obtain different hardnesses not ordinarily obtainable on a single piece.  On which three separate induction heating operations are performed.  The cam surface is hardened to 60 to 62 Rockwell C, maximum resistance to wear being desired.  The gear teeth are hardened to 53 to 55 Rockwell C for their normal service, whereas the clutch teeth are hardened to 48 to 50 Rockwell C, to resist wear and withstand shock without fracture.  Such modifications is hardnesses are obtained by slight variations in heating and quenching cycles, which automatically are timed so that uniformity is maintained.

Formerly a sprocket was carburized and required rough machining, copper plating, re-machining to remove copper plate, cutting of teeth, carburizing, hardening, chucking from pitch of teeth to grind hole, and assembling of bushing.  By using a higher carbon steel, the new processing requires only machining of blank, assembly of bushing, cutting of teeth, and induction hardening of teeth.  Since the teeth are cut after assembly of the bushing, concentricity is assured and more easily obtained than by the former method.  A portion of the induction formed to suit the contour of the teeth so that a uniform heat pattern is obtained with no excessive heat at the top of the teeth, as might happen with a flat or copper-tube coil,  The elimination of carburizing, has a broad field of application and shows the advisability of considering induction heating in design of mechanical parts.

As a matter of safety it is good practice, when changing from a through-hardened part to one on which only local surfaces are induction hardened, or from an alloy steel to a plain carbon steel, to add some thickness or size to certain sections.  The slight amount of metal added does not usually later material cost.

A typical example of such a change is a spiral bevel gear made of S.A.E. 4615 carburized, hardened, and lapped, it is not made of C-1141, induction hardened, and lapped.  Lapping is done in far less time than before, since less distortion and absolutely no scale are produced, while equal hardness is obtained.  After adding about 1/4 in. to the back face of the gear for extra strength, from a standpoint of trueness and other operation qualities it is better than before, besides costing less in both material and labor.  The heat treating cycle for this bevel gear includes an induction heating period of about 25 sec., followed by a water jet quenching of 15 sec.

A design change brought about by induction heating is the flanged shaft formerly made from forging and requiring machining of the flange profile as an integral member.  The two piece design, the flange being a stamping brazed to the shaft.  The single turn inductor used for alloy brazing in its relative position during the joining operation.  A ring of alloy is placed at the joint and with only a few seconds of heat a smooth, perfect joint is attained.

A part which utilizes induction heating for brazing and hardening here a large gear is assembled onto a shaft, and then induction brazed with the heating coil.  The small gear, of course, is made integral with the shaft.  This part formerly was made in one piece, but now is made of two separate parts, which shows the possible scope of induction heating in relation to product design.

In processing this part, the larger gear is machined and brazing to the shaft, then after assembly the gear teeth are shaved.  This operation provides for the correction of any misalignment and assures concentricity with the smaller gear.

Both sets of gear teeth are hardened by means of the coils indicated.  On this same part there also are two journals to be hardened, one at each end of the shaft, onto which roller-bearing assemblies are attached, so that the hardened areas of the shafts become the inner bearing surfaces.

Design for induction brazing requires some consideration, yet in many cases joints of a wide range can be handled effectively without modification.  The basic consideration is to analyze the parts to be joined to determine if more heat is needed at one section than another, and then design the heating coil to suit the application.  Occasionally on light sections it is better to allow the heat to flow by conduction from a heavier section, which, of course, is determined by mass and size.  However, with some parts the shape may need modifying.  For example, the joint is not particularly, suited to induction brazing since the entire flange would have to be heated in order to assure heat reaching the joint.  By providing a hub a simple one turn coil can be used and heat applied quickly to the joint without affecting the outer portion of the flange.  The modified is also practical.  A preformed ring of brazing alloy is placed in a groove cut in the shaft prior to assembling the two pieces. Here again, a single turn inductor will generate heat to the local surfaces to be joined.

When a high pressure brazed joint is required, it is possible to thread the members to be joined, then assembly them.  A ring of silver alloy is placed inside the assembly.  The heating coil is arranged to generate heat from the outside in, and when the brazing material slows it will run through the threaded area and form an exceptionally strong bond.

There are many other ways  to make high pressure joints, as well as joints which must overcome torsional strains.  Some of these would include keyways, multi-spline, serrations, pins, and various forms of threaded assemblies.

A form of brazed joint which can be used when torsional strains must be overcome the end of the shaft is provided with serrations to match the broached serrations in the hole of the arm.  When brazing alloy is applied to the joint, it flows throughout the serrations and forms an exceptionally strong bond. 

Another design change necessary for joining by induction heat is the bellows.  With this design change the coupling from the coil to the surface requiring soldering is too great, resulting in heat dissipation to the outer surfaces only.  By revising the design so that the joint is at the outside edge heat can be precisely applied to the desired surface.  Such modifications create no difficulties when the designs are originated, but clearly show why a knowledge of induction heating technique is desirable.

Another example of a two-piece design requiring brazing and hardening is the bevel gear the teeth of the gear would be difficult to cut as an integral member of the spindle.  Likewise,  since the gear teeth must be hard, any form of brazing after hardening might cause annealing or drawing.  by the induction heating method, the brazing of the two parts is done by an internal coil followed by hardening of the teeth using a multiturn coil which limits heat to the teeth.  A spray quench follows the heating portion of the cycle, and the entire hardening operation is completed at a rate of three pieces per minute.

A clutch shaft made in one-piece is induction brazing by means of a single turn coil, whereas hardening of the clutch teeth is by means of a pancake coil.  Inasmuch as the heat required for hardening the teeth is strictly localized, there is no danger of previously brazed joint's being re-heated.

An example of induction hardening where former manufacturing difficulties have been overcome.  The part represents a long spline shaft made of a 0.50 carbon steel heat treated to a hardness of 30 to 32 Rockwell C prior to final machining.  One each end of the short spline, are required hardened areas on which needle bearings operate.  These surfaces are hardened simultaneously to 61 to 63 Rockwell C by means of a series type induction heating coil.

Before the application of induction hardening is was necessary to heat the entire end of the shaft which, likewise, was quenched and drawn.  This caused the surface of the heated areas to become badly scaled and usually there was a slight amount of deformation causing a rather difficult straightening operation because of the short distance in which the warpage took place.   With induction hardening, however, heat is localized to the surface requiring hardening, thus entirely eliminating warpage.  Also, the hardened surfaces are produced practically free of scale, so that  with a minor buffing or cleaning operation, they can be used as bearing surfaces.  On other parts of this type it may be advisable to grind the journals after hardening, in which case a very small allowance has to be made.

High frequency induction heating equipment has many uses in all types of industrial plants and its application undoubtedly will play an important part in our future manufacture.  As in the case of any new process, it is necessary to follow through with certain procedures, such as the designing principles herewith described, in order to attain its full benefits - economical heat, quickly applied, with extreme uniformity.