Flux concentrator assembly for inductor

A single loop inductor for inductively heating elongated workpieces is provided with flux concentrator assemblies accurately located on the parallel conductors by lamination keepers retained in keyways in the conductor.

The present invention relates to the art of induction heating and, in 
particular, to flux concentrator assemblies for single loop inductors. 
The invention is particularly applicable to inductors for heating the 
complete length of an axle shaft and will be described with particular 
reference thereto; however, it should be appreciated that the invention 
has much broader applications and may be used for heating various other 
elongated workpieces of constant or varying cross sections. 
Induction heating followed by controlled quenching has become an accepted 
technique for surface hardening extended lengths of axle shafts. Therein, 
the axle shaft is rotated within the flux field of a stationary, single 
loop inductor. The single loop inductor is effective to uniformly, 
inductively heat the total length of the axial shaft without requiring 
movement of the inductor. This single loop inductor, commonly referred to 
as a "single shot inductor", comprises a pair of parallel conductors which 
extend substantially the complete length of the axle shaft. The ends of 
the parallel conductors are interconnected by arcuate crossover 
conductors. One of the conductors, either a parallel conductor or a 
crossover, is divided to define an electrical discontinuity. Electrical 
leads are connected at the discontinuity and therefrom to a high frequency 
power supply. In operation, the flux field from the parallel conductors 
induces eddy current heating at the surface of the rotating shaft to raise 
the temperature thereof above the critical temperature for the shaft 
material. The depth and temperature are determined by conventional control 
of the electrical parameters. After the shaft surface has reached the 
predetermined heating temperature, quenching jets rapidly cool the shaft 
at a controlled rate to provide the desired material properties over the 
length of the axle shaft. 
The control and versatility of the heating pattern for such single turn 
inductors may be enhanced by the use of flux concentrators mounted on the 
parallel conductors. These flux concentrators, generally a stack of 
U-shaped high permeability laminations, are effective to control the flux 
density along the axial length of the inductor to produce a uniform 
heating pattern and to concentrate the current density on the inner wall 
thereof. The shape of the concentrators can be modified to provide for 
cross-sectional profile variations in the workpiece without modification 
of the basic inductor design. The concentrators may also be varied in 
length and position to enable a single inductor design to heat workpieces 
of varying lengths and heating pattern locations. 
In order to be fully effective in establishing the desired uniform heating 
pattern, it is necessary that flux concentrator be accurately located on 
the inductor. Present single loop inductors are generally fabricated from 
square copper tubing with the crossover conductors brazed to the parallel 
conductors at mitered joints. Even with advanced assembly techniques, it 
is difficult to accurately control the length of parallel conductors. 
As a consequence of the conductor length variations, the number of 
laminations and locations thereof results in the heating pattern varying 
from inductor to inductor. Further, where workpiece profile changes occur, 
the proper laminations may not be accurately matched with the workpiece 
leading to local underheating or overheating with resulting non-uniform 
hardness over the length of the workpiece. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an improved flux concentrator 
assembly is provided which enables an accurate location of the laminations 
at predetermined axial locations on the conductors. More particularly, 
keyways are formed transversely in opposed surfaces of the parallel 
conductors at locations corresponding to the length over which the 
workpiece is to be heated. The keyways are referenced to the main heat 
treating apparatus such that the keyways are also referenced to the 
workpiece when the latter is at the heating position. 
U-shaped keepers corresponding to the flux concentrator shapes have spaced 
legs dimensioned for close sliding engagement with the keyways. The 
laminations are banked and arrayed to establish the desired heating 
pattern with a length corresponding to the distance between adjacent 
keepers or a keeper and an opposed abutting surface. Accordingly, in 
assembly the length of the concentrator will be accurately axially 
prescribed and the individual concentrator laminates will be accurately 
positioned with respect to the workpiece profile. As a result, a uniform 
heating pattern will be achieved from inductor to inductor. Additional 
keyways may be formed on the inductors to facilitate interchange of flux 
concentrators thereby providing multi-workpiece capability for a single 
inductor. 
Accordingly, a primary object of the present invention is to provide a flux 
concentrator for an inductor which establishes an accurately located flux 
concentrating field. 
Another object of the present invention is to provide a mounting for an 
inductor flux concentrator which accurately locates a predetermined length 
of flux concentrator elements. 
A further object of the present invention is to provide a flux concentrator 
having uniform performance characteristics from inductor to inductor. 
Yet another object of the present invention is to provide a mounting and 
keeper assembly for readily locating arrays of flux concentrator 
laminations on an inductor to establish predetermined heating profiles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings wherein the showings are for the purpose of 
illustrating a preferred embodiment of the invention only and not for the 
purpose of limiting same, FIG. 2 shows an induction heating apparatus A 
for inductively heating an elongated workpiece B, such as an axle shaft, 
having a circular flange 10 and a cylindrical shaft 12. The workpiece B is 
mounted for rotation about a horizontal axis 14 on a conventional 
induction heating unit, not shown. A predetermined length of the shaft 12 
is heated to an elevated temperature by the induction heating apparatus A 
under conditions producing a uniform heating pattern to a predetermined 
depth. The heated shaft is then quenched by conventional quenching 
apparatus, not shown, under controlled conditions to produce a uniform 
hardness and hardened depth over the predetermined length of the shaft 12. 
The heating apparatus A comprises a single-loop, or as commonly known a 
single-shot, inductor 20 connected by leads 22 and 24 to a conventional 
high frequency power supply 26. As hereinafter described, the inductor 20 
includes internal coolant passages which are fluidly connected to a 
coolant supply, not shown. 
Referring to FIG. 1, the inductor 20 generally comprises a pair of 
longitudinally extending parallel conductors 32 and 34 integrally 
connected at their outer ends to a pair of transversely extending arcuate 
crossover conductors 36 and 38. The parallel conductor 34 is 
longitudinally divided into two equal sections 34a and 34b at an 
electrical discontinuity or gap 40. The inner ends of the sections 34a and 
34b terminate with radially outwardly projecting axially spaced power 
leads 42 and 44. The power leads 42 and 44 are respectively electrically 
connected in a conventional manner to the leads 22 and 24, respectively, 
of the power supply 26. Flux concentrator assemblies 46 are carried on the 
parallel conductors 32 and 34 of the inductor 20 as hereinafter described 
in greater detail. In assembly, the inductor 20 is fixedly mounted 
coaxially with the workpiece B about the axis 14 by a conventional holding 
fixture, not shown. 
In accordance with the present invention, the inductor 20 hereinabove 
generally described, is fully machined from a single block of copper. With 
the exception of variations hereinafter noted, the inductor 20 is formed 
without mechanical or fabricated joints by conventional milling, drilling 
and turning operations. The inductor is particularly adapted for complete 
machining by computer assisted machinery centers. 
The conductors 32 and 34, as shown in FIGS. 2 through 4, have square 
transverse cross sections and extend parallel to the axis 14. The 
conductors 32 and 34 are equally spaced with respect to the axis 14 and 
disposed in a common horizontal plane. The longitudinal length of the 
conductors as measured from the inner faces of the crossover connectors 36 
and 38 is equal to the predetermined length over which the shaft 12 is to 
be uniformly hardened. Each of the conductors 34 and 36 has a vertical 
inner surface 50 which is radially spaced from the axis 14 to establish 
the requisite air gap with the outer surface of the shaft 12 thereby 
providing the required magnetic coupling during the inductive heating 
cycle. The conductors 34 and 36 have horizontal top and bottom surfaces 52 
and 54, respectively, which are symmetrically disposed above and below the 
axis 14. Together with a vertical rear surface 56, the surfaces 50, 52 and 
54 define a square transverse cross section. However, it will be apparent 
that rectangular or curvilear cross sections are also readily accomodated 
by the present invention. 
The crossover conductors 36 and 38 have outer faces 62 lying in planes 
transverse to the axis 14 and coaxially spaced from the inner surface 63. 
The width between the faces 63 and 62 is substantially the same or greater 
than the width of the parallel conductors 32 and 34 as measured between 
the surfaces 50 and 56. The crossover conductors 36, 38 have an outer 
circular cylindrical surface 64, coaxial with the axis 14, having a 
diameter substantially greater than the width between the rear surfaces 56 
of the parallel conductors 32 and 34. The crossover conductors have a 
horizontal lower surface 64 lying in a plane coextensive with the lower 
surface 54 of the parallel conductors 32 and 34. The crossover conductors 
have an inner hemi-cylindrical surface 66 coaxial with the axis 14 having 
a diameter substantially the same as the width between the inner surfaces 
50 of the conductors 32 and 34. The surface 66 downwardly terminates with 
vertically extending surfaces 68 coextensive with the lower half of the 
inner surfaces 50 of the parallel conductors 32 and 34. Horizontal fillets 
70 of a substantial radius are provided at the transition between the top 
surfaces 52 of the parallel conductors 32 and 34 and the inner faces 60 of 
the crossover conductors 36 and 38. Vertical fillets 72 of substantial 
radius are provided at the transition between the outer surfaces 56 of the 
parallel conductors 32 and 34 and the inner faces 60 of the crossover 
conductors 36 and 38. The fillets 70 and 72 reduce the stress 
concentration at the sectional transitions during operation of the 
inductor. 
The inductor 20 is provided with a plurality of coolant passages which 
extend throughout the effective electrical length of the single loop. A 
single longitudinal passage 74 is drilled axially through parallel 
conductor 32 and the adjoining sections of crossover conductors 36 and 38. 
A pair of passages 76 and 78 are drilled axially through the crossover 
conductor 36 and the parallel conductor section 34a, and the crossover 
conductor 38 and the parallel conductor section 34b, respectively. The 
tips of the drilled passages 76 and 78 terminate just short of the inner 
surfaces of the leads 42 and 44 defining the gap 40. A pair of radial 
passages 80 and 81 are drilled centrally through the leads 42 and 44, 
respectively, and intersect with and fluidly communicate with the 
associated drilled passages 76 and 78. The diameter of the aforementioned 
drilled passages, together with the exterior dimensions of the parallel 
conductors, establishes a current flow path having sufficient capacity to 
handle the power load applied to the inductor during the inductive heating 
cycle. The circular drilled hole provides for greater cross section and 
power capacity than square tubing of comparable dimensions. 
Referring to FIG. 3, a pair of arcuate milled slots 82 and 83 are formed in 
the end faces 62 of the crossover conductors 36 and 38. Each slot, 82 and 
83 spans a sector of approximately 80.degree. and has a lower end 
registering with the end of the drilled passages 74, 76 and 78 and an 
upper end which is spaced from the end of the adjacent slot. A pair of 
counterbored passages 84 are formed vertically in the crossover conductors 
36 and 38 and fluidly communicate with the upper ends of each slot. A 
recessed rim 85 is formed adjacent the sidewall of the slots. An arcuate 
cover plate 86, corresponding in dimension to the rim, is received therein 
and brazed or soldered to the end faces 62 to seal the slots. Coolant 
pipes 90 are received in the counterbores of each vertical passage 84 and 
brazed or soldered therein. Similar coolant connections, not shown, are 
provided at the outer extremities of the power leads 42 and 44. When the 
coolant pipes are connected with the supply and drain lines of the coolant 
supply, the direction of coolant flow through the coolant passages will be 
in the direction indicated by arrows. In the illustrated preferred 
embodiment, three separate coolant circuits are provided. A first coolant 
circuit extends through the passage 74 and slot 82. A second coolant 
circuit extends through the passage in conductor section 34a, the passage 
in power lead 42 and the slot 83 in the crossover conductor 36. A third 
cooling circuit extends through the passage in conductor section 34b, the 
passage in the associated power lead 44 and the slot 83 in the crossover 
conductor 38. The objective of the cooling branches is to provide high 
pressure, high flow rate cooling circuit throughout the operative length 
of the parallel conductors having sufficient heat removal capacity to keep 
the associated conductor at a controlled operating temperature. Should 
redundant circuits be desirable or required, it is apparent that parallel 
circuits can be provided within each of the cooling branches. Further, 
should only a single cooling circuit be desired, it is apparent that the 
inlet and the outlets can be in the power leads and a single hemispherical 
milled slot could be provided in the end faces 62 of the crossover 
conductors to interconnect the passages and the associated parallel 
conductors. 
The flux concentrators 46 carried by the parallel conductors 32 and 34 
provide, in a well known manner, control over the applied flux density in 
order to permit a single inductor to be used for heating, to a 
predetermined depth, workpieces having varying heat treated lengths or 
diameters. They are also effective for providing uniformity of the flux 
density on workpieces having axially cylindrical sections. 
In order to be fully effective, it is necessary to locate the flux 
concentrator elements along the axial or longitudinal length of the 
parallel conductors in a position accurately corresponding to the desirred 
heating pattern. Moreover, at the incremental axial positions, it is 
necessary to have appropriately located elements for achieving the desired 
heating depth thereat on parts having profile variations. 
As shown in FIGS. 4-8, the flux concentrator 46 are comprised of a 
plurality of U-shaped elements 100. The individual elements, in a well 
known manner, may be soft iron laminations or other high magnetic 
permeability laminates. Each element is formed with a pair of spaced legs 
102, 104 which are slidably received over the top and bottom surfaces 52 
and 54 of the associated parallel conductor sections. The effective length 
of the legs 102, 104 may be varied with respect to the parallel conductor 
to selectively vary the applied flux density therefrom. The flux 
concentrator elements are held in discrete banks of laminations by 
generally U-shaped keepers 110. The keepers 110 preferably formed of a 
high conductivity material such as copper have a peripheral profile 
corresponding to the shape of the flux concentrator elements. The width of 
the legs 112, 114 of the keeper members is slightly greater than the 
opening between the legs of the associated concentrator elements. Keyways 
116 are formed transversly across the top and bottom surfaces of the 
parallel conductor sections and slidably receive the legs of the keepers 
in assembly. The keyways 116 are accurately referenced to the machined 
surfaces of the inductor so as to define between facing surfaces of the 
individual keepers or with an end face of a crossover conductor, an 
accurately axially located spacing for receiving a predetermined number of 
flux concentrator elements. In this manner, each lamination bank can be 
precisely controlled in length and accurately located on the conductor 
thereby giving greater uniformity in the heating pattern from inductor to 
inductor. To provide for greater versatility in a single inductor design, 
various keyways may be formed along the length of inductor sections. Such 
keyways can be selectively used for mounting lamination banks of varying 
lengths and configurations thereby enabling a single inductor design to 
inductively heat workpieces of varying configurations. While the keeper 
arrangement is obviously beneficially incorporated in the fully machined 
inductor coil described above, it is apparent that the accurate locating 
of the flux concentrator lamination banks can be achieved by providing 
comparable keeper arrangements on fabricated conductors. As shown in FIGS. 
6 through 8, the keepers 110a, 110b and 110c and the concentrator elements 
100 may have legs of differing lengths and shapes for providing discrete 
changes in the heating pattern for differing part profiles and hardness 
requirements while maintaining the overall inductor configuration. 
Further, the keeper arrangement is beneficial on inductor designs other 
than the single loop configuration wherein accurate positioning of the 
flux concentrator relative to the part is desirable.