Electric heating – Inductive heating – Specific heating application
Patent
1998-12-11
2000-01-04
Leung, Philip H.
Electric heating
Inductive heating
Specific heating application
219673, 219674, 148573, 266129, H05B 640, H05B 614
Patent
active
060112468
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an induction-heating device for surface treatment of the gear of a mechanical part such as a pinion.
Attempts have already been made to subject the gear of a pinion to surface heating which exactly follows the gear profile, such that it coincides with the quenched layer.
For this purpose the part to be heated is placed in an induction coil through which an alternating current is passed.
At high frequency (>100 kHz), however, it has been observed that the induced currents best follow the tooth profile, but the thermal flux is convergent at the tip of the gear and divergent at the root thereof.
At medium current frequency (<30 kHz), the induced current density is low at the tooth tip and high at the tooth root.
In both cases, the hardened quench zone therefore cannot follow the gear profile, all the more so because the depth of the induced current also depends on the gear module.
A solution has been sought in two-stage heating comprising successive heating at low frequency and then at high frequency.
Nonetheless, difficulties are observed in regulating the heating power at low and high frequency using a single inductor.
More generally, the gear profile must be heated above a transformation temperature and must then be cooled rapidly by quenching with a liquid to achieve hardening. The factors which influence the resulting treatment thickness are depth to which the gear is heated, temperature up to which the heating temperature exceeds the transformation temperature, and cooling rate. A circular induction coil situated a short distance from the undulating gear surface generally produces greater induction heating in the regions of the gear tips than in the transition regions between the teeth. The heating temperature and depth are therefore greater in the outer regions of the teeth. The differently heated zones are therefore cooled at different rates by the quenching process.
The treatment depth thus obtained and applied to the gear of a pinion is no longer uniform. The hardening obtained is then excessive in the region of the tooth tips and insufficient in the region of the connecting surfaces between the teeth. For successful hardening of the teeth of a toothed wheel by induction heating, it is therefore necessary to heat the part uniformly to a previously chosen temperature, directly with quench-hardening to a controlled depth.
A process for hardening to a uniform depth on the surface of the teeth of the toothed wheel is described in U.S. Pat. No. 4,675,488. According to that process, a toothed wheel is disposed in an induction coil having an axial height or length corresponding to that of the toothed wheel in its axial direction. The induction coil is supplied briefly with alternating current at a frequency below 50 kHz, in order to preheat the toothed wheel. Since the heating depth obtained by induction heating is inversely proportional to the square root of the current frequency in the induction coil, preheating penetrates into the toothed wheel. The body of the tooth is therefore heated to a temperature lower than the temperature for transformation by quench-hardening of the metal comprising the toothed wheel. A second preheating stage at lower frequency continues heating of the body of the toothed wheel and the flanks of the gear.
According to this process, the toothed wheels are first introduced axially into a medium-frequency induction-heating coil.
The toothed wheel is then displaced axially into a second induction-heating coil, in which it is subjected to final heating by a greater current at high frequency.
Each coil must have an axial length longer than the axial length of the toothed wheel to permit overall heating thereof.
This process, which uses straight annular turns concentric with the pinion, permits uniform heating of the teeth of straight-gear pinions, since the induced currents then circulate parallel to the gear edges. When the gear to be treated is a helical gear with helix angle larger than 20.degree., however, circulation of the induced current in the
REFERENCES:
patent: 3196244 (1965-07-01), Wulf
patent: 4675488 (1987-06-01), Mucha et al.
patent: 4808779 (1989-02-01), Cogley
Evans: "Induction-hardening of Gears", Engineers Digest, vol. 31, No. 12, Dec. 1970, London GB, pp. 57-68.
Bonzano Giorgio
Criqui Bernard
Dias De Sousa Maria
Longeot Olivier
Ensam Bordeaux
Leung Philip H.
Renault
Saet
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