Large gear tooth-by-tooth hardening slewing rings

Large gear tooth-by-tooth hardening

Induction hardening of small-, medium, and large-size gears is done using a tooth-by-tooth technique or encircling method. Depending on the gear size, required hardness pattern and geometry, gears are induction hardened by encircling the whole gear with a coil (so-called “spin hardening of gears”), or for larger gears, heating them “tooth-by-tooth”, where a more precise hardening result can be achieved, although the process is much slower.

Tooth-by-Tooth Hardening of large gears

The tooth-by-tooth method can be done in two alternative techniques:

“tip-by-tip” applies a single-shot heating mode or scanning mode, an inductor encircles a body of a single tooth. This method is rarely used because it does not provide the required fatigue and impact strength.

A more popular “gap-by-gap” hardening technique applies only a scanning mode. It requires the inductor to be symmetrically located between two flanks of adjacent teeth. Inductor scanning rates are typically within 6mm/sec to 9mm/sec.

There are two scanning techniques used:

- the inductor is stationary and the gear is moveable

- the gear is stationary and the inductor is moveable (more popular when hardening large-size gears)

The Inductor

Inductor geometry depends upon the shape of the teeth and the required hardness pattern. Inductors can be designed to heat only the root and/or flank of the tooth, leaving the tip and tooth core soft, tough, and ductile.

Simulation helps prevent overheating

When developing tooth-by-tooth gear hardening processes, particular attention should be paid to electromagnetic end/edge effects and the ability to provide the required pattern in the gear end areas.

Upon scanning a gear tooth, the temperature is distributed within gear roots and flanks quite uniformly. At the same time, since the eddy current makes a return path through the flank and, particularly through the tooth tip, proper care should be taken to prevent overheating the tooth tip regions, in particular at the beginning and at the end of the scan hardening. 

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