Flywheel starter ring hardening at 120 kW

The problem

A flywheel starter ring gear is engaged by a starter pinion tens of thousands of times over a vehicle life. Every engagement is a small impact on a tooth flank, and the failure mode is wear on those flanks until engagement becomes unreliable.

The metallurgical requirement is straightforward: a hard, wear-resistant surface on the teeth over a core tough enough to survive the impact without cracking. The manufacturing requirement is harder. The hardness has to be the same on every tooth, all the way around the ring, on every ring.

A furnace cannot do it, because it would through-harden the ring and leave it brittle. A flame can do it, but the result follows the operator, and a ring with a soft sector is a ring that fails early in exactly one place.

Our approach

The ring is heated by an inductor that follows its circumference, so the heat lands on the tooth region and not on the body of the ring, and then quenched under a controlled sequence.

Because the treated zone is an entire circumference rather than a short feature, the machine needs real power. We built this on the ABE-120AB, our 120 kW power class, at 120 kW.

The machine is a guarded production cell with a touchscreen control station and operator access steps, rather than a bench arrangement, because a ring of this size has to be located repeatably and because quenchant has to be contained.

What this achieves

The hardness pattern is set by the coil and the cycle rather than by an operator, so it repeats. That is the whole point of the machine and it is the difference between a ring that meets a drawing and a ring that usually does.

The same 120 kW power class is also specified for shaft hardening up to 120 mm diameter, gears, bushes, JCB parts, tractor parts and axle shafts, so the machine is not single-purpose if the part family changes.