Deadlines and budgets do not always allow for “first class” gearbox rebuilds. The challenge for rebuild shops is to find the fastest and most economical way to return equipment to full functionality. Last time we reviewed housing repairs; in this installment we will cover gears.
Some gears just cannot be properly repaired. Cracks, for example, require complete removal. There are shops willing to repair weld teeth, and while I salute them for their patience and artistry, I do not want to be associated with the results. If you weld anything, even far away from the gear teeth, you have to do a thorough check for cracks following the post-weld stress relief.
For large gears with complex hub configurations, I have had success with removing all the teeth and machining the hub to serve as the center for a shrink-fitted and pinned steel “tire” that can serve as a blank for new teeth. No one likes to wait for the new-forged rolled ring to arrive, but there is often no other choice. The amount of fit needed to keep the “tire” from rotating on the hub can be calculated using the formula for a keyless coupling joint.
Worn teeth can be re-surfaced so long as the root fillets are sound. It can be tricky to grind the roots of some gears and on surface hardened gears the reduction in case depth needs to be considered.
Much of our work involved through hardened gears of “standard proportions.” We converted many of those sets to “long and short” addendum designs by turning the gear outside diameter down and making new mating pinions. Pinions — being smaller — could be made from barstock we kept in inventory. The gears usually could be recut while still mounted on their shafts, as this was quicker and reduced the chance of damaging other parts during disassembly. As an added benefit, the new geometry usually provided a higher rating.
While not recommended for new builds, these techniques can be the difference between keeping and losing a customer. Especially on custom gearboxes or prototypes, deadlines are called that for a reason.