Good news, bad news. Good news is that it wasn't a gear that broke, but
a spring/split pin holding the gear on. 'Course it was down inside the
apron and more work to get to than about anything else on the lathe!

I was surprised than the pin broke - just sheared right off. It was
small (3/32, .094, 2.4mm) and not hard, but still ...

The bad news is that there was also damage in the quick change box. A
couple of gears there had been repaired with brazed-in teeth and one of
those teeth bent. I plan on replacing them (the gears, not the teeth).

I've started experimenting with shear keys. I have some plastic that
looks promising. One test key 3/16 square x 1" long took 70 foot lbs
to shear it. I'm going to start with real small versions and work up
till I get a size that won't shear under normal conditions. Then
hopefully it will shear under damaging ones.

I thought of another approach to gear train protection (for this lathe
anyhow). The first gears off the forward-reverse toggle are two gears
on a jack shaft, keyed together. Instead of keying them, one could
sandwich some friction material between them and create a slip clutch.
It might be kind of difficult to get a consistent slip point, but it
might be worth experimenting with.

Bob