Oh, easy: that metal disc (along with the nylon worm gear and it's shaft) spits out onto the road spontaneously without permission. I learned to epoxy (JB Weld) that metal disc into the angle drive. To remove it, the tiny, weak stakings in the plastic housing can be relieved with a Dremel bur, or the epoxy relieved the same way.   Always epoxy that metal disc into the angle drive, folks!