The Safety Pinion (Fogg's patent)

A Reversing Pinion prior to assembly.

The Safety Pinion originated in 1865 when it was usually described as "Fogg's patent", it was intended to avoid damage to the movement in the event of a mainspring breakage, the spring being let down too quickly during maintenance or following failure of the click (ratchet stopping the mainspring arbor from "winding down" and forcing the barrel to move instead).

The problem was that with the very strong springs required at the time the backlash after a failure would go through the train and potentially break weaker parts, mainly I suspect the lever pallet and pallet jewels.

The solution is quite ingenious. Instead of the centre wheel pinion being fixed to the wheel / staff it is screwed on. In normal use the gear teeth on the mainspring barrel bear against the pinion and keep it screwed down.

A "Reversing Pinion" during assembly
but not yet screwed on.

If the pressure is suddenly reversed the pinion unscrews and no reverse power is applied to the rest of the movement.
This solution was initial used by Waltham and then by Elgin who presumably licenced it, but by the 1890's, if not earlier, the concept was more widely used in the USA (the example shown is from a Seth Thomas of 1896) and by a number of the larger makers in England including The Lancashire Watch Company and William Ehrhardt.

Waltham later introduced the "Safety Barrel" to address the same problem but as the amount of power a watch required reduce with the introduction of lighter movements, improved manufacturing tolerances and more jewelling, there was no longer a real need and safety pinions were discontinued.