Waltham Crescent Street Model 1870: Fogg’s Patent Pinion – Threaded Center Wheel Arbor

The safety pinion patented by Charles W. Fogg in 1865 was a critical advancement in American horology, preventing further damage to a watch movement in the event of a mainspring breakage. The ingenious design consists of a pinion that is threaded onto the center wheel arbor instead of being rigidly attached (see yesterday’s post for the patent information). When the mainspring is wound, the barrel applies continual force to the center pinion in a clockwise direction. Due to the backward-threading on the center arbor, this force tightens the center pinion securely onto the arbor, and the force is transferred through the train. However, when the mainspring breaks, the sudden recoil exerts force in the opposite direction, immediately loosening the center pinion and saving the train from damage.⠀