What Is a Safety Pinion?

The amount of potential energy in the coiled mainspring of a watch, particularly a large pocketwatch, is enormous. If the spring should break, the barrel containing the spring may turn with great velocity in the opposite direction of its usual motion. This would be almost certain to damage the train of the watch in any number of ways.

Many vintage American watches include the words "Safety Pinion" or "Safety Barrel" on the movement. The safety pinion refers to a pinion on the center shaft of the watch, which engages the mainspring barrel, and which is fitted to the center shaft with course threads. These threads are usually "left handed" meaning they tighten in the opposite direction of a usual screw. By being threaded in this way, the normal force of the wound watch holds the pinion tight and this drives the watch. But if the mainspring barrel moves the other way, the pinion is unscrewed. It will thus move rapidly down the shaft and disengage from the teeth of the mainspring barrel, saving the watch from damage.

The safety pinion is an American invention patented in 1857.