Barrel (horology)

Used in mechanical watches and clocks, a barrel is a cylindrical metal box closed by a cover, with a ring of gear teeth around it, containing a spiral spring called the mainspring, which provides power to run the timepiece.

[1] The barrel turns on an arbor (axle).

The barrel teeth engage the first pinion of the wheel train of the watch, usually the center wheel.

[2] Barrels rotate slowly: for a watch mainspring barrel, the rate is usually one rotation every 8 hours.

This construction allows the mainspring to be wound (by turning the arbour) without interrupting the tension of the spring driving the timepiece.