Barrel

A barrel is an energy-storage assembly: a toothed drum with a cover, a central arbor, and a coiled mainspring hooked between the two. A patent assigned to ETA SA Manufacture Horlogère Suisse, "Timepiece barrel" (US 9,367,037 B2, priority 18 December 2012), places it in "the field of timepiece mechanisms, and more specifically energy storage mechanisms, of the mainspring barrel, striking barrel or similar type", and describes the spring as "hooked between said drum at the outer end thereof and an arbor at the inner end thereof", with the arbor "pivotally integral with a ratchet about a pivot axis".

A Seiko Epson patent, "Spring, mainspring, hairspring, and driving mechanism and timepiece based thereon" (US 6,863,435 B2, published 8 March 2005), states the function in one line: a timepiece driving mechanism "comprises a mainspring serving as a power source, a barrel drum housing the mainspring, and a train wheel transmitting a mechanical energy of the mainspring by engaging with the barrel drum". The drum's own teeth are therefore where the going train begins.

Because a spring pushes hardest when fully wound and weakest as it runs down, the barrel is where a movement's torque curve starts. Grand Seiko's technical page describes the intended behaviour as the mainspring unwinding "gradually and evenly over a long period of time", that motion being "transmitted through the gear train". How long it lasts is the watch's power reserve.