Mainspring barrel

The mainspring barrel (German Federhaus) is the cylindrical, hat-shaped housing that contains the coiled mainspring of a mechanical movement. It is the watch's source of energy: when the watch is wound — by hand via the crown, or in an automatic by the rotor — the mainspring coils tighter around the barrel arbor. As it unwinds, the released energy drives the gear train through the escapement and ultimately moves the hands.

Construction

The barrel consists of three main parts: the barrel base with the arbor at its centre, the barrel cover and the toothed barrel wall. The mainspring itself — a long, thin strip of modern spring steel — sits between base and cover, anchored to the arbor at one end and to the barrel wall at the other. The teeth on the barrel's exterior mesh with the first wheel of the gear train and pass on the energy.

When winding, the arbor rotates; the outer end of the spring stays fixed to the barrel wall and the spring coils tighter around the arbor. As the spring unwinds, the entire barrel rotates and sets the gear train in motion.

Torque behaviour

A fully wound mainspring delivers the highest torque and gradually loses force. Two strategies address this:

• Stopwork. A mechanism that keeps the spring from reaching its weakest tension band or its highest one. Common in vintage movements; replaced by the slipping bridle in modern automatics.
• Slipping bridle. Modern in every automatic. The outer end of the spring slips along the barrel wall when the rotor winds beyond the spring's capacity. Prevents overwinding and allows continuous topping-up.

The useful torque range is typically the middle 60 to 80 percent of the wound state; the extremes produce uneven rate. On a Rolex movement the escapement therefore operates within a stable band between full wind and roughly 24 hours before stoppage.

Multiple barrels

Some constructors run two or three barrels in parallel. Examples:

• Patek Philippe calibre 240 PS. Micro-rotor with extended reserve.
• A. Lange & Söhne L901. Twin barrel in the Lange 1, around 72 hours reserve.
• Panerai P.5000 / P.2002. Three barrels, 8-day reserve.

More barrels mean longer power reserve or more consistent torque across the running time — at the cost of movement thickness and complexity.

Service and wear

At our atelier in Munich, barrel problems are usually the consequence of an overdue service. An aged or set mainspring shows as substantially reduced power reserve — a 48-hour movement running only about 30. During a complete service the barrel is opened, the spring inspected and generally replaced, and the arbor, barrel wall and cover are cleaned and re-lubricated. On an automatic the condition of the slipping bridle is also checked — a gummed bridle results in incomplete winding.