The mainspring barrel is a set of horological components that ensures the proper functioning of a mechanical watch movement. This component is the essential energy source for the operation of watch calibers.

The Mainspring Barrel: Operation and Composition

The mainspring barrel consists of several distinct components and forms a complex assembly.

A central arbor is located inside a mainspring, which in turn is housed within a hollow cylinder topped with a toothed wheel. The outer coil of the spring attaches it to the barrel, while its inner coil attaches it to the central arbor.

The mainspring and its tension are responsible for its action on the other components. It unwinds very slowly while transmitting energy to the entire gear train via the ratchet wheel (the barrel’s gear). The gear train then transmits the energy it has received to the escapement and balance wheel. Note: in one hour, the mainspring only completes one-ninth or one-sixth of a rotation!

Energy is regularly received by the escapement and balance wheel assembly thanks to the mainspring, which in turn depends on the escapement and balance wheel to prevent its energy from depleting too quickly or at an irregular rate.

How is the Mainspring Barrel Wound?

The winding of the mainspring is essential for its proper functioning. It must be well-wound at all times, as its winding provides the energy that is then transmitted to other components, enabling their movement.

The mainspring barrel can be wound via a manual or automatic mechanical movement.

Manual winding is achieved by means of a winding stem, which must then be turned to tighten the spring and thus store energy.

Automatic winding is achieved by means of a rotor or oscillating weight, which takes the form of a disc that rotates with wrist movements to wind the mainspring. It should be noted that in an automatic movement, the outer coil of the spring is not fixed. It is held by friction to prevent any risk of breakage, as the spring is continuously wound by the wearer’s movements.

Hanging or Plain Mainspring Barrel?

There are hanging or plain mainspring barrels, depending on the watch models.

The mainplate – the plate on which all components are located – is connected to the upper part of the hanging barrel.

The plain mainspring barrel, on the other hand, does not have a toothed wheel. A chain attaches it to the fusee in fusee watches.