Under the Loupe/The Barrel

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The Barrel

The barrel (more specifically, the mainspring contained within the barrel) is the source of energy in a mechanical timepiece. Energy is typically transferred to the barrel by either a manual winding system or an automatic winding system. The manual winding system relies on a conscious input of energy from the watch's bearer through the crown of the watch. An automatic winding system, on the other hand, gathers the energy necessary to wind the mainspring via the natural motions of the wearer's body.

Anatomy of the Barrel


1. Barrel Cover

Also sometimes referred to as the barrel cap, the barrel cover serves to contain the mainspring within the main body of the barrel. It also provides an active surface within which the barrel arbor can turn, and maintains the parralelism of the mainspring during winding and unwinding.

2. Barrel Arbor

The barrel arbor provides the axis of rotation of the entire barrel assembly between the mainplate and barrel bridge. It also serves to connect the winding motion of the manual or automatic winding system to mainspring via a hook that engages with a hole on the innermost point of the mainspring.

3. Mainspring

The mainspring of a watch is typically made of steel alloy and is held between the barrel cover and the base of the barrel drum. It is essentially a rectangular blade, shaped and heat treated to form a spiral or S shaped curve. As already stated, the mainspring attaches to a hook on the barrel arbor via a hole in its innermost end. When the arbor is engaged by the winding system, the mainspring is forced to tighten upon itself. The energy stored through this action is then slowly released, providing energy to the gears of the watch.

4. Barrel Drum

The barrel drum can be thought of as the first wheel in a watch. It consists of a tube-like wall that contains the mainspring within the space it defines. A series of gear teeth are cut around its base and these are responsible for transferring the power from the mainspring to the pinion of the second wheel (also often called the center wheel) of the watch. Like the hole in the barrel cover, a hole in the base of the barrel drum allows the barrel arbor to pass through the complete barrel and anchor the barrel assembly between the mainplate and barrel plate. In an automatic watch, a sophisticated lubricant is used to hold the mainspring bridle against the barrel wall until the power induced on it is too great and the bridle is allowed to slip slightly. In a manually wound timepiece, the mainspring is usually held to the barrel wall by a notch which locks the mainspring bridle in place.

Functioning of the Barrel

When engaged by the winding system of a watch, the barrel arbour turns within the complete barrel assembly, in turn winding the mainspring around the axis which it is turning. As the mainspring winds the kinetic energy of this turning action is stored as potential energy in the molecular structure of the mainspring alloy. This reserve of potential energy is then slowly converted back into kinetic energy, through the turning of the barrel drum, which in turn provides the power to the gear train which is necessary for the watch to run.

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