Barrel

The barrel is a shallow toothed drum, closed by a lid, that contains the coiled mainspring. The spring's inner end hooks to the central arbor and its outer end anchors to the barrel wall. Winding turns the arbor and coils the spring tighter; as the spring relaxes it turns the barrel, and the barrel's teeth drive the first wheel of the going train. Everything the watch does is powered from this one component, which is why how a mechanical watch works starts here rather than at the dial.

What is inside one

A barrel is an assembly rather than a single part. The drum carries teeth outside and a hooking point on the inside of its wall, a lid snaps into a groove to close it, and the arbor passes through the middle on two pivots. On top of the arbor sits the ratchet wheel, which is what the crown drives through the keyless works, held against unwinding by a click.

In a movement of 25 to 30 mm across the barrel is usually the largest component on the plate, roughly a third of the movement's diameter, and the spring inside it is a ribbon on the order of a tenth of a millimeter thick and several hundred millimeters long. The classical design rule divides the interior in three: about a third coiled spring, about a third arbor, and about a third left empty so the spring can expand. A full wind develops six to eight turns of the barrel against the arbor, given back at roughly one revolution every six to eight hours.

How to read the spec

Barrel design largely determines the quoted power reserve. A single barrel of conventional proportions gives roughly 38 to 42 hours, which is why so many movements land near two days: ETA publishes 38 hours for the 2824-2, Seiko Time Module more than 41 for the NH35, and the Miyota 9015 is quoted at about 42.

Manufacturers extend that in three ways. Rolex reached about 70 hours in the caliber 3235 without changing case size or beat rate, using thinner barrel walls and a more efficient escapement. ETA reached 80 hours in the Powermatic 80 partly by dropping the balance frequency to 3 Hz and partly through a patented thinner barrel arbor, drawn as wire rather than machined from stock, which frees volume for a longer spring. Fitting a second barrel is the third route.

Why it matters

A mainspring does not pull with constant force. Torque is highest just after winding and falls as the spring unwinds, and the balance responds by swinging less, which can shift the rate. On the bench a healthy movement commonly loses 20 to 50 degrees of amplitude over the first 24 hours from full wind. This is why accuracy is specified over that first day, why COSC rewinds a movement daily through its 15-day test rather than letting it run down, and why the last hours of a power reserve are the least trustworthy. The property being chased is isochronism: a rate that does not care how hard it is pushed.

Watchmaking has three classical answers. A fusee and chain varies the leverage as the spring runs down; stop-work prevents the spring from ever reaching its weakest coils; and most modern movements take the third route, simply making the escapement tolerant of the variation, which is cheaper and, in a wristwatch, good enough.

In practice

In an automatic watch the outer end of the spring is not fixed but pressed against the barrel wall by a slipping bridle, so winding past full simply makes it slip. The wall is often ridged to give the bridle grip, and carries a braking grease chosen so the bridle holds at full wind and lets go beyond it. That grease is the wear item: when it dries or migrates the bridle slips early, the watch winds but never reaches full amplitude, and the owner sees a reserve far short of specification, or a watch that stops overnight after a full day of wear.

Barrels are effectively sealed units in service terms. They are supplied as a barrel complete, meaning drum, lid, arbor and spring together, and at a full service most watchmakers fit a new one rather than reuse a spring that has taken a set. That part sits inside the quoted price of the job: indicative US figures for a three-hand automatic at an independent run about $250 to $500 before sales tax.

The common confusions

The barrel is not the mainspring: the barrel is the drum, the spring is the ribbon inside it. Shop copy uses the two words interchangeably, and a job sheet promising "a new barrel" almost always means a barrel complete, spring included.

Overwinding is also a manual-winding worry rather than an automatic one. On a hand-wound watch the outer end really is hooked to the wall, and forcing it can break the spring or the hook; an automatic simply slips, as can you overwind an automatic watch explains. The number of turns a full wind takes is a property of the barrel too, and it is larger than most owners expect: 55 crown turns for the NH35, per how many turns to wind a watch.