Cannon pinion

The cannon pinion is a small steel tube with a pinion cut at its lower end, pressed onto the center arbor under the dial. It carries the minute hand and drives the motion works, the slow train of wheels that gears the minute rotation down 12 to 1 for the hour hand. Its defining feature is that it is not fixed: it grips the arbor by friction alone, tightly enough to be driven by the going train and loosely enough to be twisted independently.

The tube is about a millimeter and a half across and holds on through an interference fit with the arbor. The reduction happens in two steps that multiply to twelve: the cannon pinion drives a minute wheel of about three times its tooth count, and that wheel's pinion drives an hour wheel of four times again. The hour wheel rides over the outside of it on its own tube, which is why both hands share one center.

Why it matters

That friction fit is what makes setting a watch possible. When you pull the crown out and turn it, the keyless works drives the motion works, and the cannon pinion slips on its arbor while the balance and gear train carry on underneath, undisturbed. Without it you could not move the hands without disconnecting the drive from the mainspring.

Hand setting therefore feels different from winding. Winding meets a spring that builds and then stops; setting meets a controlled slip, which should stay even and quiet through a full turn. The same joint fixes where the hands end up, so a watch that changes its date at noon has usually been re-handed twelve hours out rather than developed a fault.

How tight is tight

The grip is a specification, not an accident. It must exceed the torque needed to turn the motion works and swing the minute hand, and stay below the level at which setting strains the wheels it drives. Makers hold it as a slipping torque in fractions of a millinewton-meter and essentially never publish the figure. Some calibers move the friction off the arbor onto a sprung driving wheel, set by a spring and replaced as a unit; from outside the behavior is identical.

In practice

A cannon pinion that has lost its grip is one of the classic wristwatch faults, and its signature is that the movement is innocent. The seconds hand keeps sweeping, the rate is fine, and the hands lose time anyway; in the worst version they stop dead while the watch is audibly still running. A timegrapher cannot see it, because it listens to the escapement and knows nothing about what happens above the dial, so timing your own watch reports good health while the hands lag. Too tight is the opposite and rarer fault: setting goes stiff, and the force at the crown goes into the teeth of the minute wheel, which is how they get bent.

Chronographs and calendar watches raise the stakes, because the drive for the stopwatch and for the date advance is often taken from this same corner of the motion works. A marginal cannon pinion can then slip only under load, losing time while the chronograph runs and keeping it when the chronograph is stopped. On a Valjoux 7750, rule that out first.

Correcting one is routine bench work: a new part, or a tool that presses a small indent into the tube to close its bore. Because the job lives under the dial, the movement is partly stripped, so it is normally folded into a full service.

The common confusions

The first is treating every hand problem as a cannon pinion problem. A loose hand, a minute hand fouling an hour hand and a hand rubbing the dial are separate faults, and misaligned hands usually come from hand fitting. The second is fear of wearing the joint out by using the crown: the slip is designed in, and pinions go loose with age or after a repair. The third is what owners reach for when a watch loses hours a day and still ticks: they call it running slow and ask for regulation, which recovers nothing lost to a slipping tube.

The parts around it

The center wheel is what the cannon pinion grips, the minute wheel takes the first reduction, and the hour wheel is the tube outside it that drives the date. None of them relates to hacking seconds or the quickset date, which belong to the keyless works rather than the motion works.