· By Buffy
Anatomy 7: The Motion Works and Dial
How the hands are geared to disagree by exactly 12:1, why the cannon pinion slips, and what dial feet have to do with all of it.
Part 7 of 8 in the Anatomy series.
The gear train turns at the right speeds; the motion works is the small, exact gear cluster that turns those speeds into hand positions — and reconciles the one ratio that matters to humans: 12 hours per revolution of the minute hand.
The three wheels
- Cannon pinion — on the center wheel’s arbor; turns once per hour. The minute hand presses onto it.
- Minute wheel — an idler between cannon and hour wheel, which is why the hands move opposite to the train’s rotation direction.
- Hour wheel — turns once per 12 hours; the hour hand presses onto it.
The cannon-to-hour ratio is 12:1 exactly. The idler’s tooth count doesn’t change the ratio, only the direction and spacing — designers pick it to fit the dial real estate.
The cannon pinion is a clutch
We met it in part 2: the cannon pinion is pressed onto the center arbor with controlled friction. Winding never touches it; setting the hands forces it to slip around the arbor. Its friction must be:
- High enough that the motion works never slips during normal running (or the watch would “lose time” while the seconds kept ticking),
- Low enough that setting feels smooth and doesn’t strain the keyless works or the escapement.
Watchmakers tune it by feel with a staking tool. When it wears loose, a watch keeps perfect time and yet the hands drift — one of the great diagnostic puzzles for beginners.
Hands, dial, and motion
Hands are calibrated press-fits: hour, minute, and (seconds on the fourth wheel or a center-seconds runner). The dial is held by dial feet — two or three posts soldered to the dial’s back, locked under the movement by dial-foot screws or a locking plate. Dial feet positions define dial variants: the same calibre can be dressed in different dial layouts (sub-seconds at 6, 9, or center) by moving feet and hands.
Lume, printing, and finishing live here too — but the mechanical point is simple: between the fourth wheel and the tip of a hand there is nothing but exact ratios and controlled friction. Next, the finale: Putting It Together.