Technical Question

[MLC166]

I'm having the movement of a Seiko 7A38 serviced and I have been told the movement needs a new centre wheel and canon pinion. Can someone please explain what these parts do? Are they critical to the running of the watch?

Thanks in advance.

[Ikemi]

Stolen from Wiki:

Center or second wheel - Turns once per hour. Its pinion is turned by the teeth on the mainspring barrel in watches and spring driven clocks, and by the weight pulley in weight driven clocks. Its arbor projects through a hole in the face and drives, via a friction coupling, the cannon pinion, which carries the minute hand. It also drives the pinion of the third wheel. In wrist watches with centre seconds, that is with the seconds hand pivoted coaxially with the minute and hour hands, this wheel is positioned off centre to allow the fourth wheel to be placed at the centre of the movement. In this arrangement the wheel is called the second wheel, because it is still the second wheel in the train but no longer at the centre of the movement.

Cannon pinion - A pinion with a hollow shaft that fits friction tight over the center wheel shaft, projects through the face, and holds the minute hand. While the timepiece is not being set, this is turned by the center wheel, and drives the minute wheel. While being set it is turned by the setting mechanism, in modern clocks a setting knob on the back of the clock. In watches during setting this is turned by the minute wheel, which is turned by the keyless work. In older clocks the setting was done by opening the face and manually pushing the minute hand, which rotated the cannon pinion directly.

More here: https://en.wikipedia.org/wiki/Wheel_train

In answer to your question, fairly important!

[glazbagun]

Yes. Basically the cannon pinion is what drives your hands (and holds the Minute hand on). It is forced friction-tight onto the protruding tip of the Centre Wheel which is the part of the train mechanism that turns once per hour. It must be tight enough that the hands won't move if the watch is knocked, but not so tight as to put strain on the hand setting wheels or the timekeeping train when you're changing the time.

The friction between the two parts is the only physical connection between the time the mechanism is keeping and the time hands are showing (excluding the seconds hand, of course, which does it's own thing).

Edited by glazbagun on Thursday 29th October 11:55

[Variomatic]

On the 7a series 9as with a lot of seikos) the centre wheel and cannon pinion are combined in what the Swiss call a "cannon pinion and driver wheel". Rather than the traditional system described by wikipedia, the clutch effect is created by having the wheel clipped on to the pinion.

It's generally a reliable system but is harder to tighten if the friction gets low and is easier to damage by mishandling because the centre of the wheel is cut away to create a spring effect and it doesn't leave a lot of metal to withstand handling.

If it's loose then the first choice would usually be replacement (except that it's obsolete through official channels) because tightening runs the risk of damage and may not be successful anyway , if it's damaged then replacement is (usually) the only option.