Seiko watch movement, Part 3

This is Part 3 of my attempt to follow Nick Hacko’s Seiko 7S26 DIY project. In this post I will complete the disassembly of the time display unit. Now that the disassembly is actually starting to reveal the innards of the movement, I’ll try to understand how it works.

Removing the day-date corrector setting wheel. There’s a surprising amount of plastic in the watch.

Removing the date jumper.

With the date jumper removed, all other wheels just lift off. The intermediate date driving wheel (white) has moved from its place; it meshes with the hour wheel.

Now that the we’ve finally exposed the heart of the time display unit, it is probably a good time to try to understand how the various parts are driven. To do that, I counted the teeth of each of the wheels this side of the movement. Here’s a diagram:

A diagram of the motion work of the 7S26 movement, showing the speed of each wheel (in revolutions per day) and the number of teeth in each wheel and pinion.

Let’s start with the cannon pinion. The cannon pinion carries the minute hand so we know it makes a full revolution every hour, or 24 revolutions per day. It rotates at the same speed as the centre wheel, to which it is connected via friction coupling. When the watch is set by turning the minute wheel, the friction coupling allows the hands to turn independently of the going train.

The hour wheel carries the hour hand and thus needs to make a full revolution every 12 hours, or 2 revolutions per day. The reduction from 24 to 2 is done via the minute wheel which itself rotates at 8 revolutions per day.

Because the 7S26 movement has day and date indicators, the wheels don’t stop here. The hour wheel drives an intermediate date driving wheel (rotating at 4 rpd), whose pinion in turn drives the date indicator driving wheel. This wheel rotates at 1 rpd, and each rotation moves the date dial by one step and the day dial by two steps (to accommodate the bilingual day dial).

All parts of the time display unit (except the hands) laid out on the table. The cannon pinion is still attached.

These plastic wheels make up the day and date indicator. The date indicator driving wheel (black) rotates at 1 rpd and turns the day dial disk. The fine-toothed wheel and pinion is the intermediate date driving wheel. The small coarse-toothed wheels are part of the date corrector.

Removing the dial holding spacer is a bit tricky, and I wasn’t quite sure how to do it. It appears that as long as you start from the right place and work your way around the movement, the spacer will detach.

Dial holding spacer separated from the movement.

Removing the cannon pinion.

Cannon pinion (bottom left) and hour (top left) and minute (right) wheels.

That completes the disassembly of this side of the movement. In Part 4 we get to flip the movement over and start disassembling the automatic winding unit.