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.
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:
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).
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.