c4.1.3 A Day & Date Calendar – Seiko 7S26

In Level 4, Watch Repair Courseby Mark LovickLeave a Comment

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Lesson Transcript

Following on from the previous example which was an extremely simple calendar mechanism, let’s take a look at something a little more complicated.

This is a very common movement found in many Seiko watches, the 7S26. It’s a very well designed workhorse of a movement.

As we take a look at the design of this calendar I would like you to think back on the previous example and visualise the differences between the two.

The first thing you may have noticed here is that the calendar has a day display, and what may not be immediately apparent is that this movement also has a rapid calendar change mechanism built in.

Let’s open this up and take a closer look.

The first thing we notice is that this particular movement has a day disk with the hour wheel as it’s axis. This is a dual language day ring and so the calendar drive wheel will cause it to jump twice during it’s transition. In this case it can either show the days of the week or the day number of the week in Roman numerals.

The day ring is retained by a circlip, I am going to use hand levers to gently remove this.

And I can use a little rodico to lift the day ring away and this exposes the calendar works for inspection.

I can see that the calendar works including the day ring are secured with a cover plate. At first sight it looks like it may be secured with three screws, but upon closer inspection I can observe that there is another much smaller screw here, this seems to be of a cross head design.

Now this movement has a rapid date change mechanism, so let’s take a quick look at that.

Here is the train which governs the rapid change, on this model they seem to be made of nylon or plastic, this would not normally be a problem as these parts are not usually under any stress.

This mechanism is designed so that when the crown is pulled out to the first position, it will either advance the day or date depending on the direction the crown is turned.

We can see here that there is a spring integral to the cover plate which holds tension down on one of the intermediate wheels. And the day disk jumper spring is also integral.

And we can see here the day disk driving wheel.

As you may remember from the previous lesson, the calendar mechanism was driven by the hour wheel, and this movement is no different. There is an intermediate wheel driven by the hour wheel and this in turn drives the calendar wheel.

The calendar wheel design is different though – it has two arms rather than a single peg. One arm advances the day disk and the other advances the calendar ring. Notice the arm which advances the day disk has two notches so that it can cause the disk to jump twice.

You should also notice that these arms are quite long and are moveable. This is to allow them to ratchet over the teeth of the calendar ring or day disk in order to not cause damage should the hands be set in reverse.

So let’s remove this cover plate and look further. We will start by removing the retaining screws.

The day disk driving wheel sits loose on it’s post so let’s remove that. And we can lift away the cover plate.

Let’s take a closer look at the rapid calendar change…

When the crown is pulled out to the first position, this winding pinion is engaged with the sliding clutch. A wheel which is permanently attached to the cover plate is driven by this pinion.

This in turn drives an intermediate wheel which, depending on the direction it travels will either turn the day disk driving wheel or directly advance the calendar ring. The intermediate wheel will travel along a recess in the movement main plate depending on the direction the crown is turned. It’s a smart design.

Now you can see that there is another lower cover plate which has the calendar ring jumper spring integral. We can now remove this to expose the calendar train. But first we need to advance the calendar train so that the calendar driving wheel is not fouling this lower cover plate.

And now the lower cover plate can be removed exposing the calendar train. And as we can see here, just as in the previous video lesson we find that the hour wheel drives this intermediate wheel, which in turn drives the calendar driving wheel.

So as you can see, when compared to our previous example which was a very basic calendar mechanism, when broken down this seemingly much more complicated system is really not that much different at all. The principle of operation is very much alike.

The design is improved, there is some protection now should the hands be set anti clockwise. This allows the driving wheel to ratchet over the calendar ring if turned in reverse.

The addition of the rapid date change is a great convenience to the user of the watch. The design is simple and well designed as is the whole movement.

And we will return to this movement later on in this level as we discuss the automatic works.

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