Following the recent ‘quiz’ that I offered which involved the 59-8D SCS movement, I will try to explain how the jump second mechanism works.
But first a bit of history: Omega introduced the 59-8D movement in 1926 with 6 or 7 jewels. In 1928 it was revised with 15 jewels (which was used in a ‘chronomètre de bord’/marine chronometer). The movement was widely used, from ‘goliath’ pocket watches to buses, bedside clocks and even submarines).

In 1941 they introduced the 20-jewel ‘jump-second’ derivative, known as the 59-8D SCS. SCS is an abbreviation of ‘seconde au centre sautante’ which is French for jumping-centre-second. They kept the original base/framework of the movement (with a few changes) and effectively added a layer above it to carry the jump-second complication. If you’re interested, the patent (registered in 1943) is no. 229234. This movement was also of chronometer grade and at one stage was used as the time reference shown on TV (as illustrated in AJTT).

The mechanism used in this movement uses the regular third wheel (actually the upper of two wheels) to drive the dedicated jump-second wheel – and the lower of the wheels powers the fourth and then regular escape wheel (which is fitted with a special cam-shaped jewel) to give the intermittent (‘to-the-second’) timing to that jump-second wheel. As shown in the images, the cam-shaped jewel is surrounded by a fork, which oscillates from side-to-side as the escape wheel turns and which in turn moves a separate escape mechanism with two jewels that allow that jump‑second wheel to click every second. The ‘gearing’ provided by the cam-shaped jewel means that the jump second moves (of course) every second in spite of the escape wheel moving 5 times per second (as an 18,000 bph movement).

A revised version (transformation T1) was issued in 1942. Omega subsequently changed the calibre numbers: that 15-jewel version from 1928 was still being made and became the Cal.110. The first 59-8D SCS was no longer in production (and was therefore not included) but the 59-8D SCS T1 became the Cal.120.

The changes in the T1/Cal.120 version included a different hacking system (instead of rotating a specially-placed knob which interrupted the jump-second wheel, a bar was introduced which could be pressed down to stop the jump-second fork oscillating – see image). They also introduced a change to the third wheel which acquired a coil-spring (again see image) to replace an earlier design of spring. The spring is to allow the third wheel to continue to rotate (which is has to do, to drive the fourth and escape wheels) to avoid it being blocked by the jump second which it is also driving).

Looking at the image of the movement, the escape wheel is in the bottom left corner, controlling the balance that is in view. The cam-shaped jewel is surrounded by the fork (as in the other image) and at the other end of the fork are the two jewels that are the ‘escapement’ for the jump-second wheel.

That’s the story. I find it sad that this is not documented (as far as I can find). If anybody can offer more detail or any corrections, I’d be pleased to hear.
Tom

 

Not military huh? Well, I guess not, since it appears to be navy. Anyway. An interesting exercise, and an interesting timepiece.

 

As far as I know, the 59-8D SCS (later Cal.120) did not have any military function.

The original 59-8D (later Cal.110) (which as described was eventually used by the Navy) was not specifically a military movement. But anyway this was a different movement and did not have the jump second (which was the purpose of the thread).
Tom

 

TOM,

congrats on this very interesting write up! Much appreciated.

Here is one of my recent acquisitions with the 8D SCS



To more easily remember - merken

 

Thank's for sharing Tom, what an interesting and beautiful piece.

 

Thank you Erich.

And a lovely Ref 5506 SCS – congratulations. We need a video showing the second jumping!

Tom

 

It will need a good service first

 

Still jumpin'
Tom

 

To add to the earlier explanation, to illustrate the basic differences and similarities between the Cal.110 and Cal.120, I offer the following two images.
The first image shows the two movements as you can see them. A reminder that the Cal.110 started life as a 59-8D 6 or 7 jewel movement in 1926, in 1928 becoming the 15-jewel version (shown here). The 59-8D SCS from 1941 used a very similar basic movement but acquired the jump second complication: the various parts of the upper ‘layer’ can be seen. There were other changes, partly to accommodate the extra parts but particularly to allow for the jump second (such as a new double-tier third wheel, and an escape wheel with jewel as already discussed).

The second image shows the inside of the movements, showing the similarities of the dual-barrel arrangement. Both calibres were produced with key wind but for the Cal.110 there was also a stem wind version.

On a personal note, I like’em big! At 59mm diameter, this was one of Omega’s largest calibres, in fact the fourth largest. Largest was the 1894 Cal.30’’’ (67.6) followed by the 1896 Cal.27’’’ (61.2) – both 8-day movements; then the 1926 60.8 chronometer movement (lucky if you have one of these!) followed by the 59-8D. Even the 1932 53.7 (used for years in the Olympic watches) was described by somebody working on it as being a delight because he could climb inside it.
Tom