In this second installment of the ‘How Watches Work’ series, David joins Allen and leads him through the gears that make your mechanical watches work, generally known as “the going train.” By the end they’ve imagined complications that track time on Mercury, Jupiter and Mars
The going train is the main set, or train, of gears distributing the mainspring power to the watches mechanism. As the mainspring unwinds, it turns the cylinder it resides in which is know as the barrel. The teeth on the outside of the barrel then mesh with the next gear in the train known as the second or center wheel. In order to ensure the watch runs for many hours on a single wind, the barrel must turn slowly while being controlled by the fast moving escapement. Therefore, the job of the going train is to speed up the rotation speed from the slow moving barrel to the fast moving escape wheel while hitting the specific rotational velocities we need for useful time telling. That means having one wheel in the train (the second or center wheel) rotate precisely once per hour, and another (the fourth wheel) rotate precisely once per minute.
A simple, conventional going train will run in a straight line from the barrel towards the top of the watch to the escapement towards the bottom with the second wheel corresponding to the center of the watch and the fourth wheel landing where we traditionally find the small seconds subdial. In order to have central second hand on a watch, the going train needs to be modified so that it can drive the fourth wheel at a central location, or have additional indirect wheels added to rotate the seconds hand instead of driving it from the fourth wheel.
A third possibility to facilitate a central seconds hand is to move the second wheel away from the center but this involves passing the drive from the movement side to the dial side in a non-central location. Drive is traditionally transferred from the going train rotating on the movement side, through the main plate via the second wheel arbor, and since it is rotating once per hour, the minute hand is driven from this arbor. However, in order to set the time , that hand which is normally driven by the going train, needs to be able to rotate independently of it. This is achieved through the use of the cannon pinion.
The cannon pinion is a small steel tube which the minute hand is pressed on to. The cannon pinion is mounted on the arbor of the center wheel friction-tight so as to allow hand-setting. It has teeth at the bottom to drive the minute wheel on the dial side of the movement that in turn drives an intermediate wheel that then drives the hour wheel on which the hour hand is mounted. Taken together, this arrangement is known as the motion works and provides the 12-to-1 reduction ratio required to drive the hour wheel from the minute wheel. The time is set by rotating the minute wheel directly from the watch crown via the stem. The turning force applied via the crown through the minute wheel is enough to break the friction holding the cannon pinion to the center wheel arbor and allows the minute and hour wheels to rotate independently of the center wheel and going train.
With rotational drive transferred from the going train through the main plate to the dial side, it can power more than just the three hands. Extending the reduction concept of the motion works further, we can introduce wheels that turn even slower, for example once per day. If that day wheel then uses a single finger rather than a complete circumference of teeth to engage in the teeth on a date disk running around the outside of the dial, then we have a way to advance the date once per 24 hours.
Another complication closely tied to the going train is the chronograph which relies on a wheel with the same ratio as the fourth wheel but needs to be driven only when the buttons on the side of the watch. To achieve this, the chronograph needs to have gears (transmission ) that enter and exit the going train by swinging in from the side, or a second fourth wheel can be stacked on top of the first and engaged and disengaged using a vertical, spring loaded clutch.
For the record… the Mercurial year lasts 87.97 Earth days while the Mercurial day (one full day-night cycle) lasts 176 Earth days and so a day on Mercury takes two years. David was close with his guess that the average temperature on Mars was -40C or -50C… it is in fact -60C (-76F).
And if you are wondering what those facts have to to do with the going train of a watch, you will have to listen to the podcast!
