For a watch to run without a battery, it needs a source of energy and a way to transmit it smoothly all the way to the hands. This journey of mechanical force is made possible by a precisely calculated system of gears, called the going train.
The mainspring barrel: the power station
It all begins in the mainspring barrel, a drum housing a long, blued-steel spring coiled upon itself. Once wound (by the crown or the oscillating weight), this spring seeks to unwind: it exerts a torque on the order of a few newton-millimetres to around ten N·mm at full tension — roughly 6 to 9 N·mm (600 to 900 g·mm) on a common automatic calibre such as the ETA 2824-2. This torque, picked up by the barrel's central pinion, begins its journey toward the escapement. A standard barrel provides 40 to 50 hours of energy; certain "long power" versions reach 8 to 10 days thanks to a longer spring or several barrels in series.
The cascade: four wheels, four stages
The barrel, rotating very slowly (about one turn every 6 to 7 hours, or 6 to 8 turns over the entire power reserve), meshes with the centre wheel (1 revolution/hour — it carries the cannon pinion and the minute hand), which meshes with the third wheel (~8 revolutions/hour), which meshes with the fourth wheel (1 revolution/minute — hence the seconds hand), which finally meshes with the escape wheel. The latter spins at roughly 10 to 20 revolutions/minute depending on the frequency: the relation is A/h = 2 × number of teeth × revolutions per hour, so a 15-tooth wheel gives 10 revolutions/minute at 18,000 A/h, 12 at 21,600 A/h and 16 at 28,800 A/h — one turn every 3 to 6 seconds. Each gear stage multiplies speed and divides torque: you start with strong, slow torque at the barrel and end with weak, fast torque at the escapement, ready to be doled out in fractions.
The geometry: the involute of a circle
The profile of each tooth is not arbitrary. Since Leonhard Euler in the eighteenth century, watchmaking has used the involute of a circle profile: the curve traced by a point on a straight line as it rolls without slipping along a circle. This profile guarantees a constant transmission ratio even if the centre distance varies slightly, a normal contact pressure always oriented along the line of action, and an optimised mix of rolling and sliding. Involute teeth mesh smoothly, without jolts — an absolute prerequisite for isochronism.
The going train is the backbone of the movement: it is the first thing we dismantle in the practical modules of the online watchmaking course, and the reason a clean regulation is impossible on a worn train.
Energy efficiency: every percentage point counts
A well-made, well-lubricated going train achieves an efficiency of 85 to 92%. Losses are concentrated at the pivots (lubricated ruby/steel friction) and at the gear meshes. This is why pivots run in ruby jewels (7 pivots for a simple movement, 21 to 27 for a decorated one), and why tooth geometry remains the subject of renewed patents. Poor efficiency translates directly into a drop in amplitude.
Frequently asked questions
Why do we sometimes see 21, 25, or 27 jewels?
Each major axis pivot (typically 7 to 8 axes) runs in two jewels (one on each side), plus the pallets (2) and the roller jewel (1): that quickly adds up to 17-21. Beyond that, additional jewels equip the automatic winding mechanism or the calendar.
Does a watch gear wear out?
Very slowly, provided the oil does its job. What you mostly see are bent teeth from a violent shock, rather than meshing wear.
Can power reserve be increased without changing the barrel?
Only by lowering the frequency (thus changing the regulating organ) or by improving efficiency (finishing the pivots). Otherwise, a longer spring is required, meaning a larger barrel.
Ready to decode the secrets of the mechanical movement? Décalibre explains the transmission of energy in a simple, visual and intuitive way.