The gear train is the energetic backbone of any mechanical movement: it transfers the torque stored in the mainspring through to the escapement, and along the way it turns the hands. Without a well-built train, balance wheel and regulators receive irregular energy, and accuracy collapses.
The classic structure consists of a sequence of toothed wheels of decreasing diameter, each meshed with the pinion of the following wheel. The centre wheel makes one revolution per hour and drives the hour and minute hands; the seconds wheel makes one revolution per minute. The 12:1 reduction ratio of the motion work – technically distinct from the main train – brings the hour hand to complete its circuit in twelve hours. Each gear is held in position by a bridge, called the train bridge: a structural component not to be confused with the main plate. In German movements with a three-quarter plate, the single bridge replaces separate barrel bridge and train bridge, stiffens the assembly and offers better protection from dust.
Constructional variants are numerous. In the flying gear train – the Corum LAB 02 is the most-cited example – the gears remain suspended with no upper bridge: ball bearings are needed in place of jewels, along with longer arbors to ensure stability. In F.P. Journe's invisible going train, the train is moved to the dial side of the case, so as to create visual symmetry between the barrels without any visible mechanical link running across them. In the Genus GNS2 the very concept of the train is almost overturned: two physical trains travel across the dial following a figure-of-eight path, displaying hours and minutes as moving elements on view.
The quality of the train is read from the finishing of the bridges – sharp mirror-polished angles, sand-blasted flanks, châtons with set jewels – and from the precision of the wheel-pinion meshing. A poorly finished train produces variable friction; a train with incorrect end-shake produces torque loss. I have opened enough movements on the bench to say that a calibre can almost be understood from the geometric cleanliness of its train, even before looking at the escapement.
In constant-force mechanisms, such as those by Bernhard Lederer, the going train incorporates a device that levels the torque before it reaches the escapement: the energy always arrives the same, regardless of how much the mainspring is wound. It is a rare and costly solution, but it is the only one that solves the problem of decreasing torque at its root.
