Parts of a Movement

A movement is built around two main plates — the main plate and the top plate, the latter usually divided into separate bridges (or, in pocket watches, one larger balance cock and other bridges) — between which the wheels and pinions are pivoted. This sandwich construction lets a watchmaker remove one bridge at a time to access an individual wheel without disturbing the rest of the train, which is central to how service and repair proceed in practice.

The going train sits between the plates: the barrel (containing the mainspring), the centre wheel, the third wheel, the fourth wheel and the escape wheel, each meshing with the next. Above the top plate (dial-side components aside) sits the escapement and oscillator: the pallet fork, engaging the escape wheel and the balance's impulse jewel, and the balance wheel itself, pivoted under its own bridge — traditionally called the balance cock in a watch or clock, and often the single most decorated part of an antique movement.

The keyless works is the mechanism that lets the crown both wind the mainspring and set the hands, and includes the winding stem, the clutch or sliding pinion, the winding pinion, the crown wheel and ratchet wheel (which allow the barrel to be wound only in one direction), and the setting lever that shifts between winding and hand-setting modes when the crown is pulled out. In earlier key-wound watches this entire assembly is absent, and the mainspring is wound directly through a squared arbor exposed at the back or front of the movement.

On the dial side, beneath the dial itself, sits the motion work already introduced in the previous lesson — the cannon pinion, minute wheel and hour wheel — along with the keyless work's setting components. The dial train is deliberately kept slightly loose-fitting at the cannon pinion so the hands can be independently set without disturbing the going train's timekeeping.

Jewels — synthetic (historically natural) rubies or sapphires — appear as bearings at the points of highest friction and wear: the balance staff, the pallet fork, and often the escape wheel and train wheels in higher-grade movements. A movement's jewel count is a rough (not absolute) indicator of grade, and later lessons on reading a movement and identifying escapements return to how jewelling is used, and misused, in dating and grading a watch.