Causes of a Watch Stopping

The simplest and most common cause of a mechanical watch stopping is also the least interesting mechanically: it has simply run down, either because a hand-wound watch was not wound, or because an automatic watch was not worn or shaken enough to keep its winding rotor active. This is worth ruling out first, and doing so requires nothing beyond winding the watch (gently, and only by a crown that turns freely) and observing whether it resumes running normally.

Beyond simple running-down, a cluster of causes relate to the mainspring and barrel: a mainspring that has lost its temper and no longer delivers meaningful torque even fully wound, a spring that has become unhooked from its barrel or arbor attachment, or a barrel whose lubrication has dried and gummed to the point that the spring cannot deliver power efficiently through the train. These generally require an experienced eye and, often, specialist parts to correct, and are unlikely to be things a watch owner can distinguish from each other without opening the movement.

A second cluster relates to the escapement and balance directly: a hairspring that has become kinked, coil-bound, or is fouling against the balance cock or another part; a balance staff with a broken or badly bent pivot, which can allow the balance to sit askew and jam against the pallet fork or plate; or, especially in a watch that has been dropped or has visited a strong magnetic field, a hairspring whose coils have clung together magnetically, which can arrest the balance's swing entirely or nearly so. This last cause is common enough, and easy enough to test for without opening the case, that it has its own dedicated lesson.

A third cluster relates to contamination and lubrication generally: old, oxidised, or thickened oil anywhere along the going train or in the escapement can increase friction to the point that the watch stops, particularly in cold conditions, which is one of several reasons a watch that has not been serviced in many years is more likely to stop unpredictably even if it currently appears to run when tested briefly on the bench.

The important general lesson is that 'stopped' is a symptom with many possible mechanical causes ranging from the trivial (needs winding) to the serious (requires part replacement), and that visually distinguishing between them, let alone correcting the fault, requires opening the case and examining the movement under magnification — work that belongs with a qualified watchmaker rather than with trial-and-error at home, not least because an incorrect attempt to 'free up' a stopped movement can convert a simple, inexpensive fault into an expensive one.