Poising and Static and Dynamic Balance

A balance wheel is poised when its centre of mass coincides exactly with its pivot axis, so that gravity exerts no net turning force on it regardless of which way it is oriented. An unpoised balance — one with a slightly heavy point somewhere around its rim — will tend to settle with that heavy point downward when the watch is held still in a given orientation, and more importantly for timekeeping, it will swing very slightly differently depending on orientation as gravity assists or resists its rotation at different points in its arc, producing the position-dependent rate variation that positional regulation, covered later in this track, exists to correct.

Static poise is checked by placing the balance, complete with its hairspring removed or with the spring's influence otherwise neutralised, onto a poising tool — two perfectly level, friction-free knife-edges that the balance's own pivots rest across — and observing whether it stays still in every rotational position or consistently settles with the same point downward; a heavy point is corrected by removing a small amount of material (traditionally by cutting or drilling a balance screw, or on a plain rim by careful filing) from that point until the balance rests equally in any position.

Dynamic poise is a further, subtler consideration: even a balance that is perfectly poised at rest (static poise) can still be dynamically unbalanced if its mass distribution interacts unevenly with the hairspring's own asymmetries or with air resistance while actually oscillating at speed, producing a rate difference between positions that a static check alone will not reveal. Detecting a dynamic poise problem generally requires timing-machine observation across positions with the assembled, running balance rather than a static poising-tool check alone, which is one reason positional regulation is treated as a distinct, later stage of adjustment even after static poising is complete.

Poising work interacts closely with several other topics in this track: a poorly centred hairspring (from the previous lesson) can itself introduce a position-dependent effect that mimics an out-of-poise balance, so a careful watchmaker distinguishes between a genuine mass-poise problem and a spring-centring problem before removing any material from the balance, since removing metal is not reversible in the way that re-centring a spring is.

As with staff and hairspring work, poising is precision work performed with specialised tools under magnification, and while the underlying physics is straightforward, competent execution — judging how much material to remove, and where, without overcorrecting — is built through supervised practice on training movements before it is applied to a real, valuable balance.