TO LET DOWN

In horology, to let down has two distinct meanings. In the mechanical sense, to let down a spring means to disarm it, that is, to allow it to gradually return to its resting state by releasing the energy it has stored; thus the mainspring is let down before any intervention on the barrel, as a safety measure, to avoid a sudden release likely to injure the horologist (watchmaker) or damage the movement. In the metallurgical sense, to let down a metal consists of removing the internal tensions that have accumulated in it as a result of mechanical or thermal stresses occurring during rolling, stamping, bending or quenching. If not eliminated, these residual stresses can cause, over time, deformations or delayed ruptures. They are eliminated by an appropriate stress-relieving heat treatment: a stress-relief annealing, carried out at moderate temperature after cold working, or tempering after quenching, which stabilizes the structure without completely canceling out the desired hardening. To let down a part gives it better dimensional stability over time, an essential quality for low-tolerance components, such as the pinions, arbors and pivots of a Movement.