Double Impulse Escapement

In the classic Swiss lever escapement, the impulse to the balance wheel always travels through the fork — a passage that introduces friction and dissipates energy in both directions of oscillation. The dual impulse escapement breaks this symmetry: in one direction the impulse travels directly from the escape wheel to the balance wheel, in the other it passes through the lever. Reducing friction to just one side of the cycle is no minor detail: at 36,000 vph, every micro-loss accumulates.

The most studied case today is the Dual Impulse Escapement of the Grand Seiko 9SA5 caliber, introduced with the fifth-generation Hi-Beat family. The escape wheel has a star-shaped profile with eight arms — geometry you don't see in traditional lever escapements — and critical components are produced with MEMS technology, which allows tolerances and weights impossible with conventional machining. The system is paired with a free-spring balance wheel, which removes from the lever the task of serving as a lateral reference for the balance wheel itself: fewer contact points, less wear, better isochronal stability. The stated result is −3/+5 seconds per day, with 80 hours of power reserve across two barrels.

The principle is not new. George Daniels' Independent Double Wheel Escapement, which cleanly separates the locking and impulse functions across two distinct wheels, already demonstrated in the Seventies that impulse could be removed from lever friction. Bernhard Lederer took up that logic in his Central Impulse Escapement — presented in the 9012 caliber and awarded at the GPHG 2021 — where two independent trains, each with its own barrel and remontoir d'égalité, feed two escape wheels: a dissociation pushed to its extreme consequences. Lederer stated that he had focused precisely on the technical limits that had stopped his predecessors, and that he was able to overcome them thanks to modern machining.

Comparison with Omega's Co-Axial is frequent in specialized press, and it is well-founded: the Co-Axial also reduces friction by dissociating impulses on different planes. The difference lies in geometry and frequency context. The Co-Axial typically operates at 25,200 vph; the 9SA5 at 36,000. At high frequency, the efficiency of energy transfer weighs more heavily on the barrel's energy budget, and this explains why Grand Seiko invested in a proprietary solution rather than adapting an existing geometry.

From the bench: disassembling a 9SA5 for the first time is disorienting for someone coming from the Swiss lever. The star wheel and the lever geometry don't follow the proportions you learn by heart. It's not difficult, but it requires resetting your reflexes.