In the classical 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 not a 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 profile with eight arms — geometry you won't see in traditional lever escapement — and critical components are produced using MEMS technology, which enables tolerances and weights impossible with conventional machining. The system is paired with a free-sprung balance wheel, which removes from the lever the task of serving as a lateral reference for the balance wheel itself: fewer contacts, less wear, better isochronal stability. The declared 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 separated the locking and impulse functions onto two distinct wheels, had already demonstrated in the 1970s that impulse could be extracted from lever friction. Bernhard Lederer took up that logic in his Central Impulse Escapement — presented in caliber 9012 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 he had focused precisely on the technical limits that had stopped his predecessors, and was able to overcome them thanks to modern machining.
The comparison with Omega's Co-Axial is frequent in the specialized press, and it is 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 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 those coming from the Swiss lever. The star wheel and lever geometry do not follow the proportions you learn by heart. It is not difficult, but it requires resetting your reflexes.