Automatic Movement

Specification

Technical parameters

Overview

An automatic movement is a hand-wound movement with a winding module bolted on. An oscillating weight — the rotor — swings on a pivot at the centre of the movement as the wrist moves. Reduction gearing and a pair of click wheels convert that motion into rotation of the barrel arbor, winding the mainspring a little at a time.

Two details define how well it works. The first is winding direction: a unidirectional rotor only winds on half its travel, while a bidirectional rotor uses a reversing arrangement so both swings add tension. The second is the slipping bridle, a short extra spring clipped to the barrel wall that lets the mainspring slip once it is fully wound, so continued winding cannot break it.

Because the mainspring is held near full tension in normal wear, an automatic runs in a narrower torque band than a hand-wound movement — which is one reason it keeps more consistent time. It also means an automatic that sits in a drawer stops, and one that is only worn for short periods may never reach full wind.

What matters about it

• Oscillating rotor winds from normal wrist motion
• Slipping bridle prevents overwinding damage
• Runs closer to constant torque than a hand-wound movement
• Higher jewel count than an equivalent manual movement
• Adding complications increases height before it increases diameter

Practical notes

• Let the power down slowly through the crown or by releasing the click, never by pulling the barrel apart under tension.
• A rotor that spins freely and silently but does not wind usually has a failed reversing wheel, not a failed rotor.
• Check rotor clearance before refitting the case back — a rubbed rotor is a common cause of a sudden loss of reserve.
• When testing amplitude, measure in at least three positions; an automatic that reads well only dial-up has a problem elsewhere.

Compatible with