Budget Watch Collecting/Automatic Winding
The basic prinicple of most automatic winding systems is simple. The rotor is a relatively large unbalanced weight mounted in a way that it can turn around a pivot. As the watch moves, the rotor will shift around the pivot. The rotor is indirectly connected to the mainspring arbor in a way that causes the mainspring to be wound slightly when the rotor moves. In some watches, only movement in one direction will wind, with bidirectional winding every motion of the rotor helps wind.
Auto Winding Systems
Seiko most often uses the Magic Lever system, an extremely simple yet effective system, requiring only 3 or 4 more parts than a manual wind watch. A gear with ratchet teeth is placed so it's pinion meshes with the mainspring winding gear. The magic lever is a V shaped metal lever with flexible legs. The open end of the V is placed around the ratchet gear--one end has a hook, the other an angled point. When the magic lever moves away from the ratchet wheel, the hook pulls the wheel in the winding direction. When the magic lever moves towards the ratchet wheel, the point pushes the wheel in the winding direction. The base of the V is attached to an eccentric post, either part of the rotor or a secondary wheel driven by the rotor.
Timex used a more complex version of this system in their automatics. In the Timex system the equivalent to the Magic Lever was made of several pieces with pivots and springs, and the ratchet wheel had planetary reduction gears to increase leverage enough to wind the spring. In both of these systems, the ratchet wheel can turn without turning the rotor, allowing handwinding with no decoupling wheel needed, although many Seiko automatics do not provide handwinding capability.
Many ETA movements use reversing wheels. In this system, there are two sets of gears, each with a top and bottom gear connected by a one-way clutch. One top gear will be driven by the rotor and will in turn drive the other top gear. Both bottom gears are connected (directly or indirectly) to the mainspring arbor. When the first gear is turning in the direction that it can wind the watch, it's associated bottom gear will also turn with it, winding it. The first gear will also turn the second gear in the opposite direction. The second gear's one-way clutch will allow the bottom gear to disconnect. When the first gear moves in the wrong direction for it to wind, it's bottom gear will be allowed to disconnect, but it will be turning the second gear in the opposite direction, allowing it to wind the watch.
The rocker bar system has two gears (usually called wheels) meshed together on a pivoting rocker bar, and will be driven by a gear when the rotor turns. In this description, assume that the rocker bar is to the right of the driving gear, and the driven gear is above the rocker. When the driving wheel turns counterclockwise, the rocker will rock slightly clockwise, so that the rocker gear meshing with the driving gear will also mesh with the driven gear, turning the driven gear counterclockwise. When the driving wheel turns clockwise, it will cause the rocker to pivot slightly. The rocker gear meshed with the driving gear will move down and away from the driven gear, while the opposite rocker gear will move upwards and mesh with the driven gear, again causing the driven wheel to turn clockwise.
Most rocker-bar systems have handwinding capability, this requires a decoupling wheel so the rotor is not forced to turn with handwinding.
All of the above watches have 360 degree rotating rotors. An earlier form of automatic wind is the bumper-wind. In these, the rotor could not make a full rotation--Instead, at the ends of its allowed travel it would hit springs. These were produced until the patents for the 360 degree rotor expired.