Automatic movements can charge themselves by harnessing the energy created by the motion of the wearer’s wrist. They are often referred to as “self-winding”, and less commonly as “perpetual”. Thanks to the immense convenience of not having to wind your watch or set the time whenever you put it on, Automatics command a far higher share of the mechanical watch market than their simpler manually-wound counterparts.
But what exactly is going on inside an automatic watch, and how does it convert motion into power? In this article, we’re going to focus on the automatic function specifically, so if you haven’t read the Jomashop Academy entry for manual watches, do that first so you have the necessary foundations.
The only real difference between an automatic movement and a manual movement is how the mainspring (the power source of the watch) is wound. Then, the mainspring delivers that power to the hands, via the escapement. This latter process is the same for any watch, no matter if it is wound by hand or by motion.
Abraham-Louis Perrelet is credited with the first automatic winding movement dating from 1777. That, however, was a pocket watch, and it wasn’t until 1925 that the concept was successfully miniaturized and implemented in a wristwatch by English watchmaker John Harwood.
The very first automatic wristwatches designed by Harwood used what was effectively a buffered pendulum that swung from side-to-side, traveling an arc of 130°. A modern wristwatch with a similar design is the recently released Moritz Grossmann Hamatic watch.
Several years later in 1931, Rolex redesigned the automatic mechanism so that the mainspring would be wound by a far more efficient, centrally-mounted semi-circular rotor that could spin 360°. This innovation made the Rolex Oyster Perpetual, which featured this new technology, one of the most famous and certainly most enduring designs in history.
Perhaps the most complicated components in an automatic watch emerged as a result of the desire for bidirectional winding. Rotors, or oscillating weights, originally only wound the movement in one direction, similar to how it would be done on a manual watch. Meanwhile, rotors that are bidirectional can be wound both clockwise and counterclockwise, a much more complicated process.
There are a few ways to harness this “backward” energy, but by far the most common is via two small geared wheels known as reversers. A reverser wheel is built like a sandwich, with a springless pawl between a top and bottom disc. Two of these reverser wheels sit next to one another in a watch. When the rotor weight spins in one direction, the top and the bottom levels of the wheels turn together and energy is transferred to the mainspring in a straightforward fashion. When the weight spins the other way, however, the springless pawl lever engages with the serrated inner wall of the reverser, enabling the two levels of the reversing wheel to turn in opposite directions, which has the same desired result. Reversers can be tricky components to lubricate correctly, due to their intricate construction, but it is essential they are treated correctly as few parts in a watch are under such constant and significant material stress.
Most automatic watches have a power reserve of at least 38 hours, which means they can last just over a day and a half without being worn. In many cases automatic watches can be manually wound as well, to jump-start the movement before you strap the watch on. The advantage of giving your watch a full manual wind before putting it on is that the greater tension in the mainspring means the power being forced through the movement is greater. This causes the balance wheel (a time-keeping component found in the escapement) to reach its maximum amplitude, or amount of rotation in a single vibration. The ideal amplitude of a fully-wound watch would be around 280-300 degrees. When the watch is operating above 220 degrees amplitude, it can achieve higher accuracy as the effect of poising errors in the balance wheel and balance/hairspring is reduced.
Some of the most common automatic movements that you might expect to see in mechanical watches are the ubiquitous ETA 2824-2, its Swiss-made clone the Sellita SW200, and reliable, if less accurate Japanese workhorses the Miyota 9015 and the Seiko NH35A.