Wind up watches are mechanical watches that get their power from the user manually winding them. They are not as common as they used to be, having largely been replaced by electronic watches, but they do still exist, particularly in higher-end models. Learning how a mechanical wind-up watch works can help you learn a variety of theories, such as conservation of energy and how basic mechanical devices operate.
Crown and Winding Wheel
The crown is what the user turns in order to wind the watch. It is connected to a pinion which changes the direction of the turns. The crown will generally be at a right angle to the watch and its internal gears, so a pinion is necessary to change the direction.
The pinion is attached to a winding wheel. When you turn the crown forward, it turns the winding wheel sideways relative to the crown.
The mainspring is the key component in a mechanical watch. It is a piece of flexible metal coiled around itself. When you turn the crown, the winding wheel winds against the force of the mainspring and coils it tighter. However, the mainspring only coils tighter when you turn the crown. Once you stop winding, the mainspring starts uncoiling.
If the mainspring had its way, it would unfurl as fast as possible. Think of a rubber band pulled tight. When you let it go, does it fire slowly? It does not. However, if you ease your hands back together, you can release the tension and slow the speed of the rubber band's flight. Think of the balance in a mechanical watch as your hands. It releases the tension of the unfurling mainspring a little at a time in order to move the watch's hands. If it released it all at once, the watch's hands would spin around unchecked before coming to a complete stop. If the coil didn't release it at all, then the watch would not function.
The balance moves at one speed. It can't move at more than one speed, because it is only one gear, and it has a fixed amount of power going into it. So, it needs to utilize a gear train. Gears transfer power through their size. If a small gear is attached to a large gear and power is applied to the smaller gear, the large gear will move slower, as it does not need to move at the same rate to apply the same force. This being the case, the first gear attached to the balance is the second hand. This gear is the smallest, as it needs to move the fastest. The next, slightly larger gear, is attached to the minute hand, and the third gear is attached to the hour hand. By making each of these gears bigger than the previous, they step down the balance wheel's speed and keep accurate time.