In a continuation of posts on the fundamentals of watches here comes talk of the gear train. The first post discussed the mainspring and barrel.
A gear train is a collection of toothed wheels and pinions which interact to transfer rotational energy from one location to another. As watchmakers we usually talk about these elements in terms of “wheels” and pinions. The standard mechanical hand wound watch actually has five gear trains. They are the time train, counting train, motion works, winding train and setting train. Today, specifically I am going to talk about the time train and the counting train.
These two trains constitute the gears you most likely think about when you consider how a mechanical watch operates. The time train consists of the barrel and the center wheel pinion. The counting train consists of the center wheel through the escape wheel pinion.
The time train is the simplest of gear trains since it consists of simply one gear turning one pinion. In a traditional watch layout where the center wheel drives the minute hand of the watch these two components determine the “running time” or “power reserve” of the watch. Each rotation of the center wheel and pinion represent one hour. In a typical watch the center wheel makes about 6 or 7 rotations for each rotation of the barrel and the typical mainspring will allow for about 7 rotations of the barrel before it is wound tight. These two facts contribute to an average run time of 42 to 48 hours for most watches. In an 8 day movement an additional wheel and pinion are inserted between the barrel and center wheel. This changes the transmission ration of the train and allows the center wheel to turn faster relative to the barrel and therefore increasing the run time of the watch (by approximately 5 times).
The counting train serves to transfer the energy from the mainspring to the escapement and regulating organ (balance & hairspring) of the watch. The typical counting train consists of the center wheel, third wheel & pinion, fourth wheel & pinion, and escape wheel pinion. In a traditional layout with sub seconds hand the fourth wheel will make one rotation per minute. The balance and hairspring determine the rate of the watch, but they must correspond correctly to the counting train in order for the hands to read correctly. The necessary vibrations per hour of the balance are determined by the counting train and escape wheel. This is why it is called the counting train. In an 18,000 bph watch: there will be 18,000 interactions of escape wheel teeth and pallet stones for each rotation of the center wheel. Since each tooth interacts with two pallet stones this means that 9,000 teeth would pass a defined point for each rotation of the center wheel.
In watches that are available in multiple beats it is essential to know which beat count you are dealing with when replacing any train wheels. The wheels are usually interchangeable and if you use the wrong one the watch will appear to run well and the timing machine will register everything correctly but the hands will not rotate at the correct rate so it will not tell time accurately.
In mechanical watches the counting train is a multiplying gear train, that is to say the wheels drive the pinions. The farther from the power source you get the faster the wheels spin. In electronic analog watches the counting train is a reduction gear train, or the pinions drive the wheels. The farther from the power source you get the slower the wheels turn.
For the mechanically inclined: there are many different shapes of gear teeth. Common profiles include cycloidal, epicycloidal, hypocycloidal, and involute. ETA uses an involute gear profile in their gear train. Rolex uses what they refer to as “microgearing”. These are essentially triangle shaped teeth for many of their gears. For more on gear profiles in horology I suggest Wheel and Pinion Cutting in Horology: A Historical Guide.