Chapter 7 The Wheel Train

The primary function of a clock is to tell time. To do this, the wheels in the going train must divide the number of turns the escape wheel makes so we have one arbor somewhere in the movement that turns once per hour. This arbor carries the minute hand of our clock. It's usually located in the center of the movement and is called the center arbor. It's often the longest shaft coming out of the front of the movement as it has to poke through the center of the clock dial and hold the clock hands without rubbing on anything. In our movement, the center arbor is not a direct part of the going train. It is passively driven by the second wheel. Because the second wheel is part of the going train, the rotation of the center arbor will still be controlled by the escape wheel and pendulum. In some movements, the center arbor is a direct part of the going train and plays a role in transferring power from the mainspring to the escapement. The wheels in a train count from the power source up. In the going train, the mainspring assembly is wheel number one. The next wheel up the train that carries power from the mainspring is the second wheel, then the third wheel, and the fourth. The last wheel in this train is the fifth wheel, the escape wheel. The escape wheel is always the last wheel in the going train. Traditionally, wheel number one is the largest wheel in the going train. It carries the power source and is referred to as the great wheel. It is always the first wheel in the train. The wheel carrying the center arbor that rotates once per hour is called the center wheel, regardless of its position in the wheel train. It could be the second or the third wheel in the train, or, as in the case of our movement, completely separate from the actual wheel train. All the other wheels in the going train are referred to by their position number in relation to the great wheel. When we use the name of the wheel, its position and function in the clock is relatively unambiguous. There's only one great wheel in a clock, and it's the first wheel in the train. There's only one escape wheel, and it's always the last. There's only one center wheel, and it's always associated with the time train. If all we had was the time train in a clock, there would be no problem just using numbers for the wheels that don't have specific names and functions. Unfortunately, this is not the case. Clocks often have two or more wheel trains, and all the trains have a similar number of wheels and all count from the bottom up, starting with number one. This can cause a little confusion, as a typical three-train chiming clock could have three different wheels, all with the designation number four. To solve this problem, we put a T in front of the wheel number to indicate a wheel in the time train, an S for the strike train, and a C for the chime train. In the going train, wheels, except the center wheel, always drive pinions. The great wheel meshes with the pinion on the second arbor. The second wheel meshes with the pinion on the third arbor. The third wheel meshes with the pinion on the fourth arbor, and the fourth wheel meshes with the pinion on the escape wheel arbor. This will be important when it comes to reassembling our clock. Since the center wheel in our movement is not a direct part of the going train, it has no effect on the going train if we remove it. The pendulum in escapement would still operate even if the center arbor and its wheels and pinions were removed from the clock. There is still a direct, unbroken line of wheels and pinions to transfer power from the main spring and great wheel all the way up to the escape wheel. Of course, we couldn't tell the time because we would no longer have an arbor in the clock turning once per hour, but the going train would operate normally. Another function of the wheel train is to gear up the relatively few turns of power stored in the main spring, usually around 16 full turns, so the clock will operate a full 8 days on one winding. At the very least, our center arbor must turn once per hour or 24 times per day. If we were to use the 16 turns of power stored in our main spring directly without going through the wheel train, the power from the spring would only last 16 turns, or 16 hours. Through the gearing up process in the wheel train, our center wheel can turn 192 times or 8 full days, while the great wheel only turns 16 times during the same period. The wheels above the center wheel are accelerated even more until we get to the escape wheel. It may turn several thousand times per day. The upper gear ratio between the center arbor and the escape wheel is selected to allow shorter or longer pendulum lengths. If we have a small mantle clock, our pendulum must be short to fit in the case. A short pendulum means faster rotation of the escape wheel. A grandfather clock has a much larger case, so it can accommodate a much longer pendulum and the escape wheel would turn much slower. The gearing between the escape wheel and the center arbor converts the rotational speed of the escape wheel to the required one rotation per hour of the center arbor. So in summary, the gear ratio between the great wheel and the center wheel determines how many hours the clock will run between windings. The gear ratio between the center wheel and the escape wheel determines the length of the pendulum the clock will need to keep proper time. We need different pendulum lengths to accommodate different case designs, and that's why there's so many different movements with different wheel ratios.