Chapter 9 The Motion Works

Most clocks have an hour hand concentric with the minute hand and this hour hand makes one full turn for every twelve turns of the minute hand. This special set of wheels driven off the center arbor is called the motion works. These wheels divide the hourly rotation of the center wheel by twelve and drive the hour pipe where the hour hand is attached. It's called the hour pipe rather than the hour arbor because it is a long tube that slips over the center arbor creating the concentric hands of the clock. In this picture I've added the motion works assembly in its proper position over the center arbor. This is how it will look in your movement when it is properly assembled. The wheels and pinions on the center arbor are stacked like pancakes so from this view it's difficult to see what's going on. Let's separate the wheels so we can get a better idea of how they work. Here I've relocated the top wheel carrying the hour pipe from its normal position over the center arbor off to the side revealing the wheels and pinions below it. But I left it engaged and mating with its opposite pinion so the operational relationship remains the same. This twelve leaf pinion on the center arbor is called the cannon pinion and it will make one revolution every hour along with the center wheel and arbor. This thirty six tooth transfer wheel to the left is engaging the twelve tooth cannon pinion. The cannon pinion will have to turn three times for the thirty six tooth transfer wheel to rotate once. So the ratio of the cannon pinion to the transfer wheel in the motion works is three to one. Turning on the same arbor as the thirty six tooth transfer wheel is a ten leaf pinion. This ten leaf pinion engages the forty tooth hour pipe wheel. The ten leaf pinion will have to turn four times for the forty tooth hour pipe to rotate once. So the ratio of the transfer pinion to the hour pipe is four to one. To get the total ratio we now multiply the two individual ratios and the final ratio is three times four or twelve to one. The center wheel will make twelve revolutions for each revolution of the hour pipe. Remember this is true for all gear ratios. You multiply to get the final ratio. Now let's put it into motion so we can see how it works. The hour wheel and pipe are back over the center arbor with the cannon pinion hidden underneath. The red minute hand is attached to the center arbor. The black hour hand is attached to the hour pipe. See how the cannon pinion drives the transfer wheel? The motion works is the only place in a typical clock where we are reducing the rotational speed. Since we are reducing speed, involute tooth profiles are often used on the wheels and pinions in the motion works. Under normal conditions the whole time train of the clock is locked solid between the tremendous torque of the mainspring and the escapement. If there were no wheels or pinions that could slip, it would be impossible to move the hands of the clock to set the time. When we need to set the clock, it's the cannon pinion that provides this slipping function. In this movement the cannon pinion is fixed to the center arbor. The center wheel is an easy slip fit on the center arbor and this stout spring pushes the center wheel firmly against the cannon pinion creating a friction clutch between them. Under normal conditions this friction makes them act as though they are solidly connected together. So when the second wheel of the going train turns the center wheel, the cannon pinion and center arbor turn with it. When setting the clock, the cannon pinion slips against the tension of the spring holding the center wheel against it and allows the hands and motion works to move to set the time without interfering with the running of the going train. The cannon pinion will move by slipping against the center wheel which remains locked into the second wheel of the going train. There are many variations of the cannon pinion configuration. Just remember that every clock that has a motion works must have some sort of clutch mechanism to allow the time to be set. That clutch is one of the functions of the motion works. It's good practice to only move the hands of an antique clock in the clockwise direction when setting the time. In time only movements like ours, this is usually not a problem. But some movements with chime and strike trains have levers that will jam if the hands are turned counterclockwise. And this could damage the clock.