The CHS Tower Clock: How it Works

Or rather, “The CHS Tower Clock: How it Would Work, if it, you know, Worked”

I found myself sinking into familiar thoughts as I recently rewound the clocks in my house back an hour. Namely, “Why Batteries?!” Battery powered clocks, though obviously a great convenience in many situations, are simply Not As Cool. Where are the whirring gears, the winding of springs, or the balanced weights? There’s something about a properly working antique clock that is a joy to watch.

With that in mind, at the end of this article will be a link to a YouTube video of a professional describing a tower clock and showing it running. The clock he describes is a simpler model of the one in the Columbia Sr High School clock tower.

So how does it run? I’ll start at the part that always captivates me – how does it know how long a second is? Through the pendulum and the escapement.


On this clock the pendulum is 9′ long. Now here’s a chance for you to flex those old high school physics muscles – what’s the period of a pendulum?

Ha, trick question. In high school or college you may have taken measurements of an ideal pendulum, a point mass on a “massless” string, and found that the period T was 2 times pi times the square root of the length of the string divided by the acceleration of gravity. (For those who want a refresher, I find Wikipedia helpful.) The thing is, this isn’t a point mass and the pendulum arm itself has a nontrivial mass as well. Since I don’t feel like spending the next year making models in calculus and solving them with software, I suggest we stick with the standard and assume that the swing (the period) of the pendulum is always 1.5 seconds. Some E. Howard clocks had periods of 1 second, so that’s also possible. The important thing is, it’s consistent. So how does that control the movement of the clock? The escapement on our clock is a Graham Deadbeat escapement, or just a deadbeat.


In a way we’re working our way backwards through the clock – the escape wheel moves the escapement moves the pendulum, NOT the other way around. As this wheel turns, the escapement (the pincer-like piece above the gear) slowly swings back and forth, allowing one tick forward per swing of the pendulum and causing the “tick-tock” sound we all know and love (or hate).

The escape wheel shares an axle with a gear, which we’ll call a pinion, that is being turned by another, larger, gear. These larger gears we will call planetary gears. That planetary gear shares an axle with another pinion which in turn is being rotated by another planetary gear, etc, etc, back to the energy source. This series of gears is called the time train.


Ideally on a heavy clock like this, the gears are spaced far enough apart that one can be removed without dismantling the entire clock. On household clocks, no such luck.


Above is a clock I’ve been working on. To get at one gear, the whole thing has to come apart.

That brings us to what is running the clock, which in our case is easy – a motor. But, you ask, why can’t the motor just run the clock at the right speed? Why used the pendulum at all?


My response to you might be less than satisfactory – okay, yeah, maybe a motor could just run the time train. But it would need to be very accurate – 10 seconds lost a day, which is 0.01% off, means that your clock will be 5 minutes off after about 4 weeks. You may as well fully switch over to an electric clock and use a more precise electric team keeper.

The motor in the CHS clock does two things. One, it is there to periodically lift the weights that will then drive the time train (the trail of gears that lead to the escapement). Two, it winds the chiming cable.


The picture above shows the chain and ratchet that wind the clock. The chain is coming from the direction of the hanging weight, and the ratchet would hold the weight, particularly helpful if it is being wound by hand. Note how they are on the same axle.


The weights and the pendulum are VERY dangerous parts on tower clocks. Imagine raising the weights only to have them fall and potentially crash through the floor. Big Ben, the famous London clock at Westminster, once exploded when there was a malfunction and the clock began to spin out of control. It was pure luck plus the thickness of the floors and walls that kept everyone safe.

The winding coil that chimes the bell is also wound by the motor. Without the motor, there would be two parts of the clock to wind by hand – the chime and the time train. Thankfully the CHS clock is already equipped with a motor, though at 19 years old it likely needs to be replaced.

This next part of the clock needs to be examined by a specialist before I can be sure it is not missing parts. The tricky part about a clock like this is that you want it to chime just once at one o’clock, but increase the number of chimes as the hour changes. The rest of the clock doesn’t “know” what time it is – it just moves the minute and hour hand at a consistent rate. The chiming mechanism knows the time by using a large cam.


There is a lever, usually called a rack tail, that leans against the strange shape above, which we’ll call a snail. As the snail turns, at the first of the hour the lever is allowed to be long (it rests on the innermost edge of the snail) and then as the day goes on and the snail rotates counterclockwise, the lever is pushed further and further outward. This does not affect the time train. What concerns me about the picture above is that I don’t see a lever leaning against the snail. I’m not sure if it has been moved out of view of the picture and I failed to see it during my inspection, or if it is missing entirely. No useless part would be intentionally left on the clock, so something is definitely up. I don’t think it would have been left as part of the clock unless it served a purpose, so there must be a lever or missing lever.

The barrel around which the cord for chiming the bell is wound is on the same axis as the snail.


The winding barrel is wound with a cord that travels through the hole in the floor shown and traverses a series of pulleys to reach the bell. Another cord outside the clock shed travels through a hole in the wall and presumably once rang a series of “secondary” bells throughout the school. But where does that cord go now? It’s a mystery. Your guess is as good as mine.


Those are the basics of how the clock works. Would it work if it were cleaned up? Possibly! Only an expert performing a full servicing could tell us.



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