November 08, 2004, 19:55Kevin Pestor
pendulum wire or rod length
Please excuse my clock terminology as i ask this question.
I have a Waterbury 8 day time and strike clock.
I took it apart and rebushed some and among other things repaired 2 lantern pinions.Now for my question.I have the pendulum for it , but there was no suspension spring on it.I put one on but do not know where to make the hook on the end to put the pendulum on.I thought i would maybe cut a few inches off the rod near the gong area and start there with the pendulum on the rod.
presently i have no hands on it either as i don,t have the square hole type.I know i need the hands on this to start timing it.It runs quite well with no weight on the rod at the moment.
Thanks for all help.
November 08, 2004, 21:32Tom Seymour
Pendulum length is determined by beats per minute. I have a chart. As soon as I can find it, I will take a picture and post it.
In the meantime, a less scientific way is to start with what looks right for the case. Err to the long side.
November 08, 2004, 23:29Tom Seymour
Here is the chart. On the right side is the theoretical length matched with beats per minute.
This will give you a starting point.
November 10, 2004, 00:37Dick Feldman
It seems the more I learn about pendulums, the less I know. Theoretical pendulums are listed in all pendulum length tables. In physics (theoretical), pendulums have only one dimension, that being length. Theoretical pendulums only are subject to gravity. (No friction, no mass, etc.) A theoretical pendulum, once put into motion will never stop swinging. That being said, no human being has ever seen a theoretical pendulum operate. Theoretical pendulum lengths will approximate actual pendulum lengths. For that we are lucky.
The properties of theoretical pendulums apply to actual clock pendulums, but many other factors are present. For example, as an actual clock pendulum swings, there is friction between the pendulum and the surrounding air, and the pendulum has mass. A pendulum of a certain length will oscillate at a certain rate. For instance, a simple seconds pendulum (one that beats once per second or 3600 times per hour) will have a length of .9939 meters. (This is theoretical length). A shorter pendulum will have a shorter beat duration; a longer one will have a longer beat duration.
All clock movements can be rated in beats per hour, or beats per minute. When a mechanical clock is keeping proper time, it will beat that many beats per hour. This number can be determined by counting the number of teeth on the wheels in the clock (those between the verge and the hands) and plugging those values into a formula. If a clock is operating above its rated beats per hour, the hands will run fast. If a clock is operating below its rated beats per hour, the hands will run slow. Regulating the beats per hour on a pendulum clock is done by changing the length of the pendulum.
There are lots of approaches to fitting a pendulum to a movement. One is simple trial and error. If you know the wheel count, you can calculate the number of beats per hour the clock should run. Once a clock is fitted with a pendulum, one can count beats for an interval (like one minute or 5 minutes), convert that number of beats to beats per hour and adjust the pendulum length. You guessed it, then count again, figure again and adjust the length again.
Modern science has given us some very useful tools. I have an electronic gadget with a sensor that will “listen” to the impulse of the beat of a clock and show the beats per hour on a display. I can run the clock for a few hours or a day and compare its performance to an accurate time keeping device (my quartz watch) and adjust pendulum length using a ratio of the beats recorded versus that number that the clock should be running. I make final adjustments to the length of the pendulum by running the clock for a few days and comparing its timekeeping ability with the same watch. This is all done without knowing the Beats per hour till the clock is keeping time. Holy smokes, with this method, I end up where I started with the other method.
By now you probably think that you have been fed a lot of double talk about beats per hour and pendulum length.
A useful discussion of the workings of pendulums and the accompanying formulas can be found in The Modern Clock by Ward L. Goodrich. It is one of the standard reference books for clocks and any library should have one. As an aside, the book was published first over a hundred years ago.
Best of luck,
November 10, 2004, 20:59Kevin Pestor
Thanks Dick for that very good explanation of beats and pendulum lengths, very educational.
Tom i looked at the chart and there is a dotted line going across the page through the 30.Does that mean i need a pendulum length of aprox 30 inches?
Sorry i was not sure what the beats per min should be for this clock and where it should intersect with the cm or inches.Thanks you guys for your help.I wish we lived closer.
Also Tom i could not use the case as i don,t have the Gingerbread case this movement came from.Good idea though.