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Advice needed on spring length

Posted: Fri Mar 09, 2018 4:06 am
by phonodesbois
A friend of mine needs to install a spring in a barrel for a large clock but he has no clue on the length of the spring he has to find. So, is there a formula or a way to estimate the length of a spring based upon the barrel diameter?
Thanks for your help,
Jeff

Re: Advice needed on spring length

Posted: Fri Mar 09, 2018 7:05 am
by jboger
Once the spring is inside the barrel, the unwound spring should occupy about one-third of the radius from the center of the barrel to the barrel wall. This gives enough room for the spring to be wound and supply an equal force while the clock is running. This corresponds to about 6 turns of the spring inside the barrel. I do not have a formula for calculating the length. My source is a standard reference work for the watchmaker, "Practical Watch Repairing," by Donald de Carle, p. 83. I would continue to look for a formula, however. it would depend on the thickness of the mainspring and the diameter of the barrel. But perhaps the above can be used to make a reasonable estimate of the length.

Re: Advice needed on spring length

Posted: Fri Mar 09, 2018 4:51 pm
by Dave D
If you know the make and model of the clock, many clock parts suppliers list the spring sizes required. Installing the wrong spring can cause much wear on the movement. Too thick or even too long will not be good. Spring strength increases exponentially with thickness.

If you can tell me the make and model, I may be able to offer more help.
Dave D