I just overhauled an Ansonia oak shelf (kitchen) clock (my job no. 4357). It has the 4 pillar 8 day time and strike movement of dimensions 5 x 3-1/4 inches. The original mainsprings in these clocks are 3/4 inch wide and typically about .0155 inch thick. The previous repairer had installed a time mainspring .0185 inch thick! The loop end of the spring is labeled “USIBEL FRANCE”. I replaced this spring with a new spring that is 3/4 inch wide, .014 inch thick and 108 inches long. The pendulum motion is great (running arc slightly more than twice the escape arc).
I have seen (and discarded) these thick USIBEL FRANCE mainsprings many times before. They were sold by clock parts suppliers such as S. LaRose as “mainsprings for 8 day American Clocks” and sometimes described as the finest mainsprings, made in France. I guess they were fine for many repairers, as they are so strong that they will make a clock work without being correctly repaired! I have measured some of these springs to be as thick as .019 inches! Just by winding the clock, you can feel that they are way too strong. If used for too long, they will cause severe wear to the mainwheel teeth.
The force that a mainspring provides is proportional to the cube of the thickness: .0185 cubed divided by .0155 cubed is 1.7, so the USIBEL spring is providing 1.7 times the force of the original spring (actually it is even worse, as modern steel is better than steel made 100 years ago. So the new .0185 inch thick spring is probably at least twice as strong as necessary.
I used a no. 77.303 from R & M Imports that is specified as 3/4 x .017 x 120 inches. The springs I received have an actual thickness slightly less than .014 inch, and I shortened a spring to 108 inches and re-attached the loop end. This spring seemed so weak when uncoiled that I wondered if it would have enough power! But it works great for the time train, as mentioned in the first paragraph. NOTE TO MYSELF: Next time, leave the spring 120 inches long – there appears to be plenty of room.
Note: R & M has a no. 77.301 mainspring that is specified as 3/4 x .014 x 108 inches that may be German made, and seems stronger than the spring discussed above. I like the 77.303 the best.
It is interesting to compare the force provided by the USIBEL spring to the force provided by the .014 inch thick spring: .0185 cubed divided by .014 cubed is 2.3. This mean that the USIBEL spring provides approximately 2.3 times the force of the .014 inch thick spring. A clock running with the thick spring will wear itself out prematurely!
If you have an heirloom antique American clock that you want to pass down to future generations, make sure your clock repairer does not (or did not) install mainsprings that are too thick. Original mainsprings should be kept in the clock, unless there is a good reason for replacement (broken, too strong, rusted or damaged).