In English case, brass plaque on back says:
London, October 17, 1877
I bought this clock of
Elkington & Co.
Edward Livingston Davis
Movement approximately 3-3/4 inch diameter
Case dimensions: 17 inches H, 14-1/2 W, 7-3/16 D
5-1/2 inch dial, silvered brass, painted numerals and time track.
Measured mainspring dimensions
T: 22.3 mm wide (7/8 inch), 0.0138 – 0.014 inch thick, 54 inches long
S: 22.5 mm wide (7/8 inch), 0.013 inch thick, 46 inches long.
The strike mainspring has been shortened, so I calculated the proper size.
Mainspring thickness formula:
L = pi (B squared – A squared) / 2 T
L = Length
B = Barrel inside radius
A = Arbor radius
T = Thickness
B = 18 mm
A = 5.8 mm
Optimum calculated length: 51.3 inches
The spring was 54 inches long, it looked very old and had no problems, so it was left alone.
A = 18,9 mm
B = 6.5 mm
Optimum calculated length: 59 inches
The spring is only 46 inches long, so it must be replaced.
Looking in the R & M catalog, there were springs of .012, .0125 and .013 inches thick that looked correct for the clock. Since new spring are stronger than old springs, I chose the thinest spring, .012″. I felt that there was only a very small chance that this spring would be too weak.
I calculated the optimum length for a .012 inch thick spring: 63.9 inches.
The spring in the catalog is 7/8 inches wide, 0.012 inches thick, 61 inches long. It is shorter than optimum, but considering that a much shorter spring had worked in the clock before, and based on past experience that 8 French clocks are often designed to run around 2 weeks on a winding, I used this spring. It is being tested now, and after 9 days, the striking has not slowed down significantly.
Strike setting up: There must be only a very small run after the last hammer drops, otherwise the quarts will not be struck correctly.
My job 4353.