When replacing a clock mainspring, the repairman usually finds the closest match in his
supplier's catalog to the mainspring being replaced. Frequently this may be incorrect because it
may be too strong or too weak, or of an incorrect length. The best way to determine the
correct mainspring is to build up a database of mainspring data: record the information about
the mainspring and the clock for each clock that you overhaul. By the time you have fifty clocks
recorded, you will develop an idea of what strength of spring is suitable for what type of clock,
when a spring is too powerful and should be replaced with a weaker one, or vice-versa. In
general, the strength of a spring is proportional to the cube of the thickness: if you double (2)
the thickness, the spring is eight (=2x2x2) times stronger. The width is directly proportional to
the strength: if you double the width, you double the strength. This assumes, of course, that the
steel and the temper are the same in both springs being compared, which cannot reliably be
ascertained, but we can at least use this information as a guideline at the bench. Below is a
chart that you could use, showing the relationship between thickness and strength of two
otherwise identical springs. When the relationship is expressed in percentages, it could be
applied to any situation. where L is length, T is thickness, r(a) is the arbor radius (half the diameter of the arbor), and r(b) is the barrel radius (half the inner diameter of the barrel). This formula could be entered into a computer spreadsheet as:
or:
Below is also a spreadsheet that you could download and use for yourself. If you enter the dimensions, it would give you the answer. The diameter of the barrel and the diameter of the
arbor are measured in millimetres and should include an allowance for the barrel hook and the
arbor hook. The thickness of the spring is measured in inches because the suppliers do not all
list the thickness in millimetres. This is confusing, but I have had to learn to live with it. Enter the
data into cells B1, B2, and B3.
Here is the spreadsheet you could use to calculate mainspring lengths. You should be able to
download it into your computer and save it. This is a ZIP file and contains an MS Works 4.0 and an Excel 4.0 file (Win95) and looks like the chart below. The
numbers in bold are where you enter your data.
where f is force (or strength), b is the base (or width of the mainspring), and h is the height (or thickness of the mainspring). This means that the 0.014 inch mainspring has 47% of the strength of the 0.018 spring. I would be very careful in replacing a mainspring with one that is less than half as strong as the former.
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