mainspring formula for loop end mainsprings?

[Bean]

Hello - I know the formula for mainsprings from barrels, and I know the usual action is to measure the existing mainspring, but what if the person working on the clock before you has put in the wrong spring (and you can't find info on the web for the clock itself)? Is there really no formula that can be used?

I found this is a search:

"I come up with this. It uses the outside diameter of the spring which is sometimes easier to obtain than the inside diameter while the spring is still in the movement, but let out into a clamp.

L = 3.14 x N x (D - (N x t))

L equals the length of the coiled spring where N is the nearest whole number of coils (concentric circles), D is the outside diameter of the coiled spring and t is the spring thickness.

The spring may be the open type or in a barrel, wound or unwound, but the equation only works for that portion of the spring where the coils are touching each other."

comments?

[LaBounty]

Hey Bean-

You can use the formula for a mainspring in a barrel and substitute the distance the mainspring can unwind before hitting something as the figure for the barrel ID. Remember, this distance is a diameter so, if you measure from the pivot hole out to the second pinion, you will need to multiply by 2 to get the diameter.

The formula for determining the length of a mainspring in a barrel can be found Here (http://mb.nawcc.org/showthread.php?t=32100).

Hope that helps!

[Bean]

Thank you - that makes perfect sense.

[LaBounty]

Glad to help!

Just to add a bit... Using this method of determining the length for an open mainspring will give you the maximum length and not necessarily what was there originally. For example, when using this method on a 30 hour clock, the new mainspring was long enough to run the clock for 3 days :).