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Balance Springs

Materials & Design

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Sam Stones29/06/2010 02:01:07
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922 forum posts
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Hello all you clock lovers.
I posted some of the following in another thread on 7 June, but perhaps it was in the wrong Forum section.

The skeleton clock I am about to finish, is timed by means of a bi-metallic balance wheel and a helical spring, and driven via a lever escapement and pallets. The section of the spring as design by author John Stevens (see "A Skeleton Clock with Lever Escapement - ME Feb '72 "), has a width of 0.020" and a thickness of 0.005".

With very limited workshop facilities, and not thrilled with the idea of trimming a wider strip of spring steel, I wondered if I could instead substitute round (piano) wire for the spring of the balance wheel.

Since the spring will operate primarily in bending through its thickness and, thanks to the maths capacity of my CAD package, I have determined that the above cross-section has a moment of inertia in the thinner direction of 0.000087mm4. To have this (M of I) value, an equivalent round section would be 0.00807" dia (0.205mm). I have not determined if this size of piano wire is available.

A clock maker once advised me that a rectangular section was chosen so as not to visibly sag in the vertical sense (this helical spring hangs vertically). Also, there was some statement about the spring wobbling in operation (as opposed to `breathing'), if not wound/heat-treated/made correctly.

Any comments would be most welcome.

Sam

Edited By Sam Stones on 29/06/2010 02:06:48

ColH29/06/2010 02:21:49
28 forum posts
Hi Sam
 
Check your messages.
 
Col
Sam Stones29/06/2010 04:34:33
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922 forum posts
332 photos
Thanks Col,
 
It could be a very handy alternative. I'll wait and see if anyone replies.
 
Regards,
 
Sam

Edited By Sam Stones on 29/06/2010 04:35:23

Michael Ellis 130/06/2010 00:48:25
13 forum posts
A company I worked for years ago made balance springs of rolled wire. Your new best friend will be a jewelry maker who can flatten a medium carbon steel to the required dimensions. The next step is to lay up enough layers of the .005 thickness to equal the free space between the coils of the relaxed spring plus one, coil this assembly into a nest larger than the O.D.  Heat set (bake) the package about 400F/200+C. If the throwout is wrong, rebake at +25F steps until you get the desired O.D.
 
ellis 

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