How does a balance spring keep itself wound, or does it?
March 15, 2008 12:05 PM   Subscribe

Mechanical Watchfilter: How does a balance spring remain wound, so as to keep the balance wheel moving back and forth?

I have been a watch hound for a while now, though I don't own any quality timepieces (I have a Fossil quartz which hasn't missed a beat for 7 years now)... BUT... I'm fascinated with how mechanical watches work. One thing which has troubled me is that I fail to understand how a balance spring in a mechanical watch is consistently wound and keeps the balance wheel spinning back and forth.

Obviously, I may be misunderstanding how mechanical watches work, but I have been to countless mechanical watch sites on the net who have illustrated explanations of how gear trains, tourbillons, etc work, but none have explained this mystery of perceived-perpetual motion of the balance wheel (again, apologies if this is too easy to explain, or if I have completely misunderstood the basics of the transmission of power from the geartrain)

Here's my problem: I know that a balance wheel moves back and forth and keeps constant "rhythm" via the escape wheel which consistently and evenly moves the watch's hands... BUT the question I have is that since the balance spring is what gives the balance wheel energy to move back and forth... and the balance spring is anchored to the balance cock (without any perceived connection a gear connected to the mainspring)... shouldn't the balance spring lose energy... and if so, shouldn't the balance wheel eventually stop spinning back and forth? So how is this anchored spring kept wound?

This is the clearest I can be with the question... Thanks for anyone's help in preventing my brain from exploding! LOL
posted by lonemantis to Technology (6 answers total) 2 users marked this as a favorite
My understanding is that the watch is driven by the main spring, with it unwinding and rotating a ratchet drive. The balance spring and mass just decides how quickly and consistently the ratchet returns to be moved again by the main spring (around one second..).

The balance spring gets a relatively constant inputted load put into it by the main spring forcing (via the ratchet drive) the balance mass one way, so compressing the balance spring which pushes it back afterwards. It is a reaction spring. Energy is lost because the main spring unwinds as it drives the gears, and this bit (when rewound) puts the necessary energy back into the system. Without this the balance spring has no input to return (balance) against.
posted by Brockles at 12:22 PM on March 15, 2008

From what I understand, the balance spring is a torsional spring with very low loss which, together with the balance wheel, forms an underdamped mass-spring system. It does lose a tiny bit of energy with each oscillation cycle, but that is replenished by the energy stored in the main spring -- this energy is transferred via the arm of the escapement, which is acted on by gears that are eventually coupled to the main spring.
posted by Krrrlson at 12:38 PM on March 15, 2008

You might be interested in this site. I recall a really well done one but I can't find it right now. I was sure I book marked it in
posted by substrate at 12:42 PM on March 15, 2008

I think what you're asking is, "why doesn't the spring unwind completely, and the balance wheel stop moving?" Right?

The answer is that the balance wheel spring's natural lowest-energy state, where it would want to be if you just let it sit on the table is to be partly wound up. That represents the center point of rotation for the balance wheel. Winding the spring up more from there, and unwinding it more from there, both are higher energy states. So as long as it's driven, it will oscillate around that center lowest-energy point, back and forth.
posted by Class Goat at 3:42 PM on March 15, 2008

Best answer: Maybe this description of the escapement will help. The balance wheel rotates back and forth just like a pendulum with a precise frequency. The natural frequency of that swing is what gives accuracy to the watch or clock. In the case of the balance wheel is it the natural frequency of the balance fixed spring going back and forth as it coils and uncoils four times a second. The reason the balance wheel doesn't stop is the the escapement lever is designed to give the balance wheel a tiny shove each time it releases -- just enough to keep it going but not enough to affect the timing. The escapement allows the mainspring to unwind only one notch each swing.
posted by JackFlash at 7:47 PM on March 15, 2008

Response by poster: Fantastic!!! Thank you everyone for your input. I finally understand the "magic" which once elluded me. I now have a renewed appreciation for the intricate design and execution of mechanical watches... Thank you so much!
posted by lonemantis at 9:29 PM on March 15, 2008

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