Al "Archer" or Dennis:
apropos of something or nothing: most watch manufacturers recommend that their watch mainsprings will keep more accurate time if kept wound closer to maximum. Since the only practical way to keep a watch fully wound would be to wind it often (for a winder) or wear it constantly or store it on a properly set watch winder (for an auto) then I'm guessing this isn't a bad practice. I guess my point is that I've never come across any literature from the mechanical engineers who design and build these contraptions cautioning that you'll wear your watch out if you keep her wound up.
I think the only thing which really wears out is the mainspring from metal fatigue? Or, at least a properly serviced watch will go through multiple mainsprings before most other parts "wear out".
is that sort of correct?
Well some basics first. Constant torque from the mainspring is important for good timekeeping, and there are other factors that are all encompassed in the term "isochronosim":
In a nutshell, this is the ability of the oscillator to keep it's period, regardless of the amplitude. So as the number of degrees of rotation of the balance wheel changes, the timekeeping should not. Of course watches are not perfect, so some allowance for rate changes is made.
Now how much difference in timekeeping is there from fully wound, to say 24 hours of run time after fully winding? It depends on the watch of course, but Omega standards for modern watches are a good indication:
The positional variation at full wind, measured over 5 positions on a timing machine should be 12 seconds or less for Omega modern chronometer watches.
At full wind -24 hours (so 24 hours after being fully wound) the positional variation is allowed to be up to 15 seconds. So only 3 seconds difference after 24 hours of running, and no additional winding of the spring.
If you wear a watch during the day, and take it off at night, there should be little variation because of the mainspring running down. So the idea that it has to be almost fully wound to keep time well is not necessarily true.
Now about wear - when the watch is being wound on a winder, or on your wrist, many parts are being moved, and moving parts can wear obviously. The bearing for the rotor is moving, reversing wheels, transmission wheels, barrel arbor, and yes the mainspring is slipping inside the barrel.
Some examples of wear on those parts below. The first one is a worn reversing wheel from an ETA 7750 based watch - new wheel on the left and worn one on the right:
And here wear on the inside of the barrel wall, where the mainspring slips on a Rolex Cal. 1575 - new barrel on the left, and one very worn one on the right where you can see a deep track worn on the inner wall:
Now I replace the mainspring in every watch I service, so the spring getting worn is not really a concern - it will be replaced anyway, but the other parts can add up.
For me I always let my watch run down for 12 or even 24 hours before I put it on the winder. If the winder is set to only maintain the state of wind as it should be, then I will at least be saving the inside barrel wall from the spring slipping all the time.
Hope this helps.
Cheers, Al
