It's one of the many myths floating around in the horologist hemisphere. A good clock can never be wound past the point of no return. Although it's not a truism, there are some things to know so that you don't wind a mainspring too tightly and damage your valued timepiece.
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Why the Myth Exists
When clock owners approach their cherished chronometer, they can feel a bit worried about breaking the delicate machinations. This is the basis for this clock myth that continues to cause concern in amateur horologists. Instead, most clock owners don't wind the clock enough because they're worried they will wind it too tightly and that will be the end of the long-running piece. Although this doesn't sound too terrible, a clock is meant to run at full throttle. By not winding the clock to its maximum potential, you are cutting down on its keeping time as it should for as long as it should, causing you more work and worry as you go in to wind your clock more often.
Things to Avoid When Winding
If you're not taking a pair of vise grips and mangling the key to get it to move forward after the mainspring is obviously wound to its maximum level, then you're in no danger of overwinding your clock. If you force the key at the end of the wind continually to make sure it is completely wound, you could tear the end of the mainspring. This is only in the most extreme cases, though, as most people will stop well before they push the mainspring too far. If you hit the end and get some pushback, that's fine. Stop there and you won't damage your clock.
How to Comfortably Wind
After all of that, you don't actually have to wind the clock to the very end of its mainspring's capability. You want to stop just short of the fullest wind possible. If your timepiece has a tiny pendulum, you may need to wind it more often. The more you take the time to wind, the more wear you are putting your clock through. Some clocks will run faster if they are wound too tightly in the first 24 hours after you've tightened the mainspring. Ideally, if you stop just short of the mainspring's full potential, your clock will run just fine for a full week without having to get any attention from you.