I know that mechanical watches have a spring that they wind to store energy, and un-winding the spring produces energy for the watch. But a spring produces a lot of force when it’s very tightly wound, and very little when it’s almost completely un-wound. So how does the watch even that out with high precision?
In: 95
You’re thinking of progressive rate springs.
Not all spring are progressive.
Your question is based on a false premise.
Well the first trick is to use a *constant force spring*, which does what its name implies, or at least close to it. Then you pair that with a good *escapement* which is as insensitive as possible to variations in the driving force.
The advantage of an automatic watch vs one that needs to be wound is that it is constantly being wound, and so should stay near the same level of tension.
A mechanical watch doesn’t keep perfect time. A well regulated wrist watch will be +-2 seconds a day. Even up to 10 seconds off isn’t bad, depending on a particular movements specifics. I think there are some marine chronometers are accurate to .1 seconds a day.
To answer your specific question, all mechanical watch movements have an escapement, part of which is a spring. This is the part that “ticks”. There are various tricks watchmakers use to even out the ticking. There are even movements designed with “constant force escapements” to address this concern directly. But even if the movement isn’t dedicated to this directly, modern watch springs have a pretty flat force curve for the majority of the time the spring has enough energy to deliver power to the escapement.
the spring just provides the power, the timing is set by a wheel that back and forth, like a pendulum on wall clock.
It always swings at the same rate, and at every tick it gets just the energy needed to do it again from the spring
The spring in a watch isn’t like a typical helical spring one normally thinks of, it’s a wrapped up coil of metal. Think of a tiny flat strip wound up really tight. As you wind it it exerts a force. The neat thing is that the elastic force is mostly linear as long as it’s not completely uncoiled.
In the first place, you wind the spring really tight, and only use a small fraction of its range. The difference between a 100% wound spring and a 95% wound one is only 5%, after all. But…
The spring that *powers* the watch, the mainspring, isn’t the same spring that causes the consistent timing; that’s called the *hairsping.* It causes the *balance wheel* to twist back and forth, and that’s what meters the time.
What’s basically happening in the watch is that the whole mechanism is under tension from the mainspring, wanting to fling the hour and minute hands forward; but a mechanism called an *escapement* only allows it to move one tick each time the balance wheel swings. (This is just a summary of how the watch works, for context, not an answer to your question.)
One of the things that happens when the escapement lets the movement advance is that the movement gives the balance wheel a little kick. But the mainspring is either wound-up enough to charge that kicking mechanism up, or it isn’t. So the boost that the balance wheel gets is always the same (or missing, in which case the watch is out of juice and needs to be wound).
In addition to all the answers here, it’s important to note that most mechanical watches do not keep perfect time. They have to be adjusted as frequently as every few months or as infrequently as once a year.
But the owner doesn’t notice because you have to reset the watch at the end of every month that has less than 31 days.
Let’s look at a different clock works as all clocks operate under similar principles. You need 2 things first is a power source and the second is timing device. In grandfather clocks the power source is gravity pulling a weight down. The timing or regulated by a weighted pendulum which swings at a constant rate no matter how much force you apply to it. The rate at which a pendulum swings is called it’s “period” and period in a free hanging pendulum is only determined by the weight on the end and the distance of the weight from the pivot. It doesn’t matter how hard we push it because the farther it swings the faster it will return to the other side as gravity will accelerate it more, thus keeping the period the same as if we only pushed it a little
Now a watch works by replacing gravity with springs and the pendulum with a balance wheel. But the same still holds true. It doesn’t matter how hard the watch pushes on the balance wheel because the harder it pushes the more it will wind the balance spring (aka “hair spring”) the more the hair spring winds the harder it will swing back keeping the period of the balance roughly the same regardless of any variation on the force pushed to it.
They don’t keep perfect time, far from it, it is why quartz took over when batteries became cheap enough to put in a wrist appliance. You don’t need much power to vibrate a crystal.
In short though, the entire stereotype of the swiss watchmaker comes from the fact that they spent the time to finely tune their craft so there would be very little slop in the function while not being too tight that the metal grinds. One of the selling points with expensive watches was if they were sent to a lab and were certified as a chronometer, it meant you would lose a maximum of 4 seconds or gain a maximum of 6 seconds a day. That is laughably bad compared to a cheap quartz watch.
You’re in for a treat, Bartosz has the most amazing explanations I’ve seen answering how physical mechanisms work. https://ciechanow.ski/mechanical-watch/
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