In another thread there was a question about winding manual and automatic watches, and also about the "clutch" mechanism that is present in some watches. I offered to make a post to help explain some of these things, so hopefully you will find this post helpful.
So let's start by looking at the barrel. The barrel is the container that holds the mainspring for a watch, and this example here is from an old manual wind Zenith pocket watch I serviced some time ago. Here you can see that the barrel has a lid, and the lid has some notches in it - more about those in a minute:
Next here you can see the lid has been removed, and the coiled up mainspring, and the barrel arbor can be seen. The red arrow with the label indicates that there is a hook on the side of the barrel arbor. This hook is the inner attachment point at the center of the barrel, so the mainspring is attached to the arbor. Note that this is obviously in the "unwound" state, and at the fully wound state, the majority of the spring coils are wrapped around the barrel arbor:
Here is the mainspring now removed from the barrel - I have described it as being "set" which means it has lost some of it's shape, and therefore it's power. This example is an old blued steel spring, where most springs these days are made of white alloy. The newer springs do not develop this "set" as easily as the old springs do. This spring was replaced as part of the service:
Now let's take a close look at each end of the spring - first the inner coil where the spring attaches to the arbor - you can see the hole that the hook on the arbor attaches to:
Now let's look at the other end. Here we see that there are a couple of pieces sticking out from the side of the mainspring, and there's a hole in the end of the spring. These features are used to attach the outer end of the spring to the barrel. There are many different methods for attaching the outer end to the barrel wall - the name for this style of end is called "double brace hole end" and is seen on some vintage movements:
So if you recall in the first photo of the barrel still assembled, there were some notches in the lid. Well there's also a notch in the floor of the barrel, and a hook along the inside wall. All these serve to fix the outer end of the mainspring to the barrel wall:
A more common vintage style of attachment is shown in the next one, called the "motor barrel brace" end. This one does not have the hole for the hook, but attaches using slots in the lid and floor of the barrel:
So what about more modern wrist watches? Here is a mainspring removed from a modern wrist watch:
So right away we see some things that are different. First the overall shape of the spring forms a "S" shape. This spring is a modern alloy spring, so despite being in a watch for several years, it has not set. Now have a closer look at the end that attaches to the inner wall of the barrel:
So this is known as a "regular end" mainspring. This is the most common type of end used to anchor modern mainsprings to the barrel wall. Note that the inner attachment to the arbor is the same for all of these examples, and also the same for the automatic.
So here we see how this "regular end" is attached to the barrel wall - note the two red lines which define the notch that the tab on the end of the spring slips into:
So now we know what a manual winding spring looks like. So when winding a manual wind watch, you will feel it wind and then at some point you will feel that the crown won't wind anymore. This means that the no more spring can be coiled around the arbor, as the outer end of the spring is physically attached to the barrel.
This brings us to an automatic winding barrel. The key difference between a manual wind and an automatic is this:
Manual wind = both ends fixed
Automatic wind = inner end only fixed
Here is a photo of a modern automatic mainspring that I removed from a watch:
So note that the shape is similar to the modern manual wind spring, but here the outer end looks quite different. In this photo we have a closer look, and this is known as the "bridle." There are no additions to the spring to make it physically attach to something inside the barrel:
So in an automatic, the inner end of the coil is attached to the arbor, but the outer end is free to spin inside the barrel. So if this is the case, how does the spring generate enough power to run the watch? Well the first part of the answer is in the photo above - that bridle. That is an extra piece of mainspring mounted in such a way that when the spring is coiled into the barrel, that bridle puts quite a lot of force on the very outer coil of the spring. This creates friction between the barrel wall and the spring.
Now in some older automatics, the inside wall of the barrel was completely smooth. Many modern automatics have a series of notches in the inner barrel wall, to help that outer coil of the mainspring grab. Here they are in a modern automatic barrel, with the notches shown by the red arrows:
Now with these notches, and the bridle creating friction inside the barrel, it is still not enough to develop the power needed to run the watch. So here I show the inside of a barrel for an Omega Speedmaster Pro (Omega Cal. 1151 - ETA 7750 base with added complications) I serviced, and I have added an arrow to show the thin stripe of braking grease that I have applied to the inside barrel wall. This grease is quite tacky, and unlike most greases that are meant to lubricate, this one is meant to create more friction:
Over time this grease can break down and move around, and this is one reason why an automatic watch needs to be serviced. This will show up to the owner likely as poor timekeeping, and reduced running time.
So now what about over winding? What does this term really mean?
Well the short answer is there really is no such thing. When someone brings me a watch to service saying that they believe it's "overwound" it's usually a manual wind watch that is fully wound and not running. Somehow there is a thought that since the watch can't be wound any further, and it's not running, that it was damaged through winding too much. In reality the watch is typically so jammed with dirt and old oils, the power in the mainspring simply can't overcome the friction in the wheel train, and the watch won't run.
So can you damage a mainspring by winding it too much? In an automatic watch the answer is easy - no you can't (I suppose it could wear out the inner wall of the barrel, but this would take a LOT of winding). The oscillating weight (rotor) is constantly winding the watch when you move. The slipping mainspring means that the spring can be wound and wound and will slip when it reaches the fully wound state. I've seen this referred to as a "clutch mechanism" but I've always felt this term implied it was much more complicated than it really is.
So what about a manual wind watch? In my view there are 4 states a mainspring can be in:
1 - unwound
2 - fully wound
3 - somewhere between 1 and 2
4 - broken
So 1, 2, and 3 are no problem for the watch, as these are the states the spring should be in at various times of the day. So what does it take to break a mainspring? Well it depends on the watch, but generally quite a bit. When winding a manual wind watch, most watches let you know when they are fully wound because the crown resistance increases very quickly, so it's easy to tell when the spring has been wound. For large manual wind watches, say an ETA 6497, you would need a considerable amount of force to actually snap a mainspring that is in good condition.
Anyway, I hope this helps clear up some things about the power plant of the watch - the mainspring and barrel.
If you have any questions, please let me know and I'll do my best to answer.