The wire comes back out at a specific length, how?
it uses a ratchet mechanism.
How does that work?
On some models, you bump the trimmer on the ground to open the ratchet line begins to spool out and then the shaft rotation causes the ratchet to reengage.
Here's a discussion of the principles:
Basically a combination of centrifugal force to pull the line out and a ratchet the stops it.
Usually there is a knife that cuts the line off to the correct length. Only so much is fed out with each push of trimmer on the ground.
More mysterious is the lightweight electric model I use to trim the grass on a weedy uneven slope where a wheeled mower cannot go. Almost never do I need to tap it to feed out line. I just mow and mow for a few hours every month or two, oblivious of the nylon line, then suddenly without warning after a few months it reaches the end of the spool! Amazing. I don't know how it does it. In the past whenever I've checked, the line has been about 0.5cm out of reach of the knife. It's a brilliant design.
Though refilling the spool is always a real test of patience!
There may be different designs, but some auto-feed models have a clutch mechanism that frees the spool as you start and stop, and (so-called) centrifugal force pulls some more line from the spool. The knife then cuts off any excess line.
This can waste a lot of line, depending how you use the machine - i.e. the amount of "cutting time" between each start and stop.
The "bump feed" ones work on the same basic principle, except that the "bump" frees the spool to release the line. There is a tradeoff between the manual system being simpler, more reliable, and less likely to jam, versus the risk of damaging the machine when "bumping" it.
I meant to allude to my trimmer being very very frugal in its consumption of line. It never gets to the knife, as far as I can see. The spool of nylon lasts and lasts, incredibly. That's the mystery.
I'm not complaining!
It never APPEARS to get to the knife. When rotating it is probably right out there.
When I stretch it out tightly to see whether it has been clipping the blade, it doesn't reach. If it was being constantly clipped, I reckon consumption would be much speedier than I'm finding. An interesting test would be to remove the blade and see how things pan out. I'd need to carry a set of clippers in my pocket, though, because now and then I have to rewind a foot of line after it gets tangled in tough brier and the motor stalls, and after winding excess line back on the ends are never the exact right length.
The line trimmer is a great invention.
I just reloaded my B&D GrassHog 1000. The ratchet-clutch flyweight and trimmer line centripetal forces balance at about the desired line mass and length. Any line excess is trimmed off, but that should be seldom, with most of the reduction due to wear from the higher path obstruction density. There was no bright wear spot on the trimmer blade.
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