The spool on your weed eater houses the nylon cutting line. This spool spins rapidly inside the cutting head during operation. The spool connects to a spring, so when you bump down on the head, more line emerges. This spool is also connected to the gearbox in the head, which in turn is connected to the driveshaft inside the drive tube.
Weed Jams
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Weeds and tall grasses may wrap around the head and gearbox when cutting, causing your spool to stick in place. Sometimes these grasses may even get sucked inside the gearbox and driveshaft if they weren't removed immediately. Anything that causes the gearbox and driveshaft to stop spinning will also cause the spool to stop working properly. Clear all tangles of weeds around the spool, head and gearbox as soon as you notice them. These clogs can cause other, more serious damage in the weed eater.
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Jammed Line
The line, if installed poorly, will jam up on the spool, causing it to stick and stop spinning. The line needs to be wound tightly around the spool, with even rows and layers. If the line overlaps and crisscrosses itself or comes loose while the spool is spinning, the spool will also stop spinning. Open up the head cap, pull out the spool and remove all cutting line. Replace the cutting line, using the diameter line specified in your operator's manual, and wind it according to the operator's specifications.
Broken Spool
If the spool cracks or breaks from excessive tapping it can also cause the spool to get stuck inside the head. The spool needs to spin smoothly and without friction around the gearbox. Chips or cracks will cause the spool to stick. These spools can also get warped or bent from over tapping, which will cause the same effect.
Broken Spring
The spring underneath the spool is responsible for pushing out more cutting line. If this spring gets bent, it won't connect to the spool properly; in turn, the spool won't spin correctly and may get stuck. Springs will also lose their springiness over several seasons and will need to be replaced about the time you replace two to three spools. Make certain the spring is always properly installed in the cutting head before you reinstall the spool to ensure proper spool movement.
Problems in Your Gearbox
If the gearbox gets clogged with weeds or stops spinning for any reason, your spool will also stop spinning. Gearboxes need proper lubrication on a regular basis according to your model's operator's manual.