Here is the thing that reorganizes every string-trimmer feeding complaint, and almost no one says it first: a string trimmer has no blade. It cuts with a consumable — a length of nylon spun so fast the tip is moving at hundreds of miles an hour — and the whole head is not a cutting tool so much as a line-management system. Its only job is to hold nylon, hand out a little more as the tip wears away, and keep the rest from tangling. So when it "won't feed," "welds," or "keeps breaking," you are almost never looking at an engine or motor fault. You are looking at nylon behaving badly, and the machine faithfully failing to manage it.
And nylon has two enemies that pull in opposite directions:
- Too much heat — friction, spinning against hard surfaces, line that's too thin for the load, and vibration inside the spool. Heat makes the line melt and fuse to itself: the welded lump that won't feed.
- Too little moisture — nylon is hydrophilic (it holds water), and as it dries out with age it turns brittle. Dry line snaps: the string that breaks off constantly and disappears in an afternoon.
That single split — too hot versus too dry — sorts nearly every symptom, and it comes with a cruel twist that explains why people spend money and make it worse: the instinctive fix for each problem feeds the opposite one. Line keeps breaking, so you buy thicker line — and thicker line binds in the eyelets and won't feed, and runs hotter under load. Line won't feed, so you buy a new head or a new trimmer — and it welds up identically, because the cause was heat and how the spool was wound, not the machine. Work with the material instead of against it and the fix is usually free.
Problem one: the line welds into a lump and won't feed
You open the head and the line has fused — layers stuck to each other, sometimes a single melted knot you have to cut out. People assume the spool is defective or the line was cheap. Neither is usually true. This is line welding, and it is a heat-and-vibration problem you largely cause without knowing it.
The obvious half is heat: run the head hard, especially against fences, curbs, walls, and driveways, and friction heats the line; a thin line under a heavy load overheats faster still. Get nylon hot enough and, when it stops spinning and the layers rest against each other, they fuse. But the part almost no page leads with — and the reason two people using the same line get different results — is vibration. The line resonates at high frequency inside the spool, and that ultrasonic vibration is what actually binds the coils together; it happens more with plain round line than with shaped (square, twisted, or multi-sided) line, per trimmer-parts guidance from Sears Parts Direct. Round line is cheaper and more common, which is exactly why welding is so common.
How you wind the spool decides most of it
The single biggest lever you control is winding technique, and nearly everyone does it the way that causes welding. Most people clamp the empty spool still and wrap the line around it. That drags a twist into the line as it goes on — a rotational bind — and it's that stored twist, vibrating at speed, that fuses the coils. The better method: let the line hang free and rotate the spool around the line so no twist is introduced. Then keep the winding neat and in one direction, don't cross layers, and don't overfill.
Problem two: the line keeps breaking and disappears too fast
The opposite complaint: the line snaps off at the eyelet again and again, a full spool is gone by the time you finish the yard, and you conclude you bought bad line. Sometimes you did. Far more often the line simply dried out — and this is the fix that feels like folklore but is real materials science.
Trimmer line is monofilament nylon, and nylon is hydrophilic: it naturally holds a little water, and that water keeps it flexible. Sitting on a shelf in the garage for a season or two — especially in heat or sun — the line dries out, discolors, and goes brittle, so it shatters on impact instead of flexing. The remedy is to put the moisture back: soak the line in water for about 24 hours before you use it, or store spare spools submerged or in a sealed bag with a damp cloth. The water acts as a plasticizer — it softens the nylon back toward its original flexibility — so it resists fraying and snapping, and a heavier, water-laden line even cuts a touch better. This isn't a fringe hack; a 24-hour soak is a long-standing recommendation associated with major trimmer-line makers such as Oregon, and it costs nothing.
"Disappears in an afternoon" is often the surface, not the line
If the line vanishes fast but isn't brittle, look at what it's hitting. Edging along a concrete path, a brick wall, or a chain-link fence at full speed abrades nylon at an astonishing rate — that's grinding it away, not a defect. Ease off the throttle near hard surfaces, keep the very tip (not the whole length) doing the cutting, and a reel lasts far longer.
When it genuinely won't feed: the mechanical checklist
Sometimes the line is fine and the head still won't pay out. Before you condemn the trimmer, walk this list — every item is a few-dollar part or a free adjustment, not a new machine.
- Wound the wrong way. The spool has an arrow showing the winding direction (it must oppose the head's rotation). Wind against it and the bump mechanism can't pay line out. Rewind following the arrow.
- Overfilled or crossed layers. An overstuffed spool can't rotate freely inside the housing, and crossed windings create the exact tangle that blocks feeding. Keep the line below the spool's marked level / rim and wind it in tidy, non-overlapping rows.
- Wrong diameter. Line that's too thick for the head binds in the eyelets and won't advance. Match the diameter printed on your trimmer head's label — residential heads are typically in the 0.065″–0.105″ range — rather than assuming thicker is better.
- Worn or clogged eyelets. The eyelets the line exits through wear sharp and rough over time and start gripping the line instead of releasing it; grass can also pack them. Clean them out; replace the head if they're chewed up.
- Worn bump knob or broken feed spring. On a bump head, tapping releases centrifugal force to pay out line; a cracked knob or a tired spring means nothing advances. Both are inexpensive replacement parts — a spring is a couple of dollars — not a reason to replace the trimmer.
- Empty spool. Worth saying: sometimes it just ran out, or the last stub retracted inside. Open it and check before diagnosing anything fancier.
Read the symptom — it names the cause
Match what you've got. Notice the last column: for every row here, a new trimmer is the wrong answer, and "thicker line" is the wrong answer for most of them too.
| What's happening | Almost certainly | First move | New trimmer / thicker line? |
|---|---|---|---|
| Line fused into a lump on the spool | Welding — heat + vibration, worsened by a twisted wind | Cut out the fused part; rewind without twisting; ease off near hard surfaces | No / No |
| Snaps off constantly, especially with old line | Brittle, dried-out nylon | Soak fresh line ~24 h; store spare line damp/submerged | No / No |
| Full spool disappears in an afternoon | Grinding against concrete/fence at full throttle | Back off the throttle near hard edges; cut with the tip only | No / No |
| Bump the head, nothing feeds | Wound backwards, overfilled, or crossed layers | Rewind per the arrow, below the fill line, no crossed rows | No / No |
| Feeds nothing even when wound correctly | Worn bump knob or broken feed spring | Replace the spring/knob (a few dollars) | No / No |
| Line advances but binds or jams in the head | Line too thick for the head, or clogged/worn eyelets | Match the head's rated diameter; clean or replace eyelets | No / No |
When it really is a fault
Honesty matters, because a few trimmers genuinely have a broken head — and you should know the difference so you neither fight a dead one nor replace a good one. Suspect a real mechanical fault when:
- The spool or housing is cracked — a split spool won't hold tension or rotate properly, and no winding trick fixes it. Replace the spool/head (a cheap part, not the trimmer).
- An auto-feed (no-bump) head stops advancing despite correct line and winding — the internal feed motor or clutch can fail. Often still a replaceable head assembly.
- It still won't feed after a correct rewind (right direction, right gauge, below the fill line, no twist), clean eyelets, and a known-good spring/knob.
Even then, the fix is almost always a replacement head or spool, not a replacement trimmer. If you've concluded the whole tool is genuinely done — the engine or battery platform is tired too — then it's worth matching power source, line gauge, and shaft type to the work you actually do. The string trimmer buying guide walks through gas versus battery versus corded, line gauge by vegetation, and straight versus curved shafts so the next one fits. But get there by elimination — because the head that "won't feed" is, almost always, a spool of nylon asking to be wound right and kept from drying out.