A string trimmer spins a length of nylon line fast enough that centrifugal force makes it stiff, and the tip of that stiff line cuts grass and weeds. A motor or engine drives the head, through the shaft on most trimmers, and a guard behind the head keeps debris off you. I trim the grass strips under my plum trees and the edge of my gravel yard, and most of the mistakes I see start with someone forgetting that the line only cuts at its tip. This guide covers the parts, the cutting, the line feed, the three power types, common faults and the safety rules.
How Does a String Trimmer Work? The Short Answer
A string trimmer is a spinning head on the end of a shaft. The motor or engine turns the head, the head throws two ends of nylon line outward, and the speed makes the line stiff enough to shear soft stems. The cutting happens at the very tip of the line, which is why the tool handles grass and weeds and struggles with thick woody growth. Many people call the tool a weed eater, and the two names mean the same machine. When the growth is a hedge and not grass, my guide on hedge trimmer vs string trimmer shows which tool cuts what.
Every trimmer, whatever powers it, runs through the same five steps:
- You start the tool: on a battery or corded model you squeeze the trigger, and on a gas model you start the engine as the manual shows and then squeeze the throttle to turn the head.
- The drive reaches the cutting head at the bottom of the tool, through the shaft on a gas trimmer or straight from a motor mounted in the head.
- The line, wound on a spool inside the head, sticks out through eyelets on two sides.
- Spin speed stiffens the line, and the tip cuts the grass it touches.
- As the tip wears, the head lets out fresh line, and a small blade on the guard trims it to length.
The Parts of a String Trimmer
Five parts do the work. Knowing what each one does makes every later choice and every repair easier to follow.
The Motor or Engine
A gas trimmer puts its engine at the end of the shaft opposite the head, while electric trimmers often carry the motor in or near the head. A trigger on the handle sets the speed. Some battery models add a low and high switch: according to EGO, its ST1610T trimmer turns at 4,800 rpm on low and 6,000 rpm on high with no load. My straight shaft vs curved shaft string trimmer guide explains which shape suits which yard.
The Shaft and Handle
The shaft carries the drive from the motor to the head. Straight and curved shafts both exist, and some fold or telescope. According to EGO, its ST1610T trimmer has a hinge that locks two foldable shafts and a clamp that sets the length.
Echo warns that blades must never go on a curved shaft model. A metal blade goes only on a tool whose maker lists that blade and guard for it, with the handle and harness the manual requires. Blades belong on a brush cutter, and my guide to string trimmer vs brush cutter shows where each tool fits.
The Head and the Line
The string trimmer head holds a spool of line, and the line runs through two eyelets so that one end sticks out on each side. Each maker sets the line size its motor can swing. EGO lists 0.095 in nylon twist line for the ST1610T, while Craftsman lists 0.065 in round line for its CMCST900 and warns that heavier line overloads the motor. Use only the diameter your manual names.
EGO and DeWalt (for its DWOAS9ST) both ban metal line, wire and rope, because these can break off and become dangerous projectiles. The string trimmer line you buy should match the head, not the other way round. I explain what trimmer line to use once you have read the size in your manual.
The Guard and Its Small Blade
The guard sits behind the head and shields you from debris thrown backward. According to EGO, it reduces the risk from objects flung toward the operator. It also carries a small steel blade that trims the line to length. Echo says a trimmer run without that blade gets line that is too long and the engine will overheat, and EGO lists a dull blade as a cause of long line and a hot motor.
The guard stays on every trimmer I use, with no exceptions. Without it the line grows too long, and you lose your protection from debris.
How the Spinning Line Cuts
Nylon line is limp when it hangs still, so the speed does the real work. Centrifugal force pulls the line outward as the head spins, and the faster the head turns, the stiffer the line gets. A stiff line holds its shape long enough for the tip to hit a stem and shear it.
The circle the tip draws is the cutting swath, and makers print its width. EGO lists 16 in for the ST1610T, and Craftsman lists 13 in for the CMCST920. A wider swath cuts more grass per pass, but the maker’s figure only holds with the line at its proper length. I answer what cutting width do I need on a string trimmer yard by yard in a separate guide.
Why Only the Tip Cuts
According to EGO, you should allow the very tip of the line to do the cutting, because cutting with more than the tip reduces efficiency and may overload the motor. Husqvarna says the end of the cord does the work and warns against pressing the cord into the area to be cut. EGO and Husqvarna both say to keep the line parallel to the ground.
On the strip edges under the plums I let the tip brush the stems and keep walking, because pushing the head into the grass only slows the line. The motor stays cooler, and the cut is cleaner. A lawn edger drives a blade instead of line, and my guide on how does a lawn edger work shows the difference.
Line Length and the Swath
A longer line draws a bigger circle, and a bigger circle drags harder on the motor. Craftsman says extending the line beyond the 13 in swath of the CMCST920 hurts performance and runtime and may damage the motor. Line length also changes how the handle feels. In a test on one gas grass trimmer, the length of the line, the engine speed and the handle material each changed the vibration at the handle (1).
How the Head Feeds New Line
The tip wears away as it cuts, so the head needs a way to put out fresh line. Two systems advance the line while you cut, and the rules for each are different, as my bump feed vs automatic feed string trimmer guide shows. Your manual names the one on your head.
- Bump feed: The head stores the line and lets out more when you tap it on firm ground while it spins. Inside, a spring holds the spool against locking tabs, and a tap lets the spool turn just far enough to release a short length. EGO says to tap on bare ground or hard soil, and that tapping in tall grass may overheat the motor.
- Automatic feed: The head senses that the line is short and feeds it without a tap. Craftsman’s manual for its automatic-feed trimmer says not to bump the unit on the ground, because that can disrupt the mechanism.
Learn the rule for your own head and ignore advice written for another one.
With a bump head I tap on the bare soil of the tilled strip and never in the tall grass, because that is where the manual says the motor overheats.
Loading Line by Hand
Wind the line in the direction of the arrows on the head, and load only the diameter the manual allows. Husqvarna warns that cord wound loosely or unevenly makes the machine generate harmful vibration. EGO’s power-wound head on the ST1610T still advances line by bumping, but it also winds new line onto the spool when you press a button, and according to EGO a full spool can usually be loaded in 12 seconds. Before you touch the head to load or clear line, follow the full order. My guide to single and dual line trimmers shows how the two head layouts differ.
Stop the Tool First
Switch the trimmer off (a gas engine too) and wait for the head to stop. Then remove the battery, or pull the spark plug wire on a gas model, or unplug a corded model. Only then touch the head. A battery trimmer is ready to run until the pack is out. Let a gas engine cool before you work on it.
What Changes With Battery, Corded and Gas Power
The head and the line are the same on all three types. The drive is what differs, and it decides what you check before a job.
Battery Trimmers
A battery trimmer has an electric motor and a rechargeable pack. Use only the battery and charger the maker approves for that exact tool, charge and store the pack within the temperature range in your manual, and never use a pack that is damaged, modified or swollen. The cordless models I rate highest sit in my battery string trimmer picks.
EGO gives an operating range of 32 to 104 F for the ST1610T, and says battery machines are always in operating condition until the pack is out. Runtime depends on the pack, the speed setting and what you cut. Craftsman says its high setting shortens runtime compared with low.
Corded Trimmers
A corded trimmer runs on mains power, so there is no battery to run down. The cord is the limit. Greenworks says in its STA808 manual that an undersized extension cord drops the voltage and costs power, and tells you to use a cord rated for outdoor use. EGO says to use a GFCI-protected supply in a damp location and not to use the tool in rain or wet conditions. My corded string trimmer picks rank five plug-in models for small to mid lawns.
Gas Trimmers
A gas trimmer runs a small engine, usually a two-stroke that burns fuel mixed with oil. Echo’s manual for the SRM-225 (21.2 cc) lists a 50:1 mix of gasoline and two-stroke oil that meets ISO-L-EGD and JASO FD, 89 octane fuel and a tank of 0.44 liter. Follow the ratio, the oil type and the fuel grade in your own manual, because the Husqvarna 143R-II manual lists a 1:33 option for oil of a lower class. The differences between engine types are laid out in my 2-stroke and 4-stroke string trimmer guide.
Gas costs you in noise and exhaust. In NIOSH sampling of 80 landscaping workers, fuel-powered leaf blowers, string trimmers and chainsaws gave significantly more noise and carbon monoxide exposure than battery versions (2). EPA testing of gasoline lawn and garden engines, string trimmers among them, found short operator carbon monoxide exposures above 120 ppm for some equipment (3). A gas trimmer never runs indoors, in a shed or in a garage, even with the door open.
| Drive | What turns the shaft | What limits it | Check before use |
|---|---|---|---|
| Battery | Electric motor and pack | Pack charge and speed setting | Pack seated, temperature in range |
| Corded | Electric motor on mains power | Cord length and gauge | Cord condition, outdoor rating |
| Gas | Small engine, often two-stroke | Tank size and engine care | Fuel mix, air filter, outdoors only |
When I fuel a gas trimmer I do it outdoors on bare ground with the engine cool, and I move both the can and the trimmer away from the fuel spot, at least the distance the manual gives, before I pull the cord. The gas models I rank highest are in my gas string trimmer picks.
Why Trimmers Misbehave
Most faults trace back to the line, the load on the motor and the head. The manuals name each cause in plain terms, and the table below sums them up.
| Symptom | What is happening | What the manual says |
|---|---|---|
| Line breaks fast | Walls, curbs, wood and wire fences wear line, and a dull guard blade leaves it too long | Keep the tip off hard edges and sharpen the blade as the manual says |
| Bump head will not feed | Not enough line, line tangled, line too short or debris in the spool | Switch off, wait for the head to stop, remove the battery, pull the spark plug wire or unplug, then feed by hand as the manual shows |
| Grass wraps the head | Cutting tall grass at ground level | Cut from the top down |
| Battery trimmer cuts out | Motor overloaded by heavy line or thick growth | Lift off the grass and let the motor recover, and use the right line |
| Gas trimmer will not start | Fuel, choke or spark problem | Follow the starting steps in the manual |
Along the fence and the gravel yard edge my line wears down faster than on open lawn, so I stop and check its length there more often. That matches what the manuals say about hard edges and fences, and it is the reason I treat line as a part that wears. For cutting a defined lawn border, my string trimmers for edging picks have an edging mode or a separate edger kit.
What the Spinning Line Does to People
The same speed that makes the line cut makes everything it hits a projectile. Stones, glass, wire and bits of the line itself leave the head fast and travel farther than most people expect, so the hazards below come from the mechanism and not from carelessness alone.
Eyes, Feet and Bystanders
US emergency department data estimated 81,907 lawn trimmer injuries from 2000 to 2009, and the eye alone accounted for 42.5 percent of them (4). Most were due to foreign objects, which is why the authors stress adequate safety gear.
Gear for Every Job
Wear eye protection that meets ANSI Z87.1, hearing protection, heavy long pants, long sleeves, sturdy non-slip boots and gloves every time (Husqvarna lists steel toe-caps for its brush cutter). Keep bystanders and pets at the distance your manual gives: EGO says at least 50 ft, and DeWalt says 100 ft. Stop the tool when anyone comes closer.
Noise and Vibration
Two gas grass-trimmer engines measured about 100 and 105 dB(A) at the operator, above the recommended limits for an 8-hour day (5). Battery models are quieter, but hearing protection goes on whenever you run any trimmer. In a laboratory test with two participants, gloved hand vibration reached about 3.5 to 5.8 m/s2 on a grass trimmer, higher than on a backpack blower (6).
After a long edge my hands feel the trimmer buzz, so I rest between rows and keep both hands where the manual puts them. Breaks are the practical answer to vibration, and gloves help.
Trees, Wires and Hard Edges
EGO says tree bark, wood moldings, siding and fence posts can easily be damaged by the line. It adds that the line may contact hidden wiring, so you hold the tool by its insulated grips. Husqvarna says the cord can damage sensitive bark on trees and bushes.
My plum trees were planted in 2016 and the bark is thin near the base, so I keep the head well away from the trunks and finish those few inches with hand shears.
A Worked Example With Tall Grass
Tall grass shows the tip rule in numbers. According to EGO, grass over 8 inches should be cut by working progressively from top to bottom in small increments, removing no more than 8 inches in each pass to prevent wrapping. Take a stand of grass 24 inches tall.
| Pass | Cut from height | Cut down to |
|---|---|---|
| 1 | 24 in | 16 in |
| 2 | 16 in | 8 in |
| 3 | 8 in | Ground level |
The grass is the same, and the trimmer does the same work as before. The table is an illustration of EGO’s limit of 8 inches per pass, and small increments from the top down reduce wrapping and motor drag. I plan three passes for tall growth in the corners behind the sheds, and the head stays clear of wraps. The trimmers I trust on overgrown ground are in my tall grass string trimmer picks.
What to Remember Before You Start Trimming
The tip of the line does the cutting, so let it. The head feed rule is different on every system, so read yours. Switch off, let the head stop, then remove the battery, pull the spark plug wire or unplug before you touch the head.
Before each job I walk the strip once and pick up stones and wire, since the line throws whatever it finds. I read the feed rule in the manual again whenever I change a head, and you should open yours to the head section before the next job. A string trimmer is a stiff spinning line, a guard and a drive, and knowing that makes every fault easier to read. When you are ready to buy, my overall string trimmer ranking covers every power type.
Frequently Asked Questions
Why do people soak their weedeater string in water?
People soak weedeater string in water because nylon takes up moisture and dried-out line breaks more easily. Husqvarna lists soaking as a tip for its own cord, a couple of days before use. Check your trimmer’s manual first, because that tip belongs to one maker’s cord and is not a rule for every line.
How does a gas weed eater work?
A gas weed eater works with a small engine, usually a two-stroke running on a fuel and oil mix, that spins the shaft and head through a clutch. The clutch lets the engine idle without turning the line and joins the two when you open the throttle. Mix the fuel in the ratio your manual gives, for example 50:1 in Echo’s manual for the SRM-225.
How does the primer bulb work on a weed eater?
The primer bulb on a weed eater moves fuel from the tank toward the carburetor and back through a clear return line before a cold start. Echo’s manual says to pump it until fuel flows in that line, then pump four or five more times. Follow the full starting order in your manual, because choke position and pull count differ by model.
How does a weed eater clutch work?
A weed eater clutch connects the engine to the shaft only once the engine speeds up, so the head stays still at idle. Echo’s manual for the SRM-225 lists idle at 3,000 rpm and clutch engagement at 3,850 rpm. If the head turns at idle, stop using the trimmer and have it adjusted as the manual says or take it to a dealer.
How long should string trimmer line be?
String trimmer line should be no longer in use than your maker’s cutting swath allows, and the guard’s blade holds it to that length. Echo says the blade on the SRM-225 trims line to about 5 inches after a tap on the ground. Loading new line is a separate step with its own lengths, so follow the manual for both, because line left too long overloads the motor or overheats the engine.
References
- Mallick Z. Optimization of the operating parameters of a grass trimming machine. Appl Ergon. 2010;41(2):260-265. doi:10.1016/j.apergo.2009.07.010
- Alexander BM, Graydon PS, Pena M, Feng HA, Beamer BR. Hazardous exposures and engineering controls in the landscaping services industry. Journal of Occupational and Environmental Hygiene. 2025;22(3):189-202.
- Baldauf R, Fortune C, Weinstein J, Wheeler M, Blanchard F. Air contaminant exposures during the operation of lawn and garden equipment. J Expo Sci Environ Epidemiol. 2006;16(4):362-370.
- Leinert J, Griffin R, Blackburn J, McGwin G. The epidemiology of lawn trimmer injuries in the United States: 2000-2009. J Safety Res. 2012;43(2):137-139. doi:10.1016/j.jsr.2012.03.001
- Mallick Z, Badruddin IA, Khaleed Hussain MT, Salman Ahmed NJ, Kanesan J. Noise characteristics of grass-trimming machine engines and their effect on operators. Noise Health. 2009;11(43):98-102. doi:10.4103/1463-1741.50694
- Oh J, Chen N, Boyd C. Laboratory evaluation of occupational exposure to hand-arm vibration (HAV) during grounds maintenance equipment operations. Ann Agric Environ Med. 2023;30(2):384-389.
Safety Note: Read your manual first. A string trimmer or brush cutter throws debris with force, so clear stones, glass and wire from the area before you cut. Wear impact-rated eye protection with side coverage that meets ANSI Z87.1, hearing protection, gloves, heavy long pants, long sleeves and sturdy slip-resistant boots (steel-toe where your manual says so). Keep people and pets at least as far away as your manual says (50 feet in some manuals, 100 feet in others), and stop if anyone comes closer. Keep the correct guard fitted, use only the cutting attachments, handles and harness the maker approves for your exact tool, and never replace trimmer line with wire. A blade can kick sideways when it hits a stump or a thick stem, so fit the handlebar and harness your manual requires for blade work, hold both handles firmly and keep secure footing. Before you clear a jam, clean the tool or change line, a blade or an attachment, switch it off, wait for all movement to stop, then remove the battery, unplug the cord or disconnect the spark plug wire. Refuel a gas model outdoors with the engine off and cool, away from flames or sparks, and move away from the fueling spot before starting, as far as your manual says (often 10 to 30 feet); never run it in a garage, shed or other closed space. For a battery model, use only the maker’s battery and charger, charge it within the temperature range in your manual and never use a damaged or swollen pack. For a corded model, use a GFCI outlet, keep the cord clear of the cutting line and avoid wet conditions.