How a string trimmer works

A string trimmer cuts grass and weeds by spinning one or more flexible nylon lines at high speed. The motor turns a shaft and cutting head; the fast-moving line strikes vegetation and trims it. The guard helps deflect debris, and the feed system makes more line available as the exposed ends wear down.

How the motor turns the cutting head

When you squeeze the trigger, the motor drives a shaft that spins the head at the end of the trimmer. The shaft may be straight or curved, depending on the design. In a gas trimmer, the engine supplies the power; in a corded or battery-powered model, an electric motor does.

As the head rotates, the line extends outward. At operating speed, it becomes stiff enough to cut grass and light weeds through repeated impacts. The line does the cutting—not a sharp edge—so it gradually wears or breaks when it hits pavement, fencing, rocks, or other hard surfaces.

A guard over part of the head helps keep debris from flying toward the operator. It may also include a small blade that trims excess line to a useful length. Keep the guard installed and use the eye, face, hand, and leg protection specified in the owner’s manual.

How trimmer line feeds out

Trimmer heads use different ways to provide fresh line:

• Bump-feed head: Tapping the rotating head against the ground releases more line. Follow the manual’s instructions; bumping too hard or at the wrong angle can damage the head.
• Automatic-feed head: The head releases line as it wears, often in response to changes in operating conditions. The exact mechanism varies by model.
• Fixed-line head: Short pieces of line are inserted directly into the head. When a piece wears out, you replace it rather than unwinding line from a spool.

For spool-based heads, line is stored inside the head and routed through openings. The line diameter and loading method must match the trimmer and head. Line that is too thick may not feed properly, while line that is too thin may wear quickly. Check the owner’s manual or head markings before buying replacement line.

What changes between gas, corded, and battery models

The cutting action is similar across power types, but the source of power changes how the trimmer operates. Gas models use an engine, while corded and battery models use electric motors. Corded trimmers need a suitable outdoor-rated extension cord and a safe working area; battery models depend on a compatible, charged battery.

Power, weight, noise, and runtime vary among individual models. For a buying decision, consider the vegetation you need to cut, the size of the area, and whether you can manage a cord, fuel, or battery charging. Use the manufacturer’s guidance to determine which work a particular trimmer is designed to handle.

Why a string trimmer may stop cutting well

If the motor runs but the trimmer cuts poorly, first check that the correct line is installed and that enough line extends from the head. Remove grass or debris packed around the head only after shutting the trimmer off and following the manual’s safe-cleaning instructions. Check that the guard and head are correctly fitted, and that the spool or fixed-line pieces are loaded as directed.

If the head does not spin, or the motor repeatedly stops, turn the trimmer off and disconnect its power source before inspecting it. For a battery model, remove the battery; for a corded model, unplug it; for a gas model, follow the manual’s shutdown procedure. Do not operate a trimmer with a cracked head, damaged guard, frayed cord, or other damaged part. If basic setup checks do not solve the problem, have it inspected or replace the failed part as the manufacturer recommends.