A line trimmer, also known as a string trimmer, strimmer, brush cutter, weed wacker, weedy, or whipper snipper, is a powered handheld device that uses a flexible monofilament line instead of a blade for cutting grass and other plants near objects. It consists of a cutting head at the end of a long shaft with a handle or handles and sometimes a shoulder strap. String trimmers powered by an internal combustion engine have the engine on the opposite end of the shaft from the cutting head while electric string trimmers typically have an electric motor in the cutting head.
Electric edge trimmers have the advantage of being very light, easy to maneuver and easy-to-operate devices. However, the length of power cord that can be deployed across the ground limits them and they are usually less powerful and robust than the gasoline-engine.
Electric machines normally are limited to 2.5 mm (0.100 inch) maximum diameter nylon because of their lower power output (400 to about 1200 watts). Recently there are electric string trimmers that offer the same performance as gas powered trimmers. There are also battery-powered units available which have the benefit of being free of power cords, but the liability of weighing more due to a large battery and limited life before the battery runs down. Recently, lithium ion batteries have been able to improve runtime.
Trimmers that have nylon or metal blades usually have straight drive shafts because of the higher torque required to turn the disk and because of the shock loads that are passed back from the blade to the drive shaft and its gearbox(es). Smaller line trimmers have curved driveshafts to make holding the cutting-head at ground level much easier and with less strain on the operator.
Gasoline-engine powered trimmers usually have a minimum of 21 cc displacement motors. At this size they can easily turn 2 mm (0.080 inch) line and some have nylon blades as accessories to the line-reel. A 32 cc engine can swing a 2.75 mm (0.110) line and often has metal-blade accessories. The head contains a safety shield on the user side and a rotating hub which may also be called a head or spool. While this type of trimmer is heavier, it may use a gasoline-oil mix (if it is equipped with a two-stroke engine) and vibrates significantly, they are much more mobile (not attached to a power outlet) and are not limited in power for commercial use. Large trimmers, used for cutting roadside grass in large areas, may be quite heavy—being suspended from the body by a harness — and may be a two-hand-controlled device. These very large trimmers are often referred to as brush cutters.
A string trimmer works on the principle that a line that is turned fast enough is held out from its housing (the rotating reel) very stiffly by the string tension that exerts the centripetal force that prevents the string from flying off in a straight line under its own inertia. The faster it turns the stiffer the line. Even round-section nylon line is able to cut grass and slight, woody plants quite well. Some monofilament lines, designed for more powerful cutters, have an extruded shape—like a star—that helps the line slash the material being cut and thus it is able to cut quite large woody plants (small shrubs) or at least ring-bark them very effectively. These lines make disks less necessary for tough jobs.
The line is hand-wound onto a reel before the job is started, leaving both ends extending from the reel housing. The motor turns the reel and the line extends horizontally while the operator swings the trimmer about where the plants are to be trimmed. The operator controls the height at which cutting takes place and can trim down to ground level quite easily. As the line is worn, or breaks off, the operator knocks the reel on the ground so that a release mechanism allows some of the line in the reel to extend and replace the spent portion. The newer models have an 'auto-feed' operation where a small cutter on the line-guard ensures that the line length exposed for cutting does not exceed the length that can be swung efficiently by the motor. Newly extended line operates more efficiently because of its heavier weight and surface effects (the star-shaped edges).
The speed of the spinning hub is usually controlled by a trigger on the handle. A common mistake is to run the trimmer at full speed when near objects. High speed near objects tends to wear or break line and damage objects without working faster. Running at a slower speed can actually shorten the job by requiring fewer passes and fewer stops to reload line or untangle the hub. The speed should be varied depending on the nature of nearby objects. Also, at slower speeds the line will whip around thinner objects without grabbing, eliminating additional passes near objects like sign poles and chain link fences. For vertical cutting the whole machine can be tilted or some trimmers allow the head to be adjusted at different angles. Vertical cutting is not recommended near sidewalks or other concrete and pavement edges, because it leaves open grooves that allow water to collect and cause damage.
Most trimmers use two stroke engines and require gasoline mixed with oil. Due to pollution laws four stroke engines are becoming more popular. For instance, Honda manufacture a four stroke engine trimmer. Other companies carry Low Emission two stroke engine trimmers. Stihl manufactures a hybrid four stroke engine trimmer with no oil reservoir. This engine is lubricated using pre-mixed gasoline, like a two stroke engine.
Battery powered units are to be recharged after each use. As the recharge time is several hours long, battery powered units are ill-suited for trimming large yards. However, some models offer a quick-change battery pack so the user can have more than one battery ready to swap out when the first one runs down and also a quick charge options that cuts charging times between 30 minutes and 2 hours.