A string trimmer works on the principle that a line spun fast enough on its center stiffens by
centrifugal force; the faster the hub turns, the more rigid 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; the line is thus able to cut quite large woody plants (small shrubs) or at least
girdle them effectively. These lines make solid 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—a bump feed string trimmer, the operator knocks the reel on the ground so a release mechanism allows some of the line in the reel to replace the spent portion. Newer models "auto-feed", where a small cutter ensures the exposed length does not exceed what can be swung effectively. Newly extended line operates more effectively because of its heavier weight and surface effects. The speed of the spinning hub is controlled by a trigger-activated
throttle on the handle. A fixed-line string trimmer has a trimmer head that takes pre-cut line inserts. There is no wound spool of line in the trimmer head; the user instead feeds the pre-cut line in to the appropriate slots, making the trimmer easier to feed line into and troubleshoot than a bump feed system. For vertical cutting the whole machine can be tilted or some trimmers allow the head to be adjusted at different angles. 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, but there are other arrangements, such as where the trimmer is connected to heavy machinery and powered by a
hydraulic motor. The head contains a safety shield on the user side and a rotating hub which may also be called a head or spool. Disadvantages of a gasoline-powered string trimmer include its greater weight, the need to refuel, and the significant vibration that carries throughout the device, both of which interfere with its maneuverability and contribute to muscle fatigue. Advantages include complete mobility and higher maximum power. Large trimmers, used for cutting thick roadside grass and weeds in large areas, are both heavier and more powerful. Some are fitted with a harness to spread weight onto the shoulders. Often referred to as brush cutters, these often mount straight or circular metal blades instead of monofilament line. == Accessories ==