String trimmer

A string trimmer, also called a "weed-whip", "whipper-snipper", "weed-whacker", a "weed eater", a "line trimmer" (in Australia) or (in the UK and Ireland) a "strimmer", is a tool which uses a flexible monofilament line instead of a blade for cutting grass and other plants near objects, or on steep or irregular terrain. It consists of a cutting head at the end of a long shaft with a handle or handles and sometimes a shoulder strap.

The string trimmer was invented in the late 1960s by Tim West of Emporia, Kansas. His first model was constructed by using a 20,000 rpm router with small welded chain bolted to the center hub and mounted to a metal pole. It not only trimmed weeds and grass well, it completely obliterated the neighbor's basketball. In the early 1970s the string trimmer was later credited to inventor George Ballas of Houston, Texas, who conceived the idea while watching the revolving action of the cleaning brushes in an automatic car wash. His first trimmer was made by attaching pieces of heavy-duty fishing line to a popcorn can bolted to an edger. Ballas developed this into what he called the "Weed Eater", since it chewed up the grass and weeds around trees.

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 centrifugal force. 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; the line is thus 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 speed of the spinning hub is usually controlled by a trigger on the handle.

...
Wikipedia