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A string trimmer, also called a "weed eater" or a "weed-wacker", 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 may also be known as edge trimmers, line trimmers, whipper snippers, string trimmer, weed whips, strimmers and by many other local and brand names.
The string trimmer was invented in the early 1970s by 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, also known as a "weed whip", 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; 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.
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.
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 some other arrangements exist too. One of such is an arrangement where the trimmer is connected to heavy machinery and is powered using 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 and the significant vibration that carries throughout the device, both of which interfere with its maneuverability and contribute to muscle fatigue, as well as the requirement that motor oil be added to its fuel (if it is equipped with a two-stroke engine). Advantages of gasoline-powered trimmers include mobility (because they are not attached to a power outlet) and the higher maximum power. Large trimmers, used for cutting roadside grass in large areas, are often heavy enough to require two hands to operate, and some are even fitted with a harness enabling the user's torso to bear some of their weight. These very large trimmers are often referred to as brush cutters.
Trimmers that have nylon or metal blades usually require straight driveshafts to handle the higher torque required to turn the heavier 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.
Many string trimmers allow the hub, the head or the lower part of the shaft to be replaced with accessories. Common accessories include:
- replacing the monofilament line with metal or plastic blades.
- replacing the lower shaft with a small chain saw to create a powered pole saw.
- replacing the lower shaft with a hedge trimmer.
- replacing the lower shaft with a cultivator.
Quick release shafts are offered on many newer models which do not require any tools to switch in accessories. Typically only gasoline powered trimmers offer shaft powered accessories, as electric or battery driven units don't generate enough power.
Power and emissions 
Gasoline-engine powered trimmers usually have a minimum of 21 cc displacement motors. At this size they can easily turn 2-millimetre (0.079 in) line and some have nylon blades as accessories to the line-reel. A 32 cc engine can swing a 2.75 mm (0.108 in) line and often has metal-blade accessories. 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, MTD and Craftsman manufacture a four stroke engine trimmer. Other companies, such as John Deere, now carry Low Emission two stroke engine trimmers. Stihl manufactures a hybrid four stroke engine trimmer using a technology called 4MIX. 4MIX trimmers have no oil reservoir. This engine is lubricated using pre-mixed gasoline, like a two stroke engine.
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 ones. Electric machines normally are limited to 2.5 mm (0.098 in) 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. Battery powered units are to be recharged after each use. The recharge time is typically several hours long; some models offer a quick charge option that cuts charging time to between 30 minutes and 2 hours, or a removable battery pack so the user can have more than one battery ready to swap out when the first one runs down.
The typical two-cycle engine used on string trimmers pollutes heavily due to incomplete combustion in the cylinder. This results in unburned fuel escaping through the exhaust system and into the environment. US emission standards specifically limit emissions from small engines. Electric models produce no emissions at the point of use. Battery-powered units typically use small or large sealed lead acid, nickel metal hydride, or lithium ion batteries.
Hobbyist use of string trimmer parts 
Sometimes the engine from a string trimmer is used to power DIY motorized bicycles.
- Media related to String trimmers at Wikimedia Commons