String trimmer

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A string trimmer, also called a line trimmer, edge trimmer, Weedeater (a brandname), Weedwhacker, whipper snipper, strimmer, garden strimmer, or bush cutter 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.

"Strimmer" is correctly a tradename, but it has slid into use as a generic and spawned a back-formed verb "to strim" = "to use a string trimmer on".

1 Overview
2 Use
3 Safety
4 Accessories

[edit] Overview

Electric edge trimmers have the advantage of being very light, easy to manoeuver 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 petrol-engine ones. 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).

Petrol-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, uses a petrol-oil mix and vibrates significantly, they are much more mobile (not attached to a power outlet) and are not very limited in maximum 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.

A line-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 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. 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).

Trimmers that have nylon or metal blades usually have straight driveshafts 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 driveshaft 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.

[edit] Use

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 less passes and less 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. (Note: Vertical cutting is not recommended near sidewalks or other concrete and pavement edges. It leaves open grooves that allow water to collect and cause damage.)

Most trimmers use two stroke engines and require petrol mixed with oil while a few are four stroke engines and do not require oil to be mixed with the petrol.

[edit] Safety

While the monofilament line may be slightly safer than a metal blade in terms of cutting, the open design can make it more dangerous than a modern shrouded lawn mower. The rotating hub has a tendency to grab clothing and other items similar to the dangers of a power take-off and rotating the filaments tend to throw rocks and other hard items. Eye injuries are common for both users and bystanders. Users should always wear safety glasses. Closed toed shoes and long pants are practically required, as they will protect from flying rocks as well as the constant spray of plant bits. Hearing protection is required for internal combustion string trimmers. Long sleeves are recommended. The trimmer should be turned off or to idle when near bystanders without the proper protection. (Refer to the manufacturers manual for additional safety instructions.)

The power should be disconnected (or the engine stopped) before the line is replaced or before any significant work is done in the line-reel area (such as removing grass stalks that have wound onto the reel). The line guard should not be removed because it stops material being flung back at the operator's legs and because it carries the cutter ensuring that the line length is not too long for the trimmer. When using a disk the operator can easily overload the driveshaft and damage the machine or strip the gearbox gears.