String trimmer
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- Weedeater redirects here, for the band see Weedeater (band)
A string trimmer, also called a line trimmer, Weedeater (a brandname), Weedwhacker(a brandname), Weed Whip, weedy, whipper snipper, "Strimmer" is correctly a tradename, but it has slid into British English use as a genericized trademark and spawned a back-formed verb "to strim" = "to use a string trimmer on".</ref> garden strimmer 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.
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.
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[edit] Designs
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 (0.100 inch) maximum diameter nylon because of their lower power output (400 to about 1200 watts). 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.
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, uses a gasoline-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. These very large trimmers are often referred to as brush cutters.
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] Operation
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. <How do they work? 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 and MTD 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 with no oil resivior. 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 on
[edit] Safety
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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. Closed toed shoes and long trousers 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 for use. 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.
Using a string trimmer in areas where there is poison ivy or similar plants can result in the operators' legs being sprayed with irritant sap. Even if the legs are adequately covered, there's a risk of subsequent exposure to the harmful oils, for example, when disposing of the waste plant material or when cleaning the trimmer.
Gasoline powered units tend to introduce significant amounts of vibration to the operator which can produce discomfort and potential health risks. Professional trimmer models, used by commercial services, typically have anti-vibration features incorporated which significantly reduce the vibration effect. Electric and battery powered units are immune from vibration effects.
[edit] Environmental impact
Pollution from gas-powered groundskeeping equipment is a significant source of air pollution.[1] US emission standards specifically limit emissions from small engines. Electric models produce no emissions at the point of use, but may shift pollution to power plants. Emissions may still be reduced by the use of renewable energy in grid generation, or because central power plants generally must have stricter emissions control equipment installed. Battery powered units typically utilize large sealed lead acid or nickel metal hydride batteries, which will wear out after a few years of use and must be disposed of properly.
[edit] String breakage
One type of gross string breakage occurs when a segment, measuring back as little as an inch from the free end of the string, curves around a narrow fixed obstacle during higher speed operation. The term whiplash could be applied here, and as a function of time and stress, the elastic limit is exceeded at a sharp bend in the string, resulting in the initiation of a fracture. Segments that break off this way have a long smooth fracture angled towards parallel with the string. Care in operation can minimize this kind of breakage.
[edit] Accessories
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 are inadequate in generating enough power.
