String trimmer
Encyclopedia
Monofilament line
Monofilament fishing line is fishing line made from a single fiber of plastic. Most fishing lines are now monofilament because monofilament fibers are cheap to produce and are produced in a range of diameters which have different tensile strengths...
instead of a blade for cutting grass
Grass
Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...
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
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the 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.
History
The string trimmer was invented in the early 1970s by George BallasGeorge Ballas
George Charles Ballas was an American entrepreneur. He invented the first string trimmer, known as the Weed Eater in 1971...
of Houston, Texas
Houston, Texas
Houston is the fourth-largest city in the United States, and the largest city in the state of Texas. According to the 2010 U.S. Census, the city had a population of 2.1 million people within an area of . Houston is the seat of Harris County and the economic center of , which is the ...
, 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.
Designs
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. Recently, lithium ion batteries have been able to improve runtime.
Gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...
-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. 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 user fatigue
Muscle fatigue
Muscle fatigue is the decline in ability of a muscle to generate force. It can be a result of vigorous exercise but abnormal fatigue may be caused by barriers to or interference with the different stages of muscle contraction...
, as well as the requirement that motor oil
Motor oil
Motor oil or engine oil is an oil used for lubrication of various internal combustion engines. The main function is to lubricate moving parts; it also cleans, inhibits corrosion, improves sealing, and cools the engine by carrying heat away from moving parts.Motor oils are derived from...
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.
Operation
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 the string tension that exerts the centripetal forceCentripetal force
Centripetal force is a force that makes a body follow a curved path: it is always directed orthogonal to the velocity of the body, toward the instantaneous center of curvature of the path. The mathematical description was derived in 1659 by Dutch physicist Christiaan Huygens...
that prevents the string from flying off in a straight line under its own inertia
Inertia
Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...
. 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. 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 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
Honda
is a Japanese public multinational corporation primarily known as a manufacturer of automobiles and motorcycles.Honda has been the world's largest motorcycle manufacturer since 1959, as well as the world's largest manufacturer of internal combustion engines measured by volume, producing more than...
, MTD
MTD Products
MTD Products is a manufacturer of outdoor power equipment for the mass market. Based in Cleveland, Ohio, USA, the company began in 1932 and is a family-owned, private company. It originated as a tool and die maker...
and Craftsman
Craftsman (tools)
Craftsman is a line of tools and lawn and garden equipment controlled by Sears Holdings Corporation; the brand is owned by KCD IP, LLC, a special purpose entity created by Sears Holdings for securitization purposes....
manufacture a four stroke engine trimmer. Other companies, such as John Deere
John Deere
John Deere was an American blacksmith and manufacturer who founded Deere & Company, one of the largest and leading agricultural and construction equipment manufacturers in the world...
, now carry Low Emission two stroke engine trimmers. Stihl
Stihl
Andreas Stihl AG & Company is a manufacturer of chainsaws and other handheld power equipment including trimmers and blowers. Their headquarters are in Waiblingen, Baden-Württemberg near Stuttgart, in Germany. The company was founded in 1926 by Andreas Stihl, an important innovator in early chain...
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.
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 and also a quick charge options that cuts charging times between 30 minutes and 2 hours.
Environmental impact
The typical two-cycle engine used on string trimmers pollutes heavily as it does not burn the fuel completely in the cylinder. This results in unburned fuel escaping through the exhaust system and into the environment. US emission standardUS emission standard
In the United States, emissions standards are managed on a national level by the Environmental Protection Agency . State and local governments play a subsidiary role.- Motor vehicles :...
s 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.
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.
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 trimmerHedge trimmerA hedge trimmer, shrub trimmer, or bush trimmer, is a gardening tool or machine used for trimming hedges or solitary shrubs...
. - replacing the lower shaft with a cultivatorCultivatorA cultivator is any of several types of farm implement used for secondary tillage. One sense of the name refers to frames with teeth that pierce the soil as they are dragged through it linearly. Another sense refers to machines that use rotary motion of disks or teeth to accomplish a similar result...
.
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.