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How Does a String Trimmer Work?

By Tracy Morris ; Updated September 21, 2017

String Trimmer Basics

Trimmers are hand-held gardening tools you can use to trim grass, brush and weeds in places a lawnmower can’t reach. String trimmers are known across the country by various brand names, including Weed Eater and Weed Whacker. These trimmers use a monofilament line, similar to fishing line, that swings around a central head at a high speed. This swinging line can cut the soft tissue of grass without damaging brick or other hard or ornamental surfaces.


The idea behind the string trimmer is that the monofiliment line, when turned quickly enough, becomes stiff through centrifugal force. The faster the line turns, the stiffer it becomes until it is hard enough to cut grass and soft, woody plants.


A string trimmer consists of a trimming head, a shaft that houses electrical wire and has attached handles and a motor. Although some trimmer motors are electrical and may have an electrical cord attached, most forgo the tethering effect of an electrical cord and instead use an internal combustion engine. The most common type of internal combustion engine on string trimmers is a gasoline engine.

Electrical vs. Gas Motor Trimmers

Electrical string trimmers may have the motor in the head, while gas-powered motors are at the other end of the shaft from the string trimmer’s head. Electrical string trimmers are less powerful than their internal combustion-engine powered cousins. Typically, electrical engines are limited to a 2.5mm diameter string because they can only pull about 1200 watts of electricity. However, electrical string trimmers are lighter and quieter and require less maintenance. Newer electrical trimmers no longer require cords and instead are battery operated. However, these tend to be the most expensive trimmers. Additionally, they can only run for 15 to 20 minutes before requiring a recharge. Cordless electrical trimmers are also heavier than their corded counterparts, and can only be used for light trimming. By contrast, gas-powered trimmers are more powerful, and can cut woody brush and tall grass. Gas trimmers can also be used for edging by rotating the head of the trimmer. However, gas trimmers still create emissions due to their engines. Newer models create fewer emissions, but they are not yet emission-free.


About the Author


Tracy Morris has been a freelance writer since 2000. She has published novels and numerous online articles. Her work has appeared in national magazines and newspapers including "Ferrets," "CatFancy," "Lexington Herald Leader" and "The Tulsa World." She holds a Bachelor of Arts in journalism from the University of Arkansas.