# How the String Trimmer (Weed Eater) Actually Cuts Grass > This 1974 patent describes the mechanics of using a high-speed spinning plastic line to cut grass, replacing dangerous metal blades with flexible, non-metallic material. - **Patent:** US 3826068 - **Original title:** Rotary cutting assembly - **Owner:** Individual - **Granted:** 1974 - **Status:** Public domain (expired) - **Times cited:** 86 - **Field:** consumer_electronics, mechanical ## What it does The patent defines a rotary cutting tool that uses a flexible, non-metallic line to trim vegetation. The core mechanism involves rotating this line at a specific tip velocity, calculated based on the line's thickness (diameter), to ensure it cuts grass effectively without being dangerous to the operator. The apparatus includes a housing that stores the line, allowing it to extend through an aperture while spinning. By controlling the relationship between the line's diameter and its rotational speed, the device achieves enough energy to slice through plant stems while remaining safer than a rigid metal blade. ## What it does NOT cover - Does not cover cutting tools that use rigid metal blades or discs. - Does not cover systems where the line is not flexible or non-metallic. - Does not cover rotation speeds that fall outside the specific mathematical range defined by the line's diameter. - Does not cover manual or non-rotary cutting tools. ## The clever bit The inventors realized that if you spin a flexible plastic line fast enough, it gains enough stiffness and kinetic energy to act like a blade, but because it is flexible, it deflects upon hitting hard objects like rocks or fences instead of shattering or causing severe injury. ## Real-world examples 1. Weed Eater string trimmers 2. Electric grass trimmers 3. Gas-powered weed whackers 4. Commercial landscaping edgers ## Why it matters This invention is the foundation of the modern string trimmer, commonly known by the brand name Weed Eater. It transformed residential landscaping by replacing heavy, dangerous metal-blade mowers with a lightweight, safer alternative that could reach tight corners and edges. It is a classic example of how a simple material change—plastic line instead of steel—can create an entirely new consumer product category. ## Frequently asked questions ### What does How the String Trimmer (Weed Eater) Actually Cuts Grass cover? This 1974 patent describes the mechanics of using a high-speed spinning plastic line to cut grass, replacing dangerous metal blades with flexible, non-metallic material. ### Who owns patent US 3826068? Individual owns this patent, granted in 1974. ### When does this patent expire? This patent has expired and is now in the public domain — anyone can use the invention freely. ### What is patent US 3826068 cited by? This patent has been cited by 86 later patents that build on its ideas. ### What problem does this patent solve? This invention is the foundation of the modern string trimmer, commonly known by the brand name Weed Eater. It transformed residential landscaping by replacing heavy, dangerous metal-blade mowers with a lightweight, safer alternative that could reach tight corners and edges. It is a classic example of how a simple material change—plastic line instead of steel—can create an entirely new consumer product category. ### What does this patent NOT cover? Does not cover cutting tools that use rigid metal blades or discs. **Full plain-English explainer:** https://patentbrief.org/patent/us/3826068/weed-eater-string-trimmer **Original patent:** https://patents.google.com/patent/US3826068 --- _Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._ ## Related patents Semantically similar inventions in the PatentBrief corpus: - [How King Gillette Invented the Modern Disposable Safety Razor](https://patentbrief.org/patent/us/775134/safety-razor-gillette) — King Gillette's 1904 patent for a safety razor with a thin, replaceable, double-edged blade that changed how the world shaves. - [How the Hula Hoop Works](https://patentbrief.org/patent/us/3079728/hula-hoop-wham-o) — A 1963 patent for a lightweight, rigid plastic hoop designed to rotate around a human waist through rhythmic body movements. - [How the Modern Paper Drinking Straw Was Invented](https://patentbrief.org/patent/us/375962/drinking-straw-stone) — A 19th-century patent for a paper tube coated in wax, designed to replace natural rye grass straws for drinking beverages. - [How Modern Rollerblades Became Adjustable and Interchangeable](https://patentbrief.org/patent/us/4492385/inline-skates-rollerblade-olson) — A 1982 patent describing a skate design that allows users to swap between wheels and blades and adjust their position on the boot for better performance. - [How the First Cotton Swabs Were Mass-Produced](https://patentbrief.org/patent/us/1721815/q-tip-cotton-swab) — Leo Gerstenzang's 1929 patent for the automated manufacturing of cotton-tipped applicators, the invention that created the modern Q-Tip.