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.

What it covers

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 doesn't 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.

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.

Real-world examples

• 1.Weed Eater string trimmers
• 2.Electric grass trimmers
• 3.Gas-powered weed whackers
• 4.Commercial landscaping edgers