My old friend Jeff was always vocally upset that he didn’t come up with the idea of a string trimmer, commonly known as a Weed Eater or Weed Whacker. On the one hand, the idea is totally simple: spin some nylon line and cut grass and other relatively soft things. But, it turns out, that making the device actually usable requires a little bit of mechanical engineering.

Of course, the noisy part is a motor. The motor — driven by an engine, a battery, or a power cord — spins a flexible nylon line fast enough that the line becomes rigid from centrifugal force. That’s not the important part.

The humble spool at the bottom of the trimmer is where decades of mechanical engineering, questionable patents, consumer frustration, and genuine cleverness all meet. The earliest string trimmers were primitive. [George Ballas], who patented the Weed Eater in the early 1970s, reportedly got the idea from the rotating brushes in a car wash. Attach flexible cords to a spinning head, and they become cutting tools. In fact, the prototype used a tin can for the head. Elegant. But once the line wears down — which it does constantly — you need a way to expose fresh line. That turns out to be harder than it sounds.

The Simplest System

The easiest approach is fixed-length line. Some trimmers still work this way. You cut short pieces of heavy line (or buy it precut) and insert them into holes in the head. No spool. No springs. No moving parts.

These systems are rugged and are popular on commercial units designed to survive abuse. They also work well with thicker lines or even plastic blades. But they are annoying because every time the line wears out, you stop working and manually replace it. Spool-based systems became dominant very quickly.

The basic spool idea is straightforward enough. Wind a long nylon filament onto a reel. Some reels have two sections to feed line out on two sides of the rotating head. As the line wears away, feed out more line from the spool. But how do you do that while the thing is spinning at several thousand RPM?

Continue reading “Tech In Plain Sight: The Mechanics Of String Trimmers” →