Super Precise Weed Trimmer

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Weed trimmers cause more heartburn than any other garden tool. Depending upon centrifugal force (CF) to extend the filament when you bump the head on the ground, if you let the string get too short, there isn’t much mass for the CF to work on. Also, as dirt and grass gets into the head, you have to rev up the head faster and faster to provide enough force to pull out the string, making it more likely you will get the filament jammed up between coils on the spool where no amount of speed can get it to come out.

We’ve all done it; and we see homeowners doing it all the time. You idle the engine, grab the head with one hand while pushing the button, and pull out the string with the other hand. If only there was a way to pull on the string directly while the head is turning.
Well, there is now.

The first image shows the Angular Motion Translator (AMT) concept, Patent US06547689. It consists of mirrored planetary gear sets with the planet gear carriages connected together. The single input shaft drives the first sun gear and is the center part of the output. The two annulus ring gears are held still until you want to translate an angular motion. While the two annulus gears are held still, the input sun gear drives the input planetary gears against the input annulus gear. The rotation of these planetaries will be delivered to the output planetaries through their connected carriages. The output planetary gears work against the static output annulus to drive the output sun gear with its output sleeve. Under these conditions, the two parts of the output shaft will turn in unison, at the same speed.
When the output annulus gear is rotated, this motion is added to that from the planetaries and the output sleeve will rotate relative to the output shaft, in direct proportion to the change in the annulus gears, all with a direct mechanical connections.

The second image shows the AMT incorporated built for a weed trimmer head, while the third image shows the complete head assembly.

In the patent, the AMT is used to translate a force and motion directly to the string by driving feed capstans, all while the head is rotating under power. The output sleeve drives the body of the head; the output shaft drives the feed capstans through a series of gears. With no rotation of the input mechanism, the capstans don’t turn. But when its rotated, the capstans rotate; feeding out filament in direct proportion to the amount the input is turned. Centrifugal force has nothing to do with it. No more heartburn. No more expletives.


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  • Name:
    Paul Baker
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