13 February 2016

2.77 Seek and Geek #2: Branch Cutter

Seek and Geek #2: Branch Cutter

I found this garden cutter in my living group's backyard and it has a few design features that I find interesting.

It claims it has Power-Lever® Technology, and cuts 2X easier. I ask, 2X easier than what, exactly? A normal lever? We'll find out!

The handles have their own labels to show off their ergonomic design, presumably to catch the eye of the passerby at Home Depot. 

The handles are indeed ergonomic, as the whole thing was comfortable to hold with one hand (My hand was constraining forces and moments and still felt comfortable! )

This can cut branches up to 1.75" in diameter. 

A more detailed look at the mechanism at the end shows off this Power-Lever® Technology. There are three linkages in this system: the left handle/right blade, the left blade shown in black, and the right handle. There is a pin joint holding the left blade and the left handle/right blade together, and a pin joint holding the left handle and the right handle together. The left blade and the right handle are connected with a slotted prismatic joint, which I believe is the cause of this 2X Easier claim. 

Here is an underside view to get a better look at the slot. This is a planar three-bar linkage, with two pin joints (each allowing 1DOF: rotation) and one prismatic joint (allowing 2DOFs: translation in one dimension, and rotation). This resultant mechanism has only 1-DOF, coupled through all 3 linkages. As the handles close together to cut, the right handle linkage (shown here on the left) pivots about its joint with respect to the left handle/right cutter linkage (shown here on the right). The left cutter (in black) is pulled along for the ride through the slotted pin joint with the right handle, but is able to slide along the pin instead of moving completely.

Through this nonlinear motion, mechanical advantage is achieved, and the black left cutter ends up moving about half as fast as the handles do with respect to each other, like gearing. And, like gearing and because power is conserved, this offers about 2X the cutting force as you would have with a regular scissor-type linkage. Cool! 

In addition to the linage design offering mechanical advantage, the leverage you can get on this thing from the grips is quite large, and the ellipsoidal tubes are quite stiff while beasting this thing against a tree branch. It feels solid and sturdy, even though there is a length of about 18" from your hands to the pin joints. 

The pin joints are held down and supposedly preloaded by nylon locknuts. 

Here's a closer look at the chopper. The pressure area on the black left cutter is quite small considering the mechanical advantage this tool provides, but steel is used for the blades, which can handle all that force. The branch will be effectively sheared by this cutter. 

The other side, the right cutter, does not have a small pressure area, but is flat in order to provide a groove in which the branch can sit as it's being cut. 

The difference in pressure area between the black left cutter and the right cutter ensures that only the black cutter is seeing the highest stress, and so if one were to sharpen this tool, only the black one would need TLC. 

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