2.77 Seek and Geek #3: Hi-Lok Flush Aircraft Fastener

Seek and Geek #3: Hi-Lok Fastener

My term project for the class involves building a machine whose eventual goal is to install fasteners along the inside of an aircraft (shown above). Thousands of these fasteners connect the big single-piece composite barrel (skin) to the shear ties (latitudinal ribs) that are hoop-shaped L-brackets. 

The holes for each of the thousands of fasteners are drilled, reamed, and countersunk from the outside. The fasteners are then inserted from the OUTSIDE and are perfectly flush with the outside surface of the aircraft, with no features to hold onto. The rest of this job is done on the inside. 

The Hi-Lok fastener has a hex socket on the inside, as well as threads on the outside. By adding a nut and tightening it while using a hex key to hold the fastener in place, the fastener can be tightened completely from only the inside, while the outside, with no mating features whatsoever, remains flush for aerodynamics. 

This is the pneumatic fastening tool used to tighten the bolt onto the fastener from the inside. These are handheld, as most of the fastening is currently done by human workers, though the company has been working on making them friendly to mount on robots with air tool controls. 

One great feature of this system, a way to get precision by design and not by having to torque-control the driver tool, is the frangible collar, or breakaway nut. Once the appropriate fastening torque has been achieved, the nut will separate from the threaded collar to prevent overloading. This is freaking brilliant! When you have a human worker fastening hundreds of these at a time, they get tired or lazy, and it's easy to just beast each one with the air tool until it breaks off when the precise torque has been reached, without needing to actively feel how tight your torque wrench

So, looks kinda like my whiteboard drawing. It seems the remaining collar has no features, meaning it can't be taken out, so it's semi-permanent. I wonder if they use an epoxy as a threadlocker, or if the breakaway nut permanently deforms in such a way that the collar cannot loosen from vibration. Food for thought. 

2.77 PUPS #2: Planar Constraint

PUPS #2: Planar Constraint

Peer reviewed with Kate's comments:

MATLAB Script output. The slope changes as a function of the constraint location. 

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.