21 February 2016

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. 


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