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Comment by pjkundert | Hacker News Reader

No kidding; when he was cranking the final few inches of tension, I wonder if he realized what would happen if anything snapped: instant death was definitely one of the potential outcomes.

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Well at that point he did say "One of the things I love about working with wood is all the menacing creaking noises it makes when you load it. Really makes you appreciate how precious your fleeting life is" so I think it's safe to say he realizes this.

The proper response to that realization is to unwind the tension, not to keep pushing. Really, he just got lucky, but that's fine, it's an interesting video. I take risks myself when building stuff but I would not trust my life to something that isn't engineered for strength and in spite of all of the work that went into the computations of how to get the projectile up to speed as well as the strength of the arm there is already some proof that he misjudged how strong wood is for this application (one of the arms desintegrated).

Wood is super strong, if used properly, this looks like a setup similar to the neck of a base guitar but without the engineering of the joint to go with it, and a much nastier angle between the direction of pull and the joint.

He spent all that time engineering the speed, he forgot to engineer the base.

Safety glassed would have been a good idea, any rubber can suddenly fail. But he is absolutely right that "creaking noises" are common in wood far far far below their breaking point. I suspect he calculated the strength of all these things if he went to that much trouble for the sling. Also the markings on the bolt heads showed they were grade 8 (those 6 radial lines) which are near the best common off-the-shelf bolt strengths you can get. The OG wooden sling breaking was likely a nuanced issue stemming from it's rotational speed, so not a risk during the cranking stage. The sling wasn't even under load at that point. High-speed rotational effects are many orders of magnitude harder to predict than the needed strength of a base structure. But yes, safety glasses would have been wiser.

I don't think it's the bolts shearing we should be concerned about. Rather, the wooden beams might be split in the middle by the torque on the four bolts.

This guy seems like he's probably a very qualified engineer, yes he should be wearing some safety equipment but he also seems like he knows how close he is to the limits of the device far far more accurately than the shade tree maker.

He may very well have designed in a 50% or more safety margin on the frame.

Exactly. It would be interesting to hook the whole thing up to an electric motor and to winch it (from a safe distance) to the point where it breaks just to determine how much margin there was. Probably not a whole lot past the point that he hand cranked it.

That would be extremely interesting. Would be great to put a dummy where he was squatting in the crank position while you did it too.

I think it makes sense for the project to be powered by human energy.

I guess you could power the motor by an alternator+crank :)

Reminded me of one of Ben Krasnow's videos where he ran a hydraulic cylinder out of spec and caused it to bulge noticeably. Um, dude, there's probably a few sticks of dynamite worth of energy in that thing, you know...

Without knowing the specifics of the incident:

There is actually very little stored energy in a hydraulic system, due to the incompressibility of the fluid. Hydraulic injection injuries[0] are nasty, but catastrophic failure isn't particularly dangerous. Pneumatic systems on the other hand can reach dynamite energy levels and near-enough dynamite energy discharge rates.

[0]A not-for-the-faint-of-heart google.

Hmm, that sounds like a good (if unintuitive) point. I withdraw the criticism. :)

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