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Comment by jacquesm | Hacker News Reader

At 13:12 in that video notice the tension in the frame and how the main spars are getting out of alignment with the part the winch is mounted on. I wonder how much spare strength there still is but it can't be much. Pretty scary machine!

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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.

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 :)

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.

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. :)

I don't understand why he isn't wearing eye protection during the whole experiment. Made me incredibly nervous. Obviously won't protect you from a direct hit of the main projectile, but there are so many things that can go wrong unpredictably, your eyes are the squishiest part, and protection is easily available and not too annoying.

Yep, that was pretty terrifying to watch.

Indeed, I wasn't comfortable watching it until I realized that he was the one that posted the video. But instant desintegration was definitely on the menu there and once stuff comes apart and starts flying it's impossible to predict trajectories. That's a very large amount of energy on what looks to be two run-of-the-mill boards, they're fortunately under compression but the joint at the bottom looks (far) less than ideal for this, especially with the holes drilled in line with each other (that's a definite 'no' if you want to avoid splitting the beams).

A++ for the theory, but the implementation is not something I'd want to be near when it is in operation, it is just too unsafe judging by my somewhat experienced DIY eye.

I'm sure the guy is aware of how much stress he's putting the wood under, and about how much it can withstand.

when I was a kid, my dad made an engine hoist frame out of wood. that thing creaked out some scary sounds when he lifted whole diesel trucks with it when load testing, and it didn't even faze the hoist frame at all.

it sounded like it was going to explode into a billion supersonic toothpicks and didn't even visibly move. we painted red lines across joints to see stress movement more easily and nothing ever moved.

his material of choice? 4x4 construction lumber.

it is amazing what wood can withstand when you understand what you're asking it to do, and design so that material properties are fully exploited.

my point: loud creaking is not necessarily indicative of impending failure.

Not necessarily, but it is the only warning you get.

That's true, but you also can't know every possible point of failure of an apparatus under load just from design principles. That's why they flex the wings of new airplane designs until they break, for instance. So I expect testing under extreme tension while squatting right beside the thing without so much as a pair of safety goggles is taking unnecessary risk. I'd go for a heavy coat and helmet with face shield at least, if I needed to be that close. (But I'd rather find a way not to be.)

I'm quite aware of what you can and can not do with wood, unless he's tested one to destruction the degree of variability alone makes this a particularly dangerous stunt.

It all looks fine right until it doesn't.

> I'm quite aware of what you can and can not do with wood

yes, I'm sure you are. you always seem to know everything about everything when it comes time to argue.

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