Someone Built an Engine Crane Out of Wood. Let's Do Math to See If It's Safe

https://jalopnik.com/someone-is-fixing-their-car-using-a-homemade-wood-engin-1846550068

by NaOH • 5 years ago

27 18 5 years ago

jalopnik.com

kumarvvr• 5 years ago

As an engineer (electrical at that), there is no point in doing any math. That thing is dangerous.

Any math to calculate resultant stress is meaningless, because we have no data on how uniformly dense the wood is.

Not to mention it looks flimsy to the point of being cartoonish.

wmoser• 5 years ago

Ok, I glanced at the article and as another engineer, yes it looks sketchy and probably isn’t safe (applying the rule from rigging of: if something doesn’t look right it probably isn’t). I felt the need to address the other issues you brought up about being impossible to calculate because it’s wood. Wood is the original composite material and has been used to safety build bridges, buildings, ships etc. For the most part your calculations won’t be as exact as with metal or other engineered material but there has been lots of research into this. The US Department of Agriculture/ US Forest Service even publish a free book about it. [1] The lumber looks like it was bought from a home center and so likely has minimum strength expectations [6-3 of afore mentioned book].

I just felt the need to dispel this rumor that is quite dangerous that I see all the time at work where people use 1/4 wire rope slings with unrated hardware as part of rigging instead of 1/2 inch Manila rope because “That rope doesn’t have any rating, it’s natural”. (It does, about 1/2 the wire rope, but needs knots or splices to attach it to thing being lifted, not the unrated hardware that comes with the wire-rope.) Like someone posting further down, I’m more immediately concerned that there is no brace to prevent racking or the use of ratchet straps as a lifting device.

[1] https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr11...

js2• 5 years ago

The author is just having fun:

This whole article is a largely pointless exercise in figuring out how strong some random dude’s wood crane is, even though I have no clue what the load case is.

LorenPechtel• 5 years ago

All the numbers he worked are far within the strength limits, the imperfect measurement of wood strength doesn't matter.

What he showed is the crane is plenty strong enough to hold the engine--the failure mode is tipping, not strength.

K2h• 5 years ago

4 vertical 2x4. If they put some lateral cross bracing to keep from going all rhombus it will work. How many sticks do you think are in a wall?

howLongHowLong• 5 years ago

In the US, there is typically a vertical stud every 16 inches, and no lateral braces at all, except for windows and doors, and where dimensional lumber terminates. There's lots of lateral support from ceiling, siding, etc, but the question seemed to imply there aren't many vertical sticks in a wall. Apologies if I misunderstood.

Gravityloss2• 5 years ago

Good quality wood has very good bending strength for mass. The structure just needs some depth, like there is here.

That's what trees evolved for.

snypher• 5 years ago

By depth, do you mean cross bracing? Any discussion about lumber strength is beside the point, the fasteners will pull out as soon as it goes sideways.

Gravityloss2• 5 years ago

By depth I meant thickness. But thickness sideways doesn't help nearly as much as thickness in the vertical direction.

A beam with a bending load like here can crack from the bending at the middle for example. The fasteners at the ends are not necessarily the limit. It's different for pure compression or tension, there the fastener area sees higher load.

mikewarot• 5 years ago

There are zero triangles in that structure, it'll fold like a cardboard box.

mindslight• 5 years ago

This. This is the answer.

Compression load on the vertical members? Who cares.

Strain on the long cross member? Wood fails gracefully and you'd hear cracking.

Forward-backward stability? Probably okay if those joints are screwed really tight and ideally wood glued.

Side-side stability? Good bye!

rubyn00bie• 5 years ago

IMHO, the writing style of this article is horrible. It was truly a painful chore to read and probably the least satisfying drivel I’ll read all day.

greatgib• 5 years ago

I totally agree! Impossible to read with an ad break every paragraph.

In addition, if doing such a long and detailed article, the author could have spent a few minutes to draw on computer clean version of his schematics. Instead of unreadable pictures of hand drawn thing.

wrycoder• 5 years ago

The whole point was to get scrolls past all those ads. It was a listicle.

OnlyOneCannolo• 5 years ago

Fun article to read. The author started to get into the more likely failure modes at the end when time ran out.

Stability is probably the most important. Next is failure of the wood near the screws, which affects stability. Wood has its own considerations due to the grain. The American Wood Council has a fastener calculator which shows a few of the things that matter when doing structural design with wood.

http://www.awc.org/calculators/connections/ccstyle.asp

thiagocsf• 5 years ago

The imperial units used in the calculations made my brain hurt.

On a positive note, I suspect Americans are a lot more proficient with fractions than people who grew up learning metric.

pengaru• 5 years ago

This is why I will never own a Quattro vehicle.

Every service involving the engine begins with something like:

  Step 1: remove headlights and front bumper
  Step 2: remove myriad heat exchangers
  Step 3: remove engine

So of course you're looking at an Audi Quattro platform getting its engine pulled in a parking space. I wouldn't be surprised one bit if this was all happening just to replace an inaccessible $100 part.

djmips• 5 years ago

The author also neglected the extras that would needed to be lifted including (usually) the transmission attached to the engine (another 200 lbs right there)

Someone Built an Engine Crane Out of Wood. Let's Do Math to See If It's Safe

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