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/sci/ - Science & Math

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1894420 No.1894420 [Reply] [Original]

Not really sure where to post this, so please forgive me if I'm in the wrong place.
Like the good little geeks we are my friend and I are building trebuchets and mangonels.
We started small, but the latest model, while still table-top sized, is quite powerful.
Now we are trying to think of a good release mechanism, at this time we have a 12 mm steel rod we yank out, but the force is so big that we need to bolt the thing down, we can pick it up just holding the release pin and it won't move.

So, any idea's on how to construct a safe and reliable release mechanism?
Thanks in advance.

>> No.1894449

I made a 1.5m onager once. Shit was so cash :)

No idea on the release mechanism though, I only ever did it by hand.

>> No.1894454

By the way, I think your picture shows an onager, not a mangonel (powered by rope bundle, rather than essentially a large crossbow). Then again, nobody seems to agree on the terminology involved anyway.

>> No.1894495
File: 120 KB, 800x531, p6170030.jpg [View same] [iqdb] [saucenao] [google]
1894495

>>1894454
So it seems.
Onager/mangonel/scorpion is more or less the same thing, where one has a sling and the other doesn't. Pretty sure the scorpion is (one of) the name(s) of the sling version, since it has a pin on the arm which releases the sling, which makes the arm look kind of like the stinger of a scorpion.

The large crossbow you're referring too is the ballista, which is also powered by torsion by the way (see image, not ours this time, lol). We also build that one, but overdid it (like we usually do) and haven't figured out a way to actually 'load' it. I don't know the correct word for it, the act of bringing the arms backwards so it can actually be fired.
Anyway, the torsion on it, using just standard nylon rope, is so high we can't even get the arms halfway down.

>> No.1894507
File: 4 KB, 332x232, Catapulta_DER_1962.png [View same] [iqdb] [saucenao] [google]
[ERROR]

>>1894495
I mean like this; the device isn't a large crossbow, it's powered by one.

>> No.1894511
File: 4 KB, 332x230, Catapulta_DER_1962.png [View same] [iqdb] [saucenao] [google]
1894511

>>1894507
Stupid broken pictures.

>> No.1894535
File: 69 KB, 800x600, DSC00958.jpg [View same] [iqdb] [saucenao] [google]
1894535

>>1894511
Oh, heh. Never seen one of those before.

Pic is pre-pimped version of >>1894420
Could shoot a marble or lead ball well over 60 meters.

>>1894420
Has more rope, more torsion, longer arm, revised bucket, baseplate with holes so it can be pinned down, and changed charging/release mechanism. Now it scares us.

http://www.youtube.com/watch?v=JqoIMZcVNxE
240fps if I'm not mistaken, you can see we need a better release mechanism, the torsion is so high the pin is -really- tight

>> No.1894551

>>1894535
Nice! I never could make mine strong enough (the rope bundle pulled the whole base structure inward, greatly reducing its strength). I probably used completely wrong types of wood and stuff like that, I'm a noob with these things.

>> No.1894566

>>1894420
a ratchet would be easier to release, and easier to crank

>> No.1894585

>>1894551
My best friend is the handyman, he has all the tools and skills, I'm more of an assistant, heh. We used 20-something millimeter thick multiplex. For winding the rope we had 6mm steel rods first, but they just bend inwards, the power amazed us.

>>1894566
We now -need- a ratched to crank it down, or I need to put my whole weight down on it, which is quite the risk, this thing is now strong enough to break bones.
But how'd you go about releasing it using a ratchet..?

PS thanks for the replies so far!
/k/ rages about it being toys, and /toy/ ignores me most of the time...

>> No.1894602

>>1894585
>/k/ rages about it being toys
Heh, that reminds me that catapults like these are actually considered heavy weapons under my local weapon regulations, similar to a flamethrower or mortar. Strictly speaking, it was very illegal for me to have one.

>> No.1894609

>>1894585
In principle you release a ratchet like you release anything. The point is that a ratchet has a much smaller surface in contact so it is easier, mechanically, to release. Ever see those ratcheting straps to secure equipment in transport?

>> No.1894612

>>1894585
I think I used an M12 bolt to wind the rope around. The thread made sure the bolt wasn't going anywhere.

>> No.1894642
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1894642

>>1894602
Haha, that's awesome!
I bet /k/ would piss itself over regular catapults too, even though they can easily be lethal. Some can pierce a coconut which is how much? 2x or 3x as tough as a human skull?

>>1894609
Yes, but that doesn't sound safe or reliable/easy to use. Or I'm missing something.

>> No.1894759

Bamp.

>> No.1894793

lol, attach linear bearings, that would make it easier to slide out the rod, on the downside it will also increase the cost of the project by 200$-300$.

So that's probably not a good idea.

A typical mechanism is an L shaped hinge "thingy" which attaches to a eye bolt on the throwing arm, you pull the L unhooking it from the eyebolt to fire.

>> No.1894862

>>1894793
That's more or less what we do now, and it needs a lot of force to be pulled out, what we're looking for is more like, pull lever with < 1kg of force, and have that trigger the whole thing. Right now we need to apply more like 5kg of force to remove the pin.
Might make the whole release mechanism out of rounded metal and apply wd40 all over it.

Thanks for the input!

>> No.1894869

>>1894793
Also lol $300. So far all it cost us was maybe $20 worth of material, we got the wood for free. Did spend over 10 working hours on it all together though.