Thursday, November 14, 2013

A Practical Guide to Amateur Punkin Chunkin

This fall I decided to host a punkin chunkin event at my church, through my mens ministry, the Band of Brothers.  Well, it was quite a success, to the surprise of anyone who thought about that sort of thing.  
We had 4 devices.  The one that shot the farthest was a 10" ID air cannon.  It shot about 300 feet.  The second farthest was a trebuchet with a 10 foot arm (between pivot and knock) with about 400 lbs of weights on it.  
Unfortunately, I made an onager, and that shot about 20'.  Backward.

My onager was small, light, and easy to assemble/disassemble.  It was tethered to the ground at the front supports.  The lower ground support rails extended forward three feet, so it would keep the onager from rocking in either direction, fore and aft.  

Punkin Thrower Selection

First, decide how far you want to throw a pumpkin.  Assume it weight 8 lbs.
Depending on that, you can select a design.  Also take into consideration how many people will be helping you build and test the device.

1.  Air cannon

These are the easiest to build, if you have the materials on hand.  I would only do this if you have or can borrow a compressor, large pipe, and air tank.  If you have all that, go for it.  One person can make a cannon that will shoot 300 feet.  Any larger than that, and you'll need a helper.  The larger the device, the more helpers you'll need.  

2.  Trebuchet

This is something you should do with at least one other person - for a pumpkin.  If you are just making one to throw a small ball or stone, then you can do it yourself.  But if you want to throw a pumpkin, you'll need at least one other person.  
For one the size as the one in the pictures I have linked to,  then get two people to help you.  For a trebuchet with a 15' to 24' arm, get a 4 person team.  For a 25 to 35 foot arm, get a 5 person team.  Larger than that, get at least six people who are dependable.  Also, make a Floating arm trebuchet if you can, they are the most efficient kind.  

3.  Onager

First, I would recommend that unless you want to spend a couple hundred bucks, and have someone on your team, don't even consider an onager the size of the one I did.  If you want to throw a baseball 300', then you can make an onager yourself.  It can at most be 3' wide with a 4" diameter skein.
This is not good enough to throw a pumpkin. If you really want to launch a pumpkin, you'll need an onager at least 4' wide, with a 6" diameter skein.  
Make the onager so you don't have to screw it to the ground - there are too many rocks in the ground, and it adds to the time you can't spend with your friends.  
You have to make the modiolus line up with the hole in the board, otherwise the modiolus will move too much.  Also, its best to spend time and effort designing it to be easy to rotate.  By hand is fine, but it needs to be quick and easy.  
I suggest making a two piece modiolus like that on the mister ballista or chunk norris.  it has a smaller gear with a crank you can put it.  I suggest using the gear arm as a secondary modiolus stopper, and adding a primary one as a flat toothed piece that fits in a built up area on the scutula hole carrier (frame) and locks with the teeth. 

Don't use sheathed rope for the skein, such as this:
It got some really loose skein strands after a few throws.  But more importantly, it had a bad power curve. 
This rope was cheap, but its not true rope.  Its sheathed cigarette filters, except the filter material is polyester.  The power curve for this stuff is horrible.  I’d have been better off with Polyproplene rope, even though polypropolene rope isn’t good in sunlight. 
It was about 1500 ft-lbs of torque when pulled down to the ground, but up at the top only about 100 ft-lbs.
Each time you pull down the arm, you will have to tighten the skein.  So do this about 5 times before you fire the first pumpkin.
I recommend using a true polyester unsheathed rope with a diameter about 16 times smaller than that of the scutula hole diameter.

If you do want to attempt a larger skein, on the scale of the one I made, here is what you need to know:

  • First, you will need to have someone who is good at cutting and welding.  You will need to use some metal. 
  • Second, you need a source of scrap metal. 
  • Third, do NOT count on the skein to center the modiolus.  Make a flanged modiolus so that part of it will sit inside the skein hole.  Otherwise the modiolus will move too much.  On Siege, they suggest using a "rope sleeve".  That would be good on a 4" dia skein sized device.  Larger ones should have the sleeve welded one piece onto the modiolus.  
At the size above 3’ wide, 4” diameter skein, you will need to switch modiolus design to that of an externally toothed modiolus.  It is the only thing that can apply the torque you will need to turn the skein. 
Also, its best to spend time and effort designing it to be easy to rotate.  Rotating by hand is fine, but it needs to be quick and easy.  

For an example of what I’m talking about, see the modiolus on the mister ballista or chunk norris.  it has a smaller gear with a crank you can put it.  I suggest using the gear arm as a secondary modiolus stopper, and adding a primary one as a flat toothed piece that fits in a built up area on the side board and locks with the teeth. This will keep the modiolus from rotating when you are done tightening the skein.   

 How to not make an onager that works

First, read this, and learn all you can.
My onager had a floating wooden modiolus, a scaled up version of the 3’ wide kind of onager.  I made it an octagon shape so it could be rotated with a huge wood wrench I crafted.  I could only turn the modioli to about 1500 ft-lbs of torque before the wrench got too stretched out.  On top of that, I had to try and line up the pin in the modioli with the pin in the board under it, all while trying to hold the skein in tension.  It’s a nightmare when you are trying to take care of all the other stuff to set up and fix on the day of the chunk. 

Also, I’m not the only one who understands that grease is not enough to reduce the friction between the modiolus and the board under it.  I recommend UHMWPE, but there are other options. You could use ball bearings, if you had a bunch of them on hand.  Kamatics makes a P54 material which can take about 20,000 psi and has a friction lower than UHMWPE.  You’d only need something like this if for some reason your modiolus area was really small, or your skein diameter is huge, or a combination of the two.  

The reason for needing to make the modiolus out of metal in these larger onagers is partly due to the properties of the wood.  Although in my case, I was able to make a 1.5” thick modiolus out of two pieces of cabinet quality plywood, I would have been unable to make it toothed.  The teeth push on the wood in a direction perpendicular to the force that the bar is trying to break the modiolous.  The fact that the wood is a composite makes the fibers splinter then locally acted upon in this direction.  It would be better to have homogenous wood at the teeth, and plywood around the hole.  I’m not saying its impossible to make a working externally toothed modiolus out of wood, its just that metal would be a lot easier.  I suppose if you were forced to make it out of wood for some reason, you could do it, it would have to have a large diameter for the teeth.  A good rule of thumb for this kind of modioulus might be to have the external modiolus diameter set to 3 times the internal modiolus hole diameter.  The wood at the teeth would need to be extremely hard, so you can start with a hard wood, or use a wood hardening chemical.  The meshing smaller crank arm gear would also preferably be made of a hard wood. 

Again, wood is not recommended for onagers over 4” in skein diameter!

Advice For Many Kinds of Siege Engines, such as Onagers and Trebuchets:

The key to making a siege engine, I believe, is using what you have on hand, or can find or borrow easily. 
Obvious limits are time, money, and storage space.  After all, if my Aerospace company were so inclined, we could design and manufacture a siege engine that would shoot a pumpkin to the moon!  But no one has unlimited resources.  

You need the throwing arm to be strong.  If its made out of wood, then wrap it with rope, which will help keep it from splintering.  For the rope wrap, use a spiral hitch: watch video here:

Pay lots of attention to the knock and how the sling releases.  Make a knock that is infinitely and easily adjustable.  I'd suggest a toothed circular plate that against another toothed plate and clamped down with a bike quick bolt. 


There are various programs you can use to simulate your device:

For the onager program, my onager had the following settings:
Torque Cocked = 4500Nm
Torque Rest = 1500 Nm
Pumpkin mass = 8.4 lbs
Beam mass = 10 kg
Beam shape factor = .43
Beam length = 2m 
Sling length = 1.4m
max range calculated to  be 313.9 meters

Also note the design for my onager is lighter weight than that on most because I turn the scutula frames 90 degrees so they are C shaped.  the max load of a 4' span is the following:
2"x8" = 4,956 lbs compression
Two 2"x8" = 24,000 lbs compression
4"x4" = 16,000 lbs compression
2"x8" = 4,956 lbs in bending 

Therefore, my onager could take over 10,000 lbs in compression.  If I had needed to, I could have added a 4"x4" near the skein to take more compression load, and that would have increased the capability to about 25,000 lbs of skein compression!

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