Trebuchet (Joining Lumber)
 

Hello fellow off-season CDer's,

I'm building a trebuchet in my basement, and I want to make the swing arm out of 4x4 and 2x4, but I'm not sure how to join the two together end-to-end.

Some ideas have been tossed around, all including large bolts, some including large steel C-channel, but none of us (my brother, dad, or myself) have any real clue how to do it.

Any help would be greatly appreciated, images could validate hugs. No promises.

The attached image is what I came up with in SketchUp, feedback would be nice. The lines flanking the 2x4 are shims to keep it centered.

Re: Trebuchet (Joining Lumber)
 

It would be helpful to know what the loading is, and how it is applied. Perhaps you have a drawing or sketch of the whole machine?

Re: Trebuchet (Joining Lumber)
 

It would be better to describe the piece you want and then ask how to make it. Attaching a 4x4 end-to-end with a 2x4 is almost certainly the wrong answer to the question. If -- for example -- you really wanted a 12-foot-long arm with a 4-foot 4x4 section and an 8-foot 2x4 section, I would suggest that you make the arm up from a 12-foot 2x4 with a 4-foot section of 1x4 laminated onto each side of the butt of the arm using yellow glue and a some #8 2-1/2" screws to hold things together while the glue cures. Then, you won't be compromising on the strongest part of a wood laminate structure, which is its long fibers running longitudinally. Given the nature of a treb, I would strongly consider getting the nicest possible board for your arm. A clear piece of southern yellow pine or CVG Douglas fir would be good. Avoid anything with big growth rings or lots of knots.

Have you considered using an arm that is thinner and deeper? You could make a really nice, stiff arm out of some 3/4" plywood strips 8-inches wide and laminated with carpenter's glue. Stiffness increases hugely (the 4th power?) with thickness, and the deeper (higher) your arm, the stiffer it is.

Can you post a sketch?

Re: Trebuchet (Joining Lumber)
 

Almost immediately after I posted this question, my brother and I had the same idea for running a 2x4 the entire length and thickening the weighted end and axle.

The entire thing is southern yellow pine, primarily because its what they have at Home Depot and our budget is "it had better be on sale."

I went back into the SketchUp model and redid the arm, so here is a screen shot with a couple of dimensions for scale.

Re: Trebuchet (Joining Lumber)
 

At a Mentor Gathering following the closing ceremonies in Atlanta, we ran into a fascinating couple who participate in Pumpkin Chucking . They have teams of all ages who construct various machines and hold competitions. They have a great website http://www.punkinchunkin.com/ with forums on how to construct these machines.

Have fun and be safe!

Re: Trebuchet (Joining Lumber)
 

I don't see the need for doubling the arm, or using 4x4s for posts to support the pivot....since the arm is only 7' long. If you do reinforce it, I'd suggest putting the reinforcements on the top and bottom instead of the sides. In fact you could make the whole arm from a 1x6, taper the long end so it's about 3" wide at the tip.

The pivot shaft looks a bit long and skinny though.

Re: Trebuchet (Joining Lumber)
 

Is this thing going to stay in your basement? Or are you designing it so that you can piece it down and move it? I am reminded of my dad's friend that built a car in his basement and wound up installing an improvised car elevator (forklift hydraulics) to remove it.

Re: Trebuchet (Joining Lumber)
 

The plan from the very beginning was to have it able to break down in to 5-6 major components, most of which lay flat. (minus the counterweight box of course)

We were talking about moving the sides in a little to shorten the shaft. That's 5/8" steel from Home Depot, and bushings and shaft collars from McMaster, for the record.

The problem I see with using a 1x6 for the entire arm is warping from humidity and damage in transportation, I'd feel a lot better tossing a 2-by in the back of the van and putting stuff on top of it.

Re: Trebuchet (Joining Lumber)
 

Then use a 2x6? my main point is to make the piece larger in the dimensions that need it, and smaller in the dimensions that don't need strength.


Maybe larger diameter thinwall tubing instead of solid rod? although figuring out a bushing would take some effort.

Re: Trebuchet (Joining Lumber)
 

I went back to the model and adjusted the arm again.

There is a 2x6 running the entire length, with a 1x4 on each side where the pivots are, however those could be easily taken out of the plan. At this point they are only there because the bearings I have are too long to have one on each side of a 2-by.

Thanks for all the input, everyone.

Re: Trebuchet (Joining Lumber)
 

The other problem with a 1x6 would be that most Home Depot-quality 1x6s are suitable for trim boards, but are structurally about as strong as Kleenex. The 2x6 makes sense to me. How heavy is your counterweight?

Re: Trebuchet (Joining Lumber)
 

good point!

Might also look into buying a sheet of plywood instead of a lot of that lumber....a base plate, two triangles, and not much 2x4 could make the base structure, and it might be possible to make it easy to disassemble and lay flat.

Re: Trebuchet (Joining Lumber)
 

I've already built the two side structures and counterweight box, and the plywood probably wouldn't look as good.:rolleyes:

I planned the base with double layered 2x4's so the components can be bolted together using T-nuts.

The counterweight is as heavy as whatever we put in the box, right now we have a spare bag of gravel in the driveway (20-25 lbs? Its labeled as .5 cubic feet) but we could get another bag to experiment once its all together. We are planning on throwing SCA rocks (4 tennis balls duct taped in a tetra) so the projectile won't be really heavy.

Re: Trebuchet (Joining Lumber)
 

I can lend my personal experience here,
I built a treb very similar to what you have there, I used only one 2x4 for the arm, no extra bracing at the center and had no problems using 300lb of counterweight. For my pivot rod I used a 2" piece of steel pipe that I had laying around, my thoughts about yours is that it will flex quite a bit, especially if you start putting a lot of weight in your box. Also, as others have said, you've got bracing on things that do not really need it, such as the braces from the base to the top of the pivot support, those are good, but the others triangulating them aren't needed. Mine actually looked like yours, but take out the 4x4 post going up for the pivot. Looks good though! Just watch that arm when you (so apparently the bad word filter decided that another word for load that started with a c is bad, so PULL THE ARM BACK) , that pin is dangerous! Oh and mine was 15 feet tall with the arm pointing up. All of my dimensions were true also, as in my 2x4 was actually 2" by 4" I bought them at a local small saw mill, about the same cost as normal lowes or Home depot lumber, but the guys picked out some really good ones for me when they heard what I was doing with them.

Re: Trebuchet (Joining Lumber)
 

Yeah the more I look at the 5/8" we got, the less I trust it for this.

I just looked through our garage and spotted about 10 feet of ~1 1/2" metal pipe, but I don't know how thick it is or if its steel or aluminum. Also, its on a rack above the garage door, so I can't get it down by myself.

There should be an update when my dad gets home.

EDIT: We have a piece of 1 3/8" galvanized steel pipe that seems like it'll work. (we thinks its been in the garage since my parents moved in)

Re: Trebuchet (Joining Lumber)
 

Not much to add except the obvious -- trebuchet.com. My own is made out of coffee stirrers so is way smaller than a pumkin chucker. Ah, the fun of office supplies!

I did remember reading somewhere, and it's been a while, but putting wheels on it adds distance to the throw?

Good luck, NorviewsVeteran -- post pictures!

Re: Trebuchet (Joining Lumber)
 

3/4" plywood will offer much better support than a 1 by board. This is because in plywood the grain is oriented in difrent directions between the layers.
Also we apply construction adhesive to the plywood, and clamp it tight, than use 2" long, 1/2" crown, 16gauge staples to attach them to the 2x6, or 2x8, 2x10, etc. We have joined beems doing this to support the floor in a house.
(plywood, 2x10, plywood, 2x10, plywood. the plywood goes the entire length of the beem):D

Re: Trebuchet (Joining Lumber)
 

Nova did a show about trebuchets many years ago where two different full sized trebuchets were built using medieval technology. One had a swinging bucket and the other had a fixed weight and wheels. As I recall they both shot about the same distance.

http://www.youtube.com/watch?v=j9lhcjupnzk (great Nova episode, by the way... worth watching)

Re: Trebuchet (Joining Lumber)
 

and we saw one on our trip to Scotland in 2000! (only me and Mrs. Forbes...little Steven had to stay home)

Re: Trebuchet (Joining Lumber)
 

Yes, it does add to distance, as well as points of failure, bill of materials, construction cost and time... Maybe version 2.:D

Thanks and here's a picture as of last time I worked on it. Also a good shot of our basement. I won't be able to work on it for a while though, I'll be out of town for a few weeks at Governor's School.

Re: Trebuchet (Joining Lumber)
 

Hmmm -- squirrel's one in Scotland looks like it has stone wheels. They're probably not available at Home Depot. Though wheels do make it movable by one person.

I've been wondering about the two side supports, especially after looking at squirrel's photo. I built mine (small as it is) as two triangles with no vertical support, mostly because I've seen some that way, plus the vertical support seemed extra. The weight, at rest, goes down the two triangle pieces; when shooting, the weight goes from back to front in a more direct line. The "traditional" trebuchet -- maybe it started out as a single vertical support, to hold up the weight at rest, but quickly the side angle pieces were added when the vertical supports swayed too far during shooting, eventually getting rather complicated as in the Scottish one.

But now I'm wondering if the "traditional" style vs the "triangle" style gives more sway to add distance, like the wheels. Or maybe less strain at the joints. Sort of a wood vs steel roller coaster distinction, perhaps?

And which way is "forward" in the Sottish one? Up ramp? I'm guessing that's to reset the trebuchet after each shot, bringing it back to it's original position.

I'm thinking way too much for a Monday morning.

Re: Trebuchet (Joining Lumber)
 

Notice the Scotland trebuchet does not have a pivoting load basket. The wheels allow the whole thing to move as the weight swings around, so that the center of mass does not move so much as it fires...so it won't tip over....

With a swinging basket like NV is building, wheels aren't really necessary.

Re: Trebuchet (Joining Lumber)
 

Just a side note... you may be tempted to "dry fire" the trebuchet without a payload/projectile, just to see how it works. Nothing wrong with that, but do note that this means that your counterweight will drop very quickly, putting a larger load on the support structure and rocking the whole thing much more than it will when fired with a live load.

Trebs are awesome machines when it comes to demonstrating physics concepts... or just plain throwing stuff (water balloons work well if you build the sling right)... but even a moderately sized one packs serious momentum. Stand back and be safe!

Jason

Re: Trebuchet (Joining Lumber)
 

Yes, I know about dry firing things (bow & arrow, BB gun, paintball, slingshot) but there's always the devil's advocate thing about comparing a dry fire to a tennis ball or 12 gram BB...

And I always try to respect the danger of inanimate objects.

Thanks again for all the input and safety reminders without finding something to argue about!

Re: Trebuchet (Joining Lumber)
 

Here's one I saw in action in Wales at Caerphilly Castle with a pivoting counterweight.
It was stationed to fire into a lake moat.


For some of my homemade trebuchets I've used water as the counterweight. Makes them more easily portable.
I've also used just the angled side supports without a vertical member, but I did have stiffeners across to keep them from bowing.
Vertical supports would probably make it more robust if you're planning on moving it around a lot.

Re: Trebuchet (Joining Lumber)
 

I've had students build these in various incarnations for about 5 years now. After having built or helped students build over 150 of these I have listed my observations. We've built everything from 12" at the fulcrum using shop washers for weight to 5' at the fulcrum using weight plates and thrown atomic fire ball candy (I just liked how firing those sounded..oh the anachronism of it) to golf balls to water balloons. For water balloons we positioned ourselves at opposite sides of a parking lot and took turns firing at each other for a few hours. Much like an RPG you were allowed to move 5 feet or fire on your turn...not both.

• Wheels didn't make much difference if you were on a relatively smooth surface (smooth concrete)
  
• A wide base was good...a wide space at the fulcrum was not
  
• Looking at your frame I'd suggest a thick side-walled pipe as the axel with bolted collars or pins on either side of the frame supporting it and on each side of the lever arm. Both of these additions are to reduce side to side twisting and to stabilize the frame.
  
• Be very aware that they fire backwards sometimes just as easily as forwards...plan now on a release mechanism other than holding the lever with your hand!
  
• The pouch on the end of your sling will change based on the projectile
  
• You can put too much weight in the bucket...there is a point of diminishing or no returns where the sling whips wildly because it's going too fast to work properly.
  
• We used plate weights like you would find in a gym or home gym

Re: Trebuchet (Joining Lumber)
 

With all due respect, using plywood for a beam means that 40% of your material (in a typical sheet of plywood with an odd number of equal-thickness layers) goes in exactly the wrong direction to add any strength to your structure. Both natural sticks and plywood can be thought of composite materials: in wood, the wood fibers are held together by lignin, a natural adhesive. In plywood, layers of natural wood are glued together at a 90-degree angle. In any composite, you need to carefully layout your material to add strength in the orientation in which it will add the most strength and stiffness (where stiffness is desirable) with the least material. Since the cross plies in plywood make little contribution to resisting the rotational forces of the beam as it pivots around the shaft, it is a poor choice for a trebuchet arm.

A natural stick, on the other hand, has the bulk of its fibers running longitudinally along the long axis, which is exactly where you would want the strength. Even if you laid this up out of fiberglass or carbon fiber, nearly all the fibers would be longitudinal (although I would be tempted to use 45-45 biaxial materials just to make sure it doesn't delaminate and better resists whatever twisting forces are present).

Plywood is a wonderful material, but there are reasons that wooden studs and LVL (a glued-up wood composite with longitudinal fibers) are what are used for joists and studs in houses, while plywood is reserved for floors and roof decking. OSB has largely replaced plywood for most home building uses, and it is like lots of little plywood veneers the size of your hand (or smaller) glued up in a sandwich pointing every which way. OSB also makes an understrength beam, but a nice flat surface.

(Since a lot of construction plywood is full of voids and uses cheap/weak wood in the core veneers, I wouldn't use it anywhere in a trebuchet. It's OK for lining the bed of a pickup, but a lot of it is complete junk. If you seek out and buy good plywood you can get useful stuff, but it costs more. Shine a bright light through the plywood. Anywhere you can see light on the other side, it has voids and is unusable if you need reliable strength.)

Re: Trebuchet (Joining Lumber)
 

I have also built a trebuchet with incredibly similar dimensions, a couple of thoughts on what we found (A class project where we had to build a machine that had to fire the a bocche ball as far as possible and fit through a man door, and made completely out of donated or money given by team members, ours was $50):

- The less room between the two apposing "towers" the less flex you will get on your pipe, and thus the more consistent it will be. Even if it means you have to

- Make sure your pipe is anchored on the ends, otherwise it slides out, learned that the hard way, but if you make the pipe able to be pulled out it makes the arm able to be pulled off, then it takes one less person to carry it and stabilize the arm. You wont realize this unless you do that.

- We took a 16 foot 4x4 to make the arm, we cut a couple of feet off to make it a better length for us, then we drilled the hole in the center and we put a fiberglass tube in the wood and the the steel tube so we had a lower friction surface, we then ripped, on a table saw to just under the size of a two by two, the long end that fires the projectile so that it weighed less and then we would get a faster rotation.

- The trickiest part will be adjusting the release mechanism, so make sure you do a couple of test runs to adjust the finger to a point of release near 45 degrees