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