Trebuchet CATAPULT Project/Worklog Updated 12/12/06 Please COMMENT

The beams were standard length. I'm not sure what standard length is, but I believe it's 8 feet.

As for assembling the wood/arm... I suggest gluing them together and clamping them, and then once they're nice and even, drill the holes for the bolts. Make sure to use two rows of bolts, although it's entirely redundant since the wood panels won't slip and slide against each other after the glue is set. And I really do recommend gluing the panels because relying only on bolts *may* create extra stress around the bolts, leading to a reliability/safety issue.

For a 14 feet arm, there will be a lot of forces involved, and you MUST calculate everything before-hand. Also, if you may want to taper the arm so that its mass decreases further away from the pivot. This will greatly increase the speed of the arm, as well as make it easier to create a stable platform.

As for knowing about this stuff... I'm improvising. I'm drawing most of what I say from my physics background.

For the counter weights, how about bricks? Flat cinder blocks may be better since larger surface area per brick would allow for less sliding (more friction), and they can be drilled through for better stacking and less sliding/instability. Plus, they don't have gaps and aren't too porous so they should allow for more efficient use of space. You can have a standard weight set and then addition weights that can be secured on top so to that you can adjust the acceleration of the projectile.

 

Well the stage weights are solid lead blocks and they have holes in them already for stability so that will work. I don't know if I am going to do the laminating and bolts for the throwing arm. I might just go with a 4-8 piece of solid wood. Reinforced by some 8-2 around the axel. And yeah calculations are being done by the architect I am talking to since he has a computer simulation for stress.

 

The thing about using solid wood is that if there is a defect in the wood, that's where the stress will gang up on. But I suppose solid hardwood such as oak, could minimize this possibility.

As for calculations, you should just run a few paper and pencil figures as well. You're going to want to look at the moment of inertia of the beam as well as the torque due to the projectile. That way you can have an estimate as to how many weights you need.

You're probably going to need a few hundred pounds as a 10 lb weight at 10 feet will have the same effect as a 100 lb weight at 1 foot away. To get some decent acceleration, I imagine you're going to need maybe 400 lbs or more. The more counterweights you have, the further the projectile will go.

 

well only to a certain extent after that the more weight won't really help as much as it could. I am using 800 pounds due to a ratio I calculated of 100 to 1 for cw to projectile. So I plan on launching 8 pound projectiles max so I am going to launch a light 6 pound bowling ball.

 

You'll want it mounted on wheels, not only is it significantly more efficient but it helps to reduce the stress passing through the structure - a machine that big rocking on its base would not be a good thing...

 

well true but my counter-weight is not fixed. Plus the logistics for the wheels would be too much so there will be none.

 

OOH... DarkReaper has a very good point. When that sling goes, the momentum is going to rock the trbuchet heavily. While I don't agree that it should be mounted on wheels (b/c it might be difficult to engineer), you should think about some kind of suspension to absorb at least some of the shock.

 

this isn't an ongar either. So yeah unless if I do a horrible job most of the energy should go into the projectile. Plus the arm is free to swing any excess motion is dissapated thru that. There is no need for suspension in a trebuchet.

 

scrapheap challenge strikes again !

 

Back in the middle ages, there were two main types of trebuchet, both of which were developed to counteract rocking. One of them was to put the trebuchet on wheels. The other, which it sounds like Scarecrow is doing, is the swinging-bucket system. Instead of fastening the weights directly to the main arm, a wooden bucket (or bucket like structure) is hung from the end of the arm, and that's where the weight goes. As the arm swings, the bucket swings relative to the arm, canceling out most of the rocking force.

 

Ah, I see. So as long as the "bucket" glides smoothly about its pivot as it descends, the stresses should counter each other and balance out. LoL, now I REALLY want to make one too. Maybe a small r/c or autonomous robotic one *evil grin*.

 

I built several for my tech A-level, the rocking effect on even a 20cm heigh treb without wheels was alarming... It's more efficient with wheels because the fall path of the counterweight is closer to a vertical line - a basket or similar arrangement will help too but both is preferable.

If you really can't find wheels somewhere (scrapyard, just grab two old axles to go through holes in the base? then make sure that you never ever dry-fire the machine - at least when launching a suitably sized projectile most of the energy will go into producing distance.

There are plenty of resources out there, ensure that you know how much counterweight you have so you can calculate the projectile mass - with that much power you don't want to piss about.

 

true that but techno got it straight on the head. I am going to be having the counterweight rocking which is more efficient then if it is on wheels. This is not going to be a fixed counterweight.

Oh and I was in my basement recently found a benchpressing bar. That is going to be my axel. The benchpress bar is going to be sufficient as it is an inch in diameter of solid iron. I will not be using the whole length of coarse. So it will be plenty strong.

Dark it sounds like you made a fixed counterweight that is why it rocked. Plus the wheels will just be to hard due to the size of this machine.

 

go with the swinging arm and wheels - far superior - I've built some big ones, some up o 3m tall, firing 2-3lb objects

 

Take lots of photos of it being built. Use plents of metal brackets to strengthen the joints (gusset plates etc). Go completely nuts in terms of making sure it won't break. Don't forget when it fires there will be lots of reaction forces from accelerating the projectile, make sure you take those into consideration.

It's advisable that you use some sort of truck bearings that somone else mentioned. Wooden axles tend to be... a bit rubbish really, using proper bearings will reduce friction so much. You might consider using a thick metal axle, but if you don't make sure you use really thick wood. I just think using wood-on-wood as the bearing surface between the axle and the main structure is a bad idea.


You also need to think where to put the axle. Too far towards the weights and you might have enough torque to launch, but too far away and you will have too much torque, and will be sacrificing speed. I suggest you make lots of accurate models / computer simulations / calculations.


Picture time:

Good thinking, but why would you need to laminate your wood?

Oh please yes!

 

thanks woof and that is basically my plan as you see it there. Just I got two triangles going out so to prevent the structure from tipping over. Also my counter-weight will be swinging so the tipping up is not a prob. The eeks lots of stress is going to be 2 7 by 1's. Put around the supporting base. Thanks for the pic I will be showing one of my own soon.

Um I got my axel which I will be cutting down to be used twice. Its 1 inch diameter solid iron I think bench press. So that will be fine in holding it up. As for the bearing I don't know if I can find one so I might just put an outer pipe around the benchpress then use a **** load of grease. Will be better then metal on wood if I had metal on metal with grease. But I continue the search for bearings .

 

Come to think of it, I'm not quite sure why I recommended that. With a smooth launch with bearings in use, the load on the beam will be a lot less than I envision it to be. I also had softer woods in mind for some reason. I'm still concerned that the beam might be stressed excessively at the axle. The last thing you want is the beam to split during a launch. Some steel plating/support would probably be a lot easier to implement than laminating an entire beam

What has to be thought about now, is how he's going to get a 14 foot long hardwood beam for the arm. Two beams connected together?

 

Getting the wood is no prob. I am getting it free from a friend of mine and have seen it already. If that fails though for some dumb reason home depot sells hardwoods of that size I thinking 4 by 6 or 8 up to 22 feet. So yeah. I am uploading some pics now so yeah

 

Yay this is the stuff I got last night. The bench press is going to be cut down so I can use it for the main arm axel and the other axel the counter-weight will be using.


The ruler there is for size reference and it is a 15 inch ruler so yeah


Due to the spacers on the bar in the middle its a little off the ground. But it is an inch in diameter solid iron I believe. Plenty strong for the size I am going to have it cut down to. Its only going to be about 2.5 feet long and the gap is only going to be about 14 inches. So yeah plenty of space I got 15 inches of wood its going to sit on so yeah .


These were on the bar I might use them to make sure the bar doesn't move off the axel or that the arm doesn't move. But no real plans currently for them.

Yeah let the worklog begin I will have a detailed schematic of my design on latter today just need to finish drawing it.

 

I was going to mention the swinging weight but you've already done that, well done!

Another way to maximse the energy used and unused is to allow the base to move, i.e. on wheels or rails. This allows the trebuchet to move forwards and back during the swing. This not only helps reduce the excess energy after throw but also aids in the throw as the weight shifts forwards...

JCBeastie