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furbyjs
#1 Mar 18 2013 02:01pm
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My friend and I are working on building a catapult, and got a final design.
I was wondering if someone had any suggestions for how to improve it.
We've already assembled it and tested it, but it was only able to throw a tennis ball about 15 ft., nowhere near as far as I was hoping.

The total dimensions are about 5ft long, 2.5 ft wide, and 2.5 ft high.
For the torsion, we ran a 50ft length of rope through two holes in the arm, then around the arm after the two holes. The base, which has the holes through which the ropes run, is made of a 5 ft 2x6.
For the rope, we used polypropylene which I guess is supposed to be the best.

I think the problem is not getting enough turns from the arm for our ropes. The arms are wood, and the wood-on-wood friction is just too much to be able to get much force. I'm thinking of adding a piece of metal around the hole to reduce friction, and hopefully get a few more twists out of it.

I'd also like to enlarge the holes so that we can get more than 50 ft of rope through it. I'm guessing more rope would give me more torsion power.

If you have any suggestions, I'd love to hear them.

I'll upload some pictures as soon as I can get some.

Thanks!
furbyjs
#2 Mar 18 2013 03:00pm
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These are some older pictures. We've made slight changes since this, but basically the same.





This post was edited by furbyjs on Mar 18 2013 03:01pm
Azrad
#3 Mar 18 2013 07:29pm
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so wait, how are you create the force for each shot? are you re-twisting the rope by turning that dowel in the foreground on the last picture, then moving the arm into firing position?

Or did you just twist the rope once when you built it, and now just move the arm into firing position each time?

This post was edited by Azrad on Mar 18 2013 07:34pm
saber_x3
#4 Mar 18 2013 08:26pm
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I'm pretty sure your 1 of the problem is the contact area.
it is way too small compared to your overall rope length
I'd choose a wider arm, which basically shortens your rope length
The force transfer becomes much more instantaneous? there is a larger delay and loss of efficiency the longer your rope is, something on those lines


look at this for example, the arm is much wider

----------------
Your rope's thickness / length ratio is too low
so, I'd thicken the rope.
---------------
Try to make sure the the tennis ball is released at an angle a little bit less than 45, I wouldn't go down to 40
The exact angle isn't something I can tell you, but I do know that you want the angle to be less than 45
----------------
One of the easiest thing to test is arm length
There is a definite optimal arm length , I'd assume it is much shorter than what you have it at with your current setup
To test, basically just place the tennis ball on different places of the arm
----------------
Can you add more to it? rofl, like just a bungee from the end of the arm to bottom of the base in parallel with the arm

This post was edited by saber_x3 on Mar 18 2013 08:45pm
furbyjs
#5 Mar 18 2013 09:51pm
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Quote (Azrad @ Mar 18 2013 08:29pm)
so wait, how are you create the force for each shot? are you re-twisting the rope by turning that dowel in the foreground on the last picture, then moving the arm into firing position?

Or did you just twist the rope once when you built it, and now just move the arm into firing position each time?


The dowels are fixed after the initial twisting. You pull the arm back for each shot. We have yet to create a release mechanism, so for now it's just pull back and let go.

Quote (saber_x3 @ Mar 18 2013 09:26pm)
I'm pretty sure your 1 of the  problem is the contact area.
it is way too small  compared to your overall  rope length
I'd choose a wider arm, which basically shortens your rope length
The force transfer becomes much more instantaneous?  there is a larger delay and loss of efficiency the longer your rope is, something on those lines


look at this for example, the arm is much wider
http://www.stormthecastle.com/catapult/images/torsion_catapult/wrapping-complete.jpg
----------------
Your rope's thickness / length ratio is too low 
so, I'd thicken the rope.
---------------
Try to make sure the the tennis ball is released at an angle a little bit less than 45, I wouldn't go down to 40
The exact angle isn't something I can tell you, but I do know that you want the angle to be less than 45
----------------
One of the easiest thing to test is arm length
There is a definite optimal arm length , I'd assume it is much shorter than what you have it at with your current setup
To test, basically just place the tennis ball on different places of the arm
----------------
Can you add more to it? rofl, like just a bungee from the end of the arm to bottom of the base in parallel with the arm


Something I wanted to do was add a sort of bumper to the top of the frame (to adjust the stopping point of the arm, and the release point)
The arm is just something temporary. I wanted to use a 2x4 or 2x6, but thought it would weigh down the arm's movement too much and take away from the final force. Also worth noting: there's a bowl we attach to the arm to hold w/e is being thrown, it's just not in the pictures.
When you say thicken the rope, do you mean the actual rope thickness, or just try to get a larger bundle? I read somewhere that smaller rope thickness was better, but you need big bundles of rope. The problem is getting any more rope to fit through the holes in the base.
And I was sort of thinking the same thing, adding some bungee cords or springs just to add more force, but I'd like to just use rope if possible xD

Thanks for all the help though!
saber_x3
#6 Mar 18 2013 10:52pm
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Quote (furbyjs @ Mar 18 2013 09:51pm)
The dowels are fixed after the initial twisting. You pull the arm back for each shot. We have yet to create a release mechanism, so for now it's just pull back and let go.



Something I wanted to do was add a sort of bumper to the top of the frame (to adjust the stopping point of the arm, and the release point)
The arm is just something temporary. I wanted to use a 2x4 or 2x6, but thought it would weigh down the arm's movement too much and take away from the final force. Also worth noting: there's a bowl we attach to the arm to hold w/e is being thrown, it's just not in the pictures.
When you say thicken the rope, do you mean the actual rope thickness, or just try to get a larger bundle? I read somewhere that smaller rope thickness was better, but you need big bundles of rope. The problem is getting any more rope to fit through the holes in the base.
And I was sort of thinking the same thing, adding some bungee cords or springs just to add more force, but I'd like to just use rope if possible xD

Thanks for all the help though!

Yes, add more strands, thickening the rope

how close are you to the limit of the rope?

Azrad
#7 Mar 18 2013 10:58pm
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Quote (furbyjs @ Mar 18 2013 08:51pm)
The dowels are fixed after the initial twisting. You pull the arm back for each shot. We have yet to create a release mechanism, so for now it's just pull back and let go.
You will need to "re-do" (tighten it up) your "initial twisting" often at first.

furbyjs
#8 Mar 19 2013 05:30am
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Quote (saber_x3 @ Mar 18 2013 11:52pm)
Yes, add more strands, thickening the rope

how close are you to the limit of the rope?


What do you mean by the limit?
As of right now, I don't think I could fit any more strands in the holes, so I'm going to have to make them bigger.

Quote (Azrad @ Mar 18 2013 11:58pm)
You will need to "re-do" (tighten it up) your "initial twisting" often at first.


Yeah, we tighten it as much as we can before each firing. We just don't un-string it, because that takes forever to redo.
saber_x3
#9 Mar 19 2013 08:52am
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I mean, breaking point
Maybe you're simply not twisting enough , get a bigger lever if you have to
furbyjs
#10 Mar 19 2013 10:15am
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Quote (saber_x3 @ Mar 19 2013 09:52am)
I mean, breaking point
Maybe you're simply not twisting enough , get a bigger lever if you have to


I'd have to look at the package the rope came in, if I still have it.
It came in a 50 ft coil, in a package, from Lowe's. I want to say like 250 lbs maybe?
I'm at school and won't be back home for a couple weeks, so I can't check that out.

Also, I thought about that xD
Attaching something to the dowel rod to get more leverage.
Like I said before, I also want to put something between the dowel rod and the wooden frame, either a sheet of plastic or metal, to reduce the friction so I can get more turns out of it.
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