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Old 06-22-2007, 11:11 AM   #1 (permalink)
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Square tubing strength question

Time to make another trebuchet! I have a 2x2" carbon steel tube, 120" long. It is about 3/16" wall (9 ga?). I want to make a throwing arm out of it for a trebuchet.

I want to suspend 600 pounds of counterweight on one end, with the fulcrum point at 22 inches from said end. The other 98 inches will move the projectile and will only need to be held static while it's cocked before firing.

I have figured that at a 45* angle (600 pounds "up") the trigger load will be 71 pounds.

At the axle will be a 2x2" piece on either side (parellel to axle), 20 inches long, stiffening the section around the axle.

On the counterweight end will be 2"x2"x6" on top and bottom of the arm, stiffening that section since I will have a hanger bolt in there, weakening the wall.

Will it hold?

[img]/ubbthreads/images/graemlins/shocked.gif[/img] [img]/ubbthreads/images/graemlins/biggrin.gif[/img]
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Old 06-22-2007, 11:38 AM   #2 (permalink)
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Re: Square tubing strength question

Lets see....2x2 square tube 3/16" wall, 10 feet long with 600 pounds 22" off to one side, and 71 off to the other?
I'm not an engineer but I think it will hold just fine.
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Old 06-22-2007, 12:08 PM   #3 (permalink)
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Re: Square tubing strength question

I do too, and have had a few engineers tell me it will, but there will be quite an increase in dynamic load as it releases and swings through it's arc.

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Old 06-22-2007, 12:21 PM   #4 (permalink)
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Re: Square tubing strength question

How about putting a triangular shaped brace bar up over the top side of the long arm? This would help with the ever continuing force as it accelerates thru the arc.

Just a thought.
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Old 06-22-2007, 01:09 PM   #5 (permalink)
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Re: Square tubing strength question

I thought about that also! Similar to a portable hydraulic engine hoist. Thanks!

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Old 06-22-2007, 01:12 PM   #6 (permalink)
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Re: Square tubing strength question

[ QUOTE ]
Time to make another trebuchet! I have a 2x2" carbon steel tube, 120" long. It is about 3/16" wall (9 ga?). I want to make a throwing arm out of it for a trebuchet.

I want to suspend 600 pounds of counterweight on one end, with the fulcrum point at 22 inches from said end. The other 98 inches will move the projectile and will only need to be held static while it's cocked before firing.

I have figured that at a 45* angle (600 pounds "up") the trigger load will be 71 pounds.

At the axle will be a 2x2" piece on either side (parellel to axle), 20 inches long, stiffening the section around the axle.

On the counterweight end will be 2"x2"x6" on top and bottom of the arm, stiffening that section since I will have a hanger bolt in there, weakening the wall.

Will it hold?

[img]/ubbthreads/images/graemlins/shocked.gif[/img] [img]/ubbthreads/images/graemlins/biggrin.gif[/img]

[/ QUOTE ]


Are you starting ANOTHER crusade??? [img]/ubbthreads/images/graemlins/phoney.gif[/img]

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Old 06-22-2007, 01:15 PM   #7 (permalink)
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Re: Square tubing strength question

The technology behind them is so simple but so graceful at the same time. Watching the arm swing, sometimes almost silently, throwing an object through the air. Plus the neighbors can't complain about the noise, it's free to use, only human power or eventually a 12V battery and winch to cock it...

[img]/ubbthreads/images/graemlins/grin.gif[/img]
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Old 06-22-2007, 02:22 PM   #8 (permalink)
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Re: Square tubing strength question

Get yourself a copy of Roark's Formulas for Stress and Strain. (link)

New, it's pretty expensive but even an older edition will contain pretty much everything you'll need for a project like this.
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Old 06-23-2007, 01:12 AM   #9 (permalink)
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Re: Square tubing strength question

I'm not an engineer....but,


I don't think I would even bother with reinforcing it because I don't think the force will increase when its throwing something. I think the time the most force will be on the tube will be when its sitting there cocked and ready to fire.
The weight is already hanging on one end, so the force is there, but on the other side you have an exact counterbalance holding the other weight in the air. When you release it the one weight is allowed to fall and the only thing resisting it on the other side is the weight of the object being thrown and its own inertia. Far less then the thing holding it down. Its a very smooth and graceful acceleration too, not like if it was spring loaded or used air power or anything like that.

I may be wrong, but like I said I think the most stress you will have on it will be when its cocked and ready to fire.
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Old 06-24-2007, 11:52 AM   #10 (permalink)
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Re: Square tubing strength question

[ QUOTE ]
I'm not an engineer....but,



The weight is already hanging on one end, so the force is there, but on the other side you have an exact counterbalance holding the other weight in the air. When you release it the one weight is allowed to fall and the only thing resisting it on the other side is the weight of the object being thrown and its own inertia. Far less then the thing holding it down. Its a very smooth and graceful acceleration too, not like if it was spring loaded or used air power or anything like that.

I may be wrong, but like I said I think the most stress you will have on it will be when its cocked and ready to fire.

[/ QUOTE ]


As it swings the centrifugal force will increase exponentially with the speed of the arm,(load end) so no it will not "weigh" the same at the end of the arc as it does at rest.
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Old 06-24-2007, 09:13 PM   #11 (permalink)
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Re: Square tubing strength question

[ QUOTE ]



As it swings the centrifugal force will increase exponentially with the speed of the arm,(load end) so no it will not "weigh" the same at the end of the arc as it does at rest.

[/ QUOTE ]
Right but all the increased force will be going out, pulling on the square tube, not trying to bend it. The tube will be able to take much more of a pulling force then a bending force.
I still say it won't have any more stress (as in a bending force) on the tube whether its cocked or at any point in the throw
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Old 06-28-2007, 11:30 PM   #12 (permalink)
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Re: Square tubing strength question

GOD I love this forum
Always something cool being posted
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Old 06-29-2007, 12:53 AM   #13 (permalink)
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Re: Square tubing strength question

yes, likely the most load will be on it while it's cocked, then as it swings the force from the counter weight will decrease until the bar is perp. with the ground, then as it comes around the force will switch around slowing the arm down. Should be fine. But I'd def stand a ways away the first few times... I'd be most worried about how the piece of metal is held at the fulcrum, IE can the bearings take the stress, can the stand it's on hold up, etc...
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Old 06-29-2007, 12:04 PM   #14 (permalink)
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Re: Square tubing strength question

[ QUOTE ]
yes, likely the most load ... will decrease until the bar is perp. with the ground, then as it comes around the force will switch around slowing the arm down. Should be fine.

[/ QUOTE ]
"Load" ... that'd be tension, compression, shear or bending moment?
"Should be fine."? What's your basis for saying that?
If you don't know, don't babble.


In static conditions, the bending moment in the arm supporting the counterweight will be at its maximum when the bar is horizontal. (greatest distance between the fulcrum and the force of gravity = greatest bending moment) Once it starts moving, there will be angular acceleration to consider, as well as centrifugal force. (which increases with the square of the angular velocity, not the exponent)


[ QUOTE ]
... I'd def stand a ways away the first few times. ...

[/ QUOTE ]
That's the second most-appropriate comment I've seen here today.

[ QUOTE ]
I'm not an engineer ... I may be wrong

[/ QUOTE ]
And that's the first.


Look, I'm not sayin' that you shouldn't practice a little backyard Appalachian Engineerin' whenever you get the urge, or even that you shouldn't ever attempt something that's obviously hazardous. Hell, I play in traffic, fly airplanes, drink tap water, eat red meat, skydive, use power tools, weld, indulge in sex and a variety of other hazardous activities besides practicing engineering.

But if you do practice de facto engineering of any stripe, with or without an engineering degree you might take the Professional Engineer's Oath to heart: "To protect and promote the public health & safety at all times [including your own!] and limit your practice and public statements to your area(s) of expertise."
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Old 06-29-2007, 08:58 PM   #15 (permalink)
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Re: Square tubing strength question

Don't get so offended. Why don't you give your "engineer" opinion if it will work or not?


He is not building a skyscraper here. It will work, or the tube will bend. Its not a real big deal if it fails.
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