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Hello forum users,

I opted to use a compression tube design, which seems to work quite well. As far as the tube itself, a 1/16th sheet soaked in warm water and rolled around a dowel to pretty much any diameter can hold around 17 kg in pure compression. It weighs about 5 grams once air dried. The question I had was about how to connect the tension members to the compression member. Currently, we are using two compression members of 3/32 bass and attaching them to the sides of the tube. I feel like this is overkill and adds weight, especially since I have seen previous tube booms with only one compression member. Another possibility would be to put the tension member through a hole in the top of the boom and secure it from the inside. The problem is that this hole has to be in front of the loading block location, thus putting significant stresses on the compression member, in the way of buckling rather than pure compression. This way is also harder to manufacture and can weaken the structure of the tube considerably. If we were to use a single compression member, how would we go about attaching it to the hook? It would seem to me that our attachment would be off center with only one tension member, which could lead to tilting and early failure. Also, would we need compression tension connectors? It seems to me that the purpose of those is to prevent buckling as well as to stabilize the whole structure. We wouldn't need to prevent buckling, but stability and proper alignment on the testing wall is an issue without compression tension connectors.

The tube boom seems a very competitive design, but there are complexities in manufacturing that could significantly impair the load-bearing capabilities of this boomilever.

Interested to hear your thoughts,
arshah2

P.S. Sorry for the long post and lots of questions.

Yes, this has always been the challenge of the Tube design. Balsaman was a firm believer in it, but by the time you reinforce the end to accept the tension rod(s), and create some sort of seat for the loading block you will no longer be at 5 grams. Also, keep in mind that a tube that will support 17 KG when stood on end vertically and tested, is only carrying barely half of what is required for Div B and not even half for Div C. A structure that projects 40 CM with a 20 CM high angled tension rod with a 15 KG load applied will expose the compression "tube" to 30 KG of force (2:1 Mechanical "Disadvantage"). In Div C, its even worse... 40 CM projection, 15 CM height = 40 KG of force with a 15 KG load. But I'll be curious to hear how your progress goes!

Like dholdgreve said, the force on the compression member will be much more than 17kg. So you would have to increase the thickness of your tube sheet in order to hold a target load of 15 kg, which would most likely put it well over 7 grams. To address the problem of connecting the compression tube to the tension member, I am no expert, but I would suggest carving out a space to accommodate the tube. Use maybe, a 3/4" by 3/32" length for the tension member. I'm interested in how this'll go as well, but I would definitely suggest going for a more "normal" boomilever afterwards.

Thanks, guys,

I totally forgot about mechanical disadvantage, that means that the tube I made is actually rather weak for this purpose. The next time I buy wood, I'll try going for a 3/32 and a 1/8 balsa sheet, and see how those turn out. In addition, I'll buy wood to make a more "classic" boom as recommended. I think that with 3/32 wood, I might be able to get the hole boom under 9 grams. Another option worth exploring, I think, is to increase the diameter of the tube. Ideally, this should spread out the weight more, thus leading to more strength, right? It's like how small and fat soda cans are harder to crush from the top then tall skinny ones.

Thanks a lot for your suggestions,

arshah2

I experimented a bit with the tube design when I was a senior back in 2014. I used a solid 'plug' for the distal end that was sanded down to fit inside the tube by ~1/4 inch. The tension members were attached to the plug, which also featured a hole for the loading block assembly. Initial testing of this design yielded scores upwards of 1800 (booms just under 6g breaking around 10-11kg). These scores definitely could have been improved upon, but in the end I gave up on this design because it was too challenging for me to reproduce tube designs with high consistency (too risky for me).

I used 1/32 thick sheets for the tubes (I believe ~5/8" diameter but its hard to remember), and 1/16 bass for tensions. These pictures should pretty much explain of how the plugs were built.

Azn wrote:I experimented a bit with the tube design when I was a senior back in 2014. I used a solid 'plug' for the distal end that was sanded down to fit inside the tube by ~1/4 inch. The tension members were attached to the plug, which also featured a hole for the loading block assembly. Initial testing of this design yielded scores upwards of 1800 (booms just under 6g breaking around 10-11kg). These scores definitely could have been improved upon, but in the end I gave up on this design because it was too challenging for me to reproduce tube designs with high consistency (too risky for me).

I used 1/32 thick sheets for the tubes (I believe ~5/8" diameter but its hard to remember), and 1/16 bass for tensions. These pictures should pretty much explain of how the plugs were built.

Wow, I hadn't heard of any competitive tube booms besides the first, long back. How did you "tubify" your wood?

Azn wrote:

this is irrelevant but at first i thought you were using chopsticks to hold the wood

waffletree wrote:

Azn wrote:

this is irrelevant but at first i thought you were using chopsticks to hold the wood

^^^ me too lol

I feel that this idea not that viable this year as there is no minimum treaded rod length for the loading block and is only viable depending on the Tournament. For example, at U Chicago their loading block had an eye hook with a threaded rod length that was only long enough to pass through the wooden block. This would prevent teams form sliding the rod through the boomilever and attaching both the block and wingnut on top.

What did you use to keep the wood bent around the form to dry?