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GeorgeInNePa wrote:

thsom wrote:Compression boxes aren't necessary for a good score. I have had ok success without them (won't release too much but not MORE than 1600) However, is it in general consensus that compression boxes are better (as it's consensus that tension boomis are better than compression ones)?

EDIT: OH, and thanks iwonder!!!

EDIT: and with the new hook requirement, could you really do a single tension member design?

Yes. No problem at all with our single tension arm and mount.

We did break a tube at Regionals today. The weight of our tube boom was 9.88grams.

How do you use a single tension design with the hook? Also, how do you make sure the loading block doesn't fall off the tube or lean off center?

I think that's part of the challenge, and that part i think you should figure out yourself...

Got it thanks! (I think I finally figured it out ) Now i apologize for all of the questions but I'm kind of in my "discovery stage" of this since we are on a tight budget and I have little opportunity to test I spend an exorbitant amount of time on planning and during this time I start trying to figure most things out.
OKAY, so. If I were to use a singe tension member design, would i be correct in thinking that it would have to carry more force than when I had 2? Or does that not change it?

I believe it would have to be able to carry more.

thsom wrote:

GeorgeInNePa wrote:

thsom wrote:Compression boxes aren't necessary for a good score. I have had ok success without them (won't release too much but not MORE than 1600) However, is it in general consensus that compression boxes are better (as it's consensus that tension boomis are better than compression ones)?

EDIT: OH, and thanks iwonder!!!

EDIT: and with the new hook requirement, could you really do a single tension member design?

Yes. No problem at all with our single tension arm and mount.

We did break a tube at Regionals today. The weight of our tube boom was 9.88grams.

How do you use a single tension design with the hook? Also, how do you make sure the loading block doesn't fall off the tube or lean off center?

The loading block eye-bolt goes through the distal end of the tube. You assemble the bolt and block on the tube.

The tension arm/j-bolt interface is the hard part this year and I've been "told to shut up"...

Hi guys, I know the formula for tensile load is Tensile Strength * Cross-sectional Area. However, let's say I have 3/32" x 3/32" piece of basswood. The tensile strength I'm using is from http://www.matweb.com/search/datasheet. ... fb7&ckck=1 , which says it's 1.93 MPa. To calculate the load in kg of my load, I multiply 1.93 * 10^6 * (3/32)^2 and then divide that by 9.8 to but it into kg, which gives me 1731 kg. Which seems wrong. Is there a problem with my calculations/data? I think my tensile strength is wrong but I can't find another one. Please help!

Pascals are newtons per square meter, so you should convert inches into meters first, it'll give you about 10.9kg.

That doesn't seem right either, though. Given that we use 2 lengths of 3/32" basswood for the tension members, that means that it'll only hold about 21.8 kg. However, as Balsa Man and others have proven through calculations, the tension members are supposed to be able to withstand >40 kg which means that those 2 tension members are hardly enough. However, through experimentation, they're able to hold the full load? Why is that?

1.93 MPa is probably a little low for the wood you're using (I've also used 3/32" bass in the past for tension members).

The issue with wood (especially balsa, to an extent bass) is that it's so variable that those numbers are only rather rough estimates. Depending on the density of the wood it's strengths can vary immensely. Since you don't know what density those numbers were taken with, the water content of the wood, etc, there's no way to tell if that applies to your structure rather than through testing. This is why sometimes it's best to test individual parts of a boom when you're trying to make one, so you could build a tension testing rig and test a lot of different sizes to find the most efficient, or a compression testing rig to test different compression members, then just combine them in the final design. (watch out for glue surface areas)

Wouldn't some tests weaken or change the integrity of the wood. I know that for when testing balsa stiffness, many times the wood gets permanently bent..