Boomilever B/C

[_HenryHscioly_]

Thanks Balsaman!

I have a something like this: http://www.pitsco.com/store/detail.aspx?ID=2097&bhcp=1
just two boards that make 90 degrees, and I stick textbooks to weigh it down the base, and let boomilever hang over a table.
I tested mine so that the bucket was only ~1/8" above the floor, hopng that it'd help, but the weight of the chain and block still enough to damage my boom.
Maybe I'll build and overhangng above, like a permanent boomlever, to support the bucket if my boom that I'm testing fails.

What is a good way to tighten the bolt consistently? Would doing it by "feel" be adequate...?

I didnt use washer because I thought that I could cut weight from the base by making it smaller. If surface area matter, then means that the base will inevitably be compressed, and likely to a degree that could cause failure?

End grain sandwich sounds super ideal!

Thanks

[Balsa Man]

Glad it was helpful.

Tightening consistently.... The absolute way would be a small torque wrench. If you're good/careful, by hand should work. Figure out how tight is right; you can then reproduce by keeping track of when nut first comes snug and really starts to tighten, and turning the same amount past that each time- like first snug + 1/4 turn. You really don't want it to crush- once crushing, will keep happening- you want it stiff enough to not crush, even if that means area enough to support a washer- a little extra weight, a lot more performance.

[Acefire]

It's my first year doing Boomilever, can anyone tell me where I can get Varney's instacure glue, or some balsa wood? I'm not really sure how to make there be less compression, and my partner is only a year older than me so she isn't sure either.

[Acefire]

I found Aia's Boomilever guide very helpful, if you go onto the Wiki and scroll down to the bottom there is a link. The wiki is good too, but it's a bit more technical.

[iwonder]

Balsa wood can be found at most hobby and craft stores, I've never heard of the specific type of glue, but any superglue works well, and as to the compression forces, that was discussed in the boomilever b/c thread, it might be worth taking a look at.

[jma]

Is bracing between tension members and compression members recommended?

[juicemanman]

I would, but not too much. The mass of your boomilever counts as much as the amount it holds.

[jma]

I would, but not too much. The mass of your boomilever counts as much as the amount it holds.

Thanks.

[Balsa Man]

A few thoughts/concepts to think about regarding bracing between tension and compression members, and “the mass of your boom counts as much as it holds.”

Let’s start with scoring. Yes, both mass of boom and how much it carries “count”; but it is important to understand how. Your score is pure structural efficiency – the weight carried divided by the weight of the boom. So, a 10gram boom carrying 15kg would score 1,500; a 15 gram boom carrying 15 kg would score 1,000. A 5gram boom carrying 7.5kg would also score 1,500, and a 7.5gr boom carrying 7.5kg would also score 1,000…. So, an initial design decision becomes do you want to go for a “full load’ approach, or a lighter, partial load one?

Which ever way you go, if you’re seriously trying to be competitive, it’s all about efficiency- the least weight to carry the design load; every piece should have a specific purpose; you should understand what forces each piece has to carry at design load (tension or compression, and how much), and the wood selection for each should be the lightest that will carry design forces. Just putting some pieces in…some places is not efficient.

So, let’s consider that principle regarding bracing between tension and compression members.
First, looking just at the tension members- bracing tension members gets you absolutely nothing- there is nothing to be braced, no need for bracing, no forces trying to move the piece in tension out of alignment- the tension force is “axial”- it is along the axis- it acts to pull the piece straight.

The situation with compression members is very different- compression members are long columns under axial compression load. They initially/primarily fail by “buckling”- as you put force on, up to some load, everything is fine, then with just a little more force, the member bows out in the middle, and then breaks. You need to understand how this works. See the Ongoing Contest(Scores) thread here in the boomilever; also see discussion in the Tower thread from last year; also see wikipedia on “buckling.” Bracing columns under compression – to cut down the “effective column length” is an important concept. Done properly, it can be very efficient. If you study the information in the posts and on Wikipedia, you’ll see that the buckling strength has an inverse square relationship to the effective column length; cut the effective column in half, and its buckling strength goes up by a factor of 4 (2 squared). So, let’s say you have a compression member that without any bracing carries 2kg before buckling. If you (properly) brace at the midpoint, that buckling strength goes up to 8kg. If you then add bracing at the midpoints of the segments either side of the midpoint brace, the column’s buckling strength goes up to 32kg.

The problem is “properly” bracing. There’s lots of discussion about this in last year’s tower thread. When a member in compression first starts to buckle, the force it takes to hold it in line/to stop the initial bowing is very low; the bracing can be very light weight. To keep the braced point from moving, the bracing needs to be attached to a point (or points) that also can’t move. Also, need to understand one other thing about buckling- which direction/plane buckling is going to happen. It is going to be in the plane of the shortest cross section. In a square cross section member, that will be toward one of the sides. Which one will depend on…..little imperfections; a real member is never going to be perfectly straight- it’ll be bowing very slightly one way or another. What will not happen is bowing in the diagonal planes- toward one of the corners of thee square; that’s because the cross section across a diagonal is longer (1.414x) that the cross section between faces. So, looking end-on at a pair of boom compression members. Let’s call the plane they’re in the x plane- it’s horizontal. Let’s call the plane perpendicular to that (for each member) the y plane. It’s easy, and efficient to brace things in the x plane- ladders and Xs, or Z-bracing; it’ll work, minimal added weight. In the y plane, much more problematic. Even under tension load, the tension member will have up and down…..give to it- doesn’t really pin the end of braces attached to it (and running down to the compression member in space. Any load from the compression member starting to buckle going to the tension member will increase the tension force- meaning the member will have to be heavier/stronger. To get it stiff enough to work decently, you’d be adding significant weight. It would be much more efficient to go to a rectangular cross section in the compression members – with the long dimension of the rectangle in the y plane- that way bracing between the compression members braces both in the x plane. The bigger cross section in the y plane becomes what stiffens the members in that plane.

[ptkid]

So for attachment of the tension members to the base should I stick them on the two outer edges of the base or should I drill to holes inside of the base and stick the tension members in there? I have tried the sticking on the outside approach so far using Loctite Ultra Glue Control and almost every time it breaks there. The maximum weight I have gotten has been around 6.8 kilograms and it breaks in the same all the time. What has been working for you?