Boomilever B/C

[knightmoves]

Hello! I was wondering what people are doing for their hook connections? I tried putting a 1/4*1/8 piece between my tension members and laminating on top, bottom, sides, and back, but it broke super easily : (

What broke - the wood or the joint? How did it fail?

If the wood snapped, you need something thicker and/or stronger. If the joint failed, you need a stronger joint (better glue? more surface area? more lamination / gussets / whatever?)

[BalsaFerret]

As everyone is familiar with the "one finger buckling test", by Balsa Man himself, in order to categorize compression members, the general trend (or at least in my case) is that buckling strength is directly proportional to weight: a heavier stick would more or less have a greater buckling strength. However, as buckling only occurs when a member is under a critical compressive load, I was wondering if buckling strength has any relationship with tensile strength, as compressive strength has to do with buckling. If buckling strength increases (in this case a bass stick), would the tensile strength increase as well, or would there be no direct relationship? So far I've been using sticks with stronger buckling strengths, and I am under the impression that there might be some relationship due to greater loads held - could be placebo, you never know! I understand that tension is mainly based on the cross section as well as the angle that the tension member is subjected to, but it would be interesting to see if there are any other variables that also factor in.

[jinhusong]

We align all tension sticks parallel together, like all bar-code at one end and facing up.
Then move them to the edge of a table. Let most part of the tick hang out of table with just like 1 inch of the stick pressed to the table by fingers.

Then we just pick the ones with less drooping.

I believe we learned from Balsa Man a few years ago.

BTW, I want to make a correction of my claim one year ago. I said 1/16X1/16 bass stick never failed for tension. Now 3 or 4 boomilevers we built all broke tension around 11-13 kg load. My guess, when we did the initial pulling test, we were using very heavy sticks. They hold more than 40 kg but the pulling was ideal pulling, not in boomilever. Searching forums of 2013, 2014 boomilever event, most uses 3/32X3/32 bass for tension.

Tiger

[knightmoves]

As everyone is familiar with the "one finger buckling test", by Balsa Man himself, in order to categorize compression members, the general trend (or at least in my case) is that buckling strength is directly proportional to weight: a heavier stick would more or less have a greater buckling strength. However, as buckling only occurs when a member is under a critical compressive load, I was wondering if buckling strength has any relationship with tensile strength, as compressive strength has to do with buckling.

Look up Euler's critical load, and you'll find an expression for buckling strength of thin columns (like the sticks that you use in a boomilever). Some of the factors in that expression are related to tensile strength, and some are less directly related.

[dholdgreve]

As everyone is familiar with the "one finger buckling test", by Balsa Man himself, in order to categorize compression members, the general trend (or at least in my case) is that buckling strength is directly proportional to weight: a heavier stick would more or less have a greater buckling strength. However, as buckling only occurs when a member is under a critical compressive load, I was wondering if buckling strength has any relationship with tensile strength, as compressive strength has to do with buckling. If buckling strength increases (in this case a bass stick), would the tensile strength increase as well, or would there be no direct relationship? So far I've been using sticks with stronger buckling strengths, and I am under the impression that there might be some relationship due to greater loads held - could be placebo, you never know! I understand that tension is mainly based on the cross section as well as the angle that the tension member is subjected to, but it would be interesting to see if there are any other variables that also factor in.

Buckling Strength and Tensile Strength are apples and oranges... The tensile strength of the wood remains constant, no matter how long the piece is. Buckling strength, or compression strength is based totally on unsupported length. The shorter the unsupported length is, the stronger it is in compression. Let's say a piece of 1/4" x 1/4" x 36" balsa has a 300 gram SFPD. If this piece is then cut in half (18") I would expect each half to now have an SFPD of 1,200 grams. If each of these pieces are then braced in the center so that you now have (2) unsupported lengths on each side of the brace, I'd expect a compressive strength (SFPD) of about 4,800 grams. Now bisect each of those unsupported lengths and add (2) additional braces, so you have (4) unsupported lengths of roughly 4-1/2" each. The compressive strength now goes up to 19,200 grams.

Now keep a couple things in mind: Wood, especially balsa wood is NOT homogeneous. Just because one end tests at 1,200 grams, doesn't necessarily mean the other end will. These numbers are theoretical based on Euler's Buckling Theory. Just like a chain, boom is only as strong as it's weakest link. If you have a bad joint and the glue pops, it is not a compression failure, it is a joint failure.

It is important to analyze the load to be carried, and identify what the force exerted to each compression beam is, then identify what the SFPD of the compression stick is, then determine how many braces are required to support said load. And NO, the load to be carried is NOT the same as the force exerted on the beams.

Finally, consider this: Tensile forces do not require that the piece be subdivided to carry a greater load, however bracing may be attached to the tensile pieces to offer support to the compression pieces as needed.

[MadCow2357]

Not sure if I already asked this question (please excuse my bad memory if so), but does SPFD decrease the strength of the member tested?

Also, from skimming over the previous few posts, is the verdict that buckling strength and tensile strength have no relationship whatsoever?

[dholdgreve]

Not sure if I already asked this question (please excuse my bad memory if so), but does SPFD decrease the strength of the member tested?

Also, from skimming over the previous few posts, is the verdict that buckling strength and tensile strength have no relationship whatsoever?

Think about it this way, then you decide... Consider a piece of 1/16" x 1/16" bass wood. It makes no difference how long it is... 1" or 36" it will carry the same amount in tension.
Now consider the same piece at 1", 5", 10", and 15" Placing one end on the table, and pushing down on the other, can the same be said of the compression forces? As the unsupported length of a piece increases, the compressive strength is substantially reduced.

As for your other question, I guess it depends on how far you push down on the piece with your finger. If you push down to the point that the piece actually breaks, consider it compromised
.

[knightmoves]

As for your other question, I guess it depends on how far you push down on the piece with your finger. If you push down to the point that the piece actually breaks, consider it compromised

We've certainly had enthusiastic testing of long, light sticks of balsa that have left them with a pronounced bow. I wouldn't recommend using sticks you've distorted like that for anything important.

[PayalParikh]

We are doing Boomilever for the first time and not sure of couple of rules.
1. Do we need to prepare/submit log book for competition?
2. How do we submit the Estimated Loading Supported to the officials at the event? Is it verbal or do we need to write it down as in log?

[dholdgreve]

We are doing Boomilever for the first time and not sure of couple of rules.
1. Do we need to prepare/submit log book for competition?
2. How do we submit the Estimated Loading Supported to the officials at the event? Is it verbal or do we need to write it down as in log?

No log is actually "required" by national rules in Boomilevers, but if you are not keeping one, you are really doing yourself a disservice.
When we run a competition the estimated load held is given verbally to the check-in specialists. This may vary by competition.