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Re: Boomilever B/C
Posted: February 26th, 2020, 9:08 am
by Crtomir
Torchic wrote: ↑February 24th, 2020, 6:47 pm
Increasing the density of balsa wood drastically increases the strength. An 1/8" by 1/8" would be stronger than a 3/16" by 3/16" with the same weight because the density in the 1/8" by 1/8" is higher.
Actually, it's opposite. For two sticks with the same
weight, the stick with the bigger cross-section area is going to be the strongest under a compressive load. It's all in the Euler Critical Buckling Load Equation. So a 1/8" x 1/8" stick that weight the same as a 3/16" x 3/16" stick is going to be weaker, not stronger.
Now, if you have two sticks with the same
density, the stick with the larger cross section will be stronger, but also weigh more and therefore may or may not be less efficient than the stick with the smaller cross section.
Essentially, Euler's Critial Buckling Load Equation says that the maximum load (critical load) you can apply compressively to a column (stick) is inversely proportional to the length of that column (stick)
squared and directly proportional to the cross sectional area
squared. So if you have square sticks, the critical (max) load you can apply is directly proportional to the side length to the
fourth power! On the other hand, Euler's equation says the critical (max) load is only directly proportional to the Modulus of Elasticity (MOE) to the
first power. Typically the MOE increases linearly with density, so essentially you can say that the critical (max) load is only directly proportionally to the density to the
first power and directly proportional to the side length to the
fourth power.
That is why it is usually advantageous to increase the cross-sectional area of your stick, while keeping the same density as the thinner stick, in order to increase the maximum compressive load that the stick can handle before buckling (failing). Increasing the density, while keeping the stick thickness the same, does increase the strength, but you get more bang for your buck by increasing thickness rather than density. This is an important principle and is kind of one of the main things we hope students will learn from this event.
Hope this helps.
Re: Boomilever B/C
Posted: February 26th, 2020, 3:30 pm
by Torchic
Crtomir wrote: ↑February 26th, 2020, 9:08 am
Torchic wrote: ↑February 24th, 2020, 6:47 pm
Increasing the density of balsa wood drastically increases the strength. An 1/8" by 1/8" would be stronger than a 3/16" by 3/16" with the same weight because the density in the 1/8" by 1/8" is higher.
Actually, it's opposite. For two sticks with the same
weight, the stick with the bigger cross-section area is going to be the strongest under a compressive load. It's all in the Euler Critical Buckling Load Equation. So a 1/8" x 1/8" stick that weight the same as a 3/16" x 3/16" stick is going to be weaker, not stronger.
Now, if you have two sticks with the same
density, the stick with the larger cross section will be stronger, but also weigh more and therefore may or may not be less efficient than the stick with the smaller cross section.
Essentially, Euler's Critial Buckling Load Equation says that the maximum load (critical load) you can apply compressively to a column (stick) is inversely proportional to the length of that column (stick)
squared and directly proportional to the cross sectional area
squared. So if you have square sticks, the critical (max) load you can apply is directly proportional to the side length to the
fourth power! On the other hand, Euler's equation says the critical (max) load is only directly proportional to the Modulus of Elasticity (MOE) to the
first power. Typically the MOE increases linearly with density, so essentially you can say that the critical (max) load is only directly proportionally to the density to the
first power and directly proportional to the side length to the
fourth power.
That is why it is usually advantageous to increase the cross-sectional area of your stick, while keeping the same density as the thinner stick, in order to increase the maximum compressive load that the stick can handle before buckling (failing). Increasing the density, while keeping the stick thickness the same, does increase the strength, but you get more bang for your buck by increasing thickness rather than density. This is an important principle and is kind of one of the main things we hope students will learn from this event.
Hope this helps.
Woah thats a lot. This year we went with 1/8"s instead of 3/16"s because our density was really low, and it would be a lot less for the 3/16"s of the same weight. There's also the issue of money (3/16"s that low cost 8x as much) so we went with the 1/8"s.
Thank you for the info tho! Hopefully our towers/bridges team can use it this year, because this may be our final competition :<
EDIT: The reason why I said the 1/8s are stronger is because around the low end of the spectrum, the strength rapidly shoots up compared to density. Usually the 1 gram 3/16" we get have these air bubbles in them. Not sure if I'm right about the 1/8"s being stronger, just want to give my reasoning.
Re: Boomilever B/C
Posted: February 26th, 2020, 7:07 pm
by MadCow2357
Crtomir wrote: ↑February 26th, 2020, 9:08 am
Torchic wrote: ↑February 24th, 2020, 6:47 pm
Increasing the density of balsa wood drastically increases the strength. An 1/8" by 1/8" would be stronger than a 3/16" by 3/16" with the same weight because the density in the 1/8" by 1/8" is higher.
Actually, it's opposite. For two sticks with the same
weight, the stick with the bigger cross-section area is going to be the strongest under a compressive load. It's all in the Euler Critical Buckling Load Equation. So a 1/8" x 1/8" stick that weight the same as a 3/16" x 3/16" stick is going to be weaker, not stronger.
Now, if you have two sticks with the same
density, the stick with the larger cross section will be stronger, but also weigh more and therefore may or may not be less efficient than the stick with the smaller cross section.
Essentially, Euler's Critial Buckling Load Equation says that the maximum load (critical load) you can apply compressively to a column (stick) is inversely proportional to the length of that column (stick)
squared and directly proportional to the cross sectional area
squared. So if you have square sticks, the critical (max) load you can apply is directly proportional to the side length to the
fourth power! On the other hand, Euler's equation says the critical (max) load is only directly proportional to the Modulus of Elasticity (MOE) to the
first power. Typically the MOE increases linearly with density, so essentially you can say that the critical (max) load is only directly proportionally to the density to the
first power and directly proportional to the side length to the
fourth power.
That is why it is usually advantageous to increase the cross-sectional area of your stick, while keeping the same density as the thinner stick, in order to increase the maximum compressive load that the stick can handle before buckling (failing). Increasing the density, while keeping the stick thickness the same, does increase the strength, but you get more bang for your buck by increasing thickness rather than density. This is an important principle and is kind of one of the main things we hope students will learn from this event.
Hope this helps.
I was thinking this as well, but thought I was just imagining things (I wasn't too confident about how much I remembered from Euler's buckling lol). However, one thing worth noting is that a balance must still be achieved between density and cross-sectional area (assuming the weight of the stick is constant). If you go too light with the balsa you choose, the boom is prone to "blow-out", as dholgreve likes to call it iirc. That's when the glue joint does not fail, but the balsa wood immediately around the glue does (it kind of rips off from shear force I think). Be careful when sacrificing density for X-sectional area!
Also, nice to see you back Crtomir! From what I remember, it's been quite a while since you've been active in structure event forums!
Re: Boomilever B/C
Posted: February 27th, 2020, 5:51 am
by scioly2345
Hi Boomilever people,
So I need some advice on attaching verticals. I don’t know whether to use bass or balsa, or how to get them on properly because of the annoying angles of the compression and tension pieces. My breaks this season have mostly been compression explosions because they bend upwards. Any tips on how to do this? I just want to do one vertical on each side of the boomi so I can shrink the size of my compression to save mass. So far I’ve just made the main supports of my compression so tall and thick to the point where verticals aren’t needed, but with that comes the fact that the compression is so. heavy. Which is not cool efficiency wise.
My next steps to make my boomi lighter is 1. Learn how to properly do verticals so I can shrink my compression and 2. Change my bass size from 1/8x1/16 to either 1/8x1/32 or 3/16x1/32 for tension. I have experimented taking off lamination at either the hook area (I use a dowel) or the distal end, but that very experimenting told me that that excessive lamination, although mass consuming, is not optional, so I’m not going to mess around with lamination. I’d rather have a compression break than a lamination/dowel break for sure.
My peak efficiency this year was a 1.7k but the highest I’ve gotten at competition is 1.5k, because I’ve been too scared to go to light because I don’t want to lose the bonus. UPenn efficiencies were scary good. So I need something to save lots of mass for NYS. My guess is around 1.3k for top ten, but I have NO idea of the competitiveness of the top 5.
Re: Boomilever B/C
Posted: February 27th, 2020, 10:33 am
by JonB
scioly2345 wrote: ↑February 27th, 2020, 5:51 am
Hi Boomilever people,
So I need some advice on attaching verticals. I don’t know whether to use bass or balsa, or how to get them on properly because of the annoying angles of the compression and tension pieces. My breaks this season have mostly been compression explosions because they bend upwards. Any tips on how to do this? I just want to do one vertical on each side of the boomi so I can shrink the size of my compression to save mass. So far I’ve just made the main supports of my compression so tall and thick to the point where verticals aren’t needed, but with that comes the fact that the compression is so. heavy. Which is not cool efficiency wise.
My next steps to make my boomi lighter is 1. Learn how to properly do verticals so I can shrink my compression and 2. Change my bass size from 1/8x1/16 to either 1/8x1/32 or 3/16x1/32 for tension. I have experimented taking off lamination at either the hook area (I use a dowel) or the distal end, but that very experimenting told me that that excessive lamination, although mass consuming, is not optional, so I’m not going to mess around with lamination. I’d rather have a compression break than a lamination/dowel break for sure.
My peak efficiency this year was a 1.7k but the highest I’ve gotten at competition is 1.5k, because I’ve been too scared to go to light because I don’t want to lose the bonus. UPenn efficiencies were scary good. So I need something to save lots of mass for NYS. My guess is around 1.3k for top ten, but I have NO idea of the competitiveness of the top 5.
A few things to consider here. Learning how to properly do verticals that are light is a huge challenge but is an absolute must if you want a light design that can hold the full load. For your bass tension pieces-
do they hold full load? If so, make them lighter. Find the lightest you can use that can still hold full weight. I would move away from a dowel at the connection and experiment with some other designs. We use bass in tension and our verticals are all balsa.
I understand going "conservative" at competition but (especially this year) no risk, less reward. Our team"flirts" with disaster in boomi. We know it will break around 15kg and usually it holds, and sometimes it doesn't (looking at you, MIT). UPenn was relatively competitive with just a handful of scores at, or above, 2k but I am sure there will be more above 2k at the NY state championship.
Weigh the pieces of your boomi. Which are the heaviest? Do they need to be that heavy? Only way to know is to keep building, change one thing, and then record results. If those pieces hold up easily, make them lighter.
Re: Boomilever B/C
Posted: February 27th, 2020, 1:46 pm
by dankdecidueye
Hello everyone,
I've been stuck at 1300 for a while now for my score. Since I was using the normal boomilever design type before, should I switch to a tower chimney type design? How would they look like this season, and would height be an advantage (having a taller range of space the tower chimney occupies)?
Re: Boomilever B/C
Posted: February 27th, 2020, 2:08 pm
by MadCow2357
dankdecidueye wrote: ↑February 27th, 2020, 1:46 pm
Hello everyone,
I've been stuck at 1300 for a while now for my score. Since I was using the normal boomilever design type before, should I switch to a tower chimney type design? How would they look like this season, and would height be an advantage (having a taller range of space the tower chimney occupies)?
In short, both designs can be extremely successful. Most top teams seem to be using normal tension booms, but high scores are also very possible with tower chimney. The div B kid I'm coaching has broken 2000 with chimney, and should be approaching 2500 soon. I've achieved decent results with chimney as well, though it is quite complicated to build compared to the normal tension boomilever.
Height is certainly the TC's biggest advantage
Re: Boomilever B/C
Posted: February 27th, 2020, 2:18 pm
by xiangyu
Just curious to see what's possible, how much do y'all's tension pieces weigh? (the tension pieces + the base) My design is about 2.7 grams but I'm trying to see if I can decrease that a bit...
Xiangyu
Re: Boomilever B/C
Posted: February 27th, 2020, 3:24 pm
by MadCow2357
xiangyu wrote: ↑February 27th, 2020, 2:18 pm
Just curious to see what's possible, how much do y'all's tension pieces weigh? (the tension pieces + the base) My design is about 2.7 grams but I'm trying to see if I can decrease that a bit...
Xiangyu
I've been fine with much less (around 1.5 g), and I'm purposely overbuilding that part
Re: Boomilever B/C
Posted: February 27th, 2020, 3:38 pm
by xiangyu
MadCow2357 wrote: ↑February 27th, 2020, 3:24 pm
xiangyu wrote: ↑February 27th, 2020, 2:18 pm
Just curious to see what's possible, how much do y'all's tension pieces weigh? (the tension pieces + the base) My design is about 2.7 grams but I'm trying to see if I can decrease that a bit...
Xiangyu
I've been fine with much less (around 1.5 g), and I'm purposely overbuilding that part
Hmmm good to know. I've been using 1/8 by 1/16 for my tensions, do you think I'll be able to go lower? (About how low? I'm ordering new bass tensions soon)
Xiangyu