Boomilever B/C

[TheScienceMusician]

If you have your compression member out near the end, it will break as you add weight to the bucket. If it is too far back, roughly 5 cm from the end, the loading block will pull the end down causing it to snap. If you have it between these two points, near 2.5 cm from the end, it will hold the block up and not break. You should mainly only put gussets near the end so the compression holds without letting go.

[iwonder]

Are you keeping track of the densities of all these parts and pieces? Your problem may be density variations in the balsa, they can be significant. Other than that, an I-Beam design seems difficult due to the lack of surface area for glue and the stress concentrations at all the joints(as you're finding out), so maybe you should try really low density, but large surface area balsa gussets.

[sjwon3789]

It's funny how that, after I laminated everything in the compression (the main 2), it broke at the joints xP

[Balsa Man]

First, a couple quick comments:
by sjwon3789 on Sun Mar 03, 2013 7:46 pm It's funny how that, after I laminated everything in the compression (the main 2), it broke at the joints xP

Actually, that’s exactly the sort of thing you should expect. Failure is going to happen at “the weakest link”- the first place that sees load beyond its strength- be that a joint, or a piece of wood.; the “initial failure mode.” Before the lamination, your initial failure mode was buckling failure in the compression members. By stiffening them with lamination, they held more, meaning the joints got to see more load than they had (before the compression members buckled). If you get those joints stronger, then you’ll find out what the next failure mode is.

by TheScienceMusician on Sun Mar 03, 2013 2:50 pm If you have your compression member out near the end, it will break as you add weight to the bucket. If it is too far back, roughly 5 cm from the end, the loading block will pull the end down causing it to snap. If you have it between these two points, near 2.5 cm from the end, it will hold the block up and not break. You should mainly only put gussets near the end so the compression holds without letting go

I think what you’re talking about is where, relative to the location of the center of the load block, you have your tension members connect to the compression members. We’ve talked about this before, and how it is really important that joining be right at where the center of loading is- if, as you correctly say, the center of the block is beyond that joint, it will pull down/break off the ends of the compression members. If the center of the block is on the wall side of that joint, it will bend/bow the compression members down, causing early buckling failure in them.

Now to HenryHcsioly’s questions/discussion.
First, its great to see you working with these understandings/factors!
Second, iwonder’s insights thoughts are right-on.
Third, beyond the fact of density variation in balsa, and as iwonder points out, the absolute need to track/control density used, even in 2 pieces at the same density, there is variation in “E” - the modulus of elasticity. That variation is on the order of 20% - pretty significant. Since density can vary greatly (25, maybe even 30%+) within a piece of wood, smaller pieces cut from larger ones can have both significant density variation, and for pieces that are matched in density, the actual “E’ can also vary. Remember that buckling strength is the product of E x I (divided by effective length squared). I think the range of variation in performance vs calculated you’re seeing is the combination of these factors working.

I’m comfortable with calculating buckling strengths for single pieces (allowing for the variability noted above)- and actual results align reasonably with the theoretical. How to deal with a constructed I-beam- the combination of web and two flanges, and the glue lines is ….beyond my knowledge.

So, while no magic answers, hope this helps.
by _HenryHscioly_ on Sun Mar 03, 2013 3:04 am I am having some problems with my compression member,

(details)
by iwonder on Sun Mar 03, 2013 3:22 pm Are you keeping track of the densities of all these parts and pieces? Your problem may be density variations in the balsa, they can be significant. Other than that, an I-Beam design seems difficult due to the lack of surface area for glue and the stress concentrations at all the joints(as you're finding out), so maybe you should try really low density, but large surface area balsa gussets.

[StampingDad]

I reallly hate to interrupt a technical discussion with my first post of the year... But, my sons won B and C Div Regional with Box beam tension booms identical except for angle. So they are advancing to state. C Division failed at the distal end's tension and compression joinder. B Division failed in the beam lamination midway between wall and loading block. Failure in both cases was in the glue not the wood. Query is what is the best way way to insure proper gluing across surface without using too much glue. And is there a technique to using the Gorilla glue per instructions, e.g. wetting the surface first. Attempts to wet balsa is not user friendly to them with the relative thin pieces used.

And since both built per Balsa man's instructions on box beams, I am fairly certain we will see a response from him on the above. But Balsa Man, I have a second question. Back with the elevated bridges, I recall you were a big proponent of 1/64 bracing. My sons though avoided this after an ES broke some putting it on the scale. I, however, found a good 3x36 piece of heavy 1/64 "Colorado" balsa in our supplies and would like to know if you are advising also using it for lateral bracing of the box beams.

My concern with the beams is getting an even gluing surface across the top sides but 1/64 or 1/32 I don't think it matters they just need to work to make sure the top is smooth. We are using as the top, the side which curves up after lamination. I, however, had them redo one beam which just seemed to be too curved. So next question for Balsa Man, is there such a thing as too curved? You had mentioned about building in angle for deflection and this seems consistent with that concept. I, however, would think that if you could get the beam's absolutely straight you would want straight but after wetting them and glueing them straight is not always going to be the end product.

Their success this weekend is an absolute testament to the value of this forum. Both have drifted to competitive Vex robotics limiting SO time and both the booms were their absolute first efforts. Followed Balsa Man's instructions on box beams and Aia's wiki on the laminate base. The only assistance they needed from me was on how to mirror the angle match on the tension members. (With the angle, we first marked the expected angle across the side of the base with a sharpie and aligned it by eyeball even with the bit on the drill press and then inserted a short piece of beam in that hole as a guide to align the drill bit for the second hole).

Now that is not saying that either would be competitive at every regional. There is still a lot of work to be done on glue weight and matching wood. I would really like them to also work on building in some extra angle for that initial deflection when weight is added and they need to also work a lot on attaching and leveling. I suggested to both that they attach the base to the wall and then add the loading block as just that bit of weight stabilizes the boom such that it is not whipping in the wind as you put the level to it. However, neither heard me. Well, actually, I think both heard me but neither listened to me.

And while both holes were made precise with a forstner bit, the younger son really had a hard time getting the bolt through. I suspect that the Gorilla glue on the interior may have expanded a bit more into the hole.

[nejanimb]

StampingDad, I'd just say to not use Gorilla Glue. Too difficult to make work well, as they seem to have found.

[iwonder]

I do rather well in contests without ever using gorilla glue, especially for box beams, gorilla glue is too much of a hassle. I used to use it on the tension member joints and the base, but even there superglue works just fine if you're careful.

[Balsa Man]

I reallly hate to interrupt a technical discussion with my first post of the year... But, my sons won B and C Div Regional with Box beam tension booms identical except for angle. So they are advancing to state. C Division failed at the distal end's tension and compression joinder. B Division failed in the beam lamination midway between wall and loading block. Failure in both cases was in the glue not the wood. Query is what is the best way way to insure proper gluing across surface without using too much glue. And is there a technique to using the Gorilla glue per instructions, e.g. wetting the surface first. Attempts to wet balsa is not user friendly to them with the relative thin pieces used.

And since both built per Balsa man's instructions on box beams, I am fairly certain we will see a response from him on the above. But Balsa Man, I have a second question. Back with the elevated bridges, I recall you were a big proponent of 1/64 bracing. My sons though avoided this after an ES broke some putting it on the scale. I, however, found a good 3x36 piece of heavy 1/64 "Colorado" balsa in our supplies and would like to know if you are advising also using it for lateral bracing of the box beams.

My concern with the beams is getting an even gluing surface across the top sides but 1/64 or 1/32 I don't think it matters they just need to work to make sure the top is smooth. We are using as the top, the side which curves up after lamination. I, however, had them redo one beam which just seemed to be too curved. So next question for Balsa Man, is there such a thing as too curved? You had mentioned about building in angle for deflection and this seems consistent with that concept. I, however, would think that if you could get the beam's absolutely straight you would want straight but after wetting them and glueing them straight is not always going to be the end product.

Their success this weekend is an absolute testament to the value of this forum. Both have drifted to competitive Vex robotics limiting SO time and both the booms were their absolute first efforts. Followed Balsa Man's instructions on box beams and Aia's wiki on the laminate base. The only assistance they needed from me was on how to mirror the angle match on the tension members. (With the angle, we first marked the expected angle across the side of the base with a sharpie and aligned it by eyeball even with the bit on the drill press and then inserted a short piece of beam in that hole as a guide to align the drill bit for the second hole).

Now that is not saying that either would be competitive at every regional. There is still a lot of work to be done on glue weight and matching wood. I would really like them to also work on building in some extra angle for that initial deflection when weight is added and they need to also work a lot on attaching and leveling. I suggested to both that they attach the base to the wall and then add the loading block as just that bit of weight stabilizes the boom such that it is not whipping in the wind as you put the level to it. However, neither heard me. Well, actually, I think both heard me but neither listened to me.

And while both holes were made precise with a forstner bit, the younger son really had a hard time getting the bolt through. I suspect that the Gorilla glue on the interior may have expanded a bit more into the hole.

Congrats on the wins. Nice to hear that sharing insights is helping folk.
Gorilla Glue vs CA for box beams- pros and cons. Time to get things precisely aligned, and good glue distribution is the big plus for GG, BUT, you've got to have precise jigging, and a way to get solid, even, clamping pressure on, and maintain it....an hour+. The wetting needed doesn't need to be much- paper towel with just a little water, squeezed around to distribute, wrung out thoroughly to be just slightly damp, one brush along one piece.
With CA, you want the slowest, and you have to move decisively and quickly. "Black CA" gives you more time than any other I've found - it's called "tire glue" at Hobbytown- it's slow CA with rubber nano particles; slower than regular "slow" CA. It also has a little elasticity, as opposed to the brittleness of regular CA, which is a plus in box beams. Syringe or fine applicator tip works fine for GG- as you've found, it doesn't take much. While it is....wasteful of glue, a way to do CA is "run a (fairly fine) line on a piece of....anything-paper, plastic, aluminum- use a guide to get it straight- run to a bit over needed length. Take the piece you're wanting to get it on, and just.....bump it into the line- done right, you get a fine, but consistent line on the piece to be glued. You can, if it looks like too much, bump that glue line edge on a piece of paper to pull some off. Then into position. As w GG, you want a way to get even pressure on, but you only need it for....30 sec to a minute, as in steel bar, or a piece of wood, with hand pressure.

Getting box beams straight is really important, and can be challenging. If they're not straight,, when under load, they're already preferentially set up for buckling- the bowing has started- buckling will be at much less load than on a straight one. The "angle for deflection" I discussed was about setting the angle of (straight) compression members a bit up from horizontal, so whan tension member(s) stretch, it ends up horizontal as it approaches full load. With GG, minimal wetting will help.

1/64th "Xs" - Yes, as the tension component in a "Xs and ladders" bracing configuration - for "X-plane" bracing between two compression members, I really do believe they are a highly efficient way to go. They do need to be taught to work well- glue one end down w/ the strip aligned through where the other end needs to go. Then put glue at the second contact point, put a little pull on, and lower it into contact. ESs btw, should not be allowed to handle.....a competitive structure, unless you KNOW they know enough to handle properly. 1/64th Xs work great for what they're designed for- tenson along axis, but NOT for ...side loads.
Yes, attaching to the wall, then adding load block works- as long as someone is holding the compression memner(s) in wall contact/alignment, whail partner is doing the load block. Putting block on, then doing wall hookup also can work fine.

Hope these thoughts help.

[sjwon3789]

First, a couple quick comments:
by sjwon3789 on Sun Mar 03, 2013 7:46 pm It's funny how that, after I laminated everything in the compression (the main 2), it broke at the joints xP

Actually, that’s exactly the sort of thing you should expect. Failure is going to happen at “the weakest link”- the first place that sees load beyond its strength- be that a joint, or a piece of wood.; the “initial failure mode.” Before the lamination, your initial failure mode was buckling failure in the compression members. By stiffening them with lamination, they held more, meaning the joints got to see more load than they had (before the compression members buckled). If you get those joints stronger, then you’ll find out what the next failure mode is.

by TheScienceMusician on Sun Mar 03, 2013 2:50 pm If you have your compression member out near the end, it will break as you add weight to the bucket. If it is too far back, roughly 5 cm from the end, the loading block will pull the end down causing it to snap. If you have it between these two points, near 2.5 cm from the end, it will hold the block up and not break. You should mainly only put gussets near the end so the compression holds without letting go

I think what you’re talking about is where, relative to the location of the center of the load block, you have your tension members connect to the compression members. We’ve talked about this before, and how it is really important that joining be right at where the center of loading is- if, as you correctly say, the center of the block is beyond that joint, it will pull down/break off the ends of the compression members. If the center of the block is on the wall side of that joint, it will bend/bow the compression members down, causing early buckling failure in them.

Now to HenryHcsioly’s questions/discussion.
First, its great to see you working with these understandings/factors!
Second, iwonder’s insights thoughts are right-on.
Third, beyond the fact of density variation in balsa, and as iwonder points out, the absolute need to track/control density used, even in 2 pieces at the same density, there is variation in “E” - the modulus of elasticity. That variation is on the order of 20% - pretty significant. Since density can vary greatly (25, maybe even 30%+) within a piece of wood, smaller pieces cut from larger ones can have both significant density variation, and for pieces that are matched in density, the actual “E’ can also vary. Remember that buckling strength is the product of E x I (divided by effective length squared). I think the range of variation in performance vs calculated you’re seeing is the combination of these factors working.

I’m comfortable with calculating buckling strengths for single pieces (allowing for the variability noted above)- and actual results align reasonably with the theoretical. How to deal with a constructed I-beam- the combination of web and two flanges, and the glue lines is ….beyond my knowledge.

So, while no magic answers, hope this helps.
by _HenryHscioly_ on Sun Mar 03, 2013 3:04 am I am having some problems with my compression member,

(details)
by iwonder on Sun Mar 03, 2013 3:22 pm Are you keeping track of the densities of all these parts and pieces? Your problem may be density variations in the balsa, they can be significant. Other than that, an I-Beam design seems difficult due to the lack of surface area for glue and the stress concentrations at all the joints(as you're finding out), so maybe you should try really low density, but large surface area balsa gussets.

Hmmm...for my next step, I'll try laminating between the previous lamination and the joint then sand it off (since it'll still be connected) and see if it's a success. It only held like 1 more kg after laminating everything compared to none...

[thsom]

Hey guys, how can I make the base for my boomilever lighter? My base is currently...3-4 grams... Waaaayyyy too excessive... I use one layer of a 3/16 balsa sheet and 2 layers (on either side of the balsa) of 1/16 bass sheet. this base is about 6 cm by 3 cm... This is currently what holds my boom weight from breaking 10 grams...