How can I improve the efficiency of my Boomilever design?

[koujidaisuki76]

Hello!

I am in the Science Olympiad and I am doing the event called Boomilever where I have to make a cantilever like structure that is attached to the wal. The boomilever has to hold 15 kg or 33 pounds at one end. It can break but the purpose of this event is to have the highest efficency. Right now I'm trying to make a 10-15 gram boomilever that will actually hold. I've made three of these and they have all broken. I would think since I've made bridges and towers before it wouldn't be hard but boomilevers turns out that it's properties are completely different then that of the bridges and etc..

Specs:

It can only be made out of wood. I use bass and balsa wood..
for glue I use CA ZAP (pink) and it tends to work very well..

Size: the loding block can not be lower then 20 cm from the bolts and the boomilever can not be less then 40 cm...

and the loading bloack is 5 by 5 by 2 cms.

My observations based on my failures..:

My boomilever I have made looks like this:

(see attachment)

After I tested it, I noticed that there was a lot of force ont he bottom part of the boomilever. Compression force. The top part is tension and it seems for now I do not have problems with that.
My problem is the compression part.
How do I make a boomilever that can withstand the compresion forces?

It's hard to see in the pic but the bottom of the boomilever is made of 2 1/8 by 1/8 bass wood with X's glued in between

 

[FredGarvin]

Let's start with this question: What is the mode of failure you are seeing in the previous attempts?

 

[koujidaisuki76]

The bottom wood of the boomilever snapping because of the compression forces.

 

[Danger]

Snapping how? Could you perhaps Photoshop in some arrows and comments so we have a better understanding of what's going on? Or better yet, add a photo of the broken unit.

 

[koujidaisuki76]

the red X is where the boomilever snaps..

as the weight is pulling the boomliever.. (red arrow pointing down)
the wood is compressed and snaps where the red X's are..

 

[russ_watters]

What they mean is does it bend up, down, sideways...?

 

[koujidaisuki76]

When the weight is applied the bottom actually arches downward... to the point when it snaps..

 

[AlephZero]

When the weight is applied the bottom actually arches downward... to the point when it snaps..

Oh good, it failed the way I expected it would

There are 2 ways a rod can fail with a compressive load. One is if the axial stress is too big, but most materials can take a higher stress in compression than in tension without failing, so that's not usually the problem.

The other way is by buckling. Try getting a 1 or 2 meter length of thin wood (say a 3 or 4 mm square or round section), put it vertically with one end on the floor and push down on the top end. Nothing much happens till the load reaches some critical value, then the rod suddenly bends sideways in a random direction, and the force needed to bend it further is small.

If you look up "Euler buckling of columns" on the web you will find the limiting load for buckling depends on the length and the moment of inertia (I) for the rod. You can't change the length but you can change I. What you have done already was to make a column with a big I in the sideways direction. The two rods linked by the cross pieces all act as one "beam" structure so the value of I is proportional to the d^2 where d is the distance between the two rods. But the I in the vertical direction is small because you don't have anything similar, so it buckled in the vertical plane.

It's your competition entry not mine, so how you fix that problem is up to you...

 

[koujidaisuki76]

Could you explain the equation of Euler? and how I can use it for my designs? I tried to read it but it's very confusing since I am only in Pre-Cal right now and learning the basic levels of physics..

I understand the Young's Modulus which is E but I don't understand L(subscript e) /r...

Also how do you figure out the area moment of inertia?

"But the I in the vertical direction is small because you don't have anything similar, so it buckled in the vertical plane." So in the vertical direction inertia has to be large.. how do you determine how to make inertia larger without chaning the length of the wood?

 

[AlephZero]

If you haven't done calculus yet, don't bother about all the details. The important result is that failure force is proportional to I/l^2. You can't change the length because you have to make your structure a certain minimum size, but you can change I.

Eulers equation only applies to a perfectly straight column with the compression load perfectly lined up with the axis. What you have is more complicated than that, but the general principle still applies.

If you have a symmetrical structure, and you use two axes through the centre of gravity of the cross section, the I in Eulers equation is the sum of (area)*(distance from the axis squared).

Usually there are two different values of I for the two axes, unless it's a shape like a square or a circle.

I Hope that's enough to understand my picture. Since you haven't done calculus, take it on trust that the I for a square of side a is a^4/12.

NB these are not the same I that you would use for rotation problems. In fact I(rotation) = Ix + Iy where Ix and Iy are the moments of area about the two axes.

The bottom line is, if you want to increase M.I, you need to get some area a long way from the centroid of the section. If you look at your complete structure, actually you already have some more "area" that you could use...

Look at a tower crane on a building site. That is doing pretty much the same job as your structure needs to do.

Edit: Oops, there's a mistake on my picture. It should say a^2d^2/2 not a^2d^2/4.

 

[FredGarvin]

Have you considered using a tapered beam for the two lower ones? While not exactly the best course to cure buckling (it will probably shift the failure point towards the load) but if it is enough, it would induce the least amount of additional material. It's worth a shot.

 

[koujidaisuki76]

could you explain what taper beams are?

 

[FredGarvin]

Thicker cross sectional area towards the area that has the most stress, i.e. near the wall. It then tapers and gets smaller where you need less material. This approach is used when one wants an even stress distribution throughout the beam.

 

[koujidaisuki76]

Hi. I made several more and thought about area and everything..

For compression forces, wood (bass) and (balsa) is strong right? So if it bends its becasue there's not enough support to keep the wood in place from bending right? Thats the whole concept right? Because someone keeps telling me that bass wood is weak with compression forces and that I should be using larger wood.. Is that true?

 

[jc052341]

From What state are you from? Maybe I can help you if we are from different sates and and we don't have to go against each other.

 

[koujidaisuki76]

SOUTH CALI you??

I'm trrying to beat an effienecy of 1530

 

[Freefall]

Have you laminated your wood? It will be hard to get 1500 effiency without doing that first.

 

[koujidaisuki76]

Yes tried that...

 

[jc052341]

New York. What's this about laminating... because I've been getting 1600 without laminating.

 

[Freefall]

Tried as in it didnt work? Laminating makes the structure both stronger and lighter, give it a shot.

 

[joy771119]

where do you buy the woods?

hey~
I am a beginner for boomilever. I just wondering where do you buy the balsa or bass wood? b/c i went to Hobby Lobby, the wood is expensive for building boomilever!

 

[koujidaisuki76]

1600?? geeze. better then troy's lol.. I don't go to troy.. but lol their effefiecy is lower than yours... Could you help me? my aim sn is koujidaisuki76

 

[civil_dude]

I believer the compressive force is constant from the tension connection to the wall. Thus a tapered member would not get you much additional strength, or just shift the failure point to the narrow end.

You simply need to beef up the compressive members to withstand a higher force. Perhaps you could make mini box beams for the main compressive members, which would give you a much higher Moment of Inertia.

 

[HalfThere]

I'm struggling to get an efficiency of 100!

Geez, how are you guys managing this?

 

[giritrobbins]

From what I have seen it probably needs some sort of vertcle bracing. My logic goes like this. If it is buckling it is either going up or down so you need to do something about those directions, verticle perpendicular bracing to the bottom truss would do this. But maybe to increase the efficency and decrease the load you put some in on an angle, from the bottom truss to the top fasten point. See the image I attached. I have never done this but it seems to me like this is going to be a static load downward, what is with the cross bracing? Like I suggested to the other guy get rid of some of it, or all of it, put a couple perpendicular, it will save on weight and keep it from buckling outwards.

Also use enough glue but use a cue tip or something to wipe off excess, it may look like its not going to add much but it does.

good luck

 

[Freefall]

If you are struggling at 100 efficiency, the thing is either WAY overbuilt or you arent using good wood or something like that. Tell me about your boomi and maybe I can help you get on the right track.

What I have done with my boomi is virtually eliminate the need for vertical trusses by making the compression members into half I-beams. The horizontal trusses are absolutely necessary, however, to prevent side to side bowing, trust me. Besides, the tension members are going to be so thin that it would be nearly impossible to connect trusses to them.

 

[HalfThere]

I've had three boomilevers so far. The first was unevenly made, but mainly broke because the attachment base was, to say the least, lacking. The second was way too heavy for various reasons, but ostensibly held 13 kilograms (I think it was being aided with the bucket resting on the device that released the copper balls). The most recent one I tried weighed about 30 grams (largely, this was the attachment base) and held about 1.5 kilograms before the lower two legs of the base slipped, pulling apart all the struts rather spectacularly.

All three were designed something like the one in this topic, but upside down. I was told this was a viable way to go about it, but with my most recent test that seems to have been proven wrong. I'm going to try to emulate the design in the other boomilever topic. If that doesn't work, I'm kind of stuck.

I use entirely plain old balsa purchased from the local Hobby World, and some wood glue, with plywood for the attachment base.

 

[fredwangster]

hey do you guys have any ideas for the attachment base? I'm from louisiana 8-B and am building a stress boomilever. The attachment base keeps ripping off?

 

[koujidaisuki76]

for the base part.. glue small pieces of wood around the stick.. like gussets. : )

 

[sergealexeev]

well, i was just doing some research on this...and balsa wood has a very high variation of different weights and strengths and what-not. what might help is weighing the wood and finding out the exact strength that it is. also, use C-grain balsa for the compressive parts and A-grain for the tensiles (if you suddenly decide to go balsa for bottom parts). Anyway, the way those...uh...triangles in the bottom work is by splitting the compressive forces into smaller sections. Try rearranging the pattern so the weaker areas are split into shorter segments. and if it still breaks then you just need to rethink your design.

thanks for the pic by the way, i have to turn in 3 designs of this in 2 days cause I'm doing this in place of a physics project. just got 1 done.

 

[uppywo]

will some of you post your boomilever attachment base pictures on here i can't figure out how to make mine

 

[willzone1]

I'm doing this as part of a class, not competing in the competition.
Have any of you tried a square boomilever, like the ones on a crane?
Also, laving an L with a connecting point from the top of the vertical truss to the end of the horizontal one seems to work best; wood loses strength per length in compression, but not in tension.

Also, what is "laminating" the wood?

 

[Cheyenne]

your just putting a coat of material on it. I tried this, and it didnt really make my boomileve any more sufficient. It didn't a significant amount more, however, it did weigh ALOT more.

 

[jc052341]

Say Hello To 3100!
Woooottttt!
XD

 

[Cheyenne]

an efficiency score of 3100?? sounds a tad unbelievable. your boomilever would have to weigh less then 5 grams, and hold ALL of the weight. If your lying... -.- If your telling the truth...WAY TO GO!