Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Event called Boomilever

  1. Jan 13, 2007 #1
    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
     

    Attached Files:

    Last edited by a moderator: Feb 4, 2013
  2. jcsd
  3. Jan 13, 2007 #2

    FredGarvin

    User Avatar
    Science Advisor

    Let's start with this question: What is the mode of failure you are seeing in the previous attempts?
     
  4. Jan 13, 2007 #3
    The bottom wood of the boomilever snapping because of the compression forces.
     
  5. Jan 13, 2007 #4

    Danger

    User Avatar
    Gold Member

    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.
     
  6. Jan 13, 2007 #5
    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..
     

    Attached Files:

  7. Jan 13, 2007 #6

    russ_watters

    User Avatar

    Staff: Mentor

    What they mean is does it bend up, down, sideways...?
     
  8. Jan 13, 2007 #7
    When the weight is applied the bottom actually arches downward... to the point when it snaps..
     
  9. Jan 13, 2007 #8

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    Oh good, it failed the way I expected it would :approve:

    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...
     
  10. Jan 13, 2007 #9
    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?
     
    Last edited: Jan 14, 2007
  11. Jan 14, 2007 #10

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    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.
     

    Attached Files:

    Last edited: Jan 14, 2007
  12. Jan 14, 2007 #11

    FredGarvin

    User Avatar
    Science Advisor

    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.
     
  13. Jan 14, 2007 #12
    could you explain what taper beams are?
     
  14. Jan 15, 2007 #13

    FredGarvin

    User Avatar
    Science Advisor

    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.
     
  15. Feb 4, 2007 #14
    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 theres 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?
     
  16. Feb 8, 2007 #15
    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.
     
  17. Feb 8, 2007 #16
    SOUTH CALI you??

    I'm trrying to beat an effienecy of 1530
     
  18. Feb 9, 2007 #17
    Have you laminated your wood? It will be hard to get 1500 effiency without doing that first.
     
  19. Feb 9, 2007 #18
    Yes tried that...
     
  20. Feb 9, 2007 #19
    New York. What's this about laminating... cuz i've been getting 1600 without laminating.
     
  21. Feb 9, 2007 #20
    Tried as in it didnt work? Laminating makes the structure both stronger and lighter, give it a shot.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?