In need of some advice regarding Science Olympiad boomilever

[996gt2]

Our school is going to the state Science Olympiad competition next week, and I am responsible for construction of our boomilever. Basically, it is a crane-like device; one end is bolted to a wall and the other end carries a bucket filled with up to 15kg of sand. I have been building and testing various models since last winter, and have gotten some good advice from teachers and friends regarding vairous aspects of construction. However, I seem to have reached a plateau as far as boomilever construction is concerned. My current model is built from balsa (~90%) and basswood (~10%); it weighs in at around 10 grams and can hold most of the required weight. I do not know of any way to lighten this structure unless some of the trusses bridging the tension and compression members are removed (the wood chosen for the other parts are the lightest available pieces and each is individually weighed before use).


So...my question is: are the vertical support members between the tension and compression members of the boomilever necessary in order to support most or all of the 15kg dangling from the front of the structure? I have seen some designs that do not utilize them at all, and have heard of some schools that have constructed 7 or even 6 gram boomilever structures. I know that these supports add stiffness to the whole structure and spread the load imposed by the sand, but are would performance of the structure dwindle dramatically without them?

Attached is a picture of my latest boomilever. The top right end contains the hole for the attachment bolt, and the lower right end rests perpendicularly to the attachment wall. The weight is suspended in a bucket that is secured to the boomilever structure at the lower bottom end by a square loading block with a bolt and chain going through it.

Thanks for any help in advance

 

[HalfThere]

my best guesses, though my boomilever is floundering far behind yours, is that you can probably ditch a couple of trusses and maybe laminate the thicker pieces of wood. Beyond that, cut fat by trial and error.

 

[giritrobbins]

Well I don't know I think the vertical ones are necesary just to distribute the stresses better. Perhaps if it is a static load like you imply you can remove some of the horizontal bracing. Or the diagnals could be changed to be perpendicular to the two parallel heavy weight pieces. This could probably lighten it a couple of grams, even though it decreases the stiffness a bit.

 

[Mech_Engineer]

I'd say it would be a good idea to do a quick truss analysis on your current design, and see what the axial force on each member is. Then, vary the cross-sectional area of each spar linearly with the axial force each one experiences. Make sure to take into account the critical buckling load for each spar in compression, and maybe do a quick search on tensile/compressive strengths of Balsa wood parallel to its grains.

Which makes me want to mention: make sure all of your pieces of wood have their grains going in the same direction as the force being applied on them (axial direction).

 

[turbo]

Those of you building such load-bearing structures might be interested in this nice little program. It's fun to play around with.

http://ourworld.compuserve.com/homepages/method_comp/