This is the setup I'm currently using to test my booms:
[img]http://imageshack.us/a/img811/4889/0917021702.jpg[/img]
[size=1]Sorry for the bad quality; I could not find the digital camera so I had to use my cell phone.[/size]
I cut two pieces of wood to the same length (can be calculated using trig) to join the base and the testing wall. The wall is at a right angle to the base (with the help of brackets), which allows the base to be easily clamped down to any table/flat surface.
I will be making some changes; the holes on the testing wall makes setting up the boom difficult (as well as drawing the Contact Depth Line). Additionally, the base warped a little, so it's hard to clamp down evenly without cracking the wood.
There are easier ways to do it, but this didn't take more than a few hours to build.
fishman100 wrote:This is the setup I'm currently using to test my booms:
[img]http://imageshack.us/a/img811/4889/0917021702.jpg[/img]
[size=1]Sorry for the bad quality; I could not find the digital camera so I had to use my cell phone.[/size]
I cut two pieces of wood to the same length (can be calculated using trig) to join the base and the testing wall. The wall is at a right angle to the base (with the help of brackets), which allows the base to be easily clamped down to any table/flat surface.
I will be making some changes; the holes on the testing wall makes setting up the boom difficult (as well as drawing the Contact Depth Line). Additionally, the base warped a little, so it's hard to clamp down evenly without cracking the wood.
There are easier ways to do it, but this didn't take more than a few hours to build.
If that works for you, great, but I'd be concerned about a couple of items involving the pegboard face:
1) make sure your compression beams don't align with the holes in the pegboard, or are even close.
2) At the most, pegboard is only 1/4" thick. Unless you back it with something more substantial, it may flex. The slightest flexation of the peg board will cause the cantilever to deform which will cause unbalabced loads either in the X or Y axis.
As far as connection to a lab table, try this... Start with a 24" x 48" piece of 3/4" plywood. Cut it into a 12" x 24" and a 36" x 24" piece. Nail or screw a 24" long 2 x 2 along the 24" side of the 36" long piece, then attach the 12" x 24" perpendicularly to the 2 x 2 so you have an L shaped Now cut a 12" x 12" plywood square, then cut it diagonally to form 2 triangles and apply those to both ends of your structure. You should now be able to lay the structure on the lab table, triangles point up, with a vertical 12" face flush with the end of the table. This will be your mounting face. Cut a 2 x 4 the width of the table, and lay across the opposite end of the structure, and clamp to the table top with quick grip clamps. Quick, simple, reliable, and easily transportable.
Dan Holdgreve
Northmont Science Olympiad
Dedicated to the Memory of Len Joeris
"For the betterment of Science"
Yeah, that's what I was having trouble with; the compression members would often align with the holes and yes, the pegboard does bend (I haven't seen it bend while testing yet, but that's because I've only tested one boom that held about 1.5 kg), which is why I'm planning on replacing it with some extra plywood I have.
At nationals, and many other levels, the surface has typically been melamine or formica coated wood. VERY slick. It WILL affect your design's performance, especially as you get close to minimum weights. Testing on a rough surface will mislead you drastically!
Now, you might ask, what if MY tournament is on a rough surface. Good question. Posed as an answer without proof (but based on MUCH experience) the worst surface for almost all, if not ALL, designs is a smooth, slick surface.
I understand how slick surfaces could effect towers, but since a boom applies it's load perpendicular to the testing wall, how does the smoothness of the surface effect it's performance?
'If you're the smartest person in the room, you're in the wrong room' - Unknown
For my two cents, as the boom bends it might load one side a bit more than the other and allow it to move either sideways or worse, UP. The smaller the boom, the more problem it might be. The boomilevers are not perfect, well most of them are not.
First try on a computer model only held 5Kg. Demoralizing.
Well I got my balsa and practice balsa in recently, along with some new tools, so I think I'll be able to build a third boom, and maybe be able to have a wood failure instead of a glue failure for once
Just a couple of quick comments on some of these posts: As always, these do not count as official clarifiactions.
Slick/smooth surfaces are almost always more demanding of a structure, regardless whether it is a boom, bridge, or tower, than a coarse or tacky surface. For compression-style boomilevers it may make a difference. It is almost always to your advantage to not design with the expectation that the test surface texture will help stabilize your structure.
The range 40-45 cm for length does help with overturning moment and counterweight of testers. I've seen really long boomilevers which nearly tip them over. I don't know why anyone would build a long one, but if the rules are silent on length, then it is the choice of the competitor. Having a range makes set up at the tournament easy and predictable. Actually, though, in draft of the rules earlier this summer, there was to be a progression of lengths of booms from regional to state to nationals, getting longer each step, and the length range was intended to help control set up at each tournament and also force competitors to build differently at each level. Unfortunately, that progression was removed from the final draft. The limit of length range remained.
Height above the bolts is not relevant to the depth rule, and there is no reason to have a height limit. Unless someone has a great idea for getting a steep angle on their tension members without prohibitive excess weight, then the extra height is likely to be self-defeating. Still, it is a variable anyone can explore. You may not, however, hook the top of your boom to the top of the wall, it may only be attached via the bolts.
Boomilevers sag and bend more than other S.O. structures. It is really dramatic. Rigid joints tend to shear from the tendency to rotate. Straight members bend into curved shapes as the joints resist rotating. Design your joints with multiple shear planes and use wood with toughness to resist the bending.
In terms of design, boomilevers are relatively simple (compared to bridges, for example), but the technical issues in optimizing mass make this a great event.
