Boomilever B/C

[retired1]

Iai drilled two small holes and made them into a rectangle with the required angle with a small flat file. That article should be read several times by bot B and C participants. For something free, it is highly professional.

[JimY]

How are you guys connecting the tension members to the base?
I can't figure out how to drill the two holes that the tension members go in.

So back in 2000 when this was a trial event for the first time in B division at nationals, the team that won the event was from my current home town. I saw their device at the time and many times since. It used two anchor points to the wall, each with a custom drilled hole to match the dowel size and angle of the tension arms. The arms themselves were probably 1/8" diameter. I remember asking them how they did it, and the reply was that they had access to something that could drill compound angles. Their boom weighed something on the order of 25 grams.

Fast forward to 2008 where the winning boom at nats was more like 6 grams and went to full load. Our's was a couple grams heavier, did not go to full load, but we got 5th. We did NOT use dowels for the tension arms, but rather thin basswood strips that were simply glued to either side of a single base (and at the distal end too). There were no slits in the base or at the distal end for the tension arms to fit into either. So, this design eliminates the need to drill holes at angles that match those of the tension arms and also eliminates the need to create slits for flat tension arms. I strongly suggest going this way at both the distal end and at the wall mount.

[deezee]

I don't know if anyone else already posted this, but how far from the wall does the boomilever have to be?

[_HenryHscioly_]

right on the wall, bolted down?
oh, the block.. :]
the loading block's center needs to be 40-45cm from the wall...

what are the pros of a compression member with solid sides vs a box-beam one?
easier to bulid..?

[andrewwski]

Throwing this one together :/ (to much glue, no jig, struggling to keep it square and not taking weights...). One question... How do people know what forces are on certain members(basically how do they know when to pretension components or not)

Assuming it's a statically determinate structure, and treating all joints as true pins, it's fairly straightforward.

For each joint, write the equations of equilibrium in each direction in terms of other loads. The sum of all forces in the x direction should be zero, as should the sum of the forces in the y direction.

You then get a really big linear system of equations, which you could plug and chug by hand, or more easily solve using a program like Matlab.

I'll write up a tutorial and post it here when I have some free time.

[Balsa Man]

Looking forward to that analysis, Andrewwski!

And in the meantime, you can get a good working sense of the nature and approximate magnitude of the forces at work from our old friend, the JHU Bridge Designer app at:

http://www.jhu.edu/virtlab/bridge/truss.htm
It’s only 2-dimensional, so what you can put in is a single boom, with single tension member, but you can see the kind of forces we’re dealing with for both B-Div and C-Div booms.

Quick and easy to use:

For a “tension” boom design’, where tension member(s) angle down from the bolt(s) to the end of the horizontal boom.


To see this for a B-Div boom, ‘click in’ a right triangle. Click in one node near the upper left corner. Make it a “fixed node”. Go straight down from it 5 grid squares (so each square is = 4cm), and click in another node. Make is a “vertical sliding node.” From it, go out to the right 10 squares (i.e., 40cm) and click in a node. Join these 3 nodes with members. Put a 15kg load at the end node; click calculate, click on the image, and see the results

B-Div: 20 units vertical, 40 units horizontal- with a load at 15kg, tension on the upper member is 34kg, compression in the boom is 30kg
For C-Div, 15 units vertical, 40 units horizontal – with a load of 15kg, tension in the upper is 42kg, compression in the boom is 40kg.

You can’t use it for 3-D structures (e.g., a design with 2 boom/compression members and 2 tension members going up to the two bolt holes), but if you’re looking at a design with two tension members coming from a single bolt , it will be pretty close. If you go to two tension members going up to the two bolts, the greater angle between them will mean tension goes up some;, so say in a B- each carrying 18kg, maybe a bit more. With two boom members, they’re going to be pretty close to 15kg each.

The take away? The forces, both tension and compression, are a LOT higher than want was at work in towers last year-in C-Div,tension is roughly an order of magnitude higher.

[Barker]

Right now I'm at the point where I'm about 1/2 way done with my first boom. However, I have yet to build a testing rig... Any suggestions for designs on this because right now my design ideas are as follows:

-Drill a hole in a door or other sturdy piece of furniture
-Build an "L" shaped upright wall with bottom platform and side bracing and weight it with the lead breaks I have lying around (seriously).
-Build an "L" shaped down hanging wall to be weighted with aforementioned bricks

I feel like there has to be a better way. My main concern is in being able to remain stable torsionally when the full-load is applied (if I every manage to build something that will take the full load...)

[iwonder]

I had a piece of scrap 3/4" plywood sitting around (really narrow and tall) I simply drilled a hole in it and clamped it into a vice that I have mounted on the workbench. It's really easy to store when I'm not using it, insanely simple to build, and pretty cheap(If you've got the scrap). Plus I haven't seen the wood bend yet(and I'm pretty sure it wont, I have shelves with 40+ pounds on them made out of the stuff without much support).

[JimY]

Right now I'm at the point where I'm about 1/2 way done with my first boom. However, I have yet to build a testing rig... Any suggestions for designs on this because right now my design ideas are as follows:

-Drill a hole in a door or other sturdy piece of furniture
-Build an "L" shaped upright wall with bottom platform and side bracing and weight it with the lead breaks I have lying around (seriously).
-Build an "L" shaped down hanging wall to be weighted with aforementioned bricks

I feel like there has to be a better way. My main concern is in being able to remain stable torsionally when the full-load is applied (if I every manage to build something that will take the full load...)

The test rig that we've used since the beginning of the event consists of a homemade sawhorse made of 2x4s (40" high, 18" width at the floor, and 42" long) and about a 6 ft long 1x12 that is clamped to the end of the sawhorse using two 6" C clamps to give a vertical wall. The 1x12 is clamped to the sawhorse a bit off center, otherwise the 1x12 is not stable enough vertically. The key to this working is that the sawhorse can't be too wide at the floor or too short. Once assembled, about 20 lbs of barbell weights are added to the opposite end of the sawhorse to provide counterbalance for loading. This apparatus gives no discernible deflection in any direction while being loaded. I recall an early test in which I built a solid oak boom with rather thin basswood tension arms to see how strong the basswood is in tension as well as how strong the rig is. I'm fairly sure it went to over a 50 lb load before the arms failed. The beauty of the rig is that once it's done being used, just take the clamps off. I have a twin sawhorse, so the pair plus the 1x12 are a workbench of sorts in my basement.

While the rig does not meet the competition spec for the width of the wall (min of 40 cm), since it is not used at competitions, it doesn't matter. What you want is something that essentially does not deflect in any direction while being loaded.

[Faustina]

Speaking of testing boomilevers...

I just tested a boomilever, and after it broke, I noticed that the bolt was pretty loose from the wall. As I added weight to the boom, the wingnut "unscrewed" about 2 mm. Is this normal? To me, it seems like trouble. By the way, my configuration was

bolt head - washer - base - testing wall - wingnut.

If I put the washer next to the wingnut, will this fix my problem?