Boomilever B/C

[_HenryHscioly_]

I havent tested my boomilever yet, but on the rig at my school, there are two wingnuts on the bolt...possibly to prevent the problem you encountered

[Balsa Man]

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?

It's really important to have the bolt tight.
"Unscrewed" about 2mm is....like more than one revolution- that's a lot. I guess it's conceivable it backed off/unscrewed that much during loading, but I find that hard to believe; I've certainly never seen anything like that happen; certainly not if it was nice and tight when you started. Are you sure it was tight at the start?

One thing missing from your configuration - it's correct except you also want a washer between the wingnut and the back side of the wall.

So what's tight? As tight as it can be without distorting/crushing your base piece - nice and snug. How tight that is will depend on how you/ve designed your base piece. Good idea to have/use a wrench from the front side to hold the bolt head and keep it from turning when you tighten the wing nut on the back side.
Why's it important? To keep the.....geometry of the boom from distorting. Tension and compression forces are minimized when you have a right triangle; the boom/compression member parallel to the ground/perpendicular to the wall. The height specification (15cm for C-, 20cm for B) forms the next side of the triangle, and your tension member(s) form the hypotenuse. If the bolt loosens/is loose, the boom....drops; drops below horizontal. That configuration a) increases both tension and compression forces (for a given load), and b) introduces bending forces in both tension and compression members, and c) introduces a force trying to push the end of the boom in contact with the wall upwards. All those factors work together to take out the structure at a lower load than if it had stayed "straight." You can see this in the bridge designer app- move the tip where the load is applied down a bit.

I havent tested my boomilever yet, but on the rig at my school, there are two wingnuts on the bolt...possibly to prevent the problem you encountered

Two regular nuts- as in standard hexagonal - could be locked together (hold one with a wrench, tighten the other against it with another wrench) to prevent rotation, but two wing nuts will not allow you to do that.....

[Faustina]

Balsa Man,
The 2 mm was an estimate. I never "measured" the amount of looseness, but I can say without a doubt that the wingnut was pretty tight at the beginning and loose at the end. Thank you for the idea of using a wrench; when I tightened the wingnut, I just turned it until the bolt started turning as well. That probably wan't tight enough. And even though the rules do not specifically allow tools to be used, I assume S.O. General Rule #2 applies and that we will be allowed to use a wrench, level, etc.

But, why would I want a washer between the wingnut and wall? And do you think we can use more than one washer per mounting hole?

[chalker]

Here's another suggestion for you all... get yourself some LocTite or similar thread locking glue. The dilute stuff is designed to be removable by hand. See this for an example: http://www.loctiteproducts.com/p/10/15/ ... ue-242.htm

[dholdgreve]

Just a couple of additional thoughts:
Did the wing nut actually unscrew, or was the balsa mounting base compressed against the washer, causing it to lose 2 mm in thickness? Can you see wher the washer was? if so, that was the likely culprit.

Secondly, It may not always be a great idea to tighten the mounting block tight to the wall, especially if you are using a tension type boom... The tension rods must be perfectly straight as the penetrate through the mounting block. The slightest deflection of the tension rods at the mounting block will add shear forces that cannot be dealt with. Leaving the block a little loose may allow the mounting block to rotate as needed to keep the tension rods straight... This may not be the right solution for all designs, but it works on some, depending on how you make your connections. If you are having constant issues with the boom breaking at the connection to the mounting block, try one leaving it a little loose.

[juicemanman]

Just saying, does anyone know what the winning nats score in 2008 was?

[dholdgreve]

if I remember correctly, the projection the last time this event was run was 50 cm at 20 cm height, so a direct comparison to this year's rules would be tough.

[thsom]

With those measurements there is roughly an 80% comparison.

[Balsa Man]

if I remember correctly, the projection the last time this event was run was 50 cm at 20 cm height, so a direct comparison to this year's rules would be tough.

Actually, it was 40cm at 15cm high - same as this year for C-Div

[chalker]

Just saying, does anyone know what the winning nats score in 2008 was?

I don't have the National's data available, but the winning score at the Ohio State tournament should be somewhat comparable. It was 1113 points.