Boomilevers 2019 (B/C)

retired1
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Re: Boomilevers 2019 (B/C)

Postby retired1 » Sat Jul 07, 2018 3:15 pm

Here is a hint for next year's boomilever.
Devthane 5 (not 3) is a fantastic adhesive for connecting the tension members to the top "bar". It is moderately flexible but rather more difficult than quick set epoxy. It is very sticky and getting a small amount in just the right places is no fun. But that joint will hold nearly any load and flex that you can put on it.
It is expensive and not readily available except on line. Amazon or Ebay had the small double tube for 8 or 9 dollars which is like half price. A "gun" would be nice but it is not cheap for a 1 or 2 year working.
I refrigerate it and it has lasted well until I threw it away this year.
It is a 2 component polyurethane.

pajobubo
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Re: Boomilevers 2019 (B/C)

Postby pajobubo » Wed Jul 11, 2018 2:09 am

In a simple boomilever, where the piece in compression is perpendicular to the testing jig, isn’t the force of compression on the compression piece twice that of the force downward applied by the bucket? So 30kg at full load?

Also for the tension pieces would you guys normally make those out of dense balsa or basswood or regular balsa... what affects the strength of wood under tension?

dholdgreve
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Re: Boomilevers 2019 (B/C)

Postby dholdgreve » Fri Jul 13, 2018 12:29 pm

pajobubo wrote:In a simple boomilever, where the piece in compression is perpendicular to the testing jig, isn’t the force of compression on the compression piece twice that of the force downward applied by the bucket? So 30kg at full load?

Also for the tension pieces would you guys normally make those out of dense balsa or basswood or regular balsa... what affects the strength of wood under tension?

I believe the compressive force (as well as the tensile force) will be determined by the angle between the 2 members and the load imposed.
Think of the extreme case. If you had a boom with compression beams extending to a wall, and tension chords at 89 degrees to it, connect to... lets say a cloud, there would be very little compression force. nearly all of the load will transfer at nearly 100% to the tension rod.

Now think of the opposite extreme... Compression beam back to the wall, but the tension cord is now a cable set at 5 degrees to the beam, anchored to the wall and around a pulley at the end of the beam then continuing down to the load. You will have immense pressure on both the compression beam and the cable.
Dan Holdgreve
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