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Can_Of_Tuna wrote:

Polka-dotty wrote:I am pretty sure we are using enough glue, but maybe we need to figure out a better way to get more surface area for the glue.

Can you post a picture, or describe the boomilever with more detail?

It is an I-beam design, so two I-beams with bass on the top, balsa in the middle, and balsa on bottom. The beams are connected with balsa pieces that form x's. The tension arms start at the side of the I-beam towards the front of the Boomilever and then go up to the dowel rod so that the two arms are parallel to each other the whole way up. We drill two holes equal to the diameter of tension rod and use CA glue to connect.

Polka-dotty wrote:

Can_Of_Tuna wrote:

Polka-dotty wrote:I am pretty sure we are using enough glue, but maybe we need to figure out a better way to get more surface area for the glue.

Can you post a picture, or describe the boomilever with more detail?

It is an I-beam design, so two I-beams with bass on the top, balsa in the middle, and balsa on bottom. The beams are connected with balsa pieces that form x's. The tension arms start at the side of the I-beam towards the front of the Boomilever and then go up to the dowel rod so that the two arms are parallel to each other the whole way up. We drill two holes equal to the diameter of tension rod and use CA glue to connect.

When you said they broke at the dowel rod, am I right in assuming that the dowel rod and boomilever tore apart, or a similar scenario? If so, you may want to consider shrinking the distance between the two tension members by cutting them at an angle, which should form some sort of Isosceles(but with a blunt tip). Something else that should be used in conjunction with this is to glue on a piece of wood ontop of and below where the dowel rod goes through the wood. This will provide additional support.

Be sure to talk to other people as well, as my advice may not be correct. This is also my first year, and our school's inaugural.

Are you tension members breaking from buckling or tensile force? Are they breaking or just pulling off your base?
Case 1: Horizontal Shear (Snapping)
Make sure that your boomilever is centered or else the load will pull the boom to one side and cause premature failure in tension members.
Case 2: Tension
Use thicker tension members.
Case 3: Pulling off/out of base
Use more glue/more surface area.

chinesesushi wrote:Are you tension members breaking from buckling or tensile force? Are the breaking or just pulling off your base?
Case 1: Buckling
Make sure that your boomilever is centered or else the load will pull the boom to one side and cause premature failure in tension members.
Case 2: Tension
Use thicker tension members.
Case 3: Pulling off/out of base
Use more glue/more surface area.

Ummm, I may be wrong, but I think buckling is an indication of compression, not tension... I don't think members in tension are subject to buckling... maybe you mean shear... That can occur in either parallel or perpendicular to the grain... Parallel, of course, is simply asking the tension member to carry more force than it is capable of, and grain and lignen within the piece just pulls apart from within... Perpendicular shear can occur usually when you ask a piece in tension to transfer a force in a modified direction... Possibly the best example may be the typical tension style boom where the tension rod run at a downward angle to connect at the distal end of the compression beams... Now assume that vertical members are installed at the midway point along the boom, but maybe are just slightly too long, which causes the tension rods to not be perfectly straight... As a load is applied they are actually forced to bend over the top of the vertical brace and snap.

Tension rod "pullout" is best solved by drilling holes through the mounting rod, and running your tension rods all the way through the mounting rod, giving a full 360 degree glue surface around the tension rod.

Yes, I think I meant shear, not buckling.

Well our most recent one was perfectly fine, no cracking or anything, and then all of a sudden the right tension arm just snapped at around 11-12 kilograms. It broke about 0.3 cm away from the dowel rod and just sheared. I was thinking that maybe our loading block was resting on the right tension arm a little, and we just didn't notice it. Thank you all so much for the help!
Oh and about what efficiencies would you consider "good," or able to do decent at Nationals?

At nationals it'll probably be in the 1900's to 2000's, if not more.

Your terms might be a little strange... You said the tension member sheared but that it also broke 0.3cm from the base. Did it fail twice? Shear is like two pieces that slide whereas a tensile failure would be a tension rod just breaking at some point.

iwonder wrote:At nationals it'll probably be in the 1900's to 2000's, if not more.

Your terms might be a little strange... You said the tension member sheared but that it also broke 0.3cm from the base. Did it fail twice? Shear is like two pieces that slide whereas a tensile failure would be a tension rod just breaking at some point.

I think the longer length was meant to lower the scores in general. Not sure if it'll happen, but I think that is what is was meant for. And I think he meant it snapped, and I think it probably had an unequal balance of load which made it break on one side.

Yes, sorry! I meant snapped, but I forgot to edit that. It snapped off at 0.3 cm from the dowel rod.
I thought that the efficiencies might go down due to the longer length, but so far at all the invitationals they don't seem to be any less than last year.