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When I tested two tower crane boomilevers, the compression members in both boomilevers bent down steeply at the end with the loading block. This caused the tension arms to fail. I can't figure out what's wrong. The vertical dimensions are 2cm for one boomilever and 2.5 cm for the other. Any advise would be appreciated!

vertical dimensions for one is 2.5 cm?!?! Can you clarify? I assume you mean the dimensions of your compression members, not the entire boom itself. If the entire boom itself is only 2.5 cm tall, then that explains a lot...

nxtscholar wrote:vertical dimensions for one is 2.5 cm?!?! Can you clarify? I assume you mean the dimensions of your compression members, not the entire boom itself. If the entire boom itself is only 2.5 cm tall, then that explains a lot...

Yeah, I meant the vertical dimensions of my compression members.

Where were the tension members connected to the compression members? It should be just about where the bolt is places for loading (in line with it). It sounds like your tension members were farther out than the loading block, which causes a kind of reverse diving board thing in your compression members. (If you had the members attached closer to the wall than the loading block, it'd be exactly like a diving board and you'd see the compression members bend up).

Look back a few pages, i had the same problem. Basically it's exactly as iwonder says, your loading block was not above your tension members, and your tension members were either located to far back or too far forward.

Hey guys! Recently, I've had some issues in trying to improve my boomilever and I've hit a bit of a snafu. So my boomilever looks kinda like and build entirely of balsa:
http://imgur.com/6EhxiaL

The recent issue is that it has been breaking toward the distal end quite frequently because as it is being loaded, the balsa structure (1/4 by 1/4 for the compression member) has been bending quite significantly, causing the tension members to snap. The compression members also have pieces of wood connecting them together in the middle (kinda like a ladder) which, through vertical members, are connected to the tension members as seen in the picture.

My two solutions are:
1) Change the tension members to basswood
2) Reinforce the distal end

But both my solutions add weight to the structure, so I was wondering if either 1) there is a better solution or 2) is there something unnecessary that I have built and can remove to compensate for the weight gain?

1st, your tension members should be basswood. How heavy is your boom now, if you're concerned with weight? 2nd, 1/4" x 1/4" compression members are kind of large. 3rd, The bending of the compression part would be because you don't have enough bracings. You said it looks like a ladder, have you tried X's? 4th, move the tension members for ward a bit so that distal end is under the loading block.

You said the bending in the compression member was causing the tension members to snap, what's happening to cause that? Are the vertical pieces pushing the tension members out? Or does it kinda rotate around the tension-compression connection and the glue snap?

Either way, read my post right above yours, since it sounds like that's your problem.

Oh, and you definitely don't need bass tension members I mean, they work just fine, but I switched to balsa last year me found it was lighter and still held.

The only thing I have left to say is that 1/4'' by 1/4'' works just fine for my school. We got 1400+ scores using them, assuming 1400+ is an ok score

And to be honest, I'll state right here we actually got better efficiencies using triangular bracings rather than X's. It cut off a bit of weight and our boom still doesn't fail at the compression members

@chinesesushi: 13.5 g and the bending is in a vertical motion, so I'm not sure how much bracing I need for a 1/4 by 1/4 (right now it's in 3.5 inch gaps) and the distal end is already under the loading block

@iwonder Because the compression member (especially at the distal end) is bending, then that causes the balsa tension members to also bend with it, at which a certain point the balsa snaps due to the large bend radius it has (i.e. it can't bend much).

@nxtscholar Nice! Does it look similar to mine? Did you have the tension members connected to the compression members via vertical sticks and how would you build triangular bracings in between legs of the compression member?