General Discussion

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mrsteven
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Re: General Discussion

Post by mrsteven »

Thats questionable the call to use that scale...

Anyway, watching tonights awards online (we shant be good enough for nationals lol) gave me some fire to start workinh on boom for next year. I was toying with the idea of using a single rolled balsa sheet for the compression section without bracing and using 2 tension cords going to the tube. Issue that keeps coming up is the mounting of the loading block on the tube. Im sure cutting a circle to allow the bolt through would be a way to do it, but afraid that will take out too much structure from the circle and not be able to support the load (im working off of the assumption that the load is 15 kg again). Or to make it work use a larger diameter roll but that becomes ineffective in weight and possible issuable with getting the eye hook through enough to actually hook the chain.

Anyone in the past done boom that has toyed with tubed balsa? Is it a deadend bc too heavy?
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Re: General Discussion

Post by thsom »

Quite honestly, the trick with making a tube for compression is not exactly density or any typical factor, rather the grain of the wood. That can really make or break (no pun intended) the boom.
As for Boom, i have made 5-6 booms in the range of holding all and in the range of being light and holding little. The best efficiency received I don't want to disclose. However I tell you this, it is not hard to make a boom under 15-16 grams and holding all 15 kg, not hard at all. The worst efficiency received was a ~630, the others were in a range of 1000 up.
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Re: General Discussion

Post by mrsteven »

Well how I'm looking at it is its like towers: I want to be in the 10 gram range for boom by the end of the season.
I know the grain is super important, I'm always careful about that but have you tried using a tube for the compression member (s)? Can you use light enough wood where it becomes useful to use that over conventional bracing of members?
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Re: General Discussion

Post by LKN »

Any rough ideas on the boom dimensions/specs? I wasn't around when boom was last used in the competition cycle and I'm interested to know what we should expect for a ball park so maybe I can try a few things this summer too.

From what I recall, SLM's tube tower had a chimney 35 cm Long and about 3cm in diameter. Don't quote that, it is a guess from the picture Posted in last years forum. I believed it weighed in at 9ish grams and was somewhere around 1/32 of an inch thick. Density wise, SLM has total control over what the entire balsa sheet weighs, how uniform the density it is, and probably grain too. I don't have this luxury nearby so I might throw a few bucks to specialized balsa and see if I can custom order a few sheets and see what, if anything, might be feasible. I would think that as long as the wood didn't crack or split by rolling then thinner or lower density wood could be possible options. The other thing to remember are properties of wood and a circle... The smaller the diameter and thickness then greater the weight savings. Add on any skill to tightly and consistently roll a tube small enough to hit that perfect size you want, that an be a nightmare or impossible in itself. Balsa has a point where it can only roll so tight. Depending on how consistent tubes can be in this event, this could be the matter of tenths of grams to whole grams. Any thoughts on this?

I... Have ideas on making the circle connection work. Try some 1/16 x 1/8 at about .75g a 36" stick. Be sure to diagonally cut to nearly match the circle's curvature. Cut out 2 one cm lengths. Find out the place on the tube and mark where the 1/8 height gain from the balsa would barely exceed the total height or the tube where the block is to be loaded. Maybe trying a rubber band or affixing sandpaper to the tube itself (or whatever you jigged the tube on) and sand the piece for supporting the load block. Glue them down, make sure they are a level surface for the block. Have a hole at least 5/16 inch for the rod that goes through the tube and centered between the newly created platform for the testing block. Thoughts?
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Re: General Discussion

Post by mrsteven »

LKN wrote:I... Have ideas on making the circle connection work. Try some 1/16 x 1/8 at about .75g a 36" stick. Be sure to diagonally cut to nearly match the circle's curvature. Cut out 2 one cm lengths. Find out the place on the tube and mark where the 1/8 height gain from the balsa would barely exceed the total height or the tube where the block is to be loaded. Maybe trying a rubber band or affixing sandpaper to the tube itself (or whatever you jigged the tube on) and sand the piece for supporting the load block. Glue them down, make sure they are a level surface for the block. Have a hole at least 5/16 inch for the rod that goes through the tube and centered between the newly created platform for the testing block. Thoughts?
I can't say I understand about half of what you had just written. I cannot visualize the:
"Be sure to diagonally cut to nearly match the circle's curvature. Cut out 2 one cm lengths. Find out the place on the tube and mark where the 1/8 height gain from the balsa would barely exceed the total height or the tube where the block is to be loaded. Maybe trying a rubber band or affixing sandpaper to the tube itself (or whatever you jigged the tube on) and sand the piece for supporting the load block"
portion... are you talking about a singular tube or 2 parallel ones?

EDIT: I remember seeing that picture too, but I think he said something about how it wouldn't work... but I dont recall the location of the picture to look
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Re: General Discussion

Post by chalker »

LKN wrote: And to clarify it was a tie just between Grand Haven and Auburn. I thought it was interesting and definitely took me off guard, but when you think about the design of a competitive division b tower all of them are made so dang similar that two towers could max and easily be within a tenth of a gram of one another.
That tie gave me serious headaches in scoring. In general we ALWAYS break ties, so the entire scoring process / system is not designed to propagate ties through the various steps. And keep in mind that towers has 2 tiebreakers: tower mass and height. We were shocked to get the results and find that even height couldn't break the tie. As far as I know going back quite a while, we've never had a tie in a medal place at Nationals in ANY EVENT (I believe there have been ones very rarely in the lowest places though).

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Re: General Discussion

Post by LKN »

mrsteven wrote:
LKN wrote: I can't say I understand about half of what you had just written. I cannot visualize the:
"Be sure to diagonally cut to nearly match the circle's curvature. Cut out 2 one cm lengths. Find out the place on the tube and mark where the 1/8 height gain from the balsa would barely exceed the total height or the tube where the block is to be loaded. Maybe trying a rubber band or affixing sandpaper to the tube itself (or whatever you jigged the tube on) and sand the piece for supporting the load block"
portion... are you talking about a singular tube or 2 parallel ones?

EDIT: I remember seeing that picture too, but I think he said something about how it wouldn't work... but I dont recall the location of the picture to look
I will have a diagram up soon when I have access to a scanner in the next few days. I think it would work, but not sure, as it should distribute the forces in the same nature that a 4.5cm x 4.5cm square platform would for the loading block. There is only one tube for the body. This year in towers, the eye hook was allowed a distance of 5cm-12cm from the loading block, and was allowed to be disassembled and reassembled to set up for proper testing with no violation. With a 3 or less diameter tube, you will be cutting it close but should be able to load the boom properly.

I tried one circular chimney during the 2011 season, and I was challenged to get a straight and consistant tube shape. Although I only tried once, and practice does make better, any ideas/suggestions for this extreme warping maneuver (especially SLM)? I had a 35cm long 3cm diameter tube that was an absolute disaster, and weighed 14ish grams on its own... scrapped that idea before. Would a long piece of PVC pipe be a good device to jig the boom?

Again, any ballpark ideas for boom specs? Or where could I dig to find the old specs?
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Re: General Discussion

Post by SLM »

mrsteven wrote:...
EDIT: I remember seeing that picture too, but I think he said something about how it wouldn't work... but I dont recall the location of the picture to look
Here is an image of the cylindrical tower we experimented with last year.
Image
The chimney was about 1.5" (~3.8 cm) in diameter with a height of 35 cm and a thickness of 1/32". The chimney weighed around 9 grams. We had difficulty attaching the chimney to the base upright. I remember it easily tipping over under the applied load. However, I don't think tipping would be a issue for the boom since it has a different configuration than the tower.

If I remember correctly, we tested the chimney by itself, it held at least 45 kg of compression load, a very encouraging outcome. For what we are talking about here, the compression member is going to undergo a load of 50 kg when the boom is carrying the full (15 kg) vertical load. That is, if the boom has a length of 50 cm and a maximum height of 15 cm at the wall, and it (the boom) consists of a single tubular compression member and two tension members (connecting the tip of the boom to the wall), then the tubular member would be compressed with a force of 50 kg and the tension members collectively have to carry a force of about 53 kg.

Time permitting, we plan to experiment with the tubular section during summer. If that happens, I'll post the results here in early fall.
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Re: General Discussion

Post by SLM »

LKN wrote:... Would a long piece of PVC pipe be a good device to jig the boom?

Again, any ballpark ideas for boom specs? Or where could I dig to find the old specs?
Yes, PVC pipe is what we used. If necessary, soak the wood for a few hours, then wrap it around the pipe (use rubber bands to hold it in place) to dry...

Spec for 2008:
Attachment: 2 bolts
Distance between bolts: 20 cm
Distance from wall: 40 cm
Max height: 15 cm
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Re: General Discussion

Post by mrsteven »

What density or mass of 1/32 sheet were you using to achieve that?

I guess i dont fully understand why it has a compression load of 50 kg when only 15 is being applied. If i were to load 15 kg directly on top of the tube, would this not be the same effect in terms of compression experienced during loading? (not including stress on singular point for the loading block)

Seems that if yours held 45 kg and the 50 you say is expierenced its a route worth trying. The tricky part i foresee is stopping the circle from collapsing hortizontally under the pressure of the loading block at that point. Maybe place some wide, low density pieces in the tube to reinforce?
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