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Would anyone post a picture of a tube boomilever? I understand the basics, but I have never seen one before and might be a good example.

Balsa Man wrote:

GeorgeInNePa wrote:

Balsa Man wrote:You may not have time to play with it, but I suspect the tube failure wall you're running into is because of diameter.
Buckling strength is E x I
E is the modulus of elasticity

I =pi/4 (ro^4-ri^4)
(Ro is outer diameter, Ri is inner diameter).
When you run the numbers, that diameter is a significant factor; going from 1/2 inch to 5/8 makes a significant difference (why we went w/ 5/8ths). Lower density at 5/8ths will get you more strength than hi density at 1/2.... It's just the way the numbers work.

I agree, but my options were 1/2" and 3/4" (leftover copper pipe from a plumbing project). When I go to 3/4, the distal end block becomes way too heavy.

What did you use for a form that has a 5/8" diameter? (if you don't mind sharing just a bit more )

As I mentioned, we're not using a form - not rolling. I'll say at this point, tubes are machined in 2 pieces (from 1" x 1/2") using a router...
Wall thickness is set by the two bits used

Oh, I see. That sounds fairly involved.

I mic'd my form. 1/2" water pipe is a little bigger than 5/8", OD.

UQOnyx wrote:Would anyone post a picture of a tube boomilever? I understand the basics, but I have never seen one before and might be a good example.

I don't have any pictures of a non-busted boomilever, right now...

GeorgeInNePa wrote:Oh, I see. That sounds fairly involved.

I mic'd my form. 1/2" water pipe is a little bigger than 5/8", OD.

Actually, once you get the router set up, its 5-10 minutes to turn out a tube, and with it set up, you can turn out tube after tube at that rate.
As I noted above, using 2 pieces of 1/2" x 1" balsa- match density/weight.
First you take out the inside- using a 5/8" (5/16th radius) round nose bit- you end up with two pieces with a semi-circular cross section trough down them. Then you glue them together- edges of the troughs line up - you have a squate cross section with a 5/8" diameter hole down the middle.
Then you switch bits, adjust router height, and fence offset. You use a roundover bit with an 11/32nd radius. A roundover bit is what you use to, say round off a square corner on a piece of wood. By making 4 passes, one on each corner of your square cross section, you end up with a tube, with a 1/32nd" thick wall. You do your pieces about 2" longer than the final tube length you want. To make those outside/roundover cuts, you position the piece so that about an inch extends one side of the bit- push it into the bit, up against the fence. Then you run down the length of the piece, away from the 1" overhang. You end up with the ends still square, and the tube between those square ends. The square ends provide a way of running along the fence at the same offset for all 4 passes - which gets you a tube with the same wall thickness on all 4 quadrants... When done, cut the end square portions off.

Don't know what to think on diameter- using 1/2" water pipe; a bit over 5/8 i.d., the buckling strength should be there. Some numbers from our design sheets - for a tube length of 42cm. At 1/16th wall, even with very light balsa (5.62 #/cf, you'd have a buckling strength of 29.25kg, almost all you'd need in a B-boom (~33kg). Going up to 6.87 #/cf, that buckling strength goes up to 43.73kg. Using the 1/32nd wall as we are, if you use 12#/cf, you get a buckling strength of 43.93kg (bit more strength, bit less weight). These numbers are, of course theoretical- for homogeneous, circular cross section. Need to use at least a 20% safety factor to cover wood/grain variation.
Then there's the issue of how you align and attach the tension member as it passes through the tube. The bottom/distal end of it should come right to the bottom, inside edge of the loadblock eyebolt hole, and you need a solid plug in the zone the tension member passes through. We use very light 5/8" diameter balsa dowel; you can sand square stock sufficiently round to work...

Balsa Man wrote:

GeorgeInNePa wrote:Oh, I see. That sounds fairly involved.

I mic'd my form. 1/2" water pipe is a little bigger than 5/8", OD.

Actually, once you get the router set up, its 5-10 minutes to turn out a tube, and with it set up, you can turn out tube after tube at that rate.
As I noted above, using 2 pieces of 1/2" x 1" balsa- match density/weight.
First you take out the inside- using a 5/8" (5/16th radius) round nose bit- you end up with two pieces with a semi-circular cross section trough down them. Then you glue them together- edges of the troughs line up - you have a squate cross section with a 5/8" diameter hole down the middle.
Then you switch bits, adjust router height, and fence offset. You use a roundover bit with an 11/32nd radius. A roundover bit is what you use to, say round off a square corner on a piece of wood. By making 4 passes, one on each corner of your square cross section, you end up with a tube, with a 1/32nd" thick wall. You do your pieces about 2" longer than the final tube length you want. To make those outside/roundover cuts, you position the piece so that about an inch extends one side of the bit- push it into the bit, up against the fence. Then you run down the length of the piece, away from the 1" overhang. You end up with the ends still square, and the tube between those square ends. The square ends provide a way of running along the fence at the same offset for all 4 passes - which gets you a tube with the same wall thickness on all 4 quadrants... When done, cut the end square portions off.

Don't know what to think on diameter- using 1/2" water pipe; a bit over 5/8 i.d., the buckling strength should be there. Some numbers from our design sheets - for a tube length of 42cm. At 1/16th wall, even with very light balsa (5.62 #/cf, you'd have a buckling strength of 29.25kg, almost all you'd need in a B-boom (~33kg). Going up to 6.87 #/cf, that buckling strength goes up to 43.73kg. Using the 1/32nd wall as we are, if you use 12#/cf, you get a buckling strength of 43.93kg (bit more strength, bit less weight). These numbers are, of course theoretical- for homogeneous, circular cross section. Need to use at least a 20% safety factor to cover wood/grain variation.
Then there's the issue of how you align and attach the tension member as it passes through the tube. The bottom/distal end of it should come right to the bottom, inside edge of the loadblock eyebolt hole, and you need a solid plug in the zone the tension member passes through. We use very light 5/8" diameter balsa dowel; you can sand square stock sufficiently round to work...

Our tube ends at 41cm. The "block" extends to 42cm.

We're using two pieces of 1/4" thick stock, sanded top and bottom to fit the ID of the tube. The tension member goes directly through the "block". The lower does end right in the through hole for the eye-bolt. The eye-bolt actually bears against the end of the tension member. The tube breaks behind the "block". I wonder if the fact that the block don't attach to the "sides" of the tube is putting uneven stress on the tube?

I tested one last boomilever before my competition next Friday. It didn't do so well, but here are the results:

Held: 9 kilograms
Weighed: 9.1 grams
Efficiency: 989

It broke at the attachment base, so I'll just hope it doesn't do that at the competition . I'm hoping for 1300-1600, which I imagine would place me in the top 5.

Got 1st place at RI States today with a Boom 16g I was told it held 30ish pounds ( Didn't give me a kg weight) and scored 715 really surprised me because this was my worst score so far, while testing at home I would get scores of over 800
Also I don't know if we were still on this but when we were testing, our event supervisor like yelled anyone who didn't have permission taking pictures that took pictures.

Colorado State results.
We ended up 1st and 4th. I think if our team that was 4th had loaded faster, they'd have 100-200 points higher. Oh, well.
C-Div-
1st-11.05gr, carried full = 1357.46 (load was actually 15,600)
2nd- 13.68gr, carried full = 1096
3rd- 13.42gr, carried 14,100 =1050
4th- 9.61gr, carried 9,700 = 1009
5th- 16.5gr, carried 14,300 = 866
6th- 15.70gr, carried 10,700 = 681
7th- 5.01gr, carried 2,608 = 518
8th- 29.43gr, carried full = 509

B-Div
1st- 11.27gr, carried full = 1330.97 actually (load was actually 15,500)
2nd- 12.21gr, carried 12,700 = 1040
3rd- 13.03gr, carried 10,700 = 821
4th- 14.83gr, carried 11,900 = 802
5th- 18.49gr, carried 14,300 = 773

Top 4 in both divisions were tube designs.

Back from Illinois State. Our boom broke early, and we ended up 3rd with a 1275 (7.92g holding 10098). I heard from one of our parents that the winning boom had a score of 2029.

S4BB wrote:Back from Illinois State. Our boom broke early, and we ended up 3rd with a 1275 (7.92g holding 10098). I heard from one of our parents that the winning boom had a score of 2029.

Do you know which school won boomilever?