General Discussion

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Re: General Discussion

Post by rjm »

I was pretty disappointed to see a tie in medals placement as well. Lakeshore's tower mass was 7.75 g prior to submitting for competition (I'm their coach), and carried the entire 15 kg. No idea about Auburn's mass. I didn't see the detailed results this year. I have been assuming that the measured masses were the same, since it is unlikely that any other combination of mass and mass held would give the same quotient. I've also assumed that both towers were greater than or equal to 70.0 cm tall.

Using a .1 g resolution scale is contrary to the rules. It only gives you 2 significant figures and is too coarse to discriminate towers. If we used a 1 g resolution scale we could have even more ties, I suppose. The rules were written with two measured quantities with appropriate precision for easy judging to avoid ties, and ties are very rare. Height may have worked better as a second tie breaker if there were not a cut-off dimension (all heights above 70.0 cm were to be recorded as 70.0 cm). Height was being measured with a meter stick, presumably to the nearest millimeter.

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Re: General Discussion

Post by foreverphysics »

rjm wrote:I was pretty disappointed to see a tie in medals placement as well. Lakeshore's tower mass was 7.75 g prior to submitting for competition (I'm their coach), and carried the entire 15 kg. No idea about Auburn's mass. I didn't see the detailed results this year. I have been assuming that the measured masses were the same, since it is unlikely that any other combination of mass and mass held would give the same quotient. I've also assumed that both towers were greater than or equal to 70.0 cm tall.

Using a .1 g resolution scale is contrary to the rules. It only gives you 2 significant figures and is too coarse to discriminate towers. If we used a 1 g resolution scale we could have even more ties, I suppose. The rules were written with two measured quantities with appropriate precision for easy judging to avoid ties, and ties are very rare. Height may have worked better as a second tie breaker if there were not a cut-off dimension (all heights above 70.0 cm were to be recorded as 70.0 cm). Height was being measured with a meter stick, presumably to the nearest millimeter.

Bob Monetza
Grand Haven, MI
I have no idea what the heck happened, just that something went awry with our tower.We're pretty good with Towers, but...eh, it was all weird. I think it was slightly less than 7.5 g, though, from what I know.
Of course I could be entirely wrong; I wasn't in Towers.
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Re: General Discussion

Post by 101 »

Does anyone have scores from nationals?
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Re: General Discussion

Post by LKN »

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Scanner didn't quite work. Above is a photo of my thoughts on loading the block to the tube.

Any thoughts on whether tension strips will be feasible in the main tension members at the 16.7 degree angle on the boom? Maybe four of them total, two for each side. That should be pretty light weight, or would thin bass strips be more efficient in holding the 53ish kg of tension? If you have 4 strips, would that mean they would have to carry the load of 53/4 per tension member? If this is true it would allow you to save weight by using lighter pieces (less dense, lower cross section) to make up four tension members than two heavier (ultra dense, larger cross section) tension members. If this isnt true, than the extra tension members would not pose any savings...

How crucial is the tension members to "meet up" with the loading block? Let's say the tension members connect to the main compression member approximately 5 cm away from the applied load, toward the 90º surface (i.e. the hook and chain supporting the weight). Especially on a strong member such as a tube, will there be any impact when tension members meet the boom at a small distance from the applied load? My design above could possibly need this design to connect the tension members. If the angle between the boom and the tension members is low enough, then it is possible to connect them on the "sides" of the tube on the same vertical plane as the applied load of the loading block, directly up and down.
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Re: General Discussion

Post by mrsteven »

That is precisely what I was thinking about trying! The major issue that comes to mind with that is the circle collapsing under the point with the loading block. So to combat that I was thinking about using ultra low density, large cross section pieces inserted vertically in the tube to distribute the stress to both halves of the circle. Would this be a good idea?
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Re: General Discussion

Post by LKN »

I am not too sure if that is "entirely" accurate. I definitely agree with a large cross section and low density. But, now that I think of it, 1/16 by 1/8 is way too small for the block supports. My thinking is, the wider apart that the loading block pieces are for on the end of the boom, the more that force is being exerted in the direction tangent to the boom's curvature. Since that portion of the circle is near vertical for some amount of distance on both its left and right sides, the block supports the load without crushing (think flattening out) since the tangent line would be very close to vertical, and the moment of inertia should, I think, be significantly higher rather than letting the load be focused near the top of the tube. Ideas?
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Re: General Discussion

Post by SLM »

LKN wrote:
Scanner didn't quite work. Above is a photo of my thoughts on loading the block to the tube.

Any thoughts on whether tension strips will be feasible in the main tension members at the 16.7 degree angle on the boom? Maybe four of them total, two for each side. That should be pretty light weight, or would thin bass strips be more efficient in holding the 53ish kg of tension? If you have 4 strips, would that mean they would have to carry the load of 53/4 per tension member? If this is true it would allow you to save weight by using lighter pieces (less dense, lower cross section) to make up four tension members than two heavier (ultra dense, larger cross section) tension members. If this isnt true, than the extra tension members would not pose any savings...
I think using four (smaller) tension strips is a more practical solution than using two (larger) strips. The cross-sectional area of the tension members, added together, should be large enough (for the chosen wood density) to carry the 53 kg of load. If the load is distributed equally among the four members, each carrying a load of approximately 53/4 kg, then the cross-sectional area of each member can be 1/4 of the required area for carrying the entire load.
How crucial is the tension members to "meet up" with the loading block? Let's say the tension members connect to the main compression member approximately 5 cm away from the applied load, toward the 90º surface (i.e. the hook and chain supporting the weight). Especially on a strong member such as a tube, will there be any impact when tension members meet the boom at a small distance from the applied load?
That could pose a problem. By not placing the tension members where the 15 kg load is being applied, you are forcing the vertical load to go through the tube to get to the tension member. The problem here is that the tube is not designed to handle such a load. Since the wood grains are oriented horizontally to support the compression force in the tube, it does not offer much strength in the vertical direction.
My design above could possibly need this design to connect the tension members. If the angle between the boom and the tension members is low enough, then it is possible to connect them on the "sides" of the tube on the same vertical plane as the applied load of the loading block, directly up and down.
Why not attach the tension members directly to the vertical pieces (glued to the outside of the tube) that support the load block? There might be a way to do that.
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Re: General Discussion

Post by SLM »

mrsteven wrote:That is precisely what I was thinking about trying! The major issue that comes to mind with that is the circle collapsing under the point with the loading block. So to combat that I was thinking about using ultra low density, large cross section pieces inserted vertically in the tube to distribute the stress to both halves of the circle. Would this be a good idea?
Some experimentation is in order here. But, I doubt if the scheme would work. I would avoid applying directly a vertical load of 15 kg to a thin layer of balsa. If you place a vertical piece inside the tube, you are basically transferring the load from the top surface of the tube to the inside bottom surface through the short compression member, you are not distributing it to the top and bottom. Even if you can manage to effectively distribute the load to the top (or bottom) half of the tube, I still don't think (1/32"-thick balsa) would be strong enough to handle the vertical load. But, as I said before, we can get a better handle on this only through experimentation.
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Re: General Discussion

Post by mrsteven »

In responce to the loading block mount crushing the tube near the vertical section: i thought about that too, and instrad of the 2 balsa strips being across the tube near the edges, making 2 going actually across the curve of the circle. Possibly add the 2 you had to make a square section for the block to sit on.
Or simply get a block of balsa and cut out the curve of the circle and shave down to size and mount that on to of the tube to give a bunch of surface area contact to distribute the load

The 4 tension cords was what i was also going to do. However instead of making all 4 go to the wall mount i was thinking 1/2 way up the further tension cords connect to the main cords. Probably up the density of the main cords to support the side load being attached to it. Maybe that would save ~1/2 a gram from using the full length of cord for 2 outer cords?
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Re: General Discussion

Post by SLM »

mrsteven wrote:... The 4 tension cords was what i was also going to do. However instead of making all 4 go to the wall mount i was thinking 1/2 way up the further tension cords connect to the main cords. Probably up the density of the main cords to support the side load being attached to it. Maybe that would save ~1/2 a gram from using the full length of cord for 2 outer cords?
For a structural member to be in pure tension, the force in it must be in the same direction as the longitudinal axis of the member. By attaching the second tension member to the main tension member at its midpoint, you are introducing a non-axial force into the latter member. The member, therefore, is no longer in pure tension; it has to carry a shear force as well as a tension force. Since we want the member to be relatively light (small in cross-sectional size), it is best to keep it in pure tension. I suggest attaching each of the tension members directly to the wall.
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