General Discussion

[chalker]

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).

[LKN]

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?

[SLM]

...
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.

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.

[SLM]

... 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

[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?

[LKN]

The 50kg number comes out of the 15kg that needs to be held. If you have a perfectly perpendicular surface, than the force has a multiplier of 3.14 (pi) in a circle. Therefore, the triangle formed by the boom should have 50kg of compression force strength as SLM said. The tension members must be able to hold up this 50 kg of force that the boom is experiencing, therefore using trigonometry we find that a total of 53kg of weight needs to be held by the tension members since the direction of forces is perpendicular in 3D space and harder to support.

[SLM]

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?

In towers, the applied load acts in the direction of the member (the force pushes directly on the member). In booms the applied load is perpendicular to the longitudinal axis of the member. This causes a different force magnitude to develop in the member. The force magnitude, in part, depends on the geometry of the boom. For the specific geometry we are considering here the compression and tension members will end up having an axial force around 50 kg. For an explanation of how these forces are calculated see the pdf file here:

http://dl.dropbox.com/u/54640248/Scioly ... 202012.pdf

Also, another way to support the load block at the end of the boom is to attach a small truss-like structure to the end of the tubular member, as shown below.

[mrsteven]

In the sketch you Made, isnt the majority of the force being subdued by the tension members so not a total 53 kg stress on the joint between the tube and truss system?

[SLM]

In the sketch you Made, isnt the majority of the force being subdued by the tension members so not a total 53 kg stress on the joint between the tube and truss system?

Let's think about it this way:

The inclined tension member (which makes a 16.7 degrees angle with the horizontal axis) has to carry the vertical (15 kg) load. If the tension member were completely vertical, then the force in it would have been 15 kg, and there would be no force in the horizontal tube. But, since the member is inclined, the force in it is going to be more than 15 kg. If we think of the force as a vector having an x (horizontal) and a y (vertical) component, then the y component of the force must be 15 kg in order to balance the applied vertical load of 15 kg. So, for the y component of the tension force to be 15 kg, the (inclined) force itself has to have a magnitude of 52.2 kg. Or, mathematically speaking, F = 15/sin (16.7) = 52.2.

Therefore, since the tension member is pulling the tip of the boom up and to the left (toward the wall), then it is going to compress the tube. To be more precise, the tension member applies a horizontal (compressive) force at the tip of the boom. The magnitude of this force equals to the x component of the tension force. Since the force in the member is 52.2 kg and the angle between the member and the x axis is 16.7 degrees, then the x component of the force equals 52.2 x cos(16.7) = 50 kg.

In summary, there are three forces acting at the tip of the boom: an applied force and two member forces. The applied load is 15 kg, the horizontal compressive force in the tube (50 kg), and the inclined tension force of 52.2 kg, The algebraic sum of these forces equals zero.

[Flavorflav]

.1 scale? You got to be kidding me... that's totally silly for even a regional let alone nationals!!

A 3 way tie, is that what I saw? Thats impressive that it could even happen, even with the weak scale a tie is still a bit funny that it happened xD

One year I was running Scrambler at a B regional when I heard that the tower supervisor didn't have a scale for the sand, so I lent him the 5g/30kg postal scale I had brought as a backup. I asked him if he wanted me to go dig up another for the towers, and he said he was all set. I thought that meant he had one, but as it turned out he used the 5g scale for the towers, too. Unfortunately, nobody reported it until it was too late to do anything about it.