Ongoing Contest(Scores)

[Flavorflav]

"Distributing the weight"..... has nothing to do with anything. Water will distribute itself, as in the surface of the water will be flat. Even using a solid material; sand, gravel, etc., the distribution does NOT matter. It can be stacked up against one side, with the bucket tilting to that side, and the pull, from the single point of attachment, is still straight down.

But that is only true if the pivot is frictionless, which of course it isn't. A tilting bucket with a sticky pivot could easily make the chain lean out of plumb.

[Balsa Man]

"Distributing the weight"..... has nothing to do with anything. Water will distribute itself, as in the surface of the water will be flat. Even using a solid material; sand, gravel, etc., the distribution does NOT matter. It can be stacked up against one side, with the bucket tilting to that side, and the pull, from the single point of attachment, is still straight down.

But that is only true if the pivot is frictionless, which of course it isn't. A tilting bucket with a sticky pivot could easily make the chain lean out of plumb.

With ALL due respect, and I mean that...., has nothing to do with the pivot. You have a mass hanging from a single point of attachment (top of bucket handle S-hooked to the chain from the eye bolt. If sand gets piled to one side, the bucket will tip, whether the handle is free to pivot or not; even if the handle is locked in position. The amout of tip will vary, more if the bucket is free to pivot, less if the handle is locked. Either way, the tilt will align the center of gravity directly below the attachment point. Why, because gravity pulls straight down. It would take a.....gravity tilt pedal to change the vector of gravity acting on the loaded bucket. Its a very simple vector problem. With a single attachment point above, it is physically impossible for the chain to "lean out of plumb."

[Flavorflav]

Well, if you exaggerate the problem and imagine yourself holding a letter V by one of its points, it would seem that it would have to be out of plumb for the center of gravity to be under the attachment point. I honestly don't think it is a very big problem, though - the angle would be very small, unless the bucket were very bad indeed. The bigger problem with a high-friction pivot would be the tendency to bind and release, causing jerking.

[Balsa Man]

Well, if you exaggerate the problem and imagine yourself holding a letter V by one of its points, it would seem that it would have to be out of plumb for the center of gravity to be under the attachment point. I honestly don't think it is a very big problem, though - the angle would be very small, unless the bucket were very bad indeed. The bigger problem with a high-friction pivot would be the tendency to bind and release, causing jerking.

Oh, I totally agree a high friction pivot with the potential for bind/release – i.e., the potential for shock loading, is a significant thing to watch out for and avoid – shock loads are NOT good. A careful setup and checking procedure can pretty much eliminate the problem, though.

But, circling back to what this conversation started around. What may "seem" to be, and what IS, do not always....align. No matter how you exaggerate/configure the hanging/ off-centered load bucket problem, in absolutely every case, the chain down from the load block will be absolutely plumb – pulling vertically downward. This is really basic physics – it is how gravity, and vectors, and center of mass work. There is simply no way on this earth (unless the bucket is allowed to swing from side to side on the chain like a pendulum) that you can hang & load a bucket and not have the chain pulling precisely, vertically downward on the load block No very small off-vertical angle; no very, very, very small angle. Vertical. It may be a bit non-intuitive, but it is what it is, and its important it be understood correctly for what it is.

So, let’s take the letter V example. We can think it through, and it is really easy to experimentally confirm what’s going on. Let’s make a V out of two pieces of wood. Glue them together at the apex. Let’s punch a paper clip/loop of wire through the top of the left side, then, let’s attach a string with a loop through the wire loop. The loop through loop configuration mimics the real life chain. Attach the upper end of that string to…..something, and let it hang. The string will be vertical.

The V will, of course, be tilted a bit to the right. The point of the V, instead of pointing directly downward will be pointing a bit to the left, but the string will be vertical.

So now, let’s glue a little weight at the top of….the right side of the V (a more “unbalanced/uncentered” load in the bucket). The V will tilt more to the right. But the string will be vertical.

The more weight you add to the right side of the V, the more the V will be tilted to the right. Add enough, and the point of the V will be pointing horizontally to the left. But, repeat the mantra, the string will remain vertical.

No matter how you distribute the weight in the V, a) the line/string/chain above the attachment point/loop will be vertical, b) the line connecting the attachment point and the center of mass will be vertical, and c) the force (of gravity) applied to the string holding the V up will be vertical. The same physical laws will apply to the case of a bucket hung on a boom. This will be true no matter how the bucket is loaded- how much on- or off-center the loading is, no matter how much the bucket tips because of off-center loading, and no matter whether the bucket handle is or is not free to pivot above the bucket.

Finally, let’s go from theoretical to reality. I had a student a few years ago. It also “seemed to” him that an off-center loaded bucket would apply an off-vertical/off-plumb load to the loading block. Explanations didn’t work (“but if the bucket tips, it can’t be pulling vertically”).
We hung an empty bucket on a chain. We set up a laser level, with the light line vertical, and adjusted to shine on the chain- nice, clear red line right up and down the chain. The chain was vertical. We took a water bottle, and a hooked cup holder, and we hung it on one side of the bucket. The bucket tipped. The chain remained vertical- the red laser line exactly aligned along the chain- just like when we started. We added a second water bottle; more bucket tilt, chain still vertical. Then we took the water bottles off. We duct-taped the bucket handle in-place, so the bucket couldn’t pivot where the handle attached. Again, the chain was vertical. Put a water bottle back on. The bucket and the attached/taped down handle tipped; less tip than when the bucket was able to pivot on the handle. The chain remained vertical. Two water bottles, more tip than with one, less tip than when bucket was free to pivot on the handle. The chain remained vertical. He moved the attachment hook the bucket was hanging on to one side of the handle; bucket tilted both toward the water bottle-loaded side, and away from the ….off-set attachment side.
The chain remained vertical.

So, the moral to the story? Be careful trusting intuition- what may "seem" to you. Understand the science you're dealing with- study, ask questions; experiment.
For boom; focus attention on keeping the bucket from swinging, and don't worry if off center loading is tilting the bucket. Make sure before you start that the bucket handle is not hanging/up/binding where it attaches. Load it without bumps/shock loads.
May the force (of gravity) be with you.

[dholdgreve]

Len,
I totally agree with you, where the sand is within the bucket has absolutely nothing to do with eccentric loading, so long as the chain remains perfectly vertical. If ever there were a competition where there were two or more connection points from the buck to the device, then yes, it would matter that the load is distributed evenly... You would not believe the number of "coaches" that, over the years, I've seen instruct their students to opt for hand loading over dispenser loading because they can keep the load more balanced and centered that way... Somehow, I guess one side of the chain pulls harder than the other? (LOL)

In any case, I think I understand your concept on the rolled tube mentality, and maybe you can clarify after your regionals, but I don't see where you have gained any significant advantage with this design unless you can get substantially under 10 grams carrying the full load.

It seems it's all the rage to build trussed compression beams this year. I'm not sure if this came from a coaches clinic or where, but i can tell you the the majority of competitive boomilevers at Solon were not trussed compression beams... Every construction joint is just one more opportunity for failure... Do not over think the challenge here... Keep it simple, and focus on technique and skill... Tight joints, the right amount of glue, and make sure that every single piece installed has a specific purpose... My 2 cents worth...

[Balsa Man]

Yeah, hopefully the tilted bucket "problem" is clear for this year.

On a tube for the compression member. It all started with a conversation with a MechE friend- I usually chat with him at the start of the season to gather food for thought for our builders - basic physics/enginering, how could you do/build that, how could you get the precision needed; things like that. He helped me understand the structural advantages of thin wall tubes in compression. What played into that was the incredibly complicated jigging/building my son ended up doing for boom in '08 - single square box beam C-member, with a "Y" at the end to hold the block; 2 tension members. Also, I wanted to get to something really simple, because it was apparent our boom builders didn't want to put a whole lot of testing/development time in. We're not going to be going to Nationals, so the target we set was something EZ to build, that could do well in-State.

The tube we came up with is not a rolled tube. It is a 5/8ths i.d., 1/32nd wall tube made of 2 pieces of balsa. It is quite precise in wall thickness and straightness. Starting w/ the 2 pieces, it takes about 15 min to ...have a finished tube. Adding the tension member, another hour. Tension member is a hardwood. Couple tricks in the T-member, and how its attached to the bolt. I am going to hold off on the hows for now.

The Regional one is significantly over 10 gr- intentionally conservative. Its also significantly under 15. Density of tube wood is a bit under 12#/cf. The design calc sheets, looking at E X I at those dimensions, says that at a bit under 8#/cf, it should hold full, and get to a structure weight under 10gr. Ya never know till you hang it, though....We'll see how far we can push it between Regionals and State. Not a Nats winner, for sure; conceivably medal....maybe. It's easy to build, beautifully simple, really cool looking, and they're ready to be quite competitive here in-State.

[Flavorflav]

Okay, I've mostly wrapped my head around that as far as string goes, but I have two remaining questions:
1. Is this necessarily true for chain as well? Each link is also a joint which will have some friction. I know I have seen kinked chains before in real life, and it seems to me that a kinked chain could not be vertical if the net of the kinks had a trend.
2. It is also true that a bucket will sometimes grab and jerk. This implies that the weight is being suspended out of plumb by the force of friction of the pivot. Shouldn't this introduce a lateral force at the point of attachment, as the handle fails to rotate? Why doesn't this push the line out of plumb? Is it because the forces are equal and opposite?

I know this is not really on the topic of this thread, but I would appreciate clearing my head about this.

[iwonder]

The only joint we're worried about for find the direction that the weight is pulling in is the joint between the top s-hook and the eye bolt in the loading block. Since gravity only pulls down, and there is insignificant friction between the two, the joint can articulate(turn/move) and way it wants, but the force from gravity will always be directed down. The problem with the chain jerking around is just the shock that it puts on the boom.

[Balsa Man]

Okay, I've mostly wrapped my head around that as far as string goes, but I have two remaining questions:
1. Is this necessarily true for chain as well? Each link is also a joint which will have some friction. I know I have seen kinked chains before in real life, and it seems to me that a kinked chain could not be vertical if the net of the kinks had a trend.
2. It is also true that a bucket will sometimes grab and jerk. This implies that the weight is being suspended out of plumb by the force of friction of the pivot. Shouldn't this introduce a lateral force at the point of attachment, as the handle fails to rotate? Why doesn't this push the line out of plumb? Is it because the forces are equal and opposite?

I know this is not really on the topic of this thread, but I would appreciate clearing my head about this.

Sure.
iwonder has it right. Only joint that matters is at the load block eye. Chain to eye friction is negligible (as is chain link to chain link friction). Kinking is an entirely separate issue; a) obviously you want to make sure chain is not kinked before loading, because unkinking will cause shock, but b) a kink, or multiple kinks does NOT change the fact thet the pull (of gravity/the bucket - the center of mass of what's hanging from the eye) will always be, has to be STRAIGHT DOWN. If you go back to a physics book on pendulums, what does it say about a pendulum at rest?- center of mass at lowest point.
On your second question. The weight - the center of gravity of the weight hanging CANNOT be hanging "out of plumb"- gravity is pulling it straight down from the point of attachment- period; no "angular gravity" out there - at least in this part of the universe. The grab and jerk you note is/reflects the case in my bucket demo discussion- where (with a given off-center load) if the handle is locked, the angle of the bucket is less than it is if the handle is free to pivot. The jerk, if the handle were to pop free is the change in angle of the bucket to the greater tilt it takes if the handle is free.
The fundamental nature of a pendulum - center of mass at lowest point - is at work/applies all the way down from the block eye bolt- each link/pivot point; eye to s-hook, S-hook to first chain link, each chain link to the next, last chain link to s-hook, S-hook to bucket handle, bucket handle to bucket. At each link....point, the center of mass of what's below it is directly/straight/perpendicularly/vertically below that link point- and what matters is that that is the case at the top link- where the load is applied to the block. Regardless of the distribution/orientation/geometry of the mass below, the center of mass is at its lowest point, and the pull of that center of mass is straight down. We good?

[Flavorflav]

Not yet, if you don't mind indulging me a little further. My temrinology was imprecise - I am aware that the center of mass must always be directly below the suspension point - in fact, that is the root of my confusion. When the mass is distributed unevenly, the center of mass moves back underneath the suspension point. If the handle-bucket pivot is sticky, however, this would have to push the handle itself (not the whole system, mind you - that was never what I was thinking) out of plumb. I guess my question is, is this situation not then repeated at every joint all the way up the chain? As the handle of the bucket tries to rotate in the s-hook, is there not some tiny lateral force opposing its motion?
Now that I put my unformed thoughts into words, though, I realize that at each step such a force would be in the opposite direction which, besides being very small, would help explain why it is of no practical significance.

Okay, I think I can stop worrying about it now. Thank you for your explanation.