Re: Building fully flexible tail (several joints) with minimal actuation required
 

Casting a large piece is a challenge. Latex also rips easy. There are RTV silicone moilding compound but they're not cheap. Maybe urethane foam sheats? its tough and the edges can be bonded with contact adheasive. It bends pretty good without buckling or creasing.

Re: Building fully flexible tail (several joints) with minimal actuation required
 

Why not some thick fabric like canvas. You can even paint it so it blends in with the flowers

Re: Building fully flexible tail (several joints) with minimal actuation required
 

Dudes and Dudettes,
This only needs to last a few hours. McMaster sells latex sheets that you can stretch over the parts, paint and decorate any way you want. If you have no sharp edges it should last for several hours. You could use nylon fabric, and blow some air into it during the parade. The positive pressure will keep it somewhat inflated and virtually hide the joints.

Re: Building fully flexible tail (several joints) with minimal actuation required
 

The problem is the strains from moving the tail will spread the flowers apart possibly leaving gaps. Such gaps are not allowed in the ToR and they are pretty darned strict about it. I see two ways to deal with this.

One is to use big bunchy flowers that will spring open when they are stretched apart and squeeze together when they are pushed together. So the flowers themselves act as springs. You will probably want to install these at the last minute. I'm not sure just how long before line up you have to be finished, but you'll probably spend a good part of New Years Eve on it.

The other is to use segments that slide over one another and cover the inside segment with a really tough plant material. I'd use Yucca leaves myself, or maybe tree bark. It just has to be plant material, not necessarily flowers per se. If you're clever about it the joints become part of the design (say vertical stripes on the tail) and it looks natural.

What organization are you working with? The Cal Polys perhaps?

Re: Building fully flexible tail (several joints) with minimal actuation required
 

Yes, I'm working on Cal Poly Pomona's half of the float. (San Luis Obispo builds the other half).

I've done some experimenting and it looks like polyurethane foam (is that right?, because there is also polyethylene foam) may be the material of choice for covering. I'm talking about the same stuff they put in sofa cushions. I'm going to the fabrics store ( :ahh: ) tomorrow to play with a very large piece of it and see if it suits the needs of this project. If it turns out that this is what I will use, does anyone know where I can get lots of it for very cheap. It doesn't matter if it is "pre-owned" as long as the pieces are large and it is in relatively good shape.

Chris, I share your concern for the flowers. That's the exact issue I wanted to address. The plan is for the tail (and the rest of the dragon) to be covered primarily in Carnations. I'm thinking if they are placed with enough density, then then can "open" as you said when the one side of the tail skin stretches/bends into a convex curve. Perhaps there can be a similarly colored organic material underneath so that if we do get any gaps during movement we will not be looking at foam. And perhaps on the side of the tail that will be scrunching into a concave curve the flowers can be placed with less density to allow room for them to "scrunch." I'm just hoping the flowers will be durable enough to do this a few hundred times. Since they are only bumping into one another (and all are soft), I'm thinking it'll work out okay.

Also, I'm wondering what to use for the spring. For right now, I'm thinking of making it with the spring down one side and the cable down the other such that the spring is under minimal tension when the tail is approximately straight and when the cable is pulled the tail curves toward the cable side and the spring extends. The cable will follow a pulley at each joint so it does not make a straight line out in space between both attachment points (fixed and actuated). The goal is to have the cable roughly follow the contour of the tail. I'm thinking an elastic rope of sorts will be better than a metal spring. Do you think something like they use for bungee jumping would be good? Or does that stretch too easily or not easily enough? I've never encountered it in person.

Also:

M. Krass,
Why the double pulleys in your drawing instead of just a single one?

Thanks everyone for the excellent ideas.

Re: Building fully flexible tail (several joints) with minimal actuation required
 

There are many types of foam. Closed cell and open cell. There are many grades of stiffness You may get buckling with some. I was thinking of closed cell urathane foam. I use it to insulate water tank. Look at Mcmaster-Carr to see the selection. One other thought. There are many types of flexible round air ducting. The internal spring wire uould keep the tail round and buckling should not be a problem. There allot of types, plastice outer jacket aluminized mylar outer jacket and foil jacket. The best choice maybe aluminum flexible duct. The dryer duct stuff. It comes in 4,6,7,8". A local HVAC supplier that is well stocked would have a selection for you to look at.

Re: Building fully flexible tail (several joints) with minimal actuation required
 

Sanddrag,
The foam of which you speak does come in various thicknesses and in 1" I have seen it on rolls. It cuts well with a a straight razor, BTW.

Re: Building fully flexible tail (several joints) with minimal actuation required
 

Be sure to put some pictures up for us! :D

Re: Building fully flexible tail (several joints) with minimal actuation required
 

Perhaps if I detail a bit more about how what I sketched might work, the reason I included so many pulleys might become clearer.

The tail can swing to both the left and right in a single plane, rather than simply going from straight to curved in a single direction and is actuated by rotary motion. The circle at the leftmost end is a drum rather than a pulley so as to give and take cable out from both sides of the tail, making the device use a single cable rather than two. The drum would only have to spin some fraction of a rotation to give or take the change in cable length along a side. As I mentioned previously, there are multitudinous other ways this same give and take can be accomplished, including through the use of a cam that constantly rotates, pushing and then pulling a bar that tugs or releases two separate cables.

At the opposite end, a pair of tension springs fastened to each end of this cable would, optimally, maintain constant cable tension whether the cable be on the inside or outside of the curve.

Each joint has two pulleys per side rather than one simply as a means of capturing that cable whether it is on the inside of outside of the curve. One set keeps the cable near to the tail structure when it's on the inside of a curve. The second set, similarly, keeps the cable away from the tail structure when it's on the outside of the curve -- simply so you don't risk entangling the cable within the joints.

You could simply place the two pulleys directly on opposite sides of the cable from each other, engaging one or the other, but not both; but by wrapping the cable through the two pulleys instead, you ensure that it is always engaged for both possible conditions.

Does that make sense? It's not nearly as complicated as I pretend.

Re: Building fully flexible tail (several joints) with minimal actuation required
 

Makes perfect sense. Maddison, your design is very good; I Like it a lot. You have great talent lurking within you. If the tail is required to curve both ways, I will definitely experiment with something like that. For now, I think the tail only needs to curl one way, so I'll be experimenting with running a cable down one side and a spring down the other.

Does anyone have any ideas for what to use as cheap pulleys in my mockup design? (I'm going for something maybe a couple feet long and made probably out of wood) I to use something I alreadyu have or can buy at a local store like a hardware or office store. I just want to try the idea before going full force into an actual full scale design.

Also, does anyone have any suggestions in the way of a spring/bungee on the real thing? (For now let's say it will be cable down one side and spring donw the other and it will go from straight to a 180 degree curve)

Re: Building fully flexible tail (several joints) with minimal actuation required
 

sandrag,

i was fooling around with some wireway chain and fishing line (yes, we use fishing line in our manufacturing process) and i was able to make the wireway coil simply by tying the line to the last link running it through the wireway, and giving it a good tug. its motion is more like a tentacle than a tail, it can only curl in one direction, and it isn't very strong (obviously the moment arm is very small; less than an inch), but it would be very easy to do if you're limited in the fabrication department.

Re: Building fully flexible tail (several joints) with minimal actuation required
 

I finished building my own wood and PVC and duct tape prototype to prove the concept. It is about 2 and a half or 3 feet long. For the most part, it works. I put in mechanical stops, but they are too limiting; I want more curl than it currently has, but in designing the real thing that will be no biggie to change. The prototype turned out very sloppy since I didn't make a single measurement in the whole fabrication process. But, it works. If you hold the end link horizontal and let the rest hang out in air, it will slightly bow down because the sloppy manner in which it is constructed. But the links do curl fairly nicely one at a time.

Now here's my new question: For best performance, shoudl the chain (tail) be constructed like attachment 1 or attachment 2? Currently, my prototype is like #1 and I'm thinking maybe I should redo it to be like #2. Any thoughts?

Re: Building fully flexible tail (several joints) with minimal actuation required
 

For simplicity, the second drawing is probably the best design. It'll be easier to make several segments that are alike than to make some quantity each of two different designs.

Re: Building fully flexible tail (several joints) with minimal actuation required
 

In the first picture, the two blue segments can remain parallel with each other but offset by means of the red segment angling. It is like a geometric transversal. This isn't good. So I suppose I'll go with the second picture if we decide to actually make a fully articulated tail.

Re: Building fully flexible tail (several joints) with minimal actuation required
 

I think that number two will give a more authentic look and are easier to add mechanical stops to the motion. The first one may accordian on itself if something should go wrong. (Mr. Murphy wispers in my ear, spitting in the process he wets my shirt and the side of my face. Never forget, Murphy.)