Stengthening Wood

[neoshaakti]

Our robot is less than an inch off the ground, and we are using plywood poles of 1.4 inches by 1.4 inches in order to create a hopper (our hopper starts 42 inches off the ground, and conforms to the robot's max dimensions of 28 by 38). The poles of plywood in my opinion are very vulnerable to breaking, and the structure as a whole probably will not be sturdy enough and crumple as a result of any contact with our robot outside of the bumper zone.

How do you think we should go about strengthening our structure?

I have a few ideas myself:

1. We use steel perforated L stock from McMaster and attach it to the wood beams.

2. We buy steel square perforated tubing from McMaster and use L's, deck screws and bolts to attach the rest of the hopper to the steel perforated support beams.

3. We buy aluminum square stock, aluminum square hollow tubing and rivet/screw the faces of the square hollow tubing to block of aluminum stock... the only problem with this is it takes a lot of time to do (relatively)

I wish I had a picture to attach to this thread, unfortunately I did not bring my camera last night

Thank you for your time

[Rick TYler]

A drawing would help...

I can't picture a "pole" made up of plywood. Did you laminate plywood together to get up to 1.4" thick, and then cut square sticks out of it? If so, you did a lot of work to make a synthetic stick. A simple 2x2 would have been a lot better than fashioning plywood into poles to support something.

Could you further explain your design? There's no reason you couldn't use wood for your purpose, but please give us a little more to go on.

[travis]

Paint some of this on the sides that are perpendicular to the worst case laod:

http://www.fibreglast.com/showproduc...+Resins-5.html

Mush a strip of this in:

http://www.fibreglast.com/showproduc...+Tapes-14.html

then paint some more epoxy over it. The blue kevlar with carbon highlights would look super cool. That website and others has more specific instructions.

Travis

[Rick TYler]

Kevlar and carbon fibers both "float" on epoxy and make a weak composite bond in the absence of mechanical pressure during curing (usually either vacuum bagging or elastic bands of some kind). If you want to make composite spars, use either round poles, or generously radius the corners and consider using a biaxial fiberglass sock: http://boatbuildercentral.com/prodde...AP_U29L200x_ft instead of cloth. For what it's worth, I've never heard of "System 2000" resin. After trying out five or six brands, I use SystemThree exclusively in boats and other hobby applications. Their Silver Tip Laminating epoxy would be perfect if you want to make a composite spar for your hopper.

However, you don't really need high-tech coatings if you engineer your wooden components properly. None of those clipper ships crossing the oceans at 15-20 knots had a drop of epoxy on them.

[MrForbes]

We used 1" square, 1/16" wall aluminum tubing (available at our local hardware store) for the poles, and 1/16" aluminum sheet (not so easy to find in town) to attach our wooden top part to our wooden base. The brackets at the top have a 1" wide flange, with wood screws holding them on.

If you were to make the poles and brackets of wood, it would probably work too, but you'd have to be careful to use a strong attaching method. The trick is in the triangular brackets...that gives the structure it's strength. The bigger they are, the stronger it is.

[Tristan Lall]

Originally Posted by neoshaakti

Our robot is less than an inch off the ground, and we are using plywood poles of 1.4 inches by 1.4 inches in order to create a hopper (our hopper starts 42 inches off the ground, and conforms to the robot's max dimensions of 28 by 38).

Don't conform too tightly to the maximum dimensions. Leave some space between your robot's size and the maxumum allowable size.

Originally Posted by neoshaakti

How do you think we should go about strengthening our structure?

I have a few ideas myself:

1. We use steel perforated L stock from McMaster and attach it to the wood beams.

2. We buy steel square perforated tubing from McMaster and use L's, deck screws and bolts to attach the rest of the hopper to the steel perforated support beams.

3. We buy aluminum square stock, aluminum square hollow tubing and rivet/screw the faces of the square hollow tubing to block of aluminum stock... the only problem with this is it takes a lot of time to do (relatively)

Aluminum L-channel (angle) would probably work for you.

[spc295]

epoxy will help a lot, it also gets red of flex in wood poles.

[ChrisH]

At 1.4"x1.4" just about any wood should be sturdy enough for this competition, no matter how it is made. The problem is probably something else, like a weak attachment.

So the first thing is to make sure you have at least two bolts or other fasteners arrainged along the pole axis at each end. Side by side fasteners will not do as they tend to create a pivot point, you want a line.

Then break up any rectangular open spaces into triangles by attaching something diagonally across the corners. 1x1x1/8 aluminunm is great for this. If you want you can use cable, but then you have to go both ways. Metal pipe hanger tape will work too. Both are pretty light if weight is an issue. This will dramatically stiffen your structure.

[MrForbes]

Also, pictures of what you're working on will be very helpful, so we don't have to guess.

[Rick TYler]

Originally Posted by ChrisH

Then break up any rectangular open spaces into triangles by attaching something diagonally across the corners.

A lightweight sheer plate (like the 5-6mm luaun plywood sold at some home centers) is also an amazingly effective way to stiffen a structure. My teams must have built a dozen competition robots in the last five years with two vertical towers tied together with a lightweight stiffening plate between them. It's like magic, although Chris' x-bracing also works well if you can find the right connectors.

(Most of the advice in this thread is really good. I'm just tossing in another approach.) The other advantage of a stiffening panel is that it gives you a nice flat surface to display sponsors and your robot's name.

[Rick TYler]

Originally Posted by spc295

epoxy will help a lot, it also gets red of flex in wood poles.

I hate to disagree, but I find this to not be true in the boat-building I've done. If you coat the wood with epoxy and a layer of fiberglass, it will be a little stiffer, but not much. The only way to really stiffen a component is to externally brace it, make it from something more naturally stiff (and not many materials are stiffer than wood for a given weight) or increase the cross-sectional area. Using epoxy and glass to stiffen a plywood panel only works if the glass is pretty beefy -- like 12 ounce biaxial at least, or multiple layers of woven glass. Of course, this increases the cross-sectional area of the panel, which likely is what adds to the stiffness. If I wanted to stiffen a 6mm plywood panel I would either glue battens across it, or laminate another layer of plywood to it.

[Molten]

I'd go with 3. I'd just by 3/4 inch aluminum square tubing. Also, If you weld it that will save a lot of time. If you don't like welding, try some 80/20 though that will add some weight.

[Akash Rastogi]

Any pics of what you have so far?

[bighead1358]

wait im confused ur "hopper" wont be touched because no part of the robot may go beyond the bumper zone so nothing will make contact other than bumper to bumper.

[EricH]

Originally Posted by bighead1358

wait im confused ur "hopper" wont be touched because no part of the robot may go beyond the bumper zone so nothing will make contact other than bumper to bumper.

Methinks that you haven't seen robots rocking under forces of acceleration, sudden and otherwise. It's happened that two robots within the bumper perimeter of the year hit above the bumper zone when one or the other tipped.