pic: Cheap 6WD

[MrForbes]

Yes, we sure do!

[Dad1279]

I think I would try bronze sleeves/bushings instead of those bearings. Strong, simple, and a larger contact area with the wood.

[sdcantrell56]

Bronze bushings also have greater friction resulting in greater losses in efficiency. I'm not sure that the drivetrain with relatively high rpms would be the place to introduce extra friction.

[MrForbes]

I like the idea of bronze bushings. While they do have more friction than ball bearings, the friction is still not very much. And the added benefit of increased bearing surface on the wood is a plus.

We have also thought about making this live axle design with plywood, which is softer than oak. One problem with any non-laminated wood is that the tensile strength across the grain is rather low, so the wood can split, which would be catastrophic. Plywood doesn't have this problem as the grain goes in different directions.

[Alex Cormier]

Originally Posted by squirrel

I like the idea of bronze bushings. While they do have more friction than ball bearings, the friction is still not very much. And the added benefit of increased bearing surface on the wood is a plus.

We have also thought about making this live axle design with plywood, which is softer than oak. One problem with any non-laminated wood is that the tensile strength across the grain is rather low, so the wood can split, which would be catastrophic. Plywood doesn't have this problem as the grain goes in different directions.

I would highly recommend a chat with someone on 195, in the past they have made real nice wooden chassis and a pretty blue.

[Dad1279]

Plane the wood to 1/2" thick, Drill lighting holes, and laminate one side with 1/4" hardwood plywood?

I like where this is going.....

Wood wheels also?

We used PVC wheels a few years, the stuff used for exterior wood replacement. Worked well (won regional both years) Easy to make, drilled a hole in the middle, cut round on a bandsaw, pressed in bronze bushings, and bolted sprocket to the side (dead axle)

Bronze bushings in our wheels made it through 2 regionals, nationals, 3-4 off season competitions, and is currently a test bed. Still on the same bushings.

[Dad1279]

oops, double post...

[sdcantrell56]

2 years ago we also used bronze bushings in a 6wd layout with both sides of the shaft supported. They worked through 2 regionals and national playing extremely tough defense. They were very cheap and they worked relatively well. However, if you are interested in maximizing performance bearins are definitely the answer. Also if you are using aluminum shaft, bronze bushings will not work well and you should use some form of a plastic bushing. Finally Bushings tend to bind with a side load such as in a cantilevered driveshaft.

[Rick TYler]

Originally Posted by Dad1279

Plane the wood to 1/2" thick, Drill lighting holes, and laminate one side with 1/4" hardwood plywood?

Don't waste your time or money on hardwood plywood. The face veneers may be very thin layers of hardwood, but the cores are crappy no-name softwoods. If you want the strongest plywood available your choices are marine fir AB, baltic birch 7- or 9-ply, meranti BS1088, or okoume BS1088. Okoume will be the lightest, the fir will have the worst QA problems, and the meranti will be the strongest. Good fir marine plywood would probably be the best overall, but I haven't seen a good, void-free piece in a long time.

There are woods that are very split resistance. Hard maple and a lot of tropical hardwoods have dense, interlocking grains that don't like to split. With natural lumber, I wouldn't use thin pieces, though. About 3/4" would be the thinnest you could go to without worrying about splitting. I might go with 4/4 (usually about 13/16" thick) or 5/4 (usually a little over an inch thick) and drill speed holes in non-critical points.

Plywood doesn't split, of course, but usually has the problem that 40-60% of the wood fibers run in a direction that doesn't help the strength of your application. A perimeter frame of natural lumber eliminates that 40% of the material running vertically which adds little or nothing to the strength of the frame.

Wood is nature's own laminate material: strong, stiff and light. It suffers when you run metal fasteners through it, and from impacts and abrasion. To avoid crushing wood fibers, you can bore a hole and insert a metal sleeve into the hole slightly shorter than the thickness of the wood. Run your bolts through the sleeves, put big washers on both sides, and tighten away.

In 2005 we pressed bearings into an oak bearing block and it worked fine. We captured the bearing in two blocks of 4/4 oak which were then screwed together. The beam was made of meranti plywood, and the bearing blocks were screwed and glued into the beam. The robot had some issues, but it was structurally bullet-proof.

[Enigma's puzzle]

I don't like putting bearings in the wood personally, wood warps and cracks, when you put only a single bearing in the wood (as in the front and middle shafts), with just about any stress it will tear out your hole. The shaft will then shift off-kilter and throw chains on which ever shaft is loosened. But the bigger problem will be your hole being ruined, you would have to replace the entire side of the frame.
Maybe a 1/4 inch metal plate on the inside and outside with bearings in each. I know it increases the price but i would do it if at all possible because your out for at least a couple of matches if not the rest of the day. A little catastrophe defense.

[MrForbes]

We've been thinking about repairing a wood robot chassis if it has problems where the wheels mount, or where bearings enter it. I think you could probably make a resonable repair with thin sheet metal on the outside to restore a bearing hole, or slightly thicker metal (perhaps 1/8" x 2" steel strap) to repair a broken area where a cantilevered dead axle mounts. Or you could mount dead axle on a piece of metal that extends below the robot, and is screwed to the outside of the frame rail.

[Ivan Helmrich]

One way to keep holes through the wood from deforming is to soak the interior of the hole with thin CA (Cyano Acrylate) adhesive. It will wick into the surrounding fibers and make the area very hard and distributes the stresses to a larger area. I've done this for 1/4" dia. holes with good success, I wouldn't hesitate to do this with a larger hole though.

This also works well for holes that are threaded into wood. Drill the hole, form the thread and saturate with CA. Let the adhesive cure fully and re-tap the hole. This is especially helpful where the hole is in the edge of plywood as it helps bond the plies together.

The CA makes some nasty fumes so do this with ventilation and don't soak too large an area at one time, it's exothermic. You will already have your safety glasses on, slip on some rubber gloves as well. It's easy to glue yourself to the part you are working on.

[EricH]

Originally Posted by Ivan Helmrich

The CA makes some nasty fumes so do this with ventilation and don't soak too large an area at one time, it's exothermic. You will already have your safety glasses on, slip on some rubber gloves as well. It's easy to glue yourself to the part you are working on.

*snicker snicker* For my Aero design team, CA (thin and thick) is our glue of choice. Epoxy is a close second due to its strength. You're right on all counts; "CA and Red Bull" is a dreaded phrase due to the fumes and the caffeine, and I can attest that the fumes are quite annoying if they get to you. If you get it on anything you don't want it on, look out! (We've had glued fingers, as well as some other mishaps that I won't go into.)

CA is cured fastest by pressure, but it tends to stick to what you apply the pressure with. If there is pressure, it hardens within a minute and cures quickly after that. If not, I would guess about an hour to harden and 2 to cure. If you try anything before it's cured, be warned, you could easily be in for a very sticky situation. (There is also CA accelerant and debonder--you might want some of both if you plan to work with the stuff quite a bit.)

[MrForbes]

That's a neat tip, thanks!

[Rick TYler]

Another boat-builder trick for boring holes in wood: drill the hole oversized, and fill it with epoxy with some filler like wood flour (don't use microballoons -- they are too weak) or silica. You can put a 3"x3" patch of fiberglass cloth, not matt, on both sides if you want a little extra strength. Drill your hole right through the middle of the epoxy plug, and stick your fastener through with generous washers on both sides. This is generally done to avoid exposing end-grain to water in the hole, but it also produces a strong way to avoid crushing wood fibers.

I've had bad luck with CA as an adhesive on wood, but using it to saturate end-grain is an interesting idea. Thanks.