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I am talking about not having the transmission output shaft directly driving a wheel. Maybe put the transmission between a pair of wheels and chain them to the transmission and then chain the middle one to the last unchained wheel. As you have it right now there is a large cantilevered load on the output shaft which I would be wary of.

I am also wary of a bearing bore in wood like you and the few wood designs I have worked on have all used aluminum bearing blocks to hold the bearings.

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Wood bases are a totally viable option. Many teams have done them successfully in the past (173, 61, 195 jump to mind).

From my experience, the holes tend to "stretch" over time. We used T-nuts to anchor everything to the base to help with this. Therefore, pressing bearings into the side plates seems pretty risky.

What type of wood are you planning on using? We used 1/2" (9 ply) Baltic birch.

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It would be very easy to put a 1/16" wall aluminum tube in the bearing holes, which would eliminate the risk of the framerail cracking/widening around the bearing.

I think the transmission would like it much better if there are bearings supporting the output shaft in the frame.

Tom, the current thought is that the frame members would be oak, and the sheets would be thin plywood.

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I don't see how that would fix the problem. Maybe I'm envisioning this incorrectly?

Also, I'd advise running the chain on the outside of the frame. It's easy to lose things in this kind of "bucket" frame, and I see bolts, loose wires, and all sorts of things getting tangled in there.

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I don't think using another tube in the hole will really help the problem. You need to have the bearings in some type of plate and then the plate fastened to the wood with bolts or t-nuts to spread the load. The best thing to do that I can think of is to have to plates of aluminum sandwiching the wood and then have them bolted together with bearings in each plate. Also, why use hardwood? Baltic birch is more than strong enough and is a lot more uniform than hardwood lumber. You can always layer 2 pieces of plywood to get a thicker piece.

On a side note. I have been designing a ton of drivetrains with wood and I just keep coming back to aluminum. Wood although cheaper, seems to be a bit more difficult to build and design with to get the same functionality. Maybe I just haven;t come up with the right thing yet but so far my aluminum designs are winning in my mind.

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Exactly, there seems to be a frenzy of people wanting wood bases because of how "Simple" and "Easy" they are. So far, I've yet to see a design or setup that really is simpler or easier than a well designed base using more conventional materials.

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The load is mostly vertical, and the problem i can forsee is the wood being too soft/malleable to handle the load in the relatively concentrated area of the bearing. By using something like a 6061 aluminum tube, we could easily spread the load out and reduce the chance of wallering/widening the bearing holes.

We want to use hardwood because it is strong and available in boards the size/shape we need. We are looking at wood because it provides an easy way to do live axles with minimum machining. The robot in the rendering would require only basic cutting, gluing, screwing, and drilling. Not to mention it would be easier than previous chassis we have done, for a little gain in weight and simplicity.

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We found several appropriate sizes of oak boards at our local building supply stores, but we did not find any baltic birch. If you have birch readily available where you are, then you could use it.

The bearing issue is something that needs to be tried out. I'm still waiting for a parts request from the students so I can get some materials.....

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The last time we built this frame, a high school student assembled it in less than one day, on their own. I think that's pretty good by anyone's standards. Lots of teams have wood shops, fewer have machining capabilities.

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Wood makes for some great possibilities. Our arm in 2005 ( http://www.trobotics.ca/#MAXX ) weighed only three pounds, but could support the weight of two judges bouncing on it in a three point bending test. We brought along a prototype for them to bounce on.... the rest of the robot couldn't handle the stress, but the wooden arm could... easily! I will admit to being rather surprised how strong it was, and didn't actually believe my fellow technical mentor, Gregg, that it would work until I saw him sitting on it!

The next year ( http://www.trobotics.ca/#MAXX%202 )we machined our turret on our CNC router, and pressed the bearings right into the baltic birch turret sides. Not exactly the same impact loading as on a drivetrain, but we're still shooting nerf balls from that thing. The main sprocket on the ball loader ( http://www.chiefdelphi.com/media/photos/22924 ) was mahogany, if I recall... and you'll note the Maple bearing blocks. We also machined up a Maple motor mount for the big CIM to run our intake roller mechanism.

At this point we had pretty much fallen in love with baltic birch, so we built an elevator and actuator out of it http://www.trobotics.ca/#MAXX%202007 and last year it formed pretty much the entire superstructure of our robot http://www.trobotics.ca/#MAXX%203

So we're big fans of wood. In fact we were seriously considering a wooden chassis for last year, untill we realized that we could build an aluminum frame using the KOP parts quicker, cheaper and lighter. But then again... we have access to a full metal shop, including a TIG welder, as well as students who know how to run the mill and TIG. If we had only handsaws and drills, this design might be an ideal solution. At the very least it is a cool design exercise to see how "minimalist" one can get.

Jason

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Baltic Birch is good stuff, same for Maple. We build longboards out of both. If it's strong enough to support me standing on a 3/4inch plank of the stuff while going down a hill at 50+mph (no exaggeration) without breaking or cracking, or even flexing too much, then it should be plenty strong for FRC.

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I'm hesitant to use live axles, generally, where teams don't perhaps possess the experience or resources required to manufacture parts accurately. The setup you're describing is pretty straightforward, however, so teams may be able to get away with it.

I share everyone's concern about the bearing being seated directly into the wood because it may open up with time. I also think tat accurately drilling a ~1.125" hole for an interference fit with hand tools and that lining up the holes on each side of the frame rail are each a tall order.

Otherwise, I'd increase the ground clearance -- probably just by switching to a 6" diameter wheel -- so as to give the teams a fighting chance at climbing obstacles.

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The problem of determining the life of the bearing seated in the wood is an interesting one. The stress on the wood is only a few hundred psi, and there should not be too much motion between the bearing and the wood if the bearing is doing it's job. The fit of the bearing into the wood is also interesting, it would be nice to have an interference fit, but it would probably be OK to have a slightly loose fit. I did a quick experiment yesterday, I looked around in my home shop and found a 1-1/8" hole saw, and cut a hole in a piece of 1x6 red oak. I went to the hardware store and bought a couple cheap bearings, and put them in the hole, and put a 1/2" bolt through the bearings. There seems to be a couple thousandths of an inch clearance between the bearings and the wood hole. Playing with it, it seems to be pretty darn durable. I guess it would need to be turned into a robot and tested by students for a few days to get a better idea of how it would last, though.

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We picked up a red oak 1x4 and a Forstner bit at Lowe's today, and did some more playing around in the shop. The cheap bearings are a nice press fit in the hole drilled with the Forstner bit. Sure feels solid! although I need to do something about the dull blade in my miter saw.

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From my experience, almost everything "feels solid" by hand. To really test it you need to put some big hits on that shaft.