pic: Cheap 6WD

[s_forbes]

Originally Posted by Madison

Can you describe the power transmission components in more depth?

Assembling reliable power transmission schemes is the most difficult part of building a FIRST robot's drive train. Without learning more about why you've selected an unorthodox series of chain paths and wheel locations, I would be hesitant in recommending this to any teams with few resources.

Well I wouldn't recommend it yet... I would hope that others could offer some criticism and improvements.

Starting at the transmissions, we have two AM toughboxes with the extended output shaft option. The face of the transmission is attached directly to the wooden framerail. On the output shaft, there is a spacer, then a sprocket (12 tooth AM), and then a 5" Banebots wheel. These are held on with a cotter pin and a washer.

The other axle assemblies are similar, with the sprockets and wheels held onto the 1/2" keyed axles by washers and cotter pins. At the moment, I have the flanged bearings sitting right in a 1.125" hole in the hardwood, but this part that worries me the most at the moment. (maybe an aluminum insert?)

That's where I have it at the moment, if you see any problems please point them out.

Originally Posted by Alan Anderson

I don't see your reasoning. The way I'm looking at it, the rear wheels carry more load than if the center wheel were not lowered (up to half the robot's weight or even more, depending on the center of gravity). They just do it intermittently -- which seems likely to cause greater shock loads on the axle when the robot rocks backward.

I was making the comparison between two 6WD frames with a dropped center wheel (I might not have made that clear enough in my first post). With this configuration, the end wheels support a much smaller portion of the total robot weight, especially if it's center of mass sits right over the center wheels.

That is a good point about the shock loads to the transmission, though.

Originally Posted by sdcantrell56

Overall I like it but why wouldn't you just run all the chain on the inside? Also Maybe look into 8wd with the transmission centered and chained to the center pair of wheels. It would increase ramp-climbing ability while keeping the same center of gravity.

I considered this too, but for this particular version I wanted the transmission face to be flush with the framerail and the wheels to be close to the frame. In its current configuration, the rear wheel is about as close as you can get it to the transmission. (although, it could probably be made to work as you stated as well)

[sdcantrell56]

Running all of the chain on the inside would not increase the distance between the wheel and the bearings at all. It would simply require a longer axle on the inside of the frame.

[gburlison]

How fast do you estimate it will go?
Direct drive off of ToughBoxes seems a little fast.
Could the wheels be any smaller?

I like the chain on the outside.
It makes it easy to get to the chain and it simplifies the ToughBox mounting since you don't have to make spacers between the frame and the gearbox.

[s_forbes]

Originally Posted by sdcantrell56

Running all of the chain on the inside would not increase the distance between the wheel and the bearings at all. It would simply require a longer axle on the inside of the frame.

There isn't a bearing in the frame at the rear wheel, so the wheel is supported by the transmission bearings. A bearing could be put there though, and then the chain run on the inside. I've always thought it was a good idea not to run three bearings on the same shaft though.

Originally Posted by gburlison

How fast do you estimate it will go?
Direct drive off of ToughBoxes seems a little fast.
Could the wheels be any smaller?

I like the chain on the outside.
It makes it easy to get to the chain and it simplifies the ToughBox mounting since you don't have to make spacers between the frame and the gearbox.

I would estimate about 8 ft/sec. A 5" wheel on the toughbox gives about 9 ft/sec at the free speed of the CIMs, but there is going to be some frictional loss.

[sdcantrell56]

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.

[Tom Bottiglieri]

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.

[GUI]

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.

[Tom Bottiglieri]

Originally Posted by GUI

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 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.

[sdcantrell56]

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.

[AdamHeard]

Originally Posted by sdcantrell56

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.

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.

[Tom Bottiglieri]

Originally Posted by AdamHeard

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.

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.

[GUI]

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.

[dtengineering]

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

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.

[MrForbes]

Originally Posted by sdcantrell56

Also, why use hardwood? Baltic birch is more than strong enough and is a lot more uniform than hardwood lumber.

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.....