pic: Drivetrain Concept
 

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Some of the combat robots at Robogames tried out those new Banebots wheels.

They wear down incredibly fast. And the floor at robogames is only plate steel.

The treads are deceptivley thin, they will wear down much faster than you may think (especially the Shore A variety).

They will also turn black and lose that awesome orange color... :(

If you want to use cheap plastic wheels, the colsons are the way to go. For FIRST or for combat.

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Hmm, that's interesting about the tread wear, thanks for sharing! We were attracted to the wheels by their extremely low cost, but it looks like tread wear may be more of an issue than we expected.

The banebots wheels go up to 50 shore A, it'd be cool if someone could test them out on a first bot before the coming season to see how quickly they wear down on carpet.

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You could epoxy roughtop tread to the outside of it. We attached roughtop tread to a first kop wheel and it worked fine.

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why not just go with the andy mark kit wheels?

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I would be a little concerned about frame-torsion. Your design looks like you intend to screw into the wood, and over time the twisting of the frame may well loosen it up, especially since you're screwing into end-grain. I'd consider L-brackets with through-bolts to avoid that.

Also, unless there is a pressing reason for 8 wheels - I would avoid it. Unless you drop the middle 2 sets, you'll have trouble turning, and making 8 wheels the correct heights is really difficult. There's a reason stools have 3 legs rather than 4 (wood warps and twists over time). You can avoid that in part by using plywood, but that has problems all it's own if you plan on screwing into the end of it.

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I love the simplicity of this design. I too would suggest that the wood be made from plywood.

Another suggestion... if you make the 4 interior wheels have a slightly bigger diameter, that will work too. What I would do is put treads on the 4 interior wheels so that they have a diameter that is 0.2" larger than the outside wheels. This may be easier than varying axle heights.

Andy B.

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If all 8 wheels are driven, wouldn't that cause the outer wheels to fight the inner wheels since their smaller circumferences cause slower linear translation than the middle wheels? Is it too minor to worry about?

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Yeah, the difference is too minor to worry about. I've seen teams use this method successfully.

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I like it in general. It looks very similar to my design I posted. I too would switch to plywood, particularly a good 1088 meranti or baltic birch plywood as it is much stronger and more stable than a solid chunk of wood. Also u should look into epoxying the wood to the fiberglass plus screwing. If you do that you will have an indestructible joint.

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Another option is to use omnis on the 4 corners--that way, all eight wheels can contact the ground for stability, but still be able to turn smoothly...and I second the idea of adding L-brackets or some additional support to the 4 primary corner joints.

Question for anyone on the aluminum belly pan: what is the minimum sheet thickness one would use on such an application?

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yes use L-brackets the dont add to too much weight for all there worth

and if the ends are fiberglass why not just glass it all its not very hard and that way you would have all the great performance of wood and the added strength of the fibergalss but it would still be able to give a little allowing for a smoother ride then a metal frame:cool:


the only problem with fiberglass is its kinda messy to work with and if it gets on a tool or something you might have to throw it away


and I second what andy said about the inner wheels

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Even if it weren't too small of a difference to actually make a difference, there is a wee bit of theoretical benefit to a minor mismatch in speed. If the corner wheels are any less grippy than the specially-treaded middle ones, they'll be slipping slightly. The way friction works, they'll thus slide sideways more easily than if they were in solid lock-step with the rest of the wheels.

But it's not an issue. Andy Baker's suggestion is a proven one. The TechnoKats 2008 robot uses six-wheel drive with in-line axles and extra tread on the center wheels, and it drives great.

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does anyone use the kit wheels anymore? they work great for all kinds of stuff not just as drive wheels

at TNT i only saw 4 teams that used tham

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Good points, I forgot to cad in the top corner brackets that would reinforce those joints. It could be constructed in a very similar way to our 2008 drivetrain which was able to go through three competitions with no problems. I am not sure whether wood screws would be enough or if through bolts would be needed, that's something that may need to be tested.

Oh, and the middle four wheels are all dropped 1/8" on this model.

Last year we used a .050 or .060 sheet, I believe... not sure on the actual thickness (squirrel might know?). It held up great and provided a solid mounting surface for all of the 'guts'. Adds about five pounds to the weight, but it compensates for additional bracing and a sturdy electronics board.

We're a little hesitant to take on a project like that... like I said this particular model was designed with manufacturing taken into consideration. That means that we cut a length of wood, drill some holes, and we have a side rail. The fiberglass ends come as a pultruded channel already, so we just hack off the correct length and bolt it on. (We have this problem of being lazy and trying to avoid too much labor ;) )

Some of you also mentioned using plywood, and I'm curious as to how it would best be implemented. One team member suggested laminating several sheets of plywood together to make the siderails...


One more thing I forgot to mention before (and why I like this design so much): Since it is constructed with siderails that require just a proper sized hole for a wheel module to fit, we could really stick wheels wherever we want. So, if it turned out that we didn't want an eight wheel drive robot after all, we could just drill another hole in the center of each side and we'd have a functioning six wheel drive robot. The difference between a 6wd and an 8wd bot with this frame is just two extra wheels and chains.

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I've been discussing this with some of the students and Steve, here are a few more comments. The plan for the sides was to use a hardwood such as oak. I'm sure we'll get the "that's way too heavy!" comments...that's ok...last year our robot with it's way-too-heavy drive base was still 10 pounds lighter than it could have been, and it never wanted to fall over. Hardwood has the advantage of being dense, so it won't let the bearings open up their holes too much, and it should hold screws better than a lighter wood (although putting the screws in would be more of a challenge). We used .060" 3003 aluminum for the belly pan and corner braces last year, it worked fine, it's pretty easy to rivet to the fiberglass channel to make a strong structure.

One interesting point about this design is that it uses three quite different materials for the chassis structure, and each material is used in that place where it is most effective.

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I do like the way that you are using multiple materials and your use of fiberglass has actually gotten me thinking about incorporating it possibly. I do think that fiberglassing the side rails and bonding the whole thing together with epoxy would be your best bet. Your frame would be indestructible. Also for all the naysayers, oak is much less dense than aluminum and stronger as well for the weight so go for it. I hope you guys get a chance to prototype this and post it up on here. Maybe we should see who has a prototype frame done first :yikes:

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I think the wood will work fine for the overall stress of the base, but I worry about how about the stresses at the axels and how you are attaching the bearings. I could see big issues with however you attach the bearings for your cantilevered wheels (can someone rationally explain to me why everyone likes to have cantilevered wheels??? seriously...). I see the wood warping at the bearings (even hardwood), unless the bearings are very securely supported. For this reason it just makes more sense to me to go with two 1/8" plates on both sides of the wheels with cross-bracing.

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cantilevered wheels have benefits that i am not aware of and i will let them explain




its like the wood trim in cars, noone knows what it is but its red so they call it mahogany

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The 2 major reasons for cantilevering wheels is to ease wheel replacement, and to maximize your robot's footprint. Many teams are able to change a wheel on their cantilevered system in a matter of seconds by removing a snap ring such as 254.

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If all we had to worry about was the strength of the chassis, then I would agree with you. Unfortunately other factors come into play, like build time and ease of maintenance. Our 2007 chassis was a good example of a solid 6wd base with dead axles that were supported on each side. It was very strong, but building it took forever and it took a lot of effort to replace a wheel.

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Also how(if you decide to use this system) are you thinking about attaching bumpers, I remember on this years robot we had a nice 6 or 8 bolt attachment system but we dident factor in wrench space and ended up taking all the bolts off and putting two screws on each side in between our first and second quarter final matches

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I don't have any decent pictures of it, but last year we simply attached the bumpers on the corners and braced them in the middle with a couple of standoffs. We learned a while ago that putting bumpers on with little tiny bolts that you can't get to is a lot of work, so we used coupling nuts instead.

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For the wood, I would suggest Baltic Birch.

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Why would you suggest Baltic Birch?

(I cannot seem to find any engineering data for Baltic Birch, although I can for Yellow Birch....how different are they?)

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Baltic birch would be good for plywood but not for solid wood. A good oak or mahogany would be better suited for solid wood although you could go exotic and get bloodwood or something.

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That sounds reasonable. One advantage of using oak is that it's easily available.

Also the configuration of the wood side members in this design is such that a solid hardwood would be a good choice over plywood. For other designs, plywood might be better.

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wood is good for lots of things, but im starting to lean back towards metal side rails because thats somthing you dont want to ever have problems with

if only metal was lighter:(



does anyone know what a side rail size peice of 1/8th inch alluminum would weigh


also when i was at TNT I dont remember what team it was but they had a a piece of channel on each side and one on the bumpers and they drilled holes in both and just had pins to hold them on, it looked really nice and was very simple

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Aluminum weighs about 1/10 of a pound per cubic inch. So, if you had a 3" x 1/8" x 36" side rail, without any holes, it would weigh about 1.3 lbs. Two would weigh 2.7 lbs. That's not very heavy. But they probably ought to be made of an extruded or bent shape, such as channel or tube, to have the necessary resistance to bending.

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yes, but would a having it be channel make it hevier or lighter?

that way you could mount things easier(like bumpers)

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Imagine a channel that was 3" tall, with a 1" wide flange on the top and bottom, again 1/8" thick and 36" long. If you started with a flat sheet of aluminum and bent it into the channel shape, it would have to start out being 5" x 36" in size, so it would be quite a bit heavier than just a flat side piece.

And if it were a rectangular tube, it would be even heavier.

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Lighter or heavier depends on what you're comparing it to. A side rail like squirrel is describing is a plate, 3" high* 1/8" thick* 36" long. A typical drivetrain side using tubing has about the same weight, maybe a little more. Same length, but instead of 3" high, it's typically 1" to 1.5 " high, 1" wide, and 1/8" wall. So it will weigh a little bit more, as you effectively have to add an extra inch or two to the height of the 3" plate. Channel is lighter than tubing, because one of the sides is removed. This has other tradeoffs, though, because you lose all the support in the missing side along with the weight.

You'll end up about the same weight either way, due to holes and such like. If you really, really need the weight, a plate is easier to take material out of, but it's harder to get it removed from the right places.

edit: squirrel is right, for a channel that size. Most teams that use channel or tubing use smaller sizes or different materials.

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i just had a quick thought, couldent you use lexan for a frame? that would be awsome it becomes very stiff once you bend it into a channel shape and it would be very light ,

it dosent crack or break normaly (in sheet format) at least i have never seen it

only problem i could see with this is it developing a crack where you drilled a hole for a axle or corner bolt and it just snapping after a while

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Lexan has been done. It wasn't bent into a channel shape, it was built as a box by bending the edges and bolting. I seem to recall the 2007 design book saying that they used a Kitbot frame, or part of one, as reinforcement in the drivetrain. The team in question, IIRC, is 1714.

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wow i had figured people would say thats crazy




and what do you mean design book?

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team 126 also made lexan frame in 2007

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FIRST Robots: Rack 'n Roll. Published in 2008. 30 FRC robots that won design/innovation awards. Sequel to 2007's FIRST Robots: Behind the Design, which was robots from 2006.

I only knew about 1714 because they were in the book. Something about a see-through robot. I don't recall seeing any pictures of 126's robot...

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Correct, 1714 uses primarly lexan as their building material. They built an omni-directional "kiwi" drive this year on a lexan frame. If I remember, I'll put some better pictures, if I have any, up tonight.

http://www.morerobotics.org/Pictures...robot%20sm.JPG
http://www.morerobotics.org/Pictures...0ball%20sm.JPG

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Heh, okay guys, long time no see. But, since you brought up wood coast, I gotta speak, seeing as how we just about finished up building our own wooden robot prototype!

Oak is pretty much the best wood for a drive train, other woods compress and mar to easily to do any live axle though holes like you got here. bearings are under a lot of pressure, and they will wallow out of any softer woods.

To join the chassis together, it is recommended to use a type of cabinet or other type of joinery, these method range from the complex to the simple, and usually involve some combination of cuts, wood pins, screws, and glue, hardly any bolts. We used mortise and tenon (spelling??) joinery on the cross beams on our wood drive train and it holds up great.

the fiberglass seems to be unnecessary, wooden cross supports would work just as well with less bulk. I also think you got a little thicker wood on the side rails than you need. On ours were no thicker than 1.5 inches, and about 3 inches high, and PLENTY strong. I am sure with a little more work, you could easily cut the chassis down to about 20 lbs total.

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RoboticWanderor could you post up some more pics of the woodcoast drivetrain. Also anymore information would be appreciated. Are ya'll planning on using wood in the coming season. How much testing have you done on the new base?

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We have a fully functioning drive base using the wooden chassis, complete with a foam noodle rocket launcher turret! I will see what I can do about getting some pictures. we also used the new chassis to test chain tensioners and drivetrain layout, improving our motor mounts and the accessibility on the whole. Last year we had to rely on the smaller fingered girls on the team so they could reach into our chassis!
We really don't know what we are going to be working with for the '09 robot, and I am certainly not gonna tell if we did! We will be attending both the Dallas and Houston regionals this coming season, so everyone should have ample opportunities to come see what we have done.

Oh, and CAD work is gonna start up again here soon. keep your eyes peeled for some purple shiney!