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Looks easy to make, the only concern I would have is can it take a hit? Aside from that, what is the weight and where do I mount bumpers? (assuming they are legal) And how do you tension the chain?

Just a thought but could you save weight by using thin sheet metal as your belly pan?

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It is not completely clear from the CAD, but remember that:

1) The middle wheels need to be offset lower, or the robot won't be able to turn easily. We are using a .5 cm offset this year, but I have read about teams using 1/8 inch.

2) Many teams use omni wheels on one or both ends (not the middle) to aid in turning. The omnis compromise pushing power a bit, but make turning much easier.

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Doesn't look very stable. Maybe its the wheels or the actual frame. What would happen if it were hit on an angle? Tip over?!

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I assume the sides and ends are made of some hardwood, such as oak? and the bottom and corner braces are made of a thin plywood? Looks strong! would the weight be about 15 lbs for the wood parts? (just a guess)

Perhaps you could add some vertical pieces of aluminum angle to the corners, to easily mount bumpers to?

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What do you mean by hit on an angle? Do you mean if it were hit while going up a ramp?

I would also have to disagree and say that stability would most likely not be an issue unless they put a lot of weight up high but that would be an issue for most chassis. This looks very stable, I am assuming those are 6" wheels which puts almost all of the weight of this chassis below 6". This should be pretty stable unless they pick up like 5 of the old tetras and swing them around 8 feet in the air it should be pretty stable.

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This is basically an all wood live-axle version of our 2008 chassis, which worked great (except perhaps the strange chain path I used...). Looks good, but I think it would be preferable to mount the transmissions in the center, so you can use corner plates like on the front on all three corners.

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I'm not sure how well it would survive compared to other frames, but with bumpers I don't think it would have any problems. As squirrel pointed out, bumper mounts on the corners are easy to make and work well. For chain tensioning, I like the nylon-circle-with-offset-hole-approach. It has worked well for us in the past and is easy to do.

A sheet metal belly pan would probably save a bit of weight and could be stiffer, but I laid out this frame trying to minimize the number of parts that need to be bought online. All of the wood parts can be easily found at a home improvements store. (although if we built a chassis like this, it would probably have an aluminum base plate. :) )

It's hard to see in the rendering, but the center wheels (5" banebots wheels) are lowered 1/8".

Actually, having the transmissions in the back has some neat advantages. Since the center wheel is dropped, the rear wheels don't carry as much load. This is probably a lot nicer on the transmissions since the wheel is is about 1.5" from the transmission bearing. Also, with the chain routed this way (one on the inside, one on the outside) the wheels can sit in closer to the frame.

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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.

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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.

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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.

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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.

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.

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)

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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.

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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.

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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.

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.

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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.

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Yes, we sure do!

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

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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.

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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.

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

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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.

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oops, double post...

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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.

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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.

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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.

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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.

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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.

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*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.)

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That's a neat tip, thanks!

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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.

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We visited a friend who's into RC planes this morning, he suggested smearing some baking soda on the wood, then adding some thin CA, and beware of fumes. I guess we'll have to get some CA adhesive and make it part of our experiments.

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OK, I guess I have to confess, I learned the CA trick while building RC planes. The baking soda works much like "kicker" for the CA, accelerating the cure. This may or may not be a good thing in this case. It may cause the adhesive to cure too fast, not allowing it to wick as far into the wood. I think you are right, some experimentation may be in order.

Rick, I like epoxy for this also, but the 1 to 1 mixes found at the hardware store are too thick in my opinion. A laminating epoxy like West or Pro-set are better choices. This also takes a lot longer to cure and the protective gloves warning still stands.

I like this thread, lots of good techniques for extending the utility of an inexpensive, easy to use material.

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could use this perhaps http://www.3m.com/US/auto_marine_aer...ml?itemNbr=208

ive used it before on my hobie cat, its pretty tough sutff....

although ive found it does crack when there is high force dircted at it(such as if it was hit with a chisel
after it had cured)but it does work good for filling things

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My team is making a similar prototype with six wheels, except we have the middle wheels powered instead of the rear. I think that this this rear drive is a better design but some people on my team don't think so. Can anyone give me any reasons why one would be better than the other (possibly someone to quote) so that I can convince my team?

thanks

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I wonder if section 4.6 of the manual also applies just to the crate or everything being shipped?

http://www.usfirst.org/community/frc...nt.aspx?id=452

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To save others the trouble of looking it up:
Interesting catch if it applies, but if this rule only applies to "wood packing materials" then I don't think it would be a problem. (Our robot was denied shipment because it's full of bugs!)

I don't think either arrangement offers much over the other; they're very similar. I arrived at the configuration pictured because of some decisions I made about the way I wanted to build it:
-To make mounting the transmission easy, it would be face mounted to the wood (so no chain can run from the transmission on the inside of the frame)
-To minimize bending load on the transmission shaft, the directly-driven wheel needs to be as close in to the frame as possible. This means only one chain can be connected to the driven wheel, which means the transmission needs to be on an end.
This obviously doesn't apply to all designs, it's just the path I took for this particular one.