pic: 6wd Chassis Design

[Alex.q]

Originally Posted by Madison

Do you have a capable engineering mentor on your team or in your area that can sit with you and review your work? You'll get a lot more from that than you will from this forum, unfortunately.

I was rushed to finish this design, but will have a chance to talk with some mentors about it soon.

There are a lot of problems here -- and that's okay. We all learn by trying new things and making mistakes. In drive design, the devil is in the details a lot of the time and your modeling isn't very detailed, so forgive me if I've made incorrect assumptions in the advice I offer.

1 -- It looks like you're using Pro/E. You have my sympathy.

That is correct. Although it is hard to learn, especially because I had to teach myself, I haven't used any other programs so I cant really compare it to others.

2 -- Do not use C-channel to build your frame or otherwise expect to add a VERY rigid (i.e. thick) piece of wood or aluminum plate to the inside to add rigidity. You'll otherwise have a lot of flex in the frame -- I know this from experience. The problems you're anticipating with using rectangular or square tubing aren't real.

We have built non-cantilevered, 4 wheel frames out of 4x1 c-channel inthe past, hence my tendency to use the same shape here. I am a little worried about the flex so we will probably add a couple pieces across the middle or use plywood for the electroncs board in the middle.

3 -- The Toughbox Mini with long output shaft driving the center wheel is supported in two places in the Toughbox housing. That puts the load on that shaft at 1" (channel leg height) + spacer + sprocket + 1/2 wheel width away from the bearing. That's probably 2.5" -- too far. There's all sorts of detail missing with respect to how you're going to position the wheel on the shaft -- is it retaining clips? spacers? Retaining clips weaken the shaft. Spacers will exert an axial load from the wheel onto whatever surface (presumably the channel) the spacer works off of. How will you handle that thrust loading?

I'm not sure why you say that is 2.5" too far. the total distance from the edge of the gearbox to the outside of the far sprocket is only 3.5 inches. The shaft is supported with one of the blue pieces of channel (with a bearing) about an inch away from the gearbox, so the total cantilevered distance is under 2.5". Did I explain that setup ok? I was planning on putting a spacer on the axle; the spacer would rub against a bearing in that blue piece. Would the thrust load cause a problem here?

4 -- You've said the C-channel is 2x1 -- presumably by 1/8" thick. That means the blue parts you show as your dead-axle support are 2.25 x 1.125 x .125". That size isn't commercially available to my knowledge. How will you manufacture those parts?

You are mistaken. The blue pieces are cut from the same stock as the side pieces. They nest together with the legs of the blue piece on top of the legs of the frame piece. One of the pictures I posted kind of shows this.

5 -- It doesn't matter because they won't work. You've got a hole in blue channel and a slot in the frame channel along side slots for mounting the blue channel. I presume those mounting slots are for chain tensioning. Think about what happens when you slide the blue channel away from the center when the chain is under tension. Your axles will not remain parallel to the center axle/toughbox output and you will throw chain and add a lot of drag to the drive.

You are correct, that is what the slots are for. Would there be any way to keep the axles from pivoting as you described? Would putting a washer on the axle between the nut and frame (on the non-wheel side) possibly minimize this?

6 -- Is each Toughbox attached with only two screws? That's going to exacerbate the flex in your frame. Follow the loading -- weight of the robot passes through center wheels, bending toughbox output upward. Flex in output shaft tries to rotate the entire gearbox housing about its mounting point. This is hard to describe, so you have my apologies.

I think I understand what you are saying. I only attached the gearbox with 2 bolts because the gearbox's mounting holes are too far apart to fit more of them in the frame. How do other teams manage this (other than using custom gearboxes)?

7 -- The location of the sprockets doesn't matter. All of the loading is passing through bearings / hubs in your wheels. The sprockets are floating around the shafts and your biggest concern isn't torque applied by the chain bending your axles in a horizontal plane, but torque applied by the weight of the robot bending them in a vertical plane.

That is what I thought. The current placement of the chain was designed to try to minimize the cantilever distance with dead axles.

None of us get things right the first time -- or the second, third, or fourth. After looking at a design for a long time, it's hard to be objective about its strengths and weaknesses, so asking for advice from new sets of eyes is a good thing. If some of the things I've written don't make sense, I apologize, but I'm in a bit of a rush this morning -- I've got presentations to give this morning on drive design

Thank you your advice on all of this. I appreciate it.

[Tom Ore]

Your Pro/E work looks good - considering you are self taught it's quite good actually.

C-Channel frames can work - it depends on how they are loaded.

I believe the concern with the long shaft is regarding how far the wheel is cantilvered. In general you want the support bearing as close to the wheel as possible.

[BJC]

That wheel tensioner isn't going to work very well. 2 tightened locknuts holding a tightened drivetrain chain in tension only by keeping the bolthead and nut in compression with the hub is not going to work for any reasonable amount of time. You really need some kind of cam device to keep your chains from falling off.

[Alex.q]

Originally Posted by compwiztobe

And with box tube, you would no longer need the extra piece of c-channel to support the axle in a second spot, so you don't need those particular bolts any more anyway, AND you've gotten rid of 30 components (1 bracket, 2 bolts, 2 nuts per wheel) in the entire drive base, making the whole thing much simpler and less prone to failure.

But if I eliminated the axle brackets, I wouldn't be able to slide the axles to tension the chain. That was the idea behind using them as I did. Would box tube really make it much more rigid?

Originally Posted by BJC

That wheel tensioner isn't going to work very well. 2 tightened locknuts holding a tightened drivetrain chain in tension only by keeping the bolthead and nut in compression with the hub is not going to work for any reasonable amount of time. You really need some kind of cam device to keep your chains from falling off.

Do you mean a cam that pushes the chain down, or do you mean a cam that rotates to push the axle brackets farther out? Assuming you meant the second thing, then I was wondering why that keeps it tight. Couln't the force just cause the cam to rotate an pull the brackets together as you described before? (I'm not saying it doesn't work, just that I don't understand why it does)

[Aren Siekmeier]

Originally Posted by Alex.q

But if I eliminated the axle brackets, I wouldn't be able to slide the axles to tension the chain. That was the idea behind using them as I did.

Can't you slot both sides of the box tube like you did in one side of c-channel?

Originally Posted by Alex.q

Would box tube really make it much more rigid?

Yes. At 1/8" wall it's less likely, but C-Channel will still collapse on the open side much much sooner than box tube. Think about just the profile of each of them and which would hold up better. Closed rectangle > open c.

Originally Posted by Alex.q

Do you mean a cam that pushes the chain down, or do you mean a cam that rotates to push the axle brackets farther out? Assuming you meant the second thing, then I was wondering why that keeps it tight. Couln't the force just cause the cam to rotate an pull the brackets together as you described before? (I'm not saying it doesn't work, just that I don't understand why it does)

You can do either. The principle is that of the inclined plane. The angle of the cam and the friction between it and whatever it moves are such that no force exerted back onto the cam will turn it, but you can still turn the cam. Thinking of just a block on an inclined plane, given a downward force, the static friction force <= mu*tan(theta)*mg, so if mu*tan(theta) is greater than 1, then the frictional force will always be sufficient to keep the block in place, no matter what force you exert straight down. However, if you push sideways (up the ramp), you don't get the same force of friction and can move the block farther up (or farther down) at will. The block moves when you want it to, and not at all otherwise.

[BJC]

Originally Posted by Alex.q

Do you mean a cam that pushes the chain down, or do you mean a cam that rotates to push the axle brackets farther out? Assuming you meant the second thing, then I was wondering why that keeps it tight. Couln't the force just cause the cam to rotate an pull the brackets together as you described before? (I'm not saying it doesn't work, just that I don't understand why it does)

I meant the second. The forces that are pulling the chain are linear, if your chain tensioner is also linearly tensioned then 100% of the forces on the chain will be pulling on those bolts. However, a cam uses some sort of asymetrical piece on a pivot (like a bolt) as the cam turns around the bolt the radius changes relative to a point. This means that the forces required to loosen that chain must be applyed at almost a 90degree angle to the linear forces from the chain. This means only a fraction of the forces on the chain are actually doing work to turn the cam back out. If you use a bolt as a pivot for your cam then you can tighten it with a lock nut to couneract the fractional amount of force trying to turn the cam out.

We did this last year using seemless chain with great results. Ofcourse if you were really worried about the cam comming loose you could always have another cam to keep the tenision on the first cam.

Nice start on the drivetrain. I'll say that I know that C channel seems like it will work, however there is a reason that teams who use WCD's use box tubing as opposed to C channel. Generally when muliple very good teams do the same thing multible years in a row it is because it works really well. In this case you would probably do well to join the crowd. A phrase that is repeated often on my team: "The best indicator of if a design is good is if the team who used it uses it again next year."

Good luck, Bryan

[roystur44]

If you added two more c channels to the outside of the frame, lose the internal brackets and tied the channels into the front and back channels with some L brackets you have have a nice stiff frame.

[nssheepster]

Um, I'm not a super genius at this, but my team has had problems in the past with motors overheating. Wouldn't your motors heat up really fast if they're right next to each other like that? How are you dispersing the heat? Or is it just not an issue?

[nssheepster]

Isn't it a 2-wheel design with 4 support wheels? I mean, 6 wheels, but only 2 drive wheels? Right?

[Billfred]

Originally Posted by nssheepster

Um, I'm not a super genius at this, but my team has had problems in the past with motors overheating. Wouldn't your motors heat up really fast if they're right next to each other like that? How are you dispersing the heat? Or is it just not an issue?

Running CIMs in this style is more or less standard practice; look no further than the kit's CIMple Box gearbox as evidence. Even the CIM-Sim gearbox designed for heat-a-holic Fisher-Price motors doesn't put too much daylight between the two. If you're not abusing them* (in time or in work demanded), it usually doesn't factor into the equation.

*That's not to say you wouldn't be abusing them in the context of an FRC event; many teams that go deep into eliminations find themselves resorting to ice packs or spray dusters (or I think 433 used dry ice one year) to get the temperatures down on Fisher-Price motors simply because of the tight turn-around times.

[Ether]

Originally Posted by nssheepster

Isn't it a 2-wheel design with 4 support wheels? I mean, 6 wheels, but only 2 drive wheels? Right?

The front and rear wheels are also driven, via chain.

[nssheepster]

Ah, gotcha. Thanks. Senior year, fourth year on the team, but I'm stiiiiiilll new. Well, what is school for?