971's chassis

[cxcad]

This year I think 971 has built one of the most effective and innovative chassis, and consequently I want to emulate what they have done.

I noticed from the Flikr account that this year 971 departed from from what I presume the "original" three piece design. Could the octagonal shape not be achieved in three pieces? (image here)
That brings the questions: do the ribs provide significant rigidity?
What's the advantage of one piece modules over the traditional railed approach? (image [I know there are ribs inside it, but for the most part it is one piece?])
What is the advantage of PEM nuts, is there a closer image of them in use, how do they hold up to the beatings and vibration? (image)
Finally, what are those giant copper type plugs?

[RoboChair]

Those Copper plugs are called cleco's and they are basically a temporary rivet. You place the cleco in the holes you want clamped together and it keeps them aligned with each other. It provides an easy way to fab lots of sheet parts with less risk of assembling as you go that using rivets to start might cause.

[AustinSchuh]

Originally Posted by cxcad

This year I think 971 has built one of the most effective and innovative chassis, and consequently I want to emulate what they have done.

I noticed from the Flikr account that this year 971 departed from from what I presume the "original" three piece design. Could the octagonal shape not be achieved in three pieces? (image here)

We ran the "original" 3 piece design in 2012 and 2013, and it has served us very well.

We couldn't find a way to make an octagon with a 3 piece chassis, which means it probably isn't possible or easily to machine. We wanted an unbroken flange to help form a backbone to tie everything to and to tie everything together. The best way to do that that we found was to set the chassis up as you see this year. We started by figuring out where to put the wheels and pulleys, and then built the frame to hold everything in place. This resulted in the jog on the inner face, which resulted in an internal corner which couldn't be bent without breaking things up into more pieces. We also needed a parallel face to bolt the wheel tensioners to, which drove the inner frame rail shape that you see. You can see the set of 8 #6 holes that define the tensioner location in one of the pictures.

Originally Posted by cxcad

That brings the questions: do the ribs provide significant rigidity?

In theory, they should provide more torsional rigidity and prevent the tube from collapsing. We could probably leave them out, if we really wanted to, but I'm a fan of overkill when it doesn't hurt you... We also choose to bolt our superstructure down either to them, or right next to them, which spreads the load a lot better by spreading it through the rib.

Originally Posted by cxcad

What's the advantage of one piece modules over the traditional railed approach? (image [I know there are ribs inside it, but for the most part it is one piece?])

By railed, do you mean a classic WCD chassis? We have lots of sheet metal sponsors, and a large amount of experience working with sheet metal. We build robots that play to our strengths, and this chassis does. By building a big tube instead of 2 rails like the old AM chassis, we get a lot more structural rigidity. It is quite hard to flex one of our chassis. Our wheel tensioners are designed such that the act of tensioning the wheels locks the shaft into the frame, so it really isn't very hard to get the wheels out. The wider wheels and wider body tube lets us flip the CIMs over the wheels, which opens the belly pan up significantly.

Originally Posted by cxcad

What is the advantage of PEM nuts, is there a closer image of them in use, how do they hold up to the beatings and vibration? (image)

PEM nuts give you the strength of steel threads in aluminum. When used properly, they are quite strong and have no problems. When used improperly, they fall out and are a pain to use. When using PEM nuts, make sure that you never let any load be put into them that would try to press them out of the material. On our drive base, we only used them to mount speed controllers to our belly pan. In our superstructure, we really only used them in a couple places to let us remove parts quickly.

[cxcad]

Originally Posted by AustinSchuh

By railed, do you mean a classic WCD chassis? We have lots of sheet metal sponsors, and a large amount of experience working with sheet metal. We build robots that play to our strengths, and this chassis does. By building a big tube instead of 2 rails like the old AM chassis, we get a lot more structural rigidity. It is quite hard to flex one of our chassis. Our wheel tensioners are designed such that the act of tensioning the wheels locks the shaft into the frame, so it really isn't very hard to get the wheels out. The wider wheels and wider body tube lets us flip the CIMs over the wheels, which opens the belly pan up significantly.

I meant something like this, but I think you answered my question. I don't quite understand how the tensioners work; can you post an image of them in action? Also why not exact center to center belt?

[highlander]

I would also be very interested in learning how these tensioners work. I saw the picture you guys have, but how do you make sure that both sides of the shaft are aligned so that the wheel is straight?

I have a couple more questions:

How hard is it to replace the timing belt with this setup?
Sheet-metal wise, what tolerance does your manufacturer provide?
What thickness and alloy of sheet metal do you use?
Do you guys press in bearings into the sheetmetal? If not, what else do you add to keep them in there, and if so have you had any problems with them falling out?

Thanks

[saikiranra]

What is the advantage of having the center drive wheels offset right/left, compared to other traditional 6 wheel drive set-ups?

[AustinSchuh]

Originally Posted by saikiranra

What is the advantage of having the center drive wheels offset right/left, compared to other traditional 6 wheel drive set-ups?

We wanted our frame perimeter to be an octagon to change how the robot interacts with the field and other robots. The wheel placement was chosen to maximize the space in the center of the frame once we had decided on the frame perimeter.

[Travis Schuh]

Originally Posted by cxcad

I meant something like this, but I think you answered my question. I don't quite understand how the tensioners work; can you post an image of them in action? Also why not exact center to center belt?

Originally Posted by highlander

I would also be very interested in learning how these tensioners work. I saw the picture you guys have, but how do you make sure that both sides of the shaft are aligned so that the wheel is straight?

This is a pretty good picture of how our tensioner blocks work. The block in the upper left is positioned such that the side facing up would be bolted to the sheet metal.

The inner block slides relative to the outer block, the bolt comes from the front and provides a pull tensioner. The outer block has a lip that combined with the sheet metal fully captures the block. The springs return the block to neutral when you de-tension the wheel.

When the wheel is fully de-tensioned, the dead axle lines up with a hole in the frame. This allows us to push out the dead axle. Our wheels then drop out as one module (there should also be pictures of our wheels in the picassa albums). Once the wheels are tensioned, the dead axle no longer lines up with the hole and is captured by the frame rails. We find this to make wheel changes very easy. Also, with belts, it is critical that you are able to replace a broken belt quickly, and this does a good job of enabling this.

We align the wheel angle by eye. It turns out that the human eye is pretty good at judging this, we typically sight if the wheel is parallel to the lightening hole. We have never had an issue of having a miss-aligned wheel. We ended up going to a separated tensioner design because we really wanted a pull tensioner for various reasons (2012 was push and had lots of problems), and we couldn't come up with a way that was compact enough while maintaining accurate wheel angle.

We don't do exact center to center because it is hard to install them this way given our frame, but more importantly because you do not get the full strength performance of the belts if they are not tensioned properly. Our experience is that it is not reasonable to do this reliably with FRC tollerances. The drive belts are particularly important to get every last bit of performance out of because they are running out of spec. We do exact center to center in a lot of other mechanisms to save design and manufacturing time when the loads are low (IE the belt tying our two intake wheel sets together and the first stage motor reduction on our tusks).

Originally Posted by highlander

I would also be very interested in learning how these tensioners work. I saw the picture you guys have, but how do you make sure that both sides of the shaft are aligned so that the wheel is straight?

I have a couple more questions:

How hard is it to replace the timing belt with this setup?
Sheet-metal wise, what tolerance does your manufacturer provide?
What thickness and alloy of sheet metal do you use?
Do you guys press in bearings into the sheetmetal? If not, what else do you add to keep them in there, and if so have you had any problems with them falling out?

-Replacing timing belts is pretty easy. The center wheel is on a dead axle also, so it can drop out without us having to disassemble any of our transmission (the problem of belts in a traditional WCD).

-We always find that our sponsors are better at the tolerances than they are willing to commit to. They will give us their standard bend tolerance of /pm .015", but our experience is that they are much better than this. I think we spaced a .010" gap between anything in the drive train that was a bent part that needed to fit inside of another bent part. We didn't have any fit problems in assembly.

-We use mostly 060 on the robot, however the drivebase was half 060 and half 090. Any sheet metal that could be probably impacted by other robots (bellypan, outer rail, and maybe one or two others that I can't remember) were 090. The rest was 060 to save weight. This year was really brutal and both chassis still looks like they are new (probably helps that the bumpers also held up very well).

-We do press a lot of bearings in, however we are re-evaluating that for the future based on some bad experiences this year. We were disappointed to find that the vex pro bearings have a very large undercut by the flange that is about as thick as a piece of 090. Their bearings (especially the hex bearings) have a very poor press in sheet metal. We will likely be sourcing different bearings for next season.

[cxcad]

Originally Posted by Travis Schuh

Once the wheels are tensioned, the dead axle no longer lines up with the hole and is captured by the frame rails.

So the axle is held in place by the block and friction against the frame rail? Couldn't it just be cantilevered?

[DampRobot]

Originally Posted by cxcad

So the axle is held in place by the block and friction against the frame rail? Couldn't it just be cantilevered?

Not if they wanted to make it easily drop-outable, one of the primary advantages of dead axles. The way it makes sense to make their frame, the belt is right next to the wheel anyway, so they might as well avoid the complexity of making that shaft live. The whole retaining/dropping out is really smart, and I've never actually noticed that specifically, despite having looked at their drivetrain more times than I can count.

By the way, good to see you come out of your CD hibernation Travis and Austin.

[AustinSchuh]

Originally Posted by DampRobot

By the way, good to see you come out of your CD hibernation Travis and Austin.

The students have been doing a great job answering all questions on CD, so there hasn't really been anything to add.

[craigboez]

Based on the pictures it looks like you're adding a countersink before putting in your rivets. This seems rather time consuming, so I'm guessing there is a good reason. Care to elaborate?

[James Kuszmaul]

Originally Posted by craigboez

Based on the pictures it looks like you're adding a countersink before putting in your rivets. This seems rather time consuming, so I'm guessing there is a good reason. Care to elaborate?

If we had to do the countersinks ourselves, yes it would be time consuming, but it is relatively easy for our sponsors to quickly do the countersinks (and to do them right for the rivets). This then reduces the number of protrusions that we have to avoid interfering with or that might catch on things.

[AustinSchuh]

Originally Posted by craigboez

Based on the pictures it looks like you're adding a countersink before putting in your rivets. This seems rather time consuming, so I'm guessing there is a good reason. Care to elaborate?

We use countersunk rivets. There are punches on the CNC turret punch which will make a .129 hole that is countersunk to 120 degrees, all in 1 hit. This makes it so the bottom, front, and sides of our bot are perfectly smooth and can't catch on anything.

[roystur44]

Originally Posted by AustinSchuh

We use countersunk rivets. There are punches on the CNC turret punch which will make a .129 hole that is countersunk to 120 degrees, all in 1 hit. This makes it so the bottom, front, and sides of our bot are perfectly smooth and can't catch on anything.

The tool actually makes two hits. A prepunch hole and the tool that forms the csk. Metal is malleable. Here is a link to show some cool things you can do on a punch press. Take a look at the stiffening rib tool and the Zip Tech.



http://www.wilsontool.com/ProductSub...ng_p_spcl.aspx