971's chassis

[Madison]

Originally Posted by AustinSchuh

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.

Austin,

Is the CAD model for this available somewhere? I'd like to take a closer look at how you worked around the internal bends on that inner face.

[AustinSchuh]

Madison,

Originally Posted by Madison

Is the CAD model for this available somewhere? I'd like to take a closer look at how you worked around the internal bends on that inner face.

The CAD model isn't available, and we have no plans to release it. We subscribe to the 254 CAD release philosophy, and choose to share pictures and explain the why about what we do rather than directly share the model. We have a team meeting tonight though, and I'll (try to) have one of the students put together an exploded view that should answer your question.

[Madison]

Originally Posted by AustinSchuh

Madison,



The CAD model isn't available, and we have no plans to release it. We subscribe to the 254 CAD release philosophy, and choose to share pictures and explain the why about what we do rather than directly share the model. We have a team meeting tonight though, and I'll (try to) have one of the students put together an exploded view that should answer your question.

An exploded view would be great. I think I have a pretty good idea of what those pieces look like, but the exploded view will help me to figure out if I'm missing anything.

Thanks!

[SeanKennedy]

Pem nuts are great. They come in multiple different designs for different applications. We typically use an f- series nut because it is flush on both sides and is extremely to install correctly. They are much much stronger than just threading aluminum and they are a blind fastener when it comes to the installation aspect, so there is no need for any access to reach a nut or other piece of hardware on the back side. An f series nut can be installed with any hydraulic or arbor press, and most rivet squeezers.

When we cant access the back of a part, we used a rivnut which is a completely blind fastener as only one side of the material needs to be accessed to install the part. these parts are great for putting threads in tubes or in really odd spots. They can be found at our sponsors web site: enfasco.com

[James Kuszmaul]

Originally Posted by Madison

An exploded view would be great. I think I have a pretty good idea of what those pieces look like, but the exploded view will help me to figure out if I'm missing anything.

Thanks!

We've put up some shots of our drivebase CAD on our picasa page.

[Madison]

Originally Posted by James Kuszmaul

We've put up some shots of our drivebase CAD on our picasa page.

That was speedy. Thanks so much.

[AdamHeard]

Originally Posted by James Kuszmaul

We've put up some shots of our drivebase CAD on our picasa page.

I love that axle retention method.

[AustinSchuh]

Originally Posted by AdamHeard

I love that axle retention method.

I also like that the bushings that the axle go into. They both make it easy to get the axle out, and stick out a bit to replace spacers so that the bearing race on the wheels doesn't rub on the tensioner.

[Bryce2471]

If you don't mind, I've got a few questions.

1. How much does the assembly weigh?
2. How much is the drop center?
3. What is the gear ratios on the drive gearbox?
4. What is the diameter of you wheels?
5. What is the thickness of your sheet metal?

Thanks in advance!

[highlander]

Originally Posted by Bryce2471

If you don't mind, I've got a few questions.

1. How much does the assembly weigh?
2. How much is the drop center?
3. What is the gear ratios on the drive gearbox?
4. What is the diameter of you wheels?
5. What is the thickness of your sheet metal?

Thanks in advance!

Take a look at previous replies:

-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).

[AustinSchuh]

I'll take the easy ones. One of the students can get the harder ones. We have weekly Wednesday meetings, so I'll have someone work up some answers then if nobody does before.

Originally Posted by Bryce2471

2. How much is the drop center?

3/16"

Originally Posted by Bryce2471

4. What is the diameter of you wheels?

3.5" pitch diameter. We had one of the students roll a wheel a lot of revolutions and see how far it went to figure out the pitch thickness (is that even a real term?) of the tread.

[Thad House]

Why do you guys run 3/16 drop? I've heard 254 does this as well. With the smaller chassis, we've found that you can completely remove the drop and still have just as great of turning. Our's was actually built for a 0.1 inch drop, but because of the wear on the center wheels after a few matches our drop was basically non existent, and it seemed like our turning and pushing actually improved when this happened. Looking at Chris Hibner's old paper on turning physics, it actually makes sense because with shorter chassis the effective wheelbase is almost identical to the dropped wheelbase on the old size.

[AlecMataloni]

Originally Posted by Thad House

Why do you guys run 3/16 drop? I've heard 254 does this as well. With the smaller chassis, we've found that you can completely remove the drop and still have just as great of turning. Our's was actually built for a 0.1 inch drop, but because of the wear on the center wheels after a few matches our drop was basically non existent, and it seemed like our turning and pushing actually improved when this happened. Looking at Chris Hibner's old paper on turning physics, it actually makes sense because with shorter chassis the effective wheelbase is almost identical to the dropped wheelbase on the old size.

From my understanding, Blue Nitrile conveyor tread from mcmaster is very "grippy" and doesnt wear fast. Depending on their wheelbases AND tread width, teams like 1323/254/971 etc use 3/16 drop to minimize wheel scrub. This is because of their previous experiences designing drivetrains utilizing the material.

[AustinSchuh]

Originally Posted by Thad House

Why do you guys run 3/16 drop?

To be honest, most of it is because we've always done it that way and it hasn't caused us any problems. We ran less drop a number of years ago (2008 ish), and found that because both the front and back wheels could catch the carpet at the same time, the robot didn't handle as consistently. It works well enough, so we haven't messed with it. Nothing is worse than a poor handling robot or a robot that can't turn well.

[Thad House]

Originally Posted by AustinSchuh

To be honest, most of it is because we've always done it that way and it hasn't caused us any problems. We ran less drop a number of years ago (2008 ish), and found that because both the front and back wheels could catch the carpet at the same time, the robot didn't handle as consistently. It works well enough, so we haven't messed with it. Nothing is worse than a poor handling robot or a robot that can't turn well.

Ah OK. I was just wondering if you had tried changing with the smaller chassis. We plan on doing alot more testing with this over the summer, because if we can get rid of the rocking and still turn just as well we might just have to do that.

And I remember back in the old days where if you didn't have the drop or too little that think would hop like there was no tomorrow. Both the 2011 and 2012 robots I helped build didn't have enough and they didn't turn well at all.