26-10-2009 09:43

Andrew Schreiber

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

How grippy is the belt? Have you done any testing to see if the belt breaks when being pushed sideways? Would there be a benefit to using dual sided belt?

As an additional question, is the polycarb just for show or will the actual modules be made out of it? It looks gorgeous.

26-10-2009 10:03

EricLeifermann

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

1714's robots are always made out of polycarb, so if they were to make a crab system for their robot i don't see why they wouldn't use polycarb...

26-10-2009 10:04

Lil' Lavery

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

1714's robot from last year

26-10-2009 10:18

Andrew Schreiber

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Just learned my thing for the day then I stand by what I said about being gorgeous.

26-10-2009 10:22

Taylor

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

I'm a fan of the "carbdrive"
I'm also a glutton for pun-ishment.

26-10-2009 10:43

IKE

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

1714 the Anti-Atkins bot. All carb-all the time....

26-10-2009 10:51

Tristan Lall

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

The joints holding the top plate to the sides, and supporting the bevel gears scare me a little. Polycarbonate is acceptably strong, but rather flexible—so much so, that it's a bad idea to assume that it will be rigid under normal driving loads. This will wreak havoc with your bevel gears, unless the structure supporting the modules is rigid enough to overcome the tendency of the modules to distort. (Then again, it's more an issue of efficiency rather than failure; those gears have large, strong teeth.) At the very least, put some bracing on the open sides, to cut down on the parallelogramming action (due to side loads). You're definitely right to put a support ring on the bottom of the gearbox.

In truth, I don't think the polycarbonate is a good choice here. Aluminum gives you several times the stiffness, and several times the strength, at only about twice the weight. Besides, if you're concerned about appearance, you can always polish or anodize it.

Additionally, what's preventing axial motion of the gears and pulleys? It looks like you're using simple set screw shaft collars, and a pulley with a set screw hub. Especially if you allow the gearbox to flex, this arrangement will cause you no shortage of annoyance. Consider adding spacers to physically prevent axial motion. And I can't tell from this image, but if you're transmitting torque through set screws, you'll probably have suboptimal results. Please track down the resources necessary to add keys and keyways—they're vastly more reliable.

It looks like you're relying on that belt for traction, as well as power transmission. It would be wise to test that in the pre-season, to see how much it wears when running on carpet (and whatever else you think the 2010 floor surface might be).

26-10-2009 11:32

Chris is me

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

So I'll just ask you guys this stuff here since I'm too lazy to bump down an email.

If you guys don't call it "carb drive" I'm quitting the team.

How do the belts stay on the wheel? It seems like with tons of side load (turning at full speed, being rammed) they could wear away and pop off if they're being held on just by the teeth in the belt.

Is there going to be a full test chassis so that we can ram things into it all day and see what happens? Have you guys run this through its paces yet?

How much does it flex?

It looks wonderfully cool.

26-10-2009 11:44

NickE

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Are bearings being used? I can't really tell whats going on on the intermediate shaft.

26-10-2009 11:46

Chris is me

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

There are bearings on the intermediate shaft.

Video from other images:

http://www.youtube.com/watch?v=f2OXqa8BZX8
http://www.youtube.com/watch?v=mA23sg6UubI
http://www.youtube.com/watch?v=2qKl42vqjcU
http://www.youtube.com/watch?v=3cXdk4C9aQc

26-10-2009 12:48

J93Wagner

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Two things.

1. I see a potential problem with the crab drive not being stable enough because I don't see a place where the load would be delivered onto the module safely and effectively.
2. There might a problem if the polycarb breaks or bends too much. You would have to replace the entire drive module with new polycarb. I would suggest doing some stress anylises in the pre-season.

26-10-2009 12:51

JVN

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

What DP are those bevel gears?

Watch out for flex in the gearbox causing tip-loading on the bevel gears. Make sure you beat the heck out of those things to ensure the gears will hold up even under worst case shock load.

We uhh... had some issues... uhh... with that sort of thing at one point. Yeeeeeahhhh....

-John

26-10-2009 13:46

legotech25

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Hey, did you guys ever replace the top and bottom plates with polycarb? It looks like you're still using the laser cut acrylic parts, which would be a problem (it will shatter like no other). But once that happens, I like it!

Just a thought, maybe the bottom circle would be best made from Acetal? Just to reduce the friction and other drive forces... You might want to run that by AJ, Mr. Laabs, and/or Dad first though.

26-10-2009 14:04

Chris is me

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Yeah, I noticed that... Not something that should be on the final module for sure, probably just there for photo purposes I think.

I know a lot of teams use acetal right now to support their modules (like here). I'd recommend it solely based on what I've heard from other people.

26-10-2009 16:52

AJ R

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Yes, the final one will be made out of polycarbonate. After four years of building with it, we are confident it will hold up. As pictured, the top and bottom are acrylic because we wanted to laser cut them. American Acrylics is getting a CNC router, so they will be made out of polycarbonate eventually.

The belt is quite grippy. I dont have any numbers, but it seems to hold quite well. It is a single sided belt. We tested a double sided belt, but they have a nylon backing on both sides, and are less grippy.

Keep in mind this is only a prototype, and was put together quickly and cheaply. We do plan to put cross bracing to prevent flexing, if it needs it. Right now, we are just using set screws, but plan on using a hex shaft held in with snap rings. One of the things we like best about the module, is the belt for power transmission is also used for traction. We are planning to use a wider belt in the future.

The wheel has a groove the width of the belt for the belt to ride in. With it properly tensioned and aligned, it will stay on.

As for the DP of the gears, I will have to get back to you Wednesday.

26-10-2009 16:54

Akash Rastogi

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Is this backed up by any sort of FEA/ Stress testing?

26-10-2009 17:00

Andrew Schreiber

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Also, not to seem like I am picking on this but how are you tensioning the belt?

26-10-2009 17:07

AJ R

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

The tensioner is on the other side. It is easier to see in this picture.
http://www.chiefdelphi.com/media/photos/34298?
The belt rides on a bearing and the shaft is moved outward by turning the screws. The hole through the shaft is threaded and the hole in the module is not. Hope that clears it up. (no pun intended)

26-10-2009 17:56

Tyler Olds

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Another piece to making the ultimate Van de Graaff generator 1714?!?! Looks very elegant and awesome! All you need is to make propellers out of polycarb so you can make your robot fly (hmmmmmm maybe they are the secret manufactures of Wonder Woman's invisible jet............)

26-10-2009 18:41

Chris is me

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

shhhhh don't tell them about that prototype! We're still keeping THAT under wraps...

Maybe it's just me but seeing these pictures makes me a little more excited about swerve... If anything, you guys have certainly made something really, really cool. I hope you guys let me thoroughly ram and smash a test chassis (if one gets built in time). You know, "load testing"... yeah... mhm.

26-10-2009 22:33

xanarchyx

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

OK OK
Let me clarify some stuff

1. It WILL be made of polycarb, when fabricated properly it is as rigid as aluminum.
(but thank you for the warning, we will take it into consideration. Although 3/8 in polycarb is more rigid in small distances than people think.)

2. This is a RUFF prototype, but it is the ruff size/dimension of the real one. ( The "pulley" for the belt will be bigger for more teeth engagement.)

3. Bearings and bevel gears are just what we had laying around. They are not the final ones.
(the bearings are the really crappy kit chases ones i believe.)

4 I know I'm missing stuff, But this was just a quick update, although i would like to say. I believe it is beautiful.





OK i remembered, we are working on mounting it on the front of a frame with 2 casters in back to test it, although pre-tests of the grip and pull of the 3/8 in belt was kind of amazing. We will probably still go with thicker.

Btw, Belt on carpet = Amazing Grip.

26-10-2009 22:38

xanarchyx

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

... I cant post quick enough...

26-10-2009 23:38

J93Wagner

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Great, looks like you have a better start than I originally thought.

I also have access to last year's crab drive design if you're interested.

27-10-2009 00:54

AdamHeard

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Curious, did you guys decide to make a crab drive out of polycarb because you like polycarb, or did you do some sort of analysis, disscussion or pros/cons, etc... that lead you to this.

27-10-2009 01:30

EricH

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Fixed...

Adam, read the 2007 design book for about the first robot they made out of the stuff. If it weren't for all the stickers and lights you have to have, it would have been virtually invisible. I haven't seen one of theirs since... or seen pictures, either.

They're clearly pretty good at working with polycarb.

Bonus points to Adam for getting me started on a pun trail...

27-10-2009 08:07

xanarchyx

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Well, we mainly work with polycarb, in fact we a based out of an acrylics shop so all of our tools (even the drill bits) are designed for plastic. That is the main reason we use it. Also, over time we have found it is very unique, and as i have stated, if you know what your doing, polycarbonate can be used everywhere you use aluminum.

As for testing, we haven't done any technical testing of polycarb vs aluminum...Aluminum would be lighter, but the fact that we don't even have a wielder makes it a lot harder to work with.

27-10-2009 11:42

Chris is me

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Heh. Technically in 2007 we had a kit frame hidden in there somewhere. This year, the only metal in the whole robot was in Toughboxes, drive sprockets, motors, transmissions, shafts, and the controls; we used Poly Cord extensively to connect belt runs too (though not for drive, we're not THAT hardcore anti-metal )

I think a swerve module out of this stuff should open an interesting in-team discussion as to what the limits of our fabrication abilities are with the relative commitment to polycarbonate as a build material; projects like these that "push it" of course being extensively tested in the preseason to ensure they would work. It is significantly faster and easier for our team to fabricate out of polycarbonate; it's not all "cool factor" (though I can't say that doesn't play a small role). Every team's shops have their limits, maybe we're approaching ours? We'll find out!

Clearly.

(I was tempted to make this whole post in clear text...)

27-10-2009 18:16

J93Wagner

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

LOL, I think we got started on pun trails, which I haven't found again.

(Or did I?)

27-10-2009 18:32

xanarchyx

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

...o lord not the puns.

27-10-2009 19:18

EricH

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

So who gets the blame for those? Adam, for getting me started, me for starting them, or team 1714 for building clear robots that we can make puns about?

28-10-2009 12:38

Andrew Schreiber

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Welding is not a requirement for a competitive FRC robot. I would personally try to learn to work with Aluminum as well as plastics. Both materials have strengths and weaknesses and having more options in your arsenal means you can build the best robot you possibly can.

(And I hope I am not being too transparent about ignoring these puns, they clearly don't add anything to the discussion)

28-10-2009 12:54

Ryan Dognaux

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Loving the polycarbonate, it's fun to see a team make a crab module out of something other than the usual aluminum. I'm really interested to see how this holds up after your next modifications are made with the wider belts and everything else you mentioned. I really like the idea of using the belt for both traction purposes and motion. Please keep us posted on your development process as I'd like to see video of your drive system with the usual 130 lbs involved.

28-10-2009 17:27

Chris is me

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

While I agree with you for many potential components (I'd think making an AL swerve module would probably be a much safer option), a lot of times for our team there would be little benefit to building the robot out of aluminum when our resources for building that component out of polycarbonate are significantly greater. Take this year's robot, not a single structural failure, and we fabricated the robot faster this way than we would have if we used aluminum. In some cases it's better for our team to use polycarbonate only then. The tricky part is when we come across a game that basically requires a very strong, light part that isn't protected by foam bumpers...

28-10-2009 17:41

Andrew Schreiber

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

I agree entirely, if you have better resources for working with a material and a part can be made out of that material without impacting its performance then by all means make it out of that material. I was merely remarking on a general concept that I see too often, people assume that one material is "better" than another. For example, a lot of times people will assume metal > wood. This is wrong, metal > wood in some applications in others the converse is true. In engineering, as in life, everything is a tradeoff. For example this component, while making it out of Aluminum may be safer and lighter (may or may not be the case but this is just an example so it doesn't really matter) you can fabricate the Polycarb easier. Which one you decide is based on certain criteria. As I said, just a general statement geared more towards a general audience than your specific design.

I do like the fact that you guys are finding limits of polycarb with PolyCrab. I just would like to see how it holds up compared to Aluminum. Perhaps do some tests (either in CAD or on the actual thing) to see how it holds up. This is the off season, why not take the time to find where you can cut weight out or where you need more material?

29-10-2009 14:50

Aren_Hill

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

questions
1. How big is that lower pulley?
2. What diameter is that lower disc

niftyness

30-10-2009 10:49

xanarchyx

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

pully = ~ 7/8 (real one will be a bit bigger)
disk = ~ 3in

30-10-2009 11:31

Aren_Hill

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

by "lower pulley" i meant what your using as a wheel
by "bottom disc" i meant the side support disc diameter

01-11-2009 15:09

AJ R

Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

The lower pulley is roughly 3in, and the bottom disc is 5.5in.