pic: 1714 Polycarb prototype Crab Drive (backdrop1)



INTRODUCING TEAM 1714’s clear polycarbonate carbdrive.

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.

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…

1714’s robot from last year

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

I’m a fan of the “carbdrive”
I’m also a glutton for pun-ishment.

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

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).

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. :smiley:

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

There are bearings on the intermediate shaft.

Video from other images:




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.

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

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.

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

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.

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.

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.

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

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

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)

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…)