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Unread 10-26-2009, 08:40 AM
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pic: 1714 Polycarb prototype Crab Drive (backdrop1)

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Unread 10-26-2009, 08:43 AM
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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.
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Unread 10-26-2009, 09:03 AM
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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...
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Unread 10-26-2009, 09:04 AM
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Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

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Originally Posted by Andrew Schreiber View Post
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 robot from last year
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Unread 10-26-2009, 09:18 AM
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Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

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Originally Posted by EricLeifermann View Post
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...
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Unread 10-26-2009, 09:22 AM
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Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

I'm a fan of the "carbdrive"
I'm also a glutton for pun-ishment.
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Unread 10-26-2009, 09:43 AM
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Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

1714 the Anti-Atkins bot. All carb-all the time....
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Unread 10-26-2009, 09:51 AM
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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).
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Unread 10-26-2009, 10:32 AM
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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.
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Last edited by Chris is me : 10-26-2009 at 10:43 AM.
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Unread 10-26-2009, 10:44 AM
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Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Are bearings being used? I can't really tell whats going on on the intermediate shaft.
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Unread 10-26-2009, 10:46 AM
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Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Quote:
Originally Posted by NickE View Post
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:

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
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Last edited by Chris is me : 10-26-2009 at 12:55 PM.
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Unread 10-26-2009, 11:48 AM
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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.
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Unread 10-26-2009, 11:51 AM
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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
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Unread 10-26-2009, 12:46 PM
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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.
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Unread 10-26-2009, 01:04 PM
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Re: pic: 1714 Polycarb prototype Crab Drive (backdrop1)

Quote:
Originally Posted by legotech25 View Post
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!
Yeah, I noticed that... Not something that should be on the final module for sure, probably just there for photo purposes I think.

Quote:
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.
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