Re: What Drivetrain?
 

The "standard" definition of a WCD is a 6WD drop-center, cantilevered wheels, live-axle, center directly driven off of the gearbox (usually a 2-speed gearbox), outside wheels chain-driven off the center axle.

I don't necessarily agree that WCD is the best of the tank drives, partly because it does take some precision to make and thus might be rather difficult for some teams, but it is up there if a team can build it properly.

Re: What Drivetrain?
 

What if your team switched drive trains through the season...

No, it didn't end well.

Re: What Drivetrain?
 

Last year, our team changed drivetrains at Championships. Try to avoid that.

Re: What Drivetrain?
 

Actually... Switching drivetrains is something you could get away with, IF you've planned correctly. For example, if you've got the extra hole(s) drilled already, it'd be relatively simple to go from 8WD to 6WD, or vice versa, or 4WD to 6WD, or 4WD to mecanum (with a pair of transmissions added). Planning correctly means that you built the robot to be able to switch--though it does not necessarily mean modularity.

330 did that back in 2005--the competition robot had an extra set of transmission slots so we could swap to mecanum from 6WD if we thought it was a good idea. The testbed--a Kitbot--ran mecanum. We didn't have the weight that year to switch. To this day, 330 has yet to run mecanum on a competition robot.

Re: What Drivetrain?
 

When you say extrusion, did you use something other than the normal 2X1 tubing?

Also whats up hex bearings?

Re: What Drivetrain?
 

The one year we did mecanum (logomotion) we machined our frame with holes to easily be able to add dropped center wheels if the mecanum didn't work. We were happy with it's performance because the ease of strafing to hang tubes and that games rules limiting defense.

This year we made major drivetrain changes at our second regional and we were inspected and driving by 2:00 on Thursday. It can be done if you plan properly and practice the change, a second robot is invaluable for this.

Re: What Drivetrain?
 

Not sure if this is what he was getting at, but I'll give it a shot. There have been a couple of issues with hex bearings this year. One of the main ones was simply supply. For a long period of time this season, hex bearings were completely sold out wherever you looked. There were also problems with the hex itself, which was often oversized or undersized.

1/2" hex bearings are sold out at VexPro and West Coast Products (although their store is down for maintenance). They used to be sold out at AndyMark, although it looks like they're back in stock with an order limit of 10.

Re: What Drivetrain?
 

Yes, we use 1/8" 1 x 2 aluminum extrusion.

The hex bearings are a problem because they shouldn't exist, at least not for 4 dollars. It seems that the vex pro ones are just normal bearings broached to fit a 1/2" hex shaft. The problem with this is that the added force on the corners of the shaft crack the inner race and render the bearing useless. This happened to use every couple hours of driving in practice, and a 2 times in competition. If you use hex parts, do it with round bearings if at all possible. (you can machine down a 1/2" hex shape on 5/8th shaft)

Re: What Drivetrain?
 

Has anyone else had this problem with hex bearings?

I'm curious because I know they're other teams that run WCD's with hex bearings and to my knowledge they don't have any problems.

How exactly were your bearings configured on your WCD? What was your bearing block setup like? How close together were your bearings? Did you do anything very different from the stereotypical WCD in your drivetrain?

-Adrian

Re: What Drivetrain?
 

There was a lot of discussion about their issues this build season.

Here is an example of one thread:

http://www.chiefdelphi.com/forums/sh...d.php?t=111513

Re: What Drivetrain?
 

Interesting. We have a lot of time on both our practice bots, and 4 events on comp. All on 1/2" hex bearings from AM; no failure or issue whatsoever.

Re: What Drivetrain?
 

I've never seen a hex bearing that came "out of the tube" with significant runout issues or race damage. We've bough bearings from AM, WCP and VP. That said, poor (read: >~.008 too tight) press fits, mishandling, or excessive loads can damage the bearings. Especially damaging is when you press on the inner race while pushing the outer race into a too tight press fit.

I've never seen using 1/2" hex broached bearings per se as a problem, but there are a couple of hypothetical workarounds. First, you could turn down the 1/2" hex to 1/2" round to fit in a bearing (this would only really work if the round bit was at the ends, so it wouldn't work right for a traditional WCD). Also, you can press a 1/2" hex broached 1/2" ID 3/4" OD brass bushing into the inside of a 3/4" ID ball bearing. We've done somthing similar with 3/8" hex in a 1/2" bearing. A little excessive, but it would let you use 1/2" hex unmodified in a bearing that didn't have the inner race broached. Others have mentioned fine foxes as well.

To answer the origional question, we did 4WD skid stear. It was solid but not excellent.

Re: What Drivetrain?
 

I can only talk for the VEXpro bearings. I forgot to mention that the entire first batch we got was nowhere near concentric, the second batch was still not great but we didn't have the time to wait for more. The Andymark/WCP might not have the problem of cracking, but the VEXpro ones sure did. Next year we will definitely be using round bearings.

Re: What Drivetrain?
 

We use Six wheel drop center.

Re: What Drivetrain?
 

3539 used a 6w WCD this year. The past two years, we used 8 wheel standard tank drive, but this year we tried WCD to open up the inside of our frame for our ground intake and we were EXTREMELY happy with the results. Our drivetrain was easy to build, easy to maintain, rugged, light, and extremely nimble. We had a 1/8" drop on the center wheel, direct driven from our gearboxes. Custom 6" aluminum wheels with roughtop tread. We were geared for about 13 fps, so we were able to drive around most other robots on the field. We were very satisfied with the WCD style and will likely be visiting it in the future.

Here's a little video of our robot driving around at Troy. It was fun to watch this thing go :)
http://www.youtube.com/watch?v=gnDLFystS1Y