Aluminum

[Alan Ing]

Originally Posted by Cory

They should. We used 1/2" axles with the ends milled down with a 7/16" hex for our wheel and sprocket to mount to. Haven't had a single problem. We also used 7075 for a series of ~1.5 foot shafts in our conveyor system this year. With 7 shafts at 1.5' ea, we'd have been looking at 10.5 feet of 1/2" steel shaft otherwise.

Using aluminum in certain areas instead of steel is pretty much the only reason we made weight last year.

Cory, if you don't mind, how did you retain your wheels and sprockets? Did you use an external retaining ring with a washer, did you just tap the end of the aluminum and just use a screw with a washer? I was thinking we might try this method since we have both a 1/2" and a 7/16" hex broach. I know you guys used a key before. What in particular do you like about the hex shaft over the keyway?

I have always thought your team's drivetrain was one of the best 6 wheelers but was surprised to hear that you used 7075 aluminum for the axles. I know your machine took quite a few hits during last years competition and to hear that the axles were only 1/2" 7075 is a bit enlightening.

Oh yeah, are you also using 25 pitch chain?

[sanddrag]

Not to barge in here, but I can help get a few of the above answered. Yes, they did use number 25 chain. Sprocket count was somewhere around 20 teeth. They use snap rings to hold the wheels on, and I believe the sprockets too. My team uses screws and washers which I enjoy. I imagine they use hex instead of key, because it is easier to install and remove, there is less parts, and no little parts (the key) to lose. Not to mention it is stronger.

Also, 7075 aluminum is much stronger than 6061 with only a minimal increase in density (~.003 lb/in^3 more). 7075 has a tensile yield stress of about 73ksi whereas 6061 has a tensile yield stress of about 40ksi.

[Cory]

Originally Posted by Alan Ing

Cory, if you don't mind, how did you retain your wheels and sprockets? Did you use an external retaining ring with a washer, did you just tap the end of the aluminum and just use a screw with a washer? I was thinking we might try this method since we have both a 1/2" and a 7/16" hex broach. I know you guys used a key before. What in particular do you like about the hex shaft over the keyway?

I have always thought your team's drivetrain was one of the best 6 wheelers but was surprised to hear that you used 7075 aluminum for the axles. I know your machine took quite a few hits during last years competition and to hear that the axles were only 1/2" 7075 is a bit enlightening.

Oh yeah, are you also using 25 pitch chain?

as Dave said, we used external retaining rings. They work great provided you leave yourself a little bit of wiggle room. When cutting the grooves on your shafts you need to not put them flush with whatever you're trying to retain, or else the snapring will have a hard time properly seating itself. You could drill and tap the ends and use bolts and washers just as easily, though. This would be easier than cutting the snapring grooves.

The hex shaft is great for a couple reasons, the main one being simplified assembly. Instead of needing a key, you just slide the sprocket/wheel on the shaft. I wasn't around in 2005, but just from fixing that year's robots last year, I can tell you it takes significantly longer to seat a woodruff key, slide the sprocket/wheel on, and put on a retaining ring than it does to slide a hex broached sprocket/wheel onto the shaft. We can swap out all 6 wheels in under a minute.

We've never had a problem with keys wearing, but it transmits torque over a larger surface area, which could be beneficial as well.

The 7075 shafts have been a dream. If we were to degrease the ones on the robot right now, they'd look practically identical to when they were new. We haven't had a single failure, or wear problem.

Since you've got the broaches, I'd strongly suggest you investigate using hex shafts for power transmission this year.

P.S. We'd love to have you guys by our lab again either before or after SVR. Let us know if you'd like to come.

[Gabe]

Does anyone know of a good source for 7075 gears? Similar to the kit transmission gears, with 14.5 degree angle and 20 diametral pitch for a possible shifting transmission application.

[Cory]

Originally Posted by Gabe

Does anyone know of a good source for 7075 gears? Similar to the kit transmission gears, with 14.5 degree angle and 20 diametral pitch for a possible shifting transmission application.

Nope. You'd have to custom cut/hob them. You'd also need to look into different coatings and surface treatments for them to be suitable in a drive system.

[Alan Ing]

Cory and Dave,

Thanks for sharing the information about the 7075 axles. Providing the same type of bumpers are allowed this year, we just might give this a try.

Cory, we really appreciated last seasons unanounced visit to your Team's lab. I hope you didn't mind. We just happened to be at AMES when it occured to us that you guys were based somewhere on site and one of our team members asked if we could pay you guys a visit. Afterall, it was your team that originally traveled to Hawaii with Jason to show us your 1999 robot and got us into all this stuff. Being in Hawaii, we can't exactly visit many team sites, so yours was the first one we visited in the 7 years we've been at this. Our shop is a bit on the small side in comparison to yours, but we took away some insights on how we might organize it a bit better and store materials. It was neat seeing so many of your past robots. I remember thinking. Oh yeah, we lost in the finals to that one in 2000, to that one in the finals in 2003, to that one in the finals in 2005. Hmmm I'm starting to see a pattern. I guess losing to you guys 50% of our career in finals isn't so bad. Maybe one year we'll actually play on the same side. Good luck this season and I guess we'll see you at SVR!