What types of aluminum does your team use and where do you get it?
We have most often used 6061-T6 (round tube, angle, and flat bar) and 6063-T52 box tube. For some reason I have never been able to find box tubing in anything other than 6063-T52. Is anything else readily available in box tube?
Since we do not have a local metal supplier we order from online places like Online Metals or Metals Depot but they can be expensive.
Are there any different alloys that teams find especially useful? If so where do you get it?
McMaster lists 6061 box tube so I guess it does exist. Have you had any failure issues with your 7075 axles? I have considered 7075 axles for weight but have always chickened out and gone with steel. What diameter have you used? Last year we went with the IFI traction wheels with 3/8" ID bearings and thus 3/8" G8 bolts for dead axles. Do you think 3/8" 7075 dead axles hold up in competition?
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.
I know that there’s been a discussion about axle diameter before. Search and see what else you can find. Depending on your drive setup, your shaft requirements will vary considerably. If you use a cantilevered setup, you’ll need a much stronger/stiffer axle vs. a double supported shaft.
For a double supported shaft, I would recommend the following:
Steel axles: 3/8"
Aluminum axles:1/2" minimum
For a cantilevered setup, you’ll have to analyze your specific arrangement, as the longer the cantilever, the stronger/stiffer an axle you’ll need. Also, if you look at the weight difference b/w a 1/2" and 3/4" aluminum axle, you’ll find that there isn’t too much of a difference, when comparing it to the 120lbs you have in total.
Our frame for the past three years has been 6063 box tubing because that’s all the local supplier stocks. Works just fine, never had a problem, and polishes pretty nicely. On occassion, we’ll get metal from McMaster or other online source if it is something we need that the local place doesn’t stock. We used 6061 axles this past year. they were cantilevered out a fair distance. Bearings were <1" apart. Axle was 5/8" dia between the bearings and 1/2" for the rest. The only problem we’ve evperienced with 6061 axles is the keyway getting some backlash after a while.
We use 6061-T6 Aluminum for most things and order through OnlineMetals.com – though we don’t have to wait for it to be shipped since they’re based in Seattle. We’ll get some scrap 5052 from some other places in town when we’re in a pinch or when they’ll donate it to the team.
Typically, I design around 1x1x1/6" tubing and all plating that needn’t house a bearing is 3/16". This season, I’m trying to expand my repetoire, so I’m working with 3x1x1/8" rectangular tubing, 3x1x1/8" channel and other extrusions.
Find a local supplier and see if you can’t feed off their scrap heap. Unless it is critical, most common aluminum alloys should be adequate. How about calling Ricky’s Welding & Machine Shop in Vicksburg at a quiet time and have a heart-to-heart talk with them. 638-8238. I’m sure there are others - just look them up in the yellow pages. You’d be surprised what they can do for you if you are sincere.
We use 8020, more specifically the 1010 series. But we can get our hands on anything in the 8020 catalog for free. <3 our sponsor…our wildest dreams come true!
8020? Don’t aluminum alloys only go up to the 7x series? Also, isn’t the 1x series close to chemically pure aluminum and therefore very soft? (Reference: McMaster-Carr and Machinery’s Handbook)
[Edit] 8020 = 80/20. That could not have been more obvious. :o (This is what sleep deprivation does to you)
We try to use 7075 plate whenever we have to machine things out on the mill.
We use 6061 or 6063 when ever we have to weld things together because 7075 is difficult to weld.
As for axles we have actually used 5/8" aluminum tube dead axles on our 2004 bot and never had any issues with the axles or wheels. The wheels never had any slop and ran smoothly the whole time. I think that the wall was .090" thick. We had the axle mounts directly butted up to our flanged plain bearings that were pressed into our wheel hubs so there was practically no unsupport length and next to no torsional load (only the friction from the bearings).
We typically get most of our aluminum from either our local steel supplier or scrap metal yard or order it from mcmaster-carr. Hopefully this year we can talk to a local aluminum mill thats about 1 hour away about donating so aluminum.
Keep in mind that welded 6xxx aluminum loses 75% of its yield strength (9ksi versus 35ksi) in the heat effected zone. Be sure and overdesign the joints.
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.
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.
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.
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.
