pic: Prototype Drivetrain

[Garret]

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[Ty Tremblay]

It looks like you only have 2 CIMS powering your drivetrain. Did you do this to save weight, or do you have another purpose for them? I'm of the belief that there are enough motors available in the KOP that 4 CIMs (or at least a CIM and a FP) should be used on any drive train (with 2K9 being an anomaly).

[JamesCH95]

Replace all the 1/16th wall aluminum with 0.035" wall 4130 steel square tubing. You'll improve chassis strength, stiffness, maintain the same weight, and your welder will have an easier time.

With a steel frame you could also use weld-in nuts which could save you weight and make it easier to wrench on the robot

[sanddrag]

Originally Posted by JamesCH95

Replace all the 1/16th wall aluminum with 0.035" wall 4130 steel square tubing. You'll improve chassis strength, stiffness, maintain the same weight, and your welder will have an easier time.

Just how would this maintain the weight? 4130 Steel is about 2.83 times as dense as 6000 series aluminum. You're not even halving the wall thickness in your recommendation. This change would add a significant amount of weight. I would not recommend it.

[JamesCH95]

I'm sorry, I should have been more explicit, my brain is not 100% in this caffeine-free day

3/4"x0.35" square 4130 steel tubing would roughly maintain the weight of a 1"x0.065" aluminum square tube. The benefit is that 4130 steel still has a YS of around 70-80KSI when welded, whereas welded 6061 aluminum will be total garbage, around 8-10KSI. With further optimization a far more efficient chassis is possible if you use welded 4130 vs welded 6061 because the yield-strength:density ratio is roughly five times higher with welded 4130 compared to welded 6061.

[AdamHeard]

Originally Posted by JamesCH95

I'm sorry, I should have been more explicit, my brain is not 100% in this caffeine-free day

3/4"x0.35" square 4130 steel tubing would roughly maintain the weight of a 1"x0.065" aluminum square tube. The benefit is that 4130 steel still has a YS of around 70-80KSI when welded, whereas welded 6061 aluminum will be total garbage, around 8-10KSI. With further optimization a far more efficient chassis is possible if you use welded 4130 vs welded 6061 because the yield-strength:density ratio is roughly five times higher with welded 4130 compared to welded 6061.

If a designer knows and understands the limits of a welded 6061 frame, this is not a downside. We've been running welded 6061 frames for years, and the one broken weld we had was just horrible, awful, horrible design. Please stop saying how awful welded 6061 is every chance you get. There are a lot of advantages to teams only having to stock primarily one type of aluminum (we still stock a lot fo 2024, 7068 and 7075 for shafts however).

[JamesCH95]

Originally Posted by AdamHeard

If a designer knows and understands the limits of a welded 6061 frame, this is not a downside. We've been running welded 6061 frames for years, and the one broken weld we had was just horrible, awful, horrible design. Please stop saying how awful welded 6061 is every chance you get. There are a lot of advantages to teams only having to stock primarily one type of aluminum (we still stock a lot fo 2024, 7068 and 7075 for shafts however).

Clearly welded 6061 can be used with great results, many teams (like yours) have. I am merely suggesting that there could be a more optimal design. There are very few deviations from bolted aluminum chassis, welded aluminum chassis, and folded sheet aluminum chassis. I am just trying to encourage students and mentors to re-evaluate how they make their robots. Of course there are other factors like material cost and availability, and those do matter.

I have a suspicion that many teams just "make the chassis like we always do" without ever trying to optimize it or even try something different (if only on paper as a theoretical exercise). A CAD model is great for that, the model could made with different configurations: one with aluminum tubing, one with steel tubing. Pick a few load cases that you know the aluminum one will pass, then optimize the steel one until you have the same factor of safety and see what the weights are. It'd be a reasonably in-depth undertaking, but excellent practice for anyone who wanted to do it. I know SolidWorks has multi-configuration support, and some of the simulation packages even have built-in optimization function that are very easy to use. In the past SolidWorks has donated free copies to FIRST teams.

[Tom Line]

Originally Posted by JamesCH95

Clearly welded 6061 can be used with great results, many teams (like yours) have. I am merely suggesting that there could be a more optimal design.

There will always be a more optimal design. In fact, most people using aluminum tend to overbuild their chassis's as well.

I think you would have a hard time selling a change from aluminum to steel to many teams, not to mention quite a few air-frame firms and racecar teams.

Aluminum has a couple benefits that aren't immediately apparent.

#1 Cutting aluminum by hand, generally speaking, is far easier than steel
#2 Drilling is usually much easier
#3 Aluminum is usually used in wall thinknesses that support tapping directly without the use of additional hardware
#4 Because aluminum is generally large in size, it's that much easier to cheesehole and to make up for mis-located/designed components because you have extra leeway.

Our team learned to weld aluminum in a week, then taught the students how to do so as well. It really wasn't that bad.

That said, we built our frames out of 80-20 the first year, thin-wall steel the next two years, and aluminum the last 2. I wouldn't say any were heads-above the others. They all had their own strengths and weaknesses.

Back to the frame (I hate to make an off-topic post that isn't at least a bit on-topic), frankly I think you've overbuilt it a bit. I think you can remove a lot of your cross members and retain your rigidity and strength. Think about mounting your transmissions by their base (on a flat plate). This will remove the need for upper-long-ways beams internally, and you can move your cross-supports down onto the bottom portion of the frame.

Neat Design!

[JamesCH95]

Originally Posted by Tom Line

I think you would have a hard time selling a change from aluminum to steel to many teams, not to mention quite a few air-frame firms and racecar teams.

Funny you should mention those two industries in particular...

I learned to weld 4130 steel when I started welding for a race car team (Formula SAE) in which a vast majority of race-car frames are welded steel, the remainder are generally carbon fiber monocoque frames. In my 4 years of involvement with FSAE I never saw a race car with a welded aluminum frame. NASCAR is all welded tube steel chassis by regulation. The aluminum car chassis I have seen are not welded, like the Lotus Elise chassis is folded aluminum that is riveted and glued together. The only race vehicle that I've seen a welded aluminum chassis is a japanese sport bike, though top-end race bikes, like Ducatis, use welded steel frames.

A vast majority of small airframes and helicopter frames are welded 4130. I should know, I do welding repairs for a local helicopter shop when the mechanics need something difficult or exotic repaired. I am also in touch with a local business that plans on producing single-engine air planes, and the entire airframe is welded 4130 steel.

I completely agree with your list of aluminum benefits. Though I would add that cutting off and re-welding brackets/mounts/etc on a steel frame is not too difficult either, and I've done that PLENTY of times. One can also weld bolts (uncoated ones) directly to a steel frame, often negating the need to drill holes in the frame.

[sdcantrell56]

As a team that uses a non-standard material for the majority of our robot (wood) I particularly like the steel tube suggestion. It is much stronger than an aluminum frame as well as possibly easier to find and easier to weld. I am all for teams really investigating the best materials for the job instead of just going with what they have done in the past

[Peter Matteson]

Are you using Riv-nuts to hold on the axle pillow blocks?

[Michael Corsetto]

Instead of using washers to lower your middle wheels, try drilling the holes in all 16 pillow blocks off center by 1/16", and flip the middle ones over so the axel ends up being 1/8" lower than the outside axels. Adding washers isn't as clean/sturdy.

Also, to lower your CG (which is ALWAYS important), you might want to mount the pillow blocks on the top side of your lower frame members, that way the whole frame is lowered a total of 2 inches. And try mounting your electronics (Battery/CRio/etc) on the lower frame level, not the upper one. This will lower your CG and give the manipulator teams more area to work with. Win Win.

[M.Wong]

Instead of welding the pieces together, I would recommend using these Brunner Connectors. We've been using these for quite a while. Just make some 1/8" thickness aluminum gussets and some self-tapers in order to secure the pieces together.

[JamesCH95]

Self tapping screws?

[camtunkpa]

Originally Posted by M.Wong

Instead of welding the pieces together, I would recommend using these Brunner Connectors. We've been using these for quite a while. Just make some 1/8" thickness aluminum gussets and some self-tapers in order to secure the pieces together.

Along the same lines, we've used 80/20 quickframe which is nearly identical to the Brunner stuff but the pricing is lower. We normally rivet 1/16" lexan to tie the frame together. We've had good luck with this method the last couple years.