4 inch Wheel of Doom

[AdamHeard]

Originally Posted by Speedfiend

Looking through CD media, there's a few examples. For example, one team's wheel that I saw had an enormous amount of material taken out of it. I'm sure you could PM the creator and find out how the wheel performed.

The designer of those wheels decided to leave the community and cut off all contact; I doubt he'd reply.

[sdcantrell56]

Why wouldn't you just stick with HDPE for the wheels. The weight savings is pretty huge and hdpe or UHMW is more than strong enough for the task.

[R.C.]

Originally Posted by sdcantrell56

Why wouldn't you just stick with HDPE for the wheels. The weight savings is pretty huge and hdpe or UHMW is more than strong enough for the task.

Sdcantrell, I was not sure if UHMW was strong enough for the application. What other materials are strong enough for the application and lighter than aluminum? Plastics?? (McMaster Carr #'s Help)

Thanks everyone.

[AdamHeard]

Originally Posted by rc_cola1323

Sdcantrell, I was not sure if UHMW was strong enough for the application. What other materials are strong enough for the application and lighter than aluminum? Plastics?? (McMaster Carr #'s Help)

Thanks everyone.

You have to stop and separate the material strength and the shape/design of the wheel. You could make wheel designs where probably even glass is strong enough, and you can make wheel designs where only extremely strong steels and titaniums would be strong enough.

It's not a simple matter of saying, okay, we have a wheel designed, lets just make it out of 7068. That may work in this case, but is extreme overkill. In the workforce, if you immediately jumped to a more expensive material (compare the cost of 6061 vs 7068) , you'd probably be out of a job really quick.

I think, rather than trying to pick a material to suit this specific wheel design. Lets start over with an entirely new wheel design. First, pick the material, as it's much easier to design around a known material rather than designing a part and going "oh, I guess material XYXY" would be strong enough (and if it is, it might not be the most efficient way).

So, lets make a wheel. A real efficient design, not much heavier than it needs to be, not much more expensive than it needs to be, not more difficult to make than it needs to be (much leaves room for safety).

Okay, so in this new wheel design lets imagine the kind of stresses put on the wheel. First, you have the weight of the robot pushing from the point of contact to the hub. Second, you have the torque being transferred from the hub to the outer surface of the wheel. Finally, let's just lump in all the random forces that will happen in a match; wheel getting hit from the side, robot being twisted which would twist the wheel as well, and so on.

Now, in an ideal world you'd have an engineer on your team would know how to reasonably approximate these forces, and use a method of testing the wheel design to make sure it can withstand them (plus a safety factor). Now, you can do what I did in high school and just run Cosmos on it; but when you get older you'll realize that yeah, I got cosmos to run and give me an answer and a nice purdy graph, but in the end I had *No Idea* what I was doing really, and have no idea if those answers were reliable. Now that I've had classes that have covered some of those concepts, it really shows me how ignorant I was.

^If you have one of those engineers, go ask for his help now, and maybe stop reading the rest of this post.

So, assuming you don't we have to go assumptions; luckily there is a huge sample size of wheels that have been tested through a season for you to look at (as most robots have been wheeled pretty much since FIRST started). Look at some of the more efficient designs people have used and try to analyze what they did. Was the shape efficient at transferring the robots weight from hub to floor? Was it efficient at transferring the torque? What is it efficient at surviving the rigors of a competition?

The best example I can think of is 254/968 the past two years. For 2008, they weren't any fancy material, just plain 'ol 6061 and they worked.

Sooooooooooooooooooo...... Now that you understand what the wheel has to survive, and what has/hasn't worked and how efficient it was/wasn't; Make us an improved wheel.

You've obviously proven you have the Solidworks Skills (which can be quite difficult for some people), now lets get those design skills developed real well. You've got potential, but you've got to stop and think about how you can realize it.

[R.C.]

Adam, I understand where you're coming from. First of all the wheel was designed around 6063, because that is what we had laying around. The safety factor was about 16 with all the proper forces included (an engineer helped with all that). Then a random sponsor donated some really cool material, 7068. So we decided to make it out of that, we were going to take out more material. But a couple of fellow members from other teams asked the question "Why not plastic?" So we as a team figured that plastic would be a lot lighter but none of us really have ever worked with plastics. Also when using plastic, you need to add material. Adding or subtracting material is not the issue here. I have looked up some specs on plastic but wanted some people or teams opinion on what type of plastic they used and how it worked out for them.

Thanks Adam for the concern

[NickE]

Originally Posted by AdamHeard

The best example I can think of is 254/968 the past two years. For 2008, they weren't any fancy material, just plain 'ol 6061 and they worked.

Yes, our 6061 wheels for 2008 worked fine. This picture has an ok shot of the wheel in it if you want some reference as to how much material was removed. (The roller claw wheels are identical to the drive wheels)

[R.C.]

Originally Posted by NickE

Yes, our 6061 wheels for 2008 worked fine. This picture has an ok shot of the wheel in it if you want some reference as to how much material was removed. (The roller claw wheels are identical to the drive wheels)

Nick, how thick are those spokes???

[NickE]

Originally Posted by rc_cola1323

Nick, how thick are those spokes???

The ten spokes are .1875" wide and .1500" thick.

[Travis Covington]

In 2008 and 2007 we had identical wheels, however, the 2007 wheels were 7075 (and were optimized to utilize the stronger material) With the required bumpers in 2008, we performed another analysis and deemed the 2007 design "good enough" if we just used 6061. I should note, however, that we did have one of our 2008 wheels bend ever so slightly (still usable), when the 2007 wheels held up to much more abuse with no bending.

Also, from my past research, I couldn't find 7068 bar in any sizes large enough to make wheels. Granted, I didn't look for plate, but what I did find was all pretty small. Did you guys acquire something large enough to do the wheels?

And again, to reiterate what everyone has said so far. You should expect your design to change dramatically when using a stronger or weaker material. With a higher strength material, pockets can get way deeper, web thicknesses can be reduced greatly and you can save a ton of weight if you optimize the design and remove material in the right places. It sounds like you've done that already. As a next step, you may just do some more research to see just how strong some of the plastics are versus the materials you have used thus far and have available to you. Strength to weight ratio is the key thing to look at here, and 7068 is near the top of that list if you are only concerned with the strength (ignoring elasticity, fatigue stregth, etc)

[R.C.]

Originally Posted by Travis Covington

In 2008 and 2007 we had identical wheels, however, the 2007 wheels were 7075 (and were optimized to utilize the stronger material) With the required bumpers in 2008, we performed another analysis and deemed the 2007 design "good enough" if we just used 6061. I should note, however, that we did have one of our 2008 wheels bend ever so slightly (still usable), when the 2007 wheels held up to much more abuse with no bending.

Also, from my past research, I couldn't find 7068 bar in any sizes large enough to make wheels. Granted, I didn't look for plate, but what I did find was all pretty small. Did you guys acquire something large enough to do the wheels?

And again, to reiterate what everyone has said so far. You should expect your design to change dramatically when using a stronger or weaker material. With a higher strength material, pockets can get way deeper, web thicknesses can be reduced greatly and you can save a ton of weight if you optimize the design and remove material in the right places. It sounds like you've done that already. As a next step, you may just do some more research to see just how strong some of the plastics are versus the materials you have used thus far and have available to you. Strength to weight ratio is the key thing to look at here, and 7068 is near the top of that list if you are only concerned with the strength (ignoring elasticity, fatigue stregth, etc)

Thanks Travis for the info, the wheel previously posted was way overengineered and it weighed about .49 pounds. After talking with Adam, I got the wheel within .33 lbs. The donated 7068 was about 5.5 in diameter (rod stock), it was kinda like a early christmas gift. Not really expected. If we end up going with 7068, i'll be trying to get the weight down to .2-.28.

[Travis Covington]

I could see a highly optimized single-piece dead-axle 4" wheel approch .10 lbs. Our 4" 2 piece 7075 bead-lok style wheel weighs .23 lbs. No reason a less complicated and more optimized wheel utilizing a material with another 30ksi of yield strength couldn't cut that weight in half...