pic: Wheel 4 V1.2

[Rion Atkinson]

loading photo ...

Ok. I do apologize. I had some of the measurements off.

Spoke Depth - .375 inches

Spoke Thickness (Bottom) - .275 inches
Spoke Thickness (Top) - .15 inches
(Neither of those include the fillet.)

Your spokes seem too thick, IMO. You ought to be able to pull off at least .25" thick.

Looking good, it's nice to see the evolution of this design!

[Akash Rastogi]

I love the progression of your design, Rion. Keep it up.

What's the size of the hex?

[Rion Atkinson]

Originally Posted by CraigHickman

Your spokes seem too thick, IMO. You ought to be able to pull off at least .25" thick.

Looking good, it's nice to see the evolution of this design!

I edited the post. I had some of the measurements off. I do apologize.

Thanks. More detailed can be found here.

Originally Posted by Akash Rastogi

I love the progression of your design, Rion. Keep it up.

What's the size of the hex?

Thanks Akash.

The radius of the hex is .5 inches. Same as Andy Mark. I know. Looks big to me as well.

[Raul]

Originally Posted by CraigHickman

Your spokes seem too thick, IMO. You ought to be able to pull off at least .25" thick.
...

OK, be careful with this. If the wheels will be used in a swerve/crab application on carpet and your wheels will have a decent CoF, the spokes need to be beefier than you think.

[Brandon Holley]

Originally Posted by Raul

OK, be careful with this. If the wheels will be used in a swerve/crab application on carpet and your wheels will have a decent CoF, the spokes need to be beefier than you think.

Just to reiterate what Raul is saying here...

If you are using solidworks or inventor you have built in tools that allow you to do some quick FEA on your wheel. Instead of guessing at something being too beefy or not beefy enough, why not do some analysis?

For starters when it comes to wheels I do this:
Knowing that a wheel can only exert a certain amount of force on the ground and vice versa we can use the coefficient of friction with the wheel and ground as a starting point. The wheel will be under the most load when it is being pushed sideways along the carpet RIGHT before it loses traction with the ground. So estimate the weight of the robot and determine the normal force on one wheel. Then you will be able to determine the frictional force acting on your wheel. Use this as a baseline (ie safety factor = 1). You can then put this exact force on your wheel in Solidworks Simulation (or some other FEA software). Support the wheel right where the shaft would be and start your analysis. You are definitely going to want to bump the safety factor for a FIRST robot, and especially since this is just a rough calculation. To do that just multiply your frictional force by 3, 4 or 5 or whatever you want. Look at your stresses, compare them to the yield stress of the material you are using. Look at your deflections, make sure they seem reasonable (ie not half an inch or something like that, you want to be in the couple of thousandths range). Look at your stress concentrations and see if there is some simple geometry fixes you may be able to incorporate to alleviate them.

Analysis is a very powerful tool that starts taking things from a guessing game to concrete answers. I highly recommend learning to use it effectively. Keep in mind though that analysis isn't everything, and one small assumption you make can throw off your whole analysis.

Good luck, the wheel looks great so far.

Brando

[kramarczyk]

I second Brandon's suggestion. Additionally, be careful when looking at the visual representation of the deflected model. The deflections illustrated are typically exaggerated to make them more easily identified, so be sure to look at the numbers provided instead.

Originally Posted by Brandon Holley

why not do some analysis?

I wish I could make this bolder. Trusting your intuition only works as well as the human brain. Numbers and raw data are far more effective.

Originally Posted by Raul

OK, be careful with this. If the wheels will be used in a swerve/crab application on carpet and your wheels will have a decent CoF, the spokes need to be beefier than you think.

Cough. It's possible to pull off some VERY light wheels while still maintaining plenty of strength. Those are 1/4" Thick spokes, two 1/4"x1/4" bar per spoke area. In the bottom wheel, the ends are radiused for extra strength (and style).

[Rion Atkinson]

Originally Posted by CraigHickman

I wish I could make this bolder. Trusting your intuition only works as well as the human brain. Numbers and raw data are far more effective.



Cough. It's possible to pull off some VERY light wheels while still maintaining plenty of strength.

For one. I would like to point out that the average human only uses 10% of the brains capability.

Back on target. I honestly have no idea as to how to use the stress analysis. I know what buttons to press. Just not how they should be pressed. If that makes sense..


Yes it is possible, but I would like to leave VERY light for a later date. For now I would like to stick with getting at least one wheel out that weighs less than .35 pounds. If I can do that I'm happy.

[Alan Anderson]

Originally Posted by CraigHickman

Cough. It's possible to pull off some VERY light wheels while still maintaining plenty of strength. Those are 1/4" Thick spokes, two 1/4"x1/4" bar per spoke area. In the bottom wheel, the ends are radiused for extra strength (and style).

I believe it's side loads that Raul was worried about. Your extra strength radius doesn't seem to address that.

Originally Posted by Alan Anderson

I believe it's side loads that Raul was worried about. Your extra strength radius doesn't seem to address that.

Erm, yes it does. Please examine the lower wheel closer. The wheels were "dished" out by a .5" radiused end mill, in order to increased the side load ability of each wheel.

[Alan Anderson]

Originally Posted by CraigHickman

Erm, yes it does. Please examine the lower wheel closer. The wheels were "dished" out by a .5" radiused end mill, in order to increased the side load ability of each wheel.

It's possible that I'm not seeing something in the picture that you know is there. The extra widening on the ends of the spokes would seem to be helpful only in the normal direction of travel of the wheel. To be stronger in the sideways direction, the spoke would need to be thicker where it meets the hub.

(It's also possible that I don't know what I'm talking about, as I'm more attuned to software than hardware, but I think a couple of seasons of kit wheels from AndyMark did a good job of showing how side load stresses need to be dealt with.)

[sdcantrell56]

Craig is talking about the wheels his team built I believe 2 seasons ago that he linked to on this thread. Those wheels have a .5" radius where the spokes meet the rim. This absolutely strengthens the wheel in side impacts and can be verified by a quick FEA analysis. Adding this inner radius is a simple way to increase wheel strength without much extra work.

Originally Posted by sdcantrell56

Craig is talking about the wheels his team built I believe 2 seasons ago that he linked to on this thread. Those wheels have a .5" radius where the spokes meet the rim. This absolutely strengthens the wheel in side impacts and can be verified by a quick FEA analysis. Adding this inner radius is a simple way to increase wheel strength without much extra work.

Completely correct. My machinist was also able to use a single tool for the majority of the job, which greatly sped the process up.

Rion, when can we see the next version? I'm looking forward to seeing your progress!

[AdamHeard]

Originally Posted by CraigHickman

Completely correct. My machinist was also able to use a single tool for the majority of the job, which greatly sped the process up.

Rion, when can we see the next version? I'm looking forward to seeing your progress!

Cough. Ermm. excuse me, other rude phrases...

Alan was correct, in that the small radius there doesn't really solve the problem Raul stated. If your spoke is too thing to handle the side load, a small radius really isn't going to fix the problem; and if that little bit makes the difference, your factor of safety is too low.

Please, try to learn to have a discussion on here without being rude and insulting people.