20-12-2009 14:08

Re: pic: Wheel 4 V1.2

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!

20-12-2009 14:27

Akash Rastogi

Re: pic: Wheel 4 V1.2

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

What's the size of the hex?

20-12-2009 18:49

Rion Atkinson

Re: pic: Wheel 4 V1.2

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

Thanks. More detailed can be found here.

Thanks Akash.

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

20-12-2009 22:34

Raul

Re: pic: Wheel 4 V1.2

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.

20-12-2009 22:57

Brandon Holley

Re: pic: Wheel 4 V1.2

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

21-12-2009 07:47

kramarczyk

Re: pic: Wheel 4 V1.2

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.

21-12-2009 15:41

Re: pic: Wheel 4 V1.2

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. 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).

21-12-2009 15:49

Rion Atkinson

Re: pic: Wheel 4 V1.2

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.

21-12-2009 16:20

Alan Anderson

Re: pic: Wheel 4 V1.2

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

21-12-2009 18:40

Re: pic: Wheel 4 V1.2

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.

22-12-2009 10:44

Alan Anderson

Re: pic: Wheel 4 V1.2

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.)

22-12-2009 13:11

sdcantrell56

Re: pic: Wheel 4 V1.2

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.

22-12-2009 14:35

Re: pic: Wheel 4 V1.2

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!

22-12-2009 14:43

AdamHeard

Re: pic: Wheel 4 V1.2

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.

22-12-2009 17:07

Rion Atkinson

Re: pic: Wheel 4 V1.2

I would love to give you another version. But I have gained no advice on which to change from this thread. I have made a slight change as adviced by RC to allow easier G Code via CAM. But other than that there have been no suggestions.

If you would like to see it you can find it here. Feel free to email me any suggestions you have.

First. I would like to say that this could all be assumed as a huge lack of communication. While both sides are correct it seems that both have taken the defensive. I have not read either as rude; a little conceded maybe, but not rude. But I also do not see this going anywhere.

Second. His radii may help, but you are correct; they do not address the problem that Raul presented. It's because of the side forces that the wheel would have on it that I will not be making the wheel depth any smaller.


-Rion

EDIT: On the note of the additional radii on the side, should I put those on there our would it make the production process to slow? If I should put them, what degree?

22-12-2009 17:07

Jon Jack

Re: pic: Wheel 4 V1.2

This comes from personal experience...

In 2008 we got 'too greedy' with our wheel design and made the spokes way too thin. I think each spoke was something like .167" thick.

While practicing between LA and Atlanta, our driver turned a corner and something shinny rolled off our robot. After finishing the lap I went to go an examine what came off our robot, only to find that the wheel hub was still on the robot, but the rest of the wheel was gone.

After looking at the rest of the wheels, I found that others were starting to fatigue as can be seen in this photo.

The problem was caused by side loading and how thin our cross sectional area was. When we were doing our pre-fabrication analysis using Cosmos, we only checked the compression strength and 'normal' direction loading on the spokes. In both cases, the wheel had a factor of safety of >2. After we began breaking wheels I did another analysis on the effects of side loading and found that our FoS was <1.

If you look at the picture I linked, all the wheels broke at almost exactly the same spot, which also happened to be the smallest cross section of the spoke.

For Atlanta, we made a new set of wheels that had a thicker spoke and took the weight hit since we had 8+ lbs to work with.

Yes, weight is an important factor, but you don't want to under-build your wheels and have them break in the middle of competition. There are other places you can save weight with out sacrificing the structural integrity of your wheels.

Alan is right, where the radii are at now only help in the 'normal' direction. When it comes to side loading, they don't do anything.

22-12-2009 18:16

AdamHeard

Re: pic: Wheel 4 V1.2

Rion,

In a discussion such as this, most likely everyone has a point that is correct; this does not validate rude comments or the "I told you so!" attitude, especially when directed to some of the most respected (for good reason) mentors in FIRST.

My post was certainly immature, but I have a hard time letting things like that slide.



Yes, radii such as those are good, we do them on our wheels; they can be machined easily with a ball end mill. They certainly help make the part stronger and reduce failure at that joint. However, you can see in the picture's Jon linked that they had a radius, and thin spokes can still fail.

Raul was making a valid point, analyze what kinds of forces are on your wheels before you lighten the heck out of them. No one can say "1/4" thick spokes are strong enough" without knowing the floor surface, wheel tread, drive type, etc.

22-12-2009 18:36

Rion Atkinson

Re: pic: Wheel 4 V1.2

While you have a point (the "I told you so" attitude being wrong) I have dealt with stuff like this sense I have joined my robotics team. I guess everyone learns in different areas in different times. I do apologize for calling you out on this.

OK. Back on topic.

I honestly have no idea how to use the stress analysis on SolidWorks. Would anyone mind either looking over it for me or giving me a detailed walk through? Also. If I were to add the radii, what degree of a fillet should I put on them?

-Rion

22-12-2009 19:01

artdutra04

Re: pic: Wheel 4 V1.2

Go to the tutorials under the Help menu; you can learn how to do simple normal and shear stress tests in under ten minutes.