pic: 5 Inch Wheel
 

Re: pic: 5 Inch Wheel
 

The first thing I see missing is no key or bolt pattern for a sprocket.

Re: pic: 5 Inch Wheel
 

Ya, I didnt include a keyway but I plan to key this once we make it, if it ever gets made. Can you make keyways on a mill? I dont think you canm but might be wrong... :D

Re: pic: 5 Inch Wheel
 

The spokes look nice; but I'm curious if there is any mathematical basis (or any other basis other than artistry) for their shape?

FYI - I have no hidden agenda or Socratic teaching attempt hidden in my question - I am certainly not good enough at mechanical engineering to perform anything other than the grossest levels of analysis to answer a question like the one I just posed.

Instead, each time I see a design like this one posted as a CAD rendering, I am sincerely curious what design guidelines, or computational analysis, or empirical data the author was able to use when creating the design. Yours just happens to be the one that finally got me to ask the question.

What can you tell us?

Blake

Re: pic: 5 Inch Wheel
 

I first cut out 5 spokes that went fully from the center to the outside with all inside material cut out. I made them 1/4 inch because I felt that to be plenty to support the wheel. I filletted the edges of the spokes for extra support and thev used cosmosxpress in solidworks to run stress analysis, placing an 120 lb force on an individual wheel supported by the axle. Then after seeing it would not break I cut more weight by adding holes in an artsy way and did more stress analysis. Really I find the best way to make light weight objects is to cut a little weight and then stress test and repete until you are either happy with the weight or unhappy with how strong it is getting. :D

Re: pic: 5 Inch Wheel
 

personally i would favor a broached hex to the keyway for strength
mike d

Re: pic: 5 Inch Wheel
 

The spoke design is currently inefficient. The force exerted on the spoke via the torque applied at the hub decreases as you get farther away from the hub. So, the spokes should be thicker towards the center and thinner as you get closer to the rim. The holes in the spokes also greatly decrease the strength, If the spokes are strong enough as is with the holes, then I would redo do it with smaller spokes without holes (which if done right would weigh less than the current spoke).

But, overall, a solid start on a wheel design.

Re: pic: 5 Inch Wheel
 

Why does torque decrease as you get farther away?

Re: pic: 5 Inch Wheel
 

The torque is mainly at the shaft or the middle of the wheel. As you move farther from the center, the torque decreases.

Re: pic: 5 Inch Wheel
 

The torque doesn't change at all, but if you pick a cross section of the spoke, you can say the force on it (from the torque) is the torque divided by how far it is from the center. Think of it as a lever arm, if you apply a torque to the end of it, the force the end of the lever exerts decreases as the lever's length is increased.

And for what Mike says, YES!!!! If you can find a way to use hex axles, do it. They form a stronger connection than a keyed shaft, and are just way easier to work with... On top of that you never have to worry about losing a key, or making them in the first place.

Re: pic: 5 Inch Wheel
 

Thanks :D We might go with hex. Is it hard to find hex sprockets and what do you use as bearings with a hex shaft?

Re: pic: 5 Inch Wheel
 

pink uses flanged bearings and hex stock from mcmaster carr.
you can put the hex stock in a lathe and turn it under size so the bearing slips on and the remaining hex locates it from side to side
mike d

Re: pic: 5 Inch Wheel
 

As Adam said, the force (not the torque) on the spoke decreases as you get farther from the center.

Torque = Force x Distance, so as distance increases, force decreases.

Edit: Beat to it.

Re: pic: 5 Inch Wheel
 

Take the hex shaft to a lathe and turn it down to a circle that will fit into a bearing.

EDIT: Mike beat me.

Re: pic: 5 Inch Wheel
 

Ya thats our problem, we have a mill but no lathe. We could do it, but it is a huge hassle and we wouldnt have the freedom of doing this in shop... Hopefully we will get one sometime...:D

Re: pic: 5 Inch Wheel
 

1323 would machine it out for you if you guys needed, just email me. We machine out parts for teams in need.

Re: pic: 5 Inch Wheel
 

unless you would have to chew the shafts with your teeth, it will be worth it compared to keyways, also its a very basic machining practice with a reletivly loose tolerance if you needed to pay someone to machine them for you.
mike d

Re: pic: 5 Inch Wheel
 

Thanks, are you in atlanta? We have a source in atlanta but it is just hard to get there and back in traffic, plus if something happens we cant fix it spur of the moment.

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No, California.

Re: pic: 5 Inch Wheel
 

i would think there would be plenty of shops around atlanta that would be able to do it, if not, I am in florida, send me some dimensions and i will whip them out for you.
mike d

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nvm (how come I can't delete this post?)

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Well, you could always do it the opposite way. Machine hex shafts out of round-stock, and then use the AM hex hubs.

We did this with our mill and a dividing head. We never bought any broaches or hex shafts. Just used the parts from AM and metal bars laying around my barn.

It depends on the design.

Re: pic: 5 Inch Wheel
 

While it's true you can't really make a very good keyway on a mill, you may be able to make a hex or D-shaft bore on the mill. 1/8" end mills are available up to about 3/4" in length, so you should be able to mill a hex shaft bore out of the block.

And, now you're all about to type 'what about the rounds on the corners from the round cutter?'. Well, personally, whenever things really need to be square I do one of two things: 1, before the milling operation, drill holes at all of the points the fillet would be left from the end mill, thus removing this excess material. Or 2, If you want to be trickier and more efficient, program over-travels the length of your tool's radius away from the center of the bore you're making, also eliminating the corner radius. With a hex shaft, you should still have plenty of surface area left on all of the sides of the shaft... you should only lose 1/8" of each side.

If you're mass producing these, you can always cut your bore times by starting with a larger tool and only finishing the corners with the 1/8" tool.

-q

Re: pic: 5 Inch Wheel
 

You could also drill a 1/8" hole first (offset it the right amount from center) and then bore the center hole. Lot easier than using end-mills!

Re: pic: 5 Inch Wheel
 

A much easier solution than milling out a hex is to broach it. Mcmaster sells hex and square broaches in a variety of sizes.

http://www.mcmaster.com/#catalog/114/2462

At $128 for a 3/8" hex broach, they're not cheap. However, they will certainly save time over milling out a hex. Also, your hex come out without rounded corners.