pic: 5 Inch Wheel

[RMS11]

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[chris31]

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

[RMS11]

Originally Posted by chris31

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

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...

[gblake]

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

[RMS11]

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.

[Triple B]

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

[AdamHeard]

Originally Posted by RMS11

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.

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.

[RMS11]

Why does torque decrease as you get farther away?

[R.C.]

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

[AdamHeard]

Originally Posted by RMS11

Why does torque decrease as you get farther away?

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.

[RMS11]

Originally Posted by AdamHeard

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.

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

[Triple B]

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

[Richard McClellan]

Originally Posted by RMS11

Why does torque decrease as you get farther away?

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.

[chris31]

Originally Posted by RMS11

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

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

EDIT: Mike beat me.

[RMS11]

Originally Posted by chris31

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

EDIT: Mike beat me.

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...