Hub Motor Axle Material Choice

[hillzofvalp]

What do you guys think? I was thinking a variant of 4140 known as ETD-150

tensile strength 130,000 psi
Rockwell C32

It is supposed to machine fairly nicely, but not as nicely as 41L40:

tensile strength 85,000 psi
Rockwell B96-B100


Which should I work with? They are both about the same price. MAybe a different alloy all together?

 

[Chalo]

What do you guys think? I was thinking a variant of 4140 known as ETD-150

tensile strength 130,000 psi
Rockwell C32

It is supposed to machine fairly nicely, but not as nicely as 41L40:

tensile strength 85,000 psi
Rockwell B96-B100


Which should I work with? They are both about the same price. MAybe a different alloy all together?

If they are both versions of AISI 4140, then large tensile strength differences are probably differences in temper, mostly. Leaded steels are often machined in their normalized or annealed condition, then heat treated to yield the desired strength and hardness. They are really a lot nicer on cutting tools and your patience than the unleaded alternatives.

To put it in perspective, both those steels are stronger than what's in a commercial hub motor axle.

I've had good results machining 1144 "stressproof" steel for bike drivetrain parts. It's a roughly 100,000 psi steel that machines better than its strength would suggest. It doesn't cost too much, either.

Why are you machining a new axle for your hub motor? If there is something unusual about your requirements, that might have an effect on which material would be best.

 

[hillzofvalp]

It is for my cromotor. I'm putting it on a 175lb small motorcycle scooter thing. I want to run peaks of 12-15kW and 250 phase amps when the moment is right (cooling ready). I have enlarged the bearing on one side. I would like to make a big channel to allow 8 gauge in plus possibly water tubes. To compensate I would be putting wider, higher quality flats on similar to the John in cr hubs flats.

Goal is also to get rid of the known cromotor axle issue with the channel. Not something I want to risk with a 60mph, heavy, expensive Machine.

 

[Farfle]

I would pick 4340, as it has a higher machineability rating and similar strength. But all those series steels are pretty awesome.

 

[Chalo]

I would pick 4340, as it has a higher machineability rating and similar strength. But all those series steels are pretty awesome.

Maybe annealed 4340 is highly machinable, but the stuff I have worked with before was seriously ornery and destructive to my cutting tools. It is very strong and tough, though.

 

[Farfle]

I would pick 4340, as it has a higher machineability rating and similar strength. But all those series steels are pretty awesome.

Maybe annealed 4340 is highly machinable, but the stuff I have worked with before was seriously ornery and destructive to my cutting tools. It is very strong and tough, though.

I guess the stuff I got was annealed, as it machined like plain cold roll. but everything I machine is done on carbide insert tools, so it may be a PITA on hss. 1144 is probably a better choice as its easier to cut in a fully tempered state. But if you have the equipment to temper, the 4340 is the stuff to use.

 

[Chalo]

I have enlarged the bearing on one side. I would like to make a big channel to allow 8 gauge in plus possibly water tubes. To compensate I would be putting wider, higher quality flats on similar to the John in cr hubs flats.

As long as you have adequately large flats, a sufficient cross-sectional area, and some amount of root radius at the shoulder, any of the alloys mentioned so far would be suitable.

If you can attach the axle in such a way that you don't need nuts on it, then don't thread it. It's stronger and more fatigue-resistance without threads cut on it.

 

[hillzofvalp]

If I anneal it though it will bend. I rather not go through that anyways.

I'm using carbide tooling in a CNC lathe, so I'm not extremely worried about how bitchy it is to cut. A little bit. If you think I can leave 4340 in its raw form after machining, than I'll take your suggestion

I do like mcmaster's description of 4340:

Specially designed for applications involving severe impact, wear, heat ,and stress, this highly alloyed steel (nickel, chromium, and molybdenum) is stronger and more shock resistant than 4140.


edit:
Chalo, would clamping dropouts be adequate? this is what I have as of now.. but I was planning on shortening them. I would still worry about the axle sliding around. idk. some people say clamping axles is a no-no.

 

[bigmoose]

I have been recommending "stressproof" at 100Ksi or "fatigueproof" at 140Ksi both are slightly modified 1144 chemistry. I've personally machined stressproof and to me it machines like butter.

 

[hillzofvalp]

so the later would be better because it is harder?

 

[bigmoose]

If that question was for me, I believe you will be satisfied with stressproof/100Ksi material.

The stress concentration is where the axle necks down to the threaded portion with the two flats. The filet radius of that transition likely has more effect upon the fatigue life than the material property between 100/140Ksi.

 

[hillzofvalp]

k sounds good. But now there's a new and improved cromotor coming... so I may wait until I get it before I make a new axle. maybe I don't even need to if the new one is better.

 

[Chalo]

Those clamping dropouts look adequate to me. In the absence of a drive chain, there is no significant force to induce axle slippage anyway.

 

[NeilP]

I can not help you with mnaterial choice, I just had to go with what ever I had in teh workshop for my 5304 re - axle job. it was 'bright' steel bar of some sort, but absouluely no idea on the spec.
Living on a small island (pop. 90,000 or so) Local suppliers are non existant,...and shipping costs for the pieces I were lookn gat buying were going to be in XS of $150 USD..so I used what I had to hand.

But if you are in need of idea for how to channel the wire through take a look at my solution..a collar with 'keyway' cut for the wires.
Collar was 'sweated' on then for added security fixed with DP420.
Axle was knurled, keyed, covered with DP420 then pressed in to the stator.


http://endless-sphere.com/forums/viewtopic.php?f=3&t=40785&hilit=snapped&start=75#p614744

 

[hillzofvalp]

What about using disc brake? That would have a force on one side, right?

Neil, I've actually already done that to my cromotor... But I just used a set screw to old it in place.

 

[Chalo]

Depends on where the caliper is in relation to the axle. If the reaction force is across the slot (caliper forward of axle), no problem. If the reaction force is parallel to the slot (caliper above or below axle), then the axle can slip and must be well secured. But rear brake forces are limited to the amount that makes the wheel skid.

 

[Emoto]

I would be putting wider, higher quality flats on similar to the John in cr hubs flats.

Goal is also to get rid of the known cromotor axle issue with the channel. Not something I want to risk with a 60mph, heavy, expensive Machine.

Finally machined mine, width between flats is now 18mm, i feel more confident about it now.

 

[John in CR]

Why are you talking about waiting on the new motor unless you'd replace its axle too? 10mm flats are inappropriate on a motorcycle. Take Bigmoose's advice because there's a lot more to hubmotor axles than just strength and a hubmotor axle sees far greater extremes than other more axles. Not only do they see crazy twisting torque due to the small radius, but they also see road impacts while under side loads. Then to top it off we have to cut some of the axle away because it's also the motor mount.

If I was doing a new axle then I'd look to go with a large diameter and use something more like a pipe like Farfle did.

Emoto,
18mm flats sounds good, but what's the axle diameter? Are you sure you have a big enough flat area? I ask because my current primary motor has a 1" axle and 15mm at the flats. At 18mm I'm not sure the flat area would be wide enough.

John

 

[hillzofvalp]

I am more interested in a new bare stator to work with... I figured they had the same axle but I guess not? I disagree that 10mm is insufficient. The original 10mm axle, yes. But a much wider flat at 10mm thick would be much better than the original, no? and still be compatible with bikes. I'm no ME.. but why isn't 10mm workable?

Remember, I'm not making a new axle because I broke mine; mine is fine. I'm making a new one that is stronger and can tolerate heavier load and 250A phase torque at 14kW.

Doesn't your bike weigh in the 120-170 pound range? My "motorcycle" will be 175 pounds. If you have any further suggestions, please elaborate. I'm interested in hearing what you have to say

 

[hillzofvalp]

Is there a none-sketchy way to use a 17" motorcycle rim with the motor you sell, john?

 

[John in CR]

I am more interested in a new bare stator to work with... I figured they had the same axle but I guess not? I disagree that 10mm is insufficient. The original 10mm axle, yes. But a much wider flat at 10mm thick would be much better than the original, no? and still be compatible with bikes. I'm no ME.. but why isn't 10mm workable?

Remember, I'm not making a new axle because I broke mine; mine is fine. I'm making a new one that is stronger and can tolerate heavier load and 250A phase torque at 14kW.

If 10mm between the flats was sufficient then the Chinese wouldn't use 12-14mm on low powered scooters and 14-15mm flats on 6-7kw scooters. You can always grind them down smaller later if you decide to put it on a bicycle instead.

 

[John in CR]

Is there a none-sketchy way to use a 17" motorcycle rim with the motor you sell, john?

The resulting 23-24" wheel would be totally inappropriate for the MiniMonster. It's designed for up to a 17"OD wheel, and maybe someone under 200lb with a bike 100lb or less could push it to 20". The one I have on my SuperV, HubmonsterHE, I'm not selling yet, but soon, could probably handle it since I use a wheel over 19" in diameter, so going to 23 or 24" is reasonable for a lighter load than the 375lbs mine pushes regularly. All you'd have to do is come up with spoke flanges to bolt to the bolt on rim flange.

While it's possible the point you still miss is that it's the wrong way to go, and I would not sell one to someone intending to do it unless it was for a very light and very aerodynamic vehicle. Instead you need a smaller wheel and up the voltage. DD hubbies are geared too high in big wheels. Even the one designed for motorcycle use has to be run conservatively due to the large wheels.

Don't you think there would already be lots of motorcycle hubbies if it made sense? How do you think a fat guy like me gets performance? If you don't believe me, put a 75-100lb backpack on and go ride your ebike. Performance will suck and heat will skyrocket.

Until we have motorcycle size geared hubbies you guys will be better off focusing on how to make a smaller wheel look good to you. There's probably a way to use a large fender to create the illusion that the drive wheel is larger. As extra incentive think about the fact that I run 15kw peak input to a bone stock motor with no heat issues at all, and that extra efficiency exhibited by the lack of heat means 10-20% less battery is required for the same job in real world riding. Imagine how far I'll be able to push it with ventilation.

John

 

[hillzofvalp]

K. Those are good points. I will have to remachine my dropout extensions for swing arm to something like 15mm.. 20mm OD.

Yes, hubbies in heavier loads (my load will be 374-425lbs) and larger wheels is a bad thing in general. But what about a new and improved, 9.3kv "scooter" motor in a 23" wheel? The stamped stator would be pushing it.. Which was why I was going to go through the hassle of cooling it rather extremely. However the aluminum stator is claimed to be happy at 6kW. Could this be the right option?

Honestly I'd love to have a chain drive.. I just rather solely modify the swing arm then engineer the motor mount, chain tensioner/guide and such. It is still an option.. And the 12kW colossus seems appropriate.

 

[Farfle]

Well if you only want to mod the swingarm...

 

[Emoto]

Emoto,
18mm flats sounds good, but what's the axle diameter? Are you sure you have a big enough flat area? I ask because my current primary motor has a 1" axle and 15mm at the flats. At 18mm I'm not sure the flat area would be wide enough.

John

John the axle diameter[ before machining the new flats down to 18mm] was 24.5mm. the new flat area is 17x11m. but my plan all along was to relieve the stress area were a couple of axles have snapped.

So this is an additional clamping area, [2 torque plates/flats on the phase wire side and of course one on the sprocket side.
This looks to me the best solution on the standard axle.