1018 steel shaft strong enough

[hummina]

skateboard.

the design is solid and I havent had any problems with the shaft deforming with even the aluminum version I was selling before, but knowing a standard skate axle is made of a much harder steel I'm concerned things will deteriorate. I havent put enough rough miles on them to know for sure and would like to find out what I should be doing instead of testing till destruction.

there are two shafts and housings per motor, the bigger shaft is 42 x 7mm and the smaller is only 20x7. fitting a 42x50x7 and a 20x32x7 bearing. high heat retaining fluid and bearings have c3 internal clearnace since the motors get hot. 1018 steel with an "electroplate" finish which I need to find out what it is too.

dont want deformed shafts!! how do you think things will go? Maybe there's another similarly easily machined steel that will be stronger or can strengthened..although the mild steel does work great in the rotor as a flux ring and retains the magnetic field very well.
thank you
john

 

[BrianE22]

You need to differentiate between between deflection and strength. If you are using the same dimensions then the steel piece will deflect less than the aluminum piece. As for strength, most likely the steel will be stronger than the aluminum (especially when subject to fatigue loads). You'd need to give us the type of aluminum you are using.

 

[ornerynorsk]

Steel is stiffer and has better fatigue properties, but all products have an intended/expected lifetime under specific conditions. Nothing lasts forever. Testing under the most real-life condition possible, whether actual or simulated, will tell you what kind of lifetime of optimum performance can be expected, where performance starts to drop off, and where higher rates of failure can be expected. Testing for lifetime will tell you an expected failure rate, while test-to-failure will tell you an expected lifetime.

I've been involved in projects that approach it from both ends - design the product completely and determine expected lifetime on the back end, or determine the required lifetime and engineer the product to meet that requirement. You need to choose which is appropriate in your circumstance.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[hummina]

Doesn anyone have any experience with a 1018 shaft that's 20x7mm that I could extrapolate from instead of doing testing forever? Surely the loads on a skatewheel and the loads the metal shaft at that size can take can be somewhat figured?

I do t sell the aluminum anymore Beckie of thermal expansion and now just the 1018 stell but wonder if a harder steel would be better ?

If u look at my website u can see the shaft is part of an electric motor, so must be precision, and also very strong as it also bears the loads of the skate wheel. Very important it doesn't deform

 

[tbuelna]

For your particular application, it does not seem like raw material cost should be a significant factor in your decision process for this component. The axle component is fairly small and can be manufactured with very little waste. There are other steel materials (1144 or similar) that provide better strength/fatigue performance than 1018, without additional HT, and won't cost much more to use.

Consider whether a few cents saved for raw material of this component is worth the potential financial liability cost from someone injured as the result of this component failing due to weak material.

 

[mrfailure]

tbuelna,

Do you think the structural integrity and wear resistance might be compromised for this application in using a resulfurized free-machining 1144 steel?

 

[hummina]

I can get 1144 steel. They're calling it #40 steel. So many numbers is it a better choice for strength then?

 

[hummina]

Mrfailure do u think the 1144 is a bad
Choice for some reason?

 

[mrfailure]

I am generally not a fan of resulfurized steel - whenever I have a failure of a resulfurized part, root cause is almost always related to the compromised integrity of the part. That said, I really was asking those with practical experience what they really thought, and was not trying to make a comment (even with my bias).

 

[hummina]

Are u saying the 1144 is more prone to failure as that's surprising looking at the numbers. also trying to find the permeability compared to the 1018 I've been using

 

[mrfailure]

The large amount of sulfides has the structural effect of introducing a large amount of elongated longitudinally-oriented holes that act as stress risers and crack paths if your loading is in an unfavorable plane. They also decrease corrosion resistance, and as a free-machining steel they also decrease wear resistance. You have to determine though if this is significant in your application.

 

[dgallup]

I'm rather confused by several of your statements and how you are approaching the design of these shafts.

1) You make several references to magnetic properties of the various steels, implying this is an important consideration. However, you also state that you have used aluminum in the past. Aluminum is non-magnetic so if that worked I can't imagine how the magnetic properties of the steels can have any bearing on the design.

2) You don't say anything about the processing of the 1018 that you are using. Is it cold finished? Do you heat threat it? Both of these and other factors will affect the strength. Most alloy steels gain significant strength from heat treating.

It sounds like you are not yielding or fracturing the shafts so one can possibly conclude that the yield strength and toughness are acceptable. It sounds like you are concerned about fatigue strength. First you will need to do a stress analysis of the shafts, from that you can do a fatigue analysis. The good news is that most steels have a well defined fatigue limit so if you keep your stresses under that the life is practically infinite. That is a gross over simplification of a subject that fills many books.

Lastly, it looks like these wheels just bolt onto standard skate board trucks. The axles on those trucks are much smaller diameter (1/4"?) and appear to be more highly loaded than your shafts. When you do the stress analysis of the shafts it would not be too much more work to analyze the axles too for comparison. Do you know what material the axles are?

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[hummina]

The aluminum part I was doing before was just the shaft and the rotor was always 1018 steel. The magnets sit in the rotor. If u look on the site steelhubs.com there are good pics. The 1144 cold rolled steel, from what I read, will have less permeability and therefore not reflect the magnetic field as well as the low carbon 1018 and the motor will be a bit less efficient but strength is the most important and I'm ASSUMING the 1018 will deform. Figuring the loads on the motor shaft and housing sounds difficult as well as simulating loads. Maybe I can just extrapolate based on the 8mm steel shaft typically used for a skate axle (below)


From what I read 1018 doesn't benefit much from any treatments but I'll look again



These motors just screw and glue in a regular 8mm skate axle. The steel used for those :

http://www.eng-tips.com/viewthread.cfm?qid=327719

The new design I'm getting done is longer and the manufacturer is going to do just one to see if they can do it. Why it's harder I don't know the details but if u see my site steelhubs.com the new design will be the same almost except almost twice as long.

Thanks for all your help with this I'd be blind otherwise

 

[1gibson]

McMaster says 1144 has about twice the yield strength (100 ksi) of 1018 (54 ksi), and there is an extra strength 1144 with yield strength of 125 ksi. Don't see it in metric though.

 

[hummina]

yes I see it everywhere else except what I linked! thank you. so 1144 it is! hopefully the decrease in permeability wont be a problem and I think the differences in the metal would equate to about a 25% drop.

now...to figure out how to do the slightest interference shrink fit. next time.

 

[dgallup]

"Figuring the loads on the motor shaft and housing sounds difficult as well as simulating loads."

That's why it's called Engineering and not Guessing.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[hummina]

Thank you all for the pointers.