Some interesting axle strength calculations

[acannell]

http://www.fourwheeler.com/how-to/tr...eep-axle-tech/

interesting! actual breaking torques as well as pictures! nice find!!

The breakages appear to be pure-tensile strength failures, which is why they are breaking in the long spiral which is where the maximum tensile stress is (45 degrees to torsion axis). As opposed to the nearly square snap offs usually seen in actual axle breakages that seem to originate from cracks and propagate along the maximum shear stress plane which is perpendicular to the axle axis, as far as I understand. This is because these types of tests are really testing single-overload failures and not fatigue and crack propagation. So not very meaningful unless crack formation/propagation and fatigue are not playing a role in why your axles are breaking.

The twist angles they note are also not really important as far as understanding or improving axle design. Twist deflection angle is a result of length and shear modulus, not tensile strength or shear strength, which are both separate properties, and determine when the axle will break. Double the length and you get double the deflection angle for the same torsion. Also, shear modulus is the same between all steels regardless of temper, hardening, or alloying, so its not really something that could be changed anyway, even if it did have an impact on breaking strength, which it doesn't.

For instance, a 6al4V titanium shaft would have about half the shear modulus of any steel shaft, and so, for the same length and torque, deflect twice as much. But its tensile strength and shear strength would both be much higher than a 1018 steel shaft, and so it would break at a much higher torsion, even thought it was deflecting far more. This is just to illustrate that its possible to have much higher deflection and also a much higher breaking point, when comparing two materials. In reality I doubt anybody makes axles using 1018. Most materials I have seen mentioned for axle shafts use stronger steels that will be superior to 6al4v in tensile strength and shear strength.

[Reaper1]

Hmmm...these guys aren't far from me. I've actually been to their shop once. Maybe worth talking to them?

[Warren Stramer]

I finally had a spare hour to kill so I machined a relief in this axle stub at the spline root, to the depth of the spline valleys, polished it to a mirror finish then shot peened it with silica balls.
Don't know if it will help, but I don't think it will hurt either, Anyway it didn't cost anything.
If I can get a few more runs from an axle it will be worth it. I will report back.

Still have not had time to get the car to the track...yet.

[Vigo]

The bead blasting definitely looks like it got the desired result as far as peening. Nice looking work!

[acannell]

I finally had a spare hour to kill so I machined a relief in this axle stub at the spline root, to the depth of the spline valleys, polished it to a mirror finish then shot peened it with silica balls.
Don't know if it will help, but I don't think it will hurt either, Anyway it didn't cost anything.
If I can get a few more runs from an axle it will be worth it. I will report back.

Still have not had time to get the car to the track...yet.

nice, I would only suggest making the undercut longer away from the splines. Just eyeball finite elementing it so in other words wild a%% guess of mine. stress still rises there and spreading it out over a much longer area might help significantly

Once you start breaking axles from tensile failure with 45 degree spirals then its probably too much taken off the diameter but until then it seems to be purely crack formation/fatigue which this looks like it might address, those splines look way better than before

If you started collecting data about number of runs and what not you may have some very interesting real world, accurate data..too bad you cant get snapshots of the splines after every run easily..or cannnnn you?

[Warren Stramer]

I wish I knew how deep the case hardening is on these factory axles. I thought about widening the undercut but I didnt want to get into the softer material. Maybe I'm thinking of this wrong? What do you think? I can still machine it wider, and maybe taper up to the original diameter?

[acannell]

I wish I knew how deep the case hardening is on these factory axles. I thought about widening the undercut but I didnt want to get into the softer material. Maybe I'm thinking of this wrong? What do you think? I can still machine it wider, and maybe taper up to the original diameter?

I mean extending the undercut away from the splines along the axis of the shaft, so to the left in your most recent pictures, not deeper. Are you thinking that wouldn't really do anything unless the undercut is also made deeper? Im thinking there is some benefit from moving the two radiuses of the undercut apart, even if they stay the same depth. But there may be more benefit to making it deeper and more gradual too. Not sure....if I had finite element for solidworks this might actually be perfect to ball-park it without getting into pretend land.

So any curvature, increase radius. And any change in diameter, make happen over a longer distance. Thats got to be a true statement no matter what I think.

Im not knowledgeable enough to know the specific pros and cons in this application for the following things:

-work hardening via peening
-hardening via nitriding/carburizing/induction whatever they did at the factory
-polishing to reduce crack formation sites (along with or in lieu of peening?)

I.e. what combination of the above is best..I dunno

I wonder if any very high torque impact wrenches have splined shafts internally..and how they are designed. That would be a worst case scenario....1000 times the impact lifetime of an axle and probably a load that resembles a brick wall compared to tires/traction. Maybe worth researching that before going any further in the design. Patents? Hmm what other industrial machinery regularly subjects splined shafts to very high torques against very resistant loads?

[acannell]

I wish I knew how deep the case hardening is on these factory axles. I thought about widening the undercut but I didnt want to get into the softer material. Maybe I'm thinking of this wrong? What do you think? I can still machine it wider, and maybe taper up to the original diameter?

do you have any scrap axles? you could cut them to see how deep the hardening goes

[Warren Stramer]

I mean extending the undercut away from the splines along the axis of the shaft, so to the left in your most recent pictures, not deeper. Are you thinking that wouldn't really do anything unless the undercut is also made deeper? Im thinking there is some benefit from moving the two radiuses of the undercut apart, even if they stay the same depth. But there may be more benefit to making it deeper and more gradual too. Not sure....if I had finite element for solidworks this might actually be perfect to ball-park it without getting into pretend land.

So any curvature, increase radius. And any change in diameter, make happen over a longer distance. Thats got to be a true statement no matter what I think.

Im not knowledgeable enough to know the specific pros and cons in this application for the following things:

-work hardening via peening
-hardening via nitriding/carburizing/induction whatever they did at the factory
-polishing to reduce crack formation sites (along with or in lieu of peening?)

I.e. what combination of the above is best..I dunno

I wonder if any very high torque impact wrenches have splined shafts internally..and how they are designed. That would be a worst case scenario....1000 times the impact lifetime of an axle and probably a load that resembles a brick wall compared to tires/traction. Maybe worth researching that before going any further in the design. Patents? Hmm what other industrial machinery regularly subjects splined shafts to very high torques against very resistant loads?

I did understand what you meant, I think you are right about making the undercut wider as it would spread the torsional load over much less concentration, and allow some torsional spring action, albeit small. The longest section of the axle (center section shaft) is .941 diameter, and the undercut is .974, and I have never twisted of the long section of a shaft, nor have I ever heard anyone else break one there either.
I'm going to revise the undercut wider and post back when I get time.
PS......that shaft cuts VERY hard even with carbide, I can grind it in the lathe though.

[acannell]

I did understand what you meant, I think you are right about making the undercut wider as it would spread the torsional load over much less concentration, and allow some torsional spring action, albeit small. The longest section of the axle (center section shaft) is .941 diameter, and the undercut is .974, and I have never twisted of the long section of a shaft, nor have I ever heard anyone else break one there either.
I'm going to revise the undercut wider and post back when I get time.
PS......that shaft cuts VERY hard even with carbide, I can grind it in the lathe though.

Do you have a tool post grinder? What kinda setup you got over there anyhoo??

[Ondonti]

I did understand what you meant, I think you are right about making the undercut wider as it would spread the torsional load over much less concentration, and allow some torsional spring action, albeit small. The longest section of the axle (center section shaft) is .941 diameter, and the undercut is .974, and I have never twisted of the long section of a shaft, nor have I ever heard anyone else break one there either.
I'm going to revise the undercut wider and post back when I get time.
PS......that shaft cuts VERY hard even with carbide, I can grind it in the lathe though.

I would bug these guys

http://www.moog.com/products/actuato...light-control/

Not sure if Moog will let you see anything but splined flight control shafts/gears are made by these guys. Might all be proprietary, if I could find a picture on google I would have shared it.

Couldn't find any public resources but did find this cool airplane failures site. There was one damaged shaft in there.

http://www.flightsafetyaustralia.com...-above-5700kg/

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http://skytronicsinc.com/transmission-gearbox.html
These are pretty old. See if you can find something newer with visible splines.

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And this company shares some tiny pictures


http://www.epicos.com/EPCompanyProfi....aspx?id=30980

See the color difference on the splines but not the shaft?

The 787 U joint is the most modern thing.

[Ondonti]

Sick, here they give a bigger pic when you just search for U joints.

http://www.ashot.co.il/product/produ...versal-joints/


I wonder if one of you guys would be allowed to buy one.

I said before I would want CF axle shafts just like people use for drive shafts. Guess thats not cool though.

[acannell]

Couldn't find any public resources but did find this cool airplane failures site. There was one damaged shaft in there.

http://www.flightsafetyaustralia.com...-above-5700kg/

wow very interesting....follow that up with a viewing of charlie victor romeo, then take a flight on an old 747

hmm look familiar?

"Rudder torque tube shaft splines cracked. P/No: 21246237010300. TSN: 1655 hours/1158 cycles"

[Ondonti]

BTW the part I found a picture of above is what I was trying to find public information on. I see them around all the time and hoped it was out there for the world to see too. The spline transition on that part is also very aggressive. Interesting to see the different designs on older and then newer planes. Airbus and Boeing share suppliers on a lot of things and according to their website they make bank off both. I should point out that these are designed with the intent that they will never ever fail. If you know how long a wing is you can get an idea of how hard these have to work.

[Warren Stramer]

Do you have a tool post grinder? What kinda setup you got over there anyhoo??

I set the spindle speed on the lathe high and simply rest my die grinder on the tool post and free hand it, sorta like a wood lathe operation.

[Warren Stramer]

Ok, New spline configuration, Widened the undercut, polished the area to mirror finish and shot peened it.
I am very impressed by what Lengel built for axle assemblies, and would love to have those, But almost without exception, I usually twist the splines off on the inner left side, and I don't see how He addressed that problem. I guess I should re-read his thread.
I honestly have my doubts that undercutting and shot blasting this stock axle will help much but it cost nothing but my time, and I did the same treatment to my stock trans. input shaft and have never broke it where most other high power autos have. (knocks on wood)

I will report back when it breaks.

[acannell]

Ok, New spline configuration, Widened the undercut, polished the area to mirror finish and shot peened it.
I am very impressed by what Lengel built for axle assemblies, and would love to have those, But almost without exception, I usually twist the splines off on the inner left side, and I don't see how He addressed that problem. I guess I should re-read his thread.
I honestly have my doubts that undercutting and shot blasting this stock axle will help much but it cost nothing but my time, and I did the same treatment to my stock trans. input shaft and have never broke it where most other high power autos have. (knocks on wood)

I will report back when it breaks.

Wow that is some fine work there. I am not sure if the peened finish or the mirror finish is better. I would think peening then mirror polishing would be best yes? All the benefits of the pre-tension plus the erasure of the crack formation spots from the peening (although I would imagine the peening finish is probably pretty good)? I dunno. But that taper looks fantastic and its hard to imagine a more ideal stress reduction. Are you going to be running this with the bearings as well?

I like this better than the lengel solution, although I think whats hes doing is also interesting. But IMO when the issue is crack formation and the goal is improving material strength and reducing stress risers, welding is the very last process you want (actually I would think you wouldn't want it at all). Welding will take away the materials temper and also create uneven material properties and therefore uneven stress distribution. Not to mention if there are any voids or defects in the welds (which there will be), you've just created new stress risers and perfect crack formation sites. Plus, locating the weld at the maximum diameter of the cup puts the weakest part of the axle at the point of maximum shear stress. But I'm just pontificating who knows..

[Warren Stramer]

Wow that is some fine work there. I am not sure if the peened finish or the mirror finish is better. I would think peening then mirror polishing would be best yes? All the benefits of the pre-tension plus the erasure of the crack formation spots from the peening (although I would imagine the peening finish is probably pretty good)? I dunno. But that taper looks fantastic and its hard to imagine a more ideal stress reduction. Are you going to be running this with the bearings as well?

I like this better than the lengel solution, although I think whats hes doing is also interesting. But IMO when the issue is crack formation and the goal is improving material strength and reducing stress risers, welding is the very last process you want (actually I would think you wouldn't want it at all). Welding will take away the materials temper and also create uneven material properties and therefore uneven stress distribution. Not to mention if there are any voids or defects in the welds (which there will be), you've just created new stress risers and perfect crack formation sites. Plus, locating the weld at the maximum diameter of the cup puts the weakest part of the axle at the point of maximum shear stress. But I'm just pontificating who knows..

I think the Lengel solution is the ultimate fix, albeit expensive. I don't think the way they welded his socketed CV housings will give him any problems. My "fix" is really a Band-Aid possible improvement, we will see.
I have always been under the impression a surface should be polished and then peened to impart a compressive force on the metals surface. That is the way I've been taught anyway.
And yes I am going to use the support bearing/seal. I test fit it in the trans. and all is fine.

[acannell]

I have always been under the impression a surface should be polished and then peened to impart a compressive force on the metals surface. That is the way I've been taught anyway.

I dont know much more than the basics about shot peening. But from the basic stuff I've read, the depth goes up as hardness goes down, and of course the size of the shot and impact velocity effect depth as well. But generally its tenths of millimeters, so I would imagine that the tiny amount polishing removes wouldn't have an effect. BTW how did you polish it so nicely? Is it my imagination or are even the spline valleys polished?

[Warren Stramer]

BTW how did you polish it so nicely? Is it my imagination or are even the spline valleys polished?

All but the final polish was done in the lathe (high spindle speed). First I rough ground it with stone in die grinder, next I smoothed it with various grit and shaped cartridge rolls in my die grinder, then scotch brite cross buffs run at various angles especially perpendicular to the splines with the lathe spindle speed slowed way down. Then various grit sand paper strips, and the final hand polish was done on a hard felt wheel with polishing rouge. hope that makes sense.