TOTM: All About Axles

[BigClay]

^ wow, my head hurts now

[zjeepin]

i like the thought process cam... Paging Paul.. downtowncb..

my thinking would be to picture the shaft as any load bearing object, with its ultimate load and yield strenghts.. the more times you approach or reach the yield the likelihood of it breaking increases.. when you're trying to break something by fatiguing it, it become easier to cycle back and forth in both directions the more you cycle it..

if you consider work-hardnening, the shaft would be stronger if left in the same position experiencing the same loads.. my .02..

[downtowncb]

First I think there are a bit too many variables to say for certain. In a general case say the shaft was used hard for years but we know for certain that it hasn't endured any plastic deformation (twisted splines, necking, etc.). For fatigue analysis of a shaft subject to the random "complex" loading during it's normal use, the loading would be reduced to a simple cyclic loading based on the peak loads. Because of this I believe the shaft would be equally likely or unlikely to fail in either direction after it's initial use.

Under an axle shaft's normal operating conditions there should be no work hardening. Work hardening requires some small plastic deformation to occur. For example, bending a paperclip. If you bend it one way and try to bend it back the opposite way, it will bend separately next to the first bend. This is caused by work hardening of the grain structure in the wire. The most likely place for work hardening to occur on an outer shaft would be right after the splines exit the hub. If it were enough to be readily visible, I would consider replacing it or at least carrying a spare.

[ATL ZJ]

Wow, that's an awesome explanation and the paper clip analogy seems logical...

Here's the shaft that broke. You can see it snapped right where the splines end. The root of the splines was the smallest diameter part of the shaft, instead of having the main body of the shaft neck down from the splines like properly shaped shafts do (14b shaft for example). Interestingly enough, the line I drew down the body of the shaft when it was new was still on there, and it was still 100% straight, showing no plastic deformation past the weakest point of the shaft where it broke.



This is a real world example of how smaller diameter shafts (and their generally higher deflection characteristics) are actually tougher, even though it may seem backwards at first. The shaft that necks down is capable of absorbing more torque so the rest of the assembly may never see the higher ultimate torque numbers that a larger diameter shaft might transmit.

But... if any of you other engineers have a different theory on whether a stub is weaker when flipped, please chime in... I have one spare and it has been run on the driver side already.

[ATL ZJ]

First I think there are a bit too many variables to say for certain. In a general case say the shaft was used hard for years but we know for certain that it hasn't endured any plastic deformation (twisted splines, necking, etc.). For fatigue analysis of a shaft subject to the random "complex" loading during it's normal use, the loading would be reduced to a simple cyclic loading based on the peak loads. Because of this I believe the shaft would be equally likely or unlikely to fail in either direction after it's initial use.

Under an axle shaft's normal operating conditions there should be no work hardening. Work hardening requires some small plastic deformation to occur. For example, bending a paperclip. If you bend it one way and try to bend it back the opposite way, it will bend separately next to the first bend. This is caused by work hardening of the grain structure in the wire. The most likely place for work hardening to occur on an outer shaft would be right after the splines exit the hub. If it were enough to be readily visible, I would consider replacing it or at least carrying a spare.

OK, I've obviously been dwelling on this too much. But it's really interesting and the decision to run a used stub in a new direction could theoretically ruin a weekend...

I also came across this- do you think it applies in this situation? http://en.wikipedia.org/wiki/Bauschinger_effect

and where the F is Paul?

[Jeeptech01]

My totally pedestrian view on the subject is running a shaft backwards should be given no more thought that throwing your rig in reverse on the trail. Sure you dont use reverse as much as the forward gears but the ratio is also steeper. My .02

[ATL ZJ]

My totally pedestrian view on the subject is running a shaft backwards should be given no more thought that throwing your rig in reverse on the trail. Sure you dont use reverse as much as the forward gears but the ratio is also steeper. My .02

That's partly true, and front 44 shafts especially have been known to fail when bound up, backing up. But the difference I see is the percentage of the time you're loading the shaft in forward vs. reverse... sure it will see stress in both directions but how often are you absolutely hammering on it in reverse? rarely...

[Jeeptech01]

Hrm. Yea I figured it may relate more to the amount of time it is being used vs one hard use but I thought the paperclip description negated that line of thought.

I understood it as one beastly twist killed a shaft not the thousands of small and medium twists slowly aging the shaft until its failure.

Admittedly Im not too "up" on this type of thing but wanted to offer a different.. less intellectual line of thought LOL.

[ATL ZJ]

Well, my replacement yukon 35 spline inners came today. Which was cool in its own right because I now have two new, spare 4340 inners. But the reason I'm posting is because the short side inner is different than the one I had before.

Instead of maintaining a constant outer diameter like the one I twisted, this one has proper neckdown from the splines and a smaller operating diameter. I'm still going to run the shortside ten factory inner I have been, and hopefully I won't need to ever test the yukon, but it's good to see them change their design for the better. Figured I should correct what I said earlier. Not sure if they changed their long side shafts though- the new one I got is the same as the one I broke.

[SirFuego]

Extra pinion supports like in the 14b minimize ring and pinion deflection

Here is an interesting thread that popped up on Pirate recently. Even though it's obviously an advertisement for Currie, there is some interesting discussion cropping up regarding whether the extra pinion support really is why the 14bolt is "stronger". The theory (made by Currie and some other folks in the thread with a lot more experience than me), is that carrier deflection is usually the problem and not pinion deflection. The major advantage of the 14 bolt is due to the size of the carrier and not necessarily the 3rd pinion bearing (which probably does still make it stronger, but perhaps not really giving it much of an advantage if the pinion really doesn't deflect as is claimed).
http://www.pirate4x4.com/forum/showt...7#post12772027

[ATL ZJ]

Interesting but I think the key thing to remember about the 14b is that it just works. Out on the trail, who cares what the reason is, whether it's the pinion support or big carrier. The price/reliability ratio is unbeatable. And shaved it's gonna have almost as much clearance as that $2k center. /devilsadvocate

[SirFuego]

Yeah I didn't want to imply otherwise. The 14B is a great axle -- especially when it's shaven. Just found it interesting about the findings about the pinion support.