9” spline engagement

[boonie]

Building a rearend, forever now, don’t get much time. Either way, does anyone know what the amount of spline engagement of the axles should be into the diff? Thanks much.

 

[Yeller]

1/4” from the center pin in the difff

 

[DirtDonk]

Which means that the actual spline engagement could vary by differential type and model?
Is it a good idea to test each one with, perhaps a long wooden dowel maybe?

And curious for the reason for the question boonie. Are you experiencing an issue with the axles not going in all the way maybe? Or concerned that there’s not enough spine engagement?

 

[Yeller]

Many of the original 9” axles had minimal spline engagement like 9/16-5/8” under heavy use or extreme high mileage the splines sometimes strip. Usually 1/4” from the center pin is my minimum length and all the way to a 1/16” from the pin for maximum.

 

[Apogee]

In talking with one of the tech's at Strange (Dmitri maybe?) a few years back when I was building a 9" ARB for my EB with one of their cases, he said that 1-1/16" of spline engagement was required, because at that point the strength of the splines should be greater than the torsional strength of the shaft. I haven't done the math to confirm or deny this, but it sounded like they had. Now obviously you can run less, but the mode of failure can shift from twisting/breaking a shaft to twisting/stripping the splines. I ended up running standard F150 shafts in my F150 housing, as I kept it at 65", so my spline engagement should be whatever was stock (unless the ARB changes that).

 

[boonie]

I’m in the process of building one. As of right now I have roughly 7/8 -1” of engagement. It’s been a while since I looked at it.

 

[ntsqd]

Is that with stock or replacement axle shafts?

With the relatively low cost and now easy availability of replacement axle shafts I'd suggest buying new.

 

[JB Fab]

I was told that 1.5 X the diameter is optimal

 

[Yeller]

I was told that 1.5 X the diameter is optimal

Ford sure missed that a lot lol

 

[Apogee]

1.5X diameter is pretty much the rule of thumb I was taught in engineering for safe threaded fastener engagement, and can also be applied to splined shafts, but fits, spline profiles, etc all come into play. If you look at the width of the splined front drive slugs, they're not even a diameter in thickness typically, and it's still usually the axle that breaks first in the case of the slugs.

 

[DirtDonk]

But would at least part of that difference be due to the fact that the splined slugs are supported on the outside?
And also that outer diameter is huge compared to the rest and the forces exerted on them.
Maybe some other dynamics as well.

 

[boonie]

This is 7/8”- 1” engagement with wild horses 31 spline axles. I’m in the process of building a housing(shortening one from a van).