Axle strength vs. spline count?

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Axle strength vs. spline count?

Post by panic » Wed Jan 27, 2016 12:35 pm

I don't know how the concept of rating axles by their spline count started, but obviously a 35 spline axle is not the same as a 30 spline axle with different splines.
If the spline contour is universal, axles can be better compared for strength by assuming the diameter before the spline is proportionate to the spline count, yes (not completely accurate since the splined area is a bit larger, but close enough)?
If so, compare axles (ceteris paribus) using the cube of the spline count per the Young's Modulus vs. diameter relationship.
If axle #1 is 30 spline and axle #2 is 35:
35^3 / 30^3 = +58.8% in torsion resistance.

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Re: Axle strength vs. spline count?

Post by Greenlight » Fri Jan 29, 2016 12:04 am

Check out the Mark Williams website:
https://www.markwilliams.com/axle-txt.aspx

2. Spline count
Size does matter when it comes to axles. The higher the spline count, the larger the diameter of the axle. The larger the diameter, the stronger the axle. Most current axles are based on a 24/48 diametrical pitch. A 35-spline axle has a major diameter of 1.500"and a 40-spline axle is 1.708 inches in diameter. Here's a handy strength chart for comparison, using the GM 12-bolt as a baseline:


Number of teeth----- Common Application--- Major Diameter--- % Change in Diameter----% Change in Strength

28 -----------------------GM 10-bolt, 8.8 Ford----- 1.208" ------------------ -6.9% ------------------------ -19.3%
30 -----------------------GM 12-bolt -----------------1.291"------------------------------------------------------ -- --
31 ---------------------- 9" Ford --------------------- 1.333" ------------------- 3.5% ------------------------- 10.7%
35 -----------------------Racing ---------------------1.500" ------------------- 17.2% ------------------------- 61.2%
40 -----------------------Racing ---------------------1.708" --------------------34.5% ------------------------143.2%
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Re: Axle strength vs. spline count?

Post by panic » Fri Jan 29, 2016 8:06 am

Curious.
I already wrote them:
"I see that the diameter vs. spline count (on your axle FAQ) of 35 (1.500") and 40 (1.708") are proportionate.
My gut feeling is that the axles torsional strength varies roughly as the cube of the axle OD, which means also by the cubes of the spline counts: 40 is 49% stronger than 35 (40^3 / 35^3 = 1.49).
Am I missing something? Thanks."

and got this back:
"Hi Jeff,
I am not sure about your math but from the test we have run we have concluded that the 40 spline axles are about 30 percent stronger than the 35 spline.
Thanks,
Brian"

However, a simple comparison of diameter vs. % strength from their table shows almost exactly the OD^3 relationship (the cube of the OD % in the last column).
31 ---------------------- 9" Ford --------------------- 1.333" ------------------- 3.5% ------------------------- 10.7%-----------------103.5^3 = 10.9%
35 -----------------------Racing ---------------------1.500" ------------------- 17.2% ------------------------- 61.2%-----------------117.2^3 = 61.0%
40 -----------------------Racing ---------------------1.708" --------------------34.5% ------------------------143.2%-----------------134.5^3 = 143.3%

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Re: Axle strength vs. spline count?

Post by peejay » Sat Jan 30, 2016 12:57 pm

Spline count is like counting the bolts in a GM rearend cover. No it isn't the actual splines that make the strength, it is just a quick way of defining what you are working with.

Curiously, spline pitch seems to be the same the world over. 26 spline Mazda axles use the same pitch as American and with a small amount of clearancing, you can put a Chevy 7.5" 10-bolt diff carrier in a Ford Sierra rearend housing and the Ford stubs will spline right in.


The math in the thread does make me feel a bit better though. When I compared my old 26 spline axles to my new 31 spline, I had a sinking feeling because they aren't visually that much larger.

Image

Knowledge that people run 31 spline axles in cars weighing 1000lb more and making three times the horsepower is academic when you're trying to keep your car from breaking, you know? I lost a whole season due to axle failures.

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Re: Axle strength vs. spline count?

Post by panic » Sat Jan 30, 2016 2:52 pm

If I'm right, your new 31 spline is 69% stronger than the old 26 spline.
Here's a chart I wrote.
Axle-chart.jpg

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Re: Axle strength vs. spline count?

Post by pdq67 » Mon Feb 01, 2016 7:55 pm

It's a spline root depth deal.

Fewer splines, deeper root.

More splines, swallower root depth.

And the root depth is the actual diameter of the axle that is used for strength calculation purposes.

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