<VV> axle breakage

[djtcz at comcast.net]

----- Original Message ----- 
From: levair at aol.com 
Subject: Re: <VV> [indyscca] Autocross report 

And as expected, I had to eat humble pie today at Terre haute. 

My car broke the stub axle splines where it drives the rear wheel hub. 
I didn't even get one run in it. 
However, thanks to lots of friends and their cars, I did not miss a 
run. 

Now, did it break because of the added torque of 2+ added(30%) psi, the 
huge jump( and landing two) weeks ago at 16 th Street, or 45 years of 
age of the part, or all three?? I have run V8s through the same type 
axle hubs without failure. . 
Warren 



=========================================== 


Was the failure at the end of the splines, or part way into the hub? 



The fracture surface contains clues , unless the faces were in contact for a few frantic revolutions after complete fracture, and beat each other up. 


DJ Wulpi was an engineer for the International Harvester Corporation. He shared a batch of his failure analysis experiences in books and articles over the years. 
Some pictures based on Wulpi's work are all over the Web. 
http://www.asminternational.org/pdf/spotlights/jfap0502p011.pdf 
http://www.metallurgist.com/html/ShaftFailure2.htm 


Regardless, I'd guess it is a fatigue failure of some kind. 
It Could be "Low cycle" fatigue where some plastic or permanent deformation occurs during each loading event and relatively few hole shots are required for failure. 
Or, it could be "high cycle" fatigue where the load only causes "elastic" deformation, and takes millions of cycles for a final failure 


Repetitive loads causing stress well below "ultimate strength" or for high cycle fatigue, even "yield strength," over time, create a tiny crack, or cracks, that grow some with each additional stress cycle. 
Frequent enough magnaflux inspection would reveal initiation of cracks well in advance of failure. 


The 1965 Chassis shop manual Section 4, Fig 66 suggests The portion of an LM "drive spindle" inboard of the wheel bearings handles mostly torque, and little or no bending. A basic radius is shown at the transition of the splines. Probably plenty good enough for the intended service. 


The steps to cure or postpone "low cycle" and "high cycle" fatigue are similar. 
First reduce stress concentration thru improved geometry . No sense sending in the star quarterback wearing mittens and flip-flops. 
Second, improve material properties to raise the "endurance" limit above the highest stress the part will see. Could be a higher strength material, a nitride treatment, or the almost magic Shot-peening of the original part. 


I'd prepare Magnafluxed spindles by removing material to create a profile more like the area adjacent to the spline on this this - 
http://www.pirate4x4.com/tech/billavista/PR-35Spline/DCP_6548.JPG 
What's important is a Big smooth radius, and a minor diameter less than the spline root diameter to reduce the vicious stress concentration at the ugly transition at beginning of the splines . 
In Shop Manual Fig 66 it looks like some material could be removed at the end of the spindles spline and still provide centering diameter adjacent to the bearing, 


Then , follow up with shot peening with cast steel shot to an intensity shown on page 20 of this MIL spec 
http://www.godfreywing.com/pdf/AMS-S-13165.pdf