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Discussion Starter · #1 ·
Below are a couple pictures of my pinion shaft in two pieces. It broke right where the shaft diameter necks down, no surprise.

But I still think a material failure/defect is involved. I would have thought I would tear up the differential or u-joint before I snapped the pinion.
 

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Was this the stock set? What ratio?
 

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Well some good news and not so good news. This isn't a torque fracture nor is it a material defect either.

Looking at the bottom photo the up and down lines shown on the right side are not angled indicating a straight line fracture. The left side of the photo is the area that had fatigue after the fracture occurred.

Now what caused the fracture appears to be vibration related. I would have the driveline balance checked and driveline angles checked. There could be other causes, but not likely due to any torque related reasons.
 

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I'm wondering if a shock load could've done that like backing the truck into a curb or something of that nature.
 

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That's true. I bet it could've been a flaw from the factory.
 

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If it is a flaw in the material...you wouldn't see straight lines at the fracture...it would have been irregular lines. The fact that the lines are straight and not angled towards the center of the shaft indicate repeatative movement until failure...similiar if you were to bend a piece of metal back and forth until it breaks and then look closely at the fracture you would see the same effect. I wonder if the poster had lowered the vehicle and didn't check the drive line angle.
 

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Discussion Starter · #10 ·
Good comments guys. This was the stock 3.55 set that broke; approx. 71K miles. It has been lifted while 35's for about 25K miles.

From my years as a structural engineer, my thinking is this is a fatigue failure. The striated portion of the failure represents crack propagation through the shaft. Who knows how long this took from start to finish. Then the grainy looking part on the left is the brittle fracture when it finally broke.

This type of failure starts at some imperfection in the surface of the metal; a chip, a groove, rust, just about any irregularity in the surface can start a fatigue crack.

Had the shaft failed in torsion in one fell swoop, the whole section would have looked more grainy with some radial cracks coming in from the edges and pointing towards the center.

The vibration theory is a good one. Should I have felt a vibration while driving? The truck always rode smooth.

When I installed my lift I did not correct the pinion angle but I did not think the left was enough to creat a problem. I thought I would break a u-joint first.

The only recent shock load the truck had were a few WOT 1-2 shifts and some minor donut action in the snow the day before it broke. I think this may have contributed because there was one instance where a tire caught traction during the donut. But it didn't break then; I drove another 20 miles or so before the break.

I think the shaft was on it's way out (fatigue problems) when I caught some traction in a donut and that pushed it almost to it's rupture point. Another 20 miles of regular driving took it out.

I'm installing 4.56's with a Detroit True Trac right now and really looking forward to being on the road again.
 

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Discussion Starter · #11 ·
If it is a flaw in the material...you wouldn't see straight lines at the fracture...it would have been irregular lines. The fact that the lines are straight and not angled towards the center of the shaft indicate repeatative movement until failure...similiar if you were to bend a piece of metal back and forth until it breaks and then look closely at the fracture you would see the same effect. I wonder if the poster had lowered the vehicle and didn't check the drive line angle.
I think you're right on OuttaCtrlHemi; bending metal back and forth until it breaks is the kind of fatigue failure I'm referencing. I didn't see your post until after I put mine up.

The truck is actually lifted with no drive line angle correction. It's a 5" lift, do you think this contributed? I'm puzzled because I know of other trucks with similar set-ups and no correction that haven't had any issues.
 

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Your assessment matches mine...fatigue and not torsional or material defect. Would the vibration be felt...if severe yes...but subtle no...and the resulting damage would take some time in that case and of course prompted by some (playful fun)....we all have been there...
 

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The truck is actually lifted with no drive line angle correction. It's a 5" lift, do you think this contributed? I'm puzzled because I know of other trucks with similar set-ups and no correction that haven't had any issues.
Hmmm...typically a lift kit will have the angle taken in to account and the proper wedge would be used. I dunno...to me, any time you change the factory ride height...check the angles to be on the safe side. Most of the time you are gonna be okay...but why risk it. After you get your new gears in, take a few minutes and check the angles. Also look at the driveshafts and make sure there are no dents or missing weights. Finally check U-Joints for play or abnormal stiffness.
 

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Discussion Starter · #14 ·
The Superlift kits do not come with anything to adjust the angle of the rear axle. Is the wedge you speak of something installed between the leaf spring pack and the mounting brackets welded to the axle housing?

This sounds pretty simple, I wasn't aware of them. Who sells them?

Do you know the acceptable ranges of drive line angle?



Thanks for the help.
 

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moe in wichita ks
be sure to check the motor mounts mostly the rear mount. and check for driveshaft ballance weights missing, they are spot welded on. could the rear u-jiont have not been seated in the yoke? when i started driving my dad told me about a husdon he had that he would spin the tires on ice and hit pavement it took out the hydromatic trans. so he told me NOT to spin tires at all.
 

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Do you know the acceptable ranges of drive line angle? Thanks for the help.
Here is a page from the manual...pretty straight forward on how to calculate the driveline angle and acceptable working angles.
 

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The Superlift kits do not come with anything to adjust the angle of the rear axle. Is the wedge you speak of something installed between the leaf spring pack and the mounting brackets welded to the axle housing?

This sounds pretty simple, I wasn't aware of them. Who sells them?

Do you know the acceptable ranges of drive line angle?



Thanks for the help.
Yes; the wedges go on the bottom off the leaf pack and I would think Superlift should carry different degrees of wedges. Good luck man.
 

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Discussion Starter · #19 ·
Thanks for the help guys, I will look into some wedges to help my pinion angle. And definitely scour the driveshaft for missing weights, that's a good idea.

The u-joint on my driveshaft is a solid connection with the driveshaft so I don't think it should have caused a problem. It's different than some GM's I've had where the u-joint sat in a yoke and had a couple mains caps bolt on over it.

Prism - I noticed you're in KC; do you know of a good 4x4 shop around here? I wonder if Chux Trux would have some wedges? When I lifted my truck they were pretty cool about renting a special tool to me to remove a couple bushings. :gr_patrio
 

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IMO- it was high ratio, big tires, and lead foot.
 
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