Interesting Axle Failure

[RichardP]

Here is an interesting failure mode for an axle that I found while rebuilding my differential last year. The axle is factory SN-95 for a Ford 8.8 that saw service on my American Iron car for a few seasons.



I'm not sure how I even noticed the crack since it was mostly obscured behind the ABS exciter ring (removed for the picture). The failure wasn’t visible from the rotor side before I cleaned it up. Before the finding, there was no indication that anything was wrong. There was no looseness or wobbling noticeable or telltale sounds like the rotor rubbing the caliper bracket.



A close examination shows that the crack is a full 360 degrees around the axle and extends all the way through to the front face. The axle flange had actually rotated a small amount and was fully separated from the axle shaft. When I sectioned through the side of the flange, it fell right off of the axle. The only thing holding the flange to the shaft was the locking characteristic of the V-shaped fracture surface.



There is uniform light surface rust over the entire fracture surface. Unfortunately, since there was over six months between the last time the axle was used and when it was cut apart, that oxidation isn’t really useful for diagnosis like it could have been if it had been freshly cut apart.

There is no obvious damage or defect that the crack would have propagated from that I can see. A detailed examination from a more knowledgeable person might lend more clues.



I have no idea how long this failure had been working or how long before a complete failure would have happened. My guess is that catastrophic failure wasn't too far away and I dodged a big bullet. The other axle, with the same amount of time on it, shows no sign of any issue.

This is just something for people to keep in mind when they are inspecting their cars. Be safe out there…

Richard P.

[Larry]

You are indeed lucky. Great example of why to get forged/billet axles. Was that the right side axle?

[Auto-X Fil]

It's salvageable.





Seriously, that's scary. I don't know that suspension - would the wheel have fallen off or at least become loose? I saw that happen to an MG entering the Bus Stop at the Glen and it was a hairy ride.

[gt40]

We're talking a Fox Mustang here, so the only thing that would retain the wheel assembly if the axle had totally failed would have been the brake assembly, and I dunno how long that would have lasted at speed.

I run Mosier axles myself, but this will get put on the high-frequency inspection list now.

[Auto-X Fil]

So the brake caliper bolts onto the axle housing, and the axle and bearings are mounted together in the housing?

I've never had a car without CV joints and rear uprights. These funny sticks between the wheels confuse me.

[Mberglo]

So the brake caliper bolts onto the axle housing, and the axle and bearings are mounted together in the housing?

I've never had a car without CV joints and rear uprights. These funny sticks between the wheels confuse me.

You got it. The brake caliper actually resides in it's own bracket that's mounted to the axle housing.

Very scary indeed. [optimistic hat on] I'd like to think it was a manufacturing flaw in that one instance and not something we'll all experience. [/optimistic hat on]

[gt40]

So the brake caliper bolts onto the axle housing, and the axle and bearings are mounted together in the housing?

Only if you have c-clip eliminators or 9" housing ends welded on (or are running a 9" housing, of course.)

In this case the sliding brake caliper is held in a cage which is bolted to the axle flange. Since the failure occured beyond where the bearings would have been using a 9" setup, it doesn't matter how the axle itself is retained, the wheel would have gone for a trip in either case.

[BRGT350]

That is one scary looking failure! I will add that on my list of things to check before heading to the track. Are you going to send it out to a metalurgist to have him/her examine it? When I used to work for a camshaft manufacture, I had the 2 metalurgists working in the lab inspect my brother's crankshaft when it seperated into 3 peices. Both of them agreed that the failure was due to regrinding the crank, and a chip at the fillet along the journal started the whole mess.

This failure looks well past the ground portion of the axle, so I would doubt it was a machining error.

Is P51 around here anymore? He has help solve some metal failures on FocalJet, and I think he deals with an automotive axle supplier.

[Tob]

Boy, did you (and your competitors) get lucky. Could have been very ugly.

Here's a shot of a brand new stock SN95 axle (on left) and a Superior Axle (SN95 replacement) on right.

Note how the hub is fully machined and a bit thicker. The stocker appears to have a more generous radius where the hub meets the flange face, right where yours cracked.
From the bearing surface outward, the axles appear to use similar dimensions/chamfers, aside from the fact that the aftermarket unit is fully machined.


Most importantly (why Superior got my $) is the material they use and how it is treated. Most OEM axles are plain carbon steel shafts that are induction hardened. Superior uses a 4340 alloy that is through hardened and induction hardened.
More tech here. Considering a pair of their SN95 axles is only about $250, I'd give serious consideration were I racing with stock units now.

Tob

[swhiteh3]

And now you see why all serious race-cars use full-floater axles. Trusting all the corner loadings (and camber stiffness') to that little inside chamfer can be pretty scary.

Glad you're okay....

Scott

[Wiggawam]

Wow, talk about a scary thing to find. I wonder if something was wrong with that batch of metal or just a freak occurrence.

[SpecialService]

I had an interesting axle failure a few of days ago! The axle broke at the flange, the wheel rotor and hub departed luckily it was on a low speed corner. I was traveling over 140 mph two corners back! The axles were Moser 31 spline c clip 5 years old. The car 3000lb 91LX with 330rwhp, coilovers and 275x18" Hoosiers. I put approximately 1100 track miles on these axles. I wish to avoid this in the future, does anyone have advice on which axles I should try next Superior, Alloy USA, Moser again or something else. No matter what I put in I think I will pull the axles every year and NDT them. Blurry crappy pictures should be attached.

[Tob]

The tech can be sought after and found in a few different locations on this site. Suffice it to say, Superior has what you need.

[SpecialService]

Thanks I will definitely get the Superior axles if they are 4340. I read the tech before posting but the axle material chart is dated 2004. I checked their web site for tech http://www.superioraxle.com/tech.html it's blank, CrMo is mentioned for the front axles but no where else. I'll call to confirm they haven't switched material in the last few years.

[Wangstang]

http://www.yukongear.com/Axles.aspx

I have yukons on my car now. They do make custom axles to order for very reasonable prices. If you wanted you can call and have them use their front axle alloy to make a set of axles for you.

Wes

[SpecialService]

Thanks for the information, turns out none of the Ford 8.8 Superior axles are available in 4340 yet. The 4340 should be available in 1 to 2 months, current axles are made from 1541H.

[snipes25]

I also experienced an interesting axle failure mode, bran-spankin-new Superior 31-spline axle with only eight 20-minute sessions at Gingerman on it. When I inspected the rear brakes I discovered the driver-side axle broke at the c-clip slot. Fortunately my calipers kept everything together long enough for me to discover this off-track.

Anybody seen this before? Any ideas about the cause?

-Vern
95 Mustang

[Tob]

Differential type? Bearings, c-clip, age and condition? What color was the axle lube on teardown?

If your differential pinion shaft was worn enough (or your c-clips themselves), I could see enough axle endplay to allow the c-clips to slam back and forth enough to shear the button off the end of the axle, especially in a race type environment.

Tob

[snipes25]

Tob,

This was the first outing for this assembly, new torsion diff, new bearings, new superior axles, seals etc. all new. The lube was a little greenish, with a very fine black powdery sediment evident. I see no particles larger than powder - drained it through a paper towel to try to trap any. Everything was within spec when we assembled it. I have a few more pics of the diff, maybe you can see the coloring of the lube. Here's another shot of the broke end - maybe less glare.

-Vern

[Tob]

Hmm. Interesting. How tight was the pinion 'block' when placed between the axle ends? I gently tapped mine into place and noticed almost zero axle endplay when assembly was done. I'm using a T2R, Superior 31 spline axles, everything else new as well. If it wasn't a metallurgy issue, I'm wondering about c-clip fitment. I used new Ford clips and they fit snug. I slipped in an old one for reference and there was a hairs worth of play. At the time I pondered a failure due to lack of movement but I've not had an issue yet since assembly. Then again, I 'm not subjecting mine to the rigors of road racing.

Got a shot of either side of the c-clip that was on the failed side?

Tob

[snipes25]

The pinion block felt pretty tight to me - I attributed that to the fact that it was new, no noticable axle end play (till it broke), in fact the passengerside still feels real snug, no movement (although I haven't put an indicator on it). I haven't dis-assembled beyond whats in the pics yet. The replacement from Superior has not yet arrived. Here's a shot of the c-clip from down the tube - not much to see, but interesting that the c-clip appears to be containing the broke piece of the axle.

-Vern

[Tob]

Of note, I noticed on the new factory Ford c-clips that there was a sharp side and a radiused side. I kept the radiused 'soft side' towards the button end of the axle. Deburring the sharp side is probably a good idea, as much as I'd guess the c-clip is softer than the axle anyway. I'd still be interested in seeing the c-clip...

So how has Superior customer service been to deal with?

[snipes25]

That's a good point Tob, I'm going to take a real close look at the c-clips and pay close attention to stress risers, deburring, polish... Superior has been cool so far, the engineer I talked to wanted to know how the break point looked, and I could only describe from memory because the car was assembled and back on the trailer. He offered to do the paperwork from his end for the replacement if I ran into trouble with the vendor, so I could get the replacement more quickly. I have to say I was impressed with Jegs - one call, and they kept me on hold only long enough to ensure that a single axle was available from Superior, and they're sending it out - no substantive questions asked. I already received the return authorization in the mail, just waiting for the axle to travel from CA to MD. The guy at Superior had never seen this failure mode before (his claim anyway) and sounded anxious to get a look at it. I'm thinking the heat treat wasn't complete or there was a flaw in the bar - but I have no real emperical evidence to back that up.

On a side note, the 28-spline axles that I removed from this rear were so mushroomed that I had trouble removing the c-clip from the drivers side axle. So a few years of track use can really pound the hell out of them.

-Vern

[snipes25]

Here are photos of the broke bit, and c-clip.

-Vern

[Moving_Target]

Is that c-clip supposed to have that ridge in it? I don't recall seeing anything like that clip when I was swapping axles/repacking the t-lok etc.

[snipes25]

I don't think so. Think it's possible that it was cut by the axle pulling it against differential internals? The pinion block shows very little contact patch on that side. The other side shows more... Here's a couple of photo's of the block drivers side first, then passenger side.

[Moving_Target]

Yes, I see little machined grooves in the pinion block (unless it's just a trick of the light). It's more noticeable in the 2nd picture. Hm, I'd have thought the center of the axle end would push against the pinion block under cornering forces so why is that pinion block marked up like that?

Interesting indeed.

[snipes25]

The large radious machine marks across the entire block are from the factory, the axle end sized circles are basically where the end of the axle polished the factory machine marks smooth, there are some approx 4" diameter marks on the block where the c-clip capture disks (not sure what they are really called) also rubbed against the pinion block. [edit] I'm pretty sure the two or three long marks on the block occurred when removing the block [/edit]

What I think the pinion block may show is that the pinion block got more wear from the passenger side axle pushing inward than the driver side. Which would suggest that the drivers side axle was mostly loaded outward. This is consitent with the number of right-hand turns verses left-hand turns at Gingerman.

Anyone with thoughts on whether the outward load on the drivers side axle could have been more than the axle is designed to handle? I think it's unlikely.

Does the ridge cut into the c-clip suggest anything that would have contributed to this failure?

[Moving_Target]

I wasn't worried about the tooling marks or the scratches from removal. I was commenting on the dark concentric circles where the c-clips looks like they were contacting. It looked like the block (on both sides) had some grooving (the dark circles).

Regardless, I'm fairly certain the c-clip is worn abnormally but can't speculate as to why.

[trackbird]

The photos are now dead, but we have seen this exact same thing on Camaro/Firebird race cars with 10 bolt rear ends (and one Dana 44). The thread is here:

http://www.frrax.com/rrforum/index.p...=5876&hl=crack

It's linked to another thread about a Dana 44 failure. Same exact thing pictured at the top of this thread. They crack around the flange and are held in place by the shape of the crack (except the Dana, that one lost a wheel).

[Tob]

Interesting.
Mine...

Yours...

Both sporting 3.55's Ford gears as far as I can tell.

So I go back to my 8.8 buildup thread and start studying photos again...




From 4/19/04, during the assembly...

The block tapped in allowing for zero axle endplay. We've discussed this before in regards to the axles pushing on the calipers and now I can clearly see why. Two years ago I installed a brand new FRPP 3.55 geared axle assembly in my '88, and right off the bat there was approximately 1/8" to 3/16" of endplay at each axle. The Torsen allows for zero. There will obviously be a bit of heat here for break-in, and I'm hoping it won't be a detriment but a good thing. Fingers are crossed.

If the design were faulty, we'd be hearing/seeing a lot of failures. Ruling out any assembly woes in your case, either the Torsen machine work was off somewhere or the axle was defective. It does look like your pinion block got a bit hot, which no doubt was transferred to the button end of the axle. The groove in the c-clip must be a result of the failure, unless some foreign material somehow got in there. Does the ring/groove in the c-clip match the OD of the button? If not, its debris from somewhere.

On that note, look at the broken button end at the right of the following photo...

There appears to be a groove near the center of it. There could have been something lodged there as well. Note the following shot of one of my Superior axles, no groove.


Tob

[snipes25]

The groove on the damaged c-clip matches the inside diameter of those 1/4" thick 'disks' that nuzzle in there next to the pinion block. The replacement axle arrived today - 4 lug... now I gotta send two of them back. $@#%!it.

-Vern

[Tob]

There must have been some foreign material in there for there to be a circular depression on the clip like that. How it got in there is a mystery. An engine related issue that I can offer as a debris comparison, my 90,000 plus mile factory shortblock.




Tiny specks can cause such bad things.

[snipes25]

That's a pretty well varnished! Did that cause the pump to fail, or did you just discover it during a rebuild? I guess I'll be looking for burrs on those c-clip retaining plates. Something must have cut that groove.

[Tob]

A tiny piece of debris (1/2 the size of a small ant) got jammed in the pump cavity cauzing the pump to seize. With the camshaft still spinning the oil pump driveshaft twisted like a pretzel, shortened itself, then snapped in half. I just happened to take a gander at my oil pressure gauge and saw zero oil pressure. I drove another three miles and shut it down and pulled the engine.

[snipes25]

Jegs reported to me today that they are dropping the Superior axle line because of supply problems, apparently foote axle & gear has undergone some management or ownership changes in the last year and is having trouble keeping up with demand. They are going to send me replacements for my failed one, but won't be a Superior axle reseller in the future.

Anyone heard anything else about this?

[Tob]

I would contact Superior and tell them what the individual at Jeg's told you.

[learning_curve]

The corrosion on the hub most likely contributed to starting the crack. A reasonable preventive measure would be to paint the bare metal that is out in the weather. I am surprised how badly rusted the part is.

A closer picture of the cross section you extracted would prob indicate where the cracking started. Axle fatigue is nasty since it is fully alternating, that and the pitting corrosion provides a fast track from crack initiation to crack growth.

[fullrace]

In the 4x4 world I have seen more aftermarket axles break than stockers in some cases. Many of them from yukon, superior, etc. I would like to build a full floating 9", but I dont know how to do it light. I dont want a big heavy rearend...

[StanIROCZ]

The groove on the damaged c-clip matches the inside diameter of those 1/4" thick 'disks' that nuzzle in there next to the pinion block. The replacement axle arrived today - 4 lug... now I gotta send two of them back. $@#%!it.

-Vern

IMO, it would be a good idea to ask for 2 new axle shafts. These things are made in batches. What says that the heat treat isn't bad on both of them?