Problem with axle shaft / rear wheel bearings 308

Hello from Germany,

this evening I wanted to replace the rear wheel bearings on my 308 GTB. All went fine so far until I found, that one ring nut was not very tight - though I found the original factory paint dot on it.
Now it seems that at any time the inner ring of the inner bearing turned on the axle shaft.
There are now slight grooves on the axle shaft in the region of the inner bearing's seat. The fit is still tight, but I'm concerned about stress concentration.
What would you do ? Purchasing a new axle shaft ? And I don't know yet how the other side looks.

Best Regards and thank you in advance for your opinions

Martin
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I can't imagine that if the inner race did not get hot enough to fuse itself to the shaft, and the axle did not get scored, then the axle did not get that hot. Loctite makes a product that takes up clearance when mounting bearings. If the axle did not suffer, and it looks like yours did not, I'd Loctite it and put it together.I was doing axles on Jammin's 308 and found a loose stub axle nut. I wondered if this was from a failing bearing.
Cheers,
Tom B.

 

I've repaired a few shafts with this problem . .. I think the nut coming loose is the beginning of this failure . .. sounds like you caught it before it messed up he shaft too bad . .. I had to knurl the area where the inner race rides which isn't easy on the spline.

I fixed one like this in 1995 and just re-repaired it with the addition of loctite like other poster mentioned . .. tried to get away without knurling the spline area when I repaired it the first time and I didn't have the proper socket to tighten it all the way. Fixed the other side in 1999 and the car has seen plenty of track days.

I'd like to know at what point on the shaft they break . .. it's interesting how far the spline extends into the inner race of the bearing . . if this was a weak point seems like they could have shortened the spline.

Sean

 

Hi Sean,

I thought that they broke at the outter flange, not at the splines. I too am very curious to know what is going on, clearly, If you are going to track the car, this is a weak spot that needs to be watched and I am not even sure which end of the shaft, (if you will pardon the expression), is failing.

Need more help,
chris

 

Knowing of the few that have broken and the damage they caused, I would try as best you can to replace it with new or a good used. Its probably stretched, which is why the nut was loose, and its undersize at the bearing which is why its slipping.

 

Hello all,

thanks a lot so far for your opinions. This problem seems to be more common than I thought.
I'm still thinking about what to do. The fit of the inner race of the bearing is still tight. I just thought about, whether those grooves can cause cracking based on stress concentration.
In the meantime I found out, that the original part no. 108046 is no more available, but Ferrari offers an alternative no. 124964 at EUR 1050,-- ( $ 1500,-- ) (!!!)
In my eyes that design with half of the inner race sitting on the splines is a clear design flaw. I'm curious whether the newer part no. has this altered.
Will do the other side this evening. If it looks similar I have to take the serious decision about $ 3000,-- for new axle shafts :-(

Best Regards from Germany

Martin

 

Could a gear company or someone like that make you some new ones for less money? Or even if the price is the same maybe make them from a stronger material?

 

I know of a guy in Australia that use to make them . .. doubt there's a demand.

Like to know more details about the catastrophic failures like where they broke/were new bearings just installed or was it possible the nut had come loose? Once things get loose the shock is horrendous on the parts.

 

I recommend having the shaft Magnaflux(TM) tested to verify that there are no cracks. If it tests good, then I concur with Sean. Use Loctite(sold in the USA as Permatex) High Temperature Sleeve Retainer under the inner race, & Loctite thread locker on the nut's threads.

If you have to replace it, there should be good used ones out there. Part of the purchase agreement should be that it's returnable if it doesn't pass magnaflux testing. Then have it tested upon arrival.

 

I dug up the product I use here at work:Loctite 660 retaining compound. I do defer to Verel on the magnaflux option, and his suggestion for a high temp Loctite product is probably more better.
Tom B.

 

Great advice Verell,

What do you think about shot peening, if the part passes magnaflux? There has to be an engine builder or aircraft shop that can handle it.

Is it true that the bearing rides partially on the splines??

I don't run race tires yet, but I sure as he11 would hate to loose a rear wheel/hub - might mess up the car

worried a bit,
chris

 

The bearing definitely rides half on the splines but there needed to be some relief to machine the spline . .. . but not as much as there is.

Thinking about how this part gets loaded, race tires would imply more cornering power (limited straight line torque from engine . . .this is the 308 we're talking about ) so I would think the failure would be at the outer flange if it was in fact yielding "due to race tires" . . . there's not a lot of moment arm there though.

If it were going to break from torsional load slipping one's foot off the clutch at a reasonably high rpm in 1st gear would be a non-desctructive (hopefully) stress test perhaps? I can say from first hand experience they seem pretty robust under this kind of loading. However, if the shaft had been weakened from stretching this would be how it would break (I'm theorizing here) so might be a good test before going to the track . At the track I don't think you torsionally load this component like you do in 1st gear.

The inner bearing is the only one that's shouldered in the carrier so it really takes most of the thrust load right? It might fail . .. but the bearing I just pulled off the axle that I reapired in '95 was still thick with grease.

Sean

P.S. I've been pulling close to a 1g cornering for the last few years on repaired axles FWIW . . .if you're just having fun I would think you're okay . . I'll be running new Hoosiers at the upcoming track event so I'll defintely have my senses aware.

 

I would expect the base of the threaded nose where the ring nut fastens onto, to crack(shear) and fail first, than where the bearing sits on the splines (bearing is fully supported width-wise, on the shaft, even though some material is missing due to the spline grooves. Plus, there are no shoulders to act as stress-risers where the bearing is, but certainly the threading for the ring-nut has)... The manual calls for a high amount of torque for that ring-nut (22 kgm or almost 160 lb-ft), so it could stretch it.

Think of how a bolt breaks when tightening it too much. It'll either break right under the head, or where the shank ends and the threads begin.

And, the wheel mounting flange as has been mentioned, have been known to fail.

The only way you'll know for sure it's okay or not is to magnaflux these areas... I'll be doing that to mine once I get them out of the hubs.

 

There were some threads over the last year or so regarding these stub axles breaking, but IIRC they were all Mondials. And yes, they all broke at the outside at the flange to shaft interface point. Basically the flange broke off the shaft. The damage was pretty nasty as I recall, tore up the upright, brake rotor, broke the caliper, and tore up the sheet metal real good among other things. Try a search for Mondial stub axle, you might find it. That shaft might not seem worth $1500, but the damage it can do sure is, and thats if you dont hit anything after you lose a wheel and brakes I agree with Verell on the Magnaflux, but have someone in aviation do it. After what Dave Helms said about it I wouldnt have anyone else but those guys look at it. If its got a crack they would be better at finding it.

I do wonder if the cause could be from bearing R&R that arent torquing the nut properly. If it loosens up or wasnt tight enough initially, there is no telling what loads it could be subjected too.

 

Found it

http://www.ferrarichat.com/forum/showthread.php?t=119568&highlight=mondial+stub+axle

 

heres the other, a bit older, same issue.

http://www.ferrarichat.com/forum/showthread.php?t=64319&highlight=stub+shaft

 

Hello from Germany,

in the meantime I have dismantled the other side and though the bearings were good and the ring nut fairly tight - I needed a breaker bar to undo it - the shaft doesn't look good. Same scoring on the inner bearing's seat and:
Worse; on the highly important radius, where the flange meets the outer bearing seat, there's a significant score.
This will definitely cause stress concentration, or what I read how it's also designated ' stress riser '.
I'm curious to know what can cause this score and I begin to suspect this as the cause of some of the axle failures described in those threads Paul ( Artvonne ) referred to.
To make a long story short; if fatigue is the keyword - and my car has now 110.000 kms ( ~ 70.000 kmls ) you may not see it with Magnaflux. Beginning fatigue is difficult to observe without destroying checking methods.
This is what my collegues, which are deeper inside this business, than me, told me yesterday. I'm myself a mechanical engineer, but have not much to do with such things.
My decision now; Since my parts supplier made me a very good offer on two new axle stubs, I will go for a pair of new ones.

But after some theoretical thoughts I come to the conclusion, that I would never run the car with wheels with a wider offset or worse; track extensions, or worst; both together.
This definitely extends the moment arm from the outer bearing to the point of force application.

I suspect this even more important with early 328s which use the same bearing / axle stub design, but which have an improved seal for the outer bearing, which seems to need more space and therefore the inner bearing sits even further apart from the force application point. Not quite sure, but this what I assume after studying and comparing the parts books illustrations of both.
Since the original 308 part no. 108046 was superceded by 124964 ( just confirmed by FerrariUK some minutes ago ) - which is in fact the early 328 stub - they didn't increase the diameter of the inner bearing races, but only lengthen the seat of the outer race. Obviously for creating enough space for the outer bearing's seal.

Some words about possible stretching. I don't think, that the required torque for the ring nut ( around 220 Nm) will stretch a diameter of 40mm.
The torque will create an axial force - assumed that the thread is dry and not oiled - of around 116 kN. This creates stress of 92 N/mm². This is almost not worthy of mentioning.

Best Regards from Germany

Martin

 

Wow that's scarey . .. I did not know there had been failures on the outside of the axle like that . .. looks like Rifledriver saw this failure happen more than once .

Who knows the history of the cars that had failures (accidents, etc.) but I just found my excuse for being slow at the upcoming track event . .. no wonder guys just go race P cars . .. just when my car was getting fast I've got to think about this?

If I'm to continue down the path I'm heading I have to give this problem some serious thought.

Anyone else want in on a group buy on these things? This is assuming we think this is a life cycle/fatigue issue or does the entire design need scrapping? There is a much better way to set up the bearings in the uprights than the way it is in the 308 but from what I'm seeing it's not the bearing setup that's the failure.


Sean

 

Based on where it's breaking I wonder if magnafluxing is going to give you some indication everything is still okay? If it was a poor design wouldn't there be more stories of a wheel ripping off when someone smacks a curb? That would be way more shock/load than cornering would produce yes? I'm inclined to think it's a life cycle/fatigue issue. When airplane parts are replaced for hrs off use, is there any on set of cracking and fatigue that can be detected with magnaflux or is this replacement way before anything starts to surface?

I called a couple local aircraft guys a month or so back when D. Helms posted about it . .. the one guy I spoke to asked me if I had an inspection method in mind and didn't know what a connecting rod bolt was . .. not very impressive.

Source #2 I have yet to chat with . . . just spoke to his apprentice who seemed "okay".

Sean

 

Re: aircraft mechanics. Some absolutely hate doing any work on automotive parts. As far as a connecting rod bolt, maybe it has a more technical term we dont use, but I doubt it. Find another shop. Maybe its a fastener??

As far as Magnaflux, in the one thread I posted above there was a reference that only an X-ray would catch it.

As regards a new shaft, would it not be possible to upsize the bearings and increase the shaft diameter? Mark, we need your Monster Ferrari ideas here, could these be welded up oversize and have some bigger stuff crammed inside, or would we need whole new uprights sourced or fabricated?? Maybe the P4 parts Norwoods using could be adapted??

 

The inspection guy seemed like he wanted the work . . . he called me back a couple of times . . from chatting with him his experience seemed more inspection of parts while they are still installed like huge assemblies FWIW . . . space shuttle parts, etc. I was being somewhat general when I chatted with him but now I know exactly what I want looking at maybe I'll call him again. Other place is more of an engine shop.

So Xray will identify a possible failure for sure? No way surface inspection will tell you anything? The reason I'm asking this is I got a little deep with the guy about Xray and he made it sound like surface detection would tell you a lot . .. Xray can be somewhat "interprative" he implied but this conversation was a month ago . .. my basic deformation of materials tells me materials begin to fail on the skin etc. so is Xray really necessary? People have magnafluxed cranks the "conventional" way which is only telling you about surface imperfections yes? What would it tell us if there were no cracks in one of my axles which has seen some serious life? Anything conclusive?

So that's my thoughts on inspecting what's on our cars/what we own . . now regarding making something new:

If I was to re-design this part as opposed to just re-produce it one thing I'd do is tear my 911 apart and measure all the cross sections, wheel offset, etc. on it and decide it it is a size issue or material issue. From memory the F car parts are way bigger in this area than the P car. That's why I always thought the problem was a bearing failure. There are some things fundamentally wrong with this upright/bearing design atleast when compared to the way it's explained in "Engineer to Win" or "Prepare to Win" or one of those books by Carroll Smith. This book has the preferred bearing layout to accomodate the loads. But if it's the shaft failing . . it's irrelevant.

Another consideration is to "destructive test" the shaft . .. . see when it breaks under load . .. I'm interested in this 'cause this question needs to be answered. Anyone have a messed up shaft . . I'll pay for the test. We might need a few shafts to test different load scenarios and maybe age? . .. gutt feeling torsion is okay. I'll definitely take the P car apart and compare what's going on back there also.

 

The guy you talked to probably is a NDI tech and they check for cracks and damage you can't see.

also on most aircraft these days there are no conecting rod bolts. well I havent seen any on any of the turbine engine that I have worked on yet.

nothing like a little 6152 shaft HP for you!

 

one thing to consider, the mondial is HEAVY around 3600lbs and uses very very stiff springs in the rear. IRC 950lb or so. I should pull mine off and get them mag'd to see how they are. I've had a wheel come off and I'm running even stiffer aftermarket coils and 400hp.

 

Dave Helms was telling me about some rods he had mag checked at an automotive machine shop. They handed them back and said they were fine. Run em. He then had a aviation shop do it and they found three caps had hairline cracks. The level of knowledge "required" to work on aircraft and approve parts for return to service is enormous. The level of knowledge "required" to work on automotive engines? Zero. There is no law or regulation anywhere as regards crack testing of automotive parts, in either it being required to perform testing, any standard of knowledge of HOW that testing should be performed, or who is qualified. If your going to spend good money having them checked, find the people who know what the heck theyre doing and have them test them. Billy Bob down at Last Chance Motors aint the guy you want checking them for you.

 

Exactly! becuase when your ferrari breaks your stuck on the side of the road, when your airplane breaks you fall out of the sky, Or like I always said about my helicopters "they have the glide path of a anvil"