Yet another failed axle bolt

[Snakebitten]

That's almost more confusing to me based upon my cave man understanding of floating axles. Isn't the entire point of the "3/4 floating axle" to have more bearings in order to better resist the loads of heavy towing/hauling? My recollection is that the bearing are also conical, such that they should resist sliding along the shaft axis in either direction. If Ford put a whole other bearing into the axle, how does that result in a problem keeping it form sliding along its axis vs the 1 bearing of the regular axle? Ford also does not have this issue on Superduty trucks, so clearly they know how to design an axle. Is this axle just an example of a hybrid manifesting all of the flaws and none of the virtues of its parents?

I honestly know a lot less about axles and design than you do because I really don't have a good understanding about what "floating axle" means, nor the difference in 3/4 float vs full float.

What I do feel like I now know something about is that bolt and what it's purpose primarily is. I have also spent a few hours messing with a sheared bolt axle, along with the spline fretting that results from not discovering the sheared bolt immediately. Once it frets enough, the "press fit" of the axle into the hub is lost and the axle can be easily pushed inboard and pulled outboard with very little effort. Again, the maximum travel distance from end to end is about an inch.

Until Ford or some expert source comes along and officially explains the failure, I can only go by the evidence that I have experienced myself, up close and personal. And I've said it enough that I'm sure it will just draw eyerolls, especially from folks that need it to be more complicated. But I'm very suspect of the quality of the metalurgy of the Bolt itself, as well as the possibility that during assembly it is being over-torqued.
The one other attribute that I do leave room for being an influence would be the tolerances/flex of the hub bearing.

I also am strongly influenced to believe that the vast majority of the trucks that are affected, were built in a date range. The frequency of sheared bolts today appear to be a trickle compared to when the dilemma reared its head.

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[FaaWrenchBndr]

That's almost more confusing to me based upon my cave man understanding of floating axles. Isn't the entire point of the "3/4 floating axle" to have more bearings in order to better resist the loads of heavy towing/hauling? My recollection is that the bearing are also conical, such that they should resist sliding along the shaft axis in either direction. If Ford put a whole other bearing into the axle, how does that result in a problem keeping it form sliding along its axis vs the 1 bearing of the regular axle? Ford also does not have this issue on Superduty trucks, so clearly they know how to design an axle. Is this axle just an example of a hybrid manifesting all of the flaws and none of the virtues of its parents?

The theory behind the floating axle is to have the bearings & the hub support the weight of the load, in this case the truck and its payload.

The axle shaft itself simply transmits the rotational torque to the wheel. If the axle shaft breaks, it’s no big deal. The wheel doesn’t fall off, worst case scenario the forward drive is just lost.

With a standard axle shaft, like the non-max two models, without the bolt, if the axle shaft breaks near the flange, the wheel can depart. The shaft supports the load and provides the rotational force.

 

[Buyer2021]

Is this [3/4-float 'Max Tow'] axle just an example of a hybrid manifesting all of the flaws and none of the virtues of its parents?

Might be the case!

 

[amschind]

The theory behind the floating axle is to have the bearings & the hub support the weight of the load, in this case the truck and its payload.

The axle shaft itself simply transmits the rotational torque to the wheel. If the axle shaft breaks, it’s no big deal. The wheel doesn’t fall off, worst case scenario the forward drive is just lost.

With a standard axle shaft, like the non-max two models, without the bolt, if the axle shaft breaks near the flange, the wheel can depart. The shaft supports the load and provides the rotational force.

Yes, but....

...if the 3/4 float has conical bearing surfaces which prevent movement of the axle or a piece thereof along its axial plane, why then does the 3/4 float need a bolt under apparently large amounts of tension to remain in place while the standard axle does not? If the standard axle had a bolt that kept breaking, that would make sense to me. The 3/4 float Max Tow axle seems to have numerous advantages over the standard axle in terms of resisting unwanted slippage along its axis, and yet it not only NEEDS a bolt to prevent such movement, it also continues to snap said bolt in half. I keep pointing this out because there is a big hole in my understanding of this system, and I'm trying to grasp why 1) I can't understand it and 2) FORD can't understand it. If Ford is struggling, I fear that the answer may not be simple. I am NOT inclined to believe Snakebitten's theory because if it were simply a batch of bad bolts, then we should not see folks coming back for multiple replacements; if Ford's replacement bolts are also failing in the same manner, then the problem has to be more complex.

 

[Snakebitten]

The number of repeat shears is incredibly tiny, statistically speaking.
But I agree that even having one replacement axle assembly shearing a bolt is bizarre.
Why would the vast majority of axles not shear a bolt, yet even one truck shear the original and then the replacement?

As whether it's related to the bolt and/or the torque itself being the culprit, I can only work with what I have to work with. If my truck, which DID shear one bolt, doesn't ever shear the replacement bolts, then I will just celebrate my good fortune and have to live with not being able to prove anything. ?

 

[SilverPigeon]

Well, I can now stop asking questions about Christmas plant closures and parts bins. One of the trucks that Ford Australia imported (they are all non-PB max tow) has now had a sheared axle bolt. Build date and plant: Feb '23, Dearborn ?

 

[HammaMan]

Yes, but....

...if the 3/4 float has conical bearing surfaces which prevent movement of the axle or a piece thereof along its axial plane, why then does the 3/4 float need a bolt under apparently large amounts of tension to remain in place while the standard axle does not? If the standard axle had a bolt that kept breaking, that would make sense to me. The 3/4 float Max Tow axle seems to have numerous advantages over the standard axle in terms of resisting unwanted slippage along its axis, and yet it not only NEEDS a bolt to prevent such movement, it also continues to snap said bolt in half. I keep pointing this out because there is a big hole in my understanding of this system, and I'm trying to grasp why 1) I can't understand it and 2) FORD can't understand it. If Ford is struggling, I fear that the answer may not be simple. I am NOT inclined to believe Snakebitten's theory because if it were simply a batch of bad bolts, then we should not see folks coming back for multiple replacements; if Ford's replacement bolts are also failing in the same manner, then the problem has to be more complex.

I suspect that the axle being firmly bolted to the flange is due to moment loads (red). The bearings handle rolling and axial forces (blue), but moment loading needs the axle's length/lever. To handle moment loads the bearings need much more space between them as well as proper loading. Why is the design better for the front end than the rear? I suspect the forces during towing with the authority a trailer has combined with acceleration forces must warrant it? The front bearings are likely configured in a manner to handle the forces that are predictable in turns, as in forces in a turn are always in one direction combined with camber angle, whereas in the rear it can be moving back in forth while its always flat.

 

[FaaWrenchBndr]

Yes, but....

...if the 3/4 float has conical bearing surfaces which prevent movement of the axle or a piece thereof along its axial plane, why then does the 3/4 float need a bolt under apparently large amounts of tension to remain in place while the standard axle does not? If the standard axle had a bolt that kept breaking, that would make sense to me. The 3/4 float Max Tow axle seems to have numerous advantages over the standard axle in terms of resisting unwanted slippage along its axis, and yet it not only NEEDS a bolt to prevent such movement, it also continues to snap said bolt in half. I keep pointing this out because there is a big hole in my understanding of this system, and I'm trying to grasp why 1) I can't understand it and 2) FORD can't understand it. If Ford is struggling, I fear that the answer may not be simple. I am NOT inclined to believe Snakebitten's theory because if it were simply a batch of bad bolts, then we should not see folks coming back for multiple replacements; if Ford's replacement bolts are also failing in the same manner, then the problem has to be more complex.

The taper bearings ONLY support the hub. The axle shaft is supported by the hub on the outside and the differential on the inward side.

It’s because there is a design flaw. Whether it be in machining tolerance or material. The shaft and hub splines loosen. When this starts, the bolt shears from rotational stresses.

The bolt simply holds the axle shaft outward into the hub. If it breaks, the shaft can migrate inward. Doing so leads to less spline engagement, shearing the splines at the hub to axle joint.

 

[amschind]

The taper bearings ONLY support the hub. The axle shaft is supported by the hub on the outside and the differential on the inward side.

It’s because there is a design flaw. Whether it be in machining tolerance or material. The shaft and hub splines loosen. When this starts, the bolt shears from rotational stresses.

The bolt simply holds the axle shaft outward into the hub. If it breaks, the shaft can migrate inward. Doing so leads to less spline engagement, shearing the splines at the hub to axle joint.

So in theory, the axle itself could overtorque the bolt. I.e. small amounts of axle rotation within the hub due to spline tolerances/play could slowly ratchet the bolt tighter until it is overtorqued and snaps? If that's the culprit, I find it odd that we see only snapped bolts and not bolts that worked their way out, but I'm not sure.

 

[Snakebitten]

So in theory, the axle itself could overtorque the bolt. I.e. small amounts of axle rotation within the hub due to spline tolerances/play could slowly ratchet the bolt tighter until it is overtorqued and snaps? If that's the culprit, I find it odd that we see only snapped bolts and not bolts that worked their way out, but I'm not sure.

That oem bolt has some serious thread friction. AND comes pre-prepared with red thread lock.

It's pretty much "siezed" intentionally.
(In my opinion)

I have often wondered how many of them would just snap, rather than come out intact, if everyone attempted to remove theirs today?

 

[Buyer2021]

The whole situation remains a great mystery, apparently defying traditional physical and statistical analysis.

Quantum physics in play?

 

[Snakebitten]

The whole situation remains a great mystery, apparently defying traditional physical and statistical analysis.

Quantum physics in play?

The most amazing thing about it is that when it first popped up and grabbed the headlines, so to speak, I experienced a certain level of fear & dread.

Fortunately I wasn't one of the earliest shear'rs, so I avoided the painful experience that Ford and/or the dealership could put the customer through. By the time I sheared a bolt, it had developed into no more than an unfortunate inconvenience.

At this point I think it's pretty clear that I worry NOT! ?

Perhaps my bolt approach will prove to be folly, but worse case scenario I will extract the bolt and put another one in. But I'm kinda optimistic that no bolt of mine will be given near the opportunity to fail. $8 every 5-10k miles for a fresh bolt per axle.

 

[amschind]

That oem bolt has some serious thread friction. AND comes pre-prepared with red thread lock.

It's pretty much "siezed" intentionally.
(In my opinion)

I have often wondered how many of them would just snap, rather than come out intact, if everyone attempted to remove theirs today?

That makes a ton of sense, but if that were the issue, why didn't Ford just issue a different bolt and/or no thread locker? Perhaps the issue is that in order to avoid being overtorqued by the rotational play of the axle/hub splines, the bolt needs to be basically seized into the threads, BUT that condition prevents the bolt from adapting to some other stress which snaps them.

This is all rectally sourced speculation, so take it for what it's worth.

 

[Buyer2021]

At this point I think it's pretty clear that I worry NOT! ?

All in with that

For whatever reason even though I do a fair amount of towing I've still not suffered a bolt failure now at ~13k miles.

I don't even bother with 'knocking on wood' any more .... if it happens it happens and I'll let the dealer do their thing with it, whatever that is whenever it happens.

Sure, when the 'final remedy' is available, I'll jump on that.

 

[Snakebitten]

I don't even bother with 'knocking on wood' any more .... if it happens it happens and I'll let the dealer do their thing with it, whatever that is whenever it happens.

You had a lot of influence on my own transition from WTH, and What-If........ to whatever. ?

It'll be so interesting to finally see what Ford's solution turns out to be.

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