View Single Post
Old 12-09-2009, 03:05 AM   #437
The Road Scholar
BMWzenrider's Avatar
Joined: May 2003
Location: CheeseHead Land...
Oddometer: 1,114
Axle Failure Analysis

Originally Posted by Abenteuerfahrer
Yes, the shaft sheared flush and clean...maybe bad metalurgical composition?

Yes, we were scared shitless going 60 mph down a clean hill and suddenly boommm...crash...wheel flew off...swing arm scrapped the road..sparks a flyin'...Sharon putting her trust to her man controlling this wild bull. Didn't apply brakes of any kind, just let the friction of the sidecars swing arm do the slowing down then applied the rear brake slowly. We were praying that the swing arm would not dig into the soft asphalt surface as this would have us catapulted into either deep ditch on both shoulders. You can google the coordinates where we were stranded for 6 hours...high in the mountains...
Coordinates: N 60 15.565 W 136 58.145

Sorry that I missed this excellent RR thread. I was on the road at about the same time and was not keeping up with stuff online.

Let me start by saying that a rotating axle supported inboard of the wheel is not in and of itself a bad concept.
It has been done sucessfully for many years and a great many miles on LOTS of vehicles, including some pretty ourtrageous off-road racers in Baja and Dakar.
What do you think a driven axle is???

However, as a Mechanical Engineer with a background in machine design and looking at the total failure of the axle in question that occured, it sure appears to be a case of faulty design in this case.
I can clearly see fatigue failure propagation of the metal in the axle at the fracture site in the photos that were posted.

Let me explain briefly.

When you bend metal back and forth repeatedly you create fatigue stresses in the crystaline structure of the metal. If the stress is below the elastic limit of the metal no permanent deformation of the metal occurs and all is fine, it just goes back to its original shape like a spring.

IF however, the stress in any part of the metal get close to the plastic deformation zone for the material, that section will be permanently deformed and cause a change in the matrix of the crystal stucture of the metal. When it gets bent back the other way the process repeats. When you deform steel in this way it "work hardens" or becomes more brittle.
If you keep bending the part back and forth eventually the metal begins to tear apart, and there is less and less metal to resist the bending, which just makes the stress in the remaining material even higher, and the rate of decay faster.
Eventually there is not enough material left to resist the bending force at all and it lets go all at once.

the telltale signs of this type of failure is an area of shallow smoother cracking near the surface of the part, often times looking like tree ring type growth working from the surface inward. Followed by a stubbly brittle failure of the center of the part at the point of rapid failure.


{fun for the home viewer}
You can test fatigue failure yourself with a paperclip.
If you open it up and start bending it back and forth at one point you can see a crack starting to form before it eventually breaks in half.
And you don't have to bend it 90 or 180 degrees to make it happen. You only need to bend it in the same spot just enough to make a permenant bend in the metal. The more angle of bend the fewer bends it will take, but even at only a 1-degree permanent bend each way in that paperclip you can get it to break in two if you keep doing it long enough.

Try it!
Bend one paperclip back and forth 90-degrees and count how many times it takes to break. Then repeat that at 45-degrees, etc...


NOW, take a look at WHERE the axle broke. There is a sharp shoulder/step right there!!! BAD DESIGN!!!!!!!
Any sharp change in shape creates a large rise in the stress of the material at that point. That is why the ID of bearings have a radius on them, to allow the step on the spindle to have a RADIUS!!! That lets the stresses flow smoothly around the transition from one shaft diameter to the next and reduces the stress riser.

{To test this effect, try bending one smooth paperclip back and forth, and then bend one of the ones that have the 'notches' in them back and forth right at one of the notches and see how much faster it fails.}

Then to put that sharp stress riser just outside of where the axle was being supported put the weakest part of the shaft in the position seeing the highest possible bending stresses.

Now think about that axle rotating at hundreds of RPM's for hours per day and think about how many times it was bending back and forth every time it went around....

It wasn't just forseeable, it was inevitable!


The reason why a fixed spindle for a non-driven wheel is easier to get right with less effort/design is that it doesn't really ever see reversing stresses like a rotating shaft does. It only ever gets pushed on from one direction, up from below.
That GREATLY reduces the fatigue stressed induced into the spindle, making the design much less critical.

A rotating shaft with any kind of transitions will always need to be larger in cross section than a fixed spindle for the same wheel loads because of the reversing fatigue loads induced by the rotation of the axle.

With proper material selection, correctly designed radius' at transitions and sufficient thickness of axle to keep the stresses outside of the fatigue limits this could have been prevented.
THAT is why people who build stuff that puts life and limb in jeopardy REALLY need to hire someone who knows how to do the calculations and not just turn a machinist loose to create something that "looks" stout...


The repeated bearing failures followed by the seal failure of the third swingarm (indicating runout of the axle) would lead me to think that there is another problem at work here as well.
My first thought would be to look at the bending loads on the axle as designed. It could simply not be robust enough for the given loads, causing the axle to flex under load causing bearing misalignment. That would account for the runout issue causing the early seal breach as well.
Or it could be that the shell tube is not strong enough to hold the outer races in alignment properly when under load, again leading to premature bearing and seal failures.
But without actual numbers, I would just be guessing.
Karl Kugler
2005 BMW R1200RT w/Hannigan-LT sidecar
2002 BMW R1150RA
In Memoriam: Harley, 1993-2010 You will be missed.
BMWzenrider is offline   Reply With Quote