Re: Broken axle

Posted by Fish'n Jim On 2018/4/16 11:34:00

It's rather apparent why they got away from this design, ie, catastrophic failure possible, and we're talking decades of service later from that change.
I don't know if one can determine what exactly happens after that long without a lot of testing. A good "fat chew" subject but I'd put it in the "worn out" basket and move on. Be prudent when driving one of these old beauties.
Fatigue failure indicates was subject to cyclic(recurring) stress, such as each time the axle went around it was doing some sort of "flexing" that wasn't good for it and "pushing" on that end. The force causes the metal to "move" at the atomic scale, and according to dislocation theory, lattice "vacancies" migrate to the grain boundary, align, and cracking initiates. It'll continue as long as the stress is applied and until the cross section is narrowed to below the yield stress can support, then it shears off - catastrophically.
That force can be caused by combined or separate issues; bearings, housing/surfaces worn beyond tolerances, wheel balance, tire issues, how its driven, etc not just design. Maybe some teenager gave it a "good" holeshot, who knows?
A harmonic analysis might show some things, but who's up for that? One has to start way back at the driven end of the axle. Every shaft has a frequency where it reaches destructive harmonics. You can not operate there. Which lead to a whole host of problems to the other parts as well. There may be some archived studies from the manufactures but I'm not aware of or how to access or if they were even shared(for legal reasons) with SAE or other investigative sources.

These cars were made in the daze when they still thought left hand threads were needed, so what other poor design assumptions were made? Brand fondness out paces logic and rationality sometimes.
Taper shaft connections are widely used, even today, so I don't think there's any inherent problem. Just may not be a good application here or improper size. If you look at what changed over time, ex Packard, they put a flange on the axle end for tire connection, so that end strengthening says, there was weakness and force issues. Always look at how something evolves to understand the past.
Compare to a conventional front or trailer spindle, that connects similarly(castle nut on threads tensioning bearing races), and a tapered shaft but those cantilever shafts don't rotate. They seem to work just fine and don't require much torque nor retorquing between bearing repacks. So rotation with that type of "connection" is apparently problematic.
ps: Ross, was there a big WTF with that gape? LOL