We seem to have a pretty good picture of where these things fail. It's
inboard of the hub, between the hub and the shoulder, sometimes inside
the bearing, sometimes outside. We will diminish this problem by
induction hardening this area. We will not harden the tapered area,
because the metallurgist who is giving us advice has personal experince
with failures emanating from keyway corners in hardened parts and
advises strongly against it.
The failure that puzzles us a little bit is Jeff Snook's, which failed
inside the hub, at the stake marks.
I'd like to advance a theory on this one, but it is a theory that can
hardly be verified. For the axle shaft to fail here, the logical
starting place is the stake marks, which provide an ideal starting point
for a fatigue failure. Why were there stake marks there? Probably to
prevent the key from riding up the keyway machining radius while the hub
is being installed. If the key rode up the radius, the hub would still
tighten down and you could still apply lots of torque to the nut.
However, there would be an air space between the hub ID and the axle.
This would allow axle bending within the hub.
As a result of this thought process, I recommend that we assemble axles
differently in the future. We should install the hub on the axle and
then tap the key into the square keyway. A small chamfer ground on the
end of the keyway will help it start.
Opinions?
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