Appears to be fatigue fracture on outside with heavy wear on the fracture
face causing it to become polished. The step to the central brittle fracture
I have seen a couple times before. Of those previous occurrences at least
two were associated with high inclusion levels in the steel (free cutting
steel) combining with bending stresses. While odd looking it may not be
related to the initiation of the fatigue cracking which is your main
There is no obvious signs of the sort of fracture I would expect from a bent
shaft or eccentrically fitted wheel. The damage to the fatigue fractures
makes it difficult to determine where they initiated, i.e. did they initiate
from defects in the bar, but the most likely place seems to be the corners
of the hex bar.
There may be a seam in the bar on the left hand side of image 1, giving a
straight edge to part of the fracture running along the length of the bar.
It does not appear to be the cause of the failure.
The final fracture area is quite large. The shaft was weakened by fatigue
prior to final failure. Hex bar will fail by fatigue from the corners if
In the absence of other evidence I would be inclined to blame overstressing
of the drive at some time in the past for the failure.
Any other views?
What's up with the hacksaw looking marks on the face of the outer
My guess/senario just to get the ball rolling... an older shaft had
been cut off there and replaced with the new shaft that broke.
Or better... only part of the shaft is new and was welded up there?
Ignoring the "hacksaw marks"... the new shaft isn't a factory/stock
part, but fit... so someone used it for a replacement just long
enough to sell it to you?
No matter whether the hacksaw marks have anything to do with the
failure or not, they need to be explained. :/
Alvin in AZ
They removed the drivers off the old shaft and placed them on this shaft.
I'm not concerned about those marks really.
A welded shaft is definately not a possibility. This shaft that cracked had
no signs of being welded. The dealer could get in serious trouble for doing
that, like loosing their dealership.
I did pay for this replacement shaft that cracked, but warranty replaced
this cracked shaft and many other parts besides this time around.
A hydraulic press is used to remove and place the drivers onto the shaft.
Its probably marks from that. Also, the first shaft that was replaced due to
bending it, it's possible that they cut that shaft to get the drivers off.
It is a tight fit and maybe they couldn't be removed due to the bend in the
Thanks all for replying
You want to know what happened but just so much, not all? :/
If you don't take into account the -whole thing- you will be
jumping to a conclusion. Is that what you want? If yes, say so
now, so I can't stop working with you on this, ok? :/
Ah, that sounds right they cut it with a hacksaw, that made it
easier to pull the shaft out... I wonder how tight the driver was
after the replacement as compared to before... the bent shaft was
pulled (or even hammered?) out though the driver?
Alvin in AZ
The rubbed features on much of the fracture, and the location of the same,
suggest that fatigue cracking initiated at the outer surface; likely at the
corners of the hex form. The line or stepped features in the overload
portion of the fracture may be caused by the presence of either a rolling
seam in the bar or, as mentioned, a high number of inclusions in the steel.
The off center location of the final overload fracture certainly is typical
for rotational bending fatigue. The large final fracture region suggests
that the shaft experienced high levels of applied stress. If the shaft by
design is allowed to bend or flex appreciably as the machine is operated,
this might explain the rotational bending influence observed in the
fracture. However, if the shaft was bent, this could also explain the
off-center location of the final fracture. In any event, the failure
certainly appears to be fatigue related, initiated by cracking that formed
in the outer surface.
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