CDD uprated drive shafts

[PeteH]

First Look at this thread, I took the photo of the damaged hub up as far as Photoshop will take it. For my "money" I think there is a defect at the 12 o`clock position, the dark line. The fact that it appears to also be at the base of the thread where the shaft is undercut, might indicate a pre-existing "notch". I have seen these before, and are often the result of the Lathe operator overdoing the undercut operation. The base of the undercut should ideally be a smooth unblemished radius which is intended to reduce the possibilty of stress fracture. As Pete Lewis, also correctly observed Post hardening heat treatment is also critical, "case" hardening is a bitch to get right.

Pete

[johny]

Not sure how much case hardening is done as the needle roller bearing surface needs it but not much else. Certainly the threads are easily enough damaged so I doubt they are subject to any hardening. However machining of the keyway and step in diameter after the taper is likely to be the problem as identified by CDD then over tightening (or even just normal tightness) of the nut could start the propagation of a crack....

[PeteH]

The age of the shaft would be a factor. Over time it is not inconcieveable that the Nut be fitted by the time honoured method of Spanner and Hammer, especially back in the days of very expensive Torque tools?.

"We" had Some very large Nuts on Marine engines as you can imagine. In the early days the got "flogged" up with a sledge hammer, later makers would specify torques, and they where pulled up hydraulicaly to "x" Psi/bar.