Originally Posted by
MikeX
The fracture faces show signs of reverse bending fatigue; note that the fracture features are roughly symmetrical about a mid diameter line. If Dawes-man were to fit the broken stubs back into the same cranks and the mid diameter line was roughly parallel to the ground when the cranks are also parallel to the ground (maximum bending stress position) that would tend to confirm my hypothesis. Bonus points if the “wider” part of the fracture is “up”.
MikeX, thank you! I am fascinated by your answer.
This is a photo of the stub in another Campagnolo crank. I can take another in the same crank tomorrow but if their thread cutters are used methodically, the photo would support what you suggest, I think.
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IMG_5322 by
Dawes-man, on Flickr[/IMG]
Originally Posted by
MikeX
If it were my pedals and nuts, I’d
1) Polish the pedal shafts in the area where the fractures are occurring to a 600 grit or better finish to minimize the stress risers and make subsequent inspections easier. Various abrasive papers and cloths make that reasonable even if you have to use them “shoe shine” method. Better if you can run the final polish marks parallel to the spindle rather than circumferential.
I remember a friend saying that racing engine con rods are often polished for strength. I've just done that using an electric drill at slow speed and 400, then 600, then 1000 grit abrasive paper wrapped around the spindle and held by hand. I finished it off with longitudinal polishing with 1000 grit. This is the result.
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IMG_5325 by
Dawes-man, on Flickr[/IMG]
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IMG_5324 by
Dawes-man, on Flickr[/IMG]
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IMG_5323 by
Dawes-man, on Flickr[/IMG]
I noticed while polishing around that there seemed to be faint longitudinal ridges in the metal. Polished, you can see faint circumferential nicks and I'm wondering of I should polish until they are all gone.