Homebrew boxer twin prototype

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The fracture surface appearance indicates the crack grew primarily due to bending stresses, not torsion. Perhaps the crank was not straight?
 
Yes!
It does appear to be all in one plane doesn't it?

Dave
 
The crank was off .001 right after it was made. It was 1.125" 4140 stock turned down to 1.047 then cut using offset centers. I wouldnt rule out the possibility of any residual stresses bending it further over time that I wouldnt have remeasured. Hence I did take the time to attempt a stress relieve before making its replacement.

Its replacement is giving me no end of bother in the planning stages. I have the case and bearings installed, and taking measurements (no easy task on the inside) has told me the crank pins were off centered of the bore by ~.080. The results have thinned the center web between the pins down to .090. That, in my opinion, is not enough meat to turn between centers. This is going to require some offset jig almost 2" long, which is a heck of a lot of overhang. I am going to have to think on this one a little bit.
 
Hi Lakc, rklopp,

Actually I've been thinking about this. It's late but I think I've hit upon something, but feel free to tell me I'm wrong.

If the bearings were co-axial and the crank was "bent" then the crank would be aligned to the bearings during assembly, As it rotated, nothing would bend any further. There wouldn't be any reverse cycling. Fatique should not be an issue.

However, if the bearings were NOT colinear, then the shaft would have reverse cycling during rotation

Check the alignment of the bearings!....

Dave
 
Nice theory Dave, and worth noting. I did check the bearing alignment with a 3/8 round HSS tool blank and it slides through like butter. :)
Using my big drill on the cone starter probably was the big contributor to any (axial?) loading, so in effect, it became a victim of my earlier starting problems. The friction rubber on cone allows a significant inbalance to exist at times.
 
Jeff, you might be right there. Without a bearing on the output end to take up thrust and side loads, the starter could have put a hurt on it.
 
Greg, the latest drawing took .1 off the total overhang length of the crank, so a little less leverage might help too.
 

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