Homebrew boxer twin prototype
- Thread starter Lakc
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Lakc
Well-Known Member
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.
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
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
Lakc
Well-Known Member
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.
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.
Lakc
Well-Known Member
Greg, the latest drawing took .1 off the total overhang length of the crank, so a little less leverage might help too.
