<VV> crankshaft cracking - breaking nore

[BBRT]

 I suggest Porsche is not the best example of large journal radius. The 
other pictures are better.
The Porsche cranks have a main journal for each throw, narrower bearings and 
small radii on the journals. It is the decision to provide one rod per 
journal between main journals living with thin bearings and w/  thin block 
web structures plus other aspects, such as heavy but balanced rods that 
yield strong bottom ends.  Porsche recognized the hvy rods used, resultant 
crank stresses in competition uses and offered Ti rods early on for racing 
purposes (and I believe Aluminum too).
For what it is worth, I have seen Corvair cranks break at the main journal 
near #6.

Chuck S

----- Original Message ----- 
From: <djtcz at comcast.net>
To: <virtualvairs at corvair.org>
Sent: Sunday, February 18, 2007 5:48 PM
Subject: <VV> Re: crankshaft cracking - breaking -part 2


> Each of the broken cranks in these pictures has a radius ground at the 
> edge of the rod bearing journal to transition into the crank throw.  Some 
> are better than others.  The biggest radius that will fit is barely good 
> enough, which is why some bearings for race engines have heavily chamfered 
> edges, to allow larger than stock journal radii.  A radius adjacent to a 
> step or corner isn't very good.
>
> Folks who magnaflux cranks can report that faint indications start in the 
> underside of a rod journal, usually toward the flywheel end of the crank. 
> Magnaflux is so sensitive that it can detect crax that are still shallow 
> enough to gently grind out. When the cracks progress enough to be bright 
> and maybe longer than 3/4 inch then multiple jounals are likely to show 
> indications.
>
> There are shapes and configurations that can be machined in to a crank to 
> reduce the peak stress in the rod journal radii.  The correct hole drilled 
> to core out the rod journal, or beveling of the throw or arm are examples. 
> It is curious that a part can often be made "stronger" (less stiff, but 
> more fatigur resistant by virtue of improved stress distribution) by 
> removing material to a degree that would otherewise require a big upgrade 
> in material and heat treatement.
> http://www.masquito.be/images/crnkwear.jpg
>
> Here is how Porsche did it in the 911 series engines, but with magnificent 
> materials and heat treatment
> http://www.stomskiracing.com/sr021.jpg
> http://www.adelgigs.com/graphics/pfiles/14u.jpg
>
>