<VV> RE: VirtualVairs Digest, Vol 25, Issue 72

[Regan]

Today's Topics:

   1. crankshaft cracking - 

	Corvair crankshafts (8409) forgings have a tendency to crack because
they have a small crankpin overlay due to small journal diameters and stroke
length. That's why the early cranks never had a cracking problem. Also they
are not fully counterweighted like the Porsche so it's kind of like
comparing apples to oranges. Although, as someone stated earlier the
nitrided cranks have a much greater resistance to cracking as the
experimental airplane group finally figured out. I always wondered how long
it would take them. Not to pick on them but when I was building engines back
in the 80s I found that about 40% of the standard 8409 cranks I magnafluxed
had cracks in #6 rod journal and none of the nitrided cranks were ever
cracked. It was quite obvious which crank was the stronger one. I also
noticed that they all did not have the same consistent radius which was
common on GM stuff back in the 60s. I guess the moral to this story is if
you want the strongest 8409 crankshaft use a std/std nitrided one with the
best radius you can find and good luck because they're starting to become
nonexistent. A re-ground shaft isn't bad if it has proper radii and is
re-nitrided but it also has a decreased crankpin overlay.


REM 


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Message: 1
Date: Sun, 18 Feb 2007 20:37:28 -0500
From: "BBRT" <chsadek at comcast.net>
Subject: <VV> crankshaft cracking - breaking nore
To: <djtcz at comcast.net>,	<virtualvairs at corvair.org>
Message-ID: <001001c753c6$843b5760$86c33e47 at DCN6FF61newbond>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
	reply-type=original

 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
>
> 



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