<VV> Re: crankshaft cracking - breaking -part 2

[djtcz at comcast.net]

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

--
Dan Timberlake

-------------- Original message -------------- 
From: djtcz at comcast.net 

http://home.hiwaay.net/~langford/corvair/crank.jpg
http://www.elmnet.net/~lange_c/Talon%20Engine%20Rebuild/crank1.jpg

The maximum stress in a crankshaft is usually the underside of the rod journal in the radius.  These  Corvair  cranks' "arm" broke, but inspection of the failure would show the crack started in the rod journal radius and with each power stroke fatigued in a series of ratcheting increases until the remaining area was just too small.

http://www.bmbikes.co.uk/photos/museumphotos/Broken%20R60-6%20Crank.jpg
http://www.rotaryeng.net/broken-crank.jpg
http://www.elmnet.net/~lange_c/Talon%20Engine%20Rebuild/crank1.jpg

Before considering a crank for re-use or rework Magnafluxing is a good way to detect cracks starting in the surface, where most fatigue crax start.
"Ringing" cranks to detect cracks only detects the ones that are critically wounded.




--
Dan Timberlake

-------------- Original message -------------- 
> Message: 3 
> Date: Tue, 13 Feb 2007 15:21:29 -0800 
> From: Tony Underwood 
> Subject: Re: Hardened, (or Nitrided) Crankshafts 
> To: 
> Message-ID: <6.2.3.4.2.20070213150340.02c5a4f8 at mail.roava.net> 
> Content-Type: text/plain; charset="us-ascii"; format=flowed 
> 
> At 01:59 PM 2/12/2007, R. Stalder wrote: 
> >I'm new to corvairs so this is my first response to a question on 
> >VV. But I recently learned some stuff about cranks while building 
> >my late model turbo engine. So this is what I know, hope it 
> >helps. If your crank was originally nitrided from the factory, it 
> >will have "&" type symbol stamped on the flywheel end. If your 
> >crank has been ground it will need to be nitrided again. Nitriding 
> >hardens the surface of the crank. Grinding removes the hardened surface. 
> > 
> >However, I was told not to worry about nitriding my crank unless I 
> >was racing or planned to due a lot of hard driving. A non-nitrided 
> >late model crank would be fine for normal daily driving. 
> 
> 
> A non-hardened crank will work fine for just about any race usage as 
> well unless you get all freaky with supercharging and nitrous 
> etc. Even then there's not THAT much additional durability that 
> nitriding will actually offer up. It does help keep the crank from 
> getting scratched up if there's trash in the oil etc. In this day 
> and age, that alone is enough reason to go ahead and use a hardened 
> crank IF there's one readily available, but it's still just skim 
> icing on a large cake for all intents and purposes. 
> 
> 
> By the way: 
> 
> Remember that the *Entire* surface of the crank is hardened, via 
> nitriding. Of the Corvair cranks that I've seen actually break 
> (seen several and they all came out of 140s), NONE broke at a 
> journal, they all broke at an arm/throw. Thus, turning the journals 
> and removing the nitriding would NOT have been responsible for 
> causing those cranks to break. Now if there's a crank that was 
> severely stressed and it broke at a journal, maybe that one might 
> have been "saved" by the journals having been nitrided etc. Or 
> not. Who's to say that it wouldn't have broken at the journal 
> whether it was nitrided or not? 
> 
> ;) 
> 
> I'm still of the opinion that the nitriding is more useful in 
> protecting journal surfaces than actually keeping the crank from 
> breaking. Surface hardening as a preventative measure to combat 
> cracking is as much wishful engineering intent as it is actual 
> experience... no more, no less. 
> 
> ...still, it wouldn't hurt anything if the crank *was* hardened... 
> 
> 
> 
> tony.. 
> 
> 
>