Crankshaft material choice for durability 2

[strokersix]

I received some good general information regarding material internal damping chracteristics in the vibrations forum. I'm after some more specific information related to the actual application so I'm posing the question here as well.

In 1963 and 1964 General Motors inline six 292 inch displacement seven main bearing crankshafts had six counterweights and were forged steel. Later '60s vintage crankshafts were also six weight but nodular iron. Starting around 1968 until the present the crankshafts have 12 weights and are nodular iron. I'm told the nodular crankshafts are more durable than the steel ones and I have a difficult time believing it. So, my question is two-fold: First, assuming an equivalent geometry, is it possible for a nodular iron crankshaft to be more durable than a forged steel one? And second, what are the structural, torsional harmonic, and durability differences between a six counterweight and a 12 counterweight crankshaft design? All factory applications I'm aware of use an elastomeric damper of typical construction.

If anyone has specific knowledge of the engine in question that would certainly be welcome as well.

 

[Metalguy]

Assuming the crank would eventually fail via fatigue cracking, from higher-than-stock speeds/loads, then a good forged steel crank will last longer, esp. one made from something like 5140 or 4340 "clean" steel.

Look at what race engines use.

 

[Tmoose]

About the only time damping does >>ANY<< good is when resonances are being excited. I figure detonation will tend to excite all kinds of resonances. Simply running at the &quot;wrong&quot; rpm will excite the crank's torsional resonances. Material damping seldom exceeds .03 which although 3 times better than 0.01, but is still a tiny number. A fat juicy fluid torsional damper would seem to be the better weapon against high stresses resulting from excessive crank wind-up.

Even if the extra motions from resonance are eliminated, high power and/or high revs imply high stress in small areas of the components.

If you are making big power so the crank is simply being pushed and twisted hard, then the material endurance limit is what will make it last (endurance limit is very generally related to UTS which is VERY generally related to hardness).
High revs increase rod bearing loads, which in turn the mains may have to resist unless the counterweights are serious. I believe at some point the counterweights serve to reduce direct main bearing loads (which some mfrs have used for fuel economy too) and by reducing slight crank distortion also reduce edge loading (always good).

As you mentioned, the geometry is way important. Crummy radiuses or heat checks from grinding or scratches can raise local stresses in the radius over 3X. A 3X increase in material endurance limit would be a miracle. Compression inducing processes can be mechanical, like rolling, shot-peening or chemical, like nitriding. Putting the tiny hard working radiuses in compression can improve fatigue

Cast crank materials vary enormously - A few cranks are pretty basic iron. Some of the nodular irons (Pontiac Arma-steel?) have some impressive numbers.

 

[strokersix]

Metalguy and Tmoose,

The feeling among racers seems quite strong that the steel cranks are less durable. There is almost certainly some truth in that experience but the stated explanation of internal material damping just doesn't seem to fully explain the issue. All with engineering background, including those on this site, seem to agree that the steel crank should be more durable. There must be another explanation. Thanks to the both of you for offering possible explanations. The GM steel (1035 ???) may have had high defect levels or perhaps secondary machining process issues (heat checks, radii) both seem more plausible explanations for the observed durability difference between the steel and nodular cranks.

If anyone can confirm what carbon level and/or alloy composition GM used in the '60s for steel cranks I am interested. Likewise for the cast cranks.

Thank you.

 

[enginesrus]

It must be a stress and fatique deal. Since cast iron does have a low notch sensitivity. Also inclusions can cause sub surface cracking. I would think the nodular iron would be more forgiving.

 

[Metalguy]

There is a world of difference between a stock Chevy carbon steel crank and a high-perf. alloy one. BUT, now that I reread your post, you're running a 6 cyl. So, IF experience has shown that the cast iron cranks live longer than a carbon steel one, I'd go with them. I'd also want to remove the crank and have a good Magnaglo exam. done every so often-depending on your power/rpm levels.

I suspect you're correct, the stock Chevy steel cranks could have lots of NMI's (non-metallic inclusions), since for that type engine Chevy probably didn't see a need for anything cleaner and more $$$.

 

[evelrod]

strokersix, this is just my two cents worth---in the 28 years that I built and raced Lotus twincam engines I only had ONE crankshaft failure, a forged steel Vegantune unit with EVERYTHING counterweighted (it weighed a ton) that came from a Ford factory ralley car around 1967. After that I went back to cast iron cranks suitably 'nitrided' and prepped be Henry Velasco for timing and such. Very light weight and each was good to ~10,000 rpm and a full season. Some even lasted several seasons before signs of distress were noted. I know that current wisdom tends toward forged but I will stick with cast iron/nodular cast cranks until I encounter failure problems. &quot;If it ain't broke, don't fix it!&quot; kinda deal.

Rod

 

[motorsportsdesign]

What is the typical failure of the six and twelve CW crank?

If you could post a photo of the 6 CW crank I would be interested to see it.

Jonathan T. Schmidt

http://www.motorsportsdesign.com

 

[strokersix]

I do not know what the typical failure is. The only catastrophic failures I've had have been connecting rod (rod bolt most likely) and piston failures. Here's a photo of an early six weight steel crank:

http://users.qconline.com/users/friestad/crank.JPG

 

[comnean]

Hy,
For durability FCD 600 is used by most of engine makers (cast iron and huge serial production). Mechanical best results can be applied with precision forged steel alloy cranckshafts.

 

[motorsportsdesign]

Did I understand you to say that the six CW cranks were made in both iron and forged stell and the iron version has a reputation for longer life?

Jonathan T. Schmidt

http://www.motorsportsdesign.com

 

[strokersix]

Jonathan,

Yes you understand correctly. I personally have run all three crankshaft types without failure. I have used a fluid filled damper in all cases. I have been told the preferred crankshafts in order best-worst are: cast 12 weight, cast 6 weight, steel 6 weight.

A possible explanation in another post was dirty material (inclusions) in the steel crank or perhaps a processing problem such as small journal radii or grinding burns. These possibilities seem more plausible to me than the material damping explanation.

Mike

 

[motorsportsdesign]

What criterion is being used to determine best/worst?

Journal wear?

Jonathan T. Schmidt

http://www.motorsportsdesign.com

 

[patprimmer]

One explanation of why cast iron will outperform steel on SBC engines, stems from the fact that the people experiencing this are useingsecond hand cranks of unknown history.

Most second hand factory steel SBC cranks come from truck motors, whereby most cast cranks come from passenger sedan motors. The truck crank has most often seen much more severe use prior to being installed in the race engine.

I expect that the 12 counterweights more than offsets the material choice.

In my opinion, a good steel crank will always outperform a similar cast iron crank.

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Metalguy]

Agree with you, Pat. Somewhere I've got an article that compared bearing life under close-to-marginal lube conditions. The steel crank was superior by quite a bit.