Crankshaft material choice for durability
Crankshaft material choice for durability
(OP)
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.
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.





RE: Crankshaft material choice for durability
Look at what race engines use.
RE: Crankshaft material choice for durability
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.
RE: Crankshaft material choice for durability
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.
RE: Crankshaft material choice for durability
RE: Crankshaft material choice for durability
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 $$$.
RE: Crankshaft material choice for durability
Rod
RE: Crankshaft material choice for durability
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
RE: Crankshaft material choice for durability
http://users.qconline.com/users/friestad/crank.JPG
RE: Crankshaft material choice for durability
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.
RE: Crankshaft material choice for durability
Jonathan T. Schmidt
http://www.motorsportsdesign.com
RE: Crankshaft material choice for durability
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
RE: Crankshaft material choice for durability
Journal wear?
Jonathan T. Schmidt
http://www.motorsportsdesign.com
RE: Crankshaft material choice for durability
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.
RE: Crankshaft material choice for durability