Posted by Dennis Novotny (22.214.171.124) on 17:38:22 25/02/05
In Reply to:Porosity influence on crankshaft journal posted by Florian
Earlier in my career, I did bearing applications engineering work for Clevite Industries. During this period of my career, I did a lot of work with OEM's with regard to bearing applications work as well as technical trouble shooting.
Surface imperfections are something bearings are designed to accomodate. Babbit has perhaps the best conformability but the lowest fatigue limits.
On the other hand, overplated cast copper-lead bearings have a very high fatigue life but do not have the conformability unless they are overplated with lead or tin based babbit or lead-indium.
Again, the conformability the bearing can provide will depend on size of the imperfection on the journal and/or the "surface action" that the bearing surface can/will provide.
Surface porosity as a single entity is hard to quantify as being acceptable or not. Reason: a small porosity "bubble" may work fine under moderate loads but cause a problem when loads are extremely high and oil films are much lower in thickness.
The OEM's I worked with pretty much all had porosity limits that could be tolerated. However, they varied from engine to engine and from manufacturer to manufacturer. Each engine is different and each has its own tolerance levels for journal imperfections.
However, the following rule of thumb may be of help: If the surface imperfection on a crank journal will cause the metal from a USA "copper penny" coin dragged across the surface of a dry, unlubed journal, it would not be a surprise to see that bearing problems developed.
The above "rule of thumb" is admittedly crude but VERY effective.
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