Posted by Dennis Novotny40;18.104.22.168) on 15:53:46 01/03/05
In Reply to:Re: Porosity influence on crankshaft journal posted by Gordon England
The bearing is actually subjected to much higher hydraulic pressures that develop as a result of elastohydrodynamic films that develop between the journal and bearing.
There is a decent explanation of the peak oil film pressure (POFP) and minimum oil film thickness (MOFT) phenomenon and how they affect bearings (and perhaps porous cranks) in an SAE paper that was written by William Yahraus a number of years ago (spelling may need adjustment). The SAE should be able to find it via their database.
The problem is, the porosity that can be tolerated by one engine at one power rating may not be tolerated in another engine at the same power due to the actual POFP and MOFT that result from differences in the oiling scheme and bearing geometry of the 2 engines. Surely the same sort of thing applies to coatings applied to journals to achieve size repairs.
Generally speaking, one doesn't want any porosity or other surface anomalies in the journal. However, unless one is willing to generate perfection which can be impossible to do at an affordable price, it is difficult to provide a "generic" acceptance standard for imperfections that will work in ALL instances.
All the OEM's I worked with while doing sleeve bearing engineering for Clevit had surface imperfection allowance standards for load carrying surfaces.
Of specific note, all were different as they were all generated via empirical experiences encountered via dyno durability testing. Moreover, the tolerance was tighter for for severely loaded surfaces than for surfaces less severly loaded.
In short, can you tolerate porosity on a crankshaft journal?
Yes and no.
This depends on the load induced stresses (IE: POFP's and the resultant MOFT's) that will occur and ultimately the engines "tolerance" of what are clearly "off" conditions.
: In general most people seal crankshaft journal coatings in the belief that porosity can be detrimental to maintaining even oil film pressure, and can allow hydraulic pressure cycling within coating and cause fatigue failure.
: Regards Gordon
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