too smooth journal finish ?

[Tmoose]

There are credible but second hand reports that some industrial equipment and bearing manufacturers advise against burnished finishes on steel shafts because the finish may be "too smooth" for oil to wet the surface or otherwise be drawn into the working zone of a hydrodynamic bearing.

I'm willing to believe that trying to pull oil from a nearly wide open gravity fed low pressure zone may be a different situation than having oil pressure fed to a low pressure zone a few thousandths of an inch thick, but is there some real meat for the too smooth theory ?

MAHLE/Clevite recommends a surface finish of 10 micro inches Ra "or better" (lower) for heavy duty or highly loaded engines, with no lower limit for fine-ness of finish.
Page 35 here -

http://mahleclevite.com/publications/EB-40-07.pdf

On my old Baptist machine surface comparator 4 RMS is mighty smooth.
My recollection is a "finger nail" test of factory finish of Porsche and Mercedes cranks are certainly much smoother rms 16, and very possibly better than 8 and maybe in the 4 range.

thanks,

Dan T

 

[swall]

If I had to guess, the concern about burnished finishes may have to do more with the surface characteristics imparted by the burnishing process than the RMS number. If you look at a burnished surface with an SEM, you will see all kinds of micro pits at inclusions and carbides and you may see an "orange peel" like finish rather than a smoothly leveled finish.

 

[MikeHalloran]

I can't see a 'too smooth' finish being an issue in a hydrodynamic bearing.

I can see it being an issue under a contacting dynamic seal, where some transfer of the seal polymer to the mating surface gives best seal life. ... for some kinds of seals and some mating surfaces.







Mike Halloran
Pembroke Pines, FL, USA

 

[caoimhin1]

Also for dry running polymer bushes where PTFE transfer is necessary; e.g. Glacier recommend minimum 0.3um Ra for most materials.