importance of surface texture on shaft?

[nyvens]

hi!

i would like to know how surface texture is important on a shaft for a shaft-bearing assembly. (i am mostly interested in the practical aspect in real life situations)

i am thinking it has to do with tolerance and mating parts.
and/or retaining the right amount of lubrication for the shaft-bearing assembly in order to perform well.

help me to clarify my thoughts pls.

 

[KENAT]

It usually has little to do with tolerance and mating parts.

Deviations from nominal usually considered 'surface texture' (such as in ASME B46.1) are normally much smaller than the values for overal tolerance variation or even form control. For instance a typical machined roughness of 125 RMS to ASME B46.1, the 125 is micro inches.

It can have a lot to do with friction, as simplistically a rough surface will tend to increase coefficient of friction.

I suggest you do some googling, such as:

http://en.wikipedia.org/wiki/Roughness

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?

 

[MintJulep]

It can also affect fatigue life.

 

[eromlignod]

Transmission shafts for heavy hp are usually TG&P (turned, ground & polished). It decreases notch sensitivity. There are also recommended finishes called for by bearing manufacturers, which, ostensibly, reduce galling between the race and the shaft.

As KENAT pointed out, tolerances are a different order of magnitude: tenths rather than micro-inches.

Don
Kansas City

 

[TheTick]

Too smooth a surface can actually cause problems (higher friction) with bearing installation and other tight axial sliding fits.

 

[KENAT]

Hey Tick, I said simplistically, that was my get out clause 'cause I know that's not a 100% rule.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?

 

[hydtools]

Too rough a bearing surface and the shaft surface finish peaks will go though the lubrication film and ruin sleeve bearings/plain bushings.
If the surface lay has a 'threaded' finish it will pull lubrication out of the shaft-to-bearing running clearance. Plunge grinding will create a preferred circular lay instead of the helical lay of a travel grind.

Ted

 

[Tmoose]

http://www.engineparts.com/publications/CL77-3-402.pdf

page 2 - specifications for hard working journal bearings.
page 24-25 - bad things that can happen if the polishing is done in the wrong direction.