Looking for journal bearing clearance equation.
Looking for journal bearing clearance equation.
(OP)
I am looking for information/equation to determine the recommended clearance for Journal bearings (plain bearings) relative to RPM, temperature, and shaft diameter. I know there is a guide but can't seem to locate it.





RE: Looking for journal bearing clearance equation.
RE: Looking for journal bearing clearance equation.
Well, it'd probably go round, and all, but that is way over for an automotive crankshaft.
Cheers
Greg Locock
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RE: Looking for journal bearing clearance equation.
RE: Looking for journal bearing clearance equation.
Cheer up, it could be worse, it could be me trying to figure this out.
RE: Looking for journal bearing clearance equation.
"...there just isn't any hard equations or hardened rules on plain bearings"
I could not disagree more. There are very well defined and sensible rules/formulas for journal bearing design.
Journal-to-bearing clearance is usually a function of cooling oil flow requirements. Cooling oil flow requirements are established to maintain the bearing materials within their allowable temperature limits. Excessive bearing clearance will simply produce greater oil flow thru the bearing and is not significantly detrimental to bearing performance. Journal bearings function in the hydrodynamic regime and will produce a stable oil film with clearances even well in excess of the suggested .002 inch per inch of journal diameter.
The only draw back of excessive journal bearing clearance is that you will need a higher volume oil pump, with its associated mechanical losses.
RE: Looking for journal bearing clearance equation.
Care to post those "very well defined and sensible rules/formulas"?
RE: Looking for journal bearing clearance equation.
tbuelna is correct. there are very strict guidelines for the design of hydrodynamic bearings and the lube systems that feed them.
mayt4u is also correct, the parameters for bearing design are many.
there is a public domain book from CRC titled "handbook of lubrication" by E. Booser (i think a GE guy). and another called "fundamentals of fluid film bearings (1979) from www.umi.com (havent checked that link, it's on the cover). the first is very practical, the second one is very mathematical.
both books are probably major overkill for answering this specific question. but if you do this for a living, dropping $$$ on books is generally worth it because after you find the answer to this question, you will find another question.
regards,
magicme
------------------------------------
"not all that glitters is gold"
RE: Looking for journal bearing clearance equation.
-The future's so bright I gotta wear shades!
RE: Looking for journal bearing clearance equation.
Are you involved in a bearing design exercise or you you checking an existing installation? What is the application - rotating equipment or what?
RE: Looking for journal bearing clearance equation.
RE: Looking for journal bearing clearance equation.
You could check to see if the same bearing is fitted to both ends and compare measured clearances.
Better still, you could retrieve and loosely assemble new/spare bearing shells (from your spares store) and take id measurements for use as a reference for installed bearings.
Finally, you could try to contact the bearing supplier and ask for the spec (id) of a new bearing
RE: Looking for journal bearing clearance equation.
to machine an eliptical bore, shims are added to horizonal joint for machining, then removed for installation.
elliptical allows the vertical clearance to be reduced for better stability, but has sufficient volume on horizonal to develop wedge and cooling flow.
RE: Looking for journal bearing clearance equation.
See page 12 of 35 here:
http://www.easa.com/indus/ar100-2001.pdf
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RE: Looking for journal bearing clearance equation.
Twitchy,
I've got 20+ years experience analyzing rotor dynamics in sleeve bearings, and have studied shaft centerline movement extensively on large motors and turbomachinery.
For a motor in the 2,250 hp range (as TPL pointed out) a clearance of 1.5 mils per inch of shaft diameter will work nicely. You might want to specify something a bit tighter (1.3 or so) for new bearings. Do not size them above 1.5 mils/in, you will lose dynamic stiffness, especially with any bearing wear, and likely get running speed harmonics in your spectra at 2 mils/in, even with new bearings. And most motor bearings will be plain sleeve; ellipticals are more often used on turbomachinery.
If you keep the oil clean, large motors will run for quite a while (several years) before the bearings wear significantly. By starting in the 1.3 - 1.5 mil/in range, you'll get a long service life (all other things equal, of course).
RE: Looking for journal bearing clearance equation.
~2.5 mils/inch for 2" shaft
~ 2 mils/inch for 4" shaft
~ 1.5 mils/inch for 6" shaft
~ 1 mil per inch for 16" shaft
You didn't tell us the speed, but I'm guessing your 2250hp motor shaft diameter might be in the range 6-10" (6" for 3600rpm machine up to 10" for slow speed machine).
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