As a 'rule of thumb' you could use 1.5 thou of clearance for each 1 inch of journal diameter

Ow.

Well, it'd probably go round, and all, but that is way over for an automotive crankshaft.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

it is valid for shipping propulsion systems as well.

Sorry to say Twitchy, there just isn't any hard equations or hardened rules on plain bearings. It's, whats the application, whats the ambiant temp, what is the lowest speed/highest, what is the shaft loading, what type of material will be transported, what is the horsepower requirements, what temperture will the machine rise to, what type of oil delivery system would you have, what viscosity oil will you be using and why, what did you have for supper last night and what time will you go to bed.  I think you got the drift.  But start with a least 0.002" (TIR) and work up.  You need an oil wedge to support the shaft, so, to tight will cause rub and too large will cause oil/shaft whirl.   Depends, depends, depends...

Cheer up, it could be worse, it could be me trying to figure this out.

 

mayt4u,

"...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.

tbuelna,

Care to post those "very well defined and sensible rules/formulas"?

i can't speak for every company on the planet. however i can speak from several decades of experience with a company that has brought many good things to life. (so they advertise.)

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"

As with TPL, my standard rule of thumb for process machinery (typically 4in journal or larger, and 1800 rpm or more, and not a automobile crank shaft ) is 1.5 to 2.0 mil per inch of shaft diameter (diametral clearance). I have used this for years and it has very rarely let me down, for either tilt pad or plain journal bearings.

-The future's so bright I gotta wear shades!

Twitchy - can you let us know what you are trying to achieve?

Are you involved in a bearing design exercise or you you checking an existing installation? What is the application - rotating equipment or what?

I am trying to check the clearances of existing white metal bearings on a 2250 hp electric motor. Ambient temp. is around 86 degF (30degC) and normal operating temp of bearing is around 122 degF (50 degC). We have a check sheet for the technicians but nobody knows what is out of spec or in spec. Twitchy

1.5 thou of clearance for each 1 inch of journal diameter will do nicely for this. Get a little concerned if you have more than 2 thou per inch of diameter.

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

If you are not positive the design calls for a round bearing, you might find the readings ellictical.  that is vertical 1 to 1.5 thou/inch and horizonal 2 to 3 thou/inch.  especilly if motor is greater than 1200 rpm

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.

EASA AR100 table 2-7 will give you recommended diametrical clearance as a function of shaft size  (for use during repair of motors in the absence of OEM information):
See page 12 of 35 here:
http://www.easa.com/indus/ar100-2001.pdf

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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).

If you take the table values from the above link and convert to mils per inch, you get something like this:
~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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