Monday, August 8, 2016

Bearing Design Guide: Chapter Two: LUBRICATING MODES

           I am a mom of 9 year old twins and they are always asking what I do. The one thing I always tell them is that I try to help people.  Our recent conversation went like this: Mommy's company helps customers choose the right material and pieces of bronze to make their project with.  That's when my daughter chimes in and says " mean like an art project?" and I just say "uh, yeah sure, something like that.  So, when they saw me reading the Bearing Design Guide they were quick to ask me what it was all about.

           My sons first question was "what the heck is a bearing?"  This is a tough industry to put into words sometimes so usually having a visual aide makes it easier. My answer? of the first things I was taught when I first started here.  Think of a doughnut (mmm, I'm hungry!), it is round and has a hole in the middle.  The inside hole is called the ID and the outside is called the OD.  There is a hole in the middle because it goes over another piece of material that is part of the whole project. They seemed to have understood my explanation, but the questions didn't stop there.  The next two were, why does this one have lines in it? And why is this one polka dotted?  Polka dotted! HA! I thought that was adorable! I did my best explaining why the grooves were there and that they get filled with oil and that the "polka dots" were plug graphite and they both help the bearing when it is installed in the final project.  After explaining with a little more detail and rough pencil drawings, I think they got it.  I got a "that's pretty cool, mom" and that was it.

          I know when customers call in they are looking for a more detailed answer rather than a story about doughnuts, lines and polka dots.  The information below is definitely more detailed and informative for a bearing engineer.  Either way you explain it, the end result is still the same and I can say, hey...I helped someone today! 


Sleeve bearings operate in one of four lubrication modes or a combination of the following; the thick film or full hydrodynamic, the mixed film or thin film, boundary and the negligible speed mode such as in an oscillating speed which generally requires a solid film lubricant.

The thick film hydrodynamic mode gives the lowest friction and least wear with the lowest loss of power since the unit load is supported by a lubricant film and only permits contact of the mating surfaces during start up and shut down.

Although full hydrodynamic mode can be realized at substantially low surface speeds coupled with low loads, the greater frequency of obtaining this mode is above 70 fpm.

As an example, the sintered powdered metal self-lubricating bearing with 18% porosity must operate at about 70 fpm or more in order to generate sufficient frictional heat to allow the oil which it is impregnated with to seep from its pores and establish a reasonable film thickness.

When full film lubrication is achieved, the coefficient of friction value that can be obtained is in the range of .001 to .005.

The mixed film lubrication mode is the transitional point between fluid film and boundary where there is a partial separation of the mating surfaces generally occurring between 15 fpm and up to 70 fpm.

Wear in this mode depends upon the degree of surface roughness since the mixed film mode leads to intermittent metal to metal contact resulting in higher temperatures and increased coefficient of friction values in the range of .02 to .08.

Boundary lubrication mode generally occurs in surface speeds below 30 fpm and in oscillating and reciprocating motion. The coefficient of friction generally results in a range from .08 to .14 or   more.

Although this mode is best served by the greases, it also is served by solid lubricants such as graphite or molybdenum disulphides in collodial form as suspensions in the greases.

Solid lubrication mode* is considered when speed or velocity is negligible or where the application is to operate under abnormal temperature conditions. The solid lubricant is contained in form of plugs, filled grooves, spray coating or held in other configurations.  The coefficient  of friction generally runs in the range of .14 through .35.

 *This mode is touched upon at greater detail in later chapters.

Well...that's it for today.  I say goodbye for now.  Until next time my metal loving friends...
Next Up: Week Three, Chapter 3: Selecting The Bronze Sleeve Alloy

Don't forget...if you would like your own copy of the Bearing Design Guide Click Here and you will be taken to our websites homepage where you will be able to order a free copy.

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