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 "oooh...you 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?...one of the first things I was taught when I first started here. Think of a doughnut (mmm, yum...now 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!
Chapter Two: LUBRICATING MODES
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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