LUBRICATION THEORY
Introduction
Back in the spring of ‘98, one of our Biomedical Engineering faculty came by to guest host my Intermediate Fluid Mechanics course. Because her research expertise is in Bio-Tribology, I became inspired to write up these pages on lubrication theory for one of my Flow of the Week entries.
In general, tribology is the study of friction and wear and plays an important role in most mechanical designs. In fact, nearly all mechanical engineers are introduced to this topic in their junior level machine design courses.
The most common way to prevent wear is through lubrication, the theory of which is a subtopic within tribology as well as a subtopic within fluid mechanics. The idea of lubrication is to maintain a liquid or grease layer between moving solid surfaces. It turns out that the stresses in this layer are what keep the surfaces separated.
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The typical problem in lubrication theory is the journal bearing sketched above. In essence, this is a shaft whirling inside of an outer cylinder. You can find such a configuration in your automobile’s engine. The crankshaft is the shaft and the main bearing is the outer cylinder. Your automobile’s camshaft(s) is(are) another example.
Lateral loads such as that sketched in the figure are frequently applied either unintentionally or by design. The latter case typically occurs when the journal bearing is part of a device which converts translational motion into rotational motion or vice versa. Examples of such devices include your automobile engine or the crank arm-chain ring mechanism on your bicycle.