31-05-2012, 04:02 PM
Hydrodynamic Lubrication
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Introduction
Friction plays a large and essential role in everyday life although
we usually never think about it. For example, friction provides
the support when we walk; without it we would not be able to
move forward and indeed it would be impossible to stand up
without additional support. When we grip an object and stop it
from falling we again use friction.
In many engineering systems, however, when two surfaces slide
against one another friction is a nuisance. In this case friction has
two undesirable effects: (1) it increases wear and (2) work that is
not useful, needs to be done to overcome it. Thus, reducing
friction will not only increase the life of a component but also
increase the efficiency of the system.
Friction between surfaces may be reduced by lowering the
coefficient of friction; or it may be reduced by introducing a new
substance – a lubricant – between the surfaces. The lubricant can
be solid (e.g. graphite) or fluid (oil, water, air). The lubricant
should also be able to support a load, if any, acting normal to the
surface.
Principle
To understand the principle of hydrodynamic lubrication
consider a block sliding on a horizontal table (Figure 1a). The
force required to move it with constant velocity is equal to the
frictional force = mW; m is the coefficient of sliding friction and
W is the weight of the block. For many common surfaces m is
about 0.3. Thus we would require 0.3 kg force to slide a block
weighing 1 kg.
Floating’ drops
Now we describe the fascinating but simple experiment which
illustrates hydrodynamic (really aerodynamic) lubrication. Take
a glass tube of internal diameter 1/4" (» 6 mm) connected to one
end of a flexible (say rubber) pipe. The other end of the flexible
pipe is dipped in a container (bucket) of water. Let the water flow
out of the glass tube by siphon action. Use a valve to adjust the
flow rate.