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Friction is the resistance to
motion of one surface relative to another.
Friction creates heat. Heat can change the physical and chemical characteristics
of the lubricant.
Friction also affects the efficient operation of machinery. It also
promotes wear and wastes power.
It is estimated that between one-third and half of all the energy
produced in the world is consumed by friction.
What does this mean in terms of dollars? In the US alone, the potential
savings is estimated at over 100 billion dollars. In Australia, it
is estimated at 1 billion dollars. Therefore, by reducing friction
and wear, tremendous savings can be made through increasing the life
of machinery, reducing the lost time and lost production caused by
machinery problems and reducing the operating costs, e.g., cost of
power, increasing the output of each machine.
Selecting the right lubricant or paying a little more for a better
lubricant can result in considerable savings. A major aim in lubrication
is to minimize friction. The term minimize is used because friction
cannot be eliminated. |
There are two types of
friction; solid friction and fluid friction.
When two surfaces, such as two metal blocks, move against each other,
they experience friction. The rougher the surface of the metal blocks,
the greater the friction. This is called solid friction. In fact if
you look at the metal surface microscopically, you will see metal
peaks rather than a smooth surface. These peaks are called asperities
In fact it is estimated that due to this surface roughness, only around
20% of the surface between two metal blocks are in contact.
Solid friction can cause extremely high pressures and temperatures
to build up, such that the contacting high points of the surface asperities
actually weld together and then tear apart. This surface damage is
called ¡°wear¡±. In order to reduce the friction between the metal blocks,
we need to introduce a substance of lower friction which separates
the blocks.
The substance used, which can be made of many things, including air,
water, mineral oil, grease, soft metal, fats, waxes, etc., is referred
to as a lubricant. The lubricant forms a film between the two surfaces
and allows them to slide easily over one another, without the surfaces
touching.
Second, fluid friction is friction within the lubricant itself. This
is due to the particles/molecules rubbing against each other and,
so, creates what is termed internal friction or fluid friction. It
is a measure of the fluid¡¯s internal resistance to flow. |
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