Friction , Lubrication of Bearing
In many of the problem thus far , the student has been asked to disregard or neglect
friction . Actually , friction is present to some degree whenever two parts are in contact and
move on each other. The term friction refers to the resistance of two or more parts to
Friction is harmful or valuable depending upon where it occurs. friction is necessary
for fastening devices such as screws and rivets which depend upon friction to hold the fastener
and the parts together. Belt drivers, brakes, and tires are additional applications where friction is
The friction of moving parts in a machine is harmful because it reduces the mechanical
advantage of the device. The heat produced by friction is lost energy because no work takes
place. Also , greater power is required to overcome the increased friction. Heat is destructive
in that it causes expansion. Expansion may cause a bearing or sliding surface to fit tighter. If a
great enough pressure builds up because made from low temperature materials may melt.
There are three types of friction which must be overcome in moving parts: (1)starting,
(2)sliding, and(3)rolling. Starting friction is the friction between two solids that tend to resist
movement. When two parts are at a state of rest, the surface irregularities of both parts tend to
interlock and form a wedging action. To produce motion in these parts, the wedge-shaped
peaks and valleys of the stationary surfaces must be made to slide out and over each other. The
rougher the two surfaces, the greater is starting friction resulting from their movement .
Since there is usually no fixed pattern between the peaks and valleys of two mating parts,
the irregularities do not interlock once the parts are in motion but slide over each other. The
friction of the two surfaces is known as sliding friction. As shown in figure ,starting friction is
always greater than sliding friction .
Rolling friction occurs when roller devces are subjected to tremendous stress which cause
the parts to change shape or deform. Under these conditions, the material in front of a roller
tends to pile up and forces the object to roll slightly uphill. This changing of shape , known as
deformation, causes a movement of molecules. As a result ,heat is produced from the added
energy required to keep the parts turning and overcome friction.
The friction caused by the wedging action of surface irregularities can be overcome partly
by the precision machining of the surfaces. However, even these smooth surfaces may require
the use of a substance between them to reduce the friction still more. This substance is usuallya
lubricant which provides a fine, thin oil film. The film keeps the surfaces apart and prevents
the cohesive forces of the surfaces from coming in close contact and producing heat .
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