Friction , Lubrication Of Bearing

- Feb 06, 2020-

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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