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About Friction

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and

material elements sliding against each other. There are several types of friction:

Dry friction resists relative lateral motion of two solid surfaces in contact. Dryfriction is subdivided into:

o Static friction between non-moving surfaces, ando Kinetic friction between moving surfaces.

Fluid friction describes the friction between layers within a viscous fluid that aremoving relative to each other. Lubricated friction is a case of fluid friction where a

fluid separates two solid surfaces.

Skin friction is a component of drag, the force resisting the motion of a solid bodythrough a fluid.

Internal friction is the force resisting motion between the elements making up asolid material while it undergoes deformation.

When surfaces in contact move relative to each other, the friction between the twosurfaces converts kinetic energy into heat. This property can have dramatic

consequences, as illustrated by the use of friction created by rubbing pieces of woodtogether to start a fire. Kinetic energy is converted to heat whenever motion with friction

occurs, for example when a viscous fluid is stirred. Another important consequence of

many types of friction can be wear, which may lead to performance degradation and/or

damage to components.

Friction regimes:

Friction regimes for sliding lubricated surfaces have been broadly categorized into on the

basis of the Stribeck curve: Solid / boundary friction Fluid friction Mixed frictionThese curves clearly show the minimum value of friction as the demarcation between full

fluid-film lubrication and some solid asperity interactions.

Stribeck and others systematically studied the variation of friction between two liquidlubricated surfaces as a function of a dimensionless lubrication parameterN/P, where

is the dynamic viscosity, N the speed of the bearing and P the load projected on to the

geometrical surface.

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Dry friction

Dry friction resists relative lateral motion of two solid surfaces in contact. The tworegimes of dry friction are:

1. Static friction between non-moving surfaces, and2. Kinetic friction (sometimes called sliding friction or dynamic friction) between

moving surfaces.

In general dry friction describes the reaction between two solid bodies in contact witheach other when they are in motion (kinetic friction) and when they are not (static

friction). Both static and kinetic friction are proportional to the normal force exerted

between the two solid bodies. In all cases friction works against the motion or inclination

towards motion.

The interaction of different substances is modeled with different coefficients of friction.

This means that certain substances have higher resistance to movement than other for the

same normal force between them. These coefficients are listed in many places and arevery dependent on the state of the material like how smooth it is. Each value is

experimentally determined.

Coulomb friction, named after Charles-Augustin de Coulomb, is an approximate model

used to calculate the force of dry friction. It is governed by the equation:

where:

is the force of friction exerted by each surface on the other. It is parallel to thesurface, in a direction opposite to the net applied force.

is the coefficient of friction, which is an empirical property of the contactingmaterials,

is the normal force exerted by each surface on the other, directedperpendicular (normal) to the surface.

The Coulomb friction may take any value from zero up to , and the direction ofthe frictional force against a surface is opposite to the motion that surface would

experience in the absence of friction. Thus, in the static case, the frictional force isexactly what it must be in order to prevent motion between the surfaces; it balances the

net force tending to cause such motion. In this case, rather than providing an estimate of

the actual frictional force, the Coulomb approximation provides a threshold value for this

force, above which motion would commence. This maximum force is known as traction.

The force of friction is always exerted in a direction that opposes movement (for kinetic

friction) or potential movement (for static friction) between the two surfaces.

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Boundary Friction:

Boundary friction is the friction between surfaces that are neither completely dry nor

completely separated by a lubricant.

The Effect:When two consistent, un-lubricated surfaces slide against each other, there is a specific,

predictable amount of friction that occurs. This amount increases as velocity does, butonly up to a certain point. That increase follows the Stribeck curve. On the other hand, if

the two surfaces are completely lubricated, there is no direct friction or rubbing at all. In

real life, though, there is often a situation where the surfaces are not completely dry, butalso not so lubricated that they don't touch.

This "boundary friction" produces effects, such as, an increase in lubrication through thegeneration of shearing forces, and an oscillation effect during motion as the friction

increases and decreases.