Reducing friction

تاريخ التسجيل : 22/11/2009

Devices

Devices such as wheels, ball bearings,
roller bearings,
and air cushion or other types of fluid bearings
can change sliding friction into a much smaller type of rolling friction.

Many thermoplastic
materials such as nylon, HDPE and PTFE are commonly used in
low friction bearings.
They are especially useful because the coefficient of friction falls with increasing
imposed load. For improved wear
resistance, very high molecular weight grades are usually specified
for heavy duty or critical bearings.

Lubricants

A common way to reduce friction is by using a lubricant,
such as oil, water, or grease, which is placed between the two surfaces, often
dramatically lessening the coefficient of friction. The science of friction and
lubrication is called tribology. Lubricant technology is when lubricants are mixed
with the application of science, especially to industrial or commercial
objectives.

Superlubricity,
a recently-discovered effect, has been observed in graphite:
it is the substantial decrease of friction between two sliding objects,
approaching zero levels. A very small amount of frictional energy would still
be dissipated.

Lubricants to overcome friction need not always be
thin, turbulent fluids or powdery solids such as graphite and talc; acoustic lubrication actually uses sound as a
lubricant.

Another way to reduce friction between two parts
is to superimpose micro-scale vibration to one of the parts. This can be
sinusoidal vibration as used in ultrasound-assisted cutting or vibration noise,
known as dither.

Energy of friction

According to the law of conservation of energy, no energy is destroyed
due to friction, though it may be lost to the system of concern. Energy is
transformed from other forms into heat. A sliding hockey puck comes to rest
because friction converts its kinetic energy into heat. Since heat quickly
dissipates, many early philosophers, including Aristotle,
wrongly concluded that moving objects lose energy without a driving force.

When an object is pushed along a surface, the
energy converted to heat is given by:

Reducing friction Clip_image001

where

is the normal force,

Reducing friction Clip_image003

is the coefficient of
kinetic friction,

Reducing friction Clip_image004

is the coordinate along which the
object transverses.

Energy lost to a system as a result of friction is
a classic example of thermodynamic irreversibility.

Work of friction

In the reference frame of the interface between
two surfaces, static friction does no work,
because there is never displacement between the surfaces. In the same reference
frame, kinetic friction is always in the direction opposite the motion, and
does negative work. However, friction can do positive work in
certain frames of reference. One can see this by placing
a heavy box on a rug, then pulling on the rug quickly. In this case, the box
slides backwards relative to the rug, but moves forward relative to the frame
of reference in which the floor is stationary. Thus, the kinetic friction
between the box and rug accelerates the box in the same direction that the box
moves, doing positive work.

The work done by friction can translate into
deformation, wear,
and heat that can affect the contact surface properties (even the coefficient
of friction between the surfaces). This can be beneficial as in polishing.
The work of friction is used to mix and join materials such as in the process
of friction welding. Excessive erosion or wear of
mating surfaces occur when work due frictional forces rise to unacceptable
levels. Harder
corrosion particles caught between mating surfaces (fretting)
exacerbates wear of frictional forces. Bearing seizure or failure may result
from excessive wear due to work of friction. As surfaces are worn by work due
to friction, fit and surface finish
of an object may degrade until it no longer functions properly.

Applications

Friction is an important factor in many
engineering disciplines.

Transportation

Rail adhesion refers to the grip wheels of
a train have on the rails.
Road slipperiness is an important design
and safety factor for automobiles
- Split friction is a particularly
  dangerous condition arising due to varying friction on either side of a
  car.
- Road texture affects the interaction of
  tires and the driving surface.