FRICTION II(REDUCING FRICTION)

Devices

Devices such as wheels, ball bearings, roller bearings, air cushions, 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 uses sound as a lubricant.

Another way to reduce friction between two parts is to superimpose micro-scale vibration on 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 thermal energy. A sliding hockey puck comes to rest because friction converts its kinetic energy into heat which raises the thermal energy of the puck and the ice surface. 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 along a path C, the energy converted to heat is given by a line integral, following the definition of work

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 not 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, and then pulling on the rug quickly. In this case, the box slides backward 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 the 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 sliding surfaces occurs when work due to frictional forces rises to unacceptable levels. Harder corrosion particles caught between mating surfaces in relative motion (fretting) exacerbate 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, the 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

• Automobile brakes inherently rely on friction, slowing a vehicle by converting its kinetic energy into heat. Incidentally, dispersing this large amount of heat safely is one technical challenge in designing brake systems. Disk brakes rely on friction between a disc and brake pads that are squeezed transversely against the rotating disc. In Drum brakes, brake shoes or pads are pressed outwards against a rotating cylinder (brake drum) to create friction. Since braking discs can be more efficiently cooled than drums, disc brakes have better-stopping performance.
• Rail adhesion refers to the grip wheels of a train have on the rails, see Frictional contact mechanics.
• 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.

Assignment

1.       Define lubrication
2.       Mention 3 types of lubricant