Friction and Lubrication

By Shan Rana

Tribology

• The science of friction and lubrication is called tribology. • Tribology is an interdisciplinary approach that involves a scientific basis to

understand surfaces in contact and the lubrication needs of a given tribological system.• Applied Tribology, or tribo-technology, primarily deals with the maintenance of

machines and the minimization of wear and energy losses due to friction.• Friction creates heat, promotes wear, and wastes power, so the reduction of

friction, by any means, is vital. It is estimated, that from 1/3 to 1/2 of the total energy produced in the world is consumed by friction. It is also estimated that the cost of wear in the U .S. is equivalent to 2/3 the cost of energy. If we add these two together, we see that the cost of friction and wear in the U .S. is equivalent to the cost of energy.

FrictionWe need to understand friction, and its types, to get the basic concept of lubricant function and overcoming undesired friction with the help of lubrication.Friction: • Friction is the force that hinders or resists the relative motion of the two contacting

bodies and, depending on the application, high friction may either be desirable or undesirable.

• Friction originates from complex molecular and mechanical interactions between the contacting surfaces.

• Friction causes wear and generates heat which can lead to premature failure of the functioning machine parts.

Friction between two solids is independent of the materials and dependent upon: • The size of the contact zone• Surface roughness, asperities • Load or pressure on surfaces

Types of Friction Solid Friction

Two bodies in direct contact with each other experience dry or solid friction. Fluid Friction

When they are separated by a solid, liquid, or gaseous medium, they experience fluid friction. Mixed Friction

Between solid and fluid friction, the situation in which some parts of the two bodies are in direct contact while the others are separated by a fluid film is known as mixed friction. Internal Friction

The friction may even involve a single body, in which case it is related to the dissipation of the internal energy within the body and is called internal friction. • In lubricant-related applications, we are concerned with all three types of friction, that is, solid

friction, fluid friction, and the internal friction. • The major function of a lubricant is to minimize solid friction which it achieves by forming a fluid

film between the two contacting metal surfaces. • Usually, a fluid’s internal friction is not of any major consequence except at very low temperatures.

At these temperatures the lubricant gains viscosity which can interfere with the smooth operation of the equipment. Internal friction is important while dealing with a lubricant’s intrinsic properties, such as viscosity and pour point.

Cases of Friction

• Friction is also related to the type of motion of the two contacting bodies. • Rolling friction and sliding friction are two general cases of friction. • When force is applied to slide a steel block sitting on a steel table, both will

experience sliding friction. If a weight, or load, is placed on top of the metal block, the force necessary to cause sliding will increases significantly. • When a metal cylinder is made to roll on the surface of metal table, the cylinder

will experience rolling friction, or the rolling resistance. • Experience shows that in general less force is required to roll an object than to

slide or drag it. • For lubricated surfaces, friction is governed by different laws than those for dry

surfaces.

Friction Coefficient• Frictional force is proportional to the load, therefore

F(Force) = µ P(Load) • Friction is commonly represented by the friction coefficient µ . The coefficient of

friction is a unit-less ratio, where “F” represents the frictional force experienced by the two contacting bodies in motion, and “P” represents the normal force pressing the same two bodies together. • The value of the coefficient of friction typically ranges from 0 to 1; the higher the

value, the higher the frictional force or the resistance of the contacting bodies towards motion. Under boundary lubrication conditions, usually approaches 1. • The frictional properties of some very hard materials such as diamonds and

certain very soft materials such as Dupont's Teflon do not obey the first law. For these special materials, friction is not proportional to the load; instead it is proportional to some reduced value of the load.

Metal Surfaces• In the context of lubricants we will deal only metal surfaces.• All metal surfaces, irrespective of their finish, contain ridges, peaks, and valleys,

They stick out of the surface forming peaks and valleys at a microscopic level. These peaks are called asperities. • When two metal surfaces come in contact, solid friction, sometimes called static

or adhesive friction, ensues and the surfaces undergo adhesion and cold welding. The strength of such an association depends upon the hardness of the materials, the cleanliness of the surfaces, and the electronic structure of the metals as related to their tendency to form metal-metal solutions, or alloys. • When surfaces start to move, kinetic friction comes into play. Kinetic friction

results from plowing of the asperities of the one surface across the other surface, plastic deformation or elastic hysteresis, and wear debris getting lodged between the moving surfaces.

Friction Summary

• A positive relationship exists between the coefficient friction and wear; that is, the higher the coefficient of friction, the higher the wear. • Solid or dry friction is more severe than the mixed friction, which in turn is more

severe than the fluid friction. • Sliding friction is higher than the rolling friction, which is primarily a consequence

of the larger contact zone of the sliding surfaces.

Lubrication

LubricationLubrication is fundamental to the operation of all engineering machines.Applications that encounter metal-to-metal contact involve either no lubrication dry, solid lubrication, or liquid lubrication.• Dry lubrication (Solid lubrication):• Dry lubricants or solid lubricants are materials which despite being in the solid phase, are

able to reduce friction between two surfaces sliding against each other without the need for a liquid medium.

• Solid lubrication is common where liquid lubrication is unwanted or is difficult because of the equipment design or extremely high operating temperatures.

• Solid lubricants, exemplified by graphite and molybdenum disulfide, have multi-layered structures with low shear strength in some directions.

• Agricultural plows and certain parts of ore handling machines are examples of such equipment.

• These lubricants are applied to equipment in a number of ways, such as bonded dry films, sputtered films (metal deposit on a surface by using fast ions to eject particles of it from a target), and loose flakes.

Liquid Lubrication • When we talk about lubrication, we usually imply liquid lubrication, that is by the use of

lubricating oil, which is normally a blend of oil and additives that perform various functions. • Lubrication efficiency of an oil depends not only upon its properties, such as composition,

consistency, flow properties, and surface activity, but also on the needs of the tribological system.

Lubrication Regimes:Tribological parameters that usually define a lubrication environment are: • Friction• Lubricant viscosity• The equipment speed • LoadBearing Characteristic Number:Viscosity x velocity/unit load = a dimensionless number ZN/P = CC is known as the Bearing Characteristic Number

Lubrication Regimes• The relationship of the coefficient of friction and the oil

film thickness to lubricant viscosity Z, equipment speed N, and equipment load, or pressure P, are graphically presented by the Stribeck curve as shown in Fig. • The ratio of ZN/P is related directly to the oil film

thickness but inversely to the coefficient of friction . • This implies that high lubricant viscosity Z, high

equipment speed N, and low equipment load P will allow the formation of a thick lubricant film, and hence the equipment will encounter little or no friction. • Conversely, low lubricant viscosity, low equipment

speed, and high equipment load will create a situation where the film thickness will be inappropriate and the equipment will encounter high friction, as indicated in the figure.

Lubrication Regimes• Depending upon the lubricating environment, lubrication regimes can be divided into

fluid-film, boundary, mixed-film, and hydrostatic types.

Fluid-Film Lubrication • Fluid-film lubrication, also known as hydrodynamic lubrication, is the most desirable

type. This type of lubrication depends upon the viscosity of the lubricant and is effective only when the load in the contact zone is low. Under these circumstances, the sliding or the rolling surfaces are separated by a lubricant film several times the thickness of the surface roughness (asperities). The film thickness in this lubrication regime is estimated to be 2–100 micro(m).

• Lubrication of the thrust bearings, journal bearings, and most of the internal combustion engine parts experience fluid-film lubrication.

• Elastohydrodynamic lubrication is similar but occurs when the surfaces are in a rolling motion (relative to each other). The film layer in elastohydrodynamic conditions is much thinner than that of hydrodynamic lubrication, and the pressure on the film is greater. It is called elastohydrodynamic because the film elastically deforms the rolling surface to lubricate it.

Lubrication RegimesBoundary Lubrication• Boundary lubrication represents the opposite extreme of the lubrication environment

spectrum. Under this kind of lubrication, high loads and very slow speeds produce extreme pressures that can lead to the lack of effective lubrication.

• The film thickness in this regime is in the order of 0.0–2.0 micro(m) only, and hence maximum metal-to-metal contact occurs. If not controlled, the resulting dry metallic friction will cause catastrophic wear, and ultimately will lead to total seizure.

• Boundary lubrication is found where there are frequent starts and stops, and where shock-loading conditions are present. Examples of equipment that rely exclusively on boundary lubrication include reciprocating parts of an engine and compressor pistons, slow-moving equipment such as turbine wicket gates, and gears.

• Reactive chemicals called anti-wear and extreme pressure agents provide protection in this kind of lubrication environment.

• It is important to note that the anti-wear agents are effective only up to a maximum temperature of about 250°C, above which they essentially become ineffective.

Lubrication Regimes

Mixed-film Lubrication• Mixed-film lubrication falls between the

two extremes mentioned above and contains characteristics of both the fluid film and the boundary lubrication. There are regions of no metal-to-metal contact and of extensive metal-to-metal contact.

• While the bulk of the surfaces are separated by a lubricating layer, the asperities still make contact with each other.

Lubrication RegimesHydrostatic Lubrication• Unlike the other types of lubrication discussed above, hydrostatic

lubrication has the advantage of not depending upon the motion of the surfaces. Hence, this type of lubrication is invaluable (helpful) in applications that involve little or no surface movement. • This lubrication regime is characterized by the lack of wear, low

friction, high load capacity, and the ability to dampen vibration. • Examples of hydrostatic lubrication include lubrication of some type

of bearings, such as hydrostatic bearings, and certain metal-forming equipment involving simple pressure. • This type of lubrication allows complete separation of the surfaces by

the static film of the lubricant.

Conclusion of Lubrication Regime Process • With a better understanding of this process, it should be easier to define what

lubrication actually is. • It is a process of either separating surfaces or protecting them in a manner to

reduce friction, heat, wear and energy consumption. This can be accomplished by using oils, greases, gases or other fluids. • Therefore, next time you change the oil in your car or grease a bearing, realize

there is more going on than meets the eye.