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Tribology

tri.bol.o.gy
noun (1966):

Tribology is a study that deals with the design, friction, wear, and lubrication of interacting surfaces in relative motion (as in bearings or gears)

Tribology is the study or learning [-logy from Latin - logia] of the interaction or rubbing [from Greek Triben] of sliding surfaces.

Tribology includes three subjects:

  1. Friction
  2. Wear
  3. Lubrication

Friction

Friction is generally characterized as a branch of physics or mechanical engineering.

Friction is a force that resists the sliding or rolling of one solid object over another.

Friction is the resistance to sliding of a solid when a contacting body produces the resistance.

Friction is therefore a vital factor in the operation of most mechanisms.

Friction has been studied as a branch of mechanics for many hundreds of years and its laws, as well as satisfactory methods of estimating the magnitude of friction, have been known for nearly two centuries. The mechanism of friction, namely, the exact process by which energy is lost as two surfaces slip past each other, is understood only in an incomplete way.

The major cause of friction between metals appears to be the forces of attraction, known as adhesion , between the contact regions of the surfaces, which are always microscopically irregular. Under magnification these surface irregularities appear as hills, peaks and valleys.

Under load when two of these irregular surfaces contact, the peaks adhere or "weld" to each other, as well as, inter-lock with the valleys in the opposing surface.

Friction arises from shearing these "welded" junctions and from the action of the irregularities of the harder surface plowing across the softer surface.

Two simple experimental facts characterize the friction of sliding solids:

  1. The amount of friction is nearly independent of the area of contact
  2. Friction is proportional to the load or weight that presses the surfaces together

The ratio of friction F to load L is constant

This constant ratio is called the coefficient of friction and is usually symbolized by the Greek letter mu (μ). Mathematically therefore we can express this relationship by following formula:

μ = F/L

Because both friction and load are measured in units of force (such as pounds or Newtons), the coefficient of friction is dimensionless.

Kinetic Friction

The value of the coefficient of friction for a case of one or more bricks sliding on a clean wooden table is about 0.5, which implies that a force equal to half the weight of the bricks is required just to overcome friction in keeping the bricks moving along at a constant speed. The frictional force itself is directed oppositely to the motion of the object.

Because the friction thus described arises between surfaces in relative motion, it is called kinetic friction.

Static Friction = Stiction

Static friction acts between surfaces at rest with respect to each other. The value of static friction varies between zero and the smallest force needed to start motion.

This smallest force required to start motion, or to overcome static friction, is always greater than the force required to continue the motion, or to overcome kinetic friction.

Rolling Friction

Rolling friction occurs when a wheel, ball, or cylinder rolls freely over a surface, as in ball and roller bearings.

The main source of friction in rolling appears to be dissipation of energy involved in deformation of the objects.

If a hard ball is rolling on a level surface, the ball is somewhat flattened and the level surface somewhat indented in the regions in contact.

The elastic deformation or compression produced at the leading section of the area in contact is a hindrance to motion that is not fully compensated as the substances spring back to normal shape at the trailing section.

The internal losses in the two substances are similar to those that keep a ball from bouncing back to the level from which it is dropped.

Coefficients of sliding friction are generally 100 to 1,000 times greater than coefficients of rolling friction for corresponding materials.

This advantage was realized historically with the transition from sledge to wheel.


High Friction

High friction is needed for the satisfactory functioning of nuts and bolts, paper clips, and tongs, as well as in the familiar processes of walking, gripping objects manually, and building piles of sand or of apples.

Frictional forces, such as the traction needed to walk without slipping, may be beneficial; but they also present a great measure of opposition to motion.


For example:

About 20% of the engine power of automobile is consumed in overcoming the frictional forces in the moving parts.

Low Friction

Low friction , however, is desired in objects that are designed to move continuously, like engines, skis, and the internal mechanism of watches.


Constant Friction

Constant friction is required in brakes and clutches, as otherwise unpleasant jerky movement would be produced.


Wear

Wear is part of the material science of metallurgy.


Wear is the removal of material from a solid surface as a result of the mechanical action exerted by another solid.

Wear chiefly occurs as a progressive loss of material resulting from the mechanical interaction of two sliding surfaces under load. It is such a universal phenomenon that rarely do two solid bodies slide over each other or even touch each other without measurable material transfer or material loss.

For example:

Coins become worn as a result of continued contact with fabrics and human fingers; pencils become worn after sliding over paper; and rails become worn as a result of the continued rolling of train wheels over them.

Only living things (e.g., bone joints) are in general immune to the permanent damage caused by wear because only they have the property of healing through regrowth. And even a few living things do not heal themselves (e.g., teeth in humans).

The systematic study of wear has been severely hampered by two factors:

  1. Existence of a number of separate wear processes
    (which has led to much confusion, especially in terminology)
  2. Difficulties caused by the small amounts of material involved in wear processes.

These difficulties were greatly alleviated when radioactive isotopes of the common engineering metals (iron, copper, chromium, etc.) became available in the 1940s; tracer techniques using these radioisotopes permit measurements of wear, even in small amounts, while it is occurring.

Wear occurs on lubricated surfaces by abrasion, corrosion, and solid-to-solid contact. Proper lubricants will help combat each type. They reduce abrasive and solid-to-solid contact wear by providing a film that increases the distance between the sliding surfaces, thereby lessening the damage by abrasive contaminants and surface asperities.

The role of a lubricant in controlling corrosion of surfaces is twofold. When machinery is idle, the lubricant acts as a preservative.

This has made it possible to identify types of wear and discover some of the laws of wear.

There are four basic types of wear:

  1. Adhesive
  2. Abrasive
  3. Corrosive
  4. Surface-fatigue

Though the wear process is generally thought of as harmful, and in most practical situations it is so. Wear has some practical uses as well. For example, many methods of producing a surface on a manufactured object depend on abrasive wear, among them filing, sanding, lapping, and polishing.

Many writing instruments, principally the pencil, crayon, and chalk, depend for their effect on adhesive wear.

Another use is seen in the wear of the incisor teeth of rodents. These teeth have hard enamel covering along the outer curved surface but only soft dentine on the inner surface.

Hence, abrasive and adhesive wear, which occurs more rapidly on the softer side, acts to maintain a sharp cutting edge on the teeth.

Adhesive Wear

Adhesive wear is the most common type, it arises from the strong adhesive forces that are generated at the interface of two solid materials. When solid surfaces are pressed together, intimate contact is made over a number of small patches or junctions. During sliding, these junctions continue to be made and broken, and, if a junction does not break along the original interface, a wear particle is formed. These particles eventually break away.

Adhesive wear is undesirable for two reasons:

  1. the loss of material will eventually lead to a deterioration in the performance of the mechanism
  2. the formation of large wear particles in closely fitted sliding members may cause the mechanism to seize at an early stage in its productive life.

Adhesive wear is many times greater for unlubricated than for effectively lubricated metal surfaces.

Abrasive Wear

Abrasive wear occurs when a hard, rough surface slides over a softer one, producing grooves on the latter.

It also can be caused by loose, abrasive particles rolling between two soft sliding surfaces or by particles embedded in one of the opposing surfaces.

Abrasive fragments borne by a stream of liquid or gas may wear down a surface if they strike the surface at high speeds.

For Example: "sand blasting"

Since abrasive wear takes place when the abrading material is rough and harder than the surface to be abraded, it can be prevented either by eliminating the hard, rough constituent or by making the surface to be protected harder still.

Corrosive Wear

Corrosive wear occurs whenever a gas or liquid chemically attacks a surface left exposed by the sliding process.

Normally, when a surface corrodes, the products of corrosion (such as patina) tend to stay on the surface, thus slowing down further corrosion.

But, if continuous sliding takes place, the sliding action removes the surface deposits that would otherwise protect against further corrosion, which thus takes place more rapidly.

A surface that has experienced corrosive wear generally has a matte, relatively smooth appearance.

Surface-fatigue

Surface-fatigue wear is produced by repeated high stress attendant on a rolling motion, such as that of metal wheels on tracks or a ball bearing rolling in a machine.

The stress causes subsurface cracks to form in either the moving or the stationary component.

As these cracks grow, large particles separate from the surface and pitting ensues.

Surface-fatigue wear is the most common form of wear affecting rolling elements such as bearings or gears.

For sliding surfaces, adhesive wear usually proceeds sufficiently rapidly that there is no time for surface-fatigue wear to occur.

Lubrication

Lubrication is a branch of chemistry.


Lubrication is the action of introduction of a lubricant, i.e. any type of various substances between sliding surfaces to reduce wear and friction.

The use of lubricants, is an ancient practice, and Egyptian pictures dating back 4,000 years show the application of lubricants to reduce the friction involved in dragging heavy monuments.

In modern lubrication practice, the main concern is to reduce the wear that accompanies sliding and, at the same time, to design lubrication systems that will operate for long periods without inspection or maintenance.

A large number of different lubricants are in use at any one time, (a single major oil company may market many hundreds of different varieties), and no aspect of tribology receives as much attention as the development and testing of improved lubricants.

Nature has been applying lubrication since the evolution of synovial fluid, which lubricates the joints and bursas of vertebrate animals. Prehistoric people used mud and reeds to lubricate sledges for dragging game or timbers and rocks for construction.

Animal fat lubricated the axles of the first wagons and continued in wide use until the petroleum industry arose in the 19th century, after which crude oil became the chief source of lubricants.

The natural lubricating capacity of crude oil has been steadily improved through the development of a wide variety of products designed for the specific lubricating needs of the automobile, the airplane, the diesel locomotive, the turbojet, and power machinery of every description.

The improvements in petroleum lubricants have in turn made possible the increase in speed and capacity of industrial and other machinery.

The Jet age and the Space age, renewed the interest in synthetic lubricants because they offer superior performance in comparison to all natural lubricants. Although synthetics are now widely available, their cost is still many times higher than conventional petroleum lubricants.

The environmental concerns of recent years also renewed interests in bio-degradable lubricants, especially those based on vegetable oils.

Since 1969, a new class of lubricants sol-lubes has been developed and experimentally tested in many applications. These colloidal lubricants are suspensions of solid lubricants in liquids.

There are three basic varieties of lubrication regimes:

  1. Fluid-film
  2. Boundary
  3. Solid

Fluid-film lubrication

Interposing a fluid film that completely separates sliding surfaces results in this type of lubrication.

The fluid may be introduced intentionally, as the oil in the main bearings of an automobile, or unintentionally, as in the case of water between a smooth rubber tire and a wet pavement.

Although the fluid is usually a liquid, it may also be a gas. The gas most commonly employed is air.

To keep the parts separated, it is necessary that the pressure within the lubricating film balance the load on the sliding surfaces.

If the lubricating film's pressure is supplied by an external source, the system is lubricated hydrostatically.

If the pressure between the surfaces is generated as a result of the shape and motion of the surfaces themselves, the system is lubricated hydrodynamically.
This type of lubrication depends upon the viscous properties of the lubricant.

Boundary lubrication

A condition that lies between unlubricated sliding and fluid-film lubrication is referred to as boundary lubrication. It is also defined as that condition of lubrication in which the properties of the surfaces and properties of the lubricant, other than viscosity, determine the friction between surfaces.

Boundary lubrication encompasses a significant portion of lubrication phenomena and commonly occurs during the starting and stopping of machines.

Solid Lubrication

Graphite, Molybdenum Disulfide (Moly) and PTFE (Teflon) are widely used when normal lubricants do not possess sufficient resistance to load or temperature extremes.

But lubricants need not take only such familiar forms as fats, powders, and gases; even some metals commonly serve as sliding surfaces in some sophisticated machines.

Control functions

A lubricant primarily controls friction and wear, but it can and ordinarily does perform numerous other functions, which vary with the application and usually are interrelated.

The amount and character of the lubricant made available to sliding surfaces have a profound effect upon the friction that is encountered. For example, disregarding such related factors as heat and wear but considering friction alone between two oil-film-lubricated surfaces, the friction can be 200 times less than that between the same surfaces with no lubricant. Under fluid-film conditions, friction is directly proportional to the viscosity of the fluid.

Some lubricants, such as petroleum derivatives, are available in a great range of viscosities and thus can satisfy a broad spectrum of functional requirements. Under boundary lubrication conditions, the effect of viscosity on friction becomes less significant than the chemical nature of the lubricant. Delicate instruments, for example, must not be lubricated with fluids that would attack and corrode the finer metals.

When machinery is in use, the lubricant controls corrosion by coating lubricated parts with a protective film that may contain additives to neutralize corrosive materials. The ability of a lubricant to control corrosion is directly related to the thickness of the lubricant film remaining on the metal surfaces and the chemical composition of the lubricant.

Lubricants also can assist in controlling temperature by reducing friction and carrying off the heat that is generated. Effectiveness depends upon the amount of lubricant supplied, the ambient temperature, and the provision for external cooling. To a lesser extent, the type of lubricant also affects surface temperature.

Other functions

Various lubricants are employed as hydraulic fluids in fluid transmission devices. Others can be used to remove contaminants in mechanical systems. Detergent-dispersant additives, for instance, suspend sludges and remove them from the sliding surfaces of internal-combustion engines.

In specialized applications such as transformers and switchgear, lubricants with high dielectric constants act as electrical insulators. For maximum insulating properties, a lubricant must be kept free of contaminants and water.

Lubricants also act as shock-damping fluids in energy-transferring devices (e.g., shock absorbers) and around such machine parts as gears that are subjected to high intermittent loads.

Lubricant Types

A wide variety of lubricants are available. The principal types are reviewed here.

Liquid, oily lubricants

Animal and Vegetable

products were certainly man's first lubricants and were used in large quantities. But, because they lack chemical inertness and because lubrication requirements have become more demanding, they have been largely superseded by petroleum products and by synthetic materials. Some organic substances such as lard oil and sperm oil are still in use as additives because of their special lubricating properties.

Petroleum Lubricants

Petroleum lubricants are predominantly hydrocarbon products extracted from fluids that occur naturally within the Earth. They are used widely as lubricants because they possess a combination of the following desirable properties:

  1. availability in suitable viscosities
  2. low volatility
  3. inertness (resistance to deterioration of the lubricant)
  4. corrosion protection (resistance to deterioration of the sliding surfaces)
  5. low cost

However, petroleum lubricants loose their inertness when subjected to elevated temperatures, such as those encountered in modern engines. This causes deterioration of the lubricant by oxidation, and leads to formation of gum, varnish and other insoluble deposits. Therefore in most applications petroleum lubricants have to be frequently changed, if longevity of the equipment is desired.

Synthetic Lubricants

Synthetic lubricants generally can be characterized as oily, neutral liquid materials not usually obtained directly from petroleum but having some properties similar to petroleum lubricants.


In certain ways they are superior to hydrocarbon products.

Some synthetics exhibit greater stability of viscosity with temperature changes, resistance to scuffing and oxidation, and fire resistance.

Since the properties of different types of synthetics vary considerably, each synthetic lubricant tends to find a special application.

There is NO single synthetic lubricant type that is ideal for all lubricant applications. Commercial synthetic lubricants (Motor Oil, Gear Oil) are therefore a blend of several different types of Synthetics as well as select additives.


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SYNTHETIC

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Grease

Another form of oily lubricant is grease, a solid or semisolid substance consisting of a liquid lubricant containing a thickening agent.

The liquid lubricant is made from inedible lard, the rendered fat of waste animal parts, or is petroleum-derived or synthetic high viscosity oil.

Soaps of aluminum, barium, calcium, lithium, sodium, and strontium are the major thickening agents. Nonsoap thickeners consist of such inorganic compounds as modified clays or fine silicas, or such organic materials as arylureas or phthalocyanine pigments.

White grease is made from inedible hog fat and has a low content of free fatty acids. Yellow grease is made from darker parts of the hog and may include parts used to make white grease.

Brown grease contains beef and mutton fats as well as hog fats. Fleshing grease is the fatty material trimmed from hides and pelts. Bone grease, hide grease, and garbage grease is named according to their origin. In some factories, food offal is used along with animal carcasses, butcher-shop scraps, and garbage from restaurants for recovery of fats.

Greases of mineral or synthetic origin consist of a thickening agent dispersed in a liquid lubricant such as petroleum oil or a synthetic fluid. The thickening agent may be soap, an inorganic gel, or an organic substance. Other additives inhibit oxidation and corrosion, prevent wear, and change viscosity. The fluid component is the more important lubricant for clearances between parts that are relatively large, but for small clearances the molecular soap layers provide the lubrication.

Synthetic grease may consist of synthetic oils containing standard soaps or may be a mixture of synthetic thickeners, or bases, in petroleum oils. Silicones are greases in which both the base and the oil are synthetic. Synthetic greases are made in water-soluble and water-resistant forms and may be used over a wide temperature range. The synthetics can be used in contact with natural or other rubbers because they do not soften these materials.

Special-purpose greases may contain two or more soap bases or special additives to gain a special characteristic.

Lubrication by grease may prove more desirable than lubrication by oil under conditions when:

  1. less frequent lubricant application is necessary
  2. grease acts as a seal against loss of lubricant and ingress of contaminants
  3. less dripping or splattering of lubricant is called for
  4. less sensitivity to inaccuracies in the mating parts is needed

Solid lubricants

A solid lubricant is a film of solid material composed of:

Inorganic Compounds

There are three general kinds of inorganic compounds that serve as solid lubricants:

  1. Layer-lattice solids
  2. Soft solids
  3. Chemical conversion coatings
Layer-lattice solids

Layer-lattice solids: materials such as graphite and molybdenum disulfide, commonly called molysulfide, have a crystal lattice structure arranged in layers. Strong bonds between atoms within a layer and relatively weak bonds between atoms of different layers allow the lamina to slide on one another. Other such materials are tungsten disulfide, mica, boron nitride, borax, silver sulfate, cadmium iodide, and lead iodide. Graphite's low friction is due largely to adsorbed films; in the absence of water vapor, graphite loses its lubricating properties and becomes abrasive. Both graphite and molysulfide are chemically inert and have high thermal stability.

Soft solids

Variety of inorganic soft solids such as white lead, lime, talc, bentonite, silver iodide, and lead monoxide are used as lubricants.

Chemical conversion coatings

Many inorganic compounds can be formed on a metallic surface by chemical reaction. The best known such lubricating coatings are sulfide, chloride, oxide, phosphate, and oxalate films.

Organic Compounds

Solid organic lubricants are usually divided into two broad classes:

  1. Soaps - Waxes - Fats
  2. Polymeric Films
Soaps - Waxes - Fats

This class includes metallic soaps of calcium, sodium, lithium; animal waxes (e.g., beeswax and spermaceti wax); fatty acids (e.g., stearic and palmitic acids); and fatty esters (e.g., lard and tallow).

Polymeric Films

These are synthetic substances such as polytetrafluoroethylene (PTFE also known as Teflon®) and polychlorofluoroethylene. One major advantage of such film-type lubricants is their resistance to deterioration during exposure to the elements.

For Example: 13mm thick plates of polymeric film are used in modern prestressed concrete construction to permit thermal movement of beams resting atop columns. The long-lived polymeric film plate facilitates such expansion and contraction of the structural members.


Metals

Thin films of soft metal on a hard substrate can act as effective lubricants, if the adhesion to the substrate is good. Such metals include lead, tin, and indium.

Gaseous lubricants

Lubrication with a gas is analogous in many respects to lubrication with a liquid, since the same principles of fluid-film lubrication apply.

Although both gases and liquids are viscous fluids, they differ in two important particulars. The viscosity of gases is much lower and the compressibility much greater than for liquids.

Film thicknesses and load capacities therefore are much lower with a gas such as air.

In equipment that handles gases of various kinds, it is often desirable to lubricate the sliding surfaces with gas in order to simplify the apparatus and reduce contamination to and from the lubricant.

The list of gases used in this manner is extensive and includes air, steam, industrial gases, and liquid-metal vapors.

Properties of Lubricants

With so many types of materials capable of acting as lubricants under certain conditions, coverage of the properties of all of them is impractical. Mention is made only of those properties usually considered characteristic of commercially significant fluid lubricants.

Viscosity

Of all the properties of fluid lubricants, viscosity is the most important, since it determines the amount of friction that will be encountered between sliding surfaces and whether a thick enough film can be built up to avoid wear from solid-to-solid contact.

Viscosity customarily is measured by a viscometer, which determines the flow rate of the lubricant under standard conditions; the higher the flow rate, the lower the viscosity.

The rate is expressed in centipoises, reyns, or seconds Saybolt universal (SSU) depending, respectively, upon whether metric, English, or commercial units are used.

In most liquids, viscosity drops appreciably as the temperature is raised.

Since little change of viscosity with fluctuations in temperature is desirable to keep variations in friction at a minimum, fluids often are rated in terms of viscosity index.

The less the viscosity is changed by temperature, the higher the viscosity index.


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Pour point

The pour point, or the temperature, at which a lubricant ceases to flow, is important in appraising flow properties at low temperature. As such, it can become the determining factor in selecting one lubricant from among a group with otherwise identical properties.

Flash point

The flash point, or the temperature, at which a lubricant momentarily flashes in the pressure of a test flame, aids in evaluating fire-resistance properties. Like the pour-point factor, the flash point may in some instances become the major consideration in selecting the proper lubricant, especially in lubricating machinery handling highly flammable material.

Oiliness = Lubricity

Oiliness generally connotes relative ability to operate under boundary lubrication conditions. The term relates to a lubricant's tendency to wet and adhere to a surface. There is no formal test for the measurement of oiliness; determination of this factor is chiefly through subjective judgment and experience. The most desirable lubricant for a specific use need not necessarily be the oiliest; e.g., long-fiber grease, which is low in oiliness as compared with machine oils, is usually preferable for packing rolling bearings.

Neutralization number

The neutralization number is a measure of the acid or alkaline content of new oils and an indicator of the degree of oxidation degradation of used oils. This value is ascertained by titration, a standard analytical chemical technique, and is defined as the number of milligrams of potassium hydroxide required to neutralize one gram of the lubricant.

Penetration number

The penetration number, applied to grease, is a measure of the film characteristics of the grease.

The test consists of dropping a standard cone into the sample of grease being tested. Gradations indicate the depth of penetration: the higher the number, the more fluid the grease.


Tribology

Tribology is thus a complex interdisciplinary subject.

The phenomena considered in tribology are among the most fundamental and most common of those encountered by humans in interaction with their largely solid environment.

Many manifestations of tribology are beneficial and, indeed, make modern life possible.
The functioning of many mechanical systems depends on friction, lubrication and wear values.

While other effects of tribology constitute serious nuisances and careful design is necessary to overcome the inconvenience arising from excessive friction or wear.

On an overall basis, friction uses up, or wastes, a substantial amount of the energy generated by mankind, while a large amount of productive capacity is devoted to replacing objects made useless by wear.

Tribology is therefore receiving increasing attention, as it has become evident that the waste of resources resulting from high friction and wear is very great (more than 6% of the Gross National Product [GNP]). Correspondingly, the potential savings offered by improved tribological knowledge are also great.

Unfortunately, the background of most engineers and designers in this important area is seriously deficient.

For example:

The average mechanical engineer has had less than two hours of instruction on wear as part of his university study.

To compound the problem, coverage of tribology in most reference works is insufficient or outdated and provides little guidance to those involved in construction and design of new mechanisms.


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