A lot of theories about lubricant mechanisms or regimes were suggested by scientist but generally there have three different concepts of lubricant regimes. There are hydrodynamic lubrication, boundary lubrication and mixed lubrication.
Hydrodynamics Lubrication
Fluid film or hydrodynamics lubrication is the term given when a shaft rotating in a bearing is supported by a layer or wedge of oil so that the shaft not contact with the bearing material (Internet References,07/01/2010).Hydrodynamics lubrication is defined a system of lubrication in which the shape and relative motion of the sliding film having sufficient pressure to separate the surface.
Figure 2.1: Fluid film Lubrication
In the simple word, the surfaces are completely separated by a film of lubricant, which may be fluid, a gas or grease. The load supporting film can be created by motion of the solids (self-acting or hydrodynamic) or by a source of external pressure of the bearing (external pressurized or hydrostatic) and … hydrodynamic lubrication prevents wear in moving parts, and metal to metal contact is prevented (Internet References, 07/01/2010).
The friction and wear characteristics of bearing often indicate the mode of lubrication and a useful further guide is given by the ratio;
R = Effective film thickness
Surface roughness
This ratio not only indicates the mode of lubrication, but it has a direct bearing upon the effective life of lubricated machine components (Masjuki, 2005).The films of this lubrication are normally thicker than the dimension of the surface irregularities or protective surface films formed by boundary lubrication or chemical reaction. The film thickness ratio, R is 10 ~ 100.
Boundary Lubrication
The bodies come into closer contact at their asperities; the heat developed by the local pressures causes a condition which is called stick-slip and some asperities break off. At the elevated temperature and pressure conditions chemically reactive constituents of the lubricant react with the contact surface forming a highly resistant tenatious layer, or film on the moving solid surfaces (boundary film) which is capable of supporting the load and major wear or breakdown is avoided (Internet References, 07/01/2010). This mode of lubrication is encountered in door hinges and many machine tool slide ways. The thickness of the projective layers formed by physical and chemical reaction between the solid and the surrounding bulk lubricant; additives or atmosphere is usually small compared to the roughness of the solid surface. The length of fatty acid molecules frequently used as boundary lubricants and protective thickness of oxide films as small as 2nm. Hence for boundary lubrication, R ≤ 1.
Figure 2.2: Boundary Lubrication
There are three fundamental mechanisms of interaction between the lubricant and the solid involved to produce a protective lubricant film:
Physical adsorption
Chemical adsorption
Chemical reaction
Physical adsorption occurs when the molecules of the lubricant are held to the surface only by bongs such as van der waal's forces. No exchange of electrons takes place between the molecules of the adsorbate and the adsorbent. The molecules are weakly bonds and the formation of the boundary lubricant film is characterized by reversibility and mono and multilayer. Polar molecules, i.e. molecules that have a variation in electric charge along their length, such as long chain alcohols, aldehydes or acids, adsorb on to the surface with vertical orientation. Many molecules pack in as closely as possible and strengthen the film with lateral cohesive forces.
Chemical adsorption occurs when the lubricant molecules are held to the surface by chemical bonds. Electron exchange between the molecules of the adsorbate and the adsorbent takes places. The film might be only a monolayer thick and irreversibility and strong bounding energies characterize the formation. Lubricant films formed by polar molecules of for example fatty acids, soaps and esters lubricate effectively up to their
melting point. At temperature, typically in the range of 120 to 180 °C these lubricants
fail as result of reorientation, softening and melting. Film formed by chemical adsorption provides good lubrication at moderate loads, temperature and sliding velocities.
Chemical reactions between the lubricant molecules and the solid surface occur when there is an exchange of valence electrons and a new chemical compound is formed. The lubricating films are unlimited in thickness and characterized by high activation, high bounding energies and irreversibility. The chemical reaction boundary lubricants usually contain sulfur, chlorine or phosphorus atoms in the molecules. The films behave more like solids than viscous liquids but their shear strength is considerably lower than that of the substrates. The film is more stable than physically and chemically adsorbed films. The chemical reaction and the film formation is strongly activated at temperatures above 150 to 180 °C and the maximum temperature limits for the reactions varies in the range of 300 to 800 °C. The lubricant films formed by chemical reaction provide good lubrication for high loads, high temperature and high sliding speeds but are limited to reactive materials. The conditions are referred to as extreme pressure conditions.
Figure 2.3: Schematic of boundary lubrication
Mixed Lubrication
Mixed film lubrication is defined as a combination of hydrodynamic lubrication and boundary lubrication. This occurs when the hydrodynamic lubrication action generates pressure, which is insufficient too completely separate the two surfaces. This separation can be viewed in the ratio of the film generated by the full film mechanism to the combined surface roughness of two surfaces. The total applied load is thought to be partly carried by asperity contacts mechanism in the region of mixed lubrication this has been long poorly understood.
Figure 2.4: Mixed lubrication
VISCOSITY
The viscosity of a fluid may be defined qualitatively as its resistance of flow. Liquids are commonly described as "thick" or "thin" which is really an arbitrary visual assessment of their viscosity. The SI unit of kinematics viscosity is m²/s but the commonly used unit particle is stoked (S) or centistokes (sSt). Viscosity in lubricant is one of its most important and most evident properties.
Viscosity Stability
Excessive viscosity impedes starting at low ambient temperatures and causes unnecessary fuel consumption. A viscosity decrease can lead to wear and increased oil consumption. Oxidation can cause both viscosity increase and decrease, depending on the severity and extent. The oxidation of viscosity improvers will break them into smaller molecules, reducing their high temperature thickening, but with little effect at low temperature. If oxidation is severe, the lubricant can thicken to the extreme that it cannot be drained from the engine.
High temperature can also cause volatility losses - thickening the oil by concentration of less volatile, more viscous components. Volatility losses can often be estimated from the change in the level of detergent metals since they do not volatilize. This method however, can be misleading if the detergent metal have precipitated out of solution due to sludge formation.
Viscosity - Temperature Properties
Automatic transmission fluid is particularly valued for their excellent viscosity and temperature properties. Designed to lubricate transmission over a broad range of temperatures, they often find use in hydraulic or compressor applications where low temperature start up and operation is a problem with conventional oils. These viscosity properties are necessary, because transmissions are expected to operate over a temperature range of -40°C to +175°C.
At the high temperature end of the scale, internal leakage in pistons and valves increases to a point where pressures applied to clutches are significantly reduced. This results in delayed shifts, erratic shifts and possible clutch plate damage. One investigator reported that the limit for satisfactory operation of transmission was 3cSt. Therefore, the DEXTRON II minimum specification for ATF was set at 5.5 cSt at 100°C to allow for operation up to a maximum temperature of 175°C. However, different clearances and designs will have an effect on the minimum viscosity of 6.2 cSt at 100°C to satisfy the most critical transmission. Because most viscosity index (VI) improvers undergo some permanent shear loss, the viscosities of used ATF will be lower than the new fluid. For this reason, the 5.5 and 6.2 cSt limits for ATF are based on used fluids from transmission cycling tests.
A low temperature viscosity limit of 50 000 cP at -40°C (ASTM D-2983) is required to prevent cavitations of fluid at low temperatures good transmission performance is possible at viscosities as high as 4500 cP, but beyond that the high viscosity causes pressure fluctuations and perhaps sluggish and erratic operation.
To obtain the needed viscosity properties, it is necessary to use VI improvers, base stocks of relatively low viscosity which typically solvent extracted 100 neutrals, and sometimes pour depressants. Base stocks should, in addition, have low wax contents, which may require either special low temperature dew axing of paraffin base oils or natural low wax content naphthenic oils in combination with the paraffin oils.
INTRODUCTION TO FRICTION
Friction is the resistance to motion, which exist when a solid object is moved tangentially with respect to the surface of another, which it touches, or when an attempt is made to produce such a motion. The importance of friction may be seen in the fact that, as estimates show, a very substantial part of the total energy consumption of mankind is expended in overcoming frictional loses. Reduction of friction, either through improve design, or through the use of more suitable contacting materials, or again through the application of better lubricating substances, is thus an extremely important problem of modern technology.
It must not be overlooked, however, that very many processes of everyday life are dependent for their effectiveness on the presence of friction in large enough amounts. Hence the provision when required, of sufficiently large friction is also a task of great importance. We are all familiar with the fact that such simple process as walking or driving a car (in regard to starting, stopping and cornering), or gripping objects in our hands, cannot be readily carried out if friction is too low. When this occurs, we say that conditions are "slippery" and it becomes a friction problem to find the remedy. The maintenance of sufficiently high friction is required also in the function of such common devices as nails, screws and other fasteners.
Although the foregoing two categories comprise the two main frictional requirements, that of lowering friction when required, there is a third problem of some importance, that of maintaining friction constant within narrows limit. A typical example is provided by the brakes of an automobile, which will not stop the car rapidly enough if the friction is too low, but which will give passengers an unpleasant jerk forward if friction is too high. Other applications where friction must be under close control are in the metal-rolling industry, piston ring in cast iron bore and also in precision devices of many kinds where accurately controllable motion is desired.
Friction On Lubricant Surface
The friction on a lubricated surface is much affected by the type of lubrication exists between the rolling surfaces. In hydrodynamic and fluid lubrication, the surface that is in relative motion is separated by a lubricant layer of appreciable thickness and under ideal conditions there is no wear of the solid surface. The resistance to motion contributes the friction values. This is entirely due to the viscosity of the interposed layer.
However in practice it is impossible for us to get the hydro dynamic or fluid lubrication, particularly if the condition is with high loads and low sliding speed. In this case the thick lubricant film breaks down and the surface separated by lubricant film of only molecule dimension. This condition refers as boundary lubrication. In boundary lubrication the friction is influenced by the nature of underlying surface as well as chemical constituent of the lubricant.
INTRODUCTION TO WEAR
Wear is one of the three most commonly encountered industrial problem leading to the replacement of components and assemblies in engineering, the other being fatigue and corrosion. Wear is rarely catastrophic, but it reduces operating efficiency by increasing power loses, oil consumption, and the rate of component replacement.
Wear, as a part of the tribology scene, is now receiving considerably more attention and sufficient is known about wear mechanisms and their solution to encourage greater application of this knowledge (Eyre, T.S, 1976: 203-208).
Although some wear is to be expected during normal operation of equipment, excessive friction causes premature wear, and this creates significant economic costs due to equipment failure, cost for replacement parts and downtime. The way in which the removal of material from the surface takes place is described by several wear mechanisms such as adhesive, abrasive, fatigue, erosive and corrosive wear. It is very common that in a real situation, more than one wear mechanism is acting at the same time.
Type of Wear
(a) Abrasive wear
Abrasive wear occurs when hard particles penetrate a surface and displace materials in the form of elongated chips or slivers. Another wise smooth becomes roughened with fairly regular grooves, with or without loosely attached metallic debris. In practice, abrasive wear occurs under two or three body conditions as shown in figure below. The first generally operates under low stress conditions, with particles being transported across the surface with little breakdown in particles are being deliberately reduced in size or are trapped between two surfaces. In both cases only a small friction in the angle of attack and those particles rolling or sliding contribute 80°C and 120°C.
The two bodies' situation is most frequently encountered in the transport of loose minerals and the three bodies in mineral treatment and on the ingress of foreign particles into bearings. Volume of wear usually increases linearly with load and sliding distance. If deviations occur they are usually due to reduction in particle size of the abrasive or clogging of the surface (Eyre, T.S, 1976: 203-208).
(b) Adhesive wear
Adhesive wear occurs when surfaces slide against each other, and the pressure between the contacting asperities is high enough to cause local plastic deformation and adhesion. Adhesion is favored by clean surfaces, non oxidizing conditions, and by chemical and structural similarities between the sliding couple. Adhesion occurs between a few asperities which increase in size as motion continues. Eventually the junctions rupture at their weakest point, usually resulting in metal transfer from one surface to the other as depicted in figure above. Wear decrease if the asperity is harder, because the contact area is lower (area of contact in inversely proportional to Hv); increases is asperity is chemically clean, because bonding welding is more likely and increase if the wear couple is mutually soluble.
Figure 2.5: Surface sliding in each other
Metal adhesion and transfer may also occur due to harder surface asperities plugging through the softer counterpace. For example steel onto bronze. Metal debris then become trapped and embedded into the surface grooves on the steel surface (Eyre, T.S, 1976: 203-208).
(c) Corrosive wear
Corrosive wear occurs when sliding takes place in a corrosive environment. Ii the absence of sliding, the products of the corrosion will form a film on the surfaces. This film tends to slow down or even arrest the corrosion. However, the sliding action wears the film away, so the corrosion attack continues. When rubbing takes place in a corrosive environment, either liquid or gaseous, then surface reactions can take place.
(d) Pitting wear
Pitting wear is due to surface failure of the material as a result of stresses that exceed the endurance (fatigue) limit of the material. Metal fatigue is demonstrated by bending a piece of the metal wire, such as a paper clip, back and forth until it breaks. Whenever a metal shape is deformed repeatedly, it eventually fails. A different type of deformation occurs when a ball bearing under a load rolls along its race. While pitting is generally viewed as a mode of failure, some pitting wear is not detrimental.
(e) Surface-fatigue wear
Surface-fatigue wear is produced by repeated high stress attendant on the rolling motion, such as that of metal wheels on tracks or the bearing rolling in the 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 bearing and gears. For sliding surface, adhesive wear usually proceeds sufficiently rapidly that there is no time for surface-fatigue wear to occur.
