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MALVIYA NATIONAL INSTITUTE OF TECHNOLOGY
LUBRICANTS AND SPECIAL FLUIDS
College Id:- Batch:-
2011UEC1281 C-3 Btech. I yr
Dr. KIRTI SARASWAT
As usual large number of people deserves my thanks for the help they provided me for the preparation for this project report. First of all i would like to thanks my teacher Dr. Kirti Saraswat for her support during the preparation of this topic. I am very thankful for her guidance. I would also like to thanks my friends for the encouragement and information about the topic they provided to me during to me during my effort to prepare this topic.
LUBRICATION AND SPECIAL FLUIDS
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat. The property of reducing friction is known as lubricity.
Lubricants constitute a basic and fundamental necessity for the proper operation and maintenance of any mechanical system in general and automobiles in particular. The main function of lubricants is to decrease friction between various parts of a mechanical system having relative motion. But additionally, lubricants provide several other benefits to the system such as cooling, scavenging, improved load factor decreased wear and the tear and ecological control etc.
# Purpose of lubrication
An un-lubricated sliding part creates tremendous friction that needs great amount of power to move, slide or separate them. If friction reached the critical level, the heat will fuse the parts and will cause seizure that will ultimately bond the parts or burn them beyond use.
Proper lubrication eliminates the friction that totally contributes to this failure phenomenon. The lubricant stays in between the sliding matter and serves like roller bearings. It continuously reduces the coefficient of friction, thereby reducing the force to move and heat that leads to seizure, bonding and fire. The general purpose of lubrication is to separate the two sliding bodies to reduce friction.
Lubricant in machineries has to stay and maintain the lubricating ability to serve its purpose. This is indicated in the lubricant as drop point for grease and viscosity for oil. Load and working temperature condition are also a major consideration is lubricant selection. Oil and grease comes in basic forms as produced. The additives make the difference as to what lubrication compounds are added to satisfy the end use.
The main purpose of lubrication is to reduce friction and wear in bearings or sliding components to prevent premature failure. The effects of lubrication may be briefly explained as follows:
1. Reduction of Friction and Wear
Direct metallic contact between the bearing rings, rolling elements and cage, which are the basic components of a bearing, is prevented by an oil film that reduces the friction and wear in the contact areas. It prevents inter metallic contacts between slides by allowing film of lubricants preventing friction.
2. Extension of Fatigue Life
The sliding or rolling fatigue life of bearings depends greatly upon the viscosity and film thickness between the rolling contact surfaces. A heavy film thickness prolongs fatigue life, while insufficient film thickness shortens it.
3. Dissipation of Frictional Heat and Cooling
Circulating lubrication may be used to carry away frictional heat or heat transferred from the outside to prevent the bearing from overheating and the oil from deteriorating.
4. Other importance of lubrication
Adequate lubrication also helps to prevent foreign material from entering the bearings and guards against corrosion and rusting. Satisfactory bearing performance can be achieved by adopting the lubricating method that is most suitable for the particular application and operating conditions. In general, oil offers superior lubrication; however, grease lubrication allows a simpler structure around the bearings.
# Lubricant types and functions
Individual additives impart chemical or physical reactions that create three different types of responses. An individual lubricant additive may:
1. Enhance existing favorable base oil properties (viscosity, pour point, water release).
2. Suppress existing unfavorable base oil properties (oxidation and corrosion control).
3. Impart properties to the lubricant that the
base oil cannot provide P, AW performance).
Lubricant manufacturers use a variety of additives to support the basestock or add new properties. There are a few standardized recipes that lubricant manufacturers might use to create the common lubricant types: AW (antiwear), EP (extreme pressure), R&O (rust and oxidation inhibited).
For this reason, it is always best to avoid mixing different brands of lubricants, even within a particular viscosity grade and additive type. In addition, practitioner should know about the following specific additive properties:
1. Viscosity modifiers (VI improvers for high
temperatures; pour point depressants for low
2. Wear resistance additives (EP and AW).
3. Oxidation inhibitors.
5. Foam inhibitor
1. Viscosity modifiers
They help in changing the viscosity behavior of the lubricant across a temperature range. All base oils thin as the temperature rises. Viscosity modifiers help to slow the thinning process such that it is possible to use a lighter grade of oil for a cold-start requirement and still have sufficient oil thickness at normal operating temperature to protect the machine surfaces.
2. Wear resistance (EP and AW).
There are two classes of surface-protecting additives that are commonly used in industrial machines. AW (antiwear) additives work by forming resilient films on metal surfaces that are somewhat like the layer of tarnish that forms on silver eating utensils. AW additives often include zinc phosphorous chemicals that adsorb onto the metal surface to create
an organo-metallic oxide layer. This tarnish-like layer is easily rubbed off when the opposing metallic surfaces interact. The malleable oxide layer reforms and rubs away continuously, and at low operating component temperatures, leaving the machine component’s metal surface intact.
3. Oxidation inhibitors
They are provided to extendlubricant life cycles and reduce the formation of oxidation reaction by products in the sump. However, the primary driver for determining the typeand amount of oxidation inhibitors is the base oil quality and type. High operating temperatures, high moisture and air concentrations and the catalytic effects of wear metals all increase the need for oxidation inhibitors. The greater the additive treatment, the more complex the additive balance and the greater need for oxidation inhibitors.
They help the lubricant release moisture. This is important when the equipment is operating in very humid climates or in a plant atmosphere that is wet or humid. Paper mills, steel mills and food-processing operations have significant exposure to water-based process fluids. Although oil is hydrophobic it still retains a certain amount of water from the atmosphere
Even at low levels moisture is particularly harmful to lubricated components and increases wear from cavitation, adhesion and abrasion. In addition, when mixed with heat and wear metals, moisture rapidly accelerates the rate of oxidation. Moisture control is one of several critical contamination control parameters.
5. Foam inhibitors
They help prevent accumulation of air (formation of a foam layer) in the oil sump. Air contains oxygen, which is a primary cause of oxidation. Foaming increases the extent of air-to-oil surface contact and increases oxidation. Low viscosities do not require foam-release agents. Medium to heavy grade oils (ISO 150 and higher) tend to retain air and benefit from foam inhibitors. This type of additive is one of the few additives that can be replaced if/when it is stripped from the lubricant through filtration or normal use.
# Lubricant selection
Finished lubricants represent a carefully engineered balancing act between the strengths andweaknesses of the many types of additive agentsand basestocks. Practitioners should be well aware of the performance properties of the three common types of finished lubricants.
1. R&O (rust and oxidation inhibited) lubricants
These are selected for machines that operate without any expected metallic component interaction , where machines with interacting components run with continuous moderate to high speeds and/or relatively low loads (pumps, compressors, bearing circulation systems) and where the lubricant is needed primarily to keep surfaces wetted and protect the surfaces from moisture-induced corrosion.
2. Antiwear lubricants
These are selected for machines that operate with expected metallic component interaction but where the interactions are moderate to low loads and generally moderate to high speeds. These conditions are found almost universally in hydraulic applications where a significant amount of the AW product type is found. Other machines with
light and continuous machine interactions also may be served with AW products, such as plain and element bearing circulation systems, crankcase applications (compressor crankcases) and some instances of chain bath lubrication
3. EP lubricants
These are selected for machine applications that operate routinely (by design) with continuous component interaction or with high continuous loads and intermittent shock loading. EP lubricants are typically recommended for geared machines (excluding those with internal backstops and yellowmetal gear).
# Classification of Lubricants
Based on its physical states, you can classify lubricants as:
1. Liquid lubricants
i Vegetable oil and animal oil
ii Mineral oil from petroleum
iii Blended oil, doped oil, or compound oil
1. Liquid Lubricants or Lubricating Oils
Liquid lubricants reduce friction and wear between two moving or sliding metallic surfaces by providing a continuous fluid film in between them. They act as a cooling medium, a sealing agent, and a corrosion preventor. Liquid lubricants are classified into many types, depending on the type of base oil used.
(i) Vegetable Oil and Animal Oil
These oils are the most commonly used lubricants. They are quite oily and are absorbed by all the metallic surfaces. However, they decompose at high temperature, undergo oxidation easily, forming gummy and acidic hydrolyzed products, and thicken on coming in contact with air, restricting the smooth movement of moving surfaces. Consequently, these oils are blended with mineral oils to overcome these restrictions.
(ii) Mineral Oil from Petroleum
Mineral oils that are used as lubricants are obtained by fractional distillation of petroleum. Petroleum oils have long-chain hydrocarbons, ranging between 12-50 Carbon atoms. The shorter chain oils have lower viscosity than the longer chain hydrocarbons do. Shorter chain hydrocarbons are widely used as lubricants because they possess good stability. At very high pressure and speed, petroleum oil possesses poor oiliness but the oiliness of these oils can be improved by adding oils, such as Oleic and Stearic acid.
(iii) Blended oil or compound oil
It is obvious that no single oil can behave ideally for variety of conditions developing in lubricating systems. However carefully refined minerals do serve the purpose to a large extent. But it was found that addition of specific compounds (additives) improves the functions of lubricant significantly. So, it was established that by selective addition of these compounds (additves), desired quality of lubricant is improved. Few additives used are discussed below:
(a) Viscosity index improver
These additives prevents the lubricating oils from thinning at higher temperature and from solidifying at lower temperature. such additives are usually hydrocarbon polymers. they can be either poly isobutylene, polystyrene or long chain alkyl acrylates and polyesters.
(b) Pour-point depressants
This type of additive helps the lubricating oil to maintain fluid characterstics even at low temperatures. these additives act like a film or coating of colloidal surrounding over wax crystal nuclei. By doing so, these adiitives prevent the growth of bigger crystals, bush heap or gel like structure which may immobilize rest of oil. Few pour-point depressants are Rislone, Paraflow, etc. which are polymeric materials.
The term detergent with reference to lubricant has similarity to detergent in domestic laundry. This kind of additives clears the machine parts from dust and dirt thereby reducing friction and jamming. It is observed that octane requirement of an engine increases as the engine becomes more and more dirty. The ideal detergent wets and spreads ove the soiled surface.
Certain substances when added to the lubricant, increase the resistance of oil towards its oxidation . these anti-oxidants, those reduce affinity of oxygen towards oil, are usually organic phosphorous compounds, phenols or substituted amino type compounds. A good amount of engine dirt is contributed by oxidation product of oil. These anti-oxidants reduce engine deposit showing an improvement on octane requirement.
(e) Corroision inhibitor
These substances are added to reduce or prevent any kind of corrosion to bearings or other metal surfaces. these additives are effective by not allowing a contact of metal surfaces to corrosive substances. usually these additives belong to the organic compounds of phosphorous and antimony.
(f) Additives for higher pressure
Under extreme pressure in a system, it becomes difficlt to maintain a thick layer of oil so that metal to metal contact is avoided. Certain additives are added so that they get absorbed on surface or get chemically attached to the surface in such a way that a film is created which is easily sheared. This system helps the oils to be retained in place and carry the load. In such cases, substances which contain the groups that form strong chemical bonds with the surfaces are preferred as additives loke Diphenyl disulphide or phosphorous derivative.
2 Semi-solid Lubricants
The term grease is used to describe semisolid lubricants. Although the word grease is also used to describe rendered fat of animals, in the context of lubrication, grease typically applies to a material consisting of a soap emulsified with mineral or vegetable oil. The characteristic feature of greases is that they possess a high initial viscosity, which upon the application of shear, drops to give the effect of an oil-lubricated bearing of approximately the same viscosity as the base oil used in the grease. This change in viscosity is called thixotropy. Grease is sometimes used to describe lubricating materials that are simply soft solids or high viscosity liquids, but these materials do not exhibit the shear-thinning (thixotropic) properties characteristic of the classical grease. For example, petroleum jellies such as Vaseline are not generally classified as greases.
Greases are applied to mechanisms that can only be lubricated infrequently and where a lubricating oil would not stay in position. They also act as sealants to prevent ingress of water and incompressible materials. Grease-lubricated bearings have greater frictional characteristics due to their high viscosity.
3 Solid Lubricants
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 media.
Such lubricants, including materials such as graphite, hexagonal boron nitride, molybdenum disulfide and tungsten disulfide are also able to offer lubrication at temperatures higher than liquid and oil-based lubricants are able to operate. These lubricants are often to be found in applications such as locks or dry lubricated bearings. Such materials can operate up to 350°C in oxidizing environments and even higher in reducing / non-oxidizing environments (molybdenum disulfide up to 1100°C). The low-friction characteristics of most dry lubricants are attributed to a layered structure on the molecular level with weak bonding between layers. Such layers are able to slide relative to each other with minimal applied force, thus giving them their low friction properties. However, a layered crystal structure alone is not necessarily sufficient for lubrication. In fact, there are also some solids with non-lamellar structures that function well as dry lubricants in some applications. These include certain soft metals (indium, lead, silver, tin), polytetrafluroethylene, some solid oxides, rare-earth fluorides, and even diamond.
The four most commonly used solid lubricants are:
1. Graphite. Used in air compressors, foodstuff industry, railway track joints, open gear, ball bearings, machine-shop works etc. It is also very common for lubricating locks, since a liquid lubricant allows particles to get stuck in the lock worsening the problem.
2. Molybdenum disulfide. Used in CV joints and space vehicles. Does also lubricate in vacuum.
3. Hexagonal boron nitride. Used in space vehicles. Also called "white graphite".
4. Tungsten disulfide. Similar usage as molybdenum disulfide, but due to the high cost only found in some dry lubricated bearings.
4 Synthetic Lubricants
In large number of modern machines, the vegetable oil as well as mineral oils fail to serve as lubricant due to extreme conditions of
preesure, speed and temperature. The above mentioned oils will decompose or may catch the fire or totally freeze. To overcome these difficulties, special kind of lubricants may be required for many systems, such as die casting, furnace equipment, hot rolling mills, bearings of jet planes, and so on. Lubricants which will not only work under extreme conditions but also will bear a wide range of variable conditions have been synthesised by the help of organic and inorganic chemicals. Synthetic lubricants are designed for special job and there are nearly hundreds of commercial fluids now in the market. Synthetic oils are usually Silicon phosphates and Ester chemicals.
# Mechanism of lubrication (Types of lubrication)
Lubricant when placed between two moving or sliding surfaces, which may be circular, linear or some other kind of movement, tends to work in one of the following three ways. In general there are three types of theories or mechanisms by which role of lubricant can be understood, i.e.,
(i) Boundary lubrication (Thin film lubrication),
(ii) Extreme pressure lubrication
(iii) Fluid film or hydronamic lubrication (Thick film lubrication),
(i) Boundary or thin film lubricant
When the lubricant is not viscous enough to generate a film of sufficient thickness to separate the surfaces under heavy loads, friction may yet be reduced with the proper lubricant. Such an application is known as boundary lubrication. A thin later of lubricant is adsorbed on the metallic surfaces which avoids direct metal to metal contact. The load is carried by the later of the adsorbed lubricant on both the metal surfaces. In boundary lubrication, the distance between moving/ sliding surface is very small of the order of the height of the space asperities. The contact between the metal surfaces is possible by the squeezing out of lubricating oil film. When this occurs the load would be taken on the high spots of the journal and the bearing, and the two surfaces tend to because welded together by appreciable heat generated. This prevents motion as the two surfaces adhere together. For boundary lubrication the lubricant molecules should have ling hydrocarbon chains, high viscosity index, resistance to heat and oxidation, good oiliness, low pour and oxidation. Graphite and MoS2, Vegetable and animal oils and their soaps are used for boundary lubrication.
(ii) Extreme pressure lubrication
It is done by incorporating extreme pressure additives in mineral oils for applications in which high temperature is generated due to high speed of moving/sliding surfaces under high pressure. Chlorinated esters, sulphurised oils and tricrysyl phosphate are examples of such additives. These additives react with metallic surfaces, at prevailing high temperatures, to form metallic chlorides, sulphides or phosphates in the form of durable film. These films can withstand very high loads and high temperatures. Applications: wire drawing of titanium, in cutting fluids in machining of tough metals, for hypoid gears used in rear axle drive of cars.
(iii) Hydrodynamic or fluid film or Thick-film lubrication
In this, the moving/sliding surfaces are separated from each other by a bulk lubricant film (at least 1000oA thick).This bulk lubricant film prevents direct surface to surface contact so that the small peaks and valleys do not interlock. This consequently reduces friction and prevents wear. Fluid film lubrication is shown in figure. The small friction is only due to the internal resistance between the particles of the lubricant moving over each other. In such a system, friction depends on the thickness and viscosity of the lubricant and the relative velocity and area of the moving/sliding surfaces the co-efficient of friction is as low as 0.002 to 0.03 for fluid film lubricated system.
# Characterstics of lubricating oils
Technically, the viscosity of an oil is a measure of the oil’s resistance to shear. Viscosity is more commonly known as resistance to flow. If a lubricating oil is considered as a series of fluid layers superimposed on each other, the viscosity of the oil is a measure of the resistance to flow between the individual layers. A high viscosity implies a high resistance to flow while a low viscosity indicates low resistance to flow. Viscosity varies inversely with temperature. Viscosity is also affected by pressure; higher pressure causes the viscosity to increase, and subsequently the load-carrying capacity of the oil also increases. This property enables use of thin oils to lubricate heavy machinery. The loadcarrying capacity also increases as operating speed of the lubricated machinery is increased.
Two methods for measuring viscosity are commonly employed:
When viscosity is determined by directly measuring shear stress and shear rate, it is expressed in centipoise (cP) and is referred to as the absolute or dynamic viscosity. In the oil industry, it is more common to use kinematic viscosity, which is the absolute viscosity divided by the density of the oil being tested. Kinematic viscosity is expressed in centistokes (cSt). Viscosity in centistokes is conventionally given at two standard temperatures: 40 EC and 100 EC (104 EF and 212 EF ).
Another method used to determine oil viscosity measures the time required for an oil sample to flow through a standard orifice at a standard temperature. Viscosity is then expressed in SUS (Saybolt Universal Seconds). SUS viscosities are also conventionally given at two standard temperatures: 37 EC and 98 EC (100 EF and 210 EF). As previously noted, the units of viscosity can be expressed as centipoise (cP), centistokes (cST), or Saybolt Universal Seconds (SUS), depending on the actual test method used to
measure the viscosity.
b. Viscosity index
The viscosity index, commonly designated VI, is an arbitrary numbering scale that indicates the changes in oil viscosity with changes in temperature. Viscosity index can be classified as follows:
(i)low VI - below 35
(ii) medium VI - 35 to 80
(iii) high VI - 80 to 110
(iv) very high VI - above 110
A high viscosity index indicates small oil viscosity changes with temperature. A low viscosity index indicates high viscosity changes with temperature. Therefore, a fluid that has a high viscosity index can be expected to undergo very little change in viscosity with temperature extremes and is considered to have a stable viscosity. A fluid with a low viscosity index can be expected to undergo a significant change in viscosity as the temperature fluctuates. For a given temperature range, say -18 to 370EC ( 0 - 100 EF), the viscosity of one oil may change considerably more than another. An oil with a VI of 95 to 100 would change less than one with a VI of 80. Knowing the viscosity index of an oil is crucial when selecting a lubricant for an application, and is especially critical in extremely hot or cold climates. Failure to use an oil with the proper viscosity index when temperature extremes are expected may result in poor lubrication and equipment failure. Typically, paraffinic oils are rated at 38 EC ( 100 EF) and naphthenic oils are rated at -18 EC (0 EF). Proper selection of petroleum stocks and additives can produce oils with a very good VI.
c. Pour point
The pour point is the lowest temperature at which an oil will flow. This property is crucial for oils that must flow at low temperatures. A commonly used rule of thumb when selecting oils is to ensure that the pour point is at least 10 EC (20 EF) lower than the lowest anticipated ambient temperature.
d. Cloud point
The cloud point is the temperature at which dissolved solids in the oil, such as paraffin wax, begin to form and separate from the oil. As the temperature drops, wax crystallizes and becomes visible. Certain oils must be maintained at temperatures above the cloud point to prevent clogging of filters.
e. Flash point and fire point
The flash point is the lowest temperature to which a lubricant must be heated before its vapor, when mixed with air, will ignite but not continue to burn. The fire point is the temperature at which lubricant combustion will be sustained. The flash and fire points are useful in determining a lubricant’s volatility and fire resistance. The flash point can be used to determine the transportation and storage temperature requirements for lubricants. Lubricant producers can also use the flash point to detect potential product contamination. A lubricant exhibiting a flash point significantly lower than normal will be suspected of contamination with a volatile product. Products with a flash point less than 38 EC (100 EF) will usually require special precautions for safe handling. The fire point for a lubricant is usually 8 to 10 percent above the flash point. The flash point and fire point should not be confused with the auto-ignition temperature of a lubricant, which is the temperature at which a lubricant will ignite spontaneously without an external ignition source.
f. Acid number or neutralization number
The acid or neutralization number is a measure of the amount of potassium hydroxide required to neutralize the acid contained in a lubricant. Acids are formed as oils oxidize with age and service. The acid number for an oil sample is indicative of the age of the oil and can be used to determine when the oil must be changed.
h. Total base number
Internal combustion engine oils are formulated with a highly alkaline (base) additive package designed to neutralize the acidic byproducts of combustion. The Total Base Number (TBN) is a measure of this additive package, and it may be used as an indication of when diesel engine oil should be changed.
i. Water content
The most common contaminant in Naval lubricating systems is water. Common sources of water include lube oil cooler leaks, condensation, steam turbine gland seal leaks, and diesel engine piston blow-by and jacket water leaks. The acceleration of system corrosion by water contamination cannot be overemphasized. In addition, excessive water contamination increases the viscosity and decreases the fluid film strength of an oil. This may result in accelerated wear due to rupture of the oil film and resultant surface-tosurface contact.
A qualitative assessment of the amount of water present in some lubricants may be made by inspecting the oils’ appearance. Another method for determining water contamination levels is the Bottom Sediment & Water (B.S.& W.) test .
Demulsibility refers to a lubricant’s ability to readily separate from water. Oils used in force-feed lubrication systems should possess good water separability to prevent emulsification.
Greases are classified according to a hardness scale developed by the National Lubricating Grease Institute (NLGI). According to this system, softer greases are assigned a low NLGI number, and stiffer greases a high NLGI number . The penetration numbers refer to the depth, in tenths of millimeters, that a weighted cone penetrates the grease. Most Naval greases have NLGI numbers from 1 to 2, and are classified as medium consistency greases.
l. Dropping point
Greases exist in an essentially semi-solid form. The temperature at which a grease changes from a semi-solid to a liquid is termed its dropping point. Dropping point provides some indication of the high temperature characteristics of a grease.
m. Water washout
Greases subjected to splashing or impinging water must possess good water washout resistance. Greases with good resistance will maintain an adequate lubricating film under excessive water contamination conditions.
n. Load carrying ability
The ability of a lubricant to maintain an effective lubricating film under high loads or pressures is a measure of its load carrying or extreme pressure (EP) characteristics. The load carrying ability of a lubricant may be enhanced by the addition of EP additives.
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