Lubrication and Lubricating Oils
Lubrication and Lubricating Oils
The principle of supporting a sliding load on a friction reducing film is known as lubrication. The substance of which the film is composed is a lubricant, and to apply it is to lubricate. All liquids provide lubrication of a sort, but some do it a great deal better than others. Lubrication oils are being used for the equipment lubrication since a long time and since then they have evolved from the conventional mineral-oil based to the synthetic lubricants.
The primary purpose of lubrication is to reduce wear and heat between the contacting surfaces in relative motion. While wear and heat cannot be completely eliminated, they can be reduced to negligible or acceptable levels. Because heat and wear are associated with friction, both effects can be minimized by reducing the coefficient of friction between the contacting surfaces. Other purposes of the lubrication include (i) reduce friction, (ii) reduce oxidation and prevent corrosion, (iii) disperse contaminants, (iv) act as a sealant against dust, dirt, and water, (v) transmit mechanical power in hydraulic fluid power applications, and (vi) provide insulation in the transformer applications. The properties which the that lubricants need to have are (i) extreme pressure resistance, (ii) high thermal stability, (iii) high wear resistance, (iv) consistency, (v) very good fluidity, and (vi) rust prevention capabilities.
Lubricated friction is characterized by the presence of a thin film of the pressurized lubricant (squeeze film) between the two moving surfaces. The ratio of the squeeze film (oil film) thickness to the surface roughness determines the type of the lubrication regime
The current period of lubrication began with the work of Osborne Reynolds (1842-1912). His work was concerned with shafts rotating in bearings and cases. He found that when a lubricant is applied, a rotating shaft pulled a converging wedge of lubricant between the shaft and the bearing. He also noted that as the shaft gained velocity, the liquid flowed between the two surfaces at a greater rate. This, because the lubricant is viscous, produces a liquid pressure in the lubricant wedge which is sufficient to keep the two surfaces separated. It was shown by him that under ideal conditions, this liquid pressure is to be high enough to prevent direct contact between the two metal surfaces. The three positions of a shaft in a bearing are shown in Fig 1.
Fig 1 Three position of a shaft in a bearing
Stribeck (Stachowiak and Batchelor, 2001) demonstrated that the coefficient of friction is directly proportional to the viscosity of the lubricant and the difference in the speeds of the contact surfaces, and inversely proportional to the pressure which is exerted on the contact surfaces. In the Stribeck curve, the coefficient of friction is plotted against the expression ZN/P (sometimes referred to as the Hersey number). ZN/P is defined as (lubricant viscosity Z × shaft speed N) / bearing contact pressure P. The Stribeck curve (Fig 2) has three distinct zones known as (i) boundary lubrication, (ii) mixed lubrication, and (iii) hydrodynamic lubrication. There is also an overlapping regime of elasto-hydrodynamic lubrication.
Fig 2 Lubrication regimes
In boundary lubrication regime, the surface contact to a mono-molecular film of lubricant is between 1 nm (nano metre) and 4 nm in thickness. Boundary lubricant film becomes very thin due to heavy load on its surface where the lubricant film breaks down. The surface contact is initiated at asperities junction, and initially load is supported by it. High friction and wear are generally encountered in boundary lubrication regimes.
A transition state between boundary and hydrodynamic lubrication regimes is known as mixed lubrication. In this situation, there is frequent asperity contact but at least a portion of solid surface is partially supported by a hydrodynamic film.
The lubrication regime known as elasto-hydrodynamic lubrication occurs in the lubricated contact of non-conforming bodies with high elastic modulus, such as most metals and ceramics. The very high local pressures which result from the geometry of such contacts produce both elastic flattening of the surfaces and also, generally, a very large increase in the viscosity of the lubricant. The combination of these two effects enables lubricant entrainment into the contact to form a separating film of oil despite the very high contact pressure.
Hydrodynamic lubrication is also known as full film or fluid film lubrication. The high lubricant viscosity creates enough fluid pressure to build a supporting film and eliminates all solid-surface contact. The film thickness is quite high in this type of lubrication system compared to others.
The formation of fluid film is influenced by several factors namely (i) the contact surfaces are to meet at a slight angle to allow formation of the lubricant wedge, (ii) the fluid viscosity is to be high to maintain adequate film thickness to separate the contacting surfaces at operating speeds, (iii) the fluid is to be adhering to the contact surfaces for conveyance into the pressure area to support the load, (iv) the fluid is to be distributing itself completely within the bearing clearance area, (v) the operating speed is to be sufficient to allow formation and maintenance of the fluid film, and (vi) the contact surfaces of bearings and journals are to be smooth and free from sharp surfaces which disrupt the fluid film.
The term lubricating oil is generally used to include all those classes of lubricating materials which are applied as fluids. The history of lubricating oil began as soon as man discovered that reduced friction meant greater efficiency. There are many types of lubricating oils and each type of lubricating oil has been developed over a period of time to serve a different purpose.
All liquids generally provide lubrication of a sort, but some do it a great deal better than others. The difference between one type of lubricating oil with other type of lubricating oil is often the difference between successful operation of a machine and failure. For almost every situation, petroleum products have been found to excel as lubricating oils. Petroleum lubricants stand high in metal-wetting ability. They possess the body, or viscosity characteristics, which a substantial film requires. These lubricating oils have many additional properties which are essential to modern lubrication, such as good water resistance, inherent rust-preventive characteristics, natural adhesiveness, relatively good thermal stability, and the ability to transfer frictional heat away from lubricated parts. In addition to this, nearly all of these properties can be modified during manufacture to produce suitable lubricating oil for each of a wide variety of applications. Lubricant oils have been developed hand-in-hand with the modern plant and equipment which they lubricate.
Base stocks are refined from crude oil to obtain products with the best lubricating properties. Base stocks generally make up 80 % to 95 % of typical engine oil with 5 % of additives. Base stock is used to describe plain mineral oil. The physical properties of the lubricating oil depend on its base stock. In most cases it is chemically inert. There are three sources of base stock namely (i) biological, (ii) mineral, and (ii) synthetic. The oils manufactured from these sources show different properties and they are suitable for different applications.
Biological oils are basically the vegetable (castor, palm, or rapeseed) oils and animal based oils. Vegetable based lubricating oils are less stable (rapid oxidation) than the mineral oil based lubricating oils at high temp. These lubricating oils contain more natural boundary lubricants than the mineral oils. Animal based lubricating oils (sperm, fish, and wool oils from sheep such as lanolin) have extreme pressure properties but they have issues related to their availability. Biological based lubrication oils are suitable in applications where the risk of contamination is required to be reduced to a minimum, for example, in the food or pharmaceutical industry. They are generally applied to lubricate kilns, and bakery ovens etc.
Mineral oils are the most commonly used lubricating oils. They are petroleum based and are used in applications where temperature requirements are moderate. Typical applications of mineral oils are in gears, bearings, engines, and turbines etc.
Synthetic oils are artificially developed substitutes for mineral oils. They are specifically developed to provide lubricating oils with superior properties than the mineral oils. Synthetic lubricating oils have viscosity which does not vary as much with temperature as in mineral oil. Their rate of oxidation is much slower but they are having high cost. The temperature resistant synthetic oils are used in high performance machinery operating at high temperatures. Synthetic oils for very low temperature applications are also available.
In the present day situation, the various industries and transportation facilities are is dependent upon on the petroleum based lubricating oils. These oils are complex mixture of hydrocarbon molecules and they represent one of the important classifications of products derived from the refining of crude petroleum oils, and are readily available in a great variety of types and grades.
General classification of the mineral oil based lubricating oils
Lubricating oils are made from the more viscous portion of the crude oil which remains after removal by distillation of the gas oil and lighter fraction. Although crude oils from various parts of the world differ widely in properties and appearance, there is relatively little difference in their elemental analysis. Thus, crude oil samples generally show carbon (C) content ranging from 83 % to 87 % and hydrogen (H2) content from 11 % to 14 %. The remainder is composed of elements such as oxygen (O2), nitrogen (N2), sulphur (S), and various metallic compounds. An elemental analysis, hence, gives little indication of the extreme range of physical and chemical properties that actually exists, or of the nature of the lubricating base stocks which can be produced from a particular crude oil.
The chemical structure of lubricating oils (Fig 3) contains of hydrocarbons of the crude oils which consist of namely paraffinic components (Fig 3a and 3b) , (ii) naphthenic components (Fig 3c), aromatic components (Fig 3d), and (iv) non hydrocarbon components. The classifying of hydrocarbon as paraffinic, naphthenic and aromatic groups which are generally used for characterizing the base oil is not to be taken as absolute but as an expression of the predominating chemical tendencies of the base stocks.
Fig 2 Chemical structure of lubricating oils
The paraffinic components determine the pour point and contain not only linear but also branched paraffins. The straight chain paraffins of high molecular weights raise the pour point of the lubricating oils (waxy compounds) and are to be removed by dewaxing processes. The branched paraffins are chemically interesting hydrocarbons and they are found in large quantities in lubricating oil fractions from paraffinic crudes. Lubricating oil rich in paraffinic hydrocarbons have relatively low density and viscosity for their molecular weight and boiling range. Also, they have good viscosity / temperature characteristics. In general, paraffinic components are reasonably resistant to oxidation and have particularly good response to oxidation inhibitors.
The naphthenic components have rather higher density and viscosity for their molecular weight compared to the paraffinic components. An advantage which naphthenic components have over the paraffinic components is that they tend to have low pour point and so do not contribute to wax. But, one disadvantage is that they have inferior viscosity / temperature characteristics. Single ring alicyclics with long paraffinic side chains, however, share many properties with branched paraffins and can in fact be highly desirable components for lubricant base oils. Naphthenic components tend to have better solvency power for additives than paraffinic components but their stability to oxidative processes is inferior.
The aromatic components have densities and viscosities which are still higher. Their viscosity / temperature characteristics are in general poor but pour point is low, though they have the best solvency power for additives. Their stability to oxidation is poor. As for alicyclics, single ring aromatics with long paraffinic side chain can be very desirable base oil components.
The non hydrocarbons in lubricating oil are analogous in many ways to the hydrocarbons. S and N2 compounds are found almost entirely in ring structures such as sulphides, thiophene, pyridine, and pyrrol types. More complex molecules are also considered to exist in lubricating oil in which N2 and S atoms are found in the same molecule. As in the case of hydrocarbons, these compounds probably also have paraffinic side chains and possibly be condensed with naphthenic and aromatic ring structures. Though these non hydrocarbons can be present in only trace amounts, they often play a major role in controlling the properties of lubricating oils. In general they are chemically more active than the hydrocarbon, and hence they may markedly affect properties such as oxidation stability, thermal stability, and deposit forming tendencies.
Important properties of lubricating oils
The important properties which the lubricating oils are required to have for their performance are described below.
Physical properties of lubricating oil
Viscosity – It is the measure of the internal friction within a liquid, that is, the way the molecules interact to resist motion. It is a vital property of the lubricating oil since it influences the ability of the oil to form a lubricating film or to minimize friction. The absolute viscosity of a liquid is defined as the ratio between the applied shear stress and the resulting shear rate.
Viscosity index – It is the most frequently used method for comparing the variation of viscosity with temperature between different lubricating oils. It is a dimensionless number. The kinematic viscosity of the lubricating oil sample is measured at two different temperatures (40 deg C and 100 deg C) and the viscosity is compared with an empirical reference scale. Viscosity index is used as a convenient measure of the degree of aromatics removal during the base oil manufacturing process, but comparison of the viscosity index of different oil samples is only realistic if they are derived from the same distillate feedstock.
Low temperature properties – When a sample of the lubricating oil is cooled, its viscosity increases in a predictable manner until wax crystals start to form. The matrix of wax crystals becomes sufficiently dense with further cooling to cause an apparent solidification of the lubricating oil. Although the solidified oil does not pour under the influence of gravity, it can move if sufficient force is applied. Further decrease in temperature cause more wax to form, increasing the complexity of the wax / oil matrix. Many lubricating oils have to be capable of flow at low temperatures and a number of properties such as cloud point, and pour point etc. are to be measured.
High temperature properties – The high temperature properties of oil are governed by distillation or boiling range characteristics of the oil. These are (i) volatility which is an indication of the tendency of the lubricating oil to be lost in service by vapourization, (ii) flash point of the oil which is important from a safety point of view because it is the lowest temperature at which auto-ignition of the vapour occur above the heated oil sample.
Other physical properties – Various other physical properties can be measured, most of them relating to specialized lubricant applications. Some of the more important physical properties are (i) density which is important, since lubricating oils can be formulated by weight, but measured by volume, (ii) demulsification which is the ability of oil and water to separate, (iii) foam characteristics which is the tendency to foam formation and stability of the foam which results, (iv) pressure / viscosity characteristics, (v) thermal conductivity which is important for heat transfer fluid, (vi) electrical properties such as resistively, and dielectric constant, and (vii) surface properties as surface tension, and air separation etc.
Chemical properties of lubricating oils
Ease of starting rapidity of warming up – The main aspect in the usage of lubricating oil is its viscosity. It is not enough that the lubricating oils are to have the proper viscosity but also they are to maintain the little viscosity change within the temperature range during and after its application. Hence, the viscosity controls not only frictional and thermal effect but also oil flow as a function of the load speed, temperature and design of the equipment being lubricated. In other words, if the equipment often does not make a cold start, then it is also important that the viscosity at starting temperature is not so high that the equipment cannot be started. The rapidity with which the equipment can be put to work is dependent on the speed of circulation and supply of oil to vital components. All forms of wear and even the safety of the equipment are influenced by rapidity of circulation of the lubricating oil.
Low carbon forming tendency – This property is important for high compression ratio petrol engine where The C deposit adversely affects the combustion quality. The extent as well as the composition of such formed deposits causes noisy and rough burning which subjects the engine to high thermal and mechanical stresses resulting in lowering of performance and reduction of engine life. The typical symptoms are knocking, pre-ignition and surface ignition. These call higher octane fuels which are more expensive and do not eliminate the need for ultimate decarbonizing.
Carbon residue test methods provide some indication about the relative coke forming tendency of the oil in some application and quality-controlled lubricating oils. So, the C residue test can be helpful in selecting lubricating oils for certain industrial applications such as heat treating, lubrication of bearing subjected to high temperature and air compressors. It is claimed that the presence of viscous oil (bright stock) in the base oils plays an important role in the formation of C deposits.
High oxidation stability – One of the most important requirements of the lubricating oil is that its properties are not changed during use. The lubricating oil is often subjected to several oxidizing conditions which are primarily due to the oxidative changes of the oil. While the temperature of the lubricating oil, presence of O2, and nature by products of the composition contribute to the oxidative change properties of the lubricating oil during use. Hence, it is essential that the lubricating oil, when exposed to high temperature, does not contribute to the forming of deposits even after a long period of continuous equipment running. So, the resistance of the lubricating oil to the oxidative depends mainly on the nature of the lubricating oil and the presence of the anti-oxidant additives.
Wear reduction – Wear occurs in lubricated systems by three mechanisms namely (i) abrasion, (ii) corrosion, and (iii) metal-to metal contact (adhesion). The lubricating oil plays an important role in combating each type of wear.
The abrasive wear is caused by solid particles entering into the area between the lubricated surfaces which physically erode these surfaces and can contaminate wear fragments. To cause wear, the solid particles are to be larger than the oil-film thickness and harder than the lubricated surfaces. The flushing action of the lubricating oil, especially in forced feed or once through systems, helps in removing the potentially harmful solid particles from the area of lubricated surfaces.
The corrosive wear is normally caused by the products of oxidation of the lubricating oil. The high S content of the fuel helps the corrosive attack. In other words, corrosion is the principal cause of wear in the internal combustion engines because the products of combustion are highly acidic and contaminate the lubrication oil, and the lubricating oils function to minimize corrosive wear. It is achieved by proper refinement along with the use of oxidation inhibitors which reduces lubricant deterioration and keeps the level of corrosive oxidation products low.
The adhesive wear can significantly affect certain parts of the equipment where metal-to-metal contact takes place. Adhesive wear takes place due to the breakdown of the lubricant film. It can also be the result of excessive surface roughness or interruption of the lubricant supply. A plentiful supply of the proper viscosity of oil is often the best way to avoid these conditions. The composition of the base oil and addition of certain chemical additives are also some of the important methods for the protection of equipment parts against the adhesive wear.
Detergency and dispersant property – With the exception of detergency and dispersant property in the combustion chamber, deposits in the lubricating oil are controlled by its detergent power. The sources of the deposits found in the engines are many and their volume depends mainly on the use, the quality of combustion, the temperature of lubricating oil and coolant, and on the gas sealing of the ring in the cylinder. It these deposits are not removed from the lubricating oil when it is to be drained. Their accumulation in the engine drastically shortens the engine life. The role of the detergent additives is to reduce the amount of deposits formed and their removal easy. The detergent property imparted to the lubricating oils by additives seems to perform differently depending upon whether deposits result from high or low temperatures. Low temperature deposits are mainly comes from the fuel combustion and the detergency function of the lubricating oil is to keep them in suspension or in solution in the lubricating oil. However, high temperature deposits are mainly related to the oxidized fraction of the oil.
The role of detergency here is not only to maintain these products in suspension, but also to stop the development of those chain reactions which promote the formation of varnishes and lacquers. The physical and functional properties of the lubricating oils depend on the properties of the C atoms in the various ring structures and aliphatic side chain.
Seal compatibility – Lubricating oils are often used in equipment where they come into contact with rubber or plastic seal. The strength and degree of swell of these seals can be affected by the interaction with the lubricating oil. Various tests have been devised to measure the effect of base oils on different seals and under different test conditions. The strength and degree of swell of these seals can be affected by their interaction with the oil. Various tests measure the effects of base oils on different seals and under different test conditions.
The important functions and the properties of the lubricating oils ares given in Tab 1
|Tab 1 Important functions and properties of the lubricating oils
|Reduction of frictional resistance
|Low viscosity to provide good pumpability and not to cause undue cracking resistance
|Minimum viscosity without risk of metal to metal contact under the varying condition of temperatures, speed, and load
|Sufficiently high viscosity at high temperature with good lubrication property outside the hydrodynamic condition
|Anti-seizure properties especially during the run-in period
|Protection against corrosion and wear
|Protect metallic surface against corrosive action of fuel decomposition product (wear, SO2, HBr, and HCl etc.)
|Resist degradation (resist oxidation and have a good thermal stability)
|Counteract action of the lubricating oil decomposition product at high temperatures, especially on non-ferrous metals.
|Reduce the consequences of unavoidable metal-to-metal contact by intervention in the friction mechanism
|Resist deposit formation which affects lubrication (detergency or dispersant property).
|Contribute to the elimination of dust and other pollutants (dispersant action)
|To have sufficient viscosity at high temperature and low volatility
|To limit wear
|Not to contribute to formation of deposits and fight against such formation
|Contribute to cooling
|To have good and thermal stability and oxidation resistance
|To have low volatility
|To have viscosity which is not very high
|Facilitate suspension and eliminate undesirable products
|To be able to maintain in fine solid material at any temperature, physical, and chemical condition
Required performance characteristics of the lubricating oils
Selection and application of lubricating oil are determined by the functions which are expected for their performance. As an example, in one of the application, such as in case of delicate instrument bearing, the reduction of friction is paramount and in another application such as in case of metal cutting, the temperature control is the most important. A lubricating oil performance is to fulfill the following five important functions.
Reduction of the frictional resistance – The reduction of resistance of the moving parts to a minimum level is necessary to ensure maximum mechanical efficiency.
Protection of the equipment against all types of wear – The lubricating oil is to provide higher life to the equipment parts and hence, is to reduce its maintenance requirements.
Reduction of oil leakages – The reduction of oil leakages in an efficient manner is necessary to maintain the equipment performance and to prevent the adulterating of the oil.
Contributing the thermal equilibrium – The lubricating oil is also to function as a heat exchange medium for dissipating the heat. This is often associated with the oil viscosity with the viscous oil giving greater frictional resistance and its slow internal circulation leads to a rapid temperature raise of some vital part of the equipment. In such case, to improve the cooling efficiency, the lubricating oil is to circulate quickly.
Removal of all harmful impurities – The lubricating oil is to carry out the function of protecting the equipment against the corrosive and mechanical wear which is caused by all harmful impurities. Hence, the removal of these impurities by the lubricating oil is very important for the long life of the equipment.
Impurities and contaminants of the lubricating oils
Water content – Water content is the quantity the water present in the lubricating oil. It is generally expressed as parts per million (ppm), percent by volume, or percent by weight. It can be measured by centrifuging, distillation and voltametry. The most popular, although least accurate, method of water content assessment is the centrifuge method. In this method, a 50 % mixture of oil and solvent is centrifuged at a specified speed until the volumes of water and sediment observed are stable. Apart from water, solids and other solubles are also separated and the results obtained do not correlate well with those obtained by the other two methods.
The distillation method is a little more accurate and involves distillation of oil mixed with xylene. Any water present in the sample condenses in a graduated receiver. Voltametry method is the most accurate. It employs electrometric titration, giving the water concentration in ppm.
Corrosion and oxidation behaviour of the lubricating oils is critically related to water content. Lubricating oil mixed with water gives an emulsion. An emulsion has a much lower load carrying capacity than pure oil and lubricant failure followed by damage to the operating surfaces can result. In general, in applications such as turbine oil systems, the limit on water content is 0.2 % and for hydraulic systems 0.1 %. In dielectric systems excessive water content has a significant effect on dielectric breakdown. Normally the water content in such systems is to be less than 35 ppm.
S content – S content is the quantity of S present in the lubricating oil. It can have some beneficial, as well as some detrimental, effects on the operating equipment. S is a very good boundary agent, which can effectively operate under extreme conditions of pressure and temperature. On the other hand, it is very corrosive. A normally used technique for the determination of S content is the bomb oxidation technique. It involves the ignition and combustion of a small oil sample under pressurized O2. The S from the products of combustion is extracted and weighed.
Ash content – There is some quantity of noncombustible material present in the lubricating oil which can be determined by measuring the amount of ash remaining after combustion of the oil. The contaminants can be wear products, solid decomposition products from a fuel or lubricant, and atmospheric dust entering through a filter etc. Some of these contaminants are removed by an oil filter but some settle into the oil. To determine the amount of contaminant, the oil sample is burned in a specially designed vessel. The residue which remains is then ashed in a high temperature muffle furnace and the result displayed as a percentage of the original sample. The ash content is used as a means of monitoring lubricating oils for undesirable impurities and sometimes additives. In used lubricating oils, it can also indicate contaminants such as dirt, wear products, etc.
Chlorine content – The quantity of chlorine in the lubricating oil is to be at an optimum level. Excess chlorine causes corrosion whereas an insufficient amount of chlorine can cause wear and increase in the frictional losses. Chlorine content can be determined either by a bomb test which provides the gravimetric evaluation or by a volumetric test which gives chlorine content, after reacting with sodium metal to produce sodium chloride, then titrating with silver nitride.