Friday, 11 November 2016
VISCOSITY GENERAL PROPERTIES OF A LUBRICATING OIL
This indicates the resistance of a liquid to flow.
There are several units for measuring viscosity. Formerly, the unit commonly used in America was Saybolt Universal Second (SSU), measured at 100°F or 210°F. In Europe, the former widely used unit was Redwood I second (RWI), measured at 100°F or 210°F. At present, most countries have switched over to the metric system that employs the unit Centistokes (cSt), measured at 40°C or 100°C.
Oil with higher viscosity can stand greater pressure without being squeezed out of the lubricating surfaces. However, the high internal friction of the oil may offer greater resistance to the movement of the lubricating parts. An oil of lower viscosity offers less resistance to the moving parts but the oil can be easily squeezed out of the lubricating surfaces. It is therefore important to select a lubricating oil of appropriate viscosity to achieve optimum lubrication effect.
Viscosity changes with temperature. Hence, the measuring temperature must be specified whenever the viscosity of a liquid is stated. When temperature rises, a liquid becomes less viscous. Similarly, a liquid becomes thicker when temperature drops.
Viscosity Index (VI) is an indication of how the viscosity of a liquid varies with temperature. A high VI means the liquid does not thin out so much when temperature rises. VI improver additives that are usually high molecular weight polymers can increase the VI of lubricating oil.
Increase in oil viscosity achieved by addition of polymers can be partially lost again through degradation of the polymer molecules by shear stress such as heavily loaded gears. Oil that can resist viscosity change due to shear are said to have high shear stability.
Indicates flow characteristic at low temperature.
Depends on the wax content of the oil.
Measures the readiness of the oil to ignite momentarily in air and is a consideration regarding the fire hazard of the oil.
Oxidation of oil will produce resins and sludge that may plug filters and oil passages.
Oxidation can also produce soluble organic acids that may cause corrosion of machine parts.
A good lubricating oil should resist oxidation.
Acidity and Alkalinity (Total Acid Number and Total Base Number)
High acidic oil may cause corrosion of machine parts
Most engine oils show some alkalinity due to the addition of detergent type additives and this helps to neutralize any acid formed in the oil by oxidation.
After prolong usage, lubricating oil may contain organic acids formed by oxidation. Therefore, a measurement of the acidity of an oil can reflects its degree of oxidation.
Most engine oils contain detergent and dispersant additives to prevent dirty particular produced by incomplete combustion from accumulating and plating metal surface.
Water may seep into the lubricating system and cause rusting of machine parts.
Rust particles can act as catalyst to accelerate the oxidation of the oil.
Anti-rust additives can be absorbed onto metal surface and prevent moisture from coming into contact with the metal, thus preventing rusting.
Acidic materials in oil can cause corrosion of machine parts.
Corrosion can be minimized by the additives of corrosion inhibitor that reacts with metal to form a protective layer separating the acidic materials and the metal.
Foaming reduces the lubricity of oil because the air bubbles in the foam will create a barrier between the oil and the metal surface.
Foam can also produce resistance to the movement of machine parts.
In a hydraulic system, foam will reduce the cohesive power of the oil and cause the hydraulic pressure to drop.
Good lubricating oil will not foam easily and can disperse foam quickly. Anti-foam additives can help to reduce the foaming tendency of oil.
Emulsification and Demulsification
Emulsification is the homogenous mixing of oil and water.
Some oil requires high emulsibility so that it can mix with water easily, for example, some metal cutting oils.
The emulsibility of oil can be improved by the addition of emulsifying agent that has strong affinity for both oil and water, thus holding the oil and water molecules together.
Some other lubricants require good demulsibility so that water can be separated from the oil easily, e.g. Turbine oil. The demulsibility of oil can be achieved by good refining technique.
Some lubricating conditions may call for extremely light oil, an oil of lower viscosity than the load-speed relationship of the machine may indicate. Under such condition, wear of the metal surfaces may occur. Anti-wear additive forms a protective coating on the metal surfaces, allowing the surfaces to slide on each other with a minimum loss of metal.
Extreme Pressure Loading Property (EP)
Heavy loading, extreme pressure and intense heat may cause machine moving parts to melt and weld together, hence interfering motion.
The extreme-pressure additive in oil can react with the metal to form a compound with low melting point. The intense heat developed due to the extreme-pressure loading will be dissipated in the melting of the compound instead of welding the two metallic parts.
EP properties are usually measured by Timken method (ASTM D 2782) or FZG Gear Machine (IP 334). In the Timken method, a steel cup rotates against a steel block in a lubricant bath. The maximum load that will not cause scoring is the OK load. In the FZG Gear Machine, special gear wheels are run in the lubricants under test. The loading is increasing by stages and the stages at which gear damages occur is reported as the FZG loading stage of the lubricant.
Tacky oil contain tackiness agent and will stick to the lubricating surface for a long time without being spattered. Lubricants used in textile machinery and wire ropes usually require tackiness property.
GENERAL PROPERTIES OF GREASE
Grease is a semi-solid formed by the dispersion of a thickening agent in a liquid lubricants (base oil). Other ingredients imparting special properties may be included. Greases have advantage over oil in some applications because greases stay at the point of lubrication and will hardly be squeezed out. Sometimes, greases can also be used to seal up machine parts to prevent the entry of moisture and dust.
Base oil viscosity, hydrocarbon type, and volatility can influence the structure stability, lubricating quality, low and high temperature performance, and cost of grease. The thickener is the principal factor controlling water resistance, high temperature qualities, resistance to breakdown through continued use, and ability to stay in place. To a large extend, grease cost is determined by the type of thickener and other additives.
Thickener can be divided into several categories; soap-type, inorganic type and synthetic organic type.
The important characteristics of grease are as follows: -
This indicates the consistency (hardness or softness) of grease. It is measured by dropping a pointed cone into the grease and sees how far the cone penetrates into the sample. Different ranges of penetration are identified by the following National Lubricating Grease Institute (NLGI) Grade Numbers: 000, 00, 0, 1, 2, 3, 4, 5, and 6. Grade 000 is the softest while Grade 6 is the hardest.
Most grease thickened with soaps become softer with increase in temperature, but some greases become progressively harder upon exposure to high temperature. Non-soap thickeners, as a whole, show very little change in consistency with temperature rise.
Greases with thickeners soluble in water will emulsify and fluidize if come into contact with relatively large amount of water. In general, calcium, lithium and aluminium soaps are highly water resistance while sodium soap greases are soluble in water.
Oxidation will cause the grease to harden, form varnish like films and eventually carbonize. Additives can improve the oxidation stability of grease.
Both the oil and the thickener in soap type grease have lubricating properties. Inorganic non-soap thickener generally does not contribute to the lubricating of grease. The lubricating capability of the oil depends on its viscosity and viscosity index.
Additives may be included in a grease to promote its anti-wear properties.
Extreme Pressure Capability (EP)
Some grease contains special additives to fortify its load carrying capability so that welding and scoring of metal can be minimized.
It is the temperature at which the grease is fluid enough to drip. Grease with a dropping point below the operating temperature would not provide proper lubricant. However, the converse is not necessarily true; a dropping point above operating temperature is no guarantee of adequate lubrication since there may be change in consistency and deterioration in chemical properties of the grease at high temperatures.
API ENGINE SERVICE CLASSIFICATIONS
API Engine Service Classifications
The American Petroleum Institute (API) engine oil classification system was set up as a joint effort by API, ASTM (American Society for Testing and Materials) and SAE (Society of Automotive Engineers). The letter classification system is a method of classifying engine oils according to their performance characteristics, and relating this to their intended type of service.
The API system currently includes service classifictions for service stations/spark ignition engines ("S" series), for commercial application/compression ignition engines ("C" series), and for energy conserving engine oil ("EC" series). It is an "open-ended" system which allows for the adidtion of new designations with littlw change to existing ones.
SA – Formerly for Utility Gasoline and Diesel Engine Service (Obsolete) – Category SA denotes service typical of older engines operated under such mild conditions that the protection afforded by compounded oils is not required. This category has no performance requirements, and oils in this category should not be used in any engine unless specifically recommended by the equipment manufacturer.
SB – Minimum-Duty Gasoline Engine Service (Obsolete) - Category SB denotes service typical of older engines operated under such mild conditions that only minimum protection afforded by compounding is desired. Oils designed for this service have been used since 1930s and provide mild antiscuff capability and resistance to oil oxidation and bearing corrosion. They should not be used in any engine unless specifically recommended by the equipment manufacturer.
SC – 1964 Gasoline Engine Service (Obsolete) - Category SB denotes service typical of gasoline engines in 1964 through 1967 models of passenger cars and some trucks, operating under engine manufacturers’ warranties in effect during those model years. Oils designed for this service provide control of high and low temperature deposits, wear, rust, and corrosion in gasoline engines.
SD – 1968 Gasoline Engine Service (Obsolete) - Category SD denotes service typical of gasoline engines in 1968 through 1970 models of passenger cars and some trucks, operating under engine manufacturers’ warranties in effect during those model years. This category may also apply to certain 1971 or later models as specified (or recommended) in the owners’ manuals. Oils designed for this service provide more protection against high and low temperature deposits, wear, rust, and corrosion in gasoline engines than oils that are satisfactory for API Engine Service Category SC and may be used when API Engine Service Category SC is recommended.
SE – 1972 Gasoline Engine Service (Obsolete) - Category SE denotes service typical of gasoline engines in passenger cars and some trucks beginning with 1972 and certain 1971 through 1979 models operating under engine manufacturers’ warranties. Oils designed for this service provide more protection against oil oxidation, high temperature deposits, rust, and corrosion in gasoline engines than oils that are satisfactory for API Engine Service Category SD or SC and may be used when either of these categories is recommended.
SF – 1980 Gasoline Engine Service (Obsolete) - Category SF denotes service typical of gasoline engines in passenger cars and some trucks beginning with 1980 through 1989 models operating under engine manufacturers’ recommended maintenance procedures. Oils developed for this service provide increased oxidation stability and improved antiwear performance relative to oils that meet the minimum requirements of API Service Category SE. These oils also provide protection against engine deposits, rust, and corrosion. Oils meeting API Service Category SF may be used when API Engine Service Category SE, SD or SC are recommended.
SG – 1989 Gasoline Engine Service (Obsolete) - Category SF denotes service typical of gasoline engines in passenger cars, vans, and light trucks operating under manufacturers’ recommended maintenance procedures. Category SG oils include the performance properties of API Service Category CC. (Certain manufacturers of gasoline engines require oils that also meet the higher diesel engine Category CD.) Oils developed for this service provide improved control of engine deposits, oil oxidation, and engine wear relative to oils developed for previous categories. These oils also provide protection against rust and corrosion. Oils meeting API Service Category SG may be used when API Engine Service Category SF, SE, SF/CC or SE/CC are recommended.
SH – 1994 Gasoline Engine Service - Category SH was adopted in 1992 to describe engine oil first mandated in 1993. It is for use in service typical of gasoline engines in present and earlier passenger cars, vans, and light trucks operating under manufacturers’ recommended maintenance procedures. Engine Oils developed for this category provide performance exceeding the minimum requirements for API Service Category SG, which it is intended to replace, in the areas of deposit control, oil oxidation, wear, rust, and corrosion. Oils meeting API SH requirements have been tested according to the American Chemistry Council (ACC) Product Approval Code of Practice and may utilize the API Base Oil Interchange and Viscosity Grade Engine Testing Guidelines. They may be used where API Service Category SG and earlier categories are recommended. Effective August 1, 1997, API SH cannot be used except with API CF, CF-2, CF-4 or CG-4 when displayed in the API service symbol, and the C category must appear first.
SJ – 1997 Gasoline Engine Service – Category SJ was adopted in 1996 to describe engine oil first mandated in 1997. It is for use in service typical of gasoline engines in present and earlier passenger cars, vans, and light trucks operating under manufacturers’ recommended maintenance procedures. Oils meeting API SH requirements have been tested according to the American Chemistry Council (ACC) Product Approval Code of Practice and may utilize the API Base Oil Interchange and Viscosity Grade Engine Testing Guidelines. They may be used where API Service Category SH and earlier categories are recommended.
SL – 2001 Gasoline Engine Service – Category SL was adopted to describe engine oils for use in 2001. It is for use in service typical of gasoline engines in present and earlier passenger cars, sports utility vehicles, vans and light trucks operating under vehicle manufacturers’ recommended maintenance procedures. Oils meeting API SL requirements have been tested according to the American Chemistry Council (ACC) Product Approval Code of Practice and may utilize the API Base Oil Interchange and Viscosity Grade Engine Testing Guidelines. They may be used where API Service Category SJ and earlier categories are recommended.
SM - Introduced on 30 Novermber 2004 - Category SM oils are designed tp provide improved oxidation resistance, improved deposite protection, better wear protection, and better low-temperature performance over the life of the oil. Some SM oils may also meet the latest ILSAC specification and/or qualify as Energy Conserving. They may be used where API Service Category SJ and SL earlier categories are recommended
CA – Diesel Engine Service (Obsolete) – Service typical of diesel engines operated in mild to moderate duty with high quality fuels; occasionally has included gasoline engines in mild service. Oils designed for this service provide protection from bearing corrosion and ring-belt deposits in some naturally aspirated diesel engines when using fuels of such quality that they impose no unusual requirements for wear and deposits protection. They were widely used in the 1940s and 1950s but should not be used in any engine unless specifically recommended by the equipment manufacturer.
CB – Diesel Engine Service (Obsolete) – Service typical of diesel engines operated in mild to moderate duty, but with lower quality fuels, which necessitate more protection from wear and deposits; occasionally has included gasoline engines in mild service. Oils designed for this service were introduced in 1949. They provide necessary protection from bearing corrosion and from high temperature deposits in naturally aspirated diesel engines with higher sulfur fuels.
CC – Diesel Engine Service (Obsolete) – Service typical of certain naturally aspirated, turbocharged or supercharged diesel engines operated in moderate to severe-duty service, and certain heavy-duty gasoline engines. Oils designed for this service provide protection from bearing corrosion, rust, corrosion and from high to low temperature deposits in gasoline engines. They were introduced in 1961.
CD – Diesel Engine Service (Obsolete) – Service typical of certain naturally aspirated, turbocharged or supercharged diesel engines where highly effective control of wear and deposits is vital, or when using fuels with a wide quality range (including high-sulfur fuels). Oils designed for this service were introduced in 1955 and provide protection from high temperature deposits and bearing corrosion in these diesel engines.
CD-II – Severe-Duty Two-Stroke Cycle Diesel Engine Service (Obsolete) – Service typical of two-stroke cycle diesel engines requiring highly effective control of wear and deposits. Oils designed for this service also meet all performance requirements of API Service Category CD.
CE – 1983 Diesel Engine Service (Obsolete) – Service typical of certain turbocharged or supercharged heavy-duty diesel engines, manufactured since 1983 and operated under both low speed, high load and high speed, high load conditions. Oils designed for this service may also be used when API Service Category CD is recommended.
CF-4 – 1990 Diesel Engine Service – Service typical of high speed, four-stroke cycle diesel engines. API CF-4 oils exceed the requirements for the API CE category, providing improved control of oil consumption and piston deposits. These oils should be used in place of API CE oils. They are particularly suited for on-highway, heavy-duty truck applications. When combined with the appropriate “S” category, they can also be used in gasoline and diesel powered personal vehicles – i.e., passenger cars, light trucks and vans – when recommended by the vehicle or engine manufacturer.
CF – Indirect-Injected Diesel Engine Service – Service typical of indirect-injection diesel engines and other diesel engines that use a broad range of fuel types, including those using fuel with high sulfur content; for example, over 0.5% wt. Effective control of piston deposits, wear and copper-containing bearing corrosion is essential for these engines, which may be naturally aspirated, turbocharged or supercharged. Oils designated for this service have been in existence since 1994 and may be used when API Service Category CD is recommended.
CF-2 – Severe-Duty Two-Stroke Cycle Diesel Engine Service (Obsolete) – Service typical of two-stroke cycle diesel engines requiring highly effective control over cylinder and ring-face scuffing and deposits. Oils designed for this service have been in existence since 1994 and may be used when API Service Category CD-II is recommended. These oils do not necessarily meet the requirements of API CF or CF-4 unless they pass the test requirements for these categories.
CG-4 – 1994 Severe-Duty Diesel Engine Service – This category describes oils for use in high speed four-stroke-cycle diesel engines used in both heavy-duty on-highway(0.05% wt sulfur fuel) and off-highway (less than 0.5% wt sulfur fuel) applications. CG-4 oils provide effective control over high temperature piston deposits, wear, corrosion, foaming, oxidation stability, and soot accumulation. These oils are specially effective in engines designed to meet 1994 exhaust emission standards and may also be used in engines requiring API Service Categories CD, CE, and CF-4. Oils designed for this service have been in existence since 1994.
CH-4 Severe-Duty Diesel Engine Service – This service oils are suitable for high speed, four-stroke diesel engines designed to meet 1998 exhaust emission standards and are specifically compounded for use with diesel fuels ranging in sulfur content up to 0.5% weight. CH-4 oils are superior in performance to those meeting API CF-4 and API CG-4 and can effectively lubricate engines calling for those API Service Categories.
CI-4 – 2002 – Severe-Duty Diesel Engine Service – The CI-4 performance requirements describe oils for use in those high speed, four-stroke cycle diesel engines designed to meet 2004 exhaust emission standards, to be implemented October 2002. These oils are compounded for use in all applications with diesel fuels ranging in sulfur content up to 0.05% by weight. These oils are especially effective at sustaining engine durability where Exhaust Gas Recirculation (EGR) and other exhaust emission componentry may be used. Optimum protection is provided for control of corrosive wear tendencies, low and high temperature stability, soot handling properties, piston deposit control, valve train wear, oxidative thickening, foaming and viscosity loss due to shear. CI-4 oils are superior in performance to those meeting API CH-4, CG-4 and CF-4 and can effectively lubricate engines calling for those API Service Categories.
CI-4 Plus - 2004 - Used in comjunction with API CI-4, the " CI-4 PLUS" designation identifies oils formulated to provide a higher level of protection against soot-related viscosity increase and viscosity loss due to shear in diesel engines. Like Energy Conserving, CI-4 PLUS appears in the lower portion of the API Service Symbol "Donut."
Performance Additives - Automotive Lubricants
Additive Type Pour Point Depressant
Purpose Enable lubricant to flow at low temperatures
Additive Type Viscosity Modifier
Purpose Reduce tge rate of viscosity change with temperature
Additive Type Antifoamant
Purpose Prevent lubricant from forming a persistent foam
Additive Type Antioxidant
Purpose Retard oxidative decomposition
Additive Type Antiwear and EP Agent
Purpose Reduce friction and wear and prevent scoring and seizure
Additive Type Corrision and Rust Inhibitor
Purpose Prevent corrosion and rusting of metal parts in contact with the lubricant
Additive Type Detergent
Purpose Keep surfaces free of deposits
Additive Type Dispersant
Purpose Keep insoluble contaiminants dispersed in the lubricant
Additive Type Friction Modifier
Purpose Alter coefficient of frictioon
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