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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.