Quick Learning

Short Articles on the basics of lubrication and synthetics

How are the base oils classified?

Base Oil is the name given to lubrication grade oils initially produced from refining crude oil (mineral oil base) or through chemical synthesis (synthetic base oil). Base oil is typically defined as oil with a boiling point range between 550 and 1050 F, consisting of hydrocarbons with 18 to 40 carbon atoms. This oil can be either paraffinic or napthenic in nature depending on the chemical structure of the molecules.

Base oils in Groups I, II, II+, and III are refined mixtures of naturally occurring hydrocarbons. They are formulated with additives in order to produce petroleum or semi-synthetic fluids.

Group I

Group I base stocks contain less than 90 percent saturates and/or greater than .03 percent sulfur and have viscosity index greater than or equal to 80 and less than 120.

Group II

Group II base stocks contain greater than or equal to 90 percent saturates and less than or equal to .03 percent sulfur and have viscosity index greater than or equal to 80 and less than 120.

Group III

Group III base stocks contain greater than or equal to 90 percent saturates and less than or equal to .03 percent sulfur and have viscosity index greater than or equal to 120.

Group IV

Group IV base oils, PAOs, are chemically engineered hydrocarbons derived from alphaolefins. They possess a lower pour point, greater thermal stability, a higher viscosity index, and a higher degree of chemical inertness when compared to mineral oils.

Group V

Group V base oils encompass all other base oils including alkylated naphthalene (AN), esters, polyether, polyglycol, silicone, halogenated hydrocarbons, and other synthetic fluids. As they are considerably more expensive than mineral oils and PAOs they are most commonly used as fluid additives or otherwise used for specialized industrial applications.

How does lubrication work?

The phenomenon of lubrication can be put into these three categories: (a) Thick-Film lubrication (Fluid-Film or hydrodynamic lubrication) (b) Thin Film lubrication (Boundary lubrication) and (c) Extreme Pressure lubrication

Thick Film Lubrication

In this, moving/sliding surfaces are separated from each other by a thick film of fluid (at least 1000 A° thick), so that direct surface to surface contact and welding of welding of junctions rarely occurs. The lubricant film covers/fills the irregularities of moving/sliding surfaces and forms a thick layer between them, so that there is no direct contact between the material surfaces. This consequently reduces friction.

The lubricant chosen should have the minimum viscosity (to reduce the internal resistance between the particles of the lubricant) under working conditions and at the same time, it should remain in place and separate the surfaces.

Hydrocarbon oils (mineral oils which are lower molecular weight hydrocarbons with about 12 to 50 carbon atoms) are considered to be satisfactory lubricants for thick-film lubrication. In order to maintain the viscosity of the oil in all seasons of year, ordinary hydrocarbon lubricants are blended with selected long chain polymers.

Thin Film Lubrication

This type of lubrication is preferred where a continuous film of lubricant cannot persist. In such cases, the clearance space between the moving/sliding surfaces is lubricated by such a material which can get adsorbed on both the metallic surfaces by either physical or chemical forces. This adsorbed film helps to keep the metal surfaces away from each other at least up to the height of the peaks present on the surface. Vegetable and animal oils and their soaps can be used in this type of lubrication because they can get either physically adsorbed or chemically react in to the metal surface to form a thin film of metallic soap which can act as lubricant. Although these oils have good oiliness, they suffer from the disadvantage that they will break down at high temperatures. On the other hand, mineral oils are thermally stable and the addition of vegetable/animal oils to mineral oils, their oiliness can also be brought up. Graphite and molybdenum disulphide are also suitable for thin-film lubrication.

Extreme Pressure Lubrication

When the moving/sliding surfaces are under very high pressure and speed, a high local temperature is attained under such conditions, liquid lubricants fail to stick and may decompose and even vaporize. To meet these extreme pressure conditions, special additives are added to minerals oils. These are called extreme pressure additives. These additives form more durable films (capable of withstanding very high loads and high temperatures) on metal surfaces.

Important additives are organic compounds having active radicals or groups such as chlorine (as in chlorinated esters), sulphur (as in sulphurized oils) or phosphorus (as in tricresyl phosphate). These compounds react with metallic surfaces, at existing high temperatures, to form metallic chlorides, sulphides or phosphides.

How are lubricants classified?

Lubricants are classified on the basis of their physical state.

      • Liquid lubricants or lubricating oils
      • Semi-solid lubricants or greases
      • Solid lubricants

Liquid lubricants or Lubricating oils: Lubricating oils also known as liquid lubricants and further classified into three categories; Animal and Vegetables oils, Mineral or Petroleum oils and blended oils.

Characteristic of good lubricating oils: (1) high boiling point, (2) low freezing point,
(3) adequate viscosity for proper functioning in service, (4) high resistance to oxidation and heat,
(5) non-corrosive properties and (6) stability to decomposition at the operating temperatures.

Animal and Vegetables oils: Animal oils are extracted from the crude fat and vegetables oils such as cottonseed oil and caster oils. These oils possess good oiliness and hence they can stick on metal surfaces effectively even under elevated temperatures and heavy loads. But they suffer from the disadvantages that they are costly, undergo easy oxidation to give gummy products and hydrolyze easily on contact with moist air or water. Hence they are only rarely used these days for lubrication. But they are still used as blending agents in petroleum based lubricants to get improved oiliness.

Mineral or Petroleum oils: These are basically lower molecular weight hydrocarbons with about 12 to 50 carbon atoms. As they are cheap, available in abundance and stable under service conditions, hence they are widely used. But the oiliness of mineral oils is less, so the addition of higher molecular weight compounds like oleic acid and stearic acid increases the oiliness of mineral oil.

Blended oils: No single oil possesses all the properties required for a good lubricant and hence addition of proper additives is essential to make them perform well. Such additives added lubricating oils are called blended oils. Examples: The addition of higher molecular weight compounds like oleic acid, stearic acid, palmetic acid, etc or vegetables oil like coconut oil, castor oil, etc increases the oiliness of mineral oil.

Semi-solid Lubricants or Grease: A semi-solid lubricant obtained by combining lubricating oil with thickening agents is termed as grease. Lubricating oil is the principal component and it can be either petroleum oil or a synthetic hydrocarbon of low to high viscosity. The thickeners consist primarily of special soaps of Li, Na, Ca, Ba, Al, etc. Non-soap thickeners include carbon black, silica gel, polyureas and other synthetic polymers, clays, etc. Grease can support much heavier load at lower speed. Internal resistance of grease is much higher than that of lubricating oils; therefore it is better to use oil instead of grease. Compared to lubricating oils, grease cannot effectively dissipate heat from the bearings, so work at relatively lower temp.

Solid lubricants: They are preferred where (1) the operating conditions are such that a lubricating film cannot be secured by the use of lubricating oils or grease (2) contamination (by the entry of dust particles) of lubricating oils or grease is unacceptable (3) the operating temperature or load is too high, even for grease to remain in position and (4) combustible lubricants must be avoided. They are used either in the dry powder form or with binders to make them stick firmly to the metal surfaces while in use. They are available as dispersions in non- volatile carriers like soaps, fats, waxes, etc and as soft metal films.

The most common solid lubricants are graphite, molybdenum disulphide, tungsten disulphide and zinc oxide. They can withstand temperature upto 650° C and can be applied in continuously operating situations. They are also used as additives to mineral oils and greases in order to increase the load carrying capacity of the lubricant. Other solid lubricants in use are soapstone (talc) and mica.

Graphite: It is the most widely used of all the solid lubricants and can be used either in the powdered form or in suspension. It is soapy to touch; non-inflammable and stable up to a temperature of 375° C. Graphite has a flat plate like structure and the layers of graphite sheets are arranged one above the other and held together by weak van der Waal’s forces. These parallel layers which can easily slide one over other make graphite an effective lubricant. Also the layer of graphite has a tendency to absorb oil and to be wetted of it.

Molybdenum Disulphide: It has a sandwich- like structure with a layer of molybdenum atoms in between two layers of sulphur atoms. Poor inter laminar attraction helps these layers to slide over one another easily. It is stable up to a temperature of 400° C.

What are the main properties of lubricants?

      • Viscosity
      • Flash Point and Fire Point
      • Cloud Point and Pour Point
      • Aniline Point
      • Corrosion Stability

Viscosity: It is the property of liquid by virtue of which it offers resistance to its own flow (the resistance to flow of liquid is known as viscosity). The unit of viscosity is poise. It is the most important single property of any lubricating oil, because it is the main determinant of the operating characteristics of the lubricant. If the viscosity of the oil is too low, a liquid oil film cannot be maintained between two moving/sliding surfaces. On the other hand, if the viscosity of the oil is too high, excessive friction will result.

Effect of temperature on viscosity: Viscosity of liquids decreases with increasing temperature and, consequently, the lubricating oil becomes thinner as the operating temperature increases. Hence, viscosity of good lubricating oil should not change much with change in temperature, so that it can be used continuously, under varying conditions of temperature. If the viscosity of lubricating oil falls rapidly as the temperature is raised, it has a low viscosity index. On the other hand, if the viscosity of lubricating oil is only slightly affected on raising the temperature, its viscosity index is high.

Flash Point and Fire Point: Flash point is the lowest temperature at which the lubricant oil gives off enough vapors that ignite for a moment, when a tiny flame is brought near it; while Fire point is the lowest temperature at which the vapors of the lubricant oil burn continuously for at least five seconds, when a tiny flame is brought near it. In most cases, the fire points are 5° C to 40° C higher than the flash points. The flash and fire do not have any bearing with lubricating property of the oil, but these are important when oil is exposed to high temperature service. A good lubricant should have flash point at least above the temperature at which it is to be used. This safeguards against risk if fire, during the use of lubricant.

Cloud Point and Pour Point: When the lubricant oil is cooled slowly, the temperature at which it becomes cloudy or hazy in appearance, is called its cloud point; while the temperature at which the lubricant oil cease to flow or pour, is called its pour point. Cloud and pour points indicate the suitability of lubricant oil in cold conditions. Lubricant oil used in a machine working at low temperatures should possess low pour point; otherwise solidification of lubricant oil will cause jamming of machine. It has been found that presence of waxes in the lubricant oil raise pour point.

Aniline Point: Aniline point of the lubricant oil is defined as the minimum equilibrium solution temperature for equal volumes of aniline and lubricant oil samples. It gives an indication of the possible deterioration of the lubricant oil in contact with rubber sealing; packing, etc. Aromatic hydrocarbons have a tendency to dissolve natural rubber and certain types of synthetic rubbers. Consequently, low aromatic content in the lubricant oil is desirable. A higher aniline point means a higher percentage of paraffinic hydrocarbons and hence, a lower percentage of aromatic hydrocarbons.

Aniline point is determined by mixing mechanically equal volumes of the lubricant oil samples and aniline in a test tube. The mixture is heated, till homogenous solution is obtained. Then, the tube is allowed to cool at a controlled rate. The temperature at which the two phases (the lubricant oil and aniline) separate out is recorded at the aniline point.

Corrosion Stability: Corrosion stability of the lubricant oil is estimated by carrying out corrosion test. A polished copper strip is placed in the lubricant oil for a specified time at a particular temperature. After the stipulated time, the strip is taken out and examined for corrosion effects. If the copper strip has tarnished, it shows that the lubricant oil contains any chemically active substances, which cause the corrosion of the copper strip. A good lubricant oil should not effect the copper strip. To retard corrosion effects of the lubricant oil, certain inhibitors are added to them. Commonly used inhibitors are organic compounds containing P, As, Cr, Bi or Pb.

The perfect characteristics of a good lubricant

      • It should have a high viscosity index.
      • It should have flash and fire points higher than the operating temperature of the machine.
      • It should have high oiliness.
      • The cloud and pour points of a good lubricant should always be lower than the operating temperature of the machine.
      • The volatility of the lubricating oil should be low.
      • It should deposit least amount of carbon during use.
      • It should have higher aniline point.
      • It should possess a higher resistance towards oxidation and corrosion.
      • It should have good detergent quality

Are synthetic oil-based greases are really better than mineral oil-based greases?

YES!
      • While conventional, mineral oil-based greases can be formulated to deliver effective performance for many applications, synthetics typically deliver significant advantages, including extended oil life. Synthetic fluids provide superior lubricating capability across a much wider application temperature range, all other things being equivalent. This is true for lubricant oils as well as greases.
      • In high temperature applications, synthetic oil thins out less than comparable mineral oil, providing greater protection by forming a thicker oil film between surfaces. Additionally, these oils are less prone to degradation at elevated temperatures – a phenomenon called oxidation. Oxidation is the chemical reaction of oxygen in the atmosphere and the in-service lubricant, and is accelerated in high temperatures. Generally, the speed of oxidation doubles for every 10°C above 120°C.
      • At low temperatures, synthetic oil outperforms mineral oil by maintaining proper viscosity and better fluidity than mineral oil. Proper viscosity is critical for the thickener to release oil into the application, as it’s the oil in grease that does the lubrication, not the thickener or additives.
      • From extending regreasing intervals at a plant from weekly to quarterly, to extending the life of machine bearings, synthetic grease can improve industrial operations and help to cut costs for operators.

What are the different types and characteristics of synthetic lubricants?

Alkylated napthalene (AN)

Alkylated naphthalene (AN) is produced by the alkylation of naphthalene with an alkylating agent and an acid catalyst. AN fluids exceed the performance of PAOs in oxidizing environments as well as provide improved hydrolytic stability. They also possess low volatility and good solubility characteristics. They may be used as an additive or as a stock fluid.

Esters

Esters, diesters, and polyolesters are organic compounds formed through a reaction with an acid and an alcohol which produces an ester and water. The chemical structure of the products used dictates the properties of the ester. Hydrolytic stability can be a concern for some esters as excess water can break down the ester, reversing the chemical equation.

Diester lubricants have improved fire resistance and oxidation stability. They also resist sludge and varnish formation due to their high solvency and are commonly used as a lubricant for reciprocating air compressors.

Polyolesters exhibit excellent high temperature properties and long-term hydrolytic stability. They are used in rotary screw air compressors that operate above 180° F as is common in mining, oil well drilling, multi-stage compression, and when operating in hot ambient temperatures. They resist oil breakdown and have improved physical properties when compared to PAOs and diesters.

Polyether

Polyether or ether-based fluids, such as phenyl ether polymer or polyphenyl ethers (PPEs), are radiation-resistant fluids that offer superior thermal stability, oxidation stability, and a very low vapor pressure
PPEs are limited by a relatively high pour point, 40° F, and are commonly used in high vacuums, very high temperatures, or when a radiation-resistant fluid is required.
Polyvinyl ether (PVE) is a hydrofluorocarbon ether lubricant with excellent performance characteristics including superior lubricity and solubility with process fluids. PVE is exstensively useful in refrigerant system as it is miscible with halogenated (HFC) refrigerants.

Polyglycol

Polyglycol, glycol, polyalkyene glycol (PAG), and water-glycol fluids are often used for anti-freeze, circulating coolant, hydraulic fluids, and high water content fluids (HWCF). They offer fire resistance and excellent low-temperature properties as they dissolve moisture, prevent freezing, and have a subzero pour point.

Polyglycol lubricants are also used for hydrocarbon gas compressors as they reduce hydrocarbon solubility and have favorable viscosity and temperature characteristics. When operating under high pressures there can be issues with the formation of a hydrocarbon liquid phase. This phase can cause bearing failures if it is not separated from the lubricant before entering the compressor

Water-glycol solutions tend to have higher viscosity index values than other compositions. Zinc, cadmium, and magnesium react with water-glycol solutions and water-glycol fluids should not be used in coolant systems where these materials exist.

Silicon

Silicone-based fluids include fluorosilicones, alkylmethylsilicones, and other silicone based fluids. They contain long linear polymers that easily slide past one another. They are also compressible and exhibit a low surface tension which limits their load carrying capacity. As a lubricant they are primarily used for light loads including metal-to-plastic and plastic-to-plastic lubrication applications. Their thermal stability, chemical inertness, and oxidation resistance allow for an extended life, making them suitable for electrical, chemical, and food and beverage applications as well as for closed systems

Halogenated hydrocarbons

Fluids based on halogenated (fluorinated and/or chlorinated) hydrocarbons include chlorofluorcarbons (CFC), halogenated fluorocarbons (HFC), halogenated chlorofluorocarbon (HCFC), and perfluorocarbon (PFC) fluids. They have excellent chemical inertness and solvent resistance as is required for strong oxidizing and corrosive environments while their use is somewhat limited by high cost.

Resource Links

Petroleum Quality Institute of American
www.pqiamerica.com

National Lubricating Grease Institute
www.nlgi.org

Machinery Lubrication Magazine
machinerylubrication.com

Compressed Air Best Practices Magazine
www.airbestpractices.com

Maintenance Technology Magazine
www.maintenancetechnology.com

Independent Lubricant Manufacturers Association
www.ilma.org

Independent Petroleum Association of America
www.ipaa.org

American Society for Testing and Materials
www.astm.org

International Lubricant Standardization and Approval Committee
www.gf-5.com

Society of Automotive Engineers
www.sae.org

Society of Tribologists and Lubrication Engineers
www.stle.org

United Kingdom Lubricants Association Ltd.
www.ukla.org.uk

Union Indépendante de l’Industrie Européenne des Lubrifiants
www.ueil.org