The materials which are used to decrease the force of friction between the moving parts of a machine in contact are known as ‘lubricants’. The process of decreasing the force of friction between the moving parts of machine in contact is known as ‘lubrication’. The lubricant (also sometimes called as ‘lube’) is a substance introduced between two moving surfaces to reduce the friction between them which in turn improves the efficiency and reduces the wear. Lubrication is the process, or technique used to reduce wear of one or both surfaces in close proximity, and moving relative to each other, by interposing a substance called lubricant between the surfaces to carry or to help carry the load (pressure generated) between the opposing surfaces.
Friction is caused by the interactions at the surfaces of adjoining parts. When one surface moves over another surface in a machine, resistance to the relative motion of the surfaces takes place. The solid surface appears smooth to the naked eye, but this smooth surface shows irregularities of projections and cavities when seen under high power microscope. At a microscopic level, all surfaces are rough. When one such surface is placed over another, its projections fall into the cavities of the other and get interlocked. Because of this interlocking, there is resistance to the relative motion of the surfaces. This is called the frictional forces or frictional resistance. Surface peaks (asperities) can bond to one another or protrude into adjoining surface.
The frictional forces oppose the relative motion between the moving parts of the machine. Movement of surfaces needs an applied force high enough to overcome microscopic surface interactions. Hence, extra energy is needed to be spent to overcome the friction. The friction between the moving parts of the machine also produces heat which causes damage to the machine. Hence, friction causes wear and tear of the moving parts of the machine in contact and therefore the machine loses its efficiency. The primary purpose of using lubricant is to decrease the force of friction between the moving parts which are in contact. Fig 1 shows the causes of friction and ways to reduce friction.
Fig 1 Causes of friction and ways to reduce friction
The primary purpose for the use of lubricants is to reduce wear and heat between 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.
The lubrication regimes
The modern period of lubrication began with the work of Osborne Reynolds. Reynolds’s research was concerned with shafts rotating in bearings. When a lubricant was applied to the shaft, Reynolds found that 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 that is sufficient to keep the two surfaces separated. Under ideal conditions, Reynolds showed that this liquid pressure was great enough to prevent direct contact between the metal surfaces. Fig 2 shows the Stribeck curve which summarizes the lubrication regimes by describing the relationship between speed, load, oil viscosity, oil film thickness, and friction. It also shows types of lubrication.
Fig 2 Stribeck curve and types of lubrication
A good lubricant possesses several characteristics namely (i) high boiling point, (ii) low freezing point, (iii) high viscosity index, (iv) thermal stability, (v) hydraulic stability, (vi) demulsibility, (vii) corrosion prevention, and (viii) high resistance to oxidation. Functionality of lubricants is defined by their chemical structure and their physical properties.
Lubricants play a key role in the safety of the machine element. Besides lubricating, lubricants also carry various other functions which include (i) act as a coolant to remove the frictional heat which can be sometimes considerable, (ii) keeps moving parts apart from each other, (iii) reduce friction between moving parts, (iv) dissolve and transport contaminants and debris arising both from internal and external sources, (v) act as a hydraulic medium in some applications, (vi) protect against wear of highly loaded machine parts, (vi) prevent corrosion and rusting of machine parts, (vii) help in transmitting power, (viii) offer protection against the accumulation of deposits (sludges and varnish) in lubrication system, (ix) act as a seal for gases, (x) transport functional additives toward the surface, (xi) resist aeration and foaming, which can cause malfunctioning, (xii) resist or aid emulsion formation in wet systems, (xiii) stop the risk of smoke and fire of objects, (xiv) has the ability to separate water coming from air breathing or external sources, (xv) has the capacity to preserve oil film even in the presence of high pressures, (xvi) has resistance to hydrolysis or capacity to withstand the action of water, and (xvii) has the property of centrifugibility and filterability.
Some of the above functions are catered to by any properly refined mineral oil of suitable viscosity while for some other functions are provided by the lubricants with the help of additives. The types and amount of additive needed depend upon the performance features which a lubricant is required to be met. Typically lubricants contain more than 90 % base oil (mineral oil) and less than 10 % additives.
Types of lubricants
Lubricants can be gaseous, liquid, plastic, or solid. Their classification according to physical state includes materials and coatings which are self lubricating. The additives to the lubricants are normally not lubricants themselves but contribute important lubricating properties, when added to a lubricant.
Gaseous lubricants – Common gaseous lubricants are air, helium, and carbon dioxide etc. .
Liquid lubricants – Liquid lubricants are classified based on the origin from which liquid has been extracted and can be (i) mineral oils, (ii) fixed oils, (iii) synthetic fluids, and (iv) soluble oils and compounds.
Mineral oils extracted from petroleum crude oil and consist of widely varying mixtures of straight and branched chain paraffinic, naphthenic, and aromatic hydrocarbons. They consist of hydrocarbons (composed of 83 % to 87 % of carbon and 11 % to 14 % of hydrogen) with around 30 numbers carbon atoms in each molecule (composed of straight and cyclic carbon chains bonded together). They also contain sulphur, oxygen, and nitrogen. They have boiling points ranging from around 302 deg C to 593 deg C. Some specialty mineral oil lubricants can have boiling point extremes of 177 deg C and 815 deg C. The mineral oils can be (i) straight or unadulterated oil, (ii) compound with fixed oil or its derivatives, (iii) compound with special additives, and (iv) compounds with fixed oils or their derivatives, plus chemical additives, such as polymers and metal soaps.
Fixed oils are so called since they do not volatilize unless they decompose. They are composed of fatty acids and alcohols. On oxidation they form a gummy substance and this process is known as drying. Fixed oils which are slow to dry are used for lubrication. These oils are normally added to the mineral oils to improve film formation. Fixed oils can be (i) animal oils such as acid less tallow oil, and lard oils, etc., (ii) vegetable oils such as castor oil, rapeseed oil, and palm oil etc. and (iii) fish oil, such as sperm oil, and porpoise jaw oil etc. Animal oils have extreme pressure properties. Vegetable oils are less stable (rapid oxidation) than mineral oils at high temperatures and contain more natural boundary lubricants than mineral oils.
Synthetic fluids are selected since a mineral oil is deficient in some respect for a particular application. They are used where mineral oils are inadequate due to (i) oxidation and viscosity loss at high temperature, (ii) combustion or explosion, and (iii) solidification at low temperature. They are engineered specifically in uniformly shaped molecules with shorter carbon chains which are much more resistant to heat and stress. Synthetic fluids are expensive, but normally are more available in good quality than even the more stable of the mineral oils. Synthetic lubricants can be adjusted in the synthetic process to optimize property performance to a particular application. The flash point of synthetic fluids is higher for a certain viscosity as compared to the mineral oils. Common types of synthetic fluids are (i) synthetic hydrocarbons such as hydrogenated polyalphaolefin (PAO), (ii) esters such as diesters, trimethylolpropane, pentaerythritol, and dipentaerythritol esters, (iii) phosphate esters, (iv) silicone fluids, (v) silicate esters, (vi) silahydrocarbon fluids, (vii) Chlorotrifluoroethylene fluids (CTFEs), (viii) Polyphenylethers (PPEs), and (ix) Perfluoropolyalkylether (PFPAE) fluids.
Soluble oils or compounds are mineral oil or synthetic oils compounded with emulsifying agents.
Plastic lubricants – These lubricants are semi-liquids such as grease. Grease is a semi solid lubricant. It is a mixture consisting of natural or synthetic oil base combined with thickeners and additives. It normally consists of a soap emulsified with mineral or vegetable oil. The National Lubricating Grease Institute (NLGI) defines grease as ‘a solid to semi-solid product of dispersion of a thickening agent in a liquid lubricant. Additives imparting special properties may be included.’
Lubricating grease is composed of liquid and solid phases and is basically comprised of three components namely (i) base oil, (ii) thickener, and (iii) additives and modifiers. The thickener defines the type of the grease. The liquid phase of grease is primarily formed by the base fluid and the solid phase is formed by a network structure of soap molecules or a dispersion of solid particles such as inorganic clays or other fillers. The solid phase thickener can consist of soap molecules with or without added polymer. The base oil in the grease is immobilized by the soap molecule network structure, resulting in a semi-solid to solid appearance. The base oil solubilizes performance additives and modifiers. Base oil comprises the largest component of grease, representing 80 % to 97 %.
Solid lubricants – Solid lubricants are rarely used directly. Normally, it is added with other lubricants to increase some of its properties. Some examples of solid lubricants are graphite, molybdenum disulfide, mica, talc or soap, lead carbonate, and wax, etc.
Additives are the substances which are added to the lubricants to strengthen the properties and the performance characteristics of the lubricants. There are a large number of additives which are being used. The main families of additives are (i) antioxidants to control oil oxidation, (ii) detergents and dispersants to control deposit formation throughout the system, (iii) anti wear agents to provide the load carrying capacity and prevent scuffing of moving parts, (iv) metal deactivators, (v) corrosion inhibitors and rust inhibitors to control rust and corrosion, (vi) friction modifiers to provide oiliness and reduce friction, (vii) extreme pressure agents to provide the load carrying capacity and prevent scuffing of moving parts, (viii) anti foaming agents to control foam formation, (ix) viscosity index improvers to improve viscosity temperature relationship, (x) demulsifiers to reduce emulsion formation or for easy separation of water, (xi) emulsifying agents to reduce surface tension, (xii) stickiness improver which provides adhesive property, (xiii) pour point depressants to have gravity flow properties at low temperatures, (xiv) mist suppressors to reduce oil mist formation, to reduce loss of lubricant, and environmental pollution, (xv) biocides to control bacterial and fungus growth, and (xvi) complexing agent (in case of greases). Some of the additives which are commonly being used are metallic phosphates, metallic oleates, metallic chlorides, metallic sulphides, metallic stearates, metallic oxides and metallic oxalates.
A lubricant has a useful life since with time its properties deteriorate and as the properties deplete lubricant fails to lubricate. The following are the basic reasons for the failure of a lubricant.
- Oxidation of lubricating oil which is the result of the chemically combining of the oil molecules with oxygen of the air. The rate of oxidation increases with the increase in the working temperatures. Metal and their salts catalyze the oxidation process. Antioxidants additives used for controlling the oxidation process are of sacrificial nature which need replishment with time.
- Additives in general during use / service get depleted and lubricants lose their functional properties because of this depletion a lubricant failure can take place.
- Lubricants can get contaminated from internal and external sources.
Characteristics needed to be considered for the selection of a lubricant for a specific application include (i) operating parameters of the equipment, such as speed, load, and temperature, (ii) condition of the equipment, (iii) compatibility of the lubricant with materials in contact, (iv) operating environment such as dust, hot water, outer space, liquid oxygen, or other reactive medium etc., (v) operating condition such as continuous or intermittent, (vi) method of application of lubricant and lubricant maintenance system, (vii) clearances between moving parts, (viii) type of part to be lubricated such as gear, bearing, sliding surface etc., (ix) property requirements of lubricants which can be both physical and chemical properties with the most used physical property is the viscosity–temperature relationship, (x) boundary lubrication performance of the lubricant, (xi) stability of the lubricant in the working environment, (xii) fire resistance property of the lubricant, (xiii) biodegradability and toxicity of the lubricant, and (xiv) additive susceptibility of the lubricant.
Properties of lubricants
Important properties of lubricants are described below.
Viscosity – It is a measure of the lubricant oil’s resistance to flow. Viscosity of the lubricating oil determines its performance under operating conditions. Low viscosity oil is thin and flows easily while high viscosity oil is thick and flows slowly. As oil heats up it becomes more viscous (becomes thin). Very low viscosity of the liquid means that the lubricant film cannot be maintained between the moving surfaces and there is excessive wear. Very high viscosity of the liquid means excessive friction. Selected Lubricant is required to have proper viscosity. Viscosity is normally expressed in centipoise or centistoke.
Viscosity index – It is the average decrease in viscosity of oil per degree rise in temperature between 540 deg C to 1150 deg C. Viscosity of liquids decreases with increasing temperature. The rate at which viscosity of a lubricant changes with temperature is measured by a scale called ‘viscosity index’. Synthetic fluids have high viscosity index.
Iodine number – Iodine number is the measure of the degree of the unsaturation of the lubricating oil. It is the amount of iodine, in grams, which is taken up by 100 grams of the oil. It determines the extent of contamination of oil. Each type of the lubricating oil has its specific iodine number. Low iodine number is desirable in oils.
Aniline point – Aniline point is the minimum temperature at which oil is miscible with equal amount of aniline. Aniline point is a measure of aromatic content of the lubricating oil. Low aniline point oil has high aromatic content which attacks rubber seals. Higher aniline point means low percentage of hydrocarbons (desirable). Hence, aniline point is used as an indication of possible deterioration of rubber sealing etc.
Emulsification and demulsification – Emulsification is the property of lubricating oil to get mixed with water easily. Emulsions can be oil in water emulsion or water in oil emulsion. Good lubricating oil is to form such an emulsion with water which breaks easily. This property is called demulsification. The time in seconds in which a given volume of oil and water separates out in distinct layers is called steam demulsification number. Good lubricating oil is to have lower demulsification number. Quicker the oil separates out from the emulsion formed, better is the lubricating oil. In cutting oils, the higher the emulsification number, better the oil is. This is because the emulsion acts as a coolant as well as a lubricant.
Flash point – Flash point is the minimum temperature at which the lubricant’s vapours ignite when tiny flame is brought near. Fire point is the minimum temperature at which the lubricant’s vapours burn constantly for 5 seconds when tiny flame is brought near. Both flash point and the fire point are to be higher than the maximum achievable ambient temperatures.
Drop point – Drop point is the temperature at which grease passes from the semi-solid to the liquid state. Hence, it determines the upper temperature limit for the applicability of grease.
Cloud point and pour point – Cloud point is the temperature at which the lubricant becomes cloudy or hazy when cooled. Pour point is the temperature at which the lubricant just ceases to flow when cooled. Both indicates suitability of lubricant in cold conditions and hence are to be low. Pour point of lubricant can be lowered by dewaxing or adding suitable pour point depressant. Pour point of oil can be lowered by lowering the viscosity of the oil which is achieved by removing the viscous constituent of the oil. Lubricating oils used in capillary feed systems need to have low cloud points, otherwise impurities clog the capillary. High pour point leads to the solidification of the lubricant which can cause jamming of the machine.
Neutralization point – It determines acidity or alkalinity of oil. Acidity / acid value / acid number is milligrams of KOH (potassium hydroxide) needed to neutralize acid in 1 gram of oil. Alkalinity / base value/ base number is the milligrams of acid needed to neutralize all bases in 1 gram of oil. As the neutralization point of the oil increases, age of the oil decreases.
Saponification number – It is the milligrams of KOH needed to saponify 1 gram of oil. Saponification is hydrolysis of an easter with KOH to give alcohol and potassium salt of acid. Mineral oils do not react with KOH and are not saponifiable. Vegetable and animal oils have very high saponification values. Saponification value helps to ascertain whether the oil under reference is mineral or vegetable oil or compounded oil. All the oils have their specific saponification numbers.