FUEL TECHNOLOGY LUBRICANTS

Transcription

1 1 FUEL TECHNOLOGY Introduction to Liquid Fuels-Classification of Crude Oil- Fractional Distillation- Synthetic Petrol (Fischer-Tropschs & Bergius Process) - Knocking Anti Knocking Agents-Octane & Cetane Number. LUBRICANTS Principle and functions of lubricants Types of lubrication and mechanism Thick film or Hydrodynamic lubrication, Thin film lubrication, extreme pressure lubrication. Classification and properties of lubricants Viscosity, flash and fire points, cloud and pour points, aniline points, neutralization number and mechanical strength. UNIT-IV: FUEL TECHNOLOGY & LUBRICANTS

2 2 INTRODUCTION TO LIQUID FUELS: Petroleum or crude oil (petro=rock; ileum=oil) is a dark greenish brown viscous oil found in earth s crust. It is composed of straight chain hydrocarbons, cycloparaffins or naphthalene s, olefins and aromatic compounds together with small amount of nitrogen, oxygen and sulphur. The average composition is C=79.5 to 87.1 %; H=11.5to 14.8%, S=0.1 to 3.5%; N+O=0.5%. CLASSIFICATION OF PETROLEUM OR CRUDE OIL: 1. Paraffinic-base type crude is mainly consisting of saturated hydrocarbons from C 1-C 35 and a little of naphthalene s and aromatics. 2. Asphaltic-base type crude is mainly cyclo paraffin s with smaller amounts of paraffin s and aromatics. 3. Mixed- base type crude mainly contains both paraffinic and asphaltic hydrocarbons and also semi-solid waxes. REFINING OF PETROLEUM: The crude oil is separated into various useful fractions by fractional distillation and finally converted into desired specific products. This process is called Refining of Petroleum. This process involves three steps, Step-1: Separation of water (Cottrell s process): The crude oil from the oil well is an extremely stable emulsion of oil and salt water. This is passed through highly charged electrodes. The Colloidal water droplets coalesce to form large drops, which separate out from the oil. Step-2: Removal of harmful sulphur compounds: this involves treating of oil with copper oxide to form insoluble solid copper sulphide, which is removed by filtration. Step-3: Fractional distillation: The Petroleum oil is then heated to about C in an Iron retort, where by all volatile constituents, except the residue (asphalt or coke) is evaporated. The hot vapors are then passed up a fractionating Column which is a tall cylindrical tower containing a number of horizontal stainless steel trays at short distance. Each tray is provided with a small chimney covered with loose caps. As the vapors go up, they become gradually cooler and fractional condensation takes place at different heights of column. Higher boiling fraction condenses first; while the lower boiling fractions turn-by-turn. Various fractional products are obtained as follows; Name of fraction Boiling range omposition Uses Uncondensed gas. Below 30 0 C C 1- C 4 As domestic or industrial fuel (L.P.G)

3 3 Petroleum ether C C 5-C 7 As a solvent Gasoline or Petrol C C 5-C 9 As motor fuel, solvent Naphtha C C 9-C 10 As solvent and in dry cleaning Kerosene oil C C 10-C 16 As an illuminant, jet engine fuel Diesel oil C C 10-C 18 As diesel engine fuel Heavy oil C C 17-C 30 For getting gasoline by cracking process Heavy oil on refractionation gives: (a)lubricating oil (b)petroleum jelly Or Vaseline (c)grease (d) Paraffinic wax Residue Asphalt Tar Above C Above C 30 SYNTHETIC METHODS FOR PRODUCTION OF PETROL: POLYMERIZATION: As lubricant. Used in cosmetics and medicine. As a lubricant In candles & boot polishes. Water proofing and road making The gases obtained as a by-product from cracking of heavy oils, contains olefins and alkanes. When this gaseous mixture is subjected to high pressure and temperature, with or without the presence of catalyst, it polymerizes to form higher hydrocarbons, resembling gasoline, called polymer gasoline. CH 3.CH=CH 2 + CH 3.CH 2.CH=CH 2 CH 2=CHCH 2. CH 2. CH (CH 3) 2 Propene Butene 5-methyl hexane-1 1. FISCHER TROPSCH S PROCESS: Water gas (CO+H 2) is mixed with hydrogen. It is purified by passing through Fe 2O 3 and then in to a mixture of Fe 2O 3 + Na 2CO 3 to remove organic sulphur compounds. The purified gas is compressed to 5-25 atmospheres. The compressed gas is lead through convertor containing mixture of 100 parts of cobalt, 5 parts of thoria, 8 parts of magnesia and 200 parts of kiesel guhar earth maintained at about c. A mixture of saturated and unsaturated hydrocarbons is formed. n CO + 2 n+1 H 2 C n H 2n+2(saturated hydro carbon) + n H 2O n CO + 2 n H 2 C n H 2n (unsaturated hydro carbon) + n H 2O 2. BERGIUS PROCESS:

4 4 In this process, low ash coal pulverized to fine powder is converted into a paste with heavy oil and a catalyst nickel oleate is mixed with it. This mixture is sent to the convertor maintained at c and a pressure of atmospheres, where it meets the hydrogen gas. The hydrogen combines with carbon of the coal giving various hydrocarbons from heavy oil to gasoline. Reaction carried for one hour. The gases coming out of the convertor are passed into fractionators to get gasoline, middle oil and heavy oil. The middle oil is subjected to hydrogenation in presence of catalyst to produce gasoline. The heavy oil fraction is recycled to male paste with fresh coal powder. KNOCKING AND ANTI KNOCKING: The ratio of the gaseous volume in the cylinder at the end of the suction stroke (intake stroke) to the volume at the end of the compression stroke of the piston is known as compression ratio. If the compression ratio is high then the efficiency of the internal combustion engine is high. When the compression ratio is very high, the rate of oxidation becomes so great that the last portion of the fuel air mixture gets ignited instantaneously, producing an explosive violence, known as knocking. The knocking results in loss of efficiency. The tendency of fuel constituents to knock is: Straight-chain paraffin s > branched-chain paraffin s > olefins > cyclo paraffin s> aromatics. OCTANE RATING or OCTANE NUMBER: It was introduced by Edger in It has been found that n-heptanes knocks very badly and hence its anti-knock value has arbitrarily been given zero. On the other hand, isooctane (2, 2, 4- tri methyl pentane) gives very little knocking, so it s anti knock value has been given as 100. Thus octane number of a gasoline (or any other internal combustion engine fuel) is the percentage of isooctane in a mixture of isooctane and n- heptanes, which matches the fuel under test in knocking characteristics. It means, a 80-octane fuel is one which has the same combustion characteristics as an 80:20 mixture of iso-octane and n-heptanes. ANTI-KNOCKING AGENTS: The octane number of fuels can be raised by the addition of some anti-knocking materials like tetra ethyl lead, (C 2H 5) 4Pb or TEL and diethyl telluride, (C 2H 5) 2Te. During combustion, the added tetra ethyl lead is converted into lead oxide (PbO). They combine with hydrocarbons producing free radicals which slow down the explosive combustion. Thus the anti-knocking characteristics of the fuel are considerably increased. The lead oxide after combustion is reduced to metallic lead and is harmful to engine life,

5 5 therefore a small amount of ethylene dibromide, C 2H 4Br 2 is added to convert lead into lead bromide and remove it along with the exhaust gases. Recently new anti-knock agents which are safe and completely volatile are used rather than TEL. CETANE NUMBER: In a diesel engine, the fuel is not ignited by a spark but the compression of air increases the pressure and temperature and the incoming fuel should ignite below compression temperature to have as short an induction lag as possible. This means that the hydrocarbon molecules in a diesel fuel should be as far as possible the straight chain ones with minimum admixture of aromatic and side chain hydrocarbon molecules. The suitability of a diesel fuel is determined by its cetane value, which is the percentage of hexadecane (cetane) in a mixture of hexadecane and 2-methyl naphthalene, which has the same ignition characteristics as the diesel fuel in question. The cetane number of a diesel fuel can be raised by the addition of small quantity of certain pre-ignition dopes like ethyl nitrite, isoamyl nitrite, acetone peroxide etc. 2-Methyl naphthalene (Cetane No. = 0) n-hexadecane (Cetane No. = 100) Ignition quality order among hydrocarbon constituents of a diesel fuel is as follows: n-alkanes naphthalenes alkenes branched alkanes aromatics Thus, hydrocarbons which are poor gasoline fuels are quite good diesel fuels. PRINCIPLE OF LUBRICATION: All material surfaces, no matter how smooth they are, show many irregularities in the form of peaks (or asperities) and valleys, which are large when considered on a molecular scale (see figure below). When two solid surfaces are pressed over each other a real contact between these surfaces occurs only at a limited number of asperities i.e. peaks of the upper surface are in contact with peaks of the lower surface. Even under small loads, the local pressure at the asperities may be sufficiently great to cause appreciable deformation in ductile metals. This causes the formation of welded junctions between the asperities. It is these junction areas that carry the entire load between the two surfaces. Thus the real or true area of contact is only a small fraction of the apparent contact area between the two surfaces. In all type of machines, the surfaces of moving (or) sliding (or) rolling parts rub against each other. Due to mutual rubbing of one part against another, a resistance is offered to their moment. This resistance is known as friction. Friction causes a lot of wear and tear of surfaces of moving parts, and large amounts of energy are dissipated

6 6 in the form of heat, thereby causing loss in the efficiency of machine. Moreover, the moving parts get heated up damaged and even some times results in seizure (welding of melted surface peaks). Any substance introduced between two moving/sliding surfaces with a view to reduce the frictional resistance between them is known as a lubricant. This process is called as lubrication. FUNCTIONS OF LUBRICANT: 1. It reduces surface deformation, wear and tear 2. Reduces loss of energy in the form of heat 3. Acts as a coolant. 4. Reduces waste of energy, so efficiency of machine is enhanced. 5. Reduces expansion of metal by local frictional heat. 6. Reduces the maintenance and running cost of the machine. 7. Avoids the unsmooth relative motion of the moving parts. 8. In the ICE (Internal Combustion Engine) it acts as a seal (between piston and cylinder) And also prevent the leakage of gases under high pressure from the cylinder. MECHANISM OF LUBRICATION: There are mainly three types of mechanism by which lubrication is done. 1. Thick-film or Fluid- film or hydrodynamic lubrication. 2. Thin film or Boundary lubrication. 3. Extreme pressure lubrication. 1. THICK FILM OR FLUID FILM OR HYDRODYNAMIC LUBRICATION: For low load and high speed surfaces, thick film lubrication is used (a) In this, the moving surfaces are separated from each other by a thick film of fluid (at least 1000Å thick), so that direct surface to surface contact and welding of junctions rarely occurs. (b) The lubricant film covers the irregularities of the moving surfaces and forms a thick layer in between them. This consequently reduces wear. (c) The small friction (if any) occurs is only due to internal resistance between the particles of the lubricant moving over each other. (d) In such a system friction depends on the thickness and viscosity of the lubricant, the relative velocity and area of the moving surfaces. (e) The coefficient of friction is as low as to 0.03 for fluid film lubricant system. (f) Hydrodynamic friction occurs in the case of shaft running at a fair speed as well as in well lubricant bearings, with not too high loads. (g) In a journal bearing a film of the lubricating oil covers the irregularities of shaft as well as bearing surface and also the metal surfaces do not come into direct contact with each other. Eg. Delicate instruments, watches, guns, scientific instruments etc. are provided with this type of Lubrication. (h) Fluid film lubrication is satisfactorily done by hydrocarbon oils.

7 7 (i) (j) Hydrocarbon oils are generally blended with selected long chain polymers in order to maintain the viscosity of oil constant in all reasons of year. Moreover hydrocarbon petroleum fractions generally contain small quantities of unsaturated hydrocarbons, which get oxidized under operating conditions forming gummy products. Hence it is essential that antioxidants (like amino phenols) to be blended with hydrocarbon oils. 2. THIN FILM OR BOUNDARY LUBRICATION: When a continuous film of lubricant cannot be maintained in the moving machine surfaces due to certain reasons like high load and low speed, then a thin film lubrication process is applied. (a) In this mechanism, a thin layer of lubricant is adsorbed on the metallic surfaces which avoid direct metal to metal contact. (b) In this process the lubricant is adsorbed by physical (or) chemical forces (or) both, on both the metallic surfaces. (c) In boundary lubrication, the distance between moving surface is very small of the order of the height of the surface asperities. (d) The coefficient of friction for thin film lubrication is between 0.05 to (e) Vegetable & animal oils and their soaps possess a property of adsorption and form a thin film on metal surfaces which acts as a lubricant. However it cannot withstand high temperatures. (f) Hence mineral oils (which are thermally stable) are blended with small amounts of fatty oils or fatty acids so that they can get adsorbed and also withstand high temperatures. (g) Graphite and Molybdenum disulphid either as solid (or) as stable emulsion in oil can also be used as an efficient lubricant in thin film lubrication. These materials reduce friction between metallic surfaces by forming films on the surfaces and they can bear compression as well as high temperature. For boundary lubrication the lubricant molecule should have, (a) Long hydrocarbon chains (b) Lateral attraction between chains. (c) Polar groups or active metal atoms to promote spreading over the surface. (d) Resistance to heat and oxidation. (e) Good oiliness EXTREME PRESSURE LUBRICATION: (a) When the moving / sliding surfaces are under very high pressure and speed, a high local temperature is attained. (b) In such a condition liquid lubricants fail to stick on the surfaces, and may decompose (or) vaporize. (c) To meet these extreme pressure conditions special additives are added to mineral oils. These are called extreme pressure additives.

8 8 (d) (e) (f) (g) These additives form on metal surfaces more durable films, capable of withstanding very high loads and high temperatures. The special additives are organic compounds having active radicals (or) groups such as chlorine, sulphur or phosphorous. These compounds react with metallic surfaces at prevailing high temperatures, to form metallic Chlorides, sulphide or phosphides. These metallic compounds possess high melting points and serves as a good lubricant under extreme pressure and temperature conditions. CLASSIFICATIONS OF LUBRICANTS: On the basis of their physical state lubricants cab be broadly classified as (a) Liquid lubricants - lubricating oils (b) Semi solid lubricants - Greases and (c) Solid lubricants (a) LIQUID LUBRICANTS (OR) LUBRICATING OILS: It provides a continuous fluid film in between the moving surfaces. Good lubricating oil must possess low pressure (high boiling point), adequate viscosity at service conditions, low freezing point, high oxidation resistance, heat stability, non corrosive property and stability. Lubricating oils are further classified as (i) Animal or vegetable oils (ii) Mineral or petroleum oils (iii) Blended oils i. Animal or vegetable oils: Before discovering the petroleum fractions, animal (or) vegetable oils are the most commonly used lubricants. They possess good oiliness. But they are costly, undergo oxidation easily, forming gummy and acidic products & have some tendency to hydrolyze. So at present they are rarely used as lubricants. Now a days the animal or vegetable oils are used as blending agent with other lubricating oils ii. Mineral (or) Petroleum oils: These are obtained by distillation of petroleum (hydrocarbons). Hydrocarbon chain in petroleum oils varies between12 to 50 carbon atoms. The shorter chain oils have lower viscosity than the longer chain hydrocarbons. Now a days these are widely as lubricants because cheap, easily available, and quite stable under service conditions. Compared to animal or vegetable oils, mineral oil possesses poor oiliness. Its oiliness can be increased by adding high molecular weight compounds like oleic acid, stearic acid etc. iii. Blended oils: Desirable characteristics of lubricating oils can be improved by adding small quantities of various additives. The oils thus obtained are known as blended oils or compounded oils. The following additives are employed. Examples Additive type Purpose and function Oiliness carriers Antioxidant Corrosion preventers Extreme pressure additives Increases the oiliness character of a lubricating oil Retard oxidative decomposition, they are particularly added in lubricants used in ICE engines They protect the metal from corrosion These additives contain certain materials, which are adsorbed on the metal surface (or) react With chemically with metal, producing a surface layer of low shear strength on metal surface, (Added additives) Vegetable and fatty acids, coconut oil, palmitic acid, oleic acid etc Aromatic amines, Phenolic (or) amino compounds Organic Compounds of Phosphorous (or) Antimony Organic substance with chloride, sulphur (or) Phosphorous group

9 9 b) SEMI SOLID LUBRICANT (OR) GREASES: Lubricating grease is a semi solid. It consists of a soap being dispersed throughout liquid lubricating oil [liquid lubricant may be petroleum oil (or) synthetic oil and it contain additives for specific requirements]. Greases are prepared by saponification of fat with alkali, followed by adding hot lubricating oil while under agitation. The soap component forms the gel structure (interconnected network) within which the oil component resides. To improve the heat resistance of grease inorganic solid thickening agents are added. Ex: - carbon black, colloidal silica etc. Greases have higher frictional resistance than oils and can support much heavier loads at lower speeds. Greases are used where the lubricating oil cannot serve as a lubricating film due to high load, low speed, sudden jerks etc. (Ex: - rail axle boxes) The main function of soap is thickening agent so that grease sticks firmly to the In bearing and gears that work at high temperatures where the bearings need to be sealed (The above cases arising in any type machines generally greases are used as a lubricant), greases stick firmly to the metal surfaces due to the thickening agent of soap. Depending upon the soap component used in the manufacture of grease, they are classified as follows: (i) Calcium based greases (or) cup Greases: It is an emulsion of petroleum oils with calcium soaps. It is prepared by adding requisite amount of calcium hydroxide to hot oil, under agitation. These greases are most commonly used, cheap, insoluble in water (so water resistant) and can be used below 80 0 C only. (ii) Soda based greases: These are petroleum oils, thickened by mixing sodium soaps. They are not water resistant, because sodium soaps are soluble in water. However they withstand up to C. Suitable for lubricants in ball bearings. (iii) Lithium based greases: These are petroleum oils, thickened by mixing lithium soaps. They are water resistant and suitable for use at low temperatures [up to 15 0 C] only. (iv) Axle greases: They are very cheap resin greases, prepared by adding lime (or any heavy metal hydroxide) to resin and fatty oils. Fillers (like mica and talc) are also added to the. They are water resistant and suitable for less delicate equipments working under high loads and at low speeds. c). SOLID LUBRICANTS: 1. For high load and high velocity of moving parts where liquid and semi-solid lubricants are not suitable and efficient, the solid lubricants are preferably used. 2. Solid lubricants reduce friction by separating two moving surfaces under boundary conditions. They are used either in the dry powder form (or) mixed with oil or water. 3. Low spots on the surface of moving parts are filled by these lubricants, which form solid films having low frictional resistance. 4. The usual coefficient of friction of solid lubricants is between and Solid lubricants find application in commuter buses, electric generators and motors, where contamination of grease or lubricating oil is unacceptable. 6. In internal combustion engines (ICEs) between the piston and cylinder for increasing compression (combustible lubricants must be avoided). The two most commonly used solid lubricants are Graphite and Molybdenum disulphide. Graphite:

10 10 (i) (ii) (iii) (iv) It is one of the most widely used solid lubricants. It is (i) soapy to touch (ii) Non inflammable (ii) Not oxidized in air below C (iv) In the absence of air, it is used up to high temperature. Graphite is used either in powdered form or suspension (in oil or water with help of emulsifying agent like tannin). When graphite is dispersed in oil, it is called oil dag and when it is dispersed in water it is called aquadag. Oil dag is found particularly useful in Internal Combustion Engines while aquadag is useful where a lubricant free from oil is needed. Graphite is used as a solid lubricant at General machine shop works, Railway track joints, Lathe machines tec. Molybdenum disulphide (MoS 2): (a) It possess very low coefficient of friction (b) Stable in air up to C (c) It is used in powder form (or) along with solvents and greases PROPERTIES OF LUBRICANTS: Some of the properties of lubricating oils are (a) Viscosity (d) Cloud and pour point (b) Flash and fire point (e) Aniline point (c) Mechanical stability (f) Neutralization number VISCOSITY: It is the property of a liquid or fluid by virtue of which it offers resistance to its own flow. The force per unit area required to maintain a unit velocity gradient between two parallel layers is called coefficient of viscosity. The unit of viscosity is Poise. Viscosity is the most important property of lubricating oil, because it is the main determinant of the operating characteristics of the lubricant. Viscosity of good lubricating oil should not change much with change in temperature. MEASUREMENT OF VISCOSITY OF LUBRICATING OIL: Many Instruments are available for measuring viscosity and are known as viscometers. In a viscometer fixed volume of the liquid is allowed to flow, from a given height, through a standard capillary tube under its own weight and the time of flow in seconds is noted. The time in seconds is proportional to true viscosity. Redwood viscometers (No.1 and No. 2) and Say bolt viscometers are used for measuring the viscosity of lubricating oil. The essential difference between the two is Dimensions of orifice Red wood-1 Red wood-2 Length -10mm Diameter-1.62mm 15mm 3.8mm Useful for Low viscous oil Higher viscous oil Red wood viscometer No.1 consists of the following essential parts (a) Oil cup (b) heating bath (c) spirit level (d)kohlrausch flask and (e) leveling screws Through Redwood viscometer, the viscosity is estimated by the following manner. 1. The leveled oil cup is cleaned and ball of valve rod is placed on the agate Jet to close it. 2. The tested lubricating oil is filled in the cup up to the pointer level. 3. An empty-kohlraush flask is kept just below the jet 4. Water is filled in the bath and side tube is heated slowly with constant stirring

11 11 5. When the oil reaches to desired temperature heating is stopped and the ball valve is lifted. 6. The time taken for 50 ml of the oil to collect in the flask is noted and immediately the valve Is closed to prevent any over flow of the oil. 7. The result is expressed in Red wood No.1 seconds at the particular temperature. FLASH AND FIRE POINT: Flash point is the lowest temperature at which the oil lubricant gives off enough vapors that ignite for a moment, when a tiny flame is brought near it. Fire Point is the lowest temperature at which the vapors of the oil burn continuously for at least five seconds, when a tiny flame is brought near it. A good lubricant should have flash point at least above the temperature at which it is to be used. The flash and fire points are usually determined by using Cleveland s open cup or Penskey Marten s closed cup apparatus. CLOUD AND POUR POINT: When oil is cooled slowly, the temperature at which it becomes cloudy in appearance is called its cloud point. The temperature at which the oil ceases to flow or pour is called it pour point. Cloud & pour points indicate the suitability of lubricant in cold conditions. For machine working at low temperatures, the lubricating oil should possess low pour point. With the help of pour point apparatus easily estimate the cloud and pour point of lubricating oil. In the flat bottom test tube the lubricating oil is taken and it is surrounded by an air jacket. The jacket is surrounded by a freezing mixture ( CaCl 2 + ice) contained in a Jar. A thermostat is introduced in the test tube. As cooling proceeds slowly through air jacket and observe the lubricating oil in the test tube, at one of the temperature the cloudiness is noticed, and it is recorded as the cloud point of the lubricating oil. After this, cooling is continued and the status of the lubricating oil is observed after every 3 0C fall of temperature. The temperature at which oil does not flow in the test tube, even when it is kept horizontal for 5 seconds is recorded as the pour point. NEUTRALIZATION NUMBER (OR) ACID NUMBER: It refers to determinations of acidic (or) basic constituents of oil. Acid number (or) value is defined as the number of milligrams of KOH required to neutralize the free acids in 1gram of the oil. Generally free acids are not present in lubricants.a lubricating oil should possess acid value less than 0.1. Value greater than 0.1 indicates that the oil has been oxidized and the use of it internal combustion engine may lead to corrosion of the parts. ANILINE POINT: It is defined as the minimum temperature at which equal volumes of aniline and oil sample become homogeneous or attain equilibrium. Aromatic hydrocarbons have a tendency to dissolve natural rubber and certain types of synthetic rubbers. So, a low aromatic content in the lubricant is desirable when it is used in contact with rubber sealing s and pickings etc. Aniline point gives a rough estimate of aromatic content in the oil.

12 12 The mixture of oil and aniline which is biphasic when the aromatic content is low becomes homogeneous at certain temperature depending on the amount of aromatic content. If the aromatic content is really high, it need very low temperatures (or even room temperature) to become homogeneous. A higher aniline point means a lower percentage of aromatic hydrocarbons and hence higher percentage of paraffinic hydrocarbons. Aniline point is determined by mixing equal volumes of oil and aniline in a test tube and heating them till a homogeneous solution is obtained. Then it is cooled at a controlled rate and the temperature at which the two phases (oil and aniline) separate out is recorded as the aniline point. MECHANICAL STABILITY: To know the suitability of a lubricant under conditions of very high pressure, different mechanical tests are carried out. One such test is four-ball extreme-pressure lubrication test in which the lubricant under test is poured in a machine containing four balls. The lower three balls is stationery while upper ball is rotated. Load is gradually increased and the balls are examined at specific intervals for scale formation etc. on them. If the lubricant is satisfactory it will not form any scales otherwise the heat generated may weld the balls and lead to scratches or scale formation. Thus, this test enables us to determine the maximum load that can be carried safely by a lubricant.