Lubricants and Greases Shibu G. Pillai
Contents Introduction Functions of lubricants Mechanism of lubrication Classification of lubricants Properties of lubricants Selection of lubricants
INTRODUCTION In All types of machines,the surface of moving, rolling or sliding parts rub against each other, So resistance is produced to their movement called as FRICTION.
EFFECTS OF FRICTION Causes lot of wear and tear to surfaces of moving/rolling/sliding parts. Large amount of energy is dissipated in form of heat Ultimately reduces the efficiency of machine.
Solution is LUBRICANTS DEFINITION:- Any substance which reduces the friction when introduced between two surfaces known as lubricant and the process is termed as lubrication..
FUNCTIONS OF LUBRICANTS 1.Reduces the maintenance and running cost of the machines as it prevents from rust and corrosion.
2.Prevents foreign matter(dirt) from entering bearing and damaging the smooth surface of the journal and bearing.
3.Reduces wear, tear and surface deformation: The direct contact between the sliding/moving surface is avoided. In absence of lubricants, some of the small peaks would be sheared off as one block moves over the other.
4.As sealing Agent in Internal Combustion Engine: In the engine the lubricant used between the piston and the cylinder wall acts as a seal. This seal prevents the leakage of gases under high pressure from the cylinder.
5.Lubricant acts as a Coolant During a motion of body on surface, a considerable amount of frictional heat energy is evolved at rubbing surfaces. So work of lubricant over here is to simply flow over the heated surfaces and absorb heat from there.
6.Lubricants reduces the friction On examining a smooth metal surface under powerful microscope we may observe peaks and valleys of different heights and depths called surface roughness.
So on applying load on these surfaces due to presence of these valleys and depths there is hinderance caused due the movement.
But when lubricating oil is kept in between two surfaces, small peaks and valleys do not interlock and top part easily slides over each other i.e. reduces friction.
MECHANISM OF LUBRICATION- Considering the nature of motion between moving or sliding surfaces, there are different types of mechanisms by which the lubrication is done. They are: Hydrodynamic lubrication or thick film lubrication. Boundary lubrication or thin film lubrication. Extreme pressure lubrication. 14
Hydrodynamic Lubrication or Thick Film Lubrication- Hydrodynamic lubrication is said to exist when the moving surfaces are separated by the pressure of a continuous unbroken film or layer of lubrication. In this type of lubrication, the load is taken completely by the oil film. The basis of hydrodynamic lubrication is the formation of an oil wedge. When the journal rotates, it creates an oil taper or wedge between the two surfaces, and the pressure build up with the oil film supports the load. Hydrodynamic lubrication depends on the relative speed between the surfaces, oil viscosity, load, and clearance between the moving or sliding surfaces. 15
In hydrodynamic lubrication the lube oil film thickness is greater than outlet, pressure at the inlet increases quickly, remains fairly steady having a maximum value a little to the outside of the bearing center line, and then decreases quickly to zero at the outlet.. The coefficient of friction is as low as 0.001-0.003 for fluid lubricated system in comparison to 0.5 to 1.5 for unlubricated surfaces Hydrocarbon petroleum fraction contain small quantities of unsaturated hydrocarbons,which get oxidized under operating conditions, forming gummy products. Hence it is essential that antioxidants (aminophenols) to be blended with hydrocarbon oils. 16
Application of hydrodynamic lubrication- Delicate instruments. Light machines like watches, clocks, guns, sewing machines. Scientific instruments. Large plain bearings like pedestal bearings, main bearing of diesel engines. Hydrocarbon oils are considered to be satisfactory lubrication for fluid film lubrication. In order to maintain the viscosity of the oil in all seasons of the year, ordinary hydrocarbon lubricants are blended with selected long chain polymers. 17
Hydrodynamic Lubrication- 18
Boundary Lubrication or Thin Film Lubrication- Boundary lubrication exists when the operating condition are such that it is not possible to establish a full fluid condition, particularly at low relative speeds between the moving or sliding surfaces. The oil film thickness may be reduced to such a degree that metal to metal contact occurs between the moving surfaces. The oil film thickness is so small that oiliness becomes predominant for boundary lubrication. Boundary lubrication happens when A shaft starts moving from rest. The speed is very low. 19
The load is very high. Viscosity of the lubricant is too low. Examples for boundary lubrication: Guide and guide shoe in two stroke engine. Lubrication of the journal bearing in diesel engines (mainly during starting and stopping of engine). 20
Lubricants used for boundary lubrication are: Graphite and MoS2 Use in the form of solids or as stable emulsion, in oil. It reduces friction between metallic surfaces by forming films on the surfaces and they can bear compression as well as high temperature Mineral oils These are thermally stable and their adhesion property is improved by adding adding small amount of fatty acids or oils Vegetable and mineral oils and their soap Posses greater oiliness compared to mineral oils. They either physically adsorbed to metal surfaces or react chemically at the metal surfaces,forming a thin film of metallic soap,which acts as lubricants. 21
Extreme Pressure Lubrication- When the moving or sliding surfaces are under very high pressure and speed, a high local temperature is attained. Under such condition, liquid lubricant fails to stick to the moving parts and may decompose and even vaporize. To meet this extreme pressure condition, special additives are added to the minerals oils. These are called extreme pressure additives. These additives forms more durable films on the metal surfaces, capable of withstanding high loads and high temperature. Additives are organic compounds like chlorine (as in chlorinated esters), sulphur (as in sulphurized oils), and phosphorus (as in tricresyl phosphate). 22
Classification of Lubricants Solid lubricants (graphite, MoS 2 ) Semi-Solid Lubricants (greases, vaselines) Liquid Lubricants (all oils) Animal and Vegetable oils Mineral oils Blended Oils SYNTHETIC Oils Cutting Fluids (O/W and W/O Emulsions)
Solid lubricants are used at high temperature and heavy load where a liquid or a semi-solid lubricant film cannot be maintained contamination of oil is unacceptable where combustible lubricants must be avoided Graphite and Molybdenum Disulphide Oil dag (suspension of graphite in oil) Aqua dag (suspension of graphite in water)
CHARACTERISTICS: 1. Soapy to touch. 2. Inflammable. 3. Doesn t oxidizes under 375 C. 4. Dispersion in oil is called oildag. Dispersion in water is called aquadag. USES: 1. As lubricants in air-compressors, lathes, foodstuffs industry, open gears, chains, railway track-joints etc. 2. Graphite mixed with grease (graphite-grease) is used at higher temperatures. 3. Oildag is used in internal combustion engines & Aquadag is used in foodstuffs industries.
Semi-Solid lubricants are used machinery parts are subjected to slow speed Heavy load and produce high temperature Spilling of the oil is undesirable Greases and Vaselines are most important
Grease Semi-solid lubricant obtained by thickening of a lubricating oil by the addition of metallic soap The properties of grease depend upon the the nature and mount of thickener used the characteristics of the base oil used the way in which the grease is prepared Most of the common grease are made with soap thickener (3% - 50%) Non-soap thickeners: carbon black, asphalten
Important functions of a SOAP in a grease it act as a thickener it enables the grease to stick to the metal surface the nature of the soap determines the temperature up to which grease can be used and its consistency
Ca-Based Grease/Cup-Greases Emulsion of petroleum oil with calcium soap. 1. Insoluble in water 2. They are the cheapest 3. They can even be used at low temperatures because above 80 C, soap starts to separate USES: They are used in tractors, water pumps, etc. Na-Based 1. They are not water resistant. 2. They can be used upto 175 C USES: They are used in ball bearings.
Li-Based They are petroleum oils thickened by mixing lithium soap 1. Water Resistant 2. Expensive 3. Thermally stable USES: In Aircraft Applications. Axle Grease They are cheap resin greases prepared by adding lime to resin & fatty oils. They are water-resistant. USES: They are used in the equipment which work under high load & low speed
Liquid lubricants Mostly used Used in delicate & light machines which works at high speed but under low pressure Animal and vegetable oil Mineral oil Blended oil Synthetic oil
Animal and Vegetable Oil Extracted directly from nature They are very costly They can undergo oxidation very easily They are used where there is heavy load They are triglycerides of higher fatty acids Possess Good Oiliness Property Examples : Olive Oil, Sunflower Oil, Coconut Oil, Castor oil, Cod-liver oil
Mineral/petroleum Oil Obtained by distillation of petroleum These are basically lower molecular weight hydrocarbons with 12-50 C atoms Shorter chain have lower viscosity Most widely used Cheap, available in abundance and quite stable under any conditions Possess Poor Oiliness Property Oiliness can be improved by adding high molecular weight compunds like oleic acid, stearic acid or triglycerides of higher fatty acids
Blended Oil Additives are added to the lubricating oils in order to further improve their properties of the refined lubricating oils. They are added in amounts upto 10%.
Cutting Fluid/Oil/Emulsion In various machining operations such as drilling, cutting, threading, turning the friction is very high and this generates large amount of heat Due to overheating, the tool may lose its temper and hardness In order to prevent this, efficient lubrication and cooling have to be provided This is done by employing cutting oils or fluids or cutting emulsion
In order to provide satisfactory service, the cutting fluid should possess following characteristics: good lubricating properties low viscosity, so that lubricant can easily fill in the cracks formed on the work-piece chemical stability non-corrosive nature high thermal conductivity
Properties of Lubricants FOR OILS Viscosity and Viscosity Index Cloud and Pour-Point Flash and Fire-Point Aniline Point Oiliness Saponification number Steam Emulsion Number (SEN) Carbon Residue
FOR GREASE Penetration Index Drop Point
Viscosity & Viscosity Index Property of a liquid by virtue of which it oppose relative motion between its different layers is known as viscosity or internal friction of liquid Measure of internal resistance to flow Thickness of fluid (in laymen terms) Important in lubricant selection Two Types: Kinematic Viscosity Absolute Viscosity
Viscosity-Temperature Relation viscosity of liquids decreases with increasing temperature consequently, the lubricating oil becomes thinner as the operating temperature increases Hence, viscosity of a good lubricating oil should not change much with change in temperature
Viscosity Index The rate at which the viscosity of an oil changes with temperature is measured by an arbitrary scale If the viscosity of an oil falls rapidly as the temperature is raised, it has low viscosity index If the viscosity of an oil slightly affected on rising the temperature, it has high viscosity index For a good lubricating oil viscosity-index should be HIGH
Determination of Viscosity Index Two types of standard oils, Paraffinic-base Pennsylvanian oils (VI =100) and Naphthanicbase Gulf oils (VI =0) are used Against each of these is marked their viscosities at 100 F and 210 F Former are known as H-oils and latter as L-oils Following steps involved:
The viscosities of the oil under test at 100 F and also at 210 F are first found out, let these values be U and V respectively. The difference between the two values should be low, if the oil is good. Now from the list of H-oils (VI =100), the oil which as the same viscosity at 210 F as the oil under-test is selected, and its corresponding viscosity at 100 F is determine: say H Then, from the list of L-oils (VI = 0), the oil which as same viscosity at 100 F is determine: say L
VI = L U L H 100 U: viscosity at 100 F of the test oil (sample) L: viscosity at 100 F of the low-viscosity standard oil H: viscosity at 100 F of the high-viscosity standard oil
Viscosity-Temperature Graph
Redwood Viscometer Parameter Redwood Viscosity No. 1 Redwood Viscosity No.2 Dimension of orifice Length:10mm Diameter:1.62mm Length:50mm Diameter:3.80mm Receive flask has Smaller mouth Larger mouth Useful for Low viscous oil such as lubricating oils High viscous oil such as fuel oil Efflux time 2000 sec or less Approximately 1th/10 of that obtained with No.1 instruments.
Cloud and Pour Point When an 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 oil ceases to flow or pour, is called its POUR POINT Good lubricant should have low pour point. Polyalkylbenzene, is a pour point depressant
It gives the suitability of lubricants in cold environments The presence of waxes in the lubricating oil increases the pour point. Pour point determiners the suitability of a lubricant oil for low temperature.
Flash and Fire Point The minimum temperature at which the oil gives off sufficient vapours to ignite for a moment (flash), when a flame is brought near the surface of the oil, called its FLASH POINT The lowest at which the vapours of the oil burn continuously for atleast 5 seconds, called its FIRE POINT Good lubricant should have higher flash point w.r.t working temperature
Aniline Point gives an indication of the possible tendency of deterioration of an oil when it comes into contact with packing, rubber sealing etc. Aromatic hydrocarbon have a tendency to dissolve rubber defined as the minimum equilibrium solution temperature for equal volumes of aniline and oil sample Should be HIGH
Determination of Aniline Point Mixing equal volumes of aniline and the oil sample in a tube Heating the mixture until a homogeneous solution is obtained Allowed to cool at a specific rate, until the two phases (oil & aniline) separate out Temperature corresponding to observation is reported as ANILINE POINT
Saponification Number The number of milligram of KOH require to neutralize the fatty acids resulting from the complete hydrolysis of 1g of fat Unitless To indicate the presence of fatty materials in oil
Steam Emulsion Number (SEN) It is number of second require to demulsified Lubricant Unit: seconds For good lubricant it should be less To indicates the formation of stable emulsion or not Good lubricating oils never form stable emulsion
Penetration Index/Consistency value Consistency of the grease Defined as the distance in tenth of mm that a standard cone penetrates vertically into the sample, under the condition of load, temp and time Instrument: Penetrometer
Drop Point Defined as the temperature at which grease passes from the semi-solid to the liquid state Upper limit temp for the applicability of the grease