Lubricants and Lubrication

Lubricant

A lubricant (sometimes shortened to lube) is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, transporting foreign particles, or heating or cooling the surfaces.

The main features of lubrication are:

·         It reduces wear and tear and surface deformation, by avoiding direct contact between the rubbing surfaces.

·         It reduces the loss of energy in the form of heat by acting as a coolant.

·         It reduces the efficiency of the machine by reducing the waste of energy.

·         It reduces expansion of metal by local frictional heat.

Classification of lubricants

On the basis of state, lubricants are classified as solid, semi-solid and liquid lubricants.

Solid Lubricants

A solid lubricant is a material used as powder or thin film to provide protection from damage during relative movement and to reduce friction and wear. Solid lubrication is often termed as dry lubrication. The main purpose of solid lubricants is to build up a continuous adherent soft or hard film in the rubbing surfaces. Most commonly used solid lubricants are talc, mica, tungsten sulphide, etc. Solid lubricants are mainly used as additives to oils and greases. Solid lubricants are used where:

(i) operating conditions are such that a lubricating film cannot be secured by use of lubricating oils or greases;

(ii) contamination (by the entry of dust or grit particles) of lubricating oil or grease is unacceptable,

(iii) the operating temperatures or load is too high even for a semi-solid lubricant to remain in position; and

(iv) combustible lubricants must be avoided.

Semi-solid Lubricants

Semi-solid lubricants are a gel-like substance which reduced friction between two moving surfaces. The petroleum products such as grease, Vaseline and waxes which are neither solid nor liquid at ordinary temperature are most important semi-solid lubricants. Grease is prepared from the mixture of petroleum, oil and soaps.

Liquid lubricants

It includes animal oils, vegetable oils, petroleum oils, synthetic lubricants.

Animal oils: tallow oil, whale oil etc.

Vegetable oils: castor oil, palm oil etc

Petroleum oils: petroleum fractions

Synthetic lubricants: polyglycol, silicones etc.

Application of Lubricants

·         Lubricants are primarily used to reduce the friction between two moving surface.

·         Rust and corrosion inhibitors

·         Used in the soap and paint industries.

·         Liquid lubricants are used in medicines

·         Lubricants are also used as cutting fluid in cutting, grinding, drilling of metals.

·         Used as anti-wear, antioxidants, and antifoaming agents.

Function of lubricants

·         Keep surfaces separate under all loads, temperatures and speeds, thus minimising friction and wear.

·         Act as a cooling fluid removing the heat produced by friction or from external sources.

·         Remain adequately stable in order to guarantee constant behaviour over the forecasted useful life.

·         Protect surfaces from the attack of aggressive products formed during operation.

·         Show cleaning capability and dirt holding capacity in order to remove residue and debris that may form during operation.

Mechanism of lubrication

Fluid film lubrication

 

 

 

 

 

 

It is also known as thick film or hydrodynamic lubrication. It is carried out with the help of liquid lubricants. In this mechanism, two moving and sliding surfaces are separated by thick film of lubricant fluid of about 1000A°, applied to prevent direct surface to surface contact and consequently reduce wearing and tearing of metals. Therefore, it is known as thick film or fluid film lubrication or hydrodynamic (hydro meaning liquid and dynamic meaning relative motion) lubrication. In this case fluid is formed by mixing of hydrocarbon oils and anti-oxidants with long chain polymer so as to maintain viscosity. Fluid film lubrication is useful in delicate and light machines like watches, clocks, guns, scientific equipments.

Boundary lubrication

 

 

 

 

 

 

 

 

 

It is also known as thin film lubrication.) Thin Film or Boundary Lubrication. It is carried out with semi-solid (grease) and solid (graphite and molybdenum disulphide) lubricants. Boundary lubrication is a condition in which the lubricant film becomes too thin to provide total separation. In this type of lubrication, a thin film of lubricant is adsorbed on the surface by weak Vander Waals forces. Thin film lubrication is operating at relatively low speed and heavy loading (pressure).

Extreme Pressure (or Temperature) Lubrication

In this mechanism, moving or sliding surfaces are under high pressure and speed, therefore this is known as extreme pressure lubrication. In such a case high temperatures generated due to friction, under these condition liquid lubricants are fail to stick and decompose or vaporize. These problems are minimized by special additives are added to mineral oils. These additives form durable films on metal surfaces which can withstand high loads and high temperatures. Important additives are organic compound having group like chloride, sulphur, phosphorus etc. They react with metallic surface to form metallic compound (possess high melting points and serve as good lubricants under extreme temperatures and pressures) like chlorides, sulphides, phosphate as more durable film.

Physical characteristics of lubricants

Lubricants have a wide range of properties that impact their physical and chemical properties.  Knowing about these properties is important in determining which lubricant is best for which situation.  While there are many properties, the most important are:

Viscosity: A lubricant’s internal resistance to flow. Higher viscosity lubricants are thick and don’t flow, while lower viscosity lubricants have a closer consistency to water and do flow.  The image below demonstrates the viscosity of four different oils.  The ball sinks faster in the thinner, low viscosity oil while it sinks slower in the higher viscosity blends.  

Viscosity Index: The rate of change in viscosity with changes in temperature.  In other words, how much viscosity changes as temperature changes.

Oxidation Stability: Oxidation is a reaction that occurs when oxygen is combined with lubricating oil. Variables such as high temperatures, water and acids will accelerate the rate of oxidation. The life of a lubricant is reduced as temperatures increase, leading to varnish and sludge.

Pour Point: The lowest temperature at which a lubricant will flow or pour like a liquid.  This can differ depending on test conditions.

Demulsibility: The ability of a lubricant to separate from water.

Oiliness: Oiliness of a lubricant is a measure of its capacity to stick on to the surface of machine parts, under conditions of heavy pressure or load.

Volatility: The volatility of a lubricant is the property that defines its evaporative loss characteristics. The more volatile a lubricant is, the lower the temperature at which smaller hydrocarbon molecules will be driven off or evaporated.

Flash and fire point: Flash point is the lowest temperature at which the vapor produced by continuous heating of lubricants can be ignited. Fire point is the lowest temperature at which not momentary ignition but continuous ignition is possible.

Cloud & 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.

Chemical characteristic of point

Acid value or neutralization number: It is an indication of the acidity or alkalinity of an oil; neutralization number is the number is the weight in milligrams of the amount of acid (hydrochloric acid) or base (potassium hydroxide) required to neutralize one gram of the oil.

Emulsification: It is the property of oils to get intimately mixed with water, forming a mixture, called emulsion.

Saponification value: The saponification value is defined as the number of milligrams of KOH needed to neutralize the fatty acids obtained by complete hydrolysis of 1gram of an oil sample. Fats or oils with high saponification value (such as coconut and palm oil) are more suitable for soap making.

Selection of lubricants

Lubricants reduce wear and tear  and energy loss due to friction. For various types of machines, depending on their service conditions, different lubricants are used. Some of them are discussed as follows:

Properties of lubricants: By knowing the properties of lubricant, suitable lubricant must be selected so that its properties should not change under service conditions.

Lubricants for delicate instruments: lubricants which work perfectly fine at lower temperature are preferred for delicate instruments. Delicate instruments are not exposed to higher temperature and hence such lubricants are suitable for delicate instruments.

Lubricants for gears: Gears are usually subjected to extreme pressure. Hence lubricants containing extreme pressure additives, good oiliness are used for lubricants.

Lubricants for cutting tools: Cutting tools experience high friction and generate large amount of heat. Hence, lubricants used for cutting tools should have cooling properties. Therefore, mineral oils like fatty acid and chlorinated compound are used as lubricants.

Lubrication for electrical transformers: Lubricants used in electrical transformer must possess good dielectric properties and have good heat transfer properties and should be of low viscosity. Lubricants used for transformer coil is often referred to as transformer oil.

Characteristic of transformer oil

Transformer oil is basically an insulating oil for transformer. It is processed from fractional distillation and treatment of crude petroleum. Transfer oil must possess following characteristics:

·         Excellent dielectric properties so that it results minimum power loss in transformer.

·         Good insulation across windings.

·         High thermal stability so that is reduces evaporation losses.

·         Transformer oil should have long life under severe electrical stress.

·         Transformer oil should provide protection against corrosion.

·         Transformer oil should have wider range of operating temperature and should have good cooling capability.