LUBRICATION

 Definition and Objects:

Lubrication is the admittance of oil between two surfaces having relative motion. The objects of lubrication may be one or more of the following :

1. To reduce friction between the parts having relative motion.
2. To reduce wear of the moving part.
3. To cool the surfaces by carrying away heat generated due to friction.
4. To seal a space adjoining the surfaces.
5. To absorb shocks between bearings and other parts and consequently reduce noise.
6. To remove dirt and grit that might have crept between the rubbing parts.

 

Behaviour of a Journal in its Bearing:

 shows the behaviour of a journal rotating in a bearing, the clearance between the two being shown very much exaggerated. The clearance space is supposed to be completely filled with oil at all times, which is possible by supplying the oil as fast as it runs out. If the shaft does not rotate it will sink to the bottom of the clearance space due to the load W, and the journal and bearing will touch The way in which the rotating shaft will build up pressure in the oil sufficient to separate the surfaces  From above, the following points, about the items influencing bearing performance, can be concluded :

1. A slippery bearing material is desirable during starting and stopping, but once the oil film is established the bearing material is less important.
2. Higher is the speed of journal, more will be the oil pulled into the apex of the wedge of oil in the clearance space of Fig. 14.6 (c), and as a result, more supporting pressure will be developed.
3. An increase in supporting pressure will increase the oil film thickness ro  and decrease the eccentricity e.
4. If the eccentricity ‘e’ is decreased, the supporting pressure must decrease because the sides of the wedge are more nearly parallel.
5. Oil film thickness ro is also influenced by changing of clearance.

Film Lubrication.

It is that type of lubrication in which bearing surfaces are completely separated by a layer of film of lubricant and that the frictional resistance arises only due to relative movements of the lubricant layers.

Boundary Lubrication :

1. Under the hydro-dynamic condition the oil film supports the load. If the oil film becomes thin enough so as not to support the load without occasional metal to metal contact then journal friction developed is called boundary friction and the lubrication existing in this range is known as boundary lubrication. The word boundary friction is used because under this condition journal friction is neither completely dry and not completely fluid. In the boundary state, the kind of bearing material, the hardness and surface finish of the shaft, as well as type of lubricant all contribute to the amount of journal friction.
2. When the load acting on the bearings is very high, the material itself deforms elastically against the pressure built up of the oil film. This type of lubrication, called elastohydrodynamic lubrication, occurs between cams and followers, gear teeth, and rolling bearings when the contact pressures are extremely high.

Properties of Lubricants:

The chief qualities to be considered in selecting oil for lubrication are :

1. Viscosity
2. Flash point
3. Fire point
4. Cloud point
5. Pour point
6. Oiliness
7. Corrosion
8. Emulsification
9. Physical stability
10. Chemical stability
11. Neutralisation number
12. Adhesivenes
13. Film strength
14. Specific gravity.

1. Viscosity

It is the ability of the oil to resist internal deformation due to mechanical stresses and hence it is a measure of the ability of the oil film to carry a load. A more viscous oil can carry a greater load, but it will offer greater friction to sliding movement of the one bearing surface over the other. Viscosity varies with the temperature and hence if a surface to be lubricated is normally at high temperature it should be supplied with oil of a higher viscosity.

The viscosity is measured by viscosimeter. The important types of viscosimeters are :
1. Saybolt universal viscosimeter
2. Red wood viscosimeter
3. Engler viscosimeter
4. Barbey viscosimeter.

The contemporary approach to expressing the rate at which the viscosity of an oil will alter with temperature is by specifying its viscosity index (V.I.). The oil is compared with two reference oils having the same viscosity at 99°C. One is a paraffinic base oil (viscosity changes considerably with temperature), arbitrarily assigned an index of zero, and the other, a naphthenic base oil (little change in viscosity with temperature), assigned an index of 100.
— A high viscosity index indicates relatively smaller alterations in viscosity of the oil with temperature.
— The higher the viscosity index, the lower the rate at which its viscosity decreases with temperature increase. Though a high viscosity index is preferable in materials and much effort is made to enhance the viscosity index of oils by incorporating certain compounds, known as V.I. improvers. These are long-chain paraffinic compounds that enable the attainment of an oil with easy starting characteristics of thin oils combined with good protection against high temperature.

2. Flash point.

It is defined as the lowest temperature at which the lubricating oil will flash when a small flame is passed across its surface. The flash point of the oil should be sufficiently high to avoid flashing of oil vapours at the temperatures occurring in common use. High flash point oils are required in air compressors.

3. Fire point.

It is the lowest temperature at which the oil burns continuously. The fire point also must be high in a lubricating oil to prevent oil burning in service.

4. Cloud point.

When subjected to low temperatures, the oil changes from a liquid state to a plastic or solid state. In some cases, the oil starts solidifying which makes it appear cloudy. The temperature at which this occurs is called the cloud point.

5. Pour point.

Pour point is the lowest temperature at which the lubricating oil will pour. It is an indication of its ability to flow at low temperatures. This property must be considered because of its effect on engine starting in cold weather and on free circulation of oil through exterior feed pipes when pressure is not applied.

6. Oiliness.

This is the property that enables oil to spread over and adhere to the surface of the bearing. It is crucial in boundary lubrication.

7. Corrosion.

A lubricant should not corrode the working parts and must retain its properties even in the presence of foreign matter and additives.

8. Emulsification.

A lubricating oil, when mixed with water, is emulsified and loses its lubricating property. The emulsification number is an indicator of the tendency of an oil to emulsify with water.

9. Physical stability.

A lubricating oil must be physically stable at the lowest and highest temperatures at which the oil is to be used. At the lowest temperature, there should be no separation of solids, and at the highest temperature, it should not vaporize beyond a certain limit.

10. Chemical stability

A lubricating oil should also be chemically stable. There should be no tendency for oxide formation.

11. Neutralization number.

An oil may contain certain impurities that are not removed during refining. The neutralization number test is a simple procedure to determine the acidity or alkalinity of an oil. It is the weight in milligrams of potassium hydroxide required to neutralize the acid content of one gram of oil.

12. Adhesiveness.

It is the property of lubricating oil that causes oil particles to adhere to the metal surfaces.

13. Film strength.

It is the property of a lubricating oil that allows the oil to retain a thin film between two surfaces even at high speed and load. The film does not break, and the two surfaces do not come into direct contact. Adhesiveness and film strength cause the lubricant to enter the metal pores and cling to the surfaces of the bearings and journals, keeping them lubricated when the journals are at rest and preventing metal-to-metal contact until the lubricant film is built up.

14. Specific gravity.

It is a measure of the density of oil. It indicates the grade of lubricant by comparing one lubricant with another. It is determined by a hydrometer that floats in the oil, and the gravity is read on the hydrometer scale at the surface of the oil.

Additional Components in Lubricants

To start, there are two main types of oils: Mineral and vegetable. Both serve as lubricants meeting basic requirements. While vegetable oil outperforms mineral oil in extreme conditions, it's rarely used due to costliness. Mineral oils are widely used for lubrication purposes.
Basic mineral oils possess most essential lubricating properties. However, certain operating conditions demand specific characteristics that they lack, such as high viscosity index and resistance to oxidation and corrosion. To address these needs, additives are incorporated.

1. Detergents:

Control high-temperature deposits like gums.

When overbased, detergents neutralize acids effectively.

2. Dispersants:

Control low-temperature deposits like cold sludge and varnish.

3. Anti-wear additives:

Reduce wear and prevent scoring, galling, and seizure.

Offer extra strength for efficient lubrication under severe conditions.

Common additives include chlorine and phosphate compounds.

4. Rust inhibitors:

Minimize rusting by neutralizing acid formation or forming a protective film.

5. Viscosity index (V.I.) improver:

Prevent excessive thinning of oil as temperature rises.

High viscosity index is preferred for lubricating oils, suitable for severe weather conditions. High-molecule polymers, soluble in oil, are utilized.

Multigrade oils, containing low-viscosity oils with thickening polymers, adapt to various weather conditions.

6. Pour point additives:

At very low temperatures, base oil solidifies due to wax crystal formation. Unchecked, wax crystals can obstruct oil flow. Pour point additives hinder wax crystallization, reducing oil pour point.

Polymerized phenols or esters, up to 1 percent of the oil, serve this purpose.

7. Anti-foam agents:

Minimize oil foaming by collapsing air entrainment bubbles.

8. Anti-oxidants:

Decrease oil oxidation to safeguard alloy bearings from corrosion.

9. Oiliness improvement:

Substances like colloidal graphite and zinc oxide enhance oil film maintenance when added."

Oil contamination and sludge formation:

1. The lubricating oil after a certain operating period gets contaminated to the extent that it becomes unsuitable for further use. The contamination of the oil takes place due to oxidation, dilution, water, formation of carbon, lead compounds, metals, dust and dirt When these contaminants mix with the oil, sludge is formed in an engine.
2.Sludge is a black, brown or grey deposit having the consistency of mud. Its formation takes place as a result of operation at low engine temperatures during starting, warming up, and idling periods.

 Types of Lubricants:

Lubricants vary in types, mainly oils or greases. Yet, in specific scenarios, fluids like water, air, and solids like graphite can also serve as lubricants. Synthetic lubricants are also employed in certain situations.

1. Oils:

  

 - Lubricating oils come in different types: mineral, vegetable, and synthetic.
  (i) Mineral lubricating oils are extracted from leftover petroleum during refining. This involves a mix of gasoline, kerosene oil, and gas oil. Paraffinic crude yields more lubricating oil than naphthenic crude. The remaining residue is vacuum distilled, sometimes with steam. Separating oil from various fractions requires adjusting proportions in three diverse densities.
 (ii) Animal and vegetable fat oils are sometimes used alone but are often mixed with mineral oils. They have lower stability and tend to oxidize. They also have higher viscosity compared to mineral oils. Examples include olive oil, castor oil, rapeseed oil, lard, and fish oil, primarily used for lubrication. They're typically mixed with mineral oils in ratios ranging from 2 to 10 percent. (iii) Synthetic lubricants, not directly from petroleum, offer specific properties tailored to various applications, like a lower pour point or better viscosity-temperature characteristics.

  (iv) Synthetic lubricants, categorized into different types:
- Di-basic acid esters
- Organo-phosphate esters
- Silicate esters
- Silicone polymers

SAE Ratings of Lubricants:

Selecting the right lubricant for engines depends on operating temperature and required service. The Society of Automotive Engineers (SAE) defined engine oil viscosity using the SAE J300 standard, which introduced two primary grades for engine applications.

1. Single-Grade:

   

- These oils lack polymer viscosity index improver additives. SAE J300 standard includes eleven viscosity grades, six cold grades denoted by a W symbol. Basic viscosity measured at various cold temperatures using mPa or centipoises (cP).
   - Viscosity measured at 100°C, classified into SAE viscosity grades W20, W30, W40, W50, or W60 based on viscosity spread.

2. Multi-Grade:

  

 - Vehicles face diverse temperatures, necessitating viscosity adjustment. Special polymer additives maintain viscosity across temperature ranges.       Multi-grade oils are designated with dual viscosity grades, such as 10W W30 or 20W W40, indicating flow capacity and resistance to film breakdown. These oils provide optimal lubrication under diverse operating conditions.
   - Dual viscosity grades indicate flow capacity and resistance to film breakdown, ensuring optimal lubrication under varied conditions.
The American Petroleum Institute (API) categorizes diesel engine lubricating oils into three categories: Diesel Good, Diesel Medium, and Diesel Severe Service. Gasoline engine lubricating oils receive five service ratings: SA, SB, SC, SD, and SE, each with specific properties and compositions. Synthetic oils, known for superior lubrication, offer options derived from organic acids and alcohols from various plants, meeting modern engine needs.

Grease:

1. Lubricant Grease  is a mix of thickening agent and liquid , often solid or semi-solid. It may contain other elements for specific purposes.
2. Greases are normally used under conditions of lubrication for which oil is not as suitable or convenient.
 Greases preform better than oils under conditions requiring :

1. High bearing loads and shock loads.
2. Slow journal speed.
3. Temperature extremes.
4. Cleanliness 
 
5. Minimum attention.
6. A seal against external contaminants.
7. Large bearing clearances.
8. Avoidance of splash or drip.

The various types of greases used for lubrication are :

1. Calcium soap greases 
2. Sodium soap greases 
3. Aluminium soap greases 
4. Mixed soap greases 
5. Barium soap greases 
6. Lithium soap greases 
7. Pure petroleum greases.