Lubrication is the process, or technique employed 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. Lubricants perform the following key functions:

•  Keep moving parts apart: Lubricants are typically used to separate moving parts in a system. This has the benefit of reducing friction and surface fatigue, together with reduced heat generation, operating noise and vibrations.

• Reduce friction: Typically the lubricant-to-surface friction is much less than surface-to-surface friction in a system without any lubrication. Thus use of a lubricant reduces the overall system friction.

• Transfer heat: Typically the liquid lubricant is constantly circulated to and from a cooler part of the system, although lubricants may be used to warm as well as to cool when a regulated temperature is required.

• Carry away contaminants & debris: Lubricants for machines that regularly generate debris or contaminants such as automotive engines typically contain detergent and dispersant additives to assist in debris and contaminant transport to the filter and removal.

• Protect against wear: Lubricants prevent wear by keeping the moving parts apart. Lubricants may also contain anti-wear or extreme pressure additives to boost their performance against wear and fatigue.

• Prevent corrosion: Good quality lubricants are typically formulated with additives that form chemical bonds with surfaces, or exclude moisture, to prevent corrosion and rust, even when machine is not working.

• Seal for gases: Lubricants will occupy the clearance between moving parts through the capillary force, thus sealing the clearance. This effect can be used to seal pistons and shafts.

Fluid lubrication – Boundary lubrication

As the load increases on the contacting surfaces three distinct situations can be observed with respect to the mode of lubrication, which are called regimes of lubrication:

1. Fluid film lubrication is the lubrication regime in which through viscous forces the load is fully supported by the lubricant within the space or gap between the parts in motion relative to one another (the lubricated conjunction) and solid–solid contact is avoided.

Hydrostatic lubrication is when an external pressure is applied to the lubricant in the bearing, to maintain the fluid lubricant film where it would otherwise be squeezed out.

Hydrodynamic lubrication is where the motion of the contacting surfaces, and the exact design of the bearing is used to pump lubricant around the bearing to maintain the lubricating film. This design of bearing may wear when started, stopped or reversed, as the lubricant film breaks down.

2. Elastohydrodynamic lubrication: The opposing surfaces are separated, but there occurs some interaction between the raised solid features called asperities, and there is an elastic deformation on the contacting surface enlarging the load-bearing area whereby the viscous resistance of the lubricant becomes capable of supporting the load.

Boundary lubrication (also called boundary film lubrication): The bodies come into closer contact at their asperities; the heat developed by the local pressures causes a condition which is called stick-slip and some asperities break off. At the elevated temperature and pressure conditions chemically reactive constituents of the lubricant react with the contact surface forming a highly resistant layer, or film on the moving solid surfaces (boundary film) which is capable of supporting the load and major wear or breakdown is avoided. Boundary lubrication is also defined as that regime in which the load is carried by the surface asperities rather than by the lubricant.


Types of Lubricants

Lubricants as per their physical properties are divided in three basic categories:

1. Liquid lubricants {mineral oils, vegetable-based oils, animal oils (lanolin)
2. Solid lubricants (talc, graphite)
3. Semi-solids or consistent oils (grease)

If anyone excludes animal or vegetable-based oils, which have no application in industry, there are:

1. Mineral oils, usually made from crude petroleum and cover the greatest amount of lubricants. They present great chemical stability due to their molecular structure (hydrocarbons) which by nature have great chemical stability.

2. Synthetic oils (product of chemical reaction and not distillation). They are better than mineral oils qualitatively since there is the capability to specify the desired properties during manufacturing process. Their use is growing fast despite their high cost.

The above are used, as raw materials, in production of lubricants (finished products), which have a lot of applications. According to their applications, they can be divided into following groups:

1. Lubricants for general use: Lubricants usually without chemical additives which find use in easy applications of lubrication.
2. Lubricants for four-stroke internal combustion engines: Mono-grade and multi-grade lubricants for gasoline engines and diesel engines.
3. Lubricants for two-stroke engines: Mixing with gasoline for lubrication of two-stroke engines.
4.  Gear oils: Single grade and multi-grade lubricants for lubrication of gearboxes and differential systems.
5.  Industrial lubricants: For industrial use in hydraulic systems, air compressors, cooling systems, turbines, rotors, transformers e.t.c.
6. Marine: For diesel engines according the sulfur grade of used fuel and lubrication of other parts of engine.
7. Greases.

Characteristics of lubricants

A good lubricant possesses the following characteristics:

• High viscosity index

• Corrosion prevention

• Satisfying adhesion

• Thermal stability

• Chemical stability

The viscosity of a lubricant is simply its property to resist to friction. But the choice of a lubricant for a specific engine is based not only on the right viscosity.

Along with the right viscosity, the criterion to choose the lubricant is the stability of viscosity in the temperature variations during normal engine operation. Having a lubricant with the right viscosity during startup of the engine is not enough. When the engine is heated, after a certain operation time, the viscosity of lubricant should remain high enough so that the lubricant film does not break out. When the viscosity is higher than normal, start up of the engine is getting harder and it causes loss of power and reduction of degree of effectiveness. On the other hand, low viscosity doesn’t always maintain the lubricant film intact within contacting surfaces. The viscosity thermal stability is ensured by Viscosity Index (V.I.). The value of viscosity index is corresponding to stability of viscosity as the temperature changes. A high value of viscosity index indicates a reduced viscosity variation according to temperature variation.

The viscosity of a motor oil is probably the most important characteristic since it is related to its fluidity. SAE (Society of Automotive Engineers) ratings designate the oil’s viscosity; simply put, how “thick” or “thin” an oil is at a certain temperature and after the United States of America, many other countries have incorporated this classification system. SAE ratings start from 0 which corresponds to a very “thin” oil, probably suitable for clockwork, and reach 250 which is a very “thick” oil, suitable for lubrication of pinions.

It is important to clarify that SAE number of a motor oil is not related to its quality. It only applies to the viscosity value and in some cases, indirectly, to the viscosity index. Since viscosity varies when temperature changes, there are mono-grade and multi-grade oils.

Mono-grade oils are these which have one SAE number and find use in rare special applications. So if we used mono-grade oil in winter or in a cold weather, it would probably not have adequate fluidity to protect the engine during startup and until it reaches its operation’s temperature.

For example, a SAE 40 indicates mono-grade oil which is probably suitable for lubrication of a diesel engine in the summer. In winter conditions though, it would hinder the engine to start, due to high viscosity. On the contrary, a SAE 20W indicates an oil that in cold weather preserves an adequately high viscosity, so as to start the engine.

For these reasons, multi-grade oils which are widely used, are characterized by two numbers SAE (e.g. 5W-40). In low temperatures it behaves like a “thin” oil (5W), while in high temperatures like a “thick” oil (40).

W comes from Winter and indicates that this oil satisfy the requirements of viscosity for use in low temperatures.

The number in front of W corresponds to viscosity in low temperatures when engine is cold. Lower the viscosity, thinner the oil at the startup of the engine. This ensures the quick and easy oil circulation inside the engine, so as to protect it from friction.

The number in the end corresponds to viscosity in operation conditions of the engine. The oil has to be thicker for the conservation of pressure and lubricity, as well as low consumption.

Advances in petrochemical engineering soon led to the development of chemical additives (viscosity enhancers) which when combined with a motor oil increased its viscosity index making it possible for a single grade oil to meet both the low temperature and the high temperature grade specifications. The molecules of these additives unwind as the temperature of the oil increases and slow down the rate at which the oil’s viscosity decreases. With their help, lubricants with SAE 0W up to 50W can be produced.

Lubricants with SAE 0W up to 20W are generally “thin”, suitable for low temperatures (engine’s startup). Lubricants with SAE 30 up to SAE 60 have medium and high viscosity, while those with SAE 70 up to 250 find use in lubrication of gearboxes and differential systems (gear oils).

Simply, a lubricant with SAE 0W/30 is quite “thin”, being able to help the startup of the engine in ambient temperature until -36oC and conserve and increase its viscosity (due to developing temperature and pressure inside the engine) up to grade SAE 30 which corresponds to a medium viscosity. Another lubricant with SAE 20W/50 is able to start the engine from -24οC and increase its viscosity up to grade SAE 50 during operation of the engine.

Based on above, International Organization for Standardization (ISO) in 1975 has established a new classification system which has been approved by most countries worldwide and tends to force out all the older systems.

According to this system which is covered by ISO 3448, viscosity is expressed by Centistokes units (cSt) at 40o C temperature which is representative of operation temperature in most applications. ISO classification defines 18 classes of fluidity from 2 to 1500 cSt at 40o C. Each class is characterized and numbered from viscosity of half value of its limits with deviation ±10% from the above value. E.g. ISO 10 is a product with viscosity at 40o C from 9 to 11 cSt, which is represented by half value 10 cSt.

The advantages of ISO classification are:

a) It includes the minimum number of classes (18) which covers the lubrication requirements of all the categories of engines and machinery.

b) It eases engineers and oil suppliers to define the proper lubricant for each case.

 SAE number  –  Range of use in ambient temperature


Comparative Viscosity Classifications

AGMA: American Gear Manufacturers Association

Selection of engine oil

Selection of engine oil should be done according to use, its viscosity and specifications given by manufacturer. Every oil is designed for a specific use and this should be taken into account.

After defining the category that we have to search the suitable oil for the selected application, best thing to do is to study the specifications given by manufacturer, so that the oil should have the same or most recent specifications than the indicative ones. In most of the cases, recent specifications cover older ones as per lubrication demands. One service classifications is API (American Petroleum Institute). This is a two-letter rating beginning with “S” (Spark) for petrol engine oils, “C” (Compression) for diesel engine oils and “GL” (Gear Lubricant) for gear oils. The second letter designates the oil’s quality standard, beginning with the letter “A”. The further along the alphabet, the higher the oil’s quality. Many oils meet standards for both petrol and diesel engines and will be marked with a dual service classification, for example SH/CD, however this is not universal and it is becoming more common for oils to be specified for only one type of engine. As technology of engines and machinery evolves, the above classification grows with additions of specifications’ grades.

Demands, numerous tests and studies have created the need for creation of more oil quality control organizations like ACEA (Association des Constructeurs Européens d’Automobiles or the Association of European Automobile Manufacturers). Moreover some manufacturing companies demand certain characteristics and in many cases add their own quality specifications (e.g. VW, Mercedes, e.t.c.). The qualitative indication of oil manufacturing based on technology is a necessary clue for its identification (e.g. API, SJ, CG-4, e.t.c.).

Viscosity is another factor to be taken into consideration. Attention should be given into manufacturer recommendations related to status of machinery in climatic and environmental conditions. There is a case that manufacturer has studied the operation of the machinery in different conditions than the ones supposed to operate. In that case manufacturer’s demands have to be reconsidered.


Synthesis of engine oil

Motor oil has been created to protect the motor from wear due to friction of metal surfaces during operation. However, the gradual combustion of small oil quantities causes carbon deposits on vital engine parts, which leads to malfunction of the engine in the long term. Moreover, when the oil is blended, foam is being formed, leading to air suction by oil pump. Consequently the oil circulation is inadequate and the lubrication of the engine is reduced. If this is happening regularly, then lifetime of contacting surfaces is reduced due to inadequate lubrication.

Chemical additives are necessary for the avoidance of negative properties like foaming or partial oil combustion, as well as decreasing wear of the parts of the gears exposed to very high pressures. Some of the main categories of chemical additives are:

Pour point depressants: Improve the oil’s ability to flow at lower temperatures.

Viscosity index improvers: Make an oil’s viscosity higher at elevated temperatures, improving its viscosity index (VI). This combats the tendency of the oil to become thin at high temperature. The advantage of using less viscous oil with a VI improver is that it will have improved low temperature fluidity as well as being viscous enough to lubricate at operating temperature.

Metal deactivators: Create a film on metal surfaces to prevent the metal from causing the oil to be oxidized.

Antioxidant additives: Retard the degradation of the stock oil by oxidation.

Corrosion inhibitors: Retard the oxidation of metal inside an engine.

Friction reducers: Are used for increasing fuel economy by reducing friction between moving parts.

Anti-wear additives: Cause a film to surround metal parts, helping to keep them separated.

Extreme pressure agents: Bond to metal surfaces, keeping them from touching even at high pressure.

Dispersants: Keep contaminants (e.g. soot) suspended in the oil to prevent them from coagulating.

Anti-foam agents: Inhibit the production of air bubbles and foam in the oil which can cause a loss of lubrication, pitting, and corrosion where entrained air and combustion gases contact metal surfaces.