Lubricants
A Lubricant is any substance (solid, liquid, or semi-solid) introduced between two moving surfaces to reduce friction, heat, and wear. In engineering, lubrication is the primary method used to ensure the longevity and efficiency of machinery.
The process of reducing friction is known as Lubrication. From a molecular standpoint, the lubricant forms a thin film that prevents "asperities" (microscopic peaks on metal surfaces) from interlocking.
Functions of a Lubricant
- Friction Reduction: Converting solid friction into fluid friction.
- Wear Reduction: Preventing direct metal-to-metal contact.
- Cooling Action: Acting as a heat transfer medium to carry away heat generated by friction.
- Sealing: In internal combustion engines, lubricants help seal the gap between piston rings and cylinder walls.
- Corrosion Prevention: Protecting metal surfaces from oxidation and moisture.
- Cleaning: Carrying away dirt and wear particles to be caught in filters.
Classification of Lubricants
| Type | Examples | Best Used For |
|---|---|---|
| Liquid (Oils) | Mineral oils, Synthetic oils (Silicones), Vegetable oils. | High-speed machinery, IC engines, Gearboxes. |
| Semi-Solid (Greases) | Lithium-based grease, Calcium-based grease. | Heavy loads at low speeds, where oil won't stay in place. |
| Solid | Graphite, Molybdenum Disulphide (MoS2). | Extreme temperatures, high vacuum, or heavy loads. |
| Gaseous | Air, Nitrogen, Helium. | Ultra-high-speed bearings (e.g., dental drills). |
Critical Properties of Lubricants
A. Viscosity & Viscosity Index (V.I.)
Viscosity is the internal resistance to flow. It is the most important property. A lubricant's viscosity must be "just right"—too high and it causes drag; too low and the film breaks.
Viscosity Index: A measure of how much viscosity changes with temperature.
High V.I. = Minimal change in viscosity (Ideal for engines).
B. Flash Point & Fire Point
Flash Point: The lowest temperature at which the oil gives off enough vapor to ignite momentarily when a flame is applied.
Fire Point: The temperature at which the oil vapors burn continuously for at least 5 seconds. Usually 5–40°C higher than the flash point.
C. Cloud Point & Pour Point
Cloud Point: The temperature at which the oil becomes hazy or cloudy (wax starts to crystallize).
Pour Point: The lowest temperature at which the oil is still capable of flowing. Crucial for machines operating in cold climates.
D. Saponification Value
The number of milligrams of KOH required to saponify 1 gram of oil. It helps in identifying the presence of vegetable or animal fats in mineral oils.
Mechanisms of Lubrication
Hydrodynamic is derived from the two words, hydro and dynamic. Hydro meaning liquid and dynamic meaning relative motion. The regime of lubrication is determined by the Stribeck Curve, which relates friction to the viscosity, speed, and load of the system.
1. Hydrodynamic (Thick Film) Lubrication
In this regime, the moving surfaces are completely separated by a continuous film of liquid lubricant. The film thickness is typically 1000Å to 10,000Å. The pressure in the oil film supports the load.
- Requirement: High speed and low load.
- Application: Shafts in well-lubricated journal bearings.
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 wear and tear of metals (shown in figure). The lubricant film covers/fills the irregularities of moving/sliding surfaces and forms a thick layer between them, so that there is no direct contact between the material surfaces. This consequently reduces the friction. The lubricant chosen should have the minimum viscosity (to reduce the internal resistance between the particles of the lubricant) and should remain in place and separate the surfaces. Hydrocarbon oils (mineral oils which are lower molecular weight hydrocarbons with about 12 to 50 carbon atoms) are considered to be satisfactory lubricants for thick-film lubrication. In order to maintain the viscosity of the oil in all seasons of year, ordinary hydrocarbon lubricants are blended with selected long chain polymers. 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 equipment.
2. Boundary (Thin Film) Lubrication
This occurs when the oil film is so thin that the asperities (peaks) of the metal surfaces come into contact. Lubrication depends on the chemical adsorption of the lubricant molecules on the metal surface rather than the bulk viscosity.
- Requirement: Low speed, high load, or very low viscosity.
- Application: Starting or stopping of heavy machinery, piston rings at top-dead-center.
Boundary lubrication is a condition in which the lubricant film becomes too thin to provide total separation. Thin film lubrication is operating at relatively low speed and heavy loading or pressure (shown in figure). This type of lubrication is preferred where a continuous film of lubricant cannot persist when lubricant with lower viscosity is used. In such cases, the clearance space between the moving/sliding surfaces is lubricated by such a material which can get adsorbed on both the metallic surfaces by either physical or chemical forces or weak vander waals forces. This adsorbed film helps to keep the metal surfaces away from each other at least up to the height of the peaks present on the surface. Vegetable and animal oils and their soaps can be used in this type of lubrication because they can get either physically adsorbed or chemically react to the metal surface to form a thin film of metallic soap which can act as lubricant. Although these oils have good oiliness, but they will break down at high temperatures. On the other hand, mineral oils are thermally stable and by the addition of vegetable or animal oils to mineral oils, their oiliness can also be brought up. Graphite and molybdenum disulphide are also suitable for thin film lubrication.
3. Extreme Pressure (EP) Lubrication
Under very high loads and temperatures, even boundary films fail. EP additives (containing Chlorine, Phosphorus, or Sulfur) react with the metal surface to form a solid chemical film (like iron sulfide) that has a high melting point and low shear strength.
- Requirement: Shock loads and extreme heat.
- Application: Hypoid gears in automotive differentials, metal cutting tools.
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.
Download PDF Mechanism of Lubricants
Conclusion
Selecting the right lubricant requires balancing the operating temperature, load, and speed. While liquid oils are the "gold standard" for cooling, solids like Graphite are indispensable when liquid films fail under extreme pressure.