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
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.
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.
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.
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.