Lubrication: Definition, Role, and Types

What is Lubrication?

Lubrication is the process or technique of using a lubricant to reduce friction and wear and tear in contact between two surfaces. The study of lubrication is a discipline in the field of tribology.

Lubricants can be solids (such as molybdenum disulfide MoS2), solid/liquid dispersions (such as grease), liquids (such as oil or water), liquid-liquid dispersions, or gases.

Fluid-lubricated systems are designed so that the applied load is partially or completely carried by hydrodynamic or hydrostatic pressure, which reduces solid body interactions (and consequently friction and wear). Depending on the degree of surface separation, different lubrication regimes can be distinguished.

Adequate lubrication allows smooth, continuous operation of machine elements, reduces the rate of wear, and prevents excessive stresses or seizures at bearings. When lubrication breaks down, components can rub destructively against each other, causing heat, local welding, destructive damage, and failure.

Understanding The Lubrication

Lubrication is the control of friction and wears by the introduction of a friction-reducing film between moving surfaces in contact. The lubricant used can be a fluid, solid, or plastic substance.

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Although this is a valid definition, it fails to realize all that lubrication actually achieves.

Many different substances can be used to lubricate a surface. Oil and grease are the most common. Grease is composed of oil and a thickening agent to obtain its consistency, while the oil is what actually lubricates. Oils can be synthetic, vegetable, or mineral-based as well as a combination of these.

The application determines which oil, commonly referred to as the base oil, should be used. In extreme conditions, synthetic oils can be beneficial. Where the environment is of concern, vegetable base oils may be utilized.

Lubricants containing oil have additives that enhance, add or suppress properties within the base oil. The amount of additives depends on the type of oil and the application for which it will be used. For instance, engine oil might have a dispersant added.

A dispersant keeps insoluble matter conglomerated together to be removed by the filter upon the circulation. In environments that undergo extremes in temperature, from cold to hot, a viscosity index (VI) improver may be added. These additives are long organic molecules that stay bunched together in cold conditions and unravel in hotter environments.

This process changes the oil’s viscosity and allows it to flow better in cold conditions while still maintaining its high-temperature properties. The only problem with additives is that they can be depleted, and in order to restore them back to sufficient levels, generally the oil volume must be replaced.

The Role of a Lubricant

The primary functions of a lubricant are to:

  • Reduce friction
  • Prevent wear
  • Protect the equipment from corrosion
  • Control temperature (dissipate heat)
  • Control contamination (carry contaminants to a filter or sump)
  • Transmit power (hydraulics)
  • Provide a fluid seal

Sometimes the functions of reducing friction and preventing wear are used interchangeably. However, friction is the resistance to motion, and wear is the loss of material as a result of friction, contact fatigue, and corrosion. There is a significant difference. In fact, not all that causes friction (e.g., fluid friction) causes wear, and not all that causes wear (e.g., cavitational erosion) causes friction.

Reducing friction is a key objective of lubrication, but there are many other benefits of this process. Lubricating films can help prevent corrosion by protecting the surface from water and other corrosive substances. In addition, they play an important role in controlling contamination within systems.

The lubricant works as a conduit in which it transports contaminants to filters to be removed. These fluids also aid in temperature control by absorbing heat from surfaces and transferring it to a point of lower temperature where it can be dissipated.

Types of Lubrication

There are three different types of lubrication: boundary, mixed and full film. Each type is different, but they all rely on a lubricant and the additives within the oils to protect against wear.

  • Fluid Film Lubrication
  • Elastohydrodynamic lubrication
  • Boundary lubrication
  • Mixed lubrication

1. Fluid Film Lubrication

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 object (the lubricated conjunction), and solid-solid contact is avoided.

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

In hydrodynamic lubrication, the motion of the contacting surfaces, as well as the design of the bearing, pump lubricant around the bearing to maintain the lubricating film. This design of the bearing may wear when started, stopped, or reversed, as the lubricant film breaks down.

The basis of the hydrodynamic theory of lubrication is the Reynolds equation. The governing equations of the hydrodynamic theory of lubrication and some analytical solutions can be found in the reference.

2. Elastohydrodynamic lubrication

Mostly for non-conforming surfaces or higher load conditions, the bodies suffer elastic strains at the contact. Such strain creates a load-bearing area, which provides an almost parallel gap for the fluid to flow through.

Much as in hydrodynamic lubrication, the motion of the contacting bodies generates a flow-induced pressure, which acts as the bearing force over the contact area. In such high-pressure regimes, the viscosity of the fluid may rise considerably.

At full film elastohydrodynamic lubrication, the generated lubricant film completely separates the surfaces. Due to the strong coupling between lubricant hydrodynamic action and the elastic deformation in contacting solids, this regime of lubrication is an example of Fluid-structure interaction.

 The classical elastohydrodynamic theory considers Reynold’s equation and the elastic deflection equation to solve for the pressure and deformation in this lubrication regime. Contact between raised solid features, or asperities, can also occur, leading to a mixed-lubrication or boundary lubrication regime.

3. Boundary lubrication

The hydrodynamic effects are negligible. 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 tenacious 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.

4. Mixed lubrication

This regime is in between the full film elastohydrodynamic and boundary lubrication regimes. The generated lubricant film is not enough to separate the bodies completely, but hydrodynamic effects are considerable.

Uses of Lubrication

Lubrication is required for the correct operation of mechanical systems such as pistons, pumps, cams, bearings, turbines, gears, roller chains, cutting tools, etc. where without lubrication the pressure between the surfaces in close proximity would generate enough heat for rapid surface damage which in a coarsened condition may literally weld the surfaces together, causing seizure.

In some applications, such as piston engines, the film between the piston and the cylinder wall also seals the combustion chamber, preventing combustion gases from escaping into the crankcase.

If an engine required pressurized lubrication to, say, plain bearings, there would be an oil pump and an oil filter. On early engines (such as a Sab marine diesel), where pressurized feed was not required splash lubrication would suffice.