Types of Lubrication: Hydrodynamic, Boundary and Mixed

Lubrication reduces friction and wear between surfaces that move against each other. It also carries away heat, keeps out dirt and prevents corrosion. Tribology, the science of friction, wear and lubrication, groups lubricated contacts into regimes according to how thick the lubricant film is compared with the roughness of the surfaces. Knowing the regime explains why a bearing wears at start-up, why gears need extreme pressure additives and how to choose an oil grade.

The Stribeck curve

In the early twentieth century Richard Stribeck measured friction in bearings over a wide range of conditions. Plotting the coefficient of friction against the bearing parameter (viscosity × speed / load) gives the Stribeck curve, which shows all the regimes on one chart.

Download diagram (WebP)

At the left, low speed or high load means little or no film, and friction is high. Moving right, a film builds up and friction falls. At the minimum, the film just separates the surfaces. Further right, friction rises slowly again because shearing a thicker, more viscous film takes more energy.

Types of lubrication regimes

Hydrodynamic lubrication

A full film of lubricant completely separates the surfaces. The pressure that carries the load is generated by the motion itself: the moving surface drags oil into a converging wedge, as in a journal bearing where the shaft runs slightly off centre. The theory comes from Osborne Reynolds (1886). Friction coefficients are very low, of the order of 0.001 to 0.01, and wear is practically zero while the film is maintained. Film thickness is typically several micrometres or more.

Plain journal bearings in turbines, large motors, pumps and compressors run in this regime. Because the film depends on speed, it is lost at start and stop, which is why large machines often use hydrostatic lifting: oil is pumped in under pressure to lift the shaft before it turns.

Elastohydrodynamic lubrication (EHL)

In rolling element bearings, gear teeth and cams, the contact area is tiny and pressures reach thousands of megapascals. Two effects keep a film in place: the surfaces flatten elastically, and the oil’s viscosity rises enormously at such pressures. The film is very thin, typically well under a micrometre, so surface finish matters a great deal. EHL governs the life of rolling bearings and gear surfaces, and bearing life calculations include a factor for film adequacy.

Mixed lubrication

The film is about as thick as the surface roughness. Part of the load is carried by fluid pressure and part by contact between the highest peaks (asperities). Friction and wear are moderate. Many machine elements pass through this regime during start-up, at low speed or under heavy load.

Boundary lubrication

The film is too thin to separate the surfaces and asperities are in continuous contact. Protection comes from very thin layers formed on the metal by additives: friction modifiers, anti-wear additives (such as zinc dialkyldithiophosphate) and extreme pressure additives that react with the surface under high local temperature. Friction coefficients are higher, roughly 0.05 to 0.15, and some wear is unavoidable. Slow, heavily loaded slides, gear teeth under shock and engine cylinder walls near the end of the stroke see boundary conditions.

The film thickness ratio

A practical way to identify the regime is the film thickness ratio λ (lambda), the minimum film thickness divided by the composite roughness of the two surfaces:

$$\lambda = \frac{h_{\min}}{\sqrt{R_{q1}^2 + R_{q2}^2}}$$

Rq is the RMS surface roughness. Commonly used guide values are:

The boundaries are approximate and vary between references, but the trend is reliable: a thicker film or smoother surfaces move the contact towards full film lubrication.

Viscosity is the property that controls film thickness. Oils are graded by kinematic viscosity at 40 °C in the ISO VG system (ISO 3448). Convert between dynamic and kinematic viscosity with ν = μ / ρ:

Solid and gas lubrication

Not every lubricant is a liquid. Solid lubricants such as graphite, molybdenum disulphide and PTFE work where oils would evaporate, burn or contaminate, for example in very hot, vacuum or food environments. Gas bearings use a thin air or gas film in hydrodynamic or hydrostatic form and suit very high speed, light load spindles. Both follow the same regime ideas described above.

Choosing the right lubrication

• Know the regime for normal running and for start and stop.
• Choose viscosity to give an adequate film at operating temperature; too thin causes wear, too thick wastes power and generates heat (see modes of heat transfer for how it is removed).
• Use additives where boundary or mixed conditions are unavoidable.
• Keep it clean and dry: particles larger than the film thickness and water both destroy films.
• Grease suits rolling bearings that cannot be oil fed; its base oil does the lubricating.

Lubrication links directly to machine design: bearing life and factor of safety checks assume the bearing is properly lubricated. Pumps and compressors such as the centrifugal pump depend on it for bearing and seal life.

Related reading

• Stress and Strain: Definitions, Types and Hooke’s Law
• Vapour Compression Refrigeration Cycle Explained