Lubrication Explained

In this article...

  • What is a lubricant?
  • What function does oil provide in my machine?
  • What gets lubricated in my machine and how?

What is a lubricant, or an oil or grease?

Oil in its simplest form is a Base Stock.  It can be either a mineral base stock derived from crude petroleum or a man-made synthetic base stock. 

Additives are then added to the oil to improve existing properties, suppress unwanted properties or add new properties to the oil.  

Grease is an oil and additive package held in suspension in a soap or thickening agent.

First, we need to understand how an oil lubricates the moving surfaces before we can start to look at the oil itself.


What function does oil provide in my machine?

Wear control - to minimise surface damage to components and thus ensure long service life before the parts wear out.

Friction control –

  • to reduce the power loss overcoming friction when surfaces move against each other

  • to reduce temperature build-up between surfaces and prevent ultimate seizure of the components

  • to reduce wear induced by friction.

Corrosion control - to minimise corrosion and rusting of surfaces from acids and moisture in the oil and the environment.

Temperature control - to remove heat from the working surfaces and components and transfer the heat away to a point such as a cooler or casing.

Contamination control –

  • to stop deposits forming on surfaces

  • to hold particles in suspension and allow them to be removed by the filter or by draining.

Power transmission - to support loads or transfer power from one point to another such as on the brakes.


What gets lubricated in my machine and how?

In the case of engines and transmissions in particular there are a number of components moving relative to each other; some rolling and some sliding against each other.  At some point the components will be stationary, then starting to move, and at some stage maybe under light loading or possibly under severe loading.  Added to which, the temperature can vary partly owing to running conditions (start-up, for example) or partly due to ambient and seasonal variations.

There are two main lubrication regimes that will protect the components, namely; Boundary lubrication and Thick Film lubrication.

  • Boundary, as the name implies, is where the two component's surfaces are in direct contact but lubrication is provided by the Friction Modifiers in the additive package (adpac) acting on the component surfaces to control friction and wear.

  • Thick Film, as the name implies, is where the two component surfaces are separated by a film of oil.  This separation is dependent on various factors,

    • one being the thickness of the oil, or its viscosity

    • speed of movement of the one surface relative to the other

    • the load acting on the oil film.

Thick Film lubrication can be further sub-divided into either separation of components sliding or rolling against each other, known as Hydrodynamic and Elasto-Hydrodynamic respectively.

In the diagram below (Fig. 1) the typical scenario of a machine during start-up shows what might happen where sliding contact occurs such as at the plain bearings of the crankshaft and camshaft. 

Figure 1 – Lubrication between two sliding surfaces

  • Boundary - During the starting phase the static friction is followed by boundary friction (high friction/high wear).  The friction and wear is controlled by additives. Under extreme load it may remain boundary lubrication.

  • Mixed Film - With increasing speed the sliding surfaces are partially separated by the lubricant, this phase is called mixed film lubrication (medium friction /medium wear).  In this phase the emergency lubrication film formed by the additives protects the sliding surfaces.  In some instances this may remain as a mixed film.

  • Hydrodynamic - At even higher speeds the sliding surfaces are separated from each other by a hydrodynamic fluid film, in this phase the lowest value of wear is achieved. In this phase the viscosity acts to separate the components. Additives may not be required depending on the nature of the machine.

  • Elasto-Hydrodynamic - With rolling contact such as between the elements of an anti-friction bearing, or cam rollers, or at the pitch line of a gear tooth, immense pressure is generated at the point of contact thanks to the small contact area.  This pressure is significant to cause the components to deform elastically and trap enough of an oil film.  The pressure acting on this oil film is immense, sufficient to turn it into a solid layer momentarily.  This solid layer is only 1 or 2µm thick but this is sufficient to ensure there is separation of the two surfaces.

Both forms of Thick Film lubrication are subject to:

Particle Contamination – results in Abrasive and Surface Fatigue damage in sliding and rolling applications respectively, and is most susceptible where the particle size is equal to or slightly greater than the film size or clearance.

Moisture – results in damage to the bearing surfaces as a result of loss of film strength, and possible hydrogen embrittlement damage in rolling contacts.

Viscosity selection – crucial to ensure sufficient film strength, but too high a viscosity selection will also result in increased fluid friction and thus drag and overheating, leading to wasted power. This is one of the reasons for modern engines using lower viscosity oils. The same is true when the viscosity is too low, mixed film and boundary contact can occur resulting in increased friction and wear, as well as overheating which will exacerbate the problem. 

Correct Additive Package – the correct range of friction-modifiers are necessary to cope with all load types at all temperature ranges. 

Start-up/Shut-down – more than 70% of wear takes place during start-up owing to insufficient speed to achieve complete separation, hence the correct additive package is essential to protect during this critical period of starting and warming up.