Within the triad of tribology, lubrication is the principal solution to the phenomenon of friction and to the problem of wear, although it is not the only option. There are a number of good tribological practices that make it possible to optimise the state of the surfaces and prevent wear of critical machinery.
The main tool for preventing the different wear processes is the correct application of the lubrication. The fluid lubrication applied in the majority of industrial elements is explained by using the Stribeck curve:
Coefficient of friction; Boundary lubrication, f; Elasto-hydrodynamic lubrication; Hydrodynamic lubrication
Thin unstable film; Thick, stable film
The lubrication depends basically on three parameters: viscosity of the lubricating fluid, relative velocity between the surfaces and the load that these support. The purpose of the lubrication is to be positioned in the system known as hydrodynamic lubrication, where the oil completely separates the surfaces and the phenomenon of wear are largely attenuated. Any variation from these conditions means that the surfaces are not completely separated and wear phenomena prevail.
Another important detail is the “chain effect” of the wear. Certain types of wear can influence the generation of others, for which reason it is important to monitor the conditions of generation of the different types of wear in order to mitigate the appearance of another different type.
Wear caused by fatigue: Monitoring materials, surface finishings
The principal causes of fatigue wear stem from a failure of the surfaces in contact. After a certain period of operation, the surface microstructure suffers microcracks that weaken its behaviour.
To minimise this type of wear it is important to optimise the state of these surfaces. The metallurgical and superficial finishing must be excellent, for which reason the surfaces must be painstakingly examined in times of stoppage, and if necessary an operation must be performed to re-establish the right state. The application of high hardness surface treatments such as Bessemer process or nitriding can improve the behaviour of these items against all wear.
Corrosive wear: State of the lubricating fluid
In the case of corrosive wear, the state of the lubricating fluid is one of the critical elements that can lead to this type of wear. Checking the acidity of the fluid and contamination with other fluids that can greatly vary the acidity of the lubricant are goals for preventing the corrosion of the surfaces. The accelerated oxidation of the lubricant generates acidity, as well as the presence of water and oxides, which act as catalysts.
Corrosive wear can also be caused by a localised attack that is due to certain additives in the lubricant, when the application for which it is being used is not appropriate. For this reason, it is important to adjust the use of the lubricant and check the addition of additives to prevent corrosion associated with these items.
Adhesive wear: Ensuring good tribological conditions (viscosity, Tª, velocity)
Adhesive wear is caused fundamentally by the contact between particular surfaces at high temperature, causing micro-welds and later scratching, for which reason ensuring the correct lubrication conditions at all times is the principal goal for decreasing this type of wear.
As the Stribeck curve shows, the importance lies in the keeping control of the three influencing variables: viscosity, velocity and load. The viscosity of the fluid is influenced largely by the temperature, for which reason minimising the operating temperature can help to minimise this wear.
A variation of the operating temperature after a maintenance stoppage may be associated with an error, such as a misalignment, for which reason extreme care must be taken in these operations.
The use of high velocities can help to improve lubrication, as can reduction of the load.
Abrasive wear: Controlling contamination
As well as defective lubrication conditions, contamination of particles in the fluid is the principal cause of abrasive wear, as the particles generated act as new foci of abrasive wear.
For that reason, ensuring the cleanliness of the fluid during its use is of vital importance. The use of filtering systems in recirculation systems is recommended, as well as checking the entry of external agents that could bring in contamination, such as dust in the ambient air. During maintenance operations, it is important to check the input of external contamination.
Detailed knowledge for taking better decisions
In order to ensure the application of the best tribological practices and prevent machinery wear there is an important detail that applies across all the different sectors and applications: knowledge of the machine and of tribology. The training of the responsible staff and a knowledge of the state of the machine are the true foundation for preventing tribological problems.
With the arrival of industry 4.0, the way is opening up to a new trend: the on-line monitoring of the state of the equipment and the digitisation of these data to generate complete, consistent information in order to take better decisions.