How can tribology help extend critical machinery service life?

Tribology is the science of interacting surfaces in relative motion. Basically, it studies friction, wear and lubrication – three phenomena linked to surface degradation and, thus, to machine reliability and performance. When machines wear out, they perform worse, are less productive and, finally, begin to fail as a result of mechanical damage.

Post written in cooperation with Jorge Alarcón

A major reference covering all major aspects of tribology is the book published by Emeritus Professor Ernest Rabinowicz, PhD in Physical Chemistry, who worked for several decades at the Massachusetts Institute of Technology doing research in this field. He studied the things that cause machines used in a variety of industries to lose usefulness. According to Rabinowicz, surface degradation was the cause of over 50 per cent of loss of usefulness, as shown in the graph below:

Therefore, controlling the conditions that affect tribological behaviour is of the utmost importance to protect critical equipment. Drawing on knowledge from the fields of materials technology, physics and chemistry, tribology is a truly multidisciplinary area. Its applications aim at reducing machine downtime while increasing productivity. Tribology can help extend the service life of critical machinery, as it provides the tools required to optimise friction and wear values.

Researchers around the world have shown the huge potential savings that can be achieved with improved tribological knowledge in a variety of industries. According to Theo Mang, Kirsten Bobzin and Thorsten Bartels, authors of Industrial Tribology: Tribosystems, Friction, Wear and Surface Engineering, Lubrication (2011), gross domestic savings resulting from proper use of tribological knowledge could amount to 1.5 per cent of the GDP. In the European Union, up to 303 billion dollars could be saved with better tribological practice.

How can tribology help prevent machine failure?

Each tribological system (that is, machine and lubricant taken as a whole) has its own features. Therefore, there is no single answer to this question.

To minimise machine failure, the first step is to choose the right lubricant. Depending on the industry, there are Original Equipment Manufacturer (OEM) classifications and specific recommendations that should be followed at all times. When this information is not available, experts in lubricants can give advice on technical specifications such as viscosity, base oils, additives, and so on.

Another key to keeping machinery at optimal performance involves monitoring fluid condition and checking start-up operation. Users must be fully aware of OEM fluid cleanliness recommendations before starting a machine. Since new oils can come with high contamination levels, filtering them before starting up the machine can be an effective measure.

When partial oil changes are performed, it is important to prevent undesirable oil blends that can facilitate reactions that change the properties of the lubricating fluid or the condition of machine surfaces. A lubricating oil with inadequate viscosity values, for example, will not function as expected throughout its lifespan. The fluid film that separates the metal surfaces of tools and machines will be too thin, allowing for contact between them and thus leading to wear and, eventually, failure. In addition, potentially incompatible chemicals in the different lubricants may come into contact, in which case a complete system flush is required to drain the full volume of used oil before refilling the system with new oil.

Three basic parameters should be assessed in order to ensure good tribological behaviour: lubricant degradation, tribological system contamination and machine wear.

Monitoring all the aforementioned elements of oil lubricated equipment is one of the best tribological practices. The criticality and type of the machines being assessed will determine whether regular offline analysis can be carried out or innovative online monitoring technologies should be applied.

Online sensors can also be used for measuring critical characteristics of lubricating oils and to provide indicators of the condition of the machines the oils are used in.

How can lubricating oil service life be maximised in critical equipment?

Lubricating oil service life is affected by a number of factors. You can extend it, or at least prevent rapid degradation, by using it properly. The following best practices are recommended:

Choosing the right lubricant

ISO grades and viscosity are the most important parameters, since they are the primary indicators for the ability of fluids to keep contact areas separated. It is necessary to choose the right additive package as well, since additives cover or enhance a range of properties of the base stock, such as inhibiting the production of air bubbles or foam, preventing the metal from causing oil oxidation, or keeping metal surfaces from touching even at high pressure inside gears.

Maintaining the machine and the lubricating system in optimal working conditions

The machine-lubricant system needs to work within a specific range of parameters; otherwise, the stress on the system increases, which leads to higher temperatures and rapid degradation of the lubricating oil. Particle production and air entering the lubricating fluid might increase too, affecting the degradation process. Using the proper filters and anti-foam agents can help reduce the impact of external factors like metal particles or water, thus maximising the useful life of the lubricating oil.

Monitoring the lubricating oil can help detect critical degradation values, so that the user can take measures to extend the fluid’s lifetime and enhance the reliability of the critical machinery and the processes involved.

What measures are to be taken after monitoring and analysing the tribological condition of critical machinery?

Optimising tribological conditions prevents premature machinery wear and reduces friction, thus increasing energy efficiency and ensuring a longer service life for your machinery.

Controlling the tribological conditions in critical equipment provides useful information on probable causes for failure. Assessing the tribosystem allows for the identification of factors (surfaces, working conditions, lubricating fluids) that can be controlled by taking corrective and preventive maintenance measures. For instance, identifying the types of wear can help determine whether machine failure is the result of lubricant condition or surface damage. With this information in hand, maintenance engineers and technicians can make better-informed decisions for greater machine efficiency and optimised costs.

Likewise, corrosion as the cause of wear is associated with inadequate oil condition, while wear resulting from mechanical abrasion will lead to control of the tribological condition of the contacting surfaces.