There are certain things in life that improve with age, but unlike wine, lubricants are not designed to get better with age. They rarely fail suddenly; a lubricant quietly ages, reacts and is slowly consumed by the environment in which it operates. By the time a problem becomes obvious, the oil has usually been struggling for some time.
A lubricant doesn’t become ‘bad’ simply because it has darkened, developed an odour or been in service for a period of time. These changes are often early symptoms of degradation, but they do not on their own define failure. A good oil goes bad when it can no longer perform the functions it was designed to perform.
What does a lubricant actually do?
A fully formulated lubricant is far more than just oil. It is a carefully balanced blend of base oils and additives designed to perform several critical functions simultaneously. While the specific functions vary by application, they generally fall into five fundamental groups.
• Reduce friction and wear: By maintaining a protective film, the oil limits metal-to-metal contact and helps prevent mechanical damage.
• Dissipate heat: As machines operate, friction, load and combustion generate heat that must be removed from critical components. The lubricant acts as a heat transfer medium, carrying thermal energy away from high-temperature zones.
• Remove and suspend deposits: Contaminants like wear debris, soot and oxidation by-products must be kept in suspension and carried away from sensitive surfaces so they can be removed by filtration or oil changes.
• Protect against corrosion: Additives are included to neutralise acids, inhibit rust and prevent chemically aggressive species from attacking machine components.
• Act as a structural material: Viscosity, film strength and load-carrying capacity determine whether the oil can physically separate surfaces under operating conditions. If the oil film collapses, the lubricant can no longer do its job. As long as a lubricant can perform these functions, it is still good. When one or more is compromised, the oil may still be present in the system, but it is no longer fit for purpose.
From degradation to lubrication failure
Lubricant degradation is often the first step on the path toward lubrication failure, but the two are not the same. Degradation refers to the gradual chemical and physical changes that occur in an oil during service. Additives are consumed, base oils react with oxygen, contaminants accumulate and operating stresses take their toll, yet initially, these changes may not significantly affect machine operation.
Lubrication failure occurs when degradation has progressed to the point where the oil can no longer protect the equipment. At that stage, wear rates increase, temperatures rise, deposits form, corrosion accelerates and component damage becomes likely. The critical distinction is that degradation begins long before failure becomes visible or irreversible. Understanding this gap is what allows corrective action to be taken in time. This is where oil analysis plays a vital role as a means of detecting lubricant degradation early while intervention is still possible.
Why do oils degrade at all?
Even the best lubricant operating in well-maintained equipment under favourable conditions has a finite life. Oils degrade because they are continuously exposed to stresses that slowly change their chemistry and performance. Heat accelerates chemical reactions while oxygen drives oxidation. Contaminants such as water, fuel and soot interfere with additive performance and promote further degradation, and mechanical stresses can break down polymeric additives like viscosity index improvers. Over time, the additives designed to protect the oil and the machine are consumed doing their job.
These processes do not occur in isolation. Multiple degradation mechanisms are usually active at the same time, interacting with and accelerating one another. This is why lubricant degradation can be difficult to interpret without understanding how and why these changes occur. By understanding how lubricants degrade, it becomes easier to interpret oil analysis results, identify emerging risks and make informed maintenance decisions.
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