Lube-Tech
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.116 page 1
The difference between ‘forgiving’ and ‘less-forgiving’ lubricants in thin film tests
Svajus Joseph Asadauskas FTMC Tribology Laboratory, Center for Physical Sciences and Technology, Vilnius, Lithuania
Introduction
Many users use a term ‘Forgiving Oil’ as a praise to the lubricant, which withstands past-due oil changes, heavy loads, contamination, overheating or many other issues without major problems to the lubricated equipment. Such abuse might lead to increased wear, some corrosion and other fixable issues. But if oil is “forgiving” it would still prevent the system from catastrophic damages, such as stalling, fractured gears or broken shafts. Although “Forgiving Oil” is not a scientific term, there are some fundamental reasons it exists as a concept. Such oil is able to retain a liquid film between moving surfaces under severe degradation. The degraded film might not assure low wear anymore, but it would still have enough fluidity to prevent metal-to-metal and other types of seizures.
These oil properties cannot be easily seen when comparing samples side-by-side using just standard tests, because the results are often similar and they only show full compliance to the specifications. Longevity and durability of formulated lubricants are often equated to the oxidative stability, which is
30 LUBE MAGAZINE NO.145 JUNE 2018
typically measured by bubbling air through bulk oil at higher-than-average application temperatures. The increase in viscosity and acidity is considered critical with a number of other characteristics recorded as well: antioxidant activity, metal corrosion, colour change, etc. Often minor changes of these properties are viewed as decisive, disregarding the fact that many lubricants still perform quite well despite discoloration, increased viscosity or even acidity. Such temptation becomes even stronger at long testing durations, which sometimes exceed 1000 hours in degradation experiments.
One way to assess oil stability is to test its film rather than bulk volume. In almost all instances, lubricants form thin films on reservoir walls or equipment itself. This exposes oil molecules to air, moisture, hot metal surfaces, combustion gases and other aggressive factors [1], so lubricants degrade many times faster in films than in bulk volume. In engine oils and many other applications the importance of degradation in films is much more critical than that of degradation in bulk oil.
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