Consider Lubricant Temperature

Temperature is critical to the performance and life expectancy of lubricants and the components they protect.

In the lubrication world, temperature presents an interesting paradox and irony.

The paradox is that lubricants require heat to flow efficiently over and around surfaces, most commonly bearings. However, if the temperature gets too hot (exceeds 210 F), lubricants tend to undergo chemical changes that can drastically reduce life expectancy. Conversely, if the temperature becomes too cold, a lubricant will thicken and lose its ability to lubricate bearing surfaces. The irony is that lubricating oil is designed not merely to separate and lubricate a bearing surface, it’s also designed to absorb and carry away frictional heat from the bearing surface.

While the old adage “oil is oil and grease is grease” may have been true in agrarian societies of yesteryear, things have changed. To guarantee asset availability and reliability in today’s complex, high-speed industrial environments, lubricants must be tailored and managed to their machine-host’s specific needs and operating conditions.

The fundamental reason for lubrication is to provide a film that reduces friction between two, often metal, surfaces. If the film is insufficient, the surfaces collide and transfer energy, resulting in rapid heat buildup and metal expansion, which further retards motion until both surfaces eventually weld to one another. To avoid this worst-case scenario, the ideal intent is to guarantee the correct lubricant is available in sufficient quantity to consistently separate the moving surfaces. This ensures that temperatures stay below the magic 210 F operating temperature.

Lubricant choice

Choosing the correct lubricant for a bearing means selecting one that best matches the ambient and operating temperatures and conditions under which the component will function. This also means choosing a lubricant with the correct viscosity and additive package.

Viscosity—arguably a lubricant’s most important attribute—is a measure of its resistance to flow. A highly viscous oil is thick and resists flow. A low-viscosity oil is thin and flows easily. Again, temperature can have a dramatic impact on lubricant viscosity. A high-temperature condition, depending on the load in the bearing area, could easily collapse the film thickness of a low-viscosity product and create a metal-to-metal contact or “boundary layer” condition detrimental to the bearing and the lubricant.

Avoiding this situation means choosing a lubricant viscosity designed for the maximum operating temperature expected in the bearing area. This is achieved by paying particular attention to a lubricant’s viscosity index (VI). The VI measures the rate of viscosity change due to temperature. Better-quality lubricants have a more-desirable, narrow rate of viscosity change over a standard temperature range and allow good flow at low temperatures while maintaining their thickness at higher temperatures. Generally speaking, the higher the VI, the more stable and desirable the lubricant.

In cold-temperature conditions, hydrocarbon-based oil can thicken to the point at which it will no longer pour, largely due to its wax content. More expensive hydro-treated and synthetic-based oils will largely resolve this problem, or the user can heat the oil reservoir to a temperature that will allow it to flow again.

Heat-related lubricant failure

In the late 19th century, the Swedish Nobel Laureate Svante Arrhenius discovered a direct relationship between temperature change and the chemical-reaction rate in fluids that he put into an equation known as the Arrhenius rule. As summed up in the following statement, this rule is used in the lubrication field to express the temperature-change-dependent failure rate of oils: “For every 18-deg. F (10-deg. C) increase in oil temperature, the lubricant’s life is reduced by half.” Conversely, reducing oil temperature by the same rate doubles the lubricant’s life (see Fig. 1).

Two predominant failure mechanisms occur as oil heats up. The most common is categorized as oxidation failure and the lesser categorized as thermal failure.

Oxidation failure occurs when oxygen reacts with the lubricant base oil. Anti-foaming and anti-oxidant additives, if present in the oil, are designed to slow the process. Once they are depleted, however, the rate of oxidation will accelerate, especially in the presence of water and reactive bearing materials such as copper and iron.

In an oxidized state, hydrocarbon molecules in the oil will transform into a greasy sludge containing harmful, corrosive acids. These will cause the oil to degrade and lose its lubricating properties, effects that are manifested by an increase in lubricant viscosity, specific gravity, acidity (TAN), rapid additive depletion, darkening of the oil, a “rotten egg” odor, and varnishing of the bearing surfaces.

Thermal failure can occur when localized/external heat is transferred to the lubricant or through the adiabatic compression (a thermodynamic process that occurs when entrained air compresses and heats up according to Boyle’s law) of entrained air bubbles in pumps, bearings, and pressurized hydraulic and lubricating systems. The resulting heat causes the lubricant to decompose and its corresponding hydrogen loss to create carbon-rich particles in the form of sludge and carbon deposits. This leads to a decrease in lubricant viscosity indicated by dark fluid with greasy suspensions that smell of burned food, and evidence of coking and varnishing on the bearing surfaces.

Once a lubricant has failed, the molecular-change state is usually irreversible. At the very least, this situation calls for a lubricant change.

Oil-analysis programs monitor lubricant conditions and alert users to possible failures before they occur. The propensity of a lubricant to fail can be checked by subjecting a sample of it to a rotary pressure vessel oxidation test (RPVOT). By simulating failure through a speeded-up oxidation process, this test can provide a good indication of a lubricant’s suitability. It also can predict remaining life in virgin and used oil samples.

Prevent temperature-related failures

Whether your lubricant choice is grease or oil, once the correct product is chosen and employed it will require assistance from the maintainer to ensure that it has a fighting chance to perform and deliver a reasonable life expectancy. This can be achieved by implementing some of the following best practices:

• Keep lubricant transfer-delivery systems immaculately clean. This prevents the ingress of solid contamination that can create sludge, raise lubricant viscosity, and accelerate the oxidation process.
• Ensure the oil/grease delivery method/system is tuned to provide the correct amount of lubrication in a timely manner. Over-lubrication will create fluid-friction heat, compounded by the bearing ball/rollers working overtime to mechanically push through the excess lubricant. Both conditions cause the lubricant to heat up rapidly. Under-lubrication can allow the bearing to go into boundary lubrication, creating surface interaction frictional heat that can “cook” the lubricant.
• Maintain the oil reservoir level between the minimum (“MIN”) and maximum (“MAX”) fluid level to prevent cavitation in the oil pump or oil churn in the reservoir. Either can result in air bubbles accelerating the oxidation process.
• Keep oil reservoirs clean and free of debris. Dirt, dust, and debris can create the effect of a thermal blanket and raise the temperature of the oil inside the reservoir.
• Ensure the integrity of shaft seals. Poor shaft seals lead to excessive lubricant leakage that can quickly result in an under-lubrication state.
• Implement a lubricant test-and-control process to ensure that incompatible lubricants are not mixed together in the same bearing space, something that can lead to a variety of detrimental conditions, including overheating.
• Where hydrocarbon-base lubricants are employed in cold-weather climates, use timed block heaters or blanket-wrap element heaters for reservoir and drum/pail heating. If a lubricant is to protect a bearing surface it must readily flow across the bearing.

To learn more, see:

• “Lubricant Life-Cycle Management”
• “Winter Words: Lubrication Advice for Ensuring Desired Levels of Plant-Equipment Performance Year-Round”
• “The Lowdown on Lubricants for Roller Bearings”
• “Certification Matters: Review of Bearing Principles”