GREASES
Discover critical information lurking in grease
Emily Featherston, Product Application Supervisor and Dave Tingey, Product Application Specialist, POLARIS Laboratories®
Grease overview
When discussing the topic of equipment reliability, grease is more an “out of sight out of mind” lubricant. Grease is applied in applications where normal oil-lubricated components are impractical due to accessibility, safety, design or where there are environmental concerns. Due to these conditions, grease is often put onto a scheduled lubrication routine and the idea of maintaining these components is simply replenishing the grease. Replenishing occurs when there is an existing assumption that a wear condition cannot be remedied quickly just by adding more grease. Run to failure is the maintenance process for most greased components; however, new methods for testing in-service grease are available to provide more insight into grease maintenance recommendations.
Grease makeup is roughly 70% - 95% base oil, 3% - 30% thickener and 0% - 10% additives to make the final grease product. The base oil can and will oxidise due to age and/or catalyst reaction caused by contamination of metal wear, water or heat degrading the base oil properties. The base oil breakdown can occur from over-greasing (also called churning).
Grease consistency is dependent on the type and amount of thickener used as well as the viscosity of the base oil. Generally, grease consistency is determined by following the grades established by the National Lubricating Grease Institute (NLGI) of 000 to 6. The larger the grade, the thicker the grease.
However, two greases with the same NLGI grade does not indicate the grease has the same base oil viscosity. High-speed rotating equipment will often use a lower viscosity base oil, whereas a slow speed/
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high load equipment will require a higher viscosity base oil to provide the correct fluid film needed for proper lubrication. Age, oxidation and vibration will affect the base oil charge and will have an impact on proper lubrication. When the base oil drops out, the thickener is left behind. Loss of the base oil starves the equipment of the necessary lubricant to prevent wear of equipment while in operation.
Specification grease testing Grease testing generally consists of specification testing to determine if the grease meets the needs of specified grease properties for quality control purposes during manufacturing. However, specification testing is not relevant and only useful for in-service grease condition monitoring. One additional concern with specification testing is that the volume of grease required to conduct the testing can exceed the amount of used grease that can be extracted for testing.
Specification testing includes, but is not limited to, the following:
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