Lubricants for rotating machinery primarily serve to separate rolling and sliding contact surfaces, protect highly finished bearing surfaces from corrosion, protect against contaminants (in the case of grease) and serve as a heat-transfer medium (in the case of oil). An effective lubricant management program-grounded in proper lubricant monitoring and analysis practices-can help reinforce these functions and, in the process, deliver vital information about the machinery's condition and health.
Most notably, lubricant management programs adhering to "best practices" can confirm that the proper lubricant is used; prevent potential over- or under-lubrication; track lubricant use and waste; raise flags about quality (including inorganic contamination, debris from wear or lubricant degradation); and contribute to the desired cleanliness of machines and systems. Along the way, machinery and components can gain longer service life, unscheduled equipment downtime can be minimized and machine reliability can be maximized.
As with any predictive maintenance process, lubricant monitoring and analysis fundamentally satisfies two objectives: detecting a problem and diagnosing the source. In most cases, grease analysis will be conducted only on suspected contamination, when the wrong grease is potentially being used or when a failed component undergoes Root Cause Failure Analysis (RCFA).
With oil lubrication, however, routine day-to-day onsite inspection can tell many stories. For example, clarity and water contamination can be observed in a standing sample. Ferrous materials (filings, metal dust) can be detected using a magnet drawn up the side of a glass jar containing lubricant diluted with a solvent. Flow and discoloration can be noted in a bull's eye sight glass. Viscosity can be monitored using simple in-plant tools.
For the heavier detective work involved in analyzing lubricants, qualified labs will be enlisted to uncover suspected problems and recommend remedies, based on provided samples. These labs will especially look for signs of machinery wear particles, contamination and lubricant and/or additive degradation.
Machinery wear. All machines normally experience inorganic contamination resulting from wear. Test and measurement techniques for small wear contamination will be based largely on the predominant lubrication regimen.
In machines where the regimen is largely hydrodynamic (full fluid film) and the wearing components are nonferrous bearing surfaces (such as with sleeve and pad bearings), Rotrode Filter Spectroscopy (RFS) would be appropriate. For machines with rolling element or steel gear component wear as the primary failure modes, direct reading ferrography (DR) will prove most suitable. These techniques can also be used periodically to measure large severe wear particles.
Lubricant contamination. Contamination can be present in four different forms: gaseous, fluid, semi-solid or solid. Selection of the analytical method for contamination ultimately will depend on the machine, lubricant and operating environment.
Lubricant degradation. All lubricating oils contain additives to delay the natural degradation process. Since lubricants will lose their serviceability (and must be replaced) when required additives for an application become depleted, measuring the degradation process can help prevent related problems before they can occur.
Standard analytical methods for measuring degradation include increases in viscosity or changes in alkalinity and/or acidity. When changes in viscosity, alkalinity and/or acidity occur (from degradation instead of contamination), the indication is that sludge and varnish have already begun to form in a machine and that the oil is "overdue" for changing.
Common Tests for Oil Lubricants
When evaluating fluid lubricants, the following common tests are typically among those that may be applied, either onsite or in the lab:
Color and appearance. Such characteristics should be noted as part of routine evaluation, although some oils may be too dark for effective appraisal. If so, the oil volume observed can be reduced to a constant depth for proper observation.
Research lab where lubricant analysis is done
Viscosity. Oils found to be outside the lubricant specification are always considered abnormal. However, a change within a grade can also spell trouble. Users should be alert for 10 percent changes from new oil.
Base number. Alkalinity values (base number) of new diesel engine oil can be compared to the used oil. A general rule for oil change is when the alkalinity value of the used oil is 50 percent of the new oil.
Acid number. Acidity varies in new unused lubricating oils, based on the concentration of antiwear (AW), antiscuff (EP) or rust additives. Increases above the new oil reference will indicate oil degradation. Lubricants with additives like ZDDP and EP will generally exhibit higher acidity than those containing only rust and oxidation additives.
Emulsion. Water separability testing is primarily used to evaluate steam turbine, hydraulic and circulating oils susceptible to high water contamination.
Foam. In systems where foam is perceived to be a problem, a foam test can be performed to confirm whether the lube oil is the source. If the oil is not the problem, attention usually shifts to other influencing parameters (mechanical or operational) to determine the source.