Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/957021
to corrosion, loss of lubricating film strength and impeded function of vital additives. But what about grease? Hard particles in grease are similarly introduced to bearing and gear surfaces, resulting in the same failure modes. High moisture levels in greases can also impact lubricating films and interfere with grease and additive functionalities. Is anything being done about this? Perhaps grease cleanliness gets little attention because the problem is not obvious. When severe mois- ture levels are present in oil, the oil becomes cloudy or separates into a free water phase. Even high dirt concen- trations, if left still in a clear container to settle, will become apparent as a deposition layer, although the indi- vidual particles may not be visible. On the other hand, grease keeps its contaminants in suspension within the thickener matrix, hiding even severe cases of contamination from our eyes. Grease analysis is mostly an afterthought, a tool only used in the most specialized cases. Cleanliness Standards for Grease Nearly 15 years ago, the U.S. military recognized the flight safety impact of greases and placed require- ments to protect the performance of their assets. Two standards (Mil Spec MIL-G-81322 and MIL-G-81937) were developed for aircraft and instrumentation greases. e particle cleanliness for aircraft greases was established at a very low level, while the instrumentation greases' level was even lower. e aircraft grease cleanliness standard required less than 1,000 contaminant particles per cubic centimeter between 25 and 74 microns, and no particles larger than 75 microns. Instrumentation grease was less than 1,000 particles per cubic centimeter between 10 and 34 microns, and no particles larger than 35 microns. How do currently available commercial greases stack up against 14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 Polyurea Mineral Base Oil Elec. Motor Grease Calcium Sulfonate Mineral Base High-Temp. Lithium Complex High-Temp. High-Visc. Mineral Base Unknown Grease Type Lithium Complex High-Temp. High Visc. Mineral Base Calcium Sulfonate/Carbonate Complex Synthetic Base Food Grade Aluminum Complex Synthetic Lithium Complex High-Temp. High Visc. Mineral Base Lithium Complex Synthetic Base Oil Calcium Sulfonate Mineral Base High Temp. Calcium Sulfonate/Carbonate Complex Synthetic Base Food Grade Lithium Complex Synthetic Base Oil Calcium Sulfonate/Carbonate Complex Synthetic Base Food Grade Lithium Complex High-Temp. High Visc. Mineral Base Lithium Complex Synthetic Base Oil Lithium Complex Synthetic Base Oil Lithium Complex High-Temp. High Visc. Mineral Base Lithium Complex Mineral Base High-Water Washout Figure 1. Particle levels in new greases (green), from grease guns (orange) and from the machine (blue) Particle Levels (mg particles > 25 u/g of grease) www . machinerylubrication.com | March - April 2018 | 21 ML