Aircraft Maintenance Technology

OCT 2018

The aircraft maintenance professional's source for technological advancements, maintenance alerts, news, articles, events, and careers

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Page 50 of 63 51 grease formulations cannot substitute new additives, base oils, or thickeners without obtaining OEM, military, and civil authority endorsement. In many cases, full requalification of the grease may also be required before a change is approved. Q. Are all greases compatible with one another? No, not all greases are compatible, and the mixing of different greases should not be taken lightly. If incompatible greases are commingled, the intermixing can negatively impact grease performance as the resulting grease mixture will have an unknown composition and/or performance level. All greases contain base oils, thickening agents, and chemical additives that can react with each other and adversely impact the quality of the grease. To confirm if grease formulations are compatible, operators can consult the Standard ASTM D 6185 – Standard Practice for Evaluating Compatibility of Binary Mixtures of Lubricating Greases, which is based on performance data from testing mixtures of both greases. Standard compatibility tests may consider the stability of the oil/thickener matrix by evaluating a grease’s shear stability, dropping point, storage stability, and high-temperature performance. Most OEM, industry, and supplier recommendations suggest performing a compatibility test to assess the risk of mixing two greases. ExxonMobil Aviation has tested many combinations of greases and finds that compatibility is generally “borderline.” The normal aviation industry practice is to fully purge all old greases from any application, regardless of compatibility. This is a standard operating practice and should be followed without exception. For more information on grease compatibility and for conversion guidance, please review the technical topic at Q. How do you clean grease off an application or bearing when changing grease or making a grease conversion? A number of options exist for removing grease from your application or bearing. These can range from purging and wiping down the part to solvent washing of the part. It is generally recommended that you contact your OEM supplier to determine what would be an acceptable method to clean the application or bearing. Q. If grease incompatibility occurs, what are some initial effects that might be noticed? When incompatible greases are mixed together, the consistency typically becomes harder or softer than the individual greases. These consistency changes generally become more pronounced as operating temperatures and/or the rate of shearing increases. While in operation, incompatible greases may also exhibit excessive oil separation or bleeding tendencies. Q. If grease is contaminated with fuel, how does this impact its formulation? What else can operators do? Fuel contamination can reduce a grease’s base oil viscosity and compromise the structural stability of its thickener, affecting its load-carrying capabilities. If grease becomes contaminated by fuel, it should be fully purged and replaced. Coming up in AMT’s November/December issue, part three will focus on grease color and odor. ENGINE INTELLIGENCE. HUMAN EXPERTISE. At Pratt & Whitney's Customer Training Center, we present innovative learning experiences – including 3D and VR, online and hands-on – to support your maintenance, engine performance and fleet management needs. Backed by our EngineWiseTM global network, our team is dedicated to advancing jet engine knowledge and maintenance techniques to support you anytime, anywhere. Learn more at

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