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From: Lance (Bearing Oil and Grease Life Study)
Date: 14 Apr 2003
Time: 01:30:03
How can we estimate and predict the lube life of the products we use in the cars we service. Well, we all understand the principles of friction. Heat and contaminants are the biggest enemies of bearing grease and oil. Think of all the components that need lube, from bearings to internal Engine Component Parts. In Bearings there are three types of lubricants: Synthetic oils, mineral oils and grease. Synthetics last longer at elevated temperatures. Many have special low temperature and low flammability properties. Many also have properties that tend to absorb water (hydrolysis), but this excellent feature also tends to shorten the life of most phosphates, silicates and ester synthetic oils. I hope I am not losing anyone here. But this is important so please stay with me. Avoiding hydrolysis can be achieved with special additives, desiccated air or filtration with activated alumina or clay based fullers Earth. What kinds of Synthetic and mineral oils exist? Silicones, Alkyl silicates, alkyl phosphates, mineral oils, Polyglycols, Polyphenyl ethers, Silicones, Poly Esters, Synthetic hydrocarbons and Diesters. --- Mineral oil deteriorate quickly when they oxidize or react chemically with dissolved oxygen. This increases the oil acidity causing varnish looking deposits, this shortens bearing life. Additives can extend oil life by interrupting oxidation chain reactions. You can increase coverage by adding additives as this process takes place. How do you know when to change it? You can test it electorchemically, microscale oxidation test, differential thermal analysis, or high pressure differential scanning calorimetry. Grease contains thinkening material additives and alos contains oil, either synthetic or mineral? A grease can fail once 1/2 of it's oil content is gone. In this case noise from the bearing will alert you. A good book to read on this subject is "Applied Tribology-Bearing Design and Lubrication"
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