A practical Approach to predict Service Life and Re-lubrication Rate of Grease in Rolling Bearings

Lubrication is crucial for a sustainable operation of any machinery. A sustainable application of lubricants requires the avoidance of any waste, lubricants should not be exchanged long before their service life has ended. This refers especially to those applications where high operation temperatures are reducing the service life and require a huge consumption of lubricants. Therefore, to fulfil the requirement of sustainability a prediction of the service life at elevated temperatures is crucial. At elevated temperatures the service life of lubricants is limited by thermal aging. This results from a chemical reaction of components of the grease with the oxygen of the ambient air. Such a process follows the Arrhenius equation of chemical kinetics and is given by a straight line in a so-called Arrhenius plot. The slope of the line in the Arrhenius plot is given by the activation energy EA. The FAG FE9 test run is a typical method to assess the service life of a lubricating grease in bearings at elevated temperatures. The test is performed at the upper temperature limit of the grease and gives the service life at this maximum temperature. Once the activation energy EA is available the service life at any other temperature may be calculated. Since the temperature of Abstract *Frank Reichmann CARL BECHEM GMBH, Hagen, Germany the FE9 test run is usually the upper temperature limit the operation temperature is usually lower. Once the calculation start from a high temperature in direction a lower temperature low figures of activation energy EA are resulting in conservative service lives, as it is displayed in in this paper. In case of high figures of the activation energy EA the calculated service life becomes too high what may result in bearing failures. With FE9 test runs at different temperatures an activation energy EA = 75 kJ/mol was found for a reference grease. Laboratory methods to determine the activation energy do usually result is much higher figures. For example, HP-DSC test with this reference grease resulted in an activation energy EA > 100 kJ/mol, mostly it was even EA > 120 kJ/mol. Such a result will lead to unrealistic high calculated service lives. An activation energy of EA = 75 kJ/mol is basically a proper assumption for the calculation of service life of most greases. Generally it is always recommended to estimate a low activation energy.