Towards adaptive grease lubrication

Currently used rolling bearings are in most cases grease lubricated. The annual grease consumption at a paper mill or a steel works, for example, is typically between 3000 and 8000 kg, the majority of which is used for lubricating rolling bearings [1]. Large amounts of grease are also used to lubricate sliding bearings, guides, and other tribologically stressed components. However, the distinctive characteristics of grease lubrication are generally less well known than those of oil lubrication. Film formation on the lubricated surfaces of a grease-lubricated bearing is determined by many factors other than just base oil viscosity, which need to be considered when selecting a lubricant [2]. One way to improve grease lubrication control is by learning to identify poor lubrication conditions and lubrication system problems and thereby to be able to better predict damage development in lubricated components. This article presents a measurement-based method for identifying poor lubrication conditions in grease-lubricated rolling bearings. The study showed that poor lubrication conditions ("dry run") can be detected using acceleration sensors. The use of acceleration measurement for this purpose seems to work efficiently when there is a high natural frequency of either the bearing structure or the sensor present in the measuring chain. Quite possibly the clearest indication is obtained when the above mentioned natural frequencies are close to each other. In such a case the small impulses from metal to metal contact excite the measuring system and a clear response can be seen in acceleration spectrum. One way to make the diagnosis more reliable is to combine temperature measurement with acceleration, i.e. when the temperature decreases and acceleration increases at high frequencies, the most likely cause in grease lubricated bearings is lack of proper lubrication. Acceleration measurement, together with temperature measurement, enables active lubrication adjustment. Automatic lubrication can be controlled by means of diagnostic software. The software developed by VTT monitors bearing temperature and both high- and low-frequency vibrations in two frequency bands. At industrial sites, the use of this method is complicated by changes in operating parameters.