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Machinery & Equipment MRO
February 2022
LUBRICATION OF HYBRID BEARINGS The lubrication of ceramic bearings, what does this mean?
A
main issue is understanding the meaning of grease life. Grease life is defined as the point in time where the grease is no longer able to lubricate the bearing. By knowing grease life, we can determine the frequency by which a bearing should be re-lubricated. We do
not want to wait until the last minute to re-grease a bearing, rather we choose to do it long before it is unable to do its job. There is a general calculation method presented in many bearing manufacturers documentation, which calculates the replenishment point, which is the L1 life of the grease (the length of time that one
can have 99 per cent confidence that the grease in the bearing is still functional). At this time, we feed the bearing with a specific volume based on the bearing size with the intent of pushing out any old grease in the bearing and replacing it with fresh grease. The grease life will also give us a basis of the functional life of a sealed bearing. The general practice is to set the sealed bearing life in the same fashion as the fatigue life, in that we have a 90 per cent confidence level that a population of bearing will continue to run after this time is reached. Of course, when the grease is no longer able to lubricate, then bearing failure occurs. This differs from the traditional L10 fatigue life that most people know about. The L10 fatigue life predicts with 90 per cent confidence that a population of bearings will not suffer a failure from sub-surface fatigue. Of specific note: the calculated “life” of a sealed bearing is either the grease life or the fatigue life, whichever is a lower value. There is certainly a value to knowing how a hybrid bearing will perform compared to a traditional, all-steel bearing, especially since the hybrid bearings come at a higher price. By knowing that a hybrid bearing will last X more hours, then the cost-effectiveness of this product can be understood. It is kind of like light bulbs; yes, we pay mode for LED lights, but how much longer do they run (and less power do they take). A technical paper on the lubrication of ceramic bearing was published in Tribology Transactions, and this is what it was trying to achieve. Through testing procedures, it establishes performance factors for hybrid bearings that are grease lubricated. The tests were made with deep groove ball bearings and cylindrical roller bearings. A number of thickener types were tested, and synthetic and mineral oil. The best performance improvement with DGBBs was with a lithium complex grease and mineral oil, which saw nine times more improvement of the
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BY DOUGLAS MARTIN
