In most cluster mills the backing bearings are lubricated by the oil or water/oil emulsion used in the rolling process to remove the frictional heat as well as the heat produced by the reduction of the material thickness. This has the advantage that the rolled material is not contaminated by the bearing lubricant. The viscosity of the emulsion to be used for bearing lubrication should be at least 8 to 12 mm2/s at 40 °C. This low viscosity is not normally sufficient to provide adequate bearing lubrication, so that the flow per backing bearing must be much higher than would be needed for conventional oil lubrication. The cleanliness of the rolling oil also has a decisive influence on the service life of the backing bearings. The flow rates given in table are recommended.
Certain backing bearings, for example, the double row cylindrical roller backing bearings with integral "Fey-Ring" seals or the double row tapered roller backing bearings, must have oil mist or oil spot lubrication. For cylindrical roller backing bearings with integral radial shaft seals, circulating oil lubrication with a separate supply system should be used.
For oil-lubricated backing bearings, the use of a mineral oil with EP additives and a viscosity of 100 to 150 mm2/s at 40 °C is recommended. Table contains recommended oil flow rates for the individual lubrication methods. Higher flow rates are required under heavy loads and/or high roll speeds.