FAG Bearings Ensure Top Performance of Vertical Roller Mills(Dec 29,2005)

In terms of rolling bearing solutions, whenever a development is to break new ground, FAG is a sought-after partner. The latest example of FAG competence in this field is the production of special spherical roller bearings for a Quadropol QM61/30 vertical roller mill developed by Polysius AG in Beckum – thus far the largest and heaviest bearings of their kind. Since its first trial in the year 2000, the Quadropol mill concept has resulted in to significantly higher economic efficiency than other roller mills and has far exceeded all expectations. Even way back then FAG designed and supplied the bearings. Quadropol mills have four individual grinding rollers which are arranged evenly around the grinding table. The bearing assemblies supporting these rollers are located outside the grinding chamber, thus being protected from dust and high temperatures. Destined for a cement plant in the United Arab Emirates, the Quadropol QM61/30 vertical roller mill is the largest and most powerful vertical mill to date, for which the Application Engineers of the Heavy Industries’ Mining and Processing division had quite a few novel features in store. Produced at the FAG plant in Wuppertal, the bearings command respect by their dimensions alone: The locating bearing with its outside diameter of 1,420 mm weighs 3.4 tons. Even more impressive is the floating bearing, having an outside diameter of 1,620 mm and weighing 4.5 tons. But the two spherical roller bearings have more special features– their extended outer rings, for example, which ensure greater aligning capability than normally possible or even permissible. Edge stressing that might cause premature bearing failure is thus avoided. In order to achieve the utmost load carrying capacity, the bearings are fitted with a pin-type cage, housing the maximum number of rolling elements. The floating bearing function is ensured by means of a polytetrafluorethylene (PTFE) coating on the outer ring. This permits the smooth axial displacement of the floating bearing in the housing. The floating bearing is positioned in the middle of the shaft. To avoid the need for complicated devices to dismount the bearing inner rings from the tapered shaft seating, FAG and Polysius have come up with some creative ideas. The inner ring face was equipped with tapped holes to enable flange-mounting of a hydraulic ring. If the bearing needs to be dismounted, the inner ring is widened hydraulically and loosened from the tapered seating. The strength of the tapped holes or the bolted connection was verified with the help of the Finite Element Method (FEM), both for the load case during mounting and dismounting and for operation via widening the interference on the shaft. With its installed power of 7,000 kW, the new Quadropol 61/30 is the largest and most powerful vertical roller mill to date. In addition it boasts further special features. The bearings operate with oil circulation lubrication. The advantage of this method compared to conventional grease lubrication is that the lubricant can be better dispersed and more evenly applied inside the bearings while the bearing temperature is reduced and the oil can be filtered. A new approach was also found for the downtime issue. During maintenance work two roller units facing each other can be lifted from their position in the grinding process without having to interrupt the production for a lengthy period. The machine continues operation with the two remaining grinding roller systems, each of which weighs 180 tons and has a grinding roller diameter exceeding 3 meters.