Precision bearing supplier Schaeffler has designed, manufactured and delivered four giant-sized spherical roller bearings for use on the world’s largest roller mill, which will operate in a copper mine in Arizona, USA.
Delivered in 2011 to the manufacturer of the roller mill, Metso Minerals, the double row spherical roller bearings (designated 241/1250-B-K30-MB-C3) have a bore diameter of 1,250mm, an outside diameter of 1,950mm and a width of 710mm. The total weight of the bearing is approximately 7.85 tonnes, with each bearing roller weighing in at 50kg. Schaeffler developed the bearings in close collaboration with Metso Minerals.
The new roller mill, which covers a floor area of 60m2, is designed to crush the mined copper ore. From above, the mined copper ore falls onto two counter-rotating steel rollers. Due to the rotation, the ore is directed towards the defined milling gap between the two steel rollers, where it is crushed at milling forces of up to 27,000kN. The roller mill operates at a speed of 20rpm. Hydraulic cylinders that keep the grinding rollers in position provide the required pressure. The crushed copper ore is then transported away via conveyor systems for further processing.
The roller mill is designed to deliver an output of 10,000 tonnes per hour of crushed copper ore. Extremely large, heavy rollers are therefore required to cope with such large volumes. The rollers weigh 160 tonnes each and have a diameter of 3 metres. To suit these steel rollers, Schaeffler’s application engineers had to select spherical roller bearings of corresponding dimensions, with outside diameters of almost 2 metres.
The double row spherical roller bearings are extremely durable with solid brass cages guided on the inner ring and an optimised internal design. The bearings have a very high load carrying capacity (basic dynamic load rating is 37,500kN) and the bore is tapered with a ratio of 1:30.
The size of the spherical roller bearings created special challenges for Schaeffler’s production plant in Wuppertal, Germany. For example, in order to assemble and position the 50kg rollers into the bearings, a special lifting device had to be designed and built. The rollers were manufactured at Schaeffler’s plant in Eltmann, Germany.
Metso Minerals is also considering condition monitoring and a grease sensor for the application, as well as mounting and engineering support services. Schaeffler also supplied its ELGES spherical plain bearings and INA plain bearings for the roller mill’s hydraulic cylinders.
Spherical Roller Bearings
Spherical roller bearings are double row, self-retaining units comprising solid outer rings with a concave raceway, solid inner rings and barrel rollers with cages. The inner rings have tapered or cylindrical bores.
The symmetrical barrel rollers position themselves freely on the concave outer ring raceway, which compensates for any shaft flexing and misalignment of the bearing seats. Spherical roller bearings can support axial forces in both directions and high radial forces. They are designed for very high load carrying capacity and heavy loads. Due to the narrow osculation between the rollers and raceways, uniform stress distribution is achieved in the bearing.
For more information on spherical roller bearings from Schaeffler, please call the Marketing Department on 0121 313 5870. Alternatively, email info.uk@schaeffler.com.
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