Large steel mill split bearings replaced in just three days

FAG Industrial Services (FIS), the maintenance and condition monitoring services division of the Schaeffler Group, has successfully carried out important maintenance work on the trunnion bearings on a 140-tonne steel mill converter at ThyssenKrupp's steel production plant in Terni, Italy.

While most of the plant's 3500 employees were enjoying the annual summer shutdown in 2006, engineers from FIS set about replacing the large, heavy, split spherical roller bearings (SRBs) on the trunnion on one of the plant's two, 140-tonne high-grade steel converters. The shutdown is normally used to carry out important maintenance work, including relining and modification of the two mill converters.

ThyssenKrupp's fitters initially dismounted the drive and bull gear (a toothed gear wheel that enables tilting of the converter) on one of the converters, then took the opportunity of calling in the FIS engineers to do some preventive maintenance on the FAG-supplied trunnion bearings. Hermann Eussner, chief fitter at FIS, comments: "Prepared down to the very last detail and perfectly organised, the bearing replacement was carried out according to schedule within three days."

Heavy bearings for heavy-duty operation

The bearings on the converter are from Schaeffler's FAG range of split spherical roller bearings. With an outside diameter of 1180mm, a bore of 900mm and a weight of 800kg, these bearings are designed to withstand the high forces acting on the pressure ellipse of the loaded roller and the outer ring, which measures just a few square-centimetres. The bearings also have to withstand extremely harsh conditions - including high operating loads, dirt and high temperatures.

FAG split spherical roller bearings are mainly used for applications in which the replacement of an conventional spherical roller bearing would require intricate additional work. For example, where gearwheels or couplings have to be withdrawn, drives dismounted or shafts disassembled. With split spherical roller bearings, assembly is simplified and mounting made easier, therefore the costs of production downtime, particularly on heavy plant and machinery, are reduced.

Corus bearings

FIS is also working closely with UK-based steelmakers to ensure that production downtime is kept to a minimum. The Corus Group's steelmaking plant in Scunthorpe, for example, recently underwent major bearing replacement work. Corus Northern Engineering Services (CNES) worked closely with FIS to save at least five weeks of production.

The Scunthorpe plant uses three basic oxygen steelmaking (BOS) plant vessels that convert molten iron from the blast furnaces into steel. Each vessel weighs 1300 tonnes and is supported on two 1.75m diameter spherical roller bearings from FAG. Each vessel is able to tilt to various angles up to 150degrees from the vertical during the process for charging and discharging purposes. The arduous duty generates high local contact loads, which, over time, generate high residual stresses in the tilt bearings.

As Ian Taylor, business development engineer plant condition monitoring at CNES, says: "The delivery lead time for bearings of this magnitude can be as long as 12 to 18 months, with plant downtime for a planned bearing change requiring three weeks. The relatively high cost of the bearing becomes almost insignificant compared to the overall production outage costs. By working closely with engineers from FIS, we removed the existing bearings and fitted the new bearings. This took just four days of the three-week planned outage, before the panels and screens were refitted, the supporting frame removed and the plant recommissioned."

Potential applications for split spherical roller bearings range from shafts supported by several bearings to bearing locations of restricted access, for example, on conveyor plants, belt drives, ships, rolling mills, ventilation systems and paper mills.

FIS' range of services includes: helping customers mount and operate machines; installing and running condition monitoring systems; repairing rolling bearings and conducting analyses; studying the causes of machine damage and faults; and the early detection of problems and failures on rotating plant and machinery.