Split personality

With all the features of their solid counterparts, split roller bearing are used around the world, from a South African theme park water ride to a steel production plant in Italy. Mark Venables explains.

In Split roller bearings — where all the main components are engineered and made in halves — the inner race, roller and cage assembly, outer race, housing or flange support structure and seals, are all manufactured as separable components.

As with their solid counterparts, they are manufactured in progressively heavier duty ratings to best suit the loads, speeds, and life requirements of various applications.

Split bearings combine all the features of conventional solid bearings with the added benefit of being easily assembled around a shaft. and today’s materials and manufacturing methods ensure their reliability and long life are on a par with their solid counterparts.

because it is engineered and manufactured in halves, the complete bearing assembly allows for installation and inspection without disrupting other elements of the machinery. Equipment design can be focused on objectives without having to accommodate such things as bearing types, drive systems, couplings and gears during bearing inspections or maintenance.

Standard straight shafting can be used, eliminating the need for special shaft designs, tooling and press fits often associated with solid bearings. Unnecessary damage to adjacent equipment due to removal or replacement, and time-consuming re-alignment of machinery is greatly reduced or eliminated, as positioning of couplings, motors and gearboxes is not disturbed.

The personal safety of mechanics performing the work is improved for the same reasons. Hand tools are all that are required to install, inspect, or change split bearings. Design, maintenance and downtime are also less costly for the lifecycle of the machine. In many applications, such as large dual drive conveyors, a downtime of days can be reduced to hours, and hours to minutes. Savings are transferred directly to the user’s bottom line.

Split roller bearings are used in all major industries around the world, including mining, cement making, steel and other primary metals, power generation, water and wastewater treatment, food, chemical and petro-chemical, marine, lumber and veneer manufacturing, and glass making.

An entertaining use of split-level bearings is on the water ride at South Africa’s Gold Reef City theme park in Johannesburg. Operating under water in an abrasive solution, and at ambient temperatures up to 40ºC, Revolvo’s SRB bearings provide a reliable solution to the problem of support for the twin 11m-long Archimedes ascender screws that deliver water to the massive Zambezi Rapid Water ride.

The lower bearings on the ascender screws have replaced plain bearings that were inefficient under operating conditions of complete submersion in an abrasive sand and silt solution — the result of the constant churning action of the screw in the water.

In contrast, the upper bearings operate in dry conditions, but have to accommodate a hefty 16-tonne axial load. The twin problems of difficult operating conditions and high thrust loads are the result of the ride’s challenging application and the sheer size of the screw conveyors, which rely on the bearings for location.

The 2.7m diameter ascender screws are angled at 30º and rotate at 27.5 rev/min. Together, they are capable of supplying seven cubic metres of water/second or 25,200 tonnes/hour. The lower bearings on the ascender shafts are 200mm units from SRB’s Light series range.

They are equipped with a special sealing arrangement that allows them to operate completely submerged, excluding water and other contaminants from the bearing enclosure. This is achieved with an arrangement of two lip seals with garter springs, and centralised positive grease feed.

The positive action of the grease purge between the two seals, combined with the efficiency of the sealing arrangement, is claimed to deliver consistently reliable long-term operation. The upper bearings on the ascender screws are 250mm diameter SRB Medium series units, designed to accommodate large axial loads. This is carried between the inner race shoulder on one side and the outer race lip on the other.

Both shoulders and lips are specially designed to facilitate the generation of an oil film between the sliding surfaces of roller ends and lips, thereby reducing wear and limiting heat generation.

‘This application demonstrates how SRB can design and manufacture units to accept conditions completely outside of the normal perceived split bearing capabilities,’ said SRB’s sales and marketing director Adrian Menzies.

‘Our split roller bearings perform exceptionally well in harsh operating conditions even with shaft misalignment, whereas solid mounted roller bearings can suffer from nonconcentric ineffective seals that will rapidly lead to expensive premature bearing failure.’

With split roller bearings, the spherical location between the cartridge and pedestal support ensures that under conditions of shaft misalignment, and whatever type of seal is used, it will always remain concentric to the shaft.

A more traditional use is in operation at ThyssenKrupp’s steel production plant in Terni, Italy. 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 the 140-tonne steel mill converter.

While most of the plant’s 3,500 staff were enjoying their annual summer shutdown last year, engineers from FIS set about replacing the large, heavy, split spherical roller (trunnion) bearings on one of the plant’s two high-grade steel 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 FIS engineers to do some preventative maintenance on the FAG-supplied trunnion bearings.

‘Prepared down to the very last detail and perfectly organised, the bearing replacement was carried out according to schedule within three days,’ said FIS chief fitter Hermann Eussner.

The converter’s bearings are from Schaeffler’s FAG range of split spherical roller bearings. With an outside diameter of 1,180mm, a bore of 900mm and a weighing 800kg, these 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. They are mainly used for applications where the replacement of an ‘unsplit’ spherical roller bearing would require intricate additional work, such as where gearwheels or couplings have to be withdrawn, drives dismounted or shafts disassembled.

Because the assembly of split spherical roller bearings is simplified, and mounting made easier, the costs of production downtime — particularly on heavy plant and machinery — can be significantly reduced.