Special bearings cope with sand, silt and high loads

Operating under water, in an abrasive solution, and at ambient temperatures up to 40degC, Revolvo's SRB split roller bearings are providing a reliable solution to the problem of support for the twin 11-metre long Archimedes ascender screws that deliver water to the massive Zambezi Rapid Water ride in the Gold Reef City theme park, Johannesburg, RSA.

The lower SRB bearings on the ascender screws replace plain bearings that were inefficient under operating conditions of complete submersion in water containing abrasive sand and silt: this is the result of the constant churning action of the screw in the water. In contrast, the upper SRB bearings operate in dry conditions, but have to accommodate 16 tonnes of axial load.

These 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 SRB bearings for location. The ascender screws are 11m long and 2.7m diameter; they are angled at 30 degrees and rotate at 27.5rpm. Together, they are capable of supplying 7m3 of water per second - or 25,200 tonnes per hour.

Special lip seals and grease feed

The lower bearings on the ascender shafts are 200mm units from SRB's Light Series range. These are equipped with a special sealing arrangement that allows them to operate completely submerged, excluding water and other contaminants. This is achieved via 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, delivers consistent reliable long-term operation.

At the upper end of the ascender screw, the bearings are 250mm diameter SRB Medium Series units, designed to accommodate large axial loads. The bearing load 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.

SRB Sales & Marketing Director Adrian Menzies, states: "This application demonstrates how SRB can design and manufacture units to accept conditions completely outside of the normal perceived split bearing capabilities. Our SRB split roller bearings perform exceptionally well in harsh operating conditions even with shaft misalignment, whereas solid-mounted roller bearings can suffer from non-concentric ineffective seals that will rapidly lead to expensive premature bearing failure. Moreover the number of standard sealing solutions with SRB is far greater, which extends the range of applications for our bearings."

Guaranteed concentricity

With SRB 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. As a result, SRB split roller bearings perform exceptionally well in harsh operating conditions even with shaft misalignment.

The second major advantage provided by SRB in the area of sealing systems is the diverse range of standard options available: eight in total, including Viton high-temperature seals, aluminium triple-labyrinth seals for high speeds with high contamination, and high-performance Kevlar packing seals for extremely aggressive environments of the type encountered in quarries, mines and steelmaking plants.

Also adding to the versatility of the SRB range are seals for use in very wet environments and laminar seal rings that are highly effective in denying ingress to fine particles such as cement or ash.