MRO Magazine

Niche Supplier Solves Hot Problem

Many modern steel mills are updating their processes in an attempt to increase their efficiency. One such upgrade, inside a tunnel furnace, created an opportunity for a niche component manufacturer to...


April 1, 2009
By MRO Magazine

Many modern steel mills are updating their processes in an attempt to increase their efficiency. One such upgrade, inside a tunnel furnace, created an opportunity for a niche component manufacturer to develop simple yet effective solutions.

In this steel mill, the 600-ft-long tunnel furnace sits between the caster and the rolling mill and maintains and equalizes the temperature of the hot slabs of steel. The slabs can be 3 in. to 4 in. thick, 60 in. wide and as much as 40 ft long.

While the temperature of the molten metal at the caster may be as much as 3,000F, the slabs have been cooled to the 2,000F range by the time they are sent to the tunnel furnace.

The conveyor rolls inside the tunnel furnace are generally 12 in. to 14 in. in diameter and can be either water-cooled or dry. Water-cooled rolls are expensive and complicated to maintain. With dry rolls there is no water cooling, as they rely on the nature of the materials used to tolerate the heat.

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The facility wanted to change from wet to dry rolls, as they offer longer life and better ability to convey the slab without having a transfer of heat between the slab and roll, but certain bearing issues had to be resolved.

Each roll is supported by a pillow block bearing unit on each end and driven by a gear reducer through a coupling. The rolls are designed to turn in either direction as slabs may be moved back and forth in the furnace during the operation.

Due to the wide variation in temperature of the rolls in common tunnel furnaces, thermal shaft expansion is greater than standard bearing units can accommodate. Trying to achieve this much axial travel of the roller bearing within the housing creates a sealing problem that can lead to premature bearing failures.

Standard bearings have a limited amount of travel before the seals no longer make contact on the outside of the inner race. Additionally, in this process upgrade, another potential bearing challenge was created. When changing from wet rolls to dry ones, it is necessary to use larger-diameter rolls. Since the pass line of the slab must be maintained, it usually becomes necessary to reduce the base-to-centre height of the bearing.

On request, QM Bearings developed an innovative yet simple solution specifically for this application, using a new Blue Brute bearing capable of handling the large amount of expansion put on these bearings from the high heat. These new bearings mount, lock and seal the roller bearing in a cartridge. The cartridge assembly is then mounted inside a modified extended housing and allowed to travel large distances within it. This way the critical operation of sealing the bearing in its proven configuration is maintained, while allowing large amounts of shaft movement. Other solutions would require a seal to move long distances along very expensive roll shafts, creating wear of the seal and the shaft.

In regard to the shaft height problem posed by the transition from wet rolls to dry ones, QM Bearings was able to accommodate this request during the design of the bearings.

Managers in charge of keeping high-end production plants, such as steel mills, increasingly efficient can often benefit from the project and maintenance solutions offered by niche suppliers.

More details on QM Bearings can be found at www.qmbearings.com.


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