How Liners Affect Product Quality
Liners are important in achieving the desired quality of the product being rolled. For a mill to roll any product to an accurate shape and size, the alignment of the rolls and the roll gap must be precisely maintained. Wear on the surfaces of the roll chocks and mill housings can cause excessive clearance between the two parts, resulting in:
- Loss of gauge control
- Loss of shape control
- Reduced speed, causing reduced production
These problems are often caused by excessive clearances which lead to mispositioned rolls.
Clearances are designed purposefully between the housing and roll chocks for several reasons, including the following:
- Installation of the roll assemblies requires clearances to allow the rolls to slide between the housing posts. Traditional rules of gap size are now being modified as thinner gauges are being rolled and product quality demands increase. Experience has shown that it is nearly impossible to roll consistent gauge and shape if the gap exceeds the thickness of the sheet for cold rolling. In cold rolling, the trend is toward narrower mill clearances.
- The mill components expand due to the elevated temperatures under operating conditions.
In some applications, the gap between the housing and the backup roll chock can be virtually eliminated with the use of hydraulically expandable liners. See Figure 2.
It is important to remember that the ultimate goal is to keep the roll center lines at precisely the desired locations. The clearances between liners, housings, and chocks introduce one potential source for misalignment. Even with no gaps between the liners and the surfaces they contact, as is the case with hydraulically expandable liners, and with the chock surfaces parallel to each other, the rolls can still be misaligned. This misalignment is caused by the different center locations in chocks and the gaps in the bearing areas, which can lead to problems with shape control and vibration.
Determining the correct center lines for the rolls and modifying the liner thickness to reestablish the center lines improves product shape and gauge and chatter control. Maintenance costs are reduced due to fewer roll changes, and production speeds are increased.
Certain dimensional characteristics, especially surface finish, liner flatness, and parallelism, are also critical to product quality. Smooth surface finishes lead to low coefficients of friction (COF), which assist in proper gauge control.
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