The basic concept of a multi-roll shape-correction leveler (hereinafter also “shape-correction leveler” or just “leveler” for brevity) has been known for many years. Shape-correction levelers were developed to account for the deficiencies of known hot rolling mills and the undesirable shape defects hot rolling mills commonly impart to the metal strip produced thereby. Common but non-limiting forms of such shape defects are shown in FIGS. 1A-1D, and include coil set, cross bow, edge wave, and center buckle, respectively.
As represented in FIG. 2, known shape-correction levelers typically use opposing, substantially parallel sets of multiple work rolls 5, 10 that often are supported by back-up rolls and associated bearings designed to withstand high separating forces and to control the bending and deflection of the work rolls. The work rolls are normally positioned so that an upper row of work rolls 5 are located above a cooperating lower row of work rolls 10. A gap 15 of adjustable dimension is normally present between the upper and lower work rolls 5, 10. A metal strip to be flattened is passed through the gap 15.
During a flattening operation, metal strip material (typically from a coil) is fed into the entrance of the leveler as indicated, whereafter it is caused to pass between the opposing sets of work rolls 5, 10 (see FIG. 2) before exiting from the exit side of the leveler. Each set of work rolls is placed into contact with the metal strip by driving one set of work rolls toward the other so that a leveling (flattening) force is impressed upon the metal strip as it passes therebetween.
In known levelers, the gap 15 between the upper work rolls 5 and lower work rolls 10 at the entry side of the leveler (and work rolls) is deliberately made to be different than the gap 15 at the exit side of the leveler (and work rolls). More specifically, the gap 15 at the entry side of the leveler is set to be less than the gap at the exit side of the leveler to provide more work roll penetration, and more working of the metal strip, nearer the entry side of the leveler. In other words, the gap distance, and the amount of work roll penetration, feathers out from the entry side to the exit side of the leveler (i.e., in the direction of material flow).
As shown in FIG. 4, contact between the upper and lower work rolls of a known leveler and a metal strip material being flattened, causes the metal strip to be repeatedly bent up and down (i.e., to S-wrap) as it passes through the work rolls located near the entry side of the associated leveler. This repeated bending of the metal strip material removes shape defects from the metal strip material that result from stresses induced therein by the hot rolling process. As can also be observed in FIG. 4, the amount of work roll penetration into the metal strip material, and the degree of resulting S-wrapping, decreases as the strip material moves toward the exit side of the leveler. The feathering out of work roll penetration from the entry side to the exit side of a leveler, allows shape defects to be removed by a first group of work rolls located nearer the entry side of the leveler and coil set to be removed by a second group of work rolls located nearer the exit side of the leveler. The number of work rolls involved in each operation may vary according to the total number of work rolls present and the degree of feathering (i.e., the difference between entry side and exit side gap) employed.
A shape-correction leveler may also be operated to selectively apply forces of different magnitudes to different areas of a strip of material passing therethrough. This selective application of force bends the work rolls to a shape that causes particular zones of the strip of material (from edge to edge) to be worked more than other zones as the strip passes through the leveler. Thus, shorter zones of the strip may be selectively elongated to match the length of the longer zones. This allows a shape-correction leveler to correct a variety of different shape defects.
For purposes of illustration, a typical shape-correction leveler setup 20 for correcting center buckle is shown in FIG. 3A, while a typical setup 25 for correcting edge wave is shown in FIG. 3B. The upwardly directed arrows in FIGS. 3A-3B represent upward work roll bending forces exerted at various locations along the length of the lower work rolls 30 of the leveler as needed to correct one or more shape defects. In the known leveler examples of FIGS. 3A-3B, the work roll bending forces are produced by pairs of driven adjusting wedges 35. In known levelers, such adjusting wedges operate to bend all of the lower and/or upper work rolls present. For example, in the case of the known leveler design shown in FIGS. 3A-3B, any bending forces produced by the adjusting wedges 35 would be applied to all of the lower work rolls 30.
Each work roll of a typical shape-correction leveler is normally driven to propel the strip of material through the leveler during a leveling (flattening) operation. A shape-correction leveler drive system commonly consists of a main motor, a reduction gearbox, and a pinion gearbox, that cooperate to provide output rotation to each work roll.
An interesting phenomenon occurs when the work rolls of known shape-correction levelers penetrate into a strip of material being processed and the material S-wraps through the work rolls. With light penetration (e.g., at the exit end of the leveler) the roll surface speed substantially matches the strip speed. However, when the rolls penetrate deeper (e.g., at the entry end of the leveler), the roll surface speed tends to run slower than the strip speed. This phenomenon occurs because the material has a bend radius, (entry end of leveler) and the surface speed of the material on the inside of the bend radius is moving slower than the surface speed on the outside of the bend radius (see FIG. 4). This is analogous to the wheel speed on an automobile, wherein the wheels on both sides of the automobile rotate at the same RPM when the automobile is going straight, but the wheels on the inside of the curve will rotate slower than the wheels on the outside of the curve when the automobile is making a turn. In the case of a shape-correction leveler, the work rolls are contacting the inside bending radius of the strip material, so the rolls on the entry end of the leveler run slower to match the slower inside radius surface speed. One example of this phenomenon, from an entry to an exit end of an exemplary leveler, is depicted in FIG. 5.
The aforementioned phenomenon may be referred to as differential roll speed (DRS). When the leveler work rolls are all driven together at the same speed (see e.g., FIG. 4 and FIG. 6), the entry rolls try to push the strip material through the exit rolls, while the exit rolls try to hold the material back. DRS causes several issues in a leveler. One issue is that when the work rolls are geared together, the DRS causes high loading on the entry work rolls and internal torque windup within the roll drive system—which may cause premature failure of the drive components. Another issue is that more power tends to be consumed when the work rolls are fighting each other. Yet another issue is that DRS tends to cause a compression of the strip material rather than a stretching of the material, which reduces the effectiveness of the leveler.
Various approaches to overcoming the effects of DRS have been attempted, including but not limited to, the use of torque limiters on drive shafts; the use of torque limiting clutches on entry work roll clusters; complex and costly work roll drive systems such as systems where each work roll is individually driven, and systems utilizing split entry and exit work roll clusters with individual drive motors; and the use of two separate levelers. While torque limiters have been placed on work roll drive shafts, it has proven difficult to produce a slip torque level that is high enough to actually process strip material on levelers so equipped. Torque limiters have also proven to have a short service life and have been unreliable. Placing a torque limiter on the entry work roll cluster of a leveler so as to control the torque to the entry cluster based on total load may be effective at reducing the internal torque windup typically resulting from DRS, but torque windup still occurs within each cluster and a high torque concentration may also be present at the split between the entry and exit roll clusters. Driving each work roll of a leveler individually is very costly and can result in control difficulties when an associated leveler is used to flatten strip material across a range of material and shape defect conditions. The use of split entry and exit roll drive clusters with individual motors can also be effective at reducing the internal torque windup normally resulting from DRS, but torque windup still occurs within each work roll cluster and a high torque concentration may also be present at the split between the entry and exit roll clusters.
The desirability of overcoming the negative effects of DRS should be apparent from the foregoing remarks. It should also be apparent that improvements over the techniques previously used to mitigate or eliminate the effects of DRS would also be desirable. Exemplary embodiments presented herein overcome the effects of DRS using a single, dual-stage leveler, that allows for a simplified work roll drive system.