The inline design proves particularly useful for threading heavy-gauge material, since the leading edge of the coil enters the pinch rolls close to the nest rolls. A disadvantage of this style: Access to the nest rolls typically is blocked, requiring coil loading from the top with an overhead crane or lift truck.
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The third type of cradle—a cradle-feeder straightener (CFS)—unwinds, straightens and feeds directly into the press without the need for a slack loop, making it a very compact and self-contained coil-feeding system (Fig. 2). A CFS typically receives power from one or more closed-loop servo drives, and pays off from the bottom or top of the coil. This style provides the same advantages as the inline style, but allows the use of a loading ramp for quick coil changes. Because this style requires that coil feeding start and stop in unison with the feed motion, a CFS requires significantly more horsepower than the other cradle styles, which feed into a slack loop and unwind in a continuous operation.
Due to the vastly increased load, a CFS typically is limited to slower-speed applications, and may prove more costly than the other cradle styles due to the use of high-power servo drives. Additionally, the CFS concept generally is not recommended for use with long progressive dies, since a CFS does not allow for an effective pilot release unless the entire straightener bank is piloted.
Centering/Horizontal Reels
The second type of coil unwinder, known as a centering or horizontal reel, is the most common and versatile used by metalformers. These setups hold the coil by its inside diameter over an expanding arbor assembly that grips the coil’s inside diameter. Reels prove ideal for light- to medium-gauge material or for nonmarking applications, since their designs avoid contact with the outer wraps of the material as it unwinds. However, reels also can be used for heavy-gauge or high-strength materials if outfitted with the necessary equipment to safely contain coil clockspring and assist the threading operation.
Coil reels come in powered and nonpowered versions. Powered reels—motorized with a loop control for payoff into a slack loop—find use in applications that don’t require straightening, although they also can be used with a pull-through straightener powered by a feeder at the press.
Nonpowered reels, also known as pull-off reels (Fig. 3), typically feature only a small fixed-speed threading drive or, in some cases with smaller coils or light gauge material, no drive at all. Pull-off reels rely on a power straightener or a set of pinch rolls to pull the material off of the coil during automatic operation. These reels typically feature a drag-tensioning device to maintain back tension, preventing loop slack and material distortion.
Reels come in numerous configurations, the most common being a single-mandrel cantilevered style. These typically are offered in capacities from 500 to 60,000 lb., in widths to 78 in. Metalformers can opt for stationary or traveling models—a traveling reel can quickly be repositioned upon detection of misalignment.
Another version of the coil-reel concept is the double-ended version (Fig. 4), featuring two mandrels facing in opposite directions, with a rotating head. This style suits applications requiring quick coil changes, since the empty mandrel can be reloaded with a fresh coil while the other mandrel runs. With the addition of dual-holddown arms, double-ended reels also gain favor for running partial coils.
Yet another option is the dual-cone or double-stub-arbor version, typically used only for very wide coils or for heavy coils with small inside diameters. Here, a mandrel is inserted into both ends of the coil for support. This style, however, can carry a high purchase price, as the metalformer basically must purchase two complete traveling-reel assemblies with stubby mandrels.
Pallet Decoiler/Pan Reel
The third type of unwinder is the pallet decoiler, or pan reel. This style typically finds use only with coils of narrow, light-gauge materials. Using this style, the metalformer lays the coils on their sides and stacks them on a pallet placed on a motorized turntable for unwinding. The top coil runs first and when complete, the next coil in the stack can immediately be threaded without actually having to load a coil. The range of material width and thickness is limited, since the strip must make the transition from vertical to horizontal as it unwinds.
Advantages of this style include its efficiency in terms of run time between reloading, minimal floor-space requirements and fairly low purchase price. Pallet decoilers often get the call in applications where straightening is not required or in conjunction with a free-standing powered straightener or a pull-through straightener at the feed. MFSee also: Coe Press Equipment Corporation
Steel production and processing are continuous operations where the last step is coiling. Steelmakers and processors use tension when coiling to avoid producing “soft” or collapsing coils. Coiling induces tensile and compressive stresses into the strip, and these stresses can contribute to blank or part distortion in subsequent processes. Unless sufficient winding tension adjustments are made, the degree of these stresses change throughout the coil – whereas the outer laps of the coil may be on the order of 6 feet ( mm) in diameter, the inner laps typically are wound on a 20 inch to 24 inch (500 mm to 600 mm) diameter mandrel. In addition, the magnitude of these stresses increases with higher strength products, leading to coil shape imperfections like coil set and crossbow.
Coil set is a bow condition parallel with the rolling direction, and curves downward in the same direction as the upper outside lap of an overwound coil (Figure 1a). Here, the top surface of the coil or strip is stretched more than the bottom surface, and typically becomes more severe as the coil is processed and the lap diameter decreases. Crossbow is a bow condition perpendicular to the rolling direction, and curves downward in the same direction as the upper outside lap of an overwound coil, with the center portion of the sheet raised a measurable amount above the sheet edges (Figure 1b).
The first operation when unwinding a coil is some type of shape correction to ensure flatness before further processing. There are two main types of equipment used to create a flat coil – a straightener and a precision leveler. While these two types of equipment are similar, a precision leveler has additional capabilities. Both bend the coil back and forth over a series of work rolls to alternately stretch and compress the upper and lower surfaces (Figure 2). Critical equipment parameters include roll diameter, roll spacing, backup rolls, roll material type, gear design, backup rolls, overall system rigidity, and power requirements. The amount of force required to relieve the residual stresses is a function of the sheet thickness and yield strength. Equipment sufficient for shape correction on conventional grades may not be sufficient to completely flatten the advanced steel grades available now and in the future.
Straighteners and levelers have a series of rolls that progressively flex the strip to remove the residual stresses. Each successive roll pair has an adjustable gap to deform the sheet to a targeted amount with the goal of resulting in a flat coil once the steel passes through all the rolls. The entry end has the smallest gap, putting in the most deformation. The last pair of rolls has the largest gap, usually set for metal thickness. The gap profile varies based on thickness, yield strength, and equipment (Figure 3). Many equipment manufacturers have generated tables to guide the operator as to the best settings for various yield strength/thickness combinations.
Removing coil set requires permanent yielding in the outer 20 percent of the top and bottom surfaces of the metal. The central 80 percent of the thickness remains unchanged.T-14 Straighteners are appropriate for this type of shape correction (Figure 4). Only end bearings support the simplest straighteners, with no backup rolls used. Closing the entry roll gap risks deflection of the unsupported center, potentially leading to creating edge waves in the coil.
Eliminating crossbow and other shape imperfections like buckles or waves requires permanent yielding in the outer 80 percent of the top and bottom surfaces, with only the central core — 20 percent — remaining in the elastic range.T-14 Precision levelers, which applies tension to the strip as it bends around more smaller diameter rolls, can achieve this deformation (Figure 5). While this deformation can get the coil shape closer to flat, it also reduces the inherent formability of the grade. Processors should use only the least amount of deformation necessary to correct the shape to retain sufficient formability for stamping or other operations.
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Yield point elongation (YPE), Lüders lines, and stretcher strains are names describing the same phenomenon seen in some annealed or aged metals. A related defect called fluting occurs in V-bending. Leveling at-risk coils with repeated cycles of bending and unbending, like shown in Figure 3, may be an effective way to minimize stretcher strains or fluting. However, process control is critical, since excessive leveling work hardens the coil and results in increased strength and reduced ductility. On the other hand, insufficient leveling does not address the defects related to the yield-point phenomenon.
Recent studies K-24, K-48, K-49 describe the importance of sufficient leveling, using real-world examples as well as simulation to model the phenomena and show potential corrective actions, as shown in the following animations.K-50
Figure 6 shows an animation of V-bending without any roller leveling. The fluting defect occurs, since the formed panel shape does not conform to the punch. Figure 7 is an animation of leveling with roller penetration deep enough to produce deformation equivalent to an 85% plastic fraction. Figure 8 presents a closer view of the V-bending, highlighting improved formed panel shape conformance to the punch. The references cited above detail the simulation methodology.
The progressively higher yield strengths for AHSS are challenging the capabilities of straighteners and precision levelers that were not designed for flattening these high strength materials. Equipment manufacturers have been studying and developing solutions to address this issue. There are a series of factors related to the design of straighteners and precision levelers affected by advanced steel grades:
Roll Diameter – Leveling rolls for AHSS generally are smaller in diameter than those used for mild steel, providing a smaller radius around which to bend the material. This is because exceeding the higher yield strength of Advanced High Strength Steels requires a more aggressive bend.
Roll Spacing – Work roll center-spacing will be closer for AHSS than for comparable mild steels. Closer spacing leads to the requirement of more force to reverse-bend the material, resulting in greater power requirements for processing.
Roll Support – Larger journal diameters with larger radii and bearing capacity will withstand the greater forces and higher power required to straighten AHSS.
Roll Depth Penetration – The upper rolls must have enough travel to be able to penetrate the lower fixed rolls sufficiently so the deformation exceeds the yield strength of the AHSS grade. This penetration may need to be as much as 50 to 60 percent greater than for mild steels.
Roll Deflection – Given the greater force requirements for straightening AHSS, work roll deflection becomes a concern especially with smaller-diameter rolls more likely to flex and deflect. Processing wider sheet also increases the deflection risk. Excessive work roll deflection results in undesirable side effects such as edge waves, increased journal stresses and premature gear failure. Backup rollers prevent excessive work roll deflection.
Roll Material – Higher strength materials and special heat treatment should be employed to ensure rolls can withstand greater stresses for longer periods without experiencing fatigue failure.
Gear Materials – Gears that drive the rolls should be produced from heat treated high strength materials to produce smooth running, chatter free roll drive for long life under high loads.
Gear Positioning – Closer roll center spacing requires higher power transmission and results in a smaller gear-pitch ratio, which reduces gear power ratings.
Gear Sizes – To compensate for the gear positioning issue, flattening AHSS grades requires wider gear faces as well as stronger outboard support of journals and idler shafts to produce higher gear power ratings.
Frame Rigidity – The higher strength of advanced steels results in stresses throughout all the components of the processing unit. Frame rigidity is vital to prevent work roll deflection.
Equipment manufacturers have also developed design solutions that address processing of AHSS. As an example, several manufacturers have designed equipment with removable cartridges allowing for swapping between sets containing differently sized rolls, gears, and support structures. As they switch jobs from AHSS to conventional steels, they swap in the appropriate cartridge. This also allows for off-line roll cleaning and maintenance.
Remember that the likelihood of coil set and residual stresses in the coil increases with strength. Operators must take proper precautions when cutting the strapping banks used in coil shipment to avoid “clock-springing.”
Newer processing equipment may contain additional hold-down arms or other features to protect both plant personnel and equipment from damage.E-11
Stamping AHSS materials can affect the size, strength, power and overall configuration of every major piece of the press line, including material-handling equipment, coil straighteners, feed systems and presses.
Higher-strength materials, due to their greater yield strengths, have a greater tendency to retain coil set. This requires greater horsepower to straighten the material to an acceptable level of flatness. Straightening higher-strength coils requires larger-diameter rolls and wider roll spacing in order to work the stronger material more effectively. But increasing roll diameter and center distances on straighteners to accommodate higher-strength steels limits the range of materials that can effectively be straightened. A straightener capable of processing 600-mm-wide coils to 10 mm thick in mild steel may still straighten 1.5-mm-thick material successfully. But a straightener sized to run the same width and thickness of DP steel might only be capable of straightening 2.5 mm or 3.0-mm thick mild steel. This limitation is primarily due to the larger rolls and broadly spaced centers necessary to run AHSS materials. The larger rolls, journals and broader center distances safeguard the straightener from potential damage caused by the higher stresses.
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