Steel coil – and basically any type of sheet metal stored in coil form – is full of stress. Like good old-fashioned emotional stress, material stress is uneven and can present itself in various forms. The most common types of material stress include coil set, crossbow, edge wave and center buckle. When stress presents itself in these various forms but isn’t corrected, manufacturers can be left with a variety of headaches.
In aesthetic applications, material stress can create a wavy, distorted look that customers will reject. Perhaps worse, material stress can even damage equipment during manufacturing. If a piece of steel has a large amount of residual stress, it can be violently released during the stamping process, causing die crashes and unplanned downtime.
To avoid these costly situations, two main types of machines are used to remove internal material stress: coil straighteners and levelers. It’s important, however, not to confuse the two machine types. While both are used to improve material flatness, one can be more capable than the other in specific areas. Understanding these differences, therefore, is crucial for successful manufacturing.
Types of stress
Before diving into the differentiators between straighteners and levelers, it’s important to understand the various forms of material stress that the equipment targets. Sometimes referred to as residual or internal stress, issues such as coil set, crossbow, edge wave and center buckle are essentially hidden energy that is trapped within the material. Even though the material might look flat, this hidden energy can be released during fabrication operations, such as cutting, punching and stamping.
Some material stress isn’t necessarily hidden, however. Many common types of stress physically deform the material in a way that is clearly visible.
Coil set, as an example, is seen as curved material. It’s the result of the curvature that is retained in the material after it’s uncoiled. Crossbow is also a curvature of the material, but unlike coil set, which runs across the length of the material, it runs across the material’s width. In these situations, the material forms a smile or frown shape due to stress differences through the faces or top and bottom surfaces of the coil strip.
Conversely, edge wave and center buckle are stresses caused by length differences in the strip. When edges are longer than the center of the strip, this is referred to as an edge wave. Its name is quite accurate as it creates a rippling or wavy effect in the material.
Center buckle, however, is essentially the opposite of edge wave as it occurs when the center is longer than the edges. Because the extra material in the middle has nowhere to go while the edges remain tight, it “buckles” upward or downward, creating a series of waves or pockets down the center of the coil.
Unlike coil set, which is apparent as soon as a coil is unrolled, center buckle can sometimes remain hidden – especially if the coil is under high tension on a processing line. It will rear its ugly head, though, when tension is released, often when being cut into smaller blanks.
Types of flatteners
While similar, levelers and straighteners address different issues with the choice coming down to the type of stress that needs to be removed. Simply put, compared to levelers, straighteners have their limitations: Focusing on surface-to-surface stress, straighteners can’t fix edge wave or center buckle.
Instead, straighteners are used to remove coil set or crossbow. The stress removal is accomplished by feeding the strip through rolls that bend the strip up and down uniformly across the face, which reduces the internal stress created when the material is coiled.
When a more advanced form of flattening is required, levelers are typically the way to go. Levelers can remove coil set and crossbow like straighteners, but they can also correct length differences that create edge wave and center buckle.
Stress removal in these scenarios is accomplished using closely spaced rolls that allow for a greater bend angle. The work rolls can be flexed with adjustable backup rolls to allow the leveler to create areas of a longer path for the strip. The bending actions on the rolls allow the leveler to stretch specific areas of metal, equalizing length and stress to provide a flat strip.
Although straighteners can be incredibly effective in removing coil set and crossbow, levelers can do the same and more, improving overall flatness by correcting length issues. With brute hydraulic force, focused tension and power at the point of plasticity, levelers from The Bradbury Group equalize stress and deliver consistent, premium flatness.
As is true with most metalworking equipment, ease of use often dictates a user’s success. With intuitive controls, automatic leveler adjustment and other user-friendly options, today’s levelers are easy to operate and productive. Modern controls also help make equipment integrations with existing press feed lines easy and effective.
Companies like The Bradbury Group that are known for developing cutting-edge controls have integrated equipment into existing lines on many occasions. Jason Bartel, technical sales engineer at The Bradbury Group, explains.
“We provide controls for our straighteners and levelers that provide points for integrating control and safety,” he says. “In a stamping line, the straightener or leveler typically pulls material from an uncoiler and feeds a loop of material that is monitored. The loop monitor input adjusts the speed of the straightener or leveler accordingly.”
Yielding results
For stampers that work with high-strength steel, there is an added hurdle to overcome: the material’s higher yield point. At its core, material stress is removed by bending the steel enough to permanently reshape it where it reaches its yield point.
With mild, which has a low yield point, doesn’t have to be “bent” very far before it stays flat. High-strength steel, however, has a much higher yield point, which means it has to be bent much more aggressively. When it isn’t bent enough, the result is springback, which can affect the product quality and damage equipment. Fortunately, The Bradbury Group has developed both straighteners and levelers with the characteristics of high-strength steel in mind.
“We are able to provide a straightener that has the capability of removing coil set and crossbow where the material won’t spring back,” Bartel says. “We also provide a leveler to remove length issues, such as edge wave or center buckle, which avoids issues in the stamping die. Our leveler also provides some equalization of stress in the material within its capacity range to significantly reduce or eliminate springback in the material after stamping.”
Overall, the goal for Bartel, his team and The Bradbury Group is to provide customers with high-quality equipment and exceptional service. Whether it’s a straightener, leveler or any of the company’s other equipment types, customers can expect the highest standards for productivity, reliability and safety in coil processing.
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