March 2011 - "We’ve been seeing more requests over the last several years to supply completely stress-free material," says Chuck Damore, executive vice president, Braner USA Inc., Schiller Park, Ill. The company has more than five decades of experience with coil slitting lines, automatic slit coil packaging lines, leveling and cut-to-length lines, multiblanking lines and specialty coil processing systems.
He points out roller leveling is a good choice for most applications, but "the trend is to go to hot-rolled, high-strength, lighter materials that are more challenging to get flat with roller levelers." As a result, Braner has decided to expand its product line and offer customers the option of stretcher leveling technology.
Stretcher leveling isn’t new; it’s been around since the mid-1900s, Damore says. So when Braner began its research and development, "we investigated some of the antique machines out there and built a prototype machine on a small scale. We’d run it, test materials, see how it behaved, make changes to the machine, test more materials and make more changes until we were confident that we could bring this to a production application, and that’s when we invited Dalco Metals, Walworth, Wis., to come in and witness some sample tests on a small-scale production machine," he notes.
Dalco Metals is a family-owned flat-rolled steel processor that prides itself on facilitating consistent, reliable service from the beginning to the end of the supply chain. The company maintains close relationships with its suppliers to ensure it’s processing high-quality steel and carefully listens to customers’ requests for reliable products.
"When we started exploring this, we laid out our goals," says Bill Ring, president of Dalco Metals. "We’ve enjoyed a good partnership over time with Braner. We have other pieces of their equipment here, and we’ve found them to be reliable. [Braner] has great engineering expertise, good experience, and I’d also call their engineering creative. They’re able to take our needs, thoughts and goals and build a machine that will produce results."
The ability to participate in the stretcher leveler’s development "gave us a comfort level when dealing with Braner and the confidence that they could get the job done," Ring says. "We went down to see the test machine, and we were there during various stages of the process, talking about what they were finding and giving input as far as what’s acceptable and what’s not."
Dalco also provided Braner with material for testing to ensure the new technology could meet the company’s requirements. "Customers that install this sort of equipment are typically processing medium- to heavy-gauge hot-rolled material," Damore says. "They are supplying material to manufacturers cutting with laser machines. Those processors demand material that is stress free and remains flat when cut. Although roller levelers will supply an extremely flat product in the vast majority of applications, they don’t eliminate the trapped internal stresses entirely." This can cause problems, especially when a laser system is running lights out "over the weekend or at night when nobody is around, so when the operator comes in the next morning, he expects his material to be run, only to find that the machine got shut down because of a defective piece of material," Damore says. "With our stretcher leveler, we want to supply material that is stress free, so when somebody goes to cut it, it remains flat."
How it’s done
"Another word for stress free is memory-free," Ring adds. "We receive material in a master coil. It’s a circle, and we’re trying to turn it into a straight line, or taking away that coil memory." The control system on Dalco’s leveler allows the company to ensure that material is memory-free when it leaves the facility, and there’s no guesswork involved.
"When an operator is running material on a traditional roller leveler cut-to-length line, he looks at it in the stacker and on the machine. If it lays flat, he says it’s flat," Damore says. "He has no way of knowing if all coil memory and stress has been removed until the material gets shipped to the customer, they start cutting it and if they have springback issues, he knows it’s not flat, but then it’s too late.
"Although stretching material to eliminate trapped internal stresses has been around for many years, the hydraulic, diagnostic and software controls have not," he continues. "The machine utilizes state-of-the-art controls and software developed by Braner control engineers. A programmable logic controller and touch-screen display package calculate a recommended stretch length for a given material width, thickness and yield strength. If the operator sees the material has a defined shape problem such as an edge wave or center buckle, he can enter the wave height and frequency of the wave. Based on those entered parameters, the state-of-the-art controller automatically calculates the clamping pressure and stretch length for that particular job."
Then, as the material starts the initial stretch, "a graph is generated showing the stress versus the elongation," Damore says. "As the material is stretched, the graph extends vertically, and at the yield point, it’s going to start to curve over and level off horizontally. At that point, the operator knows he has stretched the material beyond the elastic limit. The graph will display the yield point of the material and overall elongation past the yield stress, thus visually informing him whether he has relieved all of the trapped internal stresses."
As a result, Dalco Metals can be satisfied that all material coming off the cut-to-length line is free of internal stresses before it heads out the door. In addition, "once the operator is satisfied that he has stretched the material far enough to eliminate all the trapped stresses, the controller will memorize the cycle so that the cut-to-length line can run, level, cut and stack sheets automatically," Damore notes.
Dalco Metals runs hot-rolled, hot-rolled pickled and oiled, and hot-rolled black, along with some heavier gauge cold-rolled and coated products on its leveler. The leveler itself can process 1/4-inch-thick material that’s 75 inches wide. "The maximum yield stress on the machine is 75,000 psi," says Damore. "It has two cassette modules to do this wide thickness range (3 1/2 and 2 3/16-inch diameter cassette levelers), and it will run sheets as long as 20 feet." Both Ring and his customers are pleased with the results. "Our customers are excited for what we’ve done," he says. "We have a history of reinvesting in our business, constantly upgrading and investing in new equipment, and this continues along that path. At a time when not very many people are investing, we are." MM