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Home \ Bending \ Articles \ Plate rol ls keep rol l ing heavier plate

Plate rolls keep rolling heavier plateVariable-geometry plate rolls tackle unbelievably thick plate

By Tim Heston

June 2, 2011

These days, plate rolls are forming plate 6 and 7 inches thick--and even thicker. Vessel designs that wouldhave been forged just a decade ago are now being sent to fabrication houses. The variable-geometry plateroll system, which really operates more like a press brake than a traditional plate roll, is making suchextreme rolling possible.

Plate rolls have gotten seriously big.Machine pits are deeper, the framesmassive, the rolls ever larger. Certainmachines have specifications stating theycan roll plate of certain yield strengths to4, 6, and 7, even 11 inchescold.

"I was standing next to just one roll thatwas sitting on a shop floor," said BobStasalovich, director of sales andmarketing for plate roll-maker The SertomGroup (TSG) North America, Roscoe, Ill."I'm about 6 feet, and this roll was aboutup to my chin. That gives you an idea ofjust how big these machines are."

The niches served by shops that cancold-roll plate 4 in. or thicker haveboomed (see Figure 1 and Figure 2).Superheavy rolling systems arefabricating vessels that in years pastwould have been sent to forging houses.

According to sources, several factors arespurring demand. First is increaseddemand from the sectors providinginfrastructure and power generation.

Second, a lot of old iron remains in heavy fab shops; many are upgrading, and the type of machine they'repurchasing has made such heavy rolling practical and cost-effective: the variable-geometry plate roll.

Fixed-geometry Systems

Common double-pinch, fixed-geometry plate rolls can have either three or four rolls, where the bottom rolls rise tomeet a fixed top roll. This arrangement dominates much of the industry for a reason: It's simple and efficient. But asplates get thicker, challenges arise, and most involve what all plate rolls, no matter the type, leave behind after aninitial rolling operation: the unbent flat.

All plate rolls need to pinch the plate edge, which means a small amount of material from that edge remains flat. In afixed-geometry, three-roll system, prebending minimizes these unbent flat sections. The offset rolls rise to meet thetop roll to bend the plate's leading edge upward. An operator can then run the plate flat through the open rolls andposition the trailing edge for the second prebend. If floor space is an issue, an operator may use a crane to removethe plate after the initial prebend and rotate it 180 degrees to perform the second prebend. Regardless, rollingcommences only after the leading and trailing edges are bent (see Figure 3).

Four-roll systems have three rolls on the bottom: a pinch roll in the center and an offset roll on either side. Thismeans the machine can prebend, roll, and then perform the second prebend (in this case it could be called apostbend) on the trailing edge. This gives the four-roll system a slight productivity boost, which, as sources pointedout, is why four-roll systems have become so popular in production-rolling environments such as wind tower plants(see Figure 4).

Rolling Like a Press

The variable-geometry, or variable-axis, machine is a differentanimal. Its two bottom rolls move laterally, the top roll vertically.The system really acts more like a press brake setup with avariable V die (see Figure 5 and Figure 6).

"The two side rolls of the variable-axis plate roll are very similar tothe bed, and the top roll can be operated like the ram of a pressbrake," said Orazio Davi, president of Italian plate roll manufacturerPromau/Davi, with U.S. offices in Loves Park, Ill., and a technicalcenter in Dallas.

The wider the V-die opening is, the greater tonnage a press brakecan handle, and the same holds true for a variable-geometrymachine. The two bottom offset rolls are like the two shoulders of avariable V die. To form thicker metal, the two bottom rolls moveapart, and the top roll, acting like a press brake punch, descends to

a specified position to roll the desired cylinder diameter.

Figure 1Todays machines are rated to cold-roll seriously thick plate. Here, amachine is being tested to roll 8-in.-thick plate cold. Some machines

today have been built to cold-roll up to 11-in.-thick plate. Photocourtesy of FaccinUSA

Figure 5

Variable-geometry machines resemble

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If a press brake normally bends 0.5-in.-thick plate but on occasionneeds to bend 0.75-in., "the operator would pull out a 6- or 8-in.-wide V die they used to properly bend the half-inch plate andreplace it with a larger, 12- to 14-in.-wide V dieand all of asudden it takes less tonnage per foot to bend," said Steve Bonnay,product manager at plate roll manufacturer Faccin, with homeoffices in Italy and a U.S. office in Tampa, Fla. The same logic, hesaid, applies to the variable-geometry rolling machine. The farther the two bottom rolls are away from each other, theless force per foot it takes to roll thick plate.

To prebend on a variable-geometry system, the operator can feed the plate in, parallel to the floor, and move a lowerroll virtually under the top roll to the precise position needed to produce a minimal flat section (see Figure 7).

The Unbent Flat

The physics of rolling makes a certain amount ofunbent flat sections unavoidable, but calculatingthe amount of unbent flat depends on the machinetype. A fixed-geometry system leaves unbent flatsections between 1.5 and 2 times the maximummaterial thickness the machine is rated for. So ifsuch a system is designed to roll up to 4-in.-thickplate, it would leave between 6 and 8 in. of flatregardless of the plate thickness.

This happens because the top roll is fixed and thebottom rolls follow only one axis of motion, towardand away from the top roll. A fixed-geometrysystem for 4-in. plate is designed such that thebottom rolls contact that plate at just the right spotso they can perform a prebend to create thenarrowest flat possible at the edgeagain,between 1.5 and 2 times material thickness.However, if those rolls prebend 0.75-in. plate, theywill contact the top roll in the same place as if they

were prebending that 4-in. material, leaving up to 8in. of flat. "That's a problem," Bonnay said. "Mostshops rolling 0.75-in.-thick plate couldn't live with 7or 8 in. of flat."

Variable-geometry systems don't have this problem because the two bottom rolls can be positioned under the top rollat just the right spot to leave a minimal unbent flat for the material at hand. Regardless of plate thickness, the flatusually is 1.5 to 2 times material thickness. As Davi explained, "The side rolls of the variable-axis plate rolls can beopened and closed by the operator to create the perfect opening related to the specific plate to be rolled in themachine at that time."

It's not that a fixed-geometry machine couldn't be designed to handle extreme thicknesses, but most operations don'tsee 6-in.-thick plate every day. It comes back to unbent flats. Fixed-geometry systems leave unbent flats up to 2times the maximum material thickness rated for the machine, regardless of the actual thickness of the plate betweenthe rolls. Variable-geometry systems leave unbent flats up to 2 times the thickness of the plate that's in the machine.

The variable-geometry systems also have a benefit if rerolling is required, again because of the system's pressbrake-like operation. Like a brake punch, the top roll descends to the welded seam in the middle of the unbent flatsection and rolls the metal out to the desired radius. "It's really a press roll," said TSG's Stasalovich. "The top roll isable to come down and then roll back and forth slightly over this area, and you don't lose significant thickness

capacity like you do on other styles of roll machines."

If a job requires a cylinder with no unbent flat portion at the seam, rerolling will be required regardless. Again, nomachine can roll a radius right to the plate edge. But often variable-geometry machines allow operators to tweakparameters to correct some differences in roundness, Davi explained, adding that, in some cases, such tweaking caneliminate the need for rerolling.

Production Versus Custom

All this isn't to say fixed-geometry systems don't have their place, especially for thinner material. A fixed-geometrymachine rated for up to 1.5-in. plate would leave unbent flats up to 3 in. wide, which may be acceptable for variousthin-gauge jobs. The occasional job requiring less or even no unbent flat could be welded and then rerolled.

As always, machine choice depends on application requirements. Heavy-duty applications usually fall into one of twocategories: custom or production. In production environments, such as for wind towers, reducing rolling cycle timemeans everything. These are high-volume environments processing identical or similar plate thicknesses repeatedly,which is why many plants opt for the four-roll, fixed-geometry system.

"The wind industry is the one sector where we've seen significant adoption of the four-roll systems, even forsuperheavy plate," Bonnay said.

For custom thick-plate applicationsin other words, most job shop scenariossources said that the variable-geometry systems are becoming much more popular, mainly because they can process a variety of materialthicknesses. Most feed the plate with an overhead crane. Feed tables are rare, simply because of the real estatethey require.

"If you have a 60-ft.-long feed table, and you only process 60-ft.-long plate once a month, it's not a good situation,"Bonnay said. "It's difficult to use the real estate [the feed table takes up] for anything else."

Rolling Physics

No matter how advanced plate rolls become, the physics of forming doesn't change. Like in the press brake arena, itsometimes makes sense to choose a machine rated for much thicker material than a shop would normally process.Prebending requires more concentrated force, so a machine rated for 6-in.-thick material may be able to prebendmaterial only up to 4 in. thick. It also takes more force to roll to tighter diameters, so as diameters decrease, soshould the plate thickness.

"Of course, most fabricators rolling superheavy material aren't doing tight-diameter work," Bonnay said.

Material yield strengths also affect rolling capacity, and to the frustration of fabricators everywhere, what they orderisn't necessarily what they get. Mills promise that the plates they sell will meet or exceed a minimum yield strength.Structural plate rated to be at least 36,000 PSI may in fact be 40,000 to 60,000 PSI, or even more. As sources

explained, fabricators should keep this in mind when considering machine capacity.

press brakes more than conventional rollingsystems. Like a brake punch, the top rolldescends to roll the plate to the desireddiameter. Image courtesy of TSG North

America.

Figure 7To conduct prebending on a variable-geometry machine, theoperator can instruct a bottom roll to move almost completely

under the top roll. This allows the rolls to prebend in a waythat leaves a minimal unbent flat section at the plate edge.

Image courtesy of Davi Inc.




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Related Company Showrooms:Davi Inc.Faccin

Sertom Group North America LLC

Additional Information

Taking Time to Save Time

A 20-something plate rolling machine operator, about 6 ft. tall, doesn't look it. The system, capable of cold-rollingsuperheavy plate, dwarfs him. He operates the machine cautiously, taking eight passes, easing the extraordinarilyhefty material to produce the desired diameter. Certain codes specify that the difference between the vessel'smaximum and minimum measured diameter cannot exceed 1 percent of the specified diameter. This operator gotthat variation down to less than one-quarter of 1 percent.

Bonnay visited this European shop with managers from a U.S.-based heavy fabricator. With so many passes, it tookabout 45 minutes to roll that cylinder. The U.S. fabricator accompanying Bonnay asked why the operator took somany passes.

The operator said it boiled down to time and money. Like with thick plate on a press brake, underbending can becorrected easily with another pass, but overbending would be disastrous. Pulling apart such a thick cylinder, he said,may be virtually impossible in some circumstances. Even if it were possible, it would take hours. And the cylinder

would make for some expensive scrap; the raw stock in that machine was worth more than $25,000.The operator rolled the plate so preciselywell within the tolerances specified by codethat it made downstreamfabrication much easier. An extra 20 to 30 minutes on the roll, he said, allowed for time savings of six to eight hoursin downstream fabrication.

How Thick?

"In the 1980s and 1990s, we saw about only a dozen or so large machines capable of rolling 4 in. thick and upinstalled around the world," said Davi. "About 10 years ago we began to see increased demand for these heavymachines."

He attributed the global demand surge to rising labor costs. Old mechanical machines still work, even after decadesof service, but it now costs a lot more to run them.

"Four decades ago most machines were mechanical," Davi said, adding that such machines took hours to make justone cylinder. These machines remained in operation as long as labor rates remained sufficiently low. Now, withglobal labor rates rising, the number of investments in new hydraulic plate rolls is rising, and with it the proliferation ofthick plate rolling.

With the latest equipment, shops can hot-roll insanely heavy platesmore than a foot thick in some cases. "Heating

plate may increase a machine's capacity by 40 to 60 percent, depending on the application," Stasalovich said. Butsources noted that the market for such plate is different, because heating prior to rolling changes the plate'smechanical properties.

Still, capacities seem to be ever-growing. So how thick can thick plate rolling get? Very. Studies have shown that, atleast theoretically, rolls could be designed to process plate up to 20 in. "It's a little unbelievable, but true," Davi said,"but is industry going to be capable of supplying such a plate for that giant monster?"

Today's monsters don't cold-roll 20-in.-thick plate, but they can handle 4 to 7 in. just fine, and they're quite a sight. Atshops where installers can't dig a pit to place the machinelike near the bayou of Louisianamachines tower overthe floor and sometimes require platforms for operator access and plate feeding. Other shops dig pits so deep thatplate can be fed via overhead crane virtually at floor level.

They're big, awe-inspiring. But most important, they provide designers of the world's infrastructure and power-generation equipment with another manufacturing option. For heavy fabricators, therein lies opportunity.

Tim HestonSenior EditorFMA Communications Inc.833 Featherstone RoadRockford, IL 61107Phone: 815-381-1314Contact via email

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Who remembers Phoenix Steel in Claymont, De. or Bethlehem Steel at Sparrows

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