Everything you need to know about flatteners and levellers for coil processing.
This is article is part 1 of a four-
Does your coil processing operation preserve or improve quality? Do you struggle just to keep the quality the mills put into their flat-
To maintain material quality, first we must understand flat-
Types of Shape Defects
Three categories of flat-
Edge wave, buckles, and camber are easy to understand. Twist is a little more difficult to visualize as an edge-
Sources of Shape Defects
The mills have come a long way in the last few years in controlling shape and thickness, but perfection is difficult. You need to understand this-
The producer mills would like to make crown-
At the Hot Mill
At the hot mill the crown or thickness profile can be changed without changing the shape or flatness, and vice versa.
Hot mills try to get the relationship between crown or profile and shape, but achieving both perfect shape and perfect crown control is almost impossible. With new automatic gauge control (AGC) technology, the mills are doing much better, but perfection remains an elusive goal.
At the Cold Mill
At the cold mill, every time we change the thickness profile, we also change the flatness. The cold mill roll gap should start out being set to the same profile as the crown of the incoming hot mill product. Then, as the cold mill gap is adjusted to reduce the crown, the coil also will come out flat-
If the cold mill rolls or hot mill coils have too much crown, the mills will roll out the centre, creating centre buckle in the process (see Figure 8).
If the cold mill rolls or hot mill coils have too little crown, the mills will roll out the edges and create edge wave (see Figure 9). This is fairly common in mill master coils.
Mill work rolls and backup rolls bend and compress under the vertical loads it takes to reduce plate or sheet coil thickness. In theory, if the work roll surfaces were absolutely parallel, the top and bottom surfaces of the rolled product would be parallel-
The mills deliberately crown their work rolls slightly larger in the centre to allow for deflection and compression under load. The amount of roll crown that is added is a compromise, a best guess. In addition, mills can control the gap profile by bending these huge rolls or by expanding them with hydraulic pressure like a huge steel balloon.
You've probably seen the pictures of the mill control pulpit with all its controls and computer monitors. Every time the operator pulls a lever, he changes what's going on at that mill stand and, therefore, the amounts of deflection.
The hot mill's strategy is to get the relationship between crown and shape correct so that they will both come out right in cold rolling. If that sounds difficult, it is!
The tension on the coil strand through the mill also affects the thickness reduction at the mill roll. It shouldn't surprise us then if that the heads and tails of the mill coil are thicker than the middle because they did not have full tension.
Problems Aren't Only in the Mills
Do you remember Pogo? Pogo was a small opossum and a great philosopher-
How many metal buyers would like to specify coils without coil set? The problem is that coils are in coil form. Most coils have a 20-
Where does coil set come from? It's in the coil!
Coil set should be more pronounced on the coil's inside wraps and less on the outside because the inside bend radius is smaller. In fact, it's possible to have residual reverse coil set in the outer wraps from the original master coil before it was swapped head to tail in some previous operation. We also can induce reverse coil set during our own uncoiling process by backbreaking the material over a small breaker or pass line roll.
How many coil processors can use a 60,000-
I once inspected a brand-
I watched an aluminium mill tension levelling coil on a new line. It was dead flat coming out of the leveller and then they recoiled it, under a lot of tension. The coil was crowned, meaning that the centre of the strip was thicker, so the centre of the coil on the rewind arbor had a larger OD. Big surprise! The operators were rewinding the coil over a barrel. That was pulling centre buckle back into it!
Flatness inspection had been done after levelling and before rewinding. QC insisted the material was dead flat. The customer said it was buckled when he unwound it. Engineering wanted to know about the material's "memory" or trapped stresses. Everyone was wrong.
I was asked to provide operator training on an old roller leveller that had been very badly overloaded. The service centre owner told me that the work rolls had just been reground and the machine recalibrated. His men still could not get flat material out of it.
We found that the backup roller pins were badly distorted and bent. The side frames were sprung. No amount of training was going to help them. Worse yet, the line management personnel didn't understand that they were the culprits.
Some years ago I watched a badly maintained service centre slitter producing "snakes." The recoiler arbor had been bent and was wobbling several inches as it rotated, pulling an oscillating camber into each slit mult coming from the tensioning device.
Any rolls in the system, such as pinch rolls, slitter arbors, flattener rolls, leveller rolls, and, of course, feeder rolls, that deflect or that are misaligned can produce edge wave or even camber. These rolls can put uneven pressure on part of the material and destroy the coil shape in the process.
It's surprising how many operations do not perform preventive maintenance or calibration and realignment. "If it ain't broke, don't fix it." Right!
It's surprising how many times I've found that flattener and leveller rolls haven't been reground or recalibrated in years, if ever. It's surprising how many line operators don't have access to information about machine capacities or to either nominal or actual material yield strengths of the metals they are processing.
It's surprising how many line operators have had no training on the meanings of these critical numbers.
It doesn't make sense to talk about equipment upgrades if the people running the equipment don't understand the equipment they use now. Before we start talking about new equipment, let's see what we can do to get the best out of what we already have. I'll discuss this in Parts II and III of this article series.
Remember, we need to make good stuff out of the bad stuff. Not the other way
Flattening solutions and the anatomy of a bend in flat-
This chart shows the relationship between the force on a metal and the metal's change in dimension. The vertical axis represents the amount of stress, or force, on the metal; the horizontal axis represents the amount of strain, or elongation or stretch.
Metal stretches like elastic or rubber in the elastic range. One pound of pull causes the same number of thousandths of an inch of stretch, or strain, for a given metal and cross section every time. Two pounds of pull results in twice that much stretch. If we pull it any amount up to its yield point and let go, it snaps back to its original shape, like a rubber band. Once the metal exceeds its yield point, it's in the plastic range. Then, when the metal is stretched and then released, it springs back slightly, but not to zero.
The rule of thumb for eliminating simple coil set is to stretch the upper and lower surfaces two yield strains, or twice the distance to the yield point. This produces permanent yielding in the outer 20 percent or so of the top and bottom surfaces of the metal.
Metallurgically, most metals that coil processors deal with act similarly, as does the equipment used to fabricate those metals.
The metal stretches like elastic or rubber in the elastic range. The graph in Figure 2 illustrates that 1 pound of pull, or stress, always causes the same amount of stretch, or strain, for a given metal and cross section. Two pounds of pull results in twice that much stretch. If we pull it any amount up to the yield point and let go, it snaps back to its original shape, like a rubber band.
Flatteners and levellers (as well as roll formers and press brakes) don't make any permanent change in the shape of the metal if its yield point isn't exceeded. The metal goes right back to where it was, like an old-
Once the metal is stressed past the yield point, it's in the plastic range (see Figure 2). What happens when the metal is stretched into the plastic range and then released? It does not go back to its original form. It may spring back slightly, but not back to zero.
Metal stretched past its yield point results in a permanent change in shape, or permanent set. This occurs in a flattener, leveller, or press brake die. It's also what has happened to a "sprung" machine frame.
If material is pulled or stressed past its yield strength all the way to the ultimate tensile strength (see Figure 1), it will fracture or break. That is exactly what happens when we slit, stamp, or saw metal. It's also what happens when a crankshaft or die breaks.
The rule of thumb for eliminating simple coil set is to stretch the upper and lower surfaces an amount equal to two yield strains (see Figure 3), or twice the distance from zero to the yield point. This produces permanent yielding in the outer 20 percent or so of the top and bottom surfaces of the metal. The central 80 percent of the thickness remains unchanged. Thus, coil set elimination is strictly a surface issue.
The rule of thumb for eliminating crossbow is to stretch the upper and lower surfaces an amount equal to four or five yield strains past the zero point (see Figure 4). Poissonâ€™s ratio for steel is about 0.3. To get enough crosswise elongation to eliminate crossbow, the surfaces must be elongated lengthwise 1 divided by 0.3, or about three times as far past yield. This is how we get the four to five yield strain rule.
This produces permanent yielding in the outer 80 percent or so of the top and bottom surfaces, with only the central core -
The rule of thumb for eliminating crossbow is to stretch the outer surfaces to four or five yield strains. To get enough crosswise elongation to eliminate crossbow, the surfaces must be elongated lengthwise three times as far. This produces permanent yielding in the outer 80 percent or so of the top and bottom surfaces with only the central core -
Because aluminium is more elastic than steel, it stretches more than steel with the same amount of force. A flattener or leveller must be set deeper for aluminium than for steel because aluminium must stretch farther to get past its yield point. Stretching the aluminium farther using the same line speed and yield strength takes more horsepower.
Metals are basically crystalline in structure, but engineers talk about the inner, outer, and central fibres to help us visualize what's going on. When flat-
The rule of thumb for eliminating buckles or waves on a leveller is the same -
Based on the flattener or leveller manufacturer's capacity specifications for processing steel, can you also process other metals at the same yield strength and thickness? Don't assume that you can! Aluminium that has the same yield strength as steel, for example, requires more horsepower to level it.
Aluminium is more elastic than steel, so it stretches more than steel would with the same amount of force (see Figure 5); that is, it has a different modulus of elasticity. A flattener or leveller must be set deeper than for steel because aluminium must be stretched farther to get past its yield point.
Here's the rub: Horsepower can be described as how hard and how far metal is stretched in a given period of time. Stretching aluminium farther at the same line speed and yield strength takes more horsepower. The structural load on the machine will be the same, but the horsepower must be greater. Ask the manufacturer of your levelling equipment before testing its limits on aluminium.
The bend radius of the metal going through a flattener or a leveller is determined by the machine's roll configuration, diameters, and spacing.
Metals are basically crystalline in structure. However, engineers talk about the inner, outer, and central, or neutral, fibres because it helps us to visualize what's going on (see Figure 6).
What happens when flat-
The centre of the cross section, the neutral fibber, is neither stretched nor compressed. The farther the surface fibres are from this neutral fibre, the more elongation or compression will occur for a given bend radius. That is simple geometry.
At some distance from the neutral centreline on the top and bottom of the bend the material yield point is exceeded and the material is in the plastic range. The middle is still in the elastic, or "springy," range.
The amount of elongation for the bend over the radius resulting from a particular roll configuration is a function of the distance of the metalâ€™s surface from the neutral fibre. The thinner the metal, the less surface yielding occurs -
The thinner the metal, the less is the distance from the upper and lower surfaces to the neutral fibre and the less surface yielding occurs -
The thicker the metal, the greater the distance from the upper and lower surfaces to the neutral fibre, the more elongation will occur, and thus the greater the force required to make the bend. Therefore, the upper thickness capacity limit for that machine is the machine's structural deflection under load. This becomes the maximum thickness limit.
The different colours in this computer-
The different colours in a computer-
So far we've considered only one bend. Now consider a flattener or leveller with multiple, reversing, and up-
Putting significant tension on the material as it goes through the flattener or leveller has the effect of moving the neutral fibre toward the inside of the bend. The added strip tension then adds to the tension in the outside of the bend and subtracts from the negative compression on the inside of the bend.
The result is less compression on the inside and more tension on the outside. The neutral fibre -
When the material goes through a leveller or flattener under tension, the first bend gets much more tension on the top and very little compression on the bottom. In the second bend this is reversed -
Consider the second computer analysis as shown in Figure 8, this time with tension on the strip. This stress pattern is not symmetrical. When the material goes through a leveller or flattener under tension, the first bend gets much more tension on the top and very little compression on the bottom. In the second bend this is reversed -
The process is reversed again and again as the material passes through the leveller. By the time the material comes out the exit end, more, or possibly all, of its cross section has exceeded the yield point, not just on the surfaces. All fibres, top to bottom and side to side, have been elongated past their yield points. This results in even flatter and more stable material. It also extends the lower capacity range of any roll configuration.
Producer mills and subsequent processors unwind, roll, heat, cool, and rewind the metal. They're trying to control thickness, flatness, and perhaps other parameters to comply with the customer's requirements.
Unseen but very real opposing forces are trapped inside the metal. No apparent reaction may appear until we machine, stamp, or heat the metal. These processes can break or release some of the trapped opposing forces. Then the material changes shape all by itself. Sometimes the metal just relaxes.
We know that the initial trapped stresses in a coil are random and varying from head to tail and side to side. A spread-
Note that I did not say that trapped stresses are completely eliminated in the levelling process. Trapped stresses will be more consistent and considerably reduced if we work it hard enough. The result is significantly more stable material.
AP DESIGN’S latest innovation is in conjunction with Leveltek United States is to introduce into South Africa the Stretch Levelling process. This produces steel that is stretched beyond its yield point & thus stays flat during post processing like laser cutting & turret punch blanking. Contact AP Design for more information.
If you have reviewed your existing equipment and processes as discussed in Part I of this article series and need to upgrade your flat-
We also need to clarify nomenclature. I'm not sure what the difference is between a flattener and a straightener. No two people I've talked to seem to agree on a definition. In Europe they both are called levellers. Further, all flatteners are not the same. In this article, I will define my own terms to describe the different equipment configurations.
This is the simplest roll configuration and is illustrated in Figure 1. These machines have five to nine large-
This machine is designed for removing coil set only. Rolls are configured to bend the material enough to achieve two yield strains, or twice the distance to the yield point in the outer fibres, or surfaces, of the metal. Thus, elongation past the yield point is strictly on the surface.
If we close the entry roll gap to get more bending, the unsupported centre of the work rolls may deflect, putting edge waves into the metal.
This machine is the next step up in simplicity and is illustrated in Figure 2. The work rolls are smaller in diameter and closer together, and there is very little daylight between them. There are generally 11 to 19 of them -
To eliminate crosswise crossbow in addition to lengthwise coil set, this flattener configuration must be designed to produce four to five lengthwise yield strains in the outer fibbers of the metal.
Option No. 3: A precision roller leveller for both surface-
In a sense, a precision roller leveller (see Figure 3) is similar to a backed-
In a sense, a precision roller leveller (see Figure 3) is similar to a backed-
Like the close-
Unlike a flattener, leveller backup roller flights can be vertically adjusted independently of each other, so the work rolls can be bent deliberately in a controlled manner and held there under load. On a backed-
A leveller, in the neutral setting, like a backed-
Edge wave and centre buckle are edge-
Of these three options, only a leveller with adjustable, independent backup rollers can control or eliminate edge-
The process of setting the entry and exit work roll gaps is similar for all three machine types. In general, the entry gap is set tight, feathering out to approximately material thickness at the exit gap (see Figure 4).
The entry gap setting for any material thickness and yield strength usually is supplied by the equipment builder or designer. The entry gap for a spread-
At one operation I visited, operators were instructed to close the entry gaps enough to avoid slipping when pulling slit coil off an unpowered uncoiler. In all likelihood, that setting would be much too light to get much work out of the equipment. I recommended they refer to the equipment builder's manuals for the proper settings.
Once the entry roll gap has been set, final adjustment for lengthwise coil set, up or down, is done at the exit. If, as is usually the case, the last work roll is on the bottom, closing the gap slightly will induce additional upturn. Opening the exit gap slightly will give some downturn. If the last roll is on top, the opposite will be the case.
Neutral Setting. In the neutral setting, work rolls are straight and parallel, and the backup
rollers are in a straight line, across the face of the machine. This is the setting if coil set or crossbow is the only shape problem. In practice, this would be the case with either a fixed backup flattener or an adjustable backup leveller.
Leveller Roll Bend Setting. Roll bend is available only with the leveller configuration. It is what makes a leveller a leveller. As a practical matter, most full-
The number and spacing of the backup roller flights are important.
The more flights and the closer the spacing, the better. I've seen a machine with too few flights to hold the work rolls straight under load. The unsupported work rolls deflected between the flights.
To Stretch the Edges. In the case of centre buckle or long centre, you need to stretch the edges until they are the same length as the centre (see Figure 5). Set the backup rollers so the work rolls are tight on the edges and loose in the centre.
With the proper roll gap setting, the coil will not slip over the work rolls. The work rolls are all exactly round. The coil has to go a greater distance on the outer edges than in the centre because the sink, or roll, mesh is greater on the edges. This means that we will stretch the edges in a controlled manner.
To Stretch the Centre. In the case of edge wave, or loose edges, you need to stretch the centre until it is the same length as the edges (see Figure 6). Set the backup rollers deep in the centre and shallower on the edges.
Since the material will not slip and it has to go farther in the centre, we will stretch the centre and reduce or eliminate edge waves.
Some processors compromise with a five-
The intermediate rolls on the bottom are eliminated, and the full-
With a five-
The simplest leveller configuration is four-
If such marking is a problem, a six-
Your equipment builder should supply design capacities and settings for its machine for given materials, yield strengths, and thicknesses. But what happens when we run materials beyond these capacity limits?
If the material's thickness is below the machine's minimum limit: We will not flatten or level it. There is insufficient fibre elongation to make any permanent change in shape. No damage will be done to the equipment. Also note:
â€¢ The lower thickness capacity rises in proportion to the yield strength.
â€¢ There is another lower-
Perforating weakens the material and thus raises the upper thickness capacity. It is less obvious that with perforated metals the lower capacity may also rise because of the lack of stiffness.
If the material is very narrow: We run the risk of concentrating the entire load on too few backup rollers. You may bend or break the rollers or their supports. This is why there is no real narrow-
If the material's thickness is above the machine's maximum limit: Loads on the equipment increase very quickly. Vertical separating forces between the upper and lower banks of work rolls are a function of the square of thickness. Don't do it! You may bend or otherwise permanently damage the machine's supporting structure. It is not a matter of how many times it has been overloaded. Once can do the job.
If the material's actual yield strength is higher than the machine's design yield: Machine loads increase as a function of the square of the actual yield strength.
Notice that I refer to actual yield strength, which may be much higher than the nominal yield strength. This is an easy trap. Many grades of metal have only minimum yield strengths, but their actual values can be 15 to 25 percent higher. That can be tough on the levelling equipment if you also are approaching maximum thickness at the same time.
As an example, if your machine's design capacity is based on 40,000 pounds per square inch (PSI) yield but the actual yield strength is 60,000 PSI, that is a (60/40)2 Â¥ 100 = 225 percent increase in the machine load. Maximum thickness capacity should be reduced by 100 divided by 225 percent, or by 44 percent.
The minimum thickness would be increased by (60/40) Â¥ 100 = 150 percent. The minimum thickness rises and the maximum falls. You cannot produce four to five yield strains and cannot fully level any thickness outside the revised yield range on this particular machine. You may be able to produce sufficient yield strain to remove coil set.
The best way to eliminate defects in coil shape is to buy prime material. What you get out of any leveller, flattener, or tension leveller is affected by the flatness of the material you put into it.
We can almost always improve the shape of the metal, but there are limits. If you have good material coming in, you can make it into superb material. If you have bad material coming in, you might make it into good material though probably not superb stuff. Get the best coils you can afford for your job.
Shape control equipment flatteners or levellers generally is the heart of most manufacturer's or service centre ™ coil processing and feeding lines. Don't overload it. Use proper settings. Maintain it. Align it properly. Keep it clean and calibrated.
The surfaces of flattener or leveller work rolls and backup rollers should have been case-
To clean work roll surfaces, put a Scotch-
Note that this roll cleaning can be dangerous; employees must be careful not to get dragged into the machine. An intriguing option, available from one of the leveller manufacturers, is to perform this roll cleaning by remote control so that operators aren't put at risk (see Figure 2).
Work rolls and backup rollers are expendable tooling. Regrind your work rolls more rather than less often. Work rolls and backup rollers must be reground as matched sets. If this is done regularly, only a minimum of material has to be removed with each regrind. On that basis you might get 10 to 12 regrinds out of a set.
Most major leveller manufacturers offer an optional work roll cassette or "works in a drawer." You can pull a roll cassette in and out of the leveller as a unit. You may have more than one cassette. Flip the cassette open and clean it offline. This is generally an expensive option. We sometimes recommend it for a line that's going to run continuously, three or four shifts. For a conventional service centre cut-
Another maintenance option is computer fault-
For instance, it can warn of drive motor and other overloads. If the machinery is approaching low oil level or low oil pressure, the monitor might warn you before a bearing freezes. Or it may tell you which circuit board in which panel is in trouble.
Other Options on New Levellers
Other Options on New Levellers
Optional features on new levellers include automatic roll positioning for each thickness and material so the operator doesn't have to remember the settings. The computer remembers specific coils if you want to do rebooks. Note that for any given coil, the operator still has to make minor equipment adjustments to produce dead-
The addition of an automatic roll calibration control option simplifies calibration after roll changes to adjust for the newly reground roll diameters. Recalibration of a wedge-
The most obvious and common application for levelling equipment is in a cut-
In some very old lines, the leveller is placed after the shear, which means material is levelled in plate or sheet form. While this remains typical of many European lines today, it does have some technical problems. In North America almost all modern lines level the coil before the shearing operation. The arguments about the pros and cons of this are exhaustive, but I think North America has it right.
Except on some very heavy-
Since levellers have a limited capacity range of about four times the maximum or minimum material thickness, it is common for two levellers a big one and a smaller one to be used in a cut-
Remember that the equipment uses the actual yield strength, not the nominal. The practical range for two levellers is closer to six times maximum or minimum.
Some builders have offered a single leveller with two roll cassettes having different roll diameters and spacing. This arrangement still gives a capacity range of about six times maximum or minimum. It is also very complex and can cost almost as much as two separate machines.
A European builder has proposed a multi-
There is also a question about placement of an edge trim slitter in a cut-
Capacities for Processing Heavier Thicknesses
Immediately following the cut-
A Temper Mill With Roller Levelling
A number of recent-
The temper mill cannot get the material as flat as a corrective leveller can, and the leveller cannot get the material as stable or provide as fine a hot-
Slitting Line Configurations Including Inline Levelling
When we slit a coil with crown or a thick centre, the centre coils on the recoiler will have a larger OD than the outer coils. That causes the centre strands to wind faster and the outer strands to wind slower and looser. The outer coils may be so loose that they are dangerous to handle.
Years ago slitter operators stuffed paper into these outer coils so that they pulled tighter. That was very dangerous. ANSI B11.14 (American National Standards Institute) standards for slitting line safety, which apply to line owners and operators, specify you shall not stuff paper unless the line is stopped or the operator is protected. Paper stuffing no longer is necessary. Slitting technology has evolved with the drag and pit configuration.
A friction drag device following the slitter puts tension on all the strands, or mults, going onto the rewind mandrel. The apparent excess length on the outer, looser slit strands is allowed to hang down into a looping pit (see Figure 5) between the slitter and drag device. All strands going onto the recoiler are reasonably tight.
Several different slitting line configurations are available that include corrective levelling after the slitter and before the recoiler. Synchro-
The Strand Extensioner, on the other hand, employs tension assist in the levelling process, which pulls out most of the camber, improves stability, and increases the thickness capacity range (see Figure 7). Significant necking, or narrowing, of slit strands has not been a problem. Since no friction drag is involved, the possibility of surface scratching is significantly reduced.
The line operator can stand at the main control panel, away from the dangerous recoiler nip, and change the amount of elongation in the tight mults so they all wind tightly with the flick of a control lever. Just as with a roller leveller, the line operator can elongate some parts of the coil relative to other parts by adjusting the backup roller flights to deliberately vary the sink or penetration of the upper and lower work rolls at different points along the face of the machine.
Tension levelling is an effective tension-
Tension levelling usually is restricted to gauge thicknesses of metal. Because of the massive equipment and horsepower required, this process usually is not practical for thicker, hot-
How Flat Is Flat?
Measuring flatness, or even describing it, has been a tricky issue. Out-
Producer mills say they can get less than 15 I units of flatness coming off the cold mill. With a temper mill, they can get less than 10 I units. A roller leveller, a Strand Extensioner, or a tension leveller produces less than 5 I units, and maybe less than 1 I unit. That's a big difference in flatness.