How does a coil straightener work?

Before coiled material can pass through a die to produce an acceptable part, it must be straightened. Coil straightening is accomplished by bending a strip of material around sets of rollers that alternately stretch and compress the upper and lower surfaces so that the material's yield point is exceeded. Surfaces then are the same length after springback, which results in flat material. Straightening requirements vary depending on material defects, die design, and part requirements.

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Two Ways to Get Straight

Modern straightening equipment falls into two categories: straighteners and precision levelers. Straighteners, or flatteners, are used more in the stamping process. These machines use between five and 11 rolls, depending on the material's width, thickness, strength, and type. Their large-diameter rollers are widely spaced, which removes coil set rather than levels material.

Straighteners are available in powered or pull-through versions. In powered units, an electric or hydraulic motor moves the material to the next operation. In pull-through units, the feed equipment pulls the material through the straightener, which is nonpowered.

The second classification of straightening devices is precision levelers, which, like straighteners, remove coil set but also can remove other material imperfections, such as camber, wavy edges, center buckles, and trapped stresses. Levelers are distinguished by closely spaced, small-diameter rollers with backups.

Normally, a precision leveler has more rollers than a straightener. Rollers have the ability to flex and work the material harder. Unlike straighteners, precision levelers are always powered because of the amount of work they must do to the material.

The Constant Problem of Inconstant Coil Set

Understanding basic straightening principles is necessary to obtain consistent results in the setup and operation of straightening equipment. In theory, three staggered rolls should be enough to straighten most materials. This approach will work if the amount of coil set in the material is constant throughout the coil.

Depending on material composition (thickness and yield strength), the amount of coil set can dramatically increase as the coil is depleted. In most cases, coil set is induced in the material during a previous process, such as slitting, edge conditioning, or finishing.

The outer wraps of the coil are placed under the tension and compression required to bend the material around the coil's OD, usually from 54 to 72 inches. The coil's inner wraps are placed under the tension and compression required to bend the material around the ID of the coil, usually from 16 to 24 in.

The range between the inside and outside diameters of a coil can result in a dramatic change in the amount of coil set. Using only three staggered rollers, the operator has to adjust the machine constantly to obtain an acceptable level of flatness.

Power straighteners are built with multiple work rolls to deal with varying coil set. As more work rolls are used in a straightener, the amount of coil set that can be removed becomes greater.

Straightening Through Thick, Thin

Thick materials require large-diameter rollers. The center distance spacing of these rollers can be relatively large and still do an effective job of back-bending the material. Thick materials normally require fewer straightener rollers. As the material thickness increases, the roller diameters and support journal diameters also must increase. The work rolls must be able to withstand the forces required to back-bend the material without excessive deflection across their width.

Conversely, thin materials require small-diameter rollers. The rollers' center distance spacing should be shorter to stretch and compress the material. Thin materials normally require more rollers to remove varying amounts of coil set in the material.

A consideration also should be given to the support journal diameters of work rolls on light-gauge applications. As the material and machine widths increase, the tendency for small-diameter rollers to flex and deflect also increases.

Balancing Flexibility With Productivity

Stampers struggle to increase volume, quality, and production efficiency while retaining a high level of flexibility. Most companies don't have the luxury of building their capital equipment to meet the needs of a single product line. Therefore, flexibility and capacity should be maximized to meet the demands of changing product lines, customers, and markets.

When choosing a straightener, stampers must think about the trade-offs between producing a single product versus producing a range of products. These options should be explored at the onset of a search for straightening equipment. Some fundamental decisions must be made concerning the level of the straightener's flexibility and the breadth of the demands it must meet.

When determining the level of flexibility needed for the range of materials a straightener will process, stampers must evaluate the maximum material width and the machine along with the range of material thicknesses that will be run. As the width of a straightener increases, its ability to process a material with a given thickness and width is compromised.

The tendency for work rolls and end journals to deflect becomes greater as the machine width increases. For example, a 12-in.-wide straightener with 3-in.-diameter work rolls located on 5-in. centers can process 6-in.-wide, 0.187-in.-thick material. The same straightener configuration but 36 in. wide cannot effectively process the same material because of potential roll deflection. Excessive roll deflection may result in a loss of contact surface area, decreased straightening efficiency, material slippage through the straightener, and broken work rolls.

Because of variations in material type, thickness, and width, a single straightener can't meet the demands of every application. Therefore, during the equipment specification process, all variables associated with the straightening process, such as yield strength and surface finish, should be considered.

Stampers using nonpowered pull-off coil reels, which rely on a straightener's horsepower to pull the material off of the coil and through the straightener, should consider ID, OD, coil width, and maximum coil weight variables. However, for all straightening applications, the maximum line speed must be defined.

Stampers evaluating straightening equipment may find that the more detail they can offer in defining coil variables for a specific application, the easier it will be to work with a machine supplier to select the best straightening equipment.

Bruce Grant is research and development manager for Coe Press Equipment, 40549 Brentwood, Sterling Heights, MI 48310, 586-979-4400, fax 586-979-2970, www.cpec.com. Coe manufactures pressroom feed equipment, including servo roll feeds, power straighteners, coil reels and cradles, cut-to-length systems, and complete coil processing systems.

Considerations in Equipment Selection

The modern metal stamper must build capacity and flexibility into his coil-processing and stamping machinery to meet the challenges presented by evolving product lines and new customers and markets. These overlying challenges present a substantial obstacle when specifying a new straightener. The stamper faces fundamental decisions early in the game related to the ability of the straightener to handle a variety of applications.

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 A heavy-duty straightener (shown) with seven 4-in. dia. work rolls located on 7-in. centers will straighten ¼-in.-thick cold-rolled steel, but will have minimal effect on 0.050-in. cold-rolled steel. Likewise, a straightener designed with seven 3-in.-dia. work rolls located on 5-in. centers will effectively straighten the 0.050-in.-thick steel but will lack the horsepower and roll strength to process ¼-in. material.For example, a straightener with seven 4-in.-dia. work rolls located on 7-in. centers, and given adequate power and gears, will straighten ¼-in.-thick cold-rolled steel. The same machine will have minimal effect on 0.050-in. cold-rolled steel. Likewise, a straightener designed with seven 3-in.-dia. work rolls located on 5-in. centers will effectively straighten the 0.050-in.-thick steel but will lack the horsepower and roll strength to process ¼-in. material. If an application calls for this type of variation in material thickness, a fundamental decision must be made in regards to the cost effectiveness of building a special machine to meet the full spectrum of needs, versus building a standard machine that will provide optimum straightening at either the light-gauge or heavy-gauge end.

Stampers also must consider the maximum width of the material and machine, and the range of material thicknesses to be processed. As straightener width increases so does the tendency for work rolls and journals to deflect under load, thus impacting the machine’s ability to process material with a defined thickness and width. This deflection can result in a loss of contact-surface area, decreased straightening efficiency, material slippage or broken work rolls.

Note: Do not request a machine capable of processing wide material without considering the effect that narrower material will have on the machine. A machine rated to straighten 48-in.-wide by 1⁄8-in.-thick steel may struggle to process the same thickness of steel, but as 12-in.-wide strip. The cross section and strength of the 12-in. material is substantially less than the 48-in. material, but the straightener rolls most likely will experience a greater amount of deflection when running the narrower material, as the forces and stresses are concentrated at the roll center. This area is furthest from the end journals and bearings that support the rolls. (A single row of backup rolls would allow the machine to efficiently straighten the narrower material.)

Horsepower Requirements

Although material thickness and width are fundamental, many additional factors impact the amount of horsepower required, including material yield strength. Most straighteners are rated by their capacity to process mild steel with less than 50,000-psi yield strength. Higher-strength materials will have a greater tendency to keep their coil set, demanding greater horsepower for straightening.

The combination of work-roll diameter and center-distance spacing can dramatically affect horsepower demands. For example, if two straighteners both have 3-in.-dia. work rolls and machine A has 5-in. center-distance spacing and machine B has 6-in. center spacing, machine A will require more horsepower to process material with the same thickness and width.

In a pull-off application, coil size and weight are critical variables in determining required horsepower. The maximum coil weight must be defined, since the straightener motor provides the torque and horsepower to accelerate the mass to line speed. The minimum and maximum coil outside diameter also must be defined. Though a coil has its greatest mass when at maximum outside diameter, this is not als the worst-case condition related to horsepower demands. As the coil is depleted, the straightener loses the mechanical fulcrum provided by the greater outside diameter, and its ability to overcome the drag-brake tension placed on the reel decreases. To address this issue, modern uncoiling systems include automatic drag-brake compensation.

The process requirements for throughput (in ft./min., or FPM) also are necessary to accurately calculate the requirements. To calculate required throughput, multiply the maximum speed of the press by the maximum progression length. For example, a press rated to 40 strokes/min. and a progression length of 18 in. generates a throughput of 60 FPM.

Note: Throughput often is established based on past or current production limitations, rather than on the potential of the equipment and tooling in the manufacturing process.

Achieving Maximum Effectiveness

Once a machine is specified and built, effective results depend on correct and consistent setup. The combination of pinch-roll pressure, drag-brake strength and work-roll depth setting will determine the effectiveness of the straightening operation. All straighteners use entrance-side pinch rolls to grip and pull the material; some also use exit-side pinch rolls to improve grip-and-pull capability. The amount of pinch-roll force required for a specific material depends on material width, thickness and surface condition.

Pinch-roll pressures typically are established by a combination air-pressure regulator and gauge. Heavy-gauge materials generally require greater pinch-roll forces, while thin materials tend to wrinkle under excessive pinch-roll force—which also can result in pinch-roll deflection and a loss of effective contact-surface area on the material, promoting slippage.

The drag brake maintains adequate tension on the strip between the reel and the entrance-side pinch rolls of the straightener. Optimum drag-brake strength varies with coil weight and outside diameter. When the coil is at maximum OD and there is insufficient drag-brake strength applied, the coil will tend to overspin and develop slack material between the reel and straightener. Eventually, the reel will decelerate and lose RPM due to the loss of tension in the strip. As the straightener continues to run, the slack is consumed and the strip will be snapped tight, possibly stretching or damaging the material. Excessive drag-brake strength, on the other hand, may cause material slippage through the straightener or lead to excessive tension on the material.

Establishing Roller Position

Most straighteners include a simple calibrated scale and pointer combination to establish roller position. The amount of work-roll penetration required to back-bend the material to an acceptable level of flatness varies with material thickness and type, roller diameter and roller center-distance spacing. With the optimum depth setting established for a specific material, the stamper must ensure that the work rolls return consistently to this position each time the job runs.

For those applications requiring more accurate positioning, digital roll-height indicators get the call. The upper work rolls of most straighteners are contained in precision-guiding slide-block assemblies. Methods for raising and lowering the rollers within the slide-block assemblies include fine-threaded screw and nut combinations, worm gear and screw mechanisms, and precision screw jacks.

Most often, coils are unwound from the top of the coil, so that the induced coil set naturally gives the material a downward bend. Stock straighteners typically are equipped with an odd number of work rolls, with the extra (or odd) work roll in the lower fixed bank of rolls. With proper setup, this configuration creates a slight upward bend in the material as it leaves the straightener, which helps the material slide across the die surface with a minimum amount of friction.

The guidelines for establishing proper work-roll depth settings tend to vary as much as the potential variations in material types, thickness and width. In addition, different machine builders recommend different setup practices for effective use of their machines. Here we’ll assume use of a seven-roll straightener with three adjustable upper work rolls. Position the first upper work roll to a setting that alternately stretches and compresses the upper and lower surfaces so that 60 to 70 percent of the material crosssection exceeds its yield point. Then position the second upper work roll so that 30 to 40 percent of the material crosssection exceeds yield point. Lastly, position the third upper work roll to bring the material back to a flat condition.

Use the least amount of roll penetration that produces an acceptable level of flatness. Excessive roll penetration will inhibit straightener efficiency, cause material to slip across the straightener, and place unnecessary strain on the machine’s drive components. To set roll penetration, conduct a quick visual check of material flatness before the material runs into the loop area. Then, using the threading table or similar device to support the leading edge of the material as it exits the straightener, fine tune work-roll settings to the minimum depth required to give the leading edge a slight upward bend. Document these settings for reference, to ensure the same setup is used each time the job runs.

MF

See also: Coe Press Equipment Corporation

Technologies: Coil and Sheet Handling

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