Page 38 - MetalForming June 2012

The Basics of Coil Processing Equipment, Part 2
ening 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 sub- stantially less than the 48-in. material, but the straightener rolls most likely will experience a greater amount of deflec- tion when running the narrower material, as the forces and stresses are concentrated at the roll center. This area is fur- thest 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-dis- tance 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 pro- vides the torque and horsepower to accelerate the mass to line speed. The minimum and maximum coil outside diam- eter also must be defined. Though a coil has its greatest mass when at maximum outside diameter, this is not always the worst-case condition related to horsepower demands. As the coil is depleted, the straightener loses the mechanical ful- crum provided by the greater outside diameter, and its abil- ity 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 require- ments. To calculate required throughput, multiply the max- imum 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 com- bination 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, pro- moting 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
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36 MetalForming/June 2012
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