Page 36 - MetalForming June 2012

The Basics of Coil Processing Equipment, Part 2:
Straightening the Coil
In part one of this series (January 2012 issue) on coil-processing equipment, we covered unwinding the coil. Here we focus on material straightening,
to allow the sheet to pass freely through the die.
BY JIM WARD
The purpose of straightening in a
typical coil-feed line is to prepare the material to allow it to pass freely through the die and produce acceptable parts. Requirements vary depending on any material defects, the design of the die and finished-part requirements. Straight- ening is accomplished by bending the strip around sets of rollers that alternately stretch and compress the strip’s upper and lower surfaces, exceeding material yield point so that both surfaces end up the same length after springback. The result: flat material.
Types of Straightening Machines
Straightening machines fall into two basic categories— straighteners (or flatteners) and corrective levelers. Straight- eners typically operate five to 11 work rolls whose diameter and center distances vary depending on workpiece-materi- al thickness and width. Generally, rollers are fairly large in diameter, widely spaced and not backed up. Straighteners will remove coil set from the material, allowing it to pass unre- stricted through the die and satisfying most applications.
Jim Ward is general sales manager, Coe Press Equipment, Sterling Heights, MI: 586/979-4400; www.cpec.com.
Power straighteners can be configured as part of the unwinder, as in the case of coil cradles, or for pull-off operation with coil reels. They also can be free stand- ing with a second slack loop between the straightener and unwinder—as with pallet decoilers or in cases where delicate material would be damaged by pulling off of
a large coil.
Corrective levelers will remove not only coil set but also camber, wavy edges, center buckles and trapped stresses within the material, so that it will stay flat after processing through a die. These machines are distinguished by small- diameter closely spaced rolls—with backups—and the abil- ity to flex the rolls. They normally have a greater number of work rolls than conventional straighteners, and since they work the material much harder and their rolls can be flexed, precision levelers always are powered. Therefore, they require more powerful drives than do straighteners.
Straighteners, on the other hand, can be powered or non- powered—called pull-through straighteners. As the name suggests, here the feed provides the power to pull the strip through the straightener. The advantages of this style: low cost, and, since the straightening operation occurs after the loop, loop length can be condensed without the worry that coil set will be reinduced into the material.
With pull-through straighteners, horsepower must be drawn from the feeder. This can either reduce its speed capability or greatly increase cost. Additional disadvantages
34 MetalForming/June 2012
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