EPS customers receive an important benefit beyond the clean, consistent oil-free EPS surface; they receive superior flatness due to the EPS Coil Line’s tension leveling capability. But occasionally when processors visit the TMW EPS Coil Line, they look confused and ask ‘Where exactly is the tension leveler?’ They’re looking around for a ‘traditional’ tension leveler with large bridle rolls, like what they may have seen at an acid pickling facility. They soon learn that EPS Coil Line tension leveling is done differently, but achieves the same result.
First, it’s worth noting that not all push-pull acid pickling lines use tension leveling. In fact, rather few do, even though tension leveling brings clear benefits:
- Removes shape distortions that make feeding strip through the line difficult and can damage downstream equipment;
- Breaks scale loose, which lessens duty on the acid tanks so the line can run faster;
- Produces coils with excellent flatness and reduced shape memory.
The drawback to traditional tension levelers is cost. There’s more than bending rolls and bridle rolls – it takes very large, high horsepower motors to generate the 60,000+ pounds of tension needed to achieve the 0.5% to 0.6% strip elongation typically needed to break loose scale and erase shape memory.
Traditional Tension Leveler Configuration (click on image to enlarge)
Dale Minton, President of ADS Machinery, knows. ADS designs and builds a wide array of mill-duty strip processing lines and equipment for picklers, galvanizers and coil coaters. That includes a number of tension leveling systems it has integrated into acid pickling lines. “Tension leveling is a ‘no-brainer’ for continuous pickling lines because they generally run a narrow range of lighter gauge,” explains Minton. “But for push-pull lines, tension leveling works well for 1.5mm to 5.5mm (0.060” to 0.216”) thickness, but not much beyond that. For a 12.5mm X 1830mm (0.5” X 72”) strip, it would take 200,000 pounds of force to get the elongation needed. That would require enormous equipment . . . at enormous cost.”
So how does the EPS Coil Line accomplish its tension leveling? First, EPS is a ‘tight line’ process; to descale, rinse and dry at maximum effectiveness, the strip must be very flat as it traverses the line. That flatness is accomplished through tension, but the tension is built into the EPS Coil Line, rather than requiring additional equipment. How? The line’s uncoiler generates the drag force and the recoiler generates the opposite pull force. Together, they combine to create total tension equivalent to that of the bridle rolls. That tension achieves roughly 0.6% elongation for the maximum cross section where the EPS line uses the bending rolls for scale braking: 5mm x 1830mm (0.190” X 72”). For smaller cross sections, the elongation will be more, depending on material yield strength.
EPS Coil Line Tension Leveling Configuration (click on image to enlarge)
In essence, the EPS Coil Line uses its uncoiler and recoiler to do the work of a conventional tension leveler’s bridle rolls, but without their cost or their floor space. The tension leveler bending rolls are housed in a compact unit which will typically be supplied by ADS Machinery for future EPS Coil Lines.
Why can’t acid pickling lines use the uncoiler/recoiler approach to tension level the strip? Acid pickling is decidedly not a tight-line process. The strip must sag to stay immersed in the acid and lift as it emerges from tanks. So the tension must be applied upstream of the acid tanks, using the bridle rolls, and relaxed in the tanks.
In summary, the EPS Coil Line achieves the same tension leveling as the best of push-pull acid pickling lines, but without the $4 million tension leveler price tag and the additional 14 meters (46 feet) of additional line length (and, therefore, building length) a traditional tension leveler requires.