Pushing back the web speed barrier

In web-based processes, speed is (almost) everything, so much so that competitive forces continue to drive line speeds to new records yearly. A generation ago, a go-fast paper mill whirred along at 2,200 fpm. Today, plant managers boast about the same numbers, only now they are talking in meters per minute: 3.28 times faster! That makes more products available for market, but more importantly, specific labor costs – the largest expense of many web processes – plummet.

Positive control

The one unyielding standard to advances in web speed is quality. It does no good to make product fast if it is off spec. Manufacturing engineers have learned and relearned one simple truth: The faster the speed, the better web-handling surfaces must fit together.

In an ideal sense, a web is a 2-D process; a plane that tracks around rolls that lie on parallel axes. But metrology specialists know that in real life, they have 3-D and even 4-D properties. When a web encounters rolls that are out of the design plane, it can “bag” – even a fraction of a degree of misalignment can cause a buckle or wrinkle. Also, the weight of a web can cause a roll to sag.

Paper is particularly unforgiving of misalignment. Running at 1,800 to 2,200 meters per minute, the dynamics in the sheet include heat transfer and moisture removal – while shrinking. Positive tension must be maintained across the sheet as it passes over rolls and between sections to prevent breaks.

Until recently, alignment faced a technological barrier. Optical systems could maintain roll-to-roll position and parallelism to an absolute accuracy of 0.005 in. over a distance of 50 ft.; but more realistically, global accuracy was closer to 0.060 in. In addition, these instruments require that cardinal points (3 o’clock and 9 o’clock) on a roll be visible, and the level of the roll can only be checked by a second instrument.

Also, repeatability is a problem: Two technicians can measure the same hardware and get different values. Thus, the checking and rechecking is time-consuming. As line speeds moved past 1,000 meters/minute, the lack of accuracy and repeatability were not good enough to prevent web fluctuations that led to breaks. In this respect, alignment, not horsepower, became a kind of empirical speed barrier. A better way was needed to set up the new super-fast webs.

On track

Doug Ursel figured this out on the job. He is the owner of Pine Falls Technical Services Ltd., a metrology-consulting firm in Manitoba, Canada that provides alignment services to the paper and printing industries in Central Canada and the upper U.S. Midwest. Over the years, Ursel has watched machine speeds rise and alignment tolerances shrink, to today’s roll-to-roll standard of 0.002 in./meter of machine length.

“A twist between rolls can lead to a loss of tension and positive control in the sheet. At 1,800 meters/minute, a sag becomes a wrinkle waiting to happen.” 

To evaluate alignment, Ursel uses a laser system that seems to have been designed with paper mills in mind because it can measure extreme range with extreme precision. The FARO Laser Tracker from FARO Technologies, Inc. can locate a spot 230 ft. away. In the ADM mode, distance measuring accuracy is 10 µm + 0.4 µm/m; angular accuracy is 18µm + 3 µm/m. From its tripod setup, it projects a beam that is reflected from a movable target/probe back to the sending unit where the software in the system pinpoints the target. As a technician walks the target along a machine frame or roll surface, the Tracker follows its path, recording position points as it goes, Figure 1.

Figure 1

Figure 1. Tracker follows a movable target that is placed at multiple locations on machine surfaces so that alignment can be analyzed. Photo: Falk P.L.I.

Ursel finds the FARO Tracker ideal for aligning paper machines and printing presses that stretch several hundred meters from end to end. He used to employ the optical systems that were the standard way of doing alignments, but being limited to line-of-sight measuring, they were slow. Also, they could not deliver laser precision.

By contrast, the laser measures rapidly, a feature that is becoming increasingly important. A technician can work around an obstruction, measuring long drive shafts, aligning pillow blocks, or gearbox mounts that are not visible to an optical system. Rolls with tapers do not require offsets in the instrument, and roll level is recorded at the same time the other dimensions are recorded. Eventually, a 3-D image of the measured surface is generated in the instrument’s computer. 

“Plant shutdowns used to last for a couple of weeks. Now we are lucky to get a few days,” he noted, citing a project to align a calendar stack. “We had a day to do this and finished in less than 10 hours; with an optical system, this would have taken twice as long.”

More speed from existing equipment

Most of the news about advances in machine speed comes from new installations, but aligning existing equipment to tighter standards often increases its speed. Ursel provides a couple of cases in point.

Printing presses present unique alignment problems. Because the images on brochures, magazines or newspapers have to be sharp, the four press plates have to be in registration. Ursel aligned a four-color web press to the 0.002 in./m standard and watched as machine speed was increased from 500 to 700 fpm with good registration.