The average web tension is above the desired level (too high) or below the desired level (too low) or the web is physically slack (no tension).
- Width losses leading to:
- out-of-specification width loss while handling under tension
- out-of-spec slit or die cut width increase with no tension.
- loss of edge trim
- mismatched widths at laminating
- loss of edge margin at coating
- roll sidewall misalignment at winding
- Width losses are is more significant for low modulus webs, wide webs, and webs with high Poisson’s ratios.
- Web yields.
- Web or splice breaks.
- Print or die cut patterns are out of tolerance, both shape and phase.
- Wrinkle sensitivity increases with higher web-roller traction.
- Wrinkle from roller deflection.
- Winding or unwinding rolls cinching and telescope.
- Wound rolls are too tight, leading to tight roll defects, such as blocking, core crushing, starring, spoking, and yielding (that creates bagginess).
- Crashing into air floatation systems (air turns, floatation nozzles).
- Web slips on idlers, causing scratches, lateral shifting, and guiding system failure.
- Web sags excessively in horizontal spans.
- Web fails to peel and wraps rollers, breaking the web or damaging the equipment.
- Web slips on pacer drive rollers causes a loss of speed and tension control.
- Web slips on follower drive rollers causes a loss of tension control or draw ratio.
- Web sails laterally in air flotation ovens and turns.
- Web flutters, especially in long spans.
- Wound rolls are too loose, causing telescoping in handling, winding, or unwinding.
- Wound rolls are too loose, sagging, and out-of-round.
- Wound rolls of pressure sensitive adhesive products have peaking and gapping defects.
- Web tension (high or low) is measured with a load cell roller.
High:
- Dancer roller swings to full dispensing side of motion.
- Web breaks.
- Yielding may be difficult to measure except by excessive necking (width loss) and unreasonable speed increases from elongation.
- Yielding may be measurable by permanent width loss or elongation in printed or other MD registered patterns.
- Web makes contact on air bars and turns.
Low:
- Dancer roller swings to full accumulating side of motion.
- Look for large sag in horizontal spans
- Idler rollers fail to turn.
- Web has larger shifts in air flotation ovens.
The average web tension varies with time, whether oscillating, spiking high, or dropping low.
Tension variations of +/- 10% are common. Tension variations of +/- 20% may be no problem for insensitive processes.
Tension dropping to slackness(zero tension) will cause problems for nearly any process.
- Web breaks from high tension or when slack web suddenly re-tensions.
- Web breaks or downtime from wrapping rollers.
- Coating or extrusion process variations.
- Intermittent scratching from roller slip.
- Lateral oscillation from traction shifting from ‘stick’ to ‘slip.’
- Roll telescoping from internal roll cinching (MD slip).
- Dancer roller motion increases.
- Motor torque increases or decreases.
- Web tension will vary over time as measured by a load cell roller.
- Web width will shift from wide to narrow.
- Sag in horizontal spans will increase and decrease
- Idler rollers will shift from turning to not turning.
- Visible bagginess will increase and decrease.
The average web tension changes in the machine direction (MD) from upstream to downstream location within a tension zone.
See list of problems under Tension Too High or Too Low.
- Install two load cell rollers in one zone – one at the beginning and one at the end of the zone. The differential of the tension readings will show the total MD tension variations in the zone.
- Web width will shift from wide to narrow (or narrow to wide) as it passes through a zone.
- Identical idler rollers with identical wrap angles will turn in one end of a tension zone, but not the other.
The web tension distribution varies in the cross machine (or crossweb) direction (CD) from operator to drive side of the web.
- Web breaks or yields when edge stress is too high.
- Wrinkles on rollers (especially equipment-induced one-sided tension variations or loose center baggy web in long spans).
- Wrinkles at nips (if portions of the web are at or near zero tension).
- Flutter (in portions of the web are at zero or low tension).
- Low tension lanes or edges are visibly loose, buckling ahead of rollers and nip points, sagging more in horizontal spans, or fluttering.
- Several options are available to measure crossweb tension variations, but few are used in production.
- Monitor tension roller differences of drive side vs operator side load cells. (Web must be centered on well-aligned tension roller.)
Tension may vary across the multiple narrow webs after slitting.
This section addresses tension variations in the strands between slitting and winding.
- Poor slitting quality where tension is low or zero.
- Lateral shifting of strands from low tension.
- Slit roll quality variations, both too tight (e.g., telescoping) and too loose (e.g., CD buckles).
- Low tension strands are visibly loose, sagging more in horizontal spans, fluttering, and shifting laterally.
- Slit rolls quality will vary between CD position with some percentage of rolls with too tight, too loose defects, or both.
- In advanced systems, usually with dedicated slit widths, each slit strand will have an individual load cell roller. This approach is rarely done due to the high cost of multiple load cell rollers, the productivity loss of changing rollers with slit width changes, or the lack of justification where no problem exists.