5A – Force Control Nip Variations

Force-controlled nip rollers or winding nip magnitude is too high, too low, or zero.

Problems with nip loads or pressures too high:

  • Creates roller, core, or winding roll deflection and associated problems
  • Rubber coverings wear more quickly, overheat, or come apart.
  • Product will be overly compressed and may be permanently damaged.
  • Web may break.
  • Rolls may wind too tight, forming hard roll defects.

Problems with nip loads or pressures too low:

  • Drive point slips with small tension differential.
  • Drive point air lubricates and slips.
  • Laminating forms low bond strength between laminate layers.
  • Laminate will have air bubbles.
  • Wound rolls will have air entrainment problems.
  • Embossing and microreplication process will not reach intended feature depth or height.
  • Product density is not changed enough.
  • Roll fails to wind tight enough, forming soft roll defects.
  • Nip winding forms slackness ahead of nip point, potentially forming accumulation wrinkles.
  • Calculate the load created by roller assembly effective weight, counter weights (if used), and pneumatic (or hydraulic) loads.
  • Adjust supply pressure to determine minimal pressure required to open and close the nip.
  • Measure pressure-dependent product properties.
  • Monitor roller wear, re-grinding, and re-covering rates.
  • Calibrate nip load with force gauge.
  • Calculate roll weight vs final roll size and nipping geometry.
  • Check geometry of nipping roller closing direction aligns to fixed roller rotational axis.
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5B – Gap Control Nip Variations

Position-controlled nip roller forces are too high, too low, or zero.

Problems with nip loads or pressures too high:

  • Creates roller, core, or winding roll deflection and associated problems
  • Rubber coverings wear more quickly, overheat, or fall off roller core.
  • Product will be overly compressed
  • Web may break.
  • Rolls may wind too tight, forming hard roll defects.

Problems with nip loads or pressures too low:

  • Drive point slips with small tension differential.
  • Drive point air lubricates and slips.
  • Laminating forms low bond strength between laminate layers.
  • Laminate will have air bubbles.
  • Wound rolls will have air entrainment problems.
  • Embossing and microreplication process will not reach intended feature depth or height.
  • Product density is not changed enough.
  • Roll fails to wind tight enough, forming soft roll defects.
  • Nip winding forms slackness ahead of nip point, potentially forming accumulation wrinkles.
  • Calculate load, force, or pressure from rubber compression model.
  • Use dial indicator or LVDT to calibrate the “zero” / minimal contact and measure the difference between zero position and controlled nip gapped or indented position.
  • Ensure position-controlling stops are equal on both sides of nipped process.
  • Check to ensure closed nipping roller reaches and stays on gap controlling stops.
  • Measure the force to contact with gap controlling stops.
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5C – Nip Load Transients

The force or pressure created by nipping rollers (or nipping against a winding roll) varies with time or process events and creates problems from being too high or too low.

  • Nip-controlled product properties will have MD variations
  • Drive points may have intermittent slip if the low nip load is insufficient to maintain the desired tension differential.
  • High and low nip load problems occurring over time. See Problems of Force-Loaded Nip Variations.
  • In nipped processes, nip transients may be measurable in product properties, such as bar marks or thickness variations.
  • Short of bouncing open and closed, it may be difficult to measure nip load transients in nips at drive points.
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5D – CD Nip Load Variations

The web tension distribution varies cross machine (or crossweb) direction (CD) from operator to drive side of the web.

  • Pressure-dependent product properties will have CD variations.
  • Web will have CD shift, bending or converging and wrinkling.
  • At drive points with larger tension differentials, the web will slip through in low nip load lanes and not in high load lanes, shearing, shifting, and wrinkling the web.
  • Both high and low variations will occur across the nip width. See Problems Force-Controlled Nipping Too High or Too Low.
  • Check to see if a bright light can be viewed all along the CD length of the ‘closed’ nip’s contact zone.
  • Sample and measure product pressure-dependent properties vs width.
  • Measure nip MD footprint length vs width by any of several methods.
  • Measure pressure vs MD and CD position with electronic pressure mapping system.
  • Install load cells to measure and confirm the forces of pressurized cylinders.
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