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15.1
Web 101.15SM – Nip Load Quality Control
Nip Impressions
David Roisum, Ph.D.
Finishing Technologies, Inc.
SM
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15.2
Nip - Definition
• What
– Two parallel rollers running against each other
• How
– Load control (cylinder force)
– Position control (cylinder forced to stops)
• Why
– Web Transport (move)
– Web Processing (modify)
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15.3
Nips
Transport (move web)
• Pull Rollers
• Winding
Process (modify web)
• Calendering
• Coating
• Corrugating
• Embossing
• Laminating
• Printing
• Rolling
• … etc
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15.4
Nip Themes
• Underlying physics applies to all nips
– See Mechanics of Rollers and other literature
• A uniform nip is vital for a uniform product
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15.5
What the Web Sees
• A pressure pulse
– Nip width called ‘footprint’
– Peak pressure• Sometimes does the ‘work’
– Nip Load• Sometimes does the ‘work’
• Area under curve
• Force imposed by machine
• Process setting
– Impulse (pressure x time)
– Duration (width / speed)
Footprint
Peak P
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15.6
Peak P from Nip Width
• Measure or calculate linear nip load p (lb/in)
• Measure contact width b (in)
– Nip impression paper
– Sticky notes
– Ruler
• Calculate peak pressure
σmax (lb/in^2) = 1.277 x p / b
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15.7
Nip Control
• Load Control by Cylinders
– Calendering, Pressing, Rolling
– Winding
– Etc
• Position Control by Stops
– Coaters
– Printers (many)
– etc
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15.8
Load Control
• Load Force is controlled by Cylinders
FixedRoller
MovingRoller
Pneu or Hyd.Cylinder
Pivoting ArmOr Slide Guide
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15.9
Nip Control Variation
• Load Control Variation Sources
– Cylinder Friction (pneu vs hyd)
– Control Valve Hystersis
– Pivots/Slides etc
• Load Variation Measurement
– Measure Open Pressure
– Measure Close Pressure
– Calculate Uncertainty from the Difference
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15.10
Nip Quality Measure Example
• Pressures
– 60 PSI Run
– 50 PSI Raise
– 40 PSI Lower
• Control P = 60 – 50 = 10 psi
• Uncertainty P = 50 – 40 = 10 psi
• Pk-Pk variation = 100 %
0.0
1.0
2.0
3.0
4.0
Time
Loa
d (kN
/m)
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15.11
Nip Load Control Sanity
• Nip Load Zero (0 on dial = 0 nip)
– Bias pressure one side of cyl cancels weight
– Control pressure on other side of cyl
• Nip Load Calibrated in lb/in or kN/m
– Overlays on Pressure Gage
– PLC Calculation
• Nip Load Side-to-Side Bias
– Two edged sword
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15.12
Gap/Interference Control
• Position is controlled by Stops
FixedRoller
Gap orInterference
Hand Crank orStepper Motor
Thread
Thread
Nut
PositionedRoller
Top Taper Half
Bottom Taper Half
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15.13
Position Control Variation
• Variation in
– Web caliper etc
– Roller runout
• Hysterisis of Stops
– Backlash
– Compliance
• Values
– Adjustable to millionths of an inch
– Repeatable to thousands of an inch
Input Setting Position
SpringBacklash
ReplaceRoller
One Setting
Four Results
Output Po
sition Results
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15.14
Nip Variation Sources
• Roller Radial Runout
• Roller Diameter Variations
• Roller Shell Wall Deflection
• Cover Hardness Variations
• Loading System Variations
• Roller Misalignment
• Roller Deflection
• Roller Crown/Load Mismatch
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15.15
Roller Radial Runout
• Bumps cause vibration
• Bumps may cause product Barring
• Bumps may resonate or even self-excite
• Measured as TIR (Total Indicator Runout)
• Measured with a dial indicator
Π x Dia
Barring PatternDialIndicator
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15.16
Roller Diameter Variation
• Measured with a caliper, saddle mic etc
• Measurements
– Peak-to-peak (maximum variation across roll)
– Station-to-station (maximum gradient)
• Usually most problematic
Pk-Pk
Max
Gradie
nt
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15.17
Roller Shell Wall Deflection
• Aka ‘Oil Canning’
• Thin wall bends like a squeezed pop can
• Wall may be thin when t < Dia / 20A
SectA - A
SectB - B
A
B
B More Info: MOR pp 80
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15.18
Cover Hardness Variations
• Covers may vary in hardness across the
width due to nonuniform manufacturing
• Covers may change hardness nonuniformly
due to local hardening
• Hardness of quality covers usually varies
less than 5% (P&J or Durometer)
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15.19
Roller Misalignment
• Position Misalignment
– Unintentional roller misalignment w very stiff x-tie
– Intentional? biased adjustments of stops
• Load Bias
– Intentional bias of load settings
– Unintentional bias of friction, weight etc
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15.20
Roller Deflection
• Roller deflect under nip load
• Causes a bowing away in the center
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15.21
Crown/Load Mismatch
• A crowned or barrel
shaped roller can be
cut to fill in the ‘gap’
• However, crown is
only correct for one
load
Light Nip Load and/or Heavy Crown
Heavy Nip Load and/or Light Crown
Matched Load/Crown
More Info: MOR pp 32,33,81,82
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15.22
Nip Deflection Compensation
• Roller Diameter Increase
• Roller Journal Length Decrease
• Crowning
• Skewing
• Journal Bending
• Thermal Profiling
• ‘Swimming’ Rollers
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15.23
Nip Quality Control Checks
• Roller Radial Runout
• Roller Diameter Variations
• Roller Shell Wall Deflection
• Cover Hardness Variations
• Loading System Variations
• Roller Misalignment
• Roller Deflection
• Roller Crown/Load Mismatch
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15.24
Nip Impression
• Static
– Close gap only
– Most common
– Most versatile
– Least Sensitive
• Dynamic
– Roll Through Gap
– Most Sensitive
1 2 3
1 2 3
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15.25
Reading Nip Impressions
Crown Too High for Given Load Crown Too High for Given Load
Unbalanced Loading or Misaligned Unbalanced Loading or Misaligned
Banding - Roller Wear, Grinding Banding - Roller Wear, Grinding
Crown Too Low for Given Load Crown Too Low for Given Load
Good Good
Static - Nip Width Dynamic - Nip Pressure (color)
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15.26
Nip Impression Materials
• 1 and 2 component carbonless papers
• 1 and 2 component films (Pressurex)
• Embossed foil
• Sources
– Sensor Products Inc.
– (Paper) Machinery Builders
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15.27
Other Nip Checkups
• Simple
– Sticky Note gaps
– Grease bag gaps
– Flashlight test
• Electronic
– Electronic Nip Impression
– Pressure sensitive sensors FSR
• Roller
– Saddle micrometer
– Laser micrometer
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15.28
Nip PM
• Typical
– Customer complains about streaks
– Brainstorm possible nip nonuniformity
– Where is nip paper?
– Order nip paper
– Nip uneven
• PM
– Old roller condition – probably bad
– New roller condition – should be OK
– Mid life roller condition - this is PM• Roller ID and history folder
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15.29
Nip Impression Safety
• Nip impressions should not be taken unless all people involved have been trained and tested in machine safety and nip impression safety.
• Nip impressions should not be taken unless all people involved can and will take all precautions such as listed here.
• The nip should not be closed until the operator has made certain that all personal are clear
• The nip should not be closed until the operator has a clear view of the entire length of both sides of the nip.
• The nip should not be closed until the operator has loudly yelled “Closing Nip”and waited for at least 5 seconds.
• The drive should not be engaged until the operator has made certain that all personal are clear
• The drive should not be engaged until the operator has a clear view of the entire length of both sides of the nip.
• The drive should not be engaged until the operator has loudly yelled “Drive On”and waited for at least 5 seconds.
• Consult your plant and machine builder for lockout tag out procedures for your machine.
• The nip must be blocked open when personnel are inside the machine.
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30
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