Download - AIMCAL Europe Web Handling Conference 2012
A Sensor Development Company
THE INSIDE STORY OF NIPS
Measuring Nip Pressure and Footprint
2012_1
AIMCAL Europe Web Handling Conference 2012
Stefan Wegdell
Managing Director Nip Control AB
1. Process critical nips
2. Easy-to-use hand devices
3. Printing industry background
Nip Control AB
Nips
Outline
1. Why nips are important 2. Formulas 3. Influences from nip temperature 4. Influences from roller/cylinder hardness 5. Sensor influences 6. Measurement methods & strategy 7. Measuring instruments 8. Summary
Nips
Why Nips are Important
• The meeting point of rollers/cylinders, fluids and different solid materials
• The nip itself is nip pressure, nip width, roller and material hardness; forming the nip pressure curve and specific, desired, nip characteristics
Nips
Two distinct categories: TRANSPORT or PROCESSING
Typical transport applications include: • General web feeding • Web tension • Web guiding • Winding purpose
Rollers used for TRANSPORT are not intended to modify the properties of the web in a permanent way
Nips in Different Roller Applications 1(2)
Nips
Two distinct categories: TRANSPORT or PROCESSING
Typical processing applications include: • Graphics • Coating • Priming • Lamination • Chilling
Rollers used for PROCESSING are intended to permanently modify the web or a fluid in a targeted way
Nips in Different Roller Applications 2(2)
Nips
Line Force & Average Nip Pressure
Nips
Line Force FL = Ftot / LN [ N / mm ]
Average Nip Pressure Pavg = Ftot / AN [ N / cm2 ]
F1 F2 Ftot = F1 + F2 [ N ]
Foot Print AN = L N x WN [ cm2 ]
Contact Length LN
Contact Width WN
Foot Print ( Nip ) Soft
Hard
Available information: • Total Force Ftot in [N] on shaft
• Nip Width WN in [cm]
• Nip Length LN in [cm]
CALCULATION
• Pavg = Ftot/LN*WN [N/cm2]
Quick Calculations of Average Nip Pressure
Nips
Available information: • Line Force FL in [N/mm] • Nip Width WN in [mm]
CALCULATION • Pavg = (FL/WN) * 100 [N/cm2 ]
Peak Nip Pressure PPN ≈ Average Nip Pressure Pavg x 1,4 (sinusoidal profile)
Nip Width & Pressure Profile
Nips
1 2 3
cc Δcc
WN
PPN
F
Pavg = Ftot / Anip
General Nip Pressure Relations
Nips
“Anip” increase as much as “Ftot” No Change in Nip Pressure
“Anip” increases LESS than “Ftot” Higher Nip Pressure
“Anip” increases MORE than “Ftot” Lower Nip Pressure
When rubber temperature increases, and maintaining the same impact setting, nip width increases
Approximate rule: increase of 20°C gives +50% increase in nip width
(Rollers for Graphic Printing)
Change of Rubber Temperature
Nips
Pavg = Ftot / Anip
If “Anip” increases more than “Ftot”
Lower Nip Pressure
Example: soft rubber or softening WN
PPN
When rubber hardness increases, and maintaining the same impact setting, the nip pressure increases
Approximate rule: increase of 10° Shore A gives +60% increase in nip pressure
(Rollers for Graphic Printing)
Change of Rubber Hardness
Nips
WN
PPN
Pavg = Ftot / Anip
If “F tot” increases more than “A nip”
Higher Nip Pressure
Example: hard rubber or hardening
Approx rule: +10°C doubles aging speed
Measurements Inside a Nip
Nips
• If sensor blade thickness is equal to web no influence
• If sensor blade thickness is thinner than web add substrate labels
Influence if sensor blade is thicker than web thickness
WN
PPN
Sensor Blade Thickness 0,2 – 0,4 mm
Nip Pressure increases
Nip Width increases
Absolute & Relative Measurement Values
Nips
When web thickness is equal to sensor blade thickness, with or without substrate labels Measured value is absolute and a true level
If sensor blade is thicker than the web Measured value is relative and at an elevated level
Nip Width and Pressure Level Measurements
Nips
Generic Nip Profile
Foot print between rollers
Pressure level
Nip width
Pressure profile inside nip
Sampling of pressure values
1
2
3
Static measurement
Pressure sensitive sensor element
Length sensitive sensor element
Dynamic measurements
Measuring Strategy 1(2) Where to measure
Nips
Left Middle Right
Alignment and straightness
At least three positions
Swelling
Shrinking
Concentricity At least four
positions
Shaft bending
Measuring Strategy 2(2) How to measure
Nips
To think of …… Position of sensor element
To think of …… Inching speed and reverse rotation/sideways measurement
sensor blade
Static mode Semi-dynamic mode
sensor blade
Measuring Instruments Nip pressure in Newton/cm2
Nips
320 Newton/cm2 nip pressure
Pressure Indicator™ • Metal/hard plastics to rubber • Smooth surfaces • 20 - 990 N/cm2 nip pressure (peak) • ≥ 5 mm nip width • 10 - 70°C nip temperature • 0.2 mm sensor blade thickness • Substrate labels for lamination
Measuring Instruments Nip width in mm or inches
Nips
3.5 mm nip width
Roller Nip Indicator™
• Metal/hard plastics to rubber • Smooth surfaces • 1.5 – 20/50 mm nip width • 20 - 50°C nip temperature • 0.4 mm sensor blade thickness
Summary
Nips
• Nips are often process critical
• Average nip pressure calculation formulas are available
• Peak nip pressure is approx. 1,4 times average nip pressure
• Applied force, rubber temperature, rubber hardness and aging affect nip characteristics
• To measure nips, sensor elements inside nips are needed
• To measure is to know!