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!