Web Tension Terms
Useful terms used in web printing, finishing and converting
Basis Weight: The mass per unit area of a web. Common metric
units are in g/m^2. English units for the paper industry are often in
Caliper (thickness): The thickness of a web usually
expressed in micrometers or mils (thousandths of an inch).
Caliper (brake type): A disk brake pad or braking mechanism
involving opposing brake pads that grip a rotating disc.
Converting: The processing of a web material from one form to
another. Converting processes include calendaring, coating, die cutting,
embossing, laminating, printing, punching, sheeting, slitting, treating,
winding and unwinding.
Core: A hollow tube, often of fiber, plastic or metal, upon which
a roll is wound.
Core shaft: A mandrel upon which rolls are wound
Dancer: A moving roller sensor used for feedback control of web
Idler roller: A roller which is driven by the web rather than
by an electric motor, belt or other external means
Intermediate zone: An independent tension zone typically created
between two driven nip points on a converting line
Load Cell: An electronic sensor that measures force. On converting
machinery, load cells under the ends of an undriven roller are often used
to measure web tension.
Nip: Two parallel rolls pressed together on converting machinery
between which the web passes.
PID control: Abbreviation for Proportional,
Integral and Derivative control. A common three-function algorithm found
in closed-loop controllers for automatically matching the control output
to a set value of interest. Most tension controllers on the market that
use a tension measurement input from transducers or load cells use PID
for tension control. DFE uses the terms "Gain", "Stability"
and "Response" to describe each component of PID control because
they are more descriptive of each. Read our Tension
101 Primer for a simplified explanation of PID control.
PLI: Abbreviation for Pounds
per Lineal Inch. A unit of tension measurement expressed as the
total force (in pounds) on the web in the machine direction (MD) divided
by the width (in inches) of the web. Expressing tension in PLI allows
comparison of typical tensions between various width webs or various web
substrates. See the chart for Typical Recommended
Running Tensions for Common Web Materials .
RATIO: An output feature on a tension
controller that multiplies (for unwind applications) or divides (for rewind
applications) the sampled control output by a factor adjustable between
1 and 10. The ratio feature allows an instantaneous change in control
output to correspond to the roll diameter change that takes place during
a flying splice.
Rewind Zone: A tension zone, typically on converting machinery,
created between a driven nip roll or other tensioning point and the driven
core onto which the web is wound.
Roll: A web in wound roll form. This term is also used in the
converting industry for rollers.
Roller: A rotating cylinder used for web transport. Aliases include
idler rolls, idler rollers, drums, rolls, pipe rollers.
Sample and Hold: A control feature
that locks the tension controller output at whatever level it is at when
an external contact closes. The lock is maintained until the contact opens.
Used in flying splice applications to prevent instability during the splice.
Also actuated by the RATIO function.
Soft start feature: A tension controller
feature used in unwind zones; soft start causes the controller output
to drop to a preset low level to prevent brake lockup when the machine
starts; the feature is actuated automatically upon loss of tension below
a preset trip point, by a change in machine speed, or by an external contact
Strain Gauge: A thin flat electrical transducer for measuring
strain that is bonded to a body of interest.
Substrate: The material composition of a web.
Tension Transducer: A tension sensor and variation of a load cell specifically
designed to measure exact web or filament tension in processing machinery.
Tension Limit Switch (TLS): A controller
feature that provides a relay contact closure at preset tension levels,
either high or low. TLS is often used as a web break detector or web break
Taper tension feature: A means of decreasing
web tension as roll diameter increases in a rewind zone; Taper tension
helps produce a roll of better quality by eliminating telescoping, crushed
cores, and overly tight or loose rolls.
Tension Zone: A length of machine in which the web is under nominally
the same tension, usually between driven rollers.
Unwind Zone: A tension zone created between a driven roll or
driven nip and the core from which a roll is unwound. Tension is
often created by torque applied to the unwind shaft by a pneumatic brake.
Web: A long, thin, flexible structure. Common web materials
include paper, film, foil, nonwovens and textiles.