DOVER FLEXO ELECTRONICS TENSION MEASUREMENT, DISPLAY & CONTROL
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 lb/3000ft^2.
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 tension
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 closure.
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 deterent.
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.