Tension Control Terms

Useful tension control definitions for a variety of industries, including web printing, finishing and converting.

• Accumulator (Festoon): A series of highly wrapped rollers, half of which move together on a carriage, used to store web temporarily for unwind or winding roll changes made without slowing the rest of the line

• Anvil Roller: A smooth and very hard roller which serves as the backup roller for score slitting and rotary die cutting.

• Baggy Web: A web with an uneven natural length or residual tension profile variation.

• 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.

• Braking: A negative external torque on a roller which will tend to increase the web’s tension as it goes over that roller and assist with overcoming deceleration inertia. Braking on an unwind provides a starting tension for the first span. Braking takes energy from the web machine system and turns it into heat (mechanical) or electricity (electrical).

• Buildup: The ratio of the finished wound roll diameter to its core diameter.

• Calendar: To run a web between nipped rollers.

Caliper (thickness): The thickness of a web usually expressed in micrometers or mils (thousandths of an inch).

Caliper (brake type): A disc brake pad or braking mechanism involving opposing brake pads that grip a rotating disc.

• Cantilevered: A roller which is supported on only one end.

• Chill Roll: A roller cooled with chilled circulating water.

• Coefficient of Friction (COF): The ratio of force to slide over the normal (perpendicular) force.

• Concentricity: The distance between the centers of circle and a rotating axis, such as between the shell OD and journal.

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

• Creel: A rack of spools of thread used to feed a weaving loom.

• Damping: The resistance to motion which dissipates vibratory energy.

Dancer: A moving roller sensor used for feedback control of web tension

• DC Drive: The most common drive used on web applications consisting of AC-DC power conversion, controller and DC motor.

• Drive: In web handling, a drive is a means of imparting controllable torque into a roller which may include, for example, electric motors, mechanical brakes and clutches, and transmission components such as belts and gearboxes. In the electric motor industry, the word drive is used either for the controller or for the combination of a controller and motor.

• Dead Shaft Roller: A roller with a stationary central shaft.

• Deflection: The elastic (nonpermanent) bending of a roller due to gravity, tension and nip which primarily determines the required diameter of a roller.

• Deformation: The change of length, width or shape of a material subjected to loads or stresses.

• Die Cutter: A converting component to cut complex shapes via rotary nipped rolls or stamping press.

• Differential Shaft Winder: A center or center-winder with lay-on roller whose shaft is made of slip clutches. The shaft is over sped and torque transmitted through these clutches. This nominally allows more tolerance of a winder to gauge profile problems when winding incompressible materials.

• E-Stop: Short for emergency stop. This invokes a maximum deceleration of a machine to protect an operator, product or machine from further damage after an accident.

• Elasticity: A branch of engineering mechanics that describes the elastic behavior of materials subject to loads.

• Emboss(ers): A patterned nip roller used to pattern a web or bond it together in discrete areas.

• Encoder: A rotary sensor found on drive motors and sometimes rollers. Encoders are used to measure footage, speed and wound roll diameter. A functionally equivalent sensor is a tachometer.

• Endurance Limit: The maximum stress at which a material will not experience fatigue failure at an indefinitely high number of load cycles.

• Face: The effective width of a roller.

• Feedback: Output from a sensor which is input to a controller for maintaining a control setpoint.

• Film: A thin polymer (plastic) web such as found in stretch wrap or garbage bags. Also, a thin liquid.

• Flying Splice: A machine feature which cuts and splices a web from a new unwind roll or onto a new windup core without stopping.

• Foil: A thin metal web.

• Follower: A roller which is slave to run the same (or ratioed) speed of a master roller drive. An example of a follower is a roller which is chain driven from another which is attached to a motor drive.

• FPM: Abbreviation for speed in US units of feet per minute.

• Gain: The multiplier for a sensor readout, or the aggressiveness of a controller.

• Guide Roller: A roller with one end that can be moved to tighten the front side of a web versus the back and vice versa.

• Hysteresis: The phenomenon in which the value of a physical property lags behind changes in the effect causing it, as for instance when magnetic induction lags behind the magnetizing force.

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

• Inertia: The property of a roller to resist changes in speed as quantified by mass times radius squared.

• Inertia Compensation: An automatic adjustment of the gain of a winder/unwind drive controller to avoid sluggish performance at large roll diameters and instability at small roll diameters.

• Journal: A stub shaft at the ends of live shaft rollers.

• Labyrinth Seal: A non-contacting seal used for low friction or light duty bearings.

• Lay-on Roller: A roller nipped against a wound roll. This word has aliases of pack roll (nonwovens, textiles) and rider roller (metals, paper). These terms are usually applied to moving rollers (mounted on pivots or slides) as opposed to drums which are usually stationary.

• Letoff: (Wind, Textile) Alias for unwind.

• Live Shaft Roller: A roller whose central shaft or journals rotate.

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.

• Mandrel: A cylinder upon which a roll is wound.

• Metallizing: The process of adding a thin metal coating to a web, often by electrodeposition in a vacuum.

Nip: Two parallel rolls pressed together on converting machinery between which the web passes.

• Nonwoven: A web material made of (polymer) fibers laid down in a somewhat random orientation.

• Path: The position of the center of a web as it progresses down through a machine as determined by the Normal Entry and other web handling laws.

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.

• Pillow Block: An integral bearing/housing style that mounts via two bolts through its feet.

• PLC: Abbreviation for Programmable Logic Controller. PLC’s are the ubiquitous industrial computers used to provide digital control of machinery. Analog and digital input/output (I/O) modules are used by the PLC to communicate with sensors, actuators and drives in the machine.

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.

• Pull Roller: Any roller which is driven.

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.

• Ribbon: A web whose aspect ratio (span length between rollers divided by web width) is greater than 10:1.

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.

• S Wrap: A pair of highly wrapped rollers, usually for the purpose of achieving a higher drive traction.

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.

• Segmented Roller: A series of two or more coaxially located rollers. By segmenting rollers, smaller diameters can be used to avoid the requirement of a drive.

• Segmented Tension Roll: A series of two or more coaxially located Tension Rolls installed on the same shaft, capable of sensing independent webs oriented in parallel.  A Segmented Tension Roll can also measure a series of tension points across the face of a continuous web width.

• Sheeter: A converting machine to make cuts across the web and stacks the sheets into reams.

• Slip: A mismatch in speed between a web and roller or between a belt and pulley or between a motor and its base speed.

• Slitter: A device used to cut the web lengthwise, often composed of two rotating cutting wheels.

• Slitter Rewinder: A winder equipped with an unwind and slitter.

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.

• Speed Control: The control of motor RPM such that roller surface speed maintains a setpoint or setpoint difference with respect to another roller. Also known as draw control.

• Speed Reference: The roller in a line that is chosen to set the pace for the rest of the machine. There must be one and only one speed reference in any line.

• Splice: To join together two webs end to end by adhesive tape or other means.

Strain Gage: A thin flat electrical transducer for measuring strain that is bonded to a body of interest.

• Starring: A defect, often caused by poor roll structuring, where the wound roll collapses and buckles internally. The visible result might appear as a single radial spoke caused by a handling impact, or as a series of evenly spaced radial spokes.

• Stretch: The ultimate strain of a material expressed in %.

Substrate: The material composition of a web.

• Tachometer: An RPM sensor for measuring speed. The most common tachometer is a DC generator mounted to the end of a drive motor where the output voltage is proportional to speed.

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.

• Telescope: A defect whereby the inner layers may slip or break loose on a winding or unwinding roll. MD interlayer slippage allows the roll to shift axially. This term had been (and is still) used for many distinctly different defects with a similar resulting appearance. For example, it may be used for when roll was wound crooked (dished) or shifted axially in storage.

• Tension: Stresses or forces which pull on the material. Lineal tension for webs is expressed in lb/in in US units and kN/m in metric units. Stress tension for materials is expressed as psi in US units and kPa in metric units.

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 deterrent.

Tension Zone: A length of machine in which the web is under nominally the same tension, usually between driven rollers.

• Thermal Expansion: The increase in length, width, and thickness of a material at increasing temperatures.

• Tin Canning: A defect common in thin films and seen on paper where the roll has a series of nearly uniformly spaced ridges.

• TIR: Abbreviation for Total Indicated Runout, is the change of radius as a function of rotational position on a roll or roller.

• Torque: (1) On brakes or motors, it is force x moment arm in lb-ft or N-m. (2) In winding, it is shorthand for torque differential between two motors such as on a roll and roller. In a two drum winder it is the torque differential between the front and back drums. This is also called winding force by one winder builder.

• Transducer: A sensor or load cell.

• Treater: A process of modifying a web by coating or other means. In film, this refers to bare roll, corona treat or flame treating of the surface to make it easier to print on.

• Turret Winder: A winder with two or more axis that follows a continuous maker. Each axis processes the next roll in turn without interrupting the maker.

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.

• Walking: (aka wandering) Sideways movement of a web and loss of edge position which may require an edge guide to correct.

Web: A long, thin, flexible structure. Common web materials include paper, film, foil, nonwovens and textiles.

• Web Handling: The science of web transport at maximum speed and minimum web damage.

• Wrap Angle: The angle between the ingoing and outgoing tangent of a web on a roller, or equivalently, the angle the web deflects as it goes over a roller. High wrap angles help ensure web/roller traction.

• Wrinkle: A generic term for a variety of out-of-plane web buckling defects. Wrinkles oriented in the machine direction are caused by cross direction strain, and wrinkles at a slight angle are caused by shear stresses.