A Glossary of Descriptive TermsSecond Installment

Published: October 01, 2000, By William E. Hawkins, Contributing Editor

Last month I began a glossary of terms used frequently to describe equipment, process events, and product quality problems in the converter industry. The second installment follows:

• Dancer roll

  : A roll that usually is mounted near the unwind stand and changes its position as the web tension increases or decreases. Its running position determines the amount of reverse web tension (brake pressure) to apply so that the web payoff tension is constant. Also, dancer rolls are used in special situations to control the windup tension. When a dancer roll is used to control the winding tension, another signal in addition to roll running position must be sent to the winding motor control panel. This signal must indicate winding roll diameter so that taper may be applied to the winding tension control program. The greatest asset of the dancer roll is its ability to adjust to web path length changes that may occur due to eccentricity of the web payoff roll.

• Dead band

  : An area of a sensor that does not promote change in the signal being supplied to the control station. A wider dead band in the sensor allows more web deviation from the desired set point. The sensor may be sensing alignment of the web path, web tension, transparentness of web, or web continuity. Dead band width does not increase or decrease machine response time (gain control), but a wider one can be used to reduce overcontrol of web steering devices by not disturbing the steering device with minor web edge flutter.

• Deflection

  : Usually used to describe the maximum amount of bending that takes place when a force such as a nipping roll is applied to another roll surface. Two examples where such bending occurs are tension isolation nip rolls and lay-on (contact or rider) rolls. This term sometimes is used when describing the total amount of bending that a mandrel under- goes during winding.

• Dielectric

  : A material that does not conduct electrical current through its matrix body. However, most of these materials will give up or add surface electrons easily when an incentive presents itself. Thus, significant static charge can occur on a dielectric material, especially one that is running over rolls in a coating and drying process.

• Driven rolls

  : Rolls that are driven by either the machine's main drive train or a separate auxiliary drive motor. Rolls that are driven by surface contact only are not considered driven rolls.

• Eccentricity

  : The out-of-roundness of a roll, core, or mandrel. Eccentricity usually is expressed as TIR (total indicated runout) in mils. When eccentricity is present, points on the roll surface do not rotate in the same axial circle.

• Edge sensor

  : A device used to monitor the film edge as it moves through the machine. It signals the steering roll(s) control system the web is not on the desired web path centerline. The control system uses this signal to move the web back to the machine centerline.

• Elastic limit

  : The point of elongation that a web material obtains under tension where it will not return to the original length when the tension is removed. Permanent web deformation takes place when the elastic limit has been reached or exceeded.

• Elastomer

  : A material that behaves like rubber but is made from synthetic polymers and is superior to rubber in several mechanical or chemical properties. Elastomeric roll covers can be tailor made to fit many web processes that are beyond the capabilities of natural rubber.

• Electrostatic charge

  : Electrical charges that are trapped on the web surface. The polarity of these charges may be either positive or negative. Electrostatic charges may collect on dielectric materials through the exchange of electrons across the surface interface with either conducting or non-conducting materials. Thus, electrostatic charge may build up on a web when it passes over a roller with either a conducting or non-conducting surface.

William E. Hawkins has more than 30 years of process and equipment development in web handling, including experience on all types of converting equipment. He specializes in thin web applications. Contact him at 740/474-5840; fax 740/474-3148.