- December 01, 2000, William E. Hawkins, Film Handling Solutions Ltd.
This column is a continuation of the glossary of terms that I began in the September 2000 issue for the purpose of improving communications in the converting industry. Number two was in the October issue, and the third installment follows.
: A device that measures and signals a control system how far the subject part has moved relative to a fixed reference point. This device is sometimes used to measure the pivot rotation of roll arms, linear position of sliding components, or the rotational position of rolls. The most significant feature of this device is the ability to signal location in a very short time or distance interval.
For example, it can transmit a roll's angular position to a motor control panel more than 1,000x/rev. This allows the motor to compare the actual speed with that of the desired speed and make the proper corrections more than 1,000x/sec. The encoder is one of the essential components responsible for the vast improvements in precision speed control of today's motor drive systems.
: A signal that is used in control logic to tell the control system whether or not to act on the process it is controlling. Usually a sensor(s) is used to monitor the process parameter that is most sensitive to change when control changes are made to the upstream process(es).
- Field strength
: The amount of electrical force that is generated between electrical charges on dielectric webs and any other body that has or can be induced to have an unlike charge. There are two variables that define the field strength: The field intensity varies directly with the amount of charge and inversely with the square of the distance between the two charges.
There are also two kinds of electric fields: uniform and non-uniform. The electrical field converges to a point in a non-uniform field. The field strength becomes much stronger near the point. When the strength has increased sufficiently to remove or add electrons to the boundary air molecules surrounding the point, ions are produced from the air molecules. When there are many electrons exchanging at some distance from the surface, there is a high probability of electron/molecule collisions. Other free electrons are produced, and that often produces a cascading breakdown of the dielectric capacity of the atmosphere. When this situation occurs, an arc discharges the electrical field.
- Flash wrinkles
: Foldover wrinkles in the film web that come and go quickly. Usually, the faster the machine speed, the quicker the wrinkles appear and disappear. When these wrinkles are present, there is nonalignment between the web resistance forces and the roll tracking forces. Because the wrinkles are coming and going, the web stiffness is almost able to overcome the nonaligned resistance and tracking forces.
- Foldover wrinkles
: Wrinkles in the web that stay in the folded position for a long duration. When these wrinkles are present, the tracking and resistance web forces are so badly misaligned in that area that the web stiffness is completely overcome. Machine alignment must be regained before the process can continue without this defect.
: A common name for transverse-direction (TD) web thickness variation across the width of the web. There is also machine-direction (MD) web thickness variation along the length of the web. Standing TD gauge variation is more harmful to winding smooth rolls than MD gauge because of the diameter buildup differences across the roll width.
- Idler rolls
: Rolls that are driven only by the surface friction of the running web.
: Molecules of any material that have an electrical charge. The molecules of the material have either gained (negative ion) or lost an electron (positive ion). Air is composed of gases that can be ionized in strong electric fields.
William E. Hawkins has more than 30 years of experience in 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.