Web displacement guide
The web displacement guide (offset position guide, edge guide) is a simple device used to ensure a fixed lateral position of the web in rotary print presses or in other machines that somehow process ‘endless’ stripes of a thin flexible material.
The ‘natural cause’ of web sideway movements are imperfect unwinding reels. A skewed reel, for example, may cause sine-like lateral web movements as the web is wound off the reel. However, such sideways movements are very undesirable in print presses, especially during second-pass printing.
The web offset guide unit sits just after the unwinding section of a machine and actively steers the web to counter-act any initial sideways shifting of the web. The stabilized web is then feed into the printing process.
The offset guide (aka pivot offset guide) basically consists of a single pair of parallel rollers. Those two rollers are pivoted around single non-symmetrically positioned turning point.
The guide only works well if the rotation angle is relatively small. Therefore, for large displacement amounts the unit has to be made longer. In fact, to reduce deformations of the material (may cause folding or breaking) the free-traveling entry and exit zones must be made longer for larger displacements and a wider web.
Rollers, especially the exit roller, can sometimes be coated into a high-friction material like cork or rubber to aid its steering abilities.
There are two basic configurations offset guides are used: ‘U’ and ‘Z’. In the ‘U’ configuration the web enters and exits the guide at the same side, while in the ‘Z’ configuration it enters and exits at opposite sides.
Of course, a guide can be intended to be mounted horizontally or vertically. Some of them can even work in any configuration and/or layout without modification.
A small motor is used to turn the guide left-right. Various kinds of motors are used in practice: electric DC, AC, stepper or even hydraulic or pneumatic motors.
To sense the lateral position of the web, the offset guide is equipped with a sensor, usually optical one. In simpler variant, the sensor senses the web edge. Such a sensor usually consists of two parts (light transmitter and light receiver) and aims the pivot to keep the sensor partially covered all the time. More complex sensors can read longitudinal mark printed on the web and will govern the pivot to keep the mark always steady.
A simple proportional regulator is used to govern the motor in most simple designs (although adding a small amount of derivative component might improve dynamic capabilities if the sensor doesn’t generate large noise). At small gains, the guide will become ineffective at high speeds (that is, at high disturbance frequency). On the other hand, high gains might lead to instabilities. Therefore, most designs allow for manual gain control.
You can order a web displacement guide made to fit your needs. Just send me a mail inquiry at danijel.gorupec(at)gmail.com and state as much detailed specifications as you can. At the end, you will own a new processor-controlled offset guide of lasting mechanical quality made by MIGO Company (a small Croatian company specialized in servicing and maintaining printing industry machinery).
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Danijel Gorupec, 2012.