Web Break Sensors - How They Work

Web break sensors are primarily responsible for detecting an web break on an web offset printing press.  Let's take a look at different types of web break sensors and the advantages and disadvantages of each.

Why They Are Necessary
Modern presses today run so fast that by the time a pressman realizes that the web has broken, significant damage can be done.  Therefore a web break sensor will trigger a stop if tripped.  

Different Types
Photoelectric web break sensor
There are several different types of web break sensors.  Most modern presses use a photoelectric sensor.  Here are some advantages of using this type.
  • Easy to mount and position.
  • Generally not effected by heavy ink coverage.
  • Long life.
  • Low maintenance.
Some disadvantages to the system may be that it is not able to detect a quick reduction in tension.  At times the web must clear the detector before it is noticed.  However a mechanical web break sensor does not have this problem.  Here are some advantages of this type.
  • Can sense a reduction in tension before the actual break.
  • Simple design and easy to repair
A mechanical web break detector is often used on a rollstand or flying paster.  Sometimes a longer tail or an imperfect splice may trip a photoelectric eye at the moment of the splice, whereas a mechanical web break sensor will not be fooled.  Downsides may be that it may require a little maintenance and must be checked periodically to see that it is working properly.

How To Set Them
Web break sensors should be set on each side of the paper.  The danger lies in that the web may strip on only one side and wrap around a printing cylinder.  A web break sensor on only one side will not pick this up and serious damage can occur to the printing unit.  Always have one on each side and install them about 3 to 4 inches from the edge.

Web Severing Devices
 Manufacturers of web break sensors also make web severing devices.  They consist of 3 parts.

1. A sensor to detect a break.  Usually of one of the types described above, but most often a photoelectric sensor.
Web Break Sensor
2. A web severing device.  This part will cut the paper so that it does not continue to wrap in a printing unit.  Some offset printing presses require many impressions to slow down.  This unit will discontinue paper feeding into the printing unit.
3. A web catching device.  Once a break occurs, this unit will catch the web by applying small wheels against it and push the paper through while the press decelerates.  This prevents the paper from being sucked back by one of the printing units and causing damage.

When a web break sensor triggers a break, it shuts the press down and will send a signal indicating where the break took place.  This helps to troubleshoot the source of the web break.

Folder Sensors
Some photoelectric sensors behave in such a way that if they do not receive an intermittent signal, they will shut the press down.  For example, in a high speed folder a sensor will be placed to ensure consecutive copies flow through the folder.  If a sensor does not pick up a signal that a copy has passed, it will shut the press down since this likely indicates a jam has started somewhere in the folder.  A signal will tell which sensor in the folder was tripped helping to troubleshoot.  On higher speed presses it will trigger a severing device just below the former board to prevent paper continuing to feed into the folder.

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