Web detectors are used to detect a web break and in turn stop the press.

These devices are very important since in many cases the web will fall back into the running press and wrap the plate cylinder, blanket cylinder or inking rollers. The longer the press runs after the web break the greater the risk of damage to the press unit.

To prevent even more damage when a web break occurs, web severs are used to limit the amount of paper that could possibly wrap in the unit.

These devices are located under the arch of each mono unit and another sever is placed just before the web travels in to the halfdeck portion of the unit, assuming the unit is equipped with one.

There are two basic types of web detectors, contact and non-contact. The contact type is often referred to as a spoon or ski because of its shape. It is simply an electrical switch attached to a rod and a rectangular piece of metal fixed to the other end of the rod so that it swivels.

This type of web detector is lowered onto the web after the web becomes tight. The detector glides along the surface of the web, applying a slight pressure to the web as it monitors the web’s integrity.

If the web breaks, however, the detector swings through the area where the break occurs; the web detector switch then signals the press to stop. At the same time, it will signal the web sever on the affected unit to begin operating. The sever limits the amount of paper that could possibly wrap up in the unit.

Only at a certain speed

Normally web detectors are only activated after the press has attained a pre-determined speed, normally 5,000 to 10,000 copies per hour.

This is to prevent a web that becomes a little loose on startup from causing an unnecessary stop, which would in some cause the web to be severed at the same time. So once the press reaches a pre-determined speed, web detectors are activated and ready to protect the press units in cases of a web break.

Although contact detectors are widely used, one disadvantage of the design is that the part of the detector that makes contact with the web can build up with ink and, thus, cause the web to wrinkle and lead to additional breaks. On the other hand, these designs are much less susceptible to web fluctuations.

No contact

Non-contact web directors function very much the same way as contact designs. Because these detectors do not touch the web, wrinkles and associated wrinkle-related breaks are avoided.

The disadvantage is that web fluctuations can cause these detectors to signal a break where none exists.

Despite this shortcoming, these detectors work extremely well and are more versatile than the contact type.

Non-contact detectors come in two flavors: photocell and ultrasonic.

Photocell detectors win praise because they are less sensitive to any web fluctuation. Plus, they are easy to control, thanks to an electronic control system.

Disadvantages? The device’s performance can become erratic once it becomes contaminated with ink and paper lint.

Ultrasonic detectors are very reliable and cause few problems as long as they are set properly. It’s very important to set them as close as possible to the manufacturers’ specification.

It is also important to place the detector in an area where the web has the least amount of fluctuation.

Advantages? They are unfazed by ink and paper lint buildup. If you can limit the amount of web fluctuation, then I believe this type of web detector is the most trouble-free detector on the market.

In short, web detectors are extremely valuable, especially since one or two bad cylinder wraps could pay for the entire system. That’s not even including how much money you might lose while your press is down.

Frank Bourlon has more than 30 years’ experience in the newspaper industry. He is the executive and training director for the Newspaper Production & Research Center. He can be reached at 405.524.7774 or via e-mail at fbourlon@np-rc.org.