Proximity Sensors for Paper Mills and Printing Machinery

A paper machine runs for weeks between stops. Anything that causes an unplanned stop is expensive, and a sensor that fails in the wet end is both expensive and difficult to reach.

The sensing tasks themselves are mostly straightforward. The environment is what makes the specification non obvious.

The environment

Humidity and condensation. The wet end is saturated. Air that is humid at process temperature condenses inside anything cooler, including a sensor housing and a connector. Specify sealing properly and prefer a moulded fixed cable over a connector where you can, see connector or cable and IP ratings.

Paper dust. Fine, pervasive and mildly conductive when damp. It accumulates on every surface. For optical sensing this is the main problem, because a lens coated in dust loses signal. Favour through beam over diffuse wherever the geometry allows, because through beam has the most signal margin to lose.

Chemicals. The wet end involves bleaches, sizing agents and retention aids. Cable jackets and seals need to suit. State the chemistry in your enquiry.

Continuous running. You cannot stop to change a sensor. That pushes the decision toward connectorised parts in reachable positions, so a change takes minutes at the next brief stop, and toward holding spares for every fitted part, see the spares list guide.

The sensing tasks

Reel diameter and reel change. A sensor watching the reel surface or an arm that follows it, detecting when the reel has built to diameter or run down to core. An M30 inductive sensor on the follower arm is robust; the alternative is an analogue sensor or a position transducer on the arm, giving continuous diameter rather than a point.

Web break detection. Speed matters. A break has to be detected in the time it takes for the broken web to wrap a roll, which is a fraction of a second at machine speed. A through beam photoelectric across the web, or a speed sensor on an idler roll that stops turning when the web goes slack. The idler approach is often more reliable in dust, because it is an inductive sensor on a toothed wheel rather than an optical path through a dusty atmosphere.

Core and chuck position. Inductive sensing of the chuck engaged position, straightforward M18 work, specified normally closed so a failure reads as not engaged.

Splice detection. A metallised or foil splice tape detected inductively, or a visual splice detected photoelectrically. Tell us which the mill uses, because it decides the technology outright.

Roll rotation and underspeed. A D33C speed and direction sensor on each driven roll detects a roll that has stopped or slipped before it marks the web.

Guard and access interlocks. Note the caution in replacing limit switches: a guard interlock requiring positive opening action is not a proximity switch job.

Printing specifically

Printing presses share the dust and continuous running, and add two things.

Register and mark sensing is a contrast sensing task on a moving web at speed, which is a different class of optical sensor from a general purpose photoelectric switch. Tell us the mark colour, the substrate colour and the web speed when you enquire, because the answer depends entirely on contrast.

Solvent exposure in gravure and flexo. Housing material and cable jacket both need to suit the solvent in use, and this is worth stating explicitly rather than assuming a standard part copes.

Specification summary

• IP67 minimum through the machine, higher at the wet end
• Through beam over diffuse wherever the geometry allows
• Inductive over optical wherever a metal target can be arranged, because dust does not affect it
• Normally closed on anything that should stop the machine
• Cable supported within 100 mm of the sensor, protected where the web could wrap it
• Spares held for every fitted part number, labelled by machine position

Replacing what is installed

Most Indian mills run imported sensors specified by the machine builder, often decades ago and now slow or expensive to source. Send us the part numbers and we will cross reference, see cross referencing imported switches.

Where the original is genuinely special, we manufacture to specification in modest quantities, see custom and OEM manufacturing. For comparable heavy continuous process work, see our notes on cement plants and sugar mills.