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Design Considerations in Unwind Tension Control Systems – Part 1 of 3

The following is part one of a series of 3 blogs that will go through a white paper written by New Era Converting Machinery’s Bob Pasquale. The white paper is titled “Design Considerations in Unwind Tension Control Systems.”

Part 1 will offer an overview of the entire paper and what to expect in coming blogs. Part 2 will go through sizing a braking system and various types of braking systems on the market. Part 3 will wrap the series up with types of braking systems controls and offer a brief conclusion.

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Part 1: Introduction and Background

INTRODUCTION:

A major consideration for a web processing line is the ability to properly unwind the incoming rolls of material. Critical to this operation is the ability to precisely control the material’s tension as it exits the unwind. In this presentation we will discuss the important components that make up a tension control system as well as the criteria required for the proper selection of these components. We will also discuss the advantages and disadvantages of the various components and systems that are available.

BACKGROUND:

At the beginning of most web converting lines, an unwind is used to allow rolls of the incoming web to be delivered to the process. In addition to supporting the incoming rolls so that the material can be fed into the process, the unwind must be able to deliver this material to the balance of the process in a controlled fashion. Critical to the ability to control the web as it exits the unwind is the application of the correct force, known as tension.

The proper amount of tension required is based on the mechanical properties of the material; therefore, it varies from web to web. The chart below indicates typical web tension values for various materials.

Suggested range of web tensions per mm of web thickness
Films Tension range (metric) Tension range (English)
Polyester 35 to 105 N/cm/mm 0.5 to 1.5 lb/inch/mil
Polypropylene 14 to 35 N/cm/mm 0.2 to 0.5 lb/inch.mil
BOPP 21 to 70 N/cm/mm 0.3 to 1.0 lb/inch/mil
Polyethylene 7 to 21 N/cm/mm 0.1 to 0.3 lb/inch/mil
Polystyrene 35 to 70 N/cm/mm 0.5 to 1.0 lb/inch/mil
Vinyl (uncalendared) 3.5 to 14 N/cm/mm 0.05 to 0.2 lb/inch/mil
Aluminum Foils 35 to 105 N/cm/mm 0.5 to 1.5 lb/inch/mil
Cellophane 35 to 70 N/cm/mm 0.5 to 1.0 lb/inch/mil
Nylon 7 to 21 N/cm/mm 0.1 to 0.3 lb/inch/mil
Conversion: kg/cm/mm = Newton/cm/mm x 0.1
Suggested maximum tensions for various grades of papers
Paper, basis weight Tension levels
25 g/m² (15 lb/3000 ft²) 0.88 N/cm (0.5 lb/in.)
33 g/m² (20 lb/3000 ft²) 1.31 N/cm (0.75 lb/in.)
50 g/m² (30 lb/3000 ft²) 1.75 N/cm (1.0 lb/in.)
65 g/m² (40 lb/3000 ft²) 2.6 N/cm (1.5 lb/in.)
100 g/m² (60 lb/3000 ft²) 3.5 N/cm (2.0 lb/in.)
130 g/m² (80 lb/3000 ft²) 4.4 N/cm (2.5 lb/in.)
Conversion: kg/cm = Newton/cm x 0.1
Suggested maximum tension for paperboard grades
Board thickness Tension levels
200µm (0.008 in. = 8 pt) 5.3 N/cm (3.0 lb/in.)
300µm (0.012 in. = 12 pt) 7.0 N/cm (4.0 lb/in.)
400µm (0.016 in. = 16 pt) 9.6 N/cm (5.5 lb/in.)
500µm (0.020 in. = 20 pt) 11.4 N/cm (6.5 lb/in.)
625µm (0.025 in. = 25 pt) 14.5 N/cm (8.3 lb/in.)
750µm (0.030 in. = 30 pt) 16.6 N/cm (9.5 lb/in.)
1000µm (0.040 in. = 40 pt) 21.0 N/cm (12 lb/in.)
1250µm (0.050 in. = 50 pt) 24.5 N/cm (14 lb/in.)
1500µm (0.060 in. = 60 pt) 28 N/cm (16 lb/in.)
Conversion: kg/cm = Newton/cm x 0.1

(The tension data in the above chart is based on TAPPI tables as published in TIP 0200-01 in 2011)

It is important that the web tension be properly controlled within the required range. Too little tension will allow the web to wander and go slack, which can result in issues such as wrinkling and folding-over. Too much tension can have many effects on the web including stretching it, distorting its structure, necking it down, inducing wrinkles and breaking it.

In order to properly tension the web for delivery to the process, unwinds are typically provided with a braking system that is interconnected with the material roll’s support device (air shaft, safety chuck, etc). This braking system is sized to deliver the necessary tension to the web by generating the required hold-back force on the rotating roll of material.

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*Click here to read the next blog in the series, or here to skip ahead to the third and final one!*