March 1st, 2018

How to Maintain Equal Tension on Multiple Rolls

Winding multiple rolls of material on a single shaft can result in dramatic efficiency and output boosts. However, this approach may result in unwinding and rewinding issues, unless you’re using a differential air shaft to help you maintain equal tension on multiple rolls.

Since 1999, Convertech Inc. has produced differential shafts that run on webs from 10” to 150” wide and handle rolls from 1”-12” diameters. Eager to know more, we had a conversation with Larry Taitel, CEO of Convertech, to learn more about Differential Air Shafts and how they work.

Larry Taitel, CEO of ConvertechLarry Taitel, CEO of Convertech

How converters run into trouble with multiple rolls

Picture this: You are producing some labels and you have a roll of material with a 60” width. Now you need to cut the roll into 3” wide segments but your rolls have different diameters. Taitel states, “Normally the differences are under 1/16”, but sometimes the differences in the roll diameters could be 2-3 inches.”  

You could try to wind all the rolls together on the same shaft, but they will turn at the same rate. This can lead to major troubles. The larger diameter rolls will end up being too tight and break, or the smaller diameter rolls will end up being too loose and fall apart.

How differential winding can help

Differential Winding for Different Diameters

The need for different roll widths on a single shaft can be resolved using the Differential Air Shaft, which can be used for both unwinding or rewinding. As a result, you can:

  • Laminate materials with different widths off a single roll
  • Slit material and maintain even tensions across the web
  • Slit rolls on the same shaft into different widths

Convertech’s Differential Air Shafts have bi-directional roll-lock winding, which allows converters greater flexibility. “Sometimes you want the roll to feed from the top, sometimes the roll to feed from the bottom,” Taitel said.  “The shaft has to lock in two different directions.” Other benefits of differential shafts can include:

  • No core dusting
  • The core conditions will not affect the web tension because the slipping takes place on a controlled surface
  • Lower web tensions can be achieved with heavier rolls than with direct friction type and shafts

Comparing the different winding types of differentials

Winding Types

Differentials have evolved over time and continued to become more efficient with each new winding type. Initially, core spacers were used where rolls were spaced out and pressure was applied to the sides of the core spacer for gripping. However, the pressure caused heat.

“Because the pressure was applied to such a small area, it caused extreme heat, and even led to the cores catching on fire,” Taitel said.

As the designs for differentials progressed, direct friction winding was introduced where the internal force of the differential pressed outward and the slipping came from inside the cores. While an improvement, the direct friction shafts are prone to core dust and they move from side to side, which results in jagged edges.

To solve these problems, Convertech modified the Differential Air Shaft with roll-lock winding in 1999. With their revamped shaft, the chucks expand using rewind torque to grip the core ID. By adjusting the air pressure in the bladder, the shaft has more torque control.  As a result, there is no rotational slip between the chuck and the core.

Below are more of the advantages and disadvantages of the three differential winding types:

Three Winding Systems' Pros and Cons

Be prepared and informed when retrofitting your machine

Retrofitting your machine with differential shafts can be a costly decision and you want to make sure your machine is prepared for the change. As Taitel points out, “machines have different types of controls and you have to make sure your equipment has the appropriate drive system.”

Many machines run on a torque drive, which measures the resistance of the motor, and speeds up or slows down to maintain the tension. Both the torque drive and the differential shaft operate with a clutch, so they fight each other, and this can cause the shaft to melt.

Unfortunately, customers that were unaware of their torque drive purchased custom differential shafts that couldn’t be used on their machines.

Make sure your equipment has the appropriate drive system.

To prevent this, a Differential Air Shaft needs to be run on a machine that has a speed drive. With a speed drive, “the machine measures the web speed and the roll diameter to determine how many RPMs the roll is turning,” Taitel said.  “It then tells the shaft to adjust its RPMs.”

If you ever have any questions, you should always ask. It is important to get as much information as possible so that you can make the best decision for your business.

As business booms, flexibility will be the key

Convertech has seen enough changes in the industry to know how important adaptability is and will continue to be. They’ve integrated and adapted elements in their own designs and is a perfect example of how making modifications to benefit your customers’ growth, can also spur your own.

As the converting industry continues to grow, the demands on converters will escalate.  Complicated jobs will push manufacturers further and further.  The only way to adapt is to look towards readily available technologies and equipment that will allow for continued growth.

Find out how to overcome your biggest converting and packaging challenges.

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