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Handling Multiple Narrow Slits

Published: March 24, 2026, By Trevor

By Neal Michal, Principal at Converting Expert, LLC 

Making wide rolls of material can reduce total delivered cost.  These rolls are processed on a slitter/rewinder to make finished slits for the customer. Managing multiple narrow slits can be difficult. There are three proven methods to handle multiple narrow slits: single bow roll, dual bow roll, and a spreader bar. 

Graphic Courtesy of Converting Expert, LLC

Single Bow Roll 

Bowed rolls are a go to solution for spreading a wide web that has wrinkles. They are also used to spread multiple narrow slits. 

Bowed rolls are also known as “curved axis rollers.” There are several OEMs that offer bow rolls.  A bowed roll consists of a common shaft that is bent out of plane.  A series of bearings are located along the length of the shaft. Thin steel shells are located above each bearing.  A flexible sleeve is placed over the top of shells. The surface velocity across the face of the bowed roll is constant but are angled away from the center.  This is why a bowed roller is a powerful spreading device. 

The least expensive bow roller will feature a fixed amount of bow.  OEMs also offer adjustable bow rollers at higher cost. The angle of the bow can be rotated to any position required.  Most OEMs place a pointer near the end supports to indicate the direction of bow. 

A single bow roll is most effective when located at the end of a long span with a short exit span.  Most applications only require 1/2% to 1% bow.  For reference, a 100” wide bow roll with 1” bow has 1% bow. 

It is common to use 30-45° web wrap.  Web wrap should never exceed 90°.  It is ideal for the bow roll to match the web speed to reduce wear and tear.  Low tension processes or slippery webs benefit from driving the bow roll to web speed. High tension / high traction processes may not require a motor to drive the bow roll.  

Determining how to orient the direction of bow leads to much discussion. For maximum spreading, the direction of bow is perpendicular to a line that bisects the web path. The bow roll should be pointed downstream of this bisector. Minor angular adjustments (+/- 10°) are made based on direct observation. 

Dual Bow Rolls

Dual bow rolls provide maximum spreading in a short distance. Modern slitter/rewinders feature this arrangement. Dual bow rolls provide reliable slit separation for multiple narrow slits. The web path is critical for successful operation. 

The entry web path to the first bow roll and the exit web path leaving the second bow roll must be parallel to each other. The direction of bow should point perpendicularly away from the entry & exit spans and oriented downstream. 

This arrangement will twist the slits away from the centerline in the intermediate span which are brought back parallel past the second bow roll. They should be driven at web speed to provide long term consistent slit separation. 

The primary factor that determines total spreading is the length of the intermediate span. The amount of bow plays a smaller, secondary role. 

If all slits are wound onto a common core shaft, target 3-5mm minimum slit separation. Duplex winding may still require a nominal slit separation. 

Spreader Bars

Image 1 Courtesy of Converting Expert, LLC; Image 2 Courtesy of Pearl Technologies

Bow rolls rely on sufficient web-to-roller traction to generate an effective spreading force. Their performance decreases significantly with low tension conditions, low friction substrates, or highspeed processes with significant entrained air. 

In these applications, bow rolls may not provide the traction required for stable slit separation. A nonrotating spreader bar can be used as an alternative. Depending on industry terminology, this device may be referred to as a Dbar or banana bar. Because the web slides across its smooth surface, spreader bars must not be used with materials susceptible to surface damage or scratching. They are significantly less expensive than a bow roll and can be procured from OEMs such as Pearl Technologies or fabricated inhouse using polished stainless steel tubing.

For optimal performance, the spreader bar should be positioned close to the point where spreading is required. Approximately 5° of web wrap is generally sufficient. The mounting arrangement should allow precise adjustment of the bar’s engagement depth to achieve the desired spreading effect. 

Summary 

Handling multiple narrow slits separated and wrinkle-free can be challenging. Three main methods are used:

Single Bow Roll

  • A bowed (curvedaxis) roller spreads the web by directing surface velocities outward from the center.
  • Bow amount may be fixed or adjustable.
  • Works best at the end of a long span with a short exit span.
  • Typical setup uses 0.5–1% bow and 30–45° wrap.
  • Never use more than 90° wrap around a bow roll. 
  • Bow roll must run at web speed to reduce wear.
  • Maximum spreading occurs when the direction of bow is oriented perpendicular to the bisected web path and pointed downstream.

Dual Bow Rolls

  • Provides maximum spreading in a short distance.
  • Widely used in modern slitter/rewinders for reliable slit separation.
  • Entry and exit spans must be parallel.
  • Both bow angles are perpendicular from the entry & exist spans while oriented downstream. 
  • The primary factor that determines total spreading is the length of the intermediate span. The amount of bow plays a smaller, secondary role. 
  • Target 3-5 mm minimum slit separation if winding to a common core shaft. 

Spreader Bars

  • Used when bow rolls lack traction, such as in low tension, low friction, or highspeed conditions.
  • A smooth, nonrotating bar (Dbar or banana bar) spreads the web as it slides over it.
  • Not suitable for scratch sensitive materials.
  • Cheaper than bow rolls and can be fabricated in house. 
  • Place close to the needed spread point with about 5° wrap and adjustable engagement depth.

About the Author

Neal Michal was Kimberly-Clark’s senior web handling expert and was responsible for winding and converting performance.  He is a technical advisor for the Association for Roll-to-Roll Converters and is a regular contributor to Converting Quarterly and Paper, Film and Foil Converting magazines. Neal can be contacted at This email address is being protected from spambots. You need JavaScript enabled to view it. (770) 356-7996. Learn more at: https://convertingexpert.com/