Digital Magazine

Strand Tracking Problems on Your Slitter/Rewinder

A slitter/rewinder has two goals: Divide the web into strands (slitting); and create uniform individual rolls (winding). This plan has three potential pitfalls: bad slitting, bad winding, and bad strand tracking.

Strand tracking is the forgotten pitfall. In this column I will present the most common sources of slit-to-wind tracking problems.

Slitting knife axial runout
Sometimes things aren't straight out of the starting blocks. This isn't really a tracking problem, but the result is the same: wound rolls with shifted layers. To check for this, use a dial indicator to determine if the slitting knives shift laterally during rotation or under load. You would expect this to show up as a slit width variation, but knives — especially stacked sets — can run out as a set, creating wander without width change.

Neighboring strand contact
Two slit strands running side by side on a transport roller have the potential to jump one atop the other. Once one strand rides up on a neighboring strand, the apparent diameter variation will cause it to track over more. If the strands are wound side by side and overlapped entering the winding rolls, the result is a big mess.

Slackness from web bag
All webs have some nonuniformity in their cross-web length. If the length variations are greater than the web strain from tension, some of the slit strands may become slack immediately after slitting. The lack of stiffness and traction caused by low tension will allow the web to wander.

Slackness from winding accumulation
Lock bar winding is sensitive to roll-to-roll diameter variations. Without differential winding, where winding rolls can turn at different rotational rates, thinner strands may lead to smaller diameter roll buildup and less web accumulation. The result is slack web and related web wander.

Wander from web skew
Strand length variations do not have to be greater than strain to cause tracking problems. Strands with one side longer than the other will have a tendency to track toward their tight edge (with good traction). Changes in strand skew and traction over time will create wander.

Web roller attraction
Both adhesive and electrostatic forces, if present, must be overcome to peel a web from a roller. If the forces are two high or variable, the result will be wrapped rollers at worst, or tracking variability at best.

Tracking from roller or winding roll diameter variation
The cylindricity of the downstream roller can cause a web to track to the large diameter side. A winding roll with noncylindrical profile will create the same effect, pulling the web to the larger diameter side. Pack and gap rollers at winding are used to eliminate this problem, but many slitter/rewinders do not have this design option.

Tracking from roller or winding roll alignment
The normal entry rule defines how a web will track to enter a downstream roller perpendicular to its axis of rotation. In this same manner, a misaligned or deflecting core or winding shaft will redirect the web's lateral position.

By understanding these mechanisms, you can avoid them. The result will be less waste for you and straighter rolls for your customers.

In this column we only addressed lateral shifting on the way to the winding point. We'll save web shifting after entering the wound roll for another column.

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