- March 01, 2003, Timothy J. Walker, TJWalker & Assoc. Inc.
The expression “it's a cinch” usually means something is easy, as easy as tightening a belt. However, as we learned last month, cinching is not something we want to hear about in the winding business.
How do you know if your product is cinching?
At unwinding, draw a spoke line from the core to the outside of the roll. Apply the tension and start unwinding. If the line remains in the spoke direction, the layers are not slipping relative to each other. If the spoke line turns into a spiral or forms a step, your roll is cinching.
At winding, this is a little trickier. If the speed isn't too high, try snapping a chalk line in the spoke direction. A more advanced, safe — but expensive — way to detect winder cinching is to coordinate an ink jet printer to mark the outermost layer of the winding roll once/revolution. If the series of ink jet dots form a spoke line, there is no cinching.
How is cinching eliminated? Cinching occurs when torque capacity is less than applied torque. Stop it by changing either side of this equation.
- Reduce the applied torque
Center-winding torque is the double-edged sword of cinching. If we turn it up to create a tighter roll, we increase the torque transmission demand on the roll. Luckily, there are other ways to make a roll tighter.
Surface winding — in which a driven roller or belt at the roll's surface creates tension — eliminates the need to transmit torque through the roll's layers.
A pack roller nipping the outside of a roll during winding adds to the tension of the incoming web, forming a tighter roll with minimal increase in applied torque.
You can reduce applied torque in other ways. Big changes in roll diameter usually mean increasing tension and torque; therefore, reducing roll build-up ratio with larger cores or shorter roll length may help. Always unwind a roll at a tension lower than it was wound. Don't expect an unwinding roll to have more torque capacity than when it was wound. Prevent inertial torque loads with moderate acceleration and deceleration rates.
- Increase the roll's torque capacity
Torque capacity increases with higher traction coefficient or more internal roll pressure. The obvious approach here is to wind tighter. It is counter-intuitive that higher tension could prevent cinching. It seems that more tension would increase torque capacity and applied torque equally. However, the nonlinear nature of roll buildup and the tourniquet effect means doubling tension can more than double internal roll pressure.
If your product doesn't have an inherently high friction coefficient, think of ways to increase layer-to-layer traction. Moisture, magnetism, and electrostatic pinning are some options to change the bond between roll layers.
Sometimes the wrong things save you. Hardbands, baggy web, and poor slit edges can prevent cinching. Intuitively, it doesn't seem these defects, which change cross-web tension distribution, should increase torque capacity. Therefore, the tension concentration from these nonuniform characteristics easily can stop cinching. If you have a product that occasionally cinches, look to see if that roll is too uniform.
Some products are designed with a knurl or other locking mechanism at the product edges that is trimmed off before the final product.
Watch out for cinching caused by compression or shrinkage of the core or internal roll layers. Paper cores can fall away from the product if they start too moist and dry out. Product layers can fall radially due to air bleeding out a roll or shrinkage of a film or coating over time.
Don't lose your shirt while tightening your belt; cinching can be eliminated in most products. Fight the battle on both fronts, and you may find it's a cinch to stop cinching.
Timothy J. Walker has 20+ years of experience in web handling processes. He specializes in web handling education, process development, and production problem solving. Contact him at 404/373-3771; mailto:email@example.com; tjwa.com