- February 19, 2013, Timothy J. Walker
Understanding winding, roll defects (e.g. tin-canning), and winding-related web quality (e.g., bagginess) begins with understanding how the web, winder design, and process conditions combine to create the stress and strains of the wound roll. To understand the stresses and strains within a wound roll, we must understand four things:
- What determines the web tension as it makes first contact with the winding roll?
- How is the tension controlled from the first to final lap of the winding roll?
- How do dimensional changes within a roll (e.g., compression) change roll strains and stresses?
- How do transverse direction variations in equipment or web properties affect roll structure?
Here is the second part of a four-part series on winding fundamentals.
part 2: Wound-On Tension vs. radius and speed
The wound-on tension (WOT) will normally change for each layer of a winding roll. If applied torque and nip load are constant, the torque contribution to WOT will reduce with growing roll radius. Many winding machines include options to program a changing tension or nip set point as a function of roll radius or speed.
Winding torques and tensions may be controlled to vary with roll radius. In constant tension winding, the center torque is increase proportional to roll radius. In constant torque winding, torque is held constant and winding tension will drop inversely with roll radius. Many wound rolls are wound with a tension vs. radius controlled somewhere between the constant tension and constant torque conditions. The profile of tension vs. radius is usually called winding taper tension or tension profile.
There are many logical reasons to change winding torque or nip load during the winding process. Some of these include:
- All the layers of a winding roll must transmit the applied center torque through the layer-to-layer frictional contact without slippage. The layer near the core are especially challenged in this ability to transmit torque since they have less area per layer to develop friction and they are at a mechanical disadvantage to the outer layers of a roll. Therefore, center wound and unwound rolls need more pressure in near core layers. To achieve this near core tightness, many winding processes use higher values for winding tension and nip load when winding near core layers.
- In winding smooth, non-porous products, an air layer will wind into the roll without sufficient pressure to compress or squeegee the entrained air. If excess air is allowed in a winding roll, the roll structure will loosen over time as the air escapes the wound roll, leading to soft roll defect (TD Buckling, sag, roll handling, or unwind telescoping). A winding nip roller is an effective tool to greatly reduce air captured in a winding roll. The air exclusion of a winding nip is directly a function of speed and inversely a function of effective radius (which is a factor of both nip roller radius and winding roll radius). To wind a roll with constant entrained air per layer (with a goal of controlling entrained air to less than the surface roughness of the product), nip load would need to be profiled to both speed and roll radius.
- The nip-induced tension (NIT) may be a function of roll radius with more NIT when roll radius is small.
- Entrained air will be a function of speed. Many winders, including most slitter/rewinders and all zero-speed core transfer winders must start and end the winding roll at zero speed. Should winding torque and nip load be controlled as a function of speed?
Clearly, if winding torque and nip load are held constant, they will not create a constant roll structure due these speed and radius effects. However, constant roll structure is not necessarily the goal of winding. Just as a skyscraper will have different demands for the building structure of layer near the bottom vs. top, so too will winding rolls have different structure demands from core to final layer.
Based on my experience with many film products, the best and simplest approach to creating a robust roll structure is to eliminate any speed functions on both tension from center torque and nip, to run nip load at a constant value that prevents excessive air at the maximum radius and speed conditions, and to taper tension vs. radius with final tension between 20% and 60% of starting tension on a linear tension or torque vs. radius curve. This approach creates a roll with near core layers that are tighter and have less entrained air and outer roll layers, while also reducing the torque transmission requirement in winding the final layers, but having sufficient nip load to avoid shifted layers of air lubrication.
Click here to read Part 1 of this series.
Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; email@example.com; www.webhandling.com.