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Lock up your valuables: Stop cinching and telescoping slippage in your rolls


October 1, 2007
By Pulp & Paper Canada

Tim Walker

Terminology

Terminology

Cinching and telescoping are the descriptive names for slippage between layers within rolls at winding and unwinding. Understanding the conditions that lead to cinching and telescoping will help you to keep your winding or unwinding process in control.

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Cinching is machine direction slippage, layer-to-layer relative motion tending to tighten or loosen a roll. Telescoping is crossweb or axial direction slippage, layer-to-layer relative motion that creates misalignment on the sides of a roll.

Telescoping is the obvious defect. You can spot a telescoping roll from across the room. Cinching is a sneakier defect, often invisible at first glance, but cinching is the precursor to one cause of telescoping.

What is cinching?

“Cinch” comes from the Latin word for belt. Cinch is the verb used to describe the relative motion of two layers around a curved surface like the motion used to tighten or loosen a belt. In winding, cinching is any tightening or loosening motion between the layers of a roll. This motion is sometimes called “clock-springing”, referring to how the roll’s layers will tighten or loosen like the spiral spring in an alarm clock.

Where does cinching occur in a roll?

Layers within a roll will cinch anywhere the applied torque exceeds the local torque transmission capacity. Cinching can be a localized event, happening at a distinct radial position in a roll or it can be a pervasive full-roll event. Cinching is most common in the layers near the core, where the area and friction per layer is often lower than in a roll’s outer layers.

Is cinching a defect?

Your product may cinch slightly every roll and show no ill effects, but watch out for these undesirable cinching byproducts:

1. Scratching, abrasion, and debris generation — Any time you press two surfaces together and slide them relative to each other you will have some amount of wear and debris.

2. Crepe wrinkles — Inertial cinching that drives the internal layers into compressive buckling may create permanent creases or crepe wrinkles in the web.

3. Lateral shifting of roll layers — This is a curious side effect since cinching is a machine direction event. When cinching occurs the applied torsional load has consumed the roll’s traction; therefore, there is no traction left over to oppose any internal lateral forces from crossweb non-uniformities (More on this later.)

What is torque capacity?

Torque capacity is the ability of the layers within a roll to transmit torque from layer to layer within a roll. Each layer has a unique torque capacity defined as the product of the radius, the layer-to-layer traction coefficient, the layer-to-layer pressure, and the area of contact.

The concept of torque capacity is straightforward, but calculating it is a little more difficult since it requires knowledge of the inter-layer pressure within a roll. Even without exact numbers of torque capacity, a general knowledge of winding and wound roll properties combined with observed slippage can lead in the right direction to increase torque capacity where it is needed.

Why is torque transmitted through a winding roll?

Anytime a motor or brake is applied to the core of a roll to create tension at the roll’s outer layer or to accelerate or decelerate the mass of the roll, the torque supplied at the roll’s centre must transmit layer by layer through the roll to use the torque to overcome inertia or create tension.

How is cinching eliminated?

Cinching occurs when torque capacity is less than applied torque. Stop cinching by changing either side of this equation. Increase a roll’s torque capacity or decrease the torques applied to the roll.

What reduces applied torques?

Cinching is more common on centre winders than surface winders. Whether winding or unwinding, centre winders apply torque at the core, transmitting it through the body of a roll to create tension at the roll’s outer surface. Cinching from unwinding or rewinding tension will always shift layers in a tightening direction. Tightening-direction cinching is usually self-limiting since the tightening action increases internal roll pressures and torque capacity.

In center winding, cinching may occur as the roll builds to large diameter, especially if the winding tension is constant or has little taper. Constant tension winding requires continually increasing torque. As applied torque increases, it may exceed the torque capacity of the layers near the core.

In centre unwinding, cinching may occur immediately upon tensioning the roll, again mostly likely at the layers near the core.

At unwinding, always unwind a roll at a tension lower than it was wound. Since many rolls tend to loosen in storage, don’t expect an unwinding roll to have more torque capacity than when it was wound.

Surface winding, where 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.

Cinching from inertial torque can occur in center or surface winding and may shift layers in either a tightening or loosening direction. If a winding roll is decelerated too quickly, its outer layers develop an inertial torque that resists stopping. Cinching in the loosening direction is self-promoting since the loosening action decreases internal roll pressures and torque capacity. This loosening cinching action, if great enough, can tangentially drive the web into compressive buckling, forming a crossweb or crepe wrinkle inside the roll.

Prevent inertial torque loads with moderate acceleration and deceleration rates. For slitter-rewinders, take your time getting up to speed and stopping at the end of the slit roll. In at-speed splicing processes used in many high speed winders and unwinds, again, manage acceleration and deceleration rates.

What increases a 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 may be surprising that higher tension could prevent cinching since higher tension increases applied torque. However, the non-linear nature of roll buildup and the tourniquet effect means doubling tension can more than doubles internal roll pressure and torque capacity.

If your product doesn’t have an inherently high friction coefficient think of ways to increase layer-to-layer traction. Moisture, roughness and coatings are just some factors that will change the bond between roll layers.

Sometimes the product variations are a good thing. Hardbands, baggy web and poor slit edges can prevent cinching. Since wound roll tightness is non-linear with tensioning, these defects that induce crossweb tension variation in a winding roll can create lanes of high tightness and increase a roll’s overall torque capacity. The high tension lanes in a roller will lock the roll up and prevent cinching. If you have a product occasionally that cinches, it may be that your more uniform web is missing these ‘locking lanes’ in the wound roll and are more prone to cinching (and telescoping). 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 promoted by compression or shrinkage. Paper cores or layers 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. Rolls that are wound warm may loosen as the roll cools. Anything that loosens a roll decreases its torque capacity and makes it more prone to cinching.

How do I know if my 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, then your roll is cinching.

If you can stop a roll and then restart winding, use the spoke test. At modest winding speeds, try drawing a spoke line on the turning roll or snapping a chalk line in the spoke direction and look for the spoke line slippage. A more advanced, safe, but more complicated way to detect winder cinching is to coordinate an inkjet printer to mark the outer most layer of the winding roll once per revolution. If the series of inkjet dots form a spoke line there is no cinching.

How are cinching and telescoping related?

In driving your car, once your tires slip relative to the road, you lose control in all directions. As you lose control to slippage, forces acting on your car from inertia and gravity determine where your car will go. If you are lucky, you car may continue traveling straight, but in many cases, your car will end up in the ditch or hitting a mailbox.

In winding a roll, once layers within your roll begin to slip via cinching, you lose laterally control. The slippage of cinching means other forces acting on your roll, such as uneven pressure from crossweb roll diameter variations, misalignment or uneven nip forces will determine where your wound roll layers will go. If you are lucky, the layers may slip and stay in their lateral position, but in many cases, the slipping layers will shoot laterally, taking the layers above them along for the ride. The need for friction in multiple directions is covered by the concept called the friction circle.

What is the friction circle?

The friction circle is a common term in auto racing, describing the combination of conditions that will lead to a spinout in a big turn. The friction circle says that the weight of your car and friction coefficient between the tires and the road determine the friction limit in any direction. Since friction may be needed in any direction, this limit can be considered a circle around the car. Step over the circle and slip occurs.

If you use too much friction in accelerating or braking, you slip. If you use too much friction to oppose gravity on a slide slope or to hold you against the centrifugal forces in a turn, you slip. And most importantly, if you need to use friction in more than one direction at once — look out — the friction needs may combine to push you outside the friction circle and into uncontrolled slippage.

How does the friction circle apply to cinching and telescoping?

The friction circle applies anytime you have multi-directional friction needs. Looking at a winding roll, it doesn’t immediately seem like there is any need for lateral friction since tension and torque are applied in the machine direction, but there are always small forces trying to push the layers of a winding roll sideways. Uneven winding nip pressure, uneven crossweb thickness variations, web bagginess, and misalignment are all exerting small lateral forces on the layers within a winding roll. It doesn’t take much friction to oppose these lateral forces and hold a roll laterally, but whenever any layers start cinching there isn’t any friction left over to hold the roll laterally and telescoping begins. Once telescoping starts, the uneven roll shape promote more uneven forces, so once started, telescoping will only get worse.

Surprisingly, the key to stopping telescoping, a lateral shift, is to stop the enabling machine direction shift of cinching.

Can you live with cinching?

Like any defect, a little bit may be tolerated, but too much leads to waste. Cinching may be tolerated if your product is insensitive to scratching. Telescoping may be acceptable if it is within product specification or within the range of web guiding at the next process. Some operations handle cinching and telescoping by winding with flanges or on spools to contain the shifting layers. However, with understanding of cinching and telescoping you can take by control of your winding process and create superior rolls that meet the winding, handling, and unwinding needs for both you and your customers.

Timothy J. Walker is the president of TJWalker & Associates, and is an internationally recognized expert in web handling and winding processes. Formore information, please contact 651-686-5400 or cell: 651-249-1121. Email: tjwalker@tjwa.com


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