Winding Education and Information


What is the best winding process for your product?
Specifying the winding needs of your product takes three easy steps.

Step 1 – Understand Your Winding Torque Requirements
Understanding the winding process starts with your product, not equipment options. Winding is a process that must be optimized to the product properties, both the material properties of the web and core and the desired roll geometry (diameter, core size, width). Some materials are stiff and need high winding tension. Some materials are stretchy and should be wound at lower tension. Large rolls will require more torque to create tension or overcome inertia. All products and winder designs have their limits, but finding the best match for your product will pay off in higher yields and productivity.

What is the purpose of winding?
Winding is a packaging process. The wound roll is a package that collects and protects your web product through shipping, storage, and unwinding. Just as any box or carton must meet specific packaging needs, a wound roll must have a structure that meets your roll’s winding, handling, storage, and unwinding needs.

A tensioned web pulled over a cylinder (such as a roller, core, or winding roll) will exert an inward pressure proportional to the tension (in units of force per width such as PLI) divided by the cylinder radius (P=T/r). This pressure creates friction proportional to the area of contact and coefficient of friction between the two materials (F=COF*P*A). The friction created by tension holds the roll together, keeping the layers from slipping during winding, handling, storage, and unwinding. Webs with low COF will need more internal roll pressure and more tension to prevent interlayer slippage.

What is winding torque?
Torque is a turning or twisting force. In center winding or unwinding, the web’s tension is created by applying torque to the roll’s core. At any point in winding, the winding torque will equal the force of tension times the radius of the roll. For example, if the tension force is 50 lbs and the roll radius is 3-inches, the required torque will be 150 in-lbs. If winding grows to a 10 inch radius and tension is still 50 lbs, the require torque will be 500 in-lbs.

How is torque transmitted to create winding or unwinding tension?
Torque is generated by a motor, clutch, or brake. The torque is transmitted to the winding or unwinding roll by a drive train (which includes all shafts, pulleys, belts, sprockets, chains, or coupling connecting the torque generator to the winding roll’s shaft or chuck). Once the torque reaches the shaft or chuck, there are three more steps to convert the torque into web tension. The torque needs to be transferred 1) from the shaft or chuck to the core, 2) from the core to the web, and 3) through the layers of the roll to the outermost tensioned layer. Each of these steps has a torque limit or capacity to transmit torque.

What is torque capacity and how much is needed?
Torque capacity is the maximum torque that can be transmitted from one element to another without slippage. If there is not enough torque capacity at any one of these three steps (chuck or shaft to core, core to web, and layer-to-layer within the roll), the desired torque will fail to transmit into web tension and the failing interface will slip, possibly causing debris, scratching, heat generation, or lateral shifting. In addition to creating unwinding or winding tension, torque capacity is also needed to prevent slip from inertia during acceleration or deceleration.

How is torque transmitted from the shaft or chuck to the core?
Many products are wound with a tension of approximately one pound per inch of web width (or 1 PLI – Pound per Lineal Inch), so the total web tension will be 1 PLI times the web width. The torque to create the unwinding tension on a center unwind is the total tension times the roll radius. The maximum torque required for an unwind shaft or chuck is the maximum tension times the maximum width times the maximum radius.
Most shafts or chuck transmit torque using friction between the shaft or chuck and the core. Friction is created any time two surfaces are presses together. As long as the available friction is not exceeded, the torque will transfer without slippage. The torque capacity of a friction system is the friction available times the radius it is applied. The friction available will be the normal or loading force times the coefficient of friction (COF) between the two surfaces, in this case, the COF of the core to the shaft or chuck surface.

To ensure good torque transmission, many shafts and chucks have an outer surface of rubber that has a high COF to most core materials. Other shafts and chucks, especially ones designed to grab paper or fiber cores, have a ridged or grooved surface that can ‘bite’ into the soft core material, creating a strong mechanical interlocking.

The most challenging core driving option is when a process uses a hard core (steel, aluminum, or hard polymer) and a hard shaft or chuck surface. Since the coefficient of friction typically lower between hard materials, more normal load will be required to prevent slip. In some hard chuck/core combination, a notched end or square chuck can be used to act as a keyway to prevent slip in transmitting torque.

How is the normal load created between a shaft or chuck and the winding roll?
Most cores are engaged by the shaft or clutch with either a pneumatic or mechanical expanding element or series of elements.
Pneumatic shafts and chucks have an expanding bladder that is deflated to slide the core into place and inflated to engage the core’s inner surface. Pneumatic bladder will typically be inflated with plant air pressure (usually 60-80 psi). This air pressure is exerted over the contact area of the core or a core grabbing element. The pressure times the applied contact area will equal the exerted force.

Mechanical shafts and chucks use a cam lock or sloped bearing system to fall away or engage the core depending on the direction of rotation. The force developed by a cam or sloped bearing engagement is proportional to the winding or unwinding torque and the wedging angle of the mechanism.

How is torque transmitted through the layers of the winding or unwinding roll?
To prevent slipping, scratching, and shifting layers, a wound roll should be wound tight enough to have a sufficient combination of frictional properties, layer-to-layer pressure, and contact area to opposes the external forces created during the winding, shipping, storage, and unwinding processes.

In most cases, the first wrap of web is attached with adhesive or tape to the core to ensure no-slip torque transmission. From there, the torque is transmitted layer by layer through the entire roll structure to reach the roll’s outer diameter and create the winding or unwinding web tension.

Between any two layers in a roll, the torque capacity is equal to (the radial pressure at that point in the roll) x (the area of that point in the roll) x (radius at that point) x (coefficient of friction between the side A and side B of the web). The area, radius, and COF values are easy to find, but the internal roll pressure is not.

How much pressure is needed in the roll?
The torque capacity at any pointing the roll is equal to (pressure)(area)(radius)(COF). This is a radius-squared function since radius is in the area calculation, so more pressure is needed in the layers near the core to prevent slippage.

There are two ways to get higher pressure at the core. Some webs will naturally wind with higher pressure near the core due to the tourniquet effect. Other webs will need to have the winding tension controlled from high to low as the roll builds (often called tension taper or profiling).

In a winding roll, each layer exerts pressure towards the core and as more layers are added, like stacking bricks, each layer will add to the pressure of all layers below it. The difference between stacking bricks and winding is that the pressure at the bottom of the stack is not created by gravity, but by the energy within the stretched elastic web. In stacking bricks, the pile may or compress the bottom layers in the pile, but the effect of gravity on each layer will remain the same. In winding, additional layers will compress the inner layers, reducing their radial position and stretch (or strain), reducing their contribution of the inner layers to the roll’s pressure.

Step 2 for winding >>

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