When To Use Center/Surface Winding
Center/surface winders are used when winding slippery materials, when in-line slitting, and for turret winders requiring high web tensions. On this type of winder, the web tension is pulled by the surface rolls' drive. The roll hardness is developed independently from the web tension through profiled nip from the surface roll and profile torque from the center drive, hence, "center/surface winding".
When winding slippery materials, the center/surface winder helps prevent internal web slippage, referred to as "cinching" and roll "dishing" during
the winding operation. These slippery materials often do not have the required layer-to-layer coefficient of friction to transmit the torque
required to pull the web tension at larger roll diameters without web slippage.
Center/surface winders are also required when center drives are used for in-line slitting applications. Spreading action is a function of the
amount of wrap on the bowed rolls and the amount of bow and web tension. With a center drive, the web tension is developed from the center
torque. For proper roll structure, this torque must be profiled for decreased hardness as the roll winds (see figure #1). The decreased tension
as the roll builds results in decreased amount of spreading as the slit rolls wind. With a center/surface winder, the torque can be profiled
as a function of the winding roll diameter and the web tension controlled by the surface roll drive can be held constant.
Finally, center/surface winders are used on heavier grades of rigid films, boards and composites when using a turret winder. These heavier grades require high web tensions and are normally wound to large roll diameters, typically from 1.2 to 1.8 meters. The spindle horsepower becomes extremely large as the required motor horsepower equals the tension horsepower x build-up x friction factor. On center/surface winders, the required spindle horsepower can be reduced by as much as 40 percent, while the surface roll drive is used to make up the remaining tension horsepower.
Understanding Winding Tension on a Center/Surface Winder
On a center/surface winder, the winding tension is controlled independently from the web tension coming into the winder. Typically, the winding tension at the core is approximately 50 percent greater than the unwind or process tension. As the roll builds, the tension is tapered proportional to the winding roll diameter. An example of this is as follows:
If winding 60 pound per 3000 square feet basis weight paper, then the suggested unwinding tension would be 60 x .035 (unwind tension factor) or 2.1pli. The suggested winding tension would be 60 pounds x .055 (winding tension factor) or 3.3pli. For proper roll structure, you may want to taper the winding tension by 50 percent at a 30-inch roll diameter. The drive would progressively taper the winding tension from 3.3pli at core to 1.65pli at the 30-inch diameter. This would mean that the pressure roll drive would be regenerating or pulling negative current at the start of the wind to make up the difference between the 3.3pli being pulled at the winder and the 2.1pli at the unwind (1.2pli). As the rolls wind, the tension being pulled from the center drive would be constantly decreasing and the pressure roll drive amps would be constantly increasing toward zero. At the point that the winding tension is equal to the unwinding or process tension, the pressure roll drive would be zero amps. At the 30-inch diameter the pressure roll drive would be pulling a positive current to make up the difference between the 2.1pli unwinding or process tension and the 1.65pli winding tension (0.45pli). See figure #2.
The resulting torque of the motor is proportional to the winding tension x web width x the radius of the winding roll x the taper set by the operator. For constant tension winding, the torque increases proportional to the radius roll. For constant torque winding, the torque stays constant throughout the winding cycle and the in-wound tension decreases proportional to the roll radius. Normally rolls are wound somewhere in between the constant tension and constant torque winding to achieve the proper roll hardness taper.
BC Publications is a technical newsletter designed to provide operational and maintenance information to improve the performance of your Black Clawson equipment.