A basic machine such as a slitter has one set of wind and unwind rolls. As the unwind roll gets smaller, a controller reduces brake torque to maintain constant web tension during the slitting operation.
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Web processing operations such as coating, printing, laminating, and cutting require a constant pull or tension on the material getting worked to ensure that it moves smoothly without bunching or tearing. Otherwise, under-stretched material may wrinkle and jam, while overstretched material cut to length may contract and become shorter than intended. Web tension control is critical in such operations.

In most cases, web tension is the result of forces applied to wind and unwind rolls. One roll may pull the web, while another resists, maintaining constant tension. Control systems with clutches and brakes offer an effective way to control these roll forces and web tension at lower cost than adjustablespeed drives.

Wind and unwind

A basic material-processing machine that performs just one function, like a slitter-winder, has only one set of wind and unwind rolls. Typically, a constantspeed ac motor with a clutch drives the windup roll, pulling the web off an unwind roll and through the slitter knives at speeds of 3,000 to 5,000 fpm. A brake on the unwind roll resists the winder's pull and creates tension in the web.

A master drive between a wind and unwind roll creates two operating zones, one for printing or coating at a given tension, and the other for tightly winding the material for shipment.
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Adding an adjustable-speed drive in the middle of the machine (between unwind and wind rolls) divides it into two working zones and allows speed changes. The unwind brake resists pull from the center drive to create tension in one zone and the winder motor pulls against the center drive to create tension in the other zone. This configuration makes it possible to unwind the material and perform an operation on it at one tension value, then increase tension at the winder to create a tightly wound roll for shipping.

More complex machines use additional intermediate drives along with clutches and brakes to maintain different tension levels for multiple operations.

To maintain constant web tension, clutches and brakes must compensate for constantly changing diameters of rolled material (typically a 10:1 ratio between a fully wound and unwound roll). For example, an unwind brake must provide high torque when the roll diameter is large and progressively lower torque as it unwinds and becomes smaller, based on the equation Tr = Te × Rr, where Tr is brake torque, Te is web tension, and Rr is the roll radius. A controller typically makes this happen by monitoring the diameter as it gets smaller, and gradually reducing input (air pressure or electrical current) to the brake, which reduces torque.

A typical control method uses a follower arm to sense the diameter of a roll as it unwinds. A controller uses data from the sensor to regulate brake torque, and thereby maintain web tension.

The brake simultaneously slips to let the winder pull material off the unwind roll, while producing torque to resist the winder motor.

With delicate web materials, it may be necessary to drive the unwind roll gradually up to speed to prevent tearing. A regenerative drive offers one solution. It can accelerate the roll to full speed, then reverse direction to apply back torque like a brake.

Clutches used on winding motors experience the same torque changes as unwind brakes. As the roll grows, the clutch must provide increasingly more torque. In this case, heat generated by the clutch depends on web tension and the difference in speed between input and output clutch members.

As a winding roll becomes larger, clutch output speed decreases. Thus, if a constant speed ac motor drives the clutch, the difference between the clutch's input and output speed increases as the roll gets larger. This causes the clutch to slip more, which generates more heat. As a result, a winding clutch usually needs to be larger than an unwind brake so it has enough thermal capacity to avoid overheating.

One way to minimize the difference in speed is to use an adjustable-speed motor to reduce input speed to the clutch as the roll diameter increases. This speed change need not be precise because the slipping clutch provides the final speed correction. Just keep the input speed slightly higher (about 5%) than the required output speed so the clutch only slips a small amount.

Traditionally, adjustable-speed dc drives have been used for winding applications because they provide constant torque across their speed range, whereas the torque capacity of ac drives varies with speed. However, today's ac vector drives provide constant torque, and are commonly used on winders.

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