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.
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.
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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Clutches and brakes
Designers use both friction and nonfriction type clutches and brakes in web applications. Friction types generate torque through frictional contact between mating surfaces on stationary and rotating elements. But this produces heat, especially during slipping. In most cases, air pressure actuates these devices, which helps keep them cool. Airactuated clutches provide a wide range of torque and thermal capacity.
Non-friction types, such as magnetic particle, eddy current, and hysteresis clutches, generate torque through electromagnetic attraction between stationary and rotating elements. They are actuated by electric current and the amount of torque generated depends on the amount of current in the coil. These electromagnetic devices generate less heat. They have a smaller torque range than others and are typically used for light duty applications.
A control system for a clutch or brake consists of one or more sensors and a controller. The sensor continually monitors some operating parameter that is related to web tension and transmits this information to a controller. The controller calculates the web tension, then determines any adjustment needed and sends a correcting signal to the clutch or brake.
The type of signal depends on the method of actuating the clutch or brake. For an air-actuated device, the controller adjusts the amount of air pressure to the clutch or brake. Increasing the air pressure, for example, increases the amount of torque applied, and hence the amount of tension in the web. For electrical actuation, the controller sends more current to the clutch or brake coil to increase torque and web tension.
Control systems commonly use one of three methods to determine web tension: diameter-based, dancer arm, and load cell.
Diameter-based systems operate on the principle that the torque required to generate tension in a web being wound or unwound equals the tension times the roll radius.
There are several ways to determine the roll diameter. The oldest and simplest is a follower arm that rides on the surface of the roll. As the roll diameter changes, the arm rotates through an arc. A sensor (usually a potentiometer) measures the follower arm movement and feeds this information to a controller so it can adjust the torque. The latest technique involves an ultrasonic device that reflects a sound wave off the surface of the roll and measures the time for the wave to return. This method works without contacting the roll.
Diameter-based systems have the lowest cost and least precision. They typically maintain tension within 8- 10%, which is adequate for many applications.
A dancer system regulates the torque of either an unwind or winder roll. The weight of a dancer arm roll deflects the web to create tension. The dancer moves up or down through an arc to accommodate changes in tension or speed due to variables such as jerks caused by processing the web. A sensor measures the change in dancer position and sends this information to a controller.
Dancer control systems are more complex and costly than diameterbased systems. However, they control tension more accurately, to within 4-5%.
A load cell system measures the vertical force created by a web as it wraps around a transporting roller. The controller calculates web tension from the load cell data and makes any adjustments required.
Load cell systems are the most expensive type. But they provide a high degree of accuracy (1-2% in most cases). They also tolerate high web speeds that may exceed the sensing capabilities of other systems. They can interface with PLC or PC machine controls.
John Campbell is senior technical representative, web control products, and Edd Brooks is senior technical representative, motion control products, Horton Inc., Minneapolis.
Put a lid on it
Clutches sometimes provide slipping action for other applications such as torque control. In one example, a capping head assembly uses a hysteresis clutch to apply caps on bottles at a preset torque, thus ensuring consistent cap tightness. The clutch has an adjustable ring for manually setting the torque limit.
The assembly spins freely until a cap is applied to a bottle and tightened to the preset torque limit of the clutch. When torque reaches the limit, the clutch slips, allowing the cap to stop rotating on the bottle.
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Easy does it
Machines handling delicate loads often rely on air-actuated clutches and brakes to reduce acceleration and deceleration. For example, a high-speed bottle conveyor needs to start slowly to prevent bottles from tipping and jamming the conveyor. This can be done by adjusting either air pressure or flow to a clutch that drives the conveyor.
Lower air pressure lengthens the time it takes the clutch to build up torque to start the conveyor. The longer response time reduces acceleration, giving a more gentle start. Lower pressure also reduces the maximum torque supplied.
Fixed air line restrictors or adjustable needle valves control the air flow. A small air line orifice increases response time and reduces acceleration without affecting the maximum torque. For example, decreasing the orifice from 0.187 in. to 0.046 in. increases the full-torque response time of a 6- in. diameter clutch from 30 to 146 millisec.
An air controller typically needs a separate air line with a valve and regulator for each pressure setting. However, a new microprocessorbased controller performs the same functions in a single unit, eliminating the need for multiple lines.
Clutches and brakes are also used to start and stop conveyors or turntables for operations such as cut-to-length, filling, and machining.
In a typical application, a container moves along a conveyor until it passes a photoelectric sensor as it approaches a filling station. The sensor detects the container and sends a signal to a PLC or PC that operates an air controller. The controller directs air pressure to a brake so it stops the conveyor when the container is under the filling head. When the container is filled, another sensor signals a clutch to advance the conveyor to the next container position.