Many material-processing machines use tension control to provide a constant pull of a web or filament through the machine, thereby ensuring consistent product quality.
Printing and converting industries account for most web-processing applications. In the printing industry, for example, web printing presses produce business forms, newspapers, and circulars. In the converting field, machines manufacture bags, boxes, and envelopes, plus disposable items such as napkins and tissues.
Filament-processing applications involve an even broader cross-section of industries, including those that produce wire, steel, textiles, and composite materials.
In these applications, tension control systems are typically installed at the unwind roll that feeds material into the production machinery, Figure 1.
This article covers three types of braking devices that are commonly used to apply tension in these applications: electromagnetic friction brakes, magnetic particle brakes, and air-actuated friction brakes. Other tension control devices include ac and dc drives, servomotors and step motors. AS drives were covered in the article “Understanding center-driven winders,” PTD, 5/95 and 6/95. Servomotors and step motors will be covered in a future article.
An electromagnetic friction clutchbrake typically consists of two basic parts: a rotating armature and a stationary magnet assembly with an electric coil, Figure 2. The armature and magnet have mating friction faces. Current applied to the electric coil causes a magnetic field that pulls the rotating armature and stationary magnet into contact. The resulting friction between the faces slows rotation of the shaft on which the unit is mounted.
Adjusting the amount of current supplied to the coil provides proportionately more or less braking.
Electromagnetic units are available as brakes or clutches: the operating principle for both versions is the same. The amount of running torque and thermal capacity of an electromagnetic brake depends on the number of armature discs and magnets in the unit. Running torque capacities vary with different manufacturers but typically range from about 0.5 to 414 lb-ft in sizes from 1.7 to 15-in. diam. Thermal (heat dissipation) capacities generally range from 0.02 to over 4.0 hp.
Electromagnetic brakes are typically used in web processing equipment such as printing presses, coating machines, and small slitters.
Magnetic particle units
Electromagnetic (magnetic) particle brakes rely on magnetic force between two rotating discs, which is transmitted by magnetic particles to produce a retarding force (brake torque), Figure 3. The input shaft and a cylinder form the stationary member; the output shaft and rotor comprise the rotating member. The magnetic particles are dispersed within a gap between the rotor and the cylinder. Direct current in the coil creates a magnetic field that causes the magnetic particles to attract each other. The particles rubbing against each other causes friction, which resists the relative rotation between the cylinder and rotor. The greater the current flow, the more braking torque is transmitted.
Torque capacities of these units typically range from about 0.72 to 578 lb ft. Thermal ratings range from about 0.04 to 5.5 hp.
Because they don’t use friction faces that wear in operation, magnetic particle brakes offer long service life and do not disperse friction material particles into the surrounding air. However, these brakes still experience heat build-up, caused by friction between magnetic particles rubbing together.
Magnetic particle brakes offer smooth operation at low speeds — a requirement in certain applications, such as filament coating machines, inspection machines, and small tape labeling machines. In the paper industry, these brakes enable continuous slip rewinding to produce uniform rolls of paper. They are also used in foil and film-processing machines.
Pneumatic or air-actuated brakes use air pressure to force stationary plates against rotating discs that have mating friction faces, Figure 4. Generally, torque is proportional to the air pressure, enabling these brakes to provide controlled, continuous-slip braking action.
Air-actuated brakes offer torque capacities ranging from about 4 lb ft to over 1,700 lb ft, depending on the friction coefficient of the friction material, the diameter of the friction discs, and the number of friction discs and air actuation elements in the brake. These brakes are used in paper manufacturing machines as well as machines requiring high torque, such as corrugators, corrugation slitters, and rewinders. Typical applications include unwinding heavy materials such as steel or corrugated paper rolls as used in cardboard-making machines.
Some versions have integral fans to aid heat dissipation, and the largest air brakes use water cooling through integrally cast cooling jackets.
Choosing a brake type
Selecting the type of brake depends on several factors, including the application parameters, mounting requirements, thermal and operating characteristics, type of control system to be used, costs, and in some cases customer preference.
Often, for a given application, any one of the three types will meet the application requirements. So, which one is best? Here are some factors to help you decide:
• Electromagnetic brakes have moderate torque and thermal capacities, which makes them suitable for handling light or medium-weight web materials.
• Magnetic particle units also have moderate torque and thermal capacities, so they too are suitable for light or medium weight materials. In addition, they provide smooth operation and precise control of low torques that occur with light materials and small-diameter rolls. This makes them especially wellsuited to handling films and foils.
• Air-actuated brakes have a larger range of torque and thermal capacities, which makes them suitable for web materials of various weights, especially heavy materials. They typically cannot handle very low torque values.
In general, when more than one type of brake is suitable for the application, the deciding factors are often cost and customer preference. In such cases, consult brake manufacturers for recommendations.
Keeping it under control
Tension control systems are defined as either open loop or closed loop. An open loop system is one in which the operator manually sets the tension control to a specific range. In such a system, the web tension is not measured. However, devices such as non-contact ultrasonic or photoelectric sensors, load cells, and potentiometers often provide tension-related information, such as roll diameter, to the operator.
If signals from one of the above sensing devices indicate tension variations outside of the preset range, the operator manually adjusts the control’s output (voltage or current). This causes the brake to apply more or less torque, which adjusts the web tension as needed.
In a closed loop system, the web tension is measured and this information is fed to a controller. If the tension varies from a predetermined value or range, the controller automatically adjusts the tension. A closed loop system typically contains three basic components: a sensing device to detect roll diameter (or dancer position or web tension), a controller, and an actuation device (brake).
Examples of sensing devices, Figure 1, Figure 5, and Figure 6, include a dancer arm assembly with a position sensor, load cells (either a single cantilever-type load cell or dual load cells that support each end of the sensing roller, a follower arm system (that senses roll diameter change via a potentiometer), tachometer system, and sonic systems for non-contact applications.
In a typical tensioning application, a closed loop control system works with a brake and a dancer arm assembly, Figure 1. A dancer roll rides on the web, with its weight determining the web’s tension.
When the position of the dancer roll changes, a sensor monitors the direction and speed of the change and transmits this data to the control. The control evaluates the information and adjusts the current to the disc brake so that brake torque is increased or decreased as needed. Adjusting the brake torque causes the dancer roll to either raise or lower to its correct position.
Due to the need for longer production runs and higher processing speeds, tension control systems are becoming increasingly sophisticated and responsive. New systems provide rapid response and consistent tensioning at web speeds that exceed 2,000 ft/min. To reduce material waste, they enable web material to run closer to the core, and consecutive rolls to be synchronized so their ends can be spliced together.
New microcontrollers for air-actuated brakes can turn off some of the air actuators in the brake as the roll diameter decreases, providing more performance flexibility in web processing applications. In some cases, the control system can be modified to provide a similar function for other type brakes.
Such technology trends can influence selection of a control system. You may be tempted to buy the latest, most sophisticated digital control system even though the application doesn’t require it. Avoid the temptation to buy all of the bells and whistles available, and concentrate on meeting the performance requirements of your application, both now and in the future. Conversely, don’t assume that a system that worked well in the past will be able to handle your present and future demands.
Bruce Becker is a senior applications engineer, Warner Electric, South Beloit, Ill.