Web Guiding And Spreading Systems
Web guiding and spreading systems offer improved quality and productivity in the textile industry.
Typically, web-type materials are fed from a reel to the machine, processed and then rewound. During these stages, various position errors may occur. Web guiding systems are designed to eliminate these negative process influences and assure permanent, precise web alignment and winding. Depending on the type of material application and task, a wide variety of systems are specially designed to improve quality and increase productivity.
In web guiding systems, a sensor detects the web position compared to a reference point. This is usually the web edge, but it may be a printed line or a strong color contrast on the web. The current actual web position value is compared with the set value by the controller, which then sends the difference of these two values as a signal to the system-actuating element, which, in turn, corrects web travel.
With progressive technology, detection systems have come a long way from being simple mechanical sensors to sophisticated digital infrared sensors, wide-band arrays, or charge-coupled device (CCD) cameras. The right choice of sensor specific to the application is important to guarantee optimal results.
Infrared sensors are in wide use, for example, in tenter infeed systems, highly accurate edge guiding on printing lines, and center-positioning on laminating processes. These sensors typically have a very narrow detection range and, therefore, provide highly accurate detection of any type of edge. In principle, an optoelectronic, infrared edge sensor distinguishes itself by a highly compact and stable construction with an integrated CCD chip. Due to the permanent adjustment of the sensor to ambient conditions, practically all webs can be reliably scanned, including those with very little light reflection.
Figure 1: Wide band sensors can be used for edge detection, center guiding and width monitoring.
Wide band sensors allow various-width materials to be processed without mechanical sensor position changes (See Figure 1). The sensor operates with infrared transmitters and receivers placed on opposite faces of the web, and senses the edges using special scanning techniques. It is, however, not as accurate as a stand-alone infrared edge detector and is primarily used for guiding applications in which less precision can be tolerated. Detection accuracies can vary from 3 to 7 millimeters (mm), depending on the technology used. Wide band sensors can be used for edge detection and center guiding, and, because of their construction, also as width monitors.
There is a wide range of different correction devices available in combination with the different detection systems to make a perfect fit to any process in the textile industry. Subject to the application and the fabric type, the device may be a segmented roller guide, a steering roller, a pivoting frame or a web guider.
The most economical guiders - the well-known two-roll edge guiders - have been around for decades. The web edge is detected either mechanically by a sensor lever or without contact using optoelectronics. The sensor signal controls a diaphragm cylinder or a lifting magnet that presses the control roller against the counter pressure roller. The web edge is controlled by roller offset and nip pressure. For more accurate edge guiding, a built-in broadband sensor with web edge fluctuations up to +/- 80 mm is available due to the large scanning range - 120 mm - of the infrared sensor system. For delicate webs, an additional electrical drive for the top roller ensures that the friction torque on the rollers is compensated and stretching of the web is avoided.
Figure 2: Steering roller assemblies perform two different movements in one correction procedure.
A steering roller assembly consists of two or more rollers and performs two different movements in one correction procedure (See Figure 2). First, it tilts toward the web direction of travel to achieve a continuous righting of the moving web. Second, it assumes a lateral offset vertical to the web direction of travel to effect an immediate reparation. This rectifying action is achieved by a lever system as shown in the sketch. The web must be friction-locked to the guide rollers.
Figure 3: Segemented roller guides maintain uniform distribution of tension across woven, knitted or nonwoven webs.
The segmented roller guider is the optimum web guiding system for all applications across the entire textile-processing spectrum (See Figure 3). It guides woven, knitted, and, to a certain extent, nonwoven fabrics in dry, damp or dripping wet conditions, in steamers exposed to high temperature and finishing chemicals. In the event of major corrections, the guider maintains a uniform distribution of tension across the web, thus minimizing weft distortion. The web rests on the guiding slats along its full width. The slats carry the web around the roll while moving laterally, providing a minimum offset - thus eliminating the risk of a skewed web. The guide rollers can be designed with split slats and perform web guiding and web spreading simultaneously. Scroll rollers or pneumatic uncurlers can also be added for uncurling and additional spreading capability. A good degree of friction between the segmented roller guiding slats and the fabric web is essential for an efficient web guiding process. Many modern quality fabrics feature extremely sensitive surfaces that may be slightly impaired by the contact areas of the guiding slats should these be too aggressive. Various slat coverings offer the possibility to process these fabrics safely.
Figure 4: Tenter rail guiding systems ensure precise positioning of tenter frame clips or pins to the fabric edges at the infeed of the tenter frame.
Tenter rail guiding systems ensure precise positioning of tenter frame clips or pins to the edges of the fabric at the infeed of the tenter frame, which is critical to producing quality fabric with consistent width (See Figure 4). As speeds have greatly increased, this control has been improved through the development of highly accurate infrared edge sensors, digital controllers, and positioners with encoder feedback. The rails can be moved at rates from 120 to 180 mm per second to exactly follow the fabric edges.
Many textile processes require the use of unwind or rewind systems to properly control the fabric into the process and to produce rolls of fabric with uniform edges at the rewind station. Various sensors - such as edge, line, or center-guiding - can be used; and various actuators are available for different strokes and forces.
A range of control systems consisting of sensors, controllers and actuators can be adapted for many other positioning applications, such as coater-dam positioning and slitter positioning, for example.
Despite a consistent specialization in web guiding systems for the various automation and production processes in the textile industry, a broad range of products, from individual mechanical components to multifunctional systems, is available to meet the requirements. This means that even the most complex projects may be completed cost-effectively. These efficiency solutions provide manufacturers with heightened control of processing variables, which ultimately increases productivity, reduces waste, and improves the quality of the end product.
Editor's Note: Udo Skarke is CEO of Erhardt + Leimer Inc., Duncan, S.C.