Tension control roll

Figure 18.3 is an extrusion line schematic drawing of tension control equipment for the unwinding substrate. The arrows indicate the motions of the driven tension control rolls and idlers as well as the substrate, and the direction of outward pressure-on the rolls. Figure 18.4 is what is called a flying splice on a double-station-unrolling stand where ... 

Tension Control Roil These type rolls provide the important function of material tension control. There is a proportional relationship between winding tension and lay-on-roll forces (eliminating areas were bumps, valleys, unwanted stretch, etc. develop). There are various tension control techniques available. The proper selection involves decisions on how to produce the tension, how to sense the tension, and how to control the tension. For instance, if the material has a very low tension requirement and if exact control is required, then perhaps, using a magnetic particle brake with an electrical transducer roll with appropriate electronic control is best. However, if the material is on large diameter rolls and moves at slow speed, then a roll follower system can be used effectively. 

Adjustable Roil The dancer rolls, canvas drag brakes, various pony brakes, and pneumatically operated brakes are manual adjustment systems. The most expensive would be the regenerative drive systems. The transducer rolls and dancer rolls would be a close second. These systems are usually required in high web speed applications where accurate tension control of expensive and/or sensitive material is paramount. With roll windup systems different roll or reel-change systems are used to keep the lines running at their constant high speeds. 

The main function of a winder is to guide the film evenly into the roll form. To accomplish this, it must provide tension control on the film to produce evenly wound rolls. A secondary nip, either as part of the winder or just prior to the film entering the winder, is used to provide this tension control. Tension must be sufficient that the roll will not telescope when handled, but must not be so much that it causes crushed cores or center-roll bagginess. Tensions on center drive winders are generally tapered from the core to the outer wraps to compensate for the change in diameter of the roll. On a surface winder, the compensation is automatic because of the drive mechanism. When tension is being adjusted on a winder, it must be kept at or below 0.5% of the secant modulus of the film to avoid excessive stress in the film. One reason for this low value of recommended tension is that the film is not totally at equilibrium at the time it is wound into the roll. Dimensional changes occur as the film crystallizes or cools, and induce stress. 

Fig. 4.10. Catenary furnace for heat treating metal strip. Careful strip tension control is needed to prevent strip sag to prevent strip contact with the flame. Better control can be achieved with the exit supporting roll water cooled and just within the exit end of the furnace and with a T-sensor near that roll and under the strip.

Developments in web tension control systems are providing increased capability and function to eliminate problems (Fig. 3-34). They include ultrasonic roll diameter sensors, pneumatic pressure gauge tension monitors, capston-Mt. Hope tension systems, and so on. As an example, replacement of a web-tensioning system s conventional electromechanical drive with an ordinary ac motor enables processors to lower system cost and improve web consistency, as has been done for many years. A vector control system uses a belt and pulley arrangement to remotely couple an encoder to the shaft of the ac induction motor. This approach provides closed-loop feedback, without requiring that one modify the relatively inexpensive motor by installing a special feedback device on it. 

A major part of film, sheet, coating, pipe, profile, etc. lines involve windup rolls. They include winders, dancer rolls, lip rolls, spreader rolls, textured rolls, engraved rolls, and cooling rolls. All have the common feature that they are required to be extremely precise in all their operations and measurements. Their surface conditions include commercial grade mirror finishes, precision bearings and journals are used, and, most important, controlled variable rotating speed controls to ensure uniform product tension control. 

Guide rolls give a stable run either to the paper web or to wires, fdts or belts. They are about 400 to 1000 mm in diameter and are covered with coatings (Section 5.2.4). They are also used in tension control devices to ensure that the fabrics operate at adequate tension in the machine direction and in wire and felt guide systems for continuous control of the fabric position in the cross machine direction. 

A variety of AC and DC drives are available for extruders. DC motors with solid-state eontrollers and power supplies provide the most flexible seenario for control. A speed control feedback loop on the drive will minimize variation in the extruder output. The unwind roll should be equipped with a tension control device such as an electronic regulator. 

These rolls are driven by motors and /W, whose speed is controlled through the tension of the travelling sheet. The tension of the sheet is adjusted by monitoring the diminishing diameter of the payoff roll and the thickness of the sheet. 

Before the finished sheet is finally cut into lengths as required or rolled into recoilers its exit speed and tension is monitored and controlled again by bridle no. 4, driven by motors M15 and... 

Another difficult aspect is control of the point at which material leaves the roll—firstly because the environment is hostile for measurement, and secondly because any change in the tension at that stage will change also the thickness of the product. 

The presence between film and roll of a layer of air may be demonstrated easily by putting a light roller that is free to rotate in contact with a travelling film the roll will rotate in the direction of travel but if it then is spun by hand in the opposite direction it will be seen that this has little effect on the film, and the effort necessary to cause counter-rotation is surprisingly small. Briefly, the thickness of the layer of air is controlled by the tension applied to the film, the radius of the roll, and the surface speed. The equation in Figure 43, derived from the work of Hutzenlaub,5 expresses this. 

Creases formed in the film prior to the emboss nip will become welded into it as longitudinal faults. Such creases usually are caused by release of film from the calender roll in the form of an extreme arc, or by a total failure to control the tension between the calender and the emboss nip. Momentary contact of the film with the surface of the stripping rolls also may cause creases. 

Kiss No backup roll coater is in light contact with substrate contact is controlled by web tension. Best with adhesives having good flow characteristics and relatively slow drying rates... 

The complete equipment usually consists of a mixer such as a Banbury mixer followed by the heated rolls, chilled rolls, and finally a windup roll.3 The windup roll controls the tension on the film or sheeting as it moves through the calender rolls. Calenders are generally designed to meet the specific needs of the customer. Once installed and operating continuously, the cost per pound of film or sheet is lower than by any other process such as extrusion. 

Dancer Roll These can be used as a tension-sensing device in film, sheet, and coating (wire, film, etc.) lines. They provide an even controlled rate of material movement. Type roll can have an influence on the roll s performance. As an example, chrome plated steel casting drums would seem to be very durable dancers. If used in the absence of a nip roll, should last many years. However, these rolls are in fact very soft due to the annealing which good rolls receive for stress relieving the steel. 

These tertiary crushers employ smooth or toothed heavy-duty impact and abrasion-resistant steel-rimmed rolls. The rolls are mounted inline in a horizontal manner and turn toward each other at equal speeds to create a nip into which a friable feed material is introduced (Fig. 4). Heavy-duty compression springs with automatic reset are used to dampen crushing shock and to protect the crusher from tramp iron and oversize material. An adjustable screw that adjusts spring tension changes the crusher opening. A flywheel is used to even out pulses and economize on power consumption. These crushers have a theoretical maximum reduction ratio of 4 1 and will only crush materials to about 10 mesh. Roll crushers produce a controlled product size distribution without a lot of fines. The narrow particle size distribution is achieved by controlling a combination of variables including roll speed, gap measure, differential speed, feed rate, and roll surface. 

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