provided by: Converting MagazineDraw control should be called what it actually is: Speed control. The word draw is ambiguous. It could mean the intentional and permanent elongation of material, which is a web-forming concern. Alternatively, it could be an alias for speed control. In speed control, we make adjacent motors go an ever-so-slightly different speed. If the downstream of a pair of motors goes faster, the tension in the web will tend to rise and vice versa.
Speed control is required when we form materials such as on the headbox of a paper machine or the die of an extruder. In cases such as these, we cannot pull tension in the normal sense because the material has no real strength. This may be where the word draw as an alias for speed control came from. We would also, for example, draw materials if we wanted them thinner. Molten metals are run over rollers with a positive or progressive draw. These are a few examples where speed control is used as a web-forming process.
Speed control is also required for registration. Consider two print cylinders rotating at precisely the same speed and with no high order concerns. There, one color can be aligned to another by increasing or decreasing the web length between the two color stations. The device to do this is called a compensator. Here, any thought of tension control must give way to the primary concern of registering colors.
Speed control is often used to reduce the cost of machinery as it's cheaper to supply one motor and connect it to several other rollers that need to be driven. Here, we must be quite careful. We must maintain the diameters of all rollers in that set to 4 or 5 digits. If one diameter is off, the web on one side will be tight and the other side loose. With a geared system, we can't readily change tensions if needed. For these reasons, today's machinery breaks the mechanical connections and directly drives every roller that needs driving with a separate motor.
However, the most common application of speed control is as a form of tension control. The best candidate materials for this are extremely stretchy. Materials that have a yield strain of at least 20 percent would qualify.
To make speed control work consistently, first, the material itself must be consistent. If unintentional changes in material properties occur or if you change grades, you may have to change the draw settings to make the web run without excessive slackness or tightness. Second, you must nail the speed with the motor and mechanical transmissions. Third, you must know the diameter of the driven roller, often to 4 or 5 significant digits. A hairsbreadth error in knowing the diameter of a speed-controlled roller can cause huge changes in web tension. Finally, as with any tension-control system, slippage between the web and roller cannot be tolerated.
While speed control seems simple, it's only simple for electrical engineers. For everyone else, it's the most complex of all tension-control schemes. While speed control seems inexpensive, that only considers initial cost?not operational costs or flexibility. If you choose speed control, it may be not possible to use dancer or load-cell control. You could always apply speed control later if you wished.
920/312-8466, drroisum@aol.com, www.roisum.com
author: David Roisum, Ph.D., Consulting Technical Editor
Converting Magazine. Copyright © 2008 Reed Business Information, a division of Reed Elsevier Inc. All rights reserved.