WHQuiz Answer #5 - Draw Control Pros/Cons

Published: February 29, 2012, By Timothy J. Walker

WebHandlersQuiz 5: Tension (Draw) Control – Name one advantage and one disadvantage of draw control (a.k.a. as speed ratio control)?

Advantages:

• Draw control is the simplest intermediate tensioning system, requiring no rollers or feedback between driven rollers, allowing it to fit where driven rollers must fit into a tight space. .
• Draw control lets you set a percent stretch of processing your material independent of tension, which can be important to products where elongation is a better predictor of damage than tension.
• Draw control is used in machine direction registered processes. The speed adjustments of closed-loop tension or torque control would create shifts in printing and die-cutting registration.

Disadvantages:

• Draw control is widely misunderstood, especially their dependence on upstream tension and strain as baseline for draw zone tension.
• Many people believe successive draw zones require stepped increases in speed to maintain tension. This is true for viscoelastic materials, such as cookie dough, but not for most paper, film, and foil webs.
• Draw control is dependent on relative surface speeds of input and output rollers, making them sensitive to roller wear. If speed ratios are set too high, microslip will occur.
• Draw control has a time constant equal to the time it takes the web to pass through the draw zone (Time Constant = Length/Speed). When a change occurs upstream or between the draw zone’s driven rollers, the system will require 3x this time constant to reach a new steady state.
• Draw zone don’t provide tension feedback; therefore, it can be difficult to tell if the system is working properly. 
• In many cases, the draw ratio of roller driven off a common belt or chain is set by the equipment supplier, but undocumented and unknown.  

More on this topic was covered in this archived Web Lines column: Drawing Conclusions: Part 1, Part 2 May-June 2005

If you missed it, here are all 20 questions in my Web Handler's Quiz.