Accurate Tension Control

Tension is due to forces pulling against each other and when applied to processing involving the moving web it is necesssary for the sake of production efficiency that these opposing forces are stabilized at a set value which is applicable to both the material and the process.

Tension is an invisible force so unless the correct regulating equipment is in place the result of incorrect tension may not become apparent until, in the case of some converted products, it is delivered to the end user. This is particularly so in the case of stretch films and laminates.

The correct tension, irrespective of material, is dependent not only on its modulus of elasticity but also on the process which is being carried out at the time. To cite one example, a flexographic printing press engaged in printing on one of the newer thinner gauge plastic films will require a different set of tension values for unwinding and for printing and for slitting, etc.

Unit tension, or force per unit width, is arrived at by consideration of the thickness and the type of material. Units are measured as pounds per linear inch, as N/cm or as Kg/cm. The paper industry favors the method whereby tension is quoted in terms of the unit paper weight.

Montalvo, as a specialist manufacturer of tension control related equipment, recognizes that each customer and each process needs to be evaluated on its own merit and the appropriate solution supplied. The introduction of the System 3000 range, together with other models, ensures that Montalvo customers have the widest possible choice of tension control equipment to suit their needs.

Maintenance of correct web tension is essential at every stage of the process. In the case of converting machines most will have at least two separate tension zones and sometimes as many as ten. When unwinding off a reel, the tension needs to be set at a constant value, high enough to allow the web to pass over rollers without slippage and without causing wrinkles, but not so high as to cause stretching and web breaks.

When engaged in a coating application it is important that the coating is laid down evenly. In this situation the tension gradient across the web is as critical as the total web tension in obtaining a uniform and repeatable coating weight.

Equal tension control is of prime importance in the printing of multi color work where the web is held only by tension and also when laminating two or more webs together. In the case of the latter the relative tension between the two webs can be critical to avoid sheet curl of the finished product. Accurate tension control is also needed on rewinders to make the perfectly formed reels for transportation and further processing.

It will by now be clear that for maximum production efficiency reduced waste and minimum down time tension control cannot and should not be left to change. There are a number of methods on the market which have been devised to control tension. These are: predictive systems, dancer roll systems or transducers.

The predictive method involves the operator keying in a set of predetermined values to keep tension constant. In the continuous monitoring of reel diameter the brake or drive responds to the given signal and adjusts accordingly. This can be done using some sort of mechanical arm, by electronic calculation or through the use of ultrasonic sensing devices such as Montalvo’s U-3500 system.

The predictive method can only control the brake in relation to diameter and is suitable for machines handling strong and inexpensive materials.

The dancer method is popular in that it controls web tension by using a moving roller or assembly. Although classified as a tension control system, in reality it is more of a variable speed detector with the tension produced as an effect of the force loading of the roller on the web. Dancer systems operate by balancing web tension force with the force loading of the roller. The control system keeps the dancer position constant by adjusting input or output of web speeds.

A pneumatically loaded arm generally produces output pressure in direct relation to the position of the arm. Montalvo’s Dancer Controller regulates the brake so that the position of the arm will not alter as the reel changes diameter.

The drawback to this system is that accurate results can be obtained when the control system is new but that after a time operational characteristics can change and instability can occur leading to production problems. This is not to say that there are not thousands of these systems being successfully operated. None of them can, however, measure or display web tension. They will, however, give some degree of tension control for certain applications and have web storage capability.

The only way to control any variable on the web is to measure it, then compare it with the required value and for electronic mathematical compensation to occur. This is the function of the Montalvo transducer.

The only way of measuring continuously the tension in a moving web is for the web to be bent over a path roller and to measure the force on that roller caused by the combination of tension and the angle of wrap. The resulting signal can be used simply as a measure of the tension but is nearly always compared with a signal representing desired web tension. The error between the two is then processed by a control amplifier to provide stability over a wide range of machine speeds, reel diameters and tensions.

The output is amplified and converted either to pneumatic pressure to work an air brake (generally a Montalvo air brake), electrical brake or to control a motor.

All of the systems operate in conjunction with the web to generate the tension required, which is then sensed by the transducers to form a closed loop system.

The Montalvo System 3000 is dynamically changing parameters on the regulator as the diameter of the reel changes. This is believed to be unique to Montalvo.

Selection of the correct tension control system can only be made after an on site consultation taking into account factors such as the intended process and the condition of the machine. Danarota and their European representatives’ consultative skills are almost as important as the control systems themselves. Only by heeding the advice of experts is it possible to ensure that the correct tension control system is chosen for maximum profitability.

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