Web Tension Control with Vacuum Tension Rolls
Vacuum tension rolls and belts provide web tension control by generating a web force against the roll or belt that is independent of the web tension differential (Tension In and Tension Out). The discussion that follows is based on vacuum rolls, but generally applies to vacuum belts as well.
Two things happen when the vacuum is applied to the roll. First, the vacuum under the wrap angle creates a pressure difference between the vacuum level and atmospheric conditions. This pressure difference forces the web against the roll and is independent of web tension differential.
Second, boundary or captured air carried along by the revolving roll and moving web is removed by the vacuum. This allows the web to be in intimate contact with the roll through the wrap angle.
Air removal becomes especially important as roll and web speed increase. The vacuum removes much more boundary air than can be squeezed out by the web as it begins to wrap over the roll.
To prevent the web from slipping on the roll, the frictional force must be greater than the desired tension differential. Frictional force for a vacuum roll is the product of the normal force to the surface and the coefficient of friction.
The normal force pushing the web against the roll has two components. One is the resultant force produced by web tension. The other is the force produced by the differential pressure. This second component is by far the larger force for most vacuum rolls, and accounts for the ability of vacuum rolls to achieve much higher tension differences than other tension control devices.
Three key variables affect the amount of frictional force resulting from differential pressure.
- Coefficient of friction is important. Rougher roll surfaces increase this coefficient, as does better boundary air removal. The less air that remains between web and roll, the higher the coefficient of friction will be.
- Vacuum level is also important, and should be maximized to the extent practical. But this must be carried further, since there is also the question of how much vacuum can be achieved at the interface between web and roll. That vacuum level, like coefficient of friction, depends on how well boundary air is removed.
- Surface area of contact between web and roll is another obvious factor. More contact area means more vacuum roll holding power for a given web width. Therefore, wrap angle and roll diameter are also important in determining the frictional force from the pressure difference.
Suppliers of vacuum rolls can recommend the right roll surface or covering, roll diameter, vacuum level, and hole geometry to achieve a required tension difference for a given web width and wrap angle. Hole sizes and locations help determine how well boundary air is removed so that better contact is made. At times, grooving may also be used to give boundary air an escape path.