How is tension created?

Tension is a force tending to elongate a web. Tension and elongation are directly related to each other through the web’s spring constant.
Tension can be created by a direct force, a force created by torque transmitted to a radius, or by stretching/straining the web through elongation or speed change.

Reminder: Tension may imply a tension force (defined in force units of Newtons or Pounds) or a lineal tension (defined as Force per Width in N/m of width or pounds per inch width, PLI).

Tensioning by Direct Force

The most obvious way to create tension is to hang a weight on a strip of web. The average tension will be the weight divided by the web width. In web handling, force is applied to the web via a ‘dancing’ roller (a.k.a. a dancer roller). In most cases a dancing roller is wrapped with 180 degrees, so the web tension will be half the dancer load divided by web width.

Tensioning by Frictional Force

A second direct force option is to apply a frictional load to the surface of the web. If you grab the web or the surface of an unwinding roll with your hand (please be safe if you do this), as the web slips under your hand, the web tension will build up proportional to the frictional load. This concept is used in one of the simplest constant tension systems, a weighted leather strap hanging on the outside of an unwinding roll. This technique is rarely used due to 1) damage or scratch the web, and 2) variability in friction coefficients.

Tensioning by Torque

A second direct force option is to apply a frictional load to the surface of the web. If you grab the web or the surface of an unwinding roll with your hand (please be safe if you do this), as the web slips under your hand, the web tension will build up proportional to the frictional load. This concept is used in one of the simplest constant tension systems, a weighted leather strap hanging on the outside of an unwinding roll. This technique is rarely used due to 1) damage or scratch the web, and 2) variability in friction coefficients.

Tensioning by Draw (Speed Ratio)

Since most of web handling revolves around rollers (pardon the pun), the logical way to create tension is to control torque transmitted to the shafts of unwinders, winders, and rollers. Rotary brakes and clutches are torque controlling devices. Many motors can also be operated in a torque control mode.

The tension (in force per width, such at N/m or lbs/in) created by an applied torque is equal to the torque (in N-m or lbs-in) divided by the web width (in m or in) and the radial distance (in m or in) from the web to the axis of rotation. For a given winder or unwinder torque, the tension is inversely proportional to roll radius.

Braked or clutches rollers are less common. Torque applied to a roller will attempt to create a torque differential, assuming the frictional force at the web to roller contact is high enough to avoid slippage.