Mechanical Constant Tension Systems and Their Applications in Web Processing.

Most operations in high-speed web processing require accurate maintenance of constant tension. Conventional tension-control devices rely on the sensing of a tension variation before initiating corrective action via a servo system, which system often includes complex combinations of electronic, hydraulic and/or pneumatic components. Furthermore, such systems are limited to unidirectional control, that is to say they can apply force only in one direction (such as a retarding force imparted by sliding friction) and can therefore not reduce downstream tension if it rises due to causes originating upstream of the control system. In contrast to this, the mechanical constant-tension devices described in this paper anticipate impending tension changes and take immediate preventive action. Furthermore, they are ambidirectional (can apply driving force as well as retarding force), do not depend on sliding friction, are direct-acting and, therefore, are not subject to the delays inherent in servo systems.