A tensioning device has a tension arm mounted on a base. A pulley is mounted on the tension arm for engaging a looped belt. By changing mounting bushings, one can select either a static mode wherein the tension arm is held stationary relative to the base, or a dynamic mode wherein, in response to a changing force applied between the pulley and the belt (due to vibrations or load changes, for example), the tension arm moves relative to the base in such a manner as to maintain the force substantially constant. Several such devices can be employed in a system for controlling the tension of a looped belt, one or more of the devices being in the static mode and one or more being in the dynamic mode. Several such systems can be employed in a complex machine such as a photocopier or medical apparatus for controlling the tension in a number of different belts.