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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

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 such in 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.

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1. A device for placing a looped belt under tension, the device comprising: a tension arm moveably mounted on a base, said tension arm lying substantially in a first plane;a pulley mounted on said tension arm for rotation about an axis substantially perpendicular to said first plane, said pulley ada

1. A device for placing a looped belt under tension, the device comprising: a tension arm moveably mounted on a base, said tension arm lying substantially in a first plane;a pulley mounted on said tension arm for rotation about an axis substantially perpendicular to said first plane, said pulley adapted to be engaged by the looped belt;wherein said tension arm is formed with a slot and said base is formed with a threaded hole;a screw passing through said slot and into said threaded hole and formed with a screw head; andfirst and second interchangeable bushings having different axial lengths, either of which can be interposed between said screw head and said tension arm such that said screw passes through a selected one of said first and second bushings and into said base, wherein, upon tightening said screw, said first bushing enables said screw head to clamp said tension arm to said base, thereby establishing a static mode wherein said tension arm is held stationary relative to said base, and said second mounting bushing prevents said screw head from clamping said tension arm to said base, thereby establishing a dynamic mode wherein, in response to a changing force applied between said pulley and the looped belt, said tension arm moves relative to said base in such a manner as to maintain the force substantially constant. 2. The device according to claim 1 wherein each of said first and second bushings has first and second portions, the first portion adjacent to said screw head and having a diameter too large to pass through said slot, and the second portion extending from the screw head and having a diameter small enough to pass through the slot and said tension arm toward said base; wherein said second portion of the first bushing has an axial length dimension short enough to enable said screw head to clamp said tension arm to said base, and wherein said second portion of the second bushing has an axial length dimension long enough to prevent the screw head from clamping said tension arm to said base. 3. The device according to claim 2 and further comprising a spring bracket mounted on said base and a spring engaged between the spring bracket and said tension arm, wherein said spring bracket is formed with a slot that overlaps and is narrower than the slot in said tension arm, whereby the first mounting bushing enables immobilization of both the tension arm and the spring bracket and the second mounting bushing enables immobilization of the spring bracket but prevents immobilization of the tension arm. 4. The device according to claim 1 comprising reference indicia on said tension arm facilitating adjustment of said pulley against the looped belt. 5. The device according to claim 1 wherein the force includes a component that acts in a first direction on said pulley and said tension arm, further comprising biasing means that applies a second force that has a component that acts in a second direction on said pulley and on said tension arm, the first and second directions being opposite each other. 6. The device according to claim 5 wherein the biasing means comprises a spring bracket mounted on the base and a spring engaged between said spring bracket and said tension arm. 7. The device according to claim 6 wherein said spring can be mounted to urge said tension arm in any of a plurality of first directions in order to place the looped belt under tension when run around said pulley in such a way as to tend to translate said pulley in any of a plurality of second directions opposite the first directions. 8. A device for placing a belt under tension, the device comprising: a tension arm movably mounted on a base for rotation about a first axis;a pulley mounted on the tension arm for rotation about a second axis extending substantially parallel to said first axis and substantially perpendicular to said tension arm and adapted to be engaged by the belt;a fastener extending through an arcuate slot in said tension arm and into said base; andplural selectively interchangeable bushings of different axial lengths, one of which is adapted to be interposed between said fastener and said tension arm so as to enable said fastener to clamp said tension arm to said base; and another of which is adapted to be interposed between said fastener and the tension arm so as to permit rotation of said tension arm relative to said base in response to a changing force applied between said pulley and the looped belt. 9. The device of claim 8 wherein said plural interchangeable bushings comprise a pair of bushings of different axial lengths.