ARCO is an automatic chain or belt tightener working in rotation. The elastic movement is produced by a spiral spring of zinc plated steel that, conveniently guided, is subject to bending and torsional stress. This particularity confers it a very high index of reaction and a good constant for the ratio between the rotation angle and the developed force. As ARCO is entirely made out of zinc plated steel, it can also be used in extreme situations like for high temperatures (up to about +250°C) and low temperatures (up to about -50°C). Due to its specially designed cubic base one can assemble it in at least five different ways. The catalogue, by means of schemas, suggests some simple and economical usages showing that special anchoring brackets or supports are not necessary. ARCO can be provided with many interface kits: polyethylene sliding blocks, pinions and rollers made of zinc plated steel or plastic. ARCO is available in two versions with different maximum permitted rotation angles: ±40° and ±90° from the rest position.