BLU and BLUD are automatic axial chain or belt tighteners designed to work in damp environment or in contact with corrosive agents. BLU is composed of a plastic column thrusted by a zinc plated or stainless steel spring which slides inside a box-shaped plastic body,. BLU can be provided with a large amount of interface kits: polyethylene sliding blocks, polyethylene wheels on fork, plastic or steel pinions on fork, rollers of zinc plated steel or plastic on fork, shielded rollers made of stainless steel or plastic on a stainless steel fork. BLUD presents the advantage of working on two columns thus developing a double force compared to that of the base element BLU. BLUD can be provided with special anchoring brackets of different shapes in order to facilitate its positioning. Besides it can be equipped with a polyethylene sliding block with elevated radius of curvature for a high winding degree and with large connectors at the extremities which even in angled positions. make the entrance and exit of the chain easier BLU and BLUD are provided with a preloading system in order to facilitate the installation operations. BLU and BLUD can be equipped, furthermore, with an electric travel-end switch and with a light signal (LUX or ALUX) positioned inside the column. These last two options are used to increase the operators’ security at work and to fully respect the rules of accident prevention at work. Within the range of BLU products are also the axial grazing slide guides GS and GD. Although these products do not work with a high precision, they are easy to use and versatile linear guides, realized in self-lubricating material, apt to confer an economical answer to most construction requirements. GS and GD, indeed, were designed to slide along conventional retail bars made of steel or aluminium.