Abstract: A hydro-pneumatic tensioning unit, particularly for use on a tracked vehicle for pressing a guide wheel against the tensed track comprising a cylinder and a tensioning piston, which piston defines a working space in the cylinder. A volume of gas is separated from said working space by a partition and a membrane which will withstand compression and acts against the partition. The edge of the membrane is attached either to the cylinder itself or to the interior of a hollow piston, by a holding member employing prestressing or compressive force. At least one penetration opening is provided in the partition which is attached to the cylinder or the piston. The partition limits the excursion of the membrane in the direction of the working space and its surface which faces the membrane is contoured to match the shape of the membrane.