The oil-tight tensioning system (10) has an oil reservoir (12) containing hermetic oil and closed off by a diaphragm (16). A pressure device (26) is fitted to the side of the diaphragm exposed to atmospheric pressure. A cylindrical holding region (22) is provided on the side of the diaphragm exposed to atmospheric pressure, and is so in engagement with the pressure device that any free movement between the diaphragm and the pressure device is effectively prevented.
