TENSIONING DEVICE FOR A SAFETY BELT

The invention relates to a tensioning device (10) for a safety belt, in particular in a motor vehicle, comprising a gas generator (17), a piston (21) guided in a tube (16) which seals a pressure chamber (20) in the tube (16), wherein the pressure chamber (20) can be subjected to pressure by the gas generator (17). The piston (21) is drivable by the pressure present in the pressure chamber (20) at least indirectly to perform a tensioning movement, wherein the piston (21) has a, preferably dome-shaped, end face (3) for engaging a mass body (19) that is displaceable in the tube (16). The piston (21) has a narrowed channel, which can be enlarged by a pressure increase in the pressure chamber (20), by a thermal effect associated with a pressure increase in the pressure chamber (20), or by material removal caused by a pressure increase in the pressure chamber (20). The channel is formed on the end face (3) of the piston (21).