mechanism 4 is generally designed as a chain or as a belt if a chain is used gearwheels are employed as wheels to ensure a secure revolution of traction mechanism 4 it has proven to be advantageous to tension traction mechanism 4 for this purpose a tensioning rail 2 as illustrated in fig2 through 4 is pressed against continuously revolving traction mechanism 4 until traction mechanism 4 reaches the desired pretension tensioning rail 2 generally presses with a front side of a base wall 6 on slack span 42 of traction mechanism 4 so that traction mechanism 4 slides across tensioning rail 2 during operation it has proven to be advantageous if tensioning rail 2 or base wall 6 is arched in longitudinal direction l in other words the front side of base wall 6 has a convex design thus it is not possible for traction mechanism 4 to jam in tensioning rail 2 or in base wall 6 in addition a fairly large contact surface for traction mechanism 4 is achieved on the front side it may advantageously also be provided that tensioning rail 2 or base wall 6 has an arched configuration and or design in the transverse direction on the front side facing traction mechanism 4 this ensures a particularly problem free operation base wall 6 of tensioning rail 2 is advantageously planar so that a longitudinal extension and or a transverse extension of base wall 6 in.