On the frame of the apparatus are pivoted the upper ends of two lateral levers of which the lower ends are respectively pivoted at the two ends of an intermediate horizontal lever, these levers defining three sides of a deformable parallelogram. Three rollers ar carried on a lever which rollers are adapted for clamping the strap circulating around the periphery of a pulley, while another lever carries a deflector roller which co-acts with the loaded strand of the strap for applying the rollers against the pulley. Besides the deflector roller are mounted two guide rollers which are urged by a spring. The loaded strand is guided between the deflector roller and one guide roller while the other strand, which is clamped between the guide rollers, is driven at a speed identical to the speed of the loaded strand but of opposite direction.