the present invention relates to an endless chain structure incorporating a series of cages with a mechanism for varying the distance between the cages . the cages are arranged in two rows , one above the other , and are very close together in the straight stretches . each of the cages is attached to a mechanism for varying the distances which determine an increase or a reduction in the separation between the cages before or after , respectively their passage over end pulleys or wheels of the structure , so that a cage begins its ascent towards the upper level or its descent towards the lower level , following a curved line determined by the rim of the pulley , to avoid the cage in question from being reached or touched by the cage immediately following and travelling in a direction that is still rectilinear .