A closure apparatus for openings for access to industrial machines includes a door which can move, by virtue of the action of a respective actuator which includes a motorized shaft which supports at least two mutually opposite pulleys for winding flexible elements connected to respective substantially upper and external portions of the door. The actuator includes substantially at an intermediate section of the shaft, at least one third idle pulley for winding at least one third flexible element connected to a substantially upper and central portion of the door. The third pulley is provided with at least one position sensor for detecting an unrolling of the third flexible element.