Azhar; I have used a slack box when working with fragile threads. It consists of a fixed rod with independant pulleys on it and a movable rod in guides with more independant pulleys on it. the thread is wraped around the pulleys and creates a constant tension on the thread as the supply or takeup(depending on which end of the machine you are at) is designed to keep the movable rod within the designed range. If a break occures the moveable rod falls to the bottom breaking a beam or tripping a switch. I suspect that your machine will have a similar tension regulating system that you can use for your detection point. hth Terry