32 33 from the barrel or roll 11 due to the barrel 11 and shaft 21 being rotated initially as a unit thereby allowing the barrel to rotate freely until bolthead 51 comes into contact with the back end of pawl 47 this is enough to unwind a sufficient portion of the door to enable it to continue unrolling by gravity at this time the rotational inertia of barrel 11 and the downward travel of slatted door 2 disengages pawl 68 from lug 67 fig5 and springs 31 32 33 are re introduced into the system but with a somewhat lesser amount of tension to aid in controlling the downward descent of the door and to assist in lifting the door for emergency exit at the time of release of the chain 57 which is connected to the chain 101 the weight 99 is permitted to drop thereby reacting through the cams 87 and 92 to move the arm 88 rightwardly as seen in fig8 and disengage the jaw clutch means 82 and 91 this frees the centrifugal brake 78 and the sprocket system 7 associated therewith from any influence by the windup means 94 112 with the adjustment of the centrifugal brake appropriately selected it will limit the speed of unwinding of the roll.