To keep the same size motor when panel numbers are varied, the invention includes a torsion spring counter balancing mechanism for a stacking panel shutter door, wherein one end of the or each torsion spring is fixable to a drive transmission shaft of the stacking panel shutter door, the drive transmission shaft being adapted to raise and lower the or each shutter of the stacking panel shutter door; and the other end of the or each torsion spring is fixable to a separate rotatable member of the stacking panel shutter door; and whereby the mechanism is provided with means adapted to enable both the drive transmission shaft and the rotatable member to rotate, during opening and closing of the stacking panel shutter door, in such a manner as to cause respective ends of the or each torsion spring to travel at different speeds during the opening and closing sequence.
