a fabric strip when the fabric strip is subjected to considerable negative air pressure due to installation between a duct and an inlet of an axial flow fan or blower application_number US 69757996 A description in fig1 and 2 two metal strips 1 and 3 clasp the fabric portion 2 of a conventional pre fabricated flexible duct connector assembly with continuous single or double fold seams fig1 shows the connector assembly in its flat condition as it emerges from the roll forming equipment fig2 shows the same connector formed into a circular collar fastened with overlap seams and ready for installation the clenched fabric portion 2 is free to be manipulated axially like a bellows or laterally so as to compensate for duct to equipment misalignment only the width of the fabric portion restricts its freedom to move in all directions fig3 shows the collar of fig2 in cutaway section one metal end 1 placed tightly over a round duct end 4 the other end 3 over a circular inlet flange and axial flow fan assembly 5 the metal ends 1 and 3 are positioned in a non extended mode to permit slack in the fabric sleeve 2 air movement through the duct shown by an arrow creates a suction at the fabric portion causing it to invaginate reducing its free x sectional area thereby restricting the passage of air through it the degree of constriction which can be considerable is dependant upon the physical size of the duct and equipment the amount of slack in the fabric sleeve and the air volume and static pressure of the system in fig4 the same air movement conditions exist as in fig3 except that a flat metal ring 6 has been introduced substantially centrally.