Abstract: An automatically braking mechanism for a rolling door system is used in connection with en elongate support shaft for selectively rolling up or lowering the rolling door formed of a series of connected slats. A driven wheel is fixed on the support shaft and a motor drive is provided for driving a chain drive sprocket wheel and a continuous roller chain couples the drive and driven sprocket wheels. The roller chain normally moves along a predetermined path prior to failure of the roller chain when the roller chain is under tension. A braking mechanism includes a first elongate sensing arm pivotally mounted at one end and carries a follower roller at the other end which is normally in direct contact wit the roller chain. A second elongate actuator arm is integrally formed with the sensing arm so as to be pivotally mounted for sharing common angular movements with the sensing arm.