Patent ID: 8905113

Claim:
A door mechanism comprising: a rotatably mounted door shaft to operate a door, the door shaft rotating in a first direction in response to the door being raised, the door shaft rotating in a second direction in response to door being lowered, the door being biased to close; a drive sprocket fixed to the door shaft; a holding brake assembly to counter the closing bias of the door, the holding brake assembly comprising: a holding brake sprocket rotationally coupled to the drive sprocket; a freewheel coupled to the holding brake sprocket, the freewheel configured to provide free rotation of the holding brake sprocket in the first direction; and a first brake coupled to the holding brake sprocket via the freewheel, the first brake providing resistance to rotation of the holding brake sprocket in the second direction; a door release comprising: a drop wheel coupled to the holding brake sprocket via the first brake; a brake drive sprocket fixed to the drop wheel; and a drop arm having a tooth to mesh with the drop wheel, the drop arm having an engaged position and a disengaged position, in response to the drop arm being in the engaged position, rotation of the brake drive sprocket is prevented, in response to the drop arm being in the disengaged position, rotation of the brake drive sprocket is unimpeded by the drop wheel, wherein the drop arm moves from the engaged position to the disengaged position in response to an event; and a second brake having a brake sprocket rotationally coupled to the brake drive sprocket, the second brake being configured to provide resistance to rotation of the brake sprocket in response to rotation of the brake sprocket exceeding a predetermined braking speed.