Door release mechanism
A door mechanism includes a bracket and a door shaft, rotationally mounted to the bracket, to operate a door. The door mechanism includes a gear assembly having a ring gear, set of planetary gears, sun gear, and drive sprocket. The ring gear fixed to the door shaft. The door mechanism includes a holding brake to counter the closing bias of the door. The door release has an engaged position in which rotation of the door shaft in the second direction is impeded by a first brake; and a disengaged position in which rotation of the door shaft in the second direction is unimpeded by the first brake. The door mechanism includes a link and a second brake. The link is configured to melt in response to a predetermined temperature. The second brake is configured to provide resistance in response to component rotation exceeding a predetermined speed.
1. A door mechanism comprising:
a bracket;
a door shaft to operate a door, the door shaft being rotationally mounted to the bracket, 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 gear assembly comprising:
a ring gear fixed to the door shaft;
a set of planetary gears mated to the ring gear, a set of respective axes of the set of planetary gears being fixed relative to the bracket;
a sun gear mated to the set of planetary gears; and
a drive sprocket fixed to the sun gear;
a holding brake to counter the closing bias of the door, the holding brake comprising:
a holding brake sprocket rotationally coupled to the drive sprocket;
a freewheel coupled to the holding brake sprocket to facilitate free rotation of the door shaft in the first direction; and
a first brake coupled to the holding brake sprocket via the freewheel to provide resistance to rotation of the door shaft in the second direction;
a door release having an engaged position and a disengaged position, in response to the door release being in the engaged position, rotation of the door shaft in the second direction is impeded by the first brake, in response to the door release being in the disengaged position, rotation of the door shaft in the second direction is unimpeded by the first brake;
a link to secure the door release in the engaged position, the link being configured to melt in response to an ambient temperature exceeding a predetermined temperature; and
a second brake having a brake sprocket rotationally coupled to the drive sprocket, the second brake being configured to provide resistance to the brake sprocket in response to rotation of the brake sprocket exceeding a predetermined braking speed.
2. The door mechanism according to claim 1 , wherein the freewheel includes a ratchet and pawl.
3. The door mechanism according to claim 1 , wherein the first brake is a frictional brake comprising a friction plate sandwiched between a first plate and a second plate, the first plate is fixed to rotate with the holding brake sprocket, the second plate is fixed to rotate with the drop wheel.
4. The door mechanism according to claim 3 , wherein the first brake includes a spring to urge the first plate towards the second plate.
5. The door mechanism according to claim 1 , wherein the second brake is a centrifugal brake.
6. The door mechanism according to claim 1 , wherein the second brake is a viscous brake.