Door release mechanism

1. 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.

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 second brake is a centrifugal brake.

4. The door mechanism according to claim 1 , wherein the second brake is a viscous brake.

5. The door mechanism according to claim 1 , further comprising a worm gear connected to the door shaft and a worm mated to the worm gear, the worm being fixed to an operating shaft configured to receive a crank-arm.

6. The door mechanism according to claim 1 , further comprising a link to secure the drop arm in the engaged position, the link being configured to melt in response to an ambient temperature exceeding a predetermined temperature and to move the drop arm to the disengaged position.

7. 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 rotate with the drop wheel.

8. The door mechanism according to claim 7 , wherein the first brake includes a spring to urge the first plate towards the second plate.

9. A door mechanism comprising:

a door shaft rotatably mounted to operate a door, the door being raised in response to the door shaft rotating in a first direction, the door being lowered in response to the door shaft rotating in a second direction, 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;

a door release comprising:

a drop wheel coupled to the holding brake sprocket via the first brake;

a brake drive sprocket fixed the drop wheel; and

a drop arm selectively engaging 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 holding brake sprocket in the second direction urges the brake drive sprocket to rotate, wherein the drop arm moves from the engaged position to the disengaged position in response to an event; and

a second brake rotationally coupled to the door shaft, the second brake being configured to provide resistance to rotation of the door shaft in response to the door shaft exceeding a predetermined rotational velocity.

10. The door mechanism according to claim 9 , wherein the freewheel includes a ratchet and pawl.

11. The door mechanism according to claim 9 , wherein the second brake is a centrifugal brake.

12. The door mechanism according to claim 9 , wherein the second brake is a viscous brake.

13. The door mechanism according to claim 9 , further comprising a link to secure the drop arm in the engaged position, the link being configured to melt in response to an ambient temperature exceeding a predetermined temperature and to move the drop arm to the disengaged position.

14. The door mechanism according to claim 9 further comprising a gear assembly, comprising:

a first gear fixed to the door shaft; and

a second gear mated to the first gear, the second gear being fixed to an operating shaft configured to receive a crank-arm.

15. The door mechanism according to claim 14 , wherein the first gear is a worm gear and the second gear is a worm.

16. The door mechanism according to claim 9 , 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 and the second plate is fixed rotate with the drop wheel.

17. The door mechanism according to claim 16 , wherein the first brake includes a spring to urge the first plate towards the second plate.