| 摘要 |
A rolling or overhead door assembly is disclosed comprising a shaft having an external thread bounded by a first and second bearing surface, a hub screwingly engaging the shaft and adapted to travel along the shaft, a first sprocket rotatably mounted on the first bearing surface and operably connected with a take-down device for unrolling a flexible door and a second sprocket mounted on the second bearing surface and operably connected with a take-up roller for rolling up the door, and abutments on opposite sides of the first and second sprocket. Upon rotation of the shaft the hub advances along the shaft and frictionally engages the first sprocket until the first sprocket and shaft rotate together and upon counter rotation of the shaft the hub advances back along the shaft and frictionally engages the second sprocket until the second sprocket and shaft rotate together. |
