Patent ID: 8286685

Claim:
A limit mechanism for a door machine for controlling a door curtain to stop accurately at an upper stop level and a lower stop level, said door machine including an output pulley and a reel actuated by the output pulley for rolling the door curtain, said limit mechanism comprising: a screw rod arranged in parallel with the reel, both ends of which are rotatably supported on a frame, an end portion of the screw rod being provided with a driven pulley which is rotated by the reel; a pair of nut blocks composed of a first nut block and a second nut block, each of the first and the second nut blocks having a threaded hole for joining with the screw rod; a pair of lock devices for locking the first nut block and the second nut block; a guiding cassette fixed on the frame for transforming a rotation of the pair of nut blocks into a linear motion in axial directions of the screw rod when the screw rod is rotated; and a pair of elastic buffer devices arranged respectively at the both ends of the screw rod for defining sliding strokes of the first and the second nut blocks; wherein a polygon-flanged cap portion is provided on one end of each of the first and the second nut blocks, and a knurl portion is formed on a circumferential surface on the other end of each of the first and the second nut blocks; each of the lock device includes a stationary member and a movable member, the stationary member being a rectangular plate with a central hole and a pair of lock grooves each formed on an upper portion and a lower portion of the rectangular plate, the movable member being a square plate with a polygonal accommodation bore centrally arranged and a pair of protruding lugs each formed corresponding in shape to the lock grooves and extending outwardly form an upper end and a lower end of the square plate; the movable member is biased by an elastic element such that the protruding lugs are constantly inserted toward said lock grooves when the stationary member is coupled with the movable member; each of the nut block is formed with a shoulder between the cap portion and the knurl portion; and when said knurl portion is inserted through the central hole of the stationary member, the cap portion is correspondingly received within the accommodation bore of the movable member to the effect that the edge of said cap portion is constrained by said polygonal accommodation bore of said movable member and the nut blocks are prevented from turning with the screw rod.