| 摘要 |
A positional control device is provided to prevent accidental drifting of an overhead sectional door upward or downward from a fully opened, partly opened or fully closed position. The device allows the door to be manually moved to a different elevation without requiring a separate operation to release the door from its original position. This is achieved by a mechanism which couples the upward and downward motion of the door to the rotation of a sprocket wheel engaged with a spring loaded ratchet arm with a bifurcated head specially shaped for ratcheting engagement with the teeth of the sprocket wheel. Suitable adjustment of the spring allows the ratchet mechanism to hold the door in its given position by exerting a force which can readily be overcome by manual raising or lowering the door to a different position.
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