| 摘要 |
A positive drive for window shades and the like is characterized, first of all, by an opposed pair of channel-shaped tracks confining the right and left side margins of the shade and each also housing one run of an endless chain reaved under a pulley at the bottom and over a sprocket at the top. The bottom of the shade carries a rigid hem bar having eyelets in its ends sized to pass the chain but not a pair of stops carried by the latter that engage the hem bar and unwind the shade off the roller it is wound upon at the top when an exposed run of one of the chains is pulled. Right and lefthand one-way clutches are interposed between the sprockets and roller operative when the shade is lowered to disengage and permit relative rotational movement between the shade drive mechanism and the roller until the stops engage the eyelets in the hem bar. These same clutches engage when the shade is raised to link the drive mechanism and shade roller together for conjoint rotation.
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