Patent ID: 8893763

Claim:
A pull cord device adapted to control raising and lowering of a base rail relative to a header rail of a window covering in an upright direction, said pull cord device comprising: a container to be disposed on one of the base rail and the header rail; a spring driving unit disposed in said container and including a cord spool and a spring motor that biases said cord spool to rotate in a cord-winding direction; a pull cord having one end coupled to said cord spool, extending out of said container, and further having an opposite end to be coupled to the other one of the base rail and the header rail; a braking unit disposed in said container and operatively associated with said cord spool and said pull cord for retaining releasably the base rail at a desired height relative to the header rail; and a one-way damper unit including: a housing mounted to said container; a unidirectional clutch disposed in said housing, forming a gap with said housing, and having: a casing having an inner casing surface formed with a groove that extends at an angle with respect to an axis and that has a deep groove portion and a shallow groove portion angularly spaced apart from said deep groove portion, said shallow groove portion having a depth with respect to the axis that is shallower than that of said deep groove portion; a main shaft having an insert segment extending into said casing along the axis, and a connecting segment extending from said insert segment outwardly of said casing; and a clutching member disposed in said casing and including: a fixing frame formed with a mounting slot; a clutching body disposed in said mounting slot, contacting said insert segment of said main shaft, and extending into said groove, said clutching body having a cross-sectional dimension shorter than a smallest distance between an outer surface of said insert segment and a groove bottom of said deep groove portion, and longer than a largest distance between said outer surface of said insert segment and a groove bottom of said shallow groove portion; and a spring member disposed between said fixing frame and said clutching body for biasing said clutching body toward said shallow groove portion; a damping liquid that fills said gap between said housing and said unidirectional clutch; and a driven wheel coupled to said connecting segment of said main shaft and driven rotatably by said spring driving unit.