Patent ID: 6761203
Filing Date: 2004-07-13
Classification: E06B

Abstract:
A window blind comprising:a head rail; a bottom rail; an expandable window covering between said head rail and said bottom rail; a pair of pull cords interconnecting said head rail, said bottom rail and said expandable window covering; and a spring motor including a frame mounted on one of said head rail and said bottom rail, a drive drum mounted rotatably on said frame and provided with a drive gear, a pair of cord spools mounted rotatably on said frame and disposed on opposite sides of said drive drum, each of said cord spools being provided with a driven gear that meshes with said drive gear and being connected to a respective one of said pull cords, a spiral spring wound on said drive drum and having opposite ends connected to said drive drum and said frame, respectively, said spiral spring providing a biasing force for biasing said drive drum to rotate in a direction for winding said pull cords on said cord spools, and a friction imposing mechanism mounted on said frame and operable so as to provide a friction force that acts on one of said drive drum and said pull cords; wherein said spiral spring deforms from an initial state to an extent corresponding to vertical distance of said bottom rail from said head rail; and wherein said biasing force of said spiral spring and said friction force attributed to said friction imposing mechanism cooperate to support the weight of said bottom rail and the weight of said expandable window covering that acts on said bottom rail so as to retain said bottom rail at a desired vertical distance relative to said head rail.