Window blinds all generally work in the same fashion. A window covering is manipulated by pulling a lift cord or by pulling the covering itself. In the case of a lift cord, strings that connect to the bottom of the window covering are pulled by the cord. The bottom of the window covering moves higher if the cord is pulled and moves lower if the cord is released. The window covering either folds, rolls or collapses. In applications such as venetian blinds, the blinds must first be aligned so that they can be lifted; failure to first align the blinds may damage them. Additionally, there must be some method for the lift cord to remain stationary once the window covering has been raised or lowered to the desired position. In some applications, the cord is wrapped around a hook fastened into the wall. Two common types of blinds, venetian blinds and roller shades, offer different solutions to this problem, which are discussed in the sections below.
Raising and Lowering the Blinds
Venetian blinds contain many individual blinds, called slats. The slats are always parallel with each other, and when the blind is fully lowered, they are the same distance apart. The slats also can all be turned at the same time in the same direction. When the slats are turned so that they are parallel to the window and all touching, most of the light from outside a window is blocked. Turning the slats perpendicular to the window allows light to pass between them. Turning the slats is accomplished by turning a rod that rotates the slats up or down. Typically three sets of three strings run through each slat; each set of strings operates the same way. The middle string raises and lowers the blinds, while the outer two strings form a ladder with a slat on each rung. Turning the rod tilts the slats. Lifting the blinds lets the most amount of light through the window, since all the slats are moved out of the way. The slats must first be turned perpendicular to the window, which allows them to collapse against each other when the bottom of the blinds is lifted. When the lift cord is pulled, the blinds rise. To keep the blinds lifted at any given level, a cog with teeth sits inside the rail into which the the lift cord runs. When the lift cord is pulled towards the cog, usually towards the right, the cog's teeth catch on the cord. When the lift cord is released, gravity causes the blinds to fall and the lift cord to retract.When the cog is caught on the lift cord, which then gets caught between the rail as well, it keeps the lift cord from retracting further and holds the blinds in place.
Another common form of blinds is the rolling blind or roller shade. Roller shades consist of a single sheet of fabric or other thin material that warps around a roll; when the roll is unwound, the sheet covers the window. Inside the roll is a mechanism that creates tension, such as a spring, that keeps the shades rolled up. Tension in the spring increases as the shade is pulled down. A ratchet and pin allow the shades to stay in place when pulled down, despite the spring tension. When the pin is released, usually by pulling the shade down to its limit, the spring tension automatically retracts the shade.