Clock springs, or power springs, are helically wound springs that are naturally inclined to expand; the expansion generates torque or circular movement. However, once the expansion occurs and the spiral length has run out, the clock spring must be rewound.
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Clock Spring Design
Clock springs consist of a flat coil strip and are wound, either tightly or openly, similar to a snail shell in that each coil is nested within a larger coil. Flat coil strips provide a stronger spring than round wire, although round wire is more commonly used in springs. Often, clock springs are wound and mounted on an arbor, which is a piece of round stock that has one end formed to accept the inside end of the clock spring, while the outside end is attached to a stationary post. The power spring is also housed within a cylindrical casing, or barrel. The clock spring is wound by turning the arbor, but it drives the object’s movement by the barrel; this arrangement allows the clock spring to continue powering the object while it is being wound. Winding the object turns the arbor, which tightens the clock spring, wrapping it closer around the arbor. Clock springs are most commonly formed through the cold rolling process. A type of roll forming, cold rolling utilizes roll forming machines, which consist of sequences of calendars, or roller die pairs, positioned both above and below the metal coil that is being formed. As the metal moves through the machine, the rollers bend the material along the linear axis, pressing the metal into a more uniform grain flow while shaping the metal into flat coil strips. Cold rolled clock springs are created at temperatures below the recrystallization point of the metal. The cold rolling process increases the tensile strength of the metal. The clock springs are then annealed or made softer through heating to be wound or coiled. Next, the clock spring is heat treated in order to reduce stresses and fractures and thus further strengthen the spring.
Applications for Clock Springs
The coils of a clock spring are formed by winding spring wire around a cylinder into helically shaped metal springs. There are two coil types for clock springs: tight coils and open coils. Clock springs with tight coils are, in normal applications, completely without friction. Tight coil clock springs are often used as locking mechanisms. Clock springs with open coils are the most common type and must be mounted in housings. Open coil clock springs offer a low increase of force and are often used in applications such as retractable reels, retracting seat belts, mechanical motors, tape measures, timing devices, vehicle suspensions, galvanometers, and electrical switches. Typical industries that utilize clock springs include military, medical, automotive, electronics, agricultural, commercial, industrial, and utilities. Clock springs are most commonly made from a certain type of spring steel referred to as blue or clock spring steel. Blue steel is tempered and polished spring steel, which has a very high carbon content and is generally dark blue in color.