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Constant Force Springs
Constant force springs are a special type of extension spring that provide a long linear reach with minimum force buildup as well as being able to store power indefinitely when extended. In general, constant force springs are used for the storing and absorption of energy, similar to suspension systems, and the maintaining of tension or force. Unlike coil and flat springs, constant force springs act as retracting mechanisms, providing even, uniform load through its entire travel length as it is uncoiled and recoiled. Constant force springs can also provide mechanical motion in applications such as clock springs, which must be rewound once the spring length has run out; they are also used in applications such as electric motors, fitness equipment, gardening equipment, toys, medical devices and commercial planes. Unlike other types of springs, constant force springs do not obey Hooke’s Law of Elasticity. Hooke’s Law states that how much a spring can stretch is directly related to its load; so long as its load does not go over what the spring can stretch. Constant force compression springs do not obey this law because the inherent stress of the springs resists the loading force at a nearly constant, or zero, rate of force. This allows constant force springs to be able to provide mechanical force simply by unwinding the spring. Constant force springs will naturally want to recoil and must be unwound by another force, such as a gear. Able to help remove the effect of gravity, constant force springs also have very small space requirements and can be beneficial for a variety of industries include aviation, electronics, furniture, hardware, medical, commercial, industrial, defense and retail. Constant force springs are a combination of a flat spring and a coil spring, being constructed of one long flat coil strip that has been wound and coiled like a snail shell, with each coil nesting inside of a larger coil. The constant force spring is then heat treated to retain this coiled shape. Flat coil strips provide a stronger spring than round wire; although, round wire is more commonly used in springs. Constant force springs are typically mounted to the object by tightly wrapping the spring on a drum and then attaching the free end of the spring to the loading force. However, this relationship can also be reversed. Spring steel is often cold-rolled to make constant force springs since spring steel is a low alloy, medium carbon steel with a very high tensile strength that lends itself well to heat treatment. Also, spring steel is well known for its excellent elastic properties. However, materials such as stainless steel, copper, bronze, stainless steel, titanium, molybdenum, hastelloy and magnet wire can also be cold-rolled to make constant force springs. In cold rolling, roll forming machines consist of sequences of calenders, or roller die pairs positioned both above and below the metal sheet or coil which 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 grainflow while shaping the metal into flat coil strips. Cold rolled parts are created at room temperature, but can be at higher or lower temperatures as long as it is below the recrystallization point of the metal. The cold rolling process increases the tensile strength of the metal. The constant force springs are then annealed, or made softer through heating, so that they can be wound or coiled. Next, the constant force spring is heat treated in order to reduce stresses and fractures and thus further strengthen the spring.
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