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# Design of constant force springs

source：Qianye Precision time：2022-9-14
A constant force spring is a spring that exerts a constant force over its range of motion. That is, it does not obey Hooke’s law. Typically, a constant force spring is constructed as a coiled band of spring steel so that the spring relaxes when fully coiled. When it is unrolled, the restoring force comes mainly from the portion of the belt that is close to the roll. Since the geometry of this region remains nearly constant as the spring unfolds, the resultant force is nearly constant.
By creating a spring with a variable pitch, you can make a push-shape spring with a constant velocity. Placing a larger pitch in a larger outer diameter coil and a smaller pitch in a smaller outer diameter coil will force the single spring to fold all coils at the same rate when compressed.
Design of Constant Force Springs: A constant force spring is a coil of prestressed tape that exerts a nearly constant restraining force against unwinding (Figures 1 and 2). The force is constant because the change in the radius of curvature is constant. This is the case if the change in coil diameter due to buildup is not taken into account. Long reach, constant torque and virtually no friction have led many designers to specify constant force springs in applications such as sliding doors and windows and height adjustment of computer monitors and fans, store display shelves or motor carbon brush motors . Constant force motor springs are also used to drive mechanisms for timers, movie cameras and cable retractors.

Tension type constant force tension springs are supplied in coils with a natural radius of curvature R (Fig. 3). If such a spring is installed on the drum, the drum diameter should be 10% to 20% larger than its natural diameter. There should be one and a half turns on the reel. The active portion of the material is approximately equal to 1.25 times the diameter D. Therefore, the spring will not reach its rated load until the elongation is greater than 1.25D (Figure 1). The radius of curvature of the active part goes from R to infinity and a clearance is required. The recommended distance from the center of the drum to the straight section is 0.8 times the diameter D (Figure 3). Strips can become unstable when stretched for extended periods of time and should be guided to prevent twisting or kinking during recoil. The diameter of the idler must be larger than the natural diameter and must not be used to reverse the natural radius of curvature (Figure 4).

Depending on your application, a constant force spring may be ideal. Consultation with the spring manufacturer for constant force springs is recommended at the design stage. Qianye Precision specializes in the production and manufacture of high-precision clockwork springs, which are widely used in various electronic devices, household appliances, small toys, office supplies, communication equipment, medical machinery, electronic computers, windows, doors, vehicles, etc. The imported raw materials and domestic raw materials will continue to meet your requirements. The company has more than 20 years of experience in product development, production and manufacturing, and has become a well-known brand distributor in China.

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