• Material - Typically stainless steel Type 301 is selected, but high-carbon steel, inconel and other materials are also suitable for constant force springs.

  • Fatigue - A moderate or low number of required cycles will permit the use of a heavily pre-stressed spring with surprisingly high force available from a small package. On the other hand, where space is not restricted, these springs can be designed to provide millions of cycles.

  • Backbend - Unless specifically designed to do so, the constant force extension spring cannot normally be back-bent or reverse wound without permanent deformation.

  • Tensile load - Normally, the spring material should not be loaded in tension by restraining the coil and increasing the load. If it is necessary to place a stop on the travel, it should be applied in the mechanism in such a way as to avoid using the spring itself to transmit a holding force other than that exerted by its natural re-coiling force.

  • Speed and Acceleration - If the two members to be connected by the coiled-band spring are expected to approach each other or separate at high speed or acceleration, testing may be necessary to determine whether or not the spring will perform favorably under such conditions.

  • Torque - The drum and coiled body of the extension spring must be completely free to rotate. No torque load or friction load should be applied to the spool or shaft, as the extended portion must be taken up by the coil readily to maintain full tension. A torque or friction load will also cause a "hysteresis" effect. In an extreme case this condition can render the spring non-functioning.

  • Stability - Unless properly designed, a long, free extended portion of a spring may have a tendency to buckle or curl, particularly at the start of a return stroke. While there is no theoretical limit to such a spring's extension, it is advisable to confine any long extended portion in a way that will prevent curling. This can be done by providing some external support or guide. Stability increases as the width is increased in its relation to thickness. Width should be at least 50 times the thickness. A width to-thickness ratio of 250 is a recommended maximum. Good drum design, good bearings and careful attention to avoid high mass inertia, will contribute to stability.

  • Initial load - From its relaxed condition the coiled-band extension spring must be withdrawn at least 1.5 times its drum diameter before developing full rated load. After this extension is made, the force will be maintained at any further extension.

  • Environment - corrosive atmospheres and/or extreme temperatures will affect the life of a spring.

  • End configuration can be custom designed if readily available end configurations do not suit certain specific applications

  • Method of mounting - should be carefully chosen for maximum performance and consistency.

  • Safety - Uncoiling or extending the spring stores energy in the extended section. If the energized spring is released from all restraint, it will try to return to its normal free condition with considerable force and speed. This movement may be uncontrolled in direction and/or speed. Serious injury can result from contact with an edge, end or coiled section of the spring. A constant force spring must not be over extended. Caution should be exercised when removing a spring from the equipment. Since spring failure may result in injury or damage to other equipment failsafe travel-limiting devices must be built into the unit.



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