Extension Springs General Data: Extension springs are springs which absorb and store energy by offering resistance to a pulling force. Various types of ends are used to attach the extension spring to the source of the force. Special considerations should be given when choosing a hook for your extension spring. Keep in mind, the suggested allowable hook stress in torsion is 30-45% (depending on material) of its MTS (Minimal Tensile Strength), while that for bending is 75% of its MTS. The Spring Team engineering department will gladly help you with your end configuration. The functional design characteristics of the spring should be given as mandatory specifications. Secondary characteristics, which may well be useful for reference, should be identified as advisory data. This practice controls the essential requirements, while providing as much design flexibility as possible to the spring manufacturer meeting these requirements. Spring Team always verifies incoming designs to insure manufacturability and informs customers when designs are over stressed or are not capable of being produced. Designs are always verified using SMI's (Spring Manufacturers Institute) most current released computer design program and our engineering department will share the derived printouts with those interested. Spring Team has a design handbook which we will gladly ship out to engineers to assist in their spring development needs.