Helical extension springs store energy and exert a pulling force. Usually they are made from round wire and are close-wound with initial tension. Extension springs are stressed in torsion in the body. Design procedures for the body are similar to those of compression springs with the following major exceptions. Most helical extension springs are coiled with initial tension equal to the minimum force required to separate adjacent coils. Extension springs do not normally have set removed. Furthermore, unlike compression springs, extension springs do not have a solid stop to prevent them from overloading. For these reasons, design stress levels are generally lower for extension than for compression springs.
Extension springs require a method of attachment to other parts in an assembly. A wide variety of ends have been developed and used successfully for many years. Examples of this include threaded inserts, swivel hooks, twist loops, side loops, cross over center loops, and extended hooks. Loops are ends which have little or no gap while hooks are loops with large gaps. The most common configurations are those that can be formed during the manufacturing operation such as twist, side, cross over center and extended hooks and loops. Many of these hooks are made by bending the last coils of an extension spring to form the ends. Other types of ends are made using straight wire and forming hooks which are then inserted into the ends of the extension spring with the end coils then being closed down around the hooks or loops.
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