Helical Tension Spring :
Extension springs are helical Tension springs designed to store energy and resist a pulling force. The spring used on a screen door is good example of the most common extension spring. Extension springs must have a means for attaching them to components in your device or assembly. Ends can be loops, hooks, threaded inserts or swivels. The end configurations on extension springs are unlimited but always perform the same basic function. Position of the ends can be random or can be designed with specific orientation to each other and to the extension spring body. Stresses present in the loops can be higher than those in the extension spring body. Calculation of these stresses is complex but it is a vital component or your final extension spring design as they can strongly affect performance, especially in cyclic applications