Total number of coils is counted from tip to tip. Springs with closed ends or with closed and ground ends have one inactive coil at each end. Springs with open ends are considered to have virtually no inactive coil. Springs with open ends ground are considered to have about one-half inactive coil at each end.

The active coils are what make a spring a spring. The active term should really be applied to any portion of a spring that stores and releases energy. In the case of a compression spring, the active portion will expand as the spring is compressed. The opposite is typical of an extension spring. The action of a clutch spring tightening down on a shaft is typical of the diameter change as a torsion spring winds up.

When designing a spring and specifying its dimensions, it is critical that the number of coils is counted correctly, as this can have a huge effect on the strength of the spring. It is a straightforward process - simply start at one end of the spring, where the wire has been cut, then follow the wire round every time you go through 360o that counts as a full coil (180o = coil; 90o = coil etc.) The compression spring pictured right has five total coils (not six). The same method applies to extension springs and torsion springs.



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