1. Loop types should be very clearly specified. The term “regular loops” should not be used, as this term is not universally stan­dard.
  2. Loop lengths should be specified within the limits shown above. If they are above these limits, they should be considered hooks. (Note: Usually, the end is considered a loop when the opening is less than one wire size; the end is considered a hook when the opening is greater than one wire size).
  3. Loop position should be specified as either random or to a specified position.
  4. Loop openings should be specified as either approximate (to one wire size) or as a specified opening (e.g., 0°, 90°, 180°, 270° etc).

In designing extension springs, it is important to be aware that as the space occupied by the machine loop is shortened, the transition radius is reduced and a substantial stress concentration occurs. This contributes to shortened spring life and premature failure. Most extension spring failures occur in the area of the end. To maximize the life of the spring, the path of the wire should be smooth and gradual as it flows into the end. Tool marks and other stress concentrations should be held to a minimum. A minimum bend radius of 1.5 times the wire diameter is recommended.

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