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Fatigue Life of ASTM A228 Music Wire Tension Springs Near Elastic Limit

We are working to maximize the spring tension on a stretch hose similar to a vacuum cleaner hose, with a very total cycle life of 10,000 extensions and retractions, but will have slight local stretching and extending during use that can be 20% of the fully range of motion of the spring and have hundreds of these minor stress changes near, but below, the elastic limit of the wire used.

We are using 0.047" music wire ASTM A228 to form a coiled tension spring with a 1.0 inch diameter that will be excessively pre-biased to have a strong retracting force even when fully retracted. During the first use the spring, the spring will be extended to its final working length for the first time, which will be beyond the elastic limit of the spring with its initial pre-biasing. At this first use the spring will set its final biasing. During operation, the spring will be stretched up to this working length repeatedly and return to a retracted length (spring is always tension biased). Also, when at its working length, the spring may retract and extend as much as 20% of its full range of motion hundreds of times during one extension and retraction cycle of the hose. Tests have shown that the 0.047" wire under this type of pre-biasing and set, can provide 2.0 lbs of retracting force when retraced and 3.5lbs of force at its full working length. This however is taking the wire to its elastic limit and will cause problems with the life of the spring.

Question: What surface quality, and/or wire preparation is needed to insure that the above described tension spring can survive 10,000 cycles each with hundreds of minor motion cycling with loading up to and near its set elastic limit as described above?

If this is not possible to achieve, how much pre-bias is acceptable or how close can we get to the elastic limit to maximize retracting force?

Gary Ragner
Manufacture - Gainesville, Florida, USA
June 21, 2010