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Industry News - Technical Article
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Initial Tension - What Is It?
posted February 25, 2008
One of the least understood elements of spring design is initial tension in extension springs. So often designers will guess at the amount of force necessary to overcome this one time load, or they will disregard it entirely. Though two springs may be identical in physical size, they can vary substantially in load at a given length depending on the amount of initial tension.
A highly technical description of initial tension is coiling the spring against the plain of its own axis. However, most spring-makers simplify the description by saying that initial tension is simply the amount of force that is required to begin deflection.
Generally, the higher the index (Mean Diameter/Wire Size) the lower the initial tension will be, and conversely, the lower indexes produce higher initial tension.
However, the range of initial tension will vary in each case. Initial tension is the most achievable and controllable when the resulting stress is between 30% and 60% of the tensile of the material, divided by the spring index (D/d). Requirements outside this range cause difficulty in the manufacturing and control of the process and should be discussed with the spring manufacturer.
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