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 plane 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 spring index (Mean Diameter/Wire Size) the lower the initial tension will be, and conversely, the lower indexes produce higher initial tension.
Please contact the Newcomb Spring facility nearest you for more information on initial tension and other considerations of spring design.
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