Press Release 2-15-12
Katy Spring & Manufacturing, Inc
Compression Springs · Extension Springs · Torsion Springs

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Extension Spring Geometry

Extension springs are typically manufactured with an initial tension. Extension spring initial tension is the force required to break apart the adjacent coils. This fabrication method allows consistent free length.
Since the extension spring initial tension is not zero, the spring rate is not truly linear when measured from the unloaded position. However once the initial tension is overcome, the extension spring does behave linearly.

Extension Spring Shear Stress

extension springs

in their resting state, they also have a shear stress in their coils while at rest. The maximum shear stress, in its free position occurs on the inner face of the coils.

After the initial tension is overcome, the extension spring can be analyzed as a compression spring with a negative force. The maximum shear stress in the extension spring increases with the load.

Extension Spring End Stress

When considering a regular extension spring hook, the geometry of the hook often causes stress concentration which leads to failure.

Extension Spring Safety

With an extension spring, there is no such safety geometry, as with a compression spring, since the extension spring is in tension. For this and other reasons, extension spring maximum working stresses are typically limited to three fourths of those for compression springs of similar geometry and spring material.

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Katy Spring & Mfg. Inc.