Extension Spring Geometry
Extension springs are typically manufactured with 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
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.