Spring failures (breakage) can occur due to a variety of reasons; stress, tool marks, wire imperfections, to name a few. Katy Spring has been examining spring failures and making recommendations to customers for years that can prevent recurrence. One issue looked at frequently is the spot of the fracture or failure of the spring. Many times tool marks or wire imperfections are blamed for the cause of compression spring failure, however it is common for compression springs to initially break on the ID of the spring.
The reason compression springs break on the ID is attributed to direct shear stress and wire curvature. When a compression spring deflects there is a rotational stress placed on the wire also known as torsion shear stress. The maximum stress on the wire when a compression spring deflects is on the surface of the wire. The torsion stress is applied to the inner diameter of the wire and is opposite to the applied force of the outer diameter of the wire.
Springs that break, normally break on the ID of the compression spring or extension spring because of direct shear stress on the ID and spring curvature.
Springs that breaks initiate on the spring OD are usually the result of abuse or material defect.