During the assembly process of the spring washer, the diameter of the spring washer becomes larger and expands outward. This phenomenon is called ring expansion. This phenomenon has a long history, and the reason for it has been reported as early as half a century ago, but so far there are different opinions and no clear conclusion.
In 2000, the fifth issue of "Aerospace Standardization" also gave a detailed explanation on the discussion of "the reason for the expansion of the spring washer". The main reason is that the spring washer is gradually flattened with the rotation of the nut. The force of the spring washer gradually increases, and it reaches half of the elastic test load value when it is flattened. Continue to apply pressure to the nut until the generated axial preload reaches the elastic test load value of the spring washer, and then apply pressure to the nut, and the nut will A radially outward frictional force is generated on the spring washer, which can easily cause the spring to expand outward due to the radially outward frictional force, which is difficult to solve at present.