For disc springs with a material thickness greater than 6mm, Din 2093 specifies that a contact flat should be applied to the top inside diameter and the bottom outside diameter of the disc spring, see above drawing. These contact flats aid alignment of the disc springs during stacking, but cause a reduction in the lever arm length and therefore an increase in the spring force. This is compensated by reducing the material thickness, which doesn't alter the overall height or spring force at 75% from the original disc but does increase the cone angle.
