What does maximum safe deflection mean? (Stress value, calculated number cycles)

In a static application, this is the maximum safe deflection from free position that will not result in the spring taking a permanent set. For a compression spring, the permanent set will result in reduced free length and force output. For an extension spring, the permanent set will reduce force output by reducing initial tension or increasing the free length.

In cyclic compression spring applications, this would be the maximum deflection to which a spring could be compressed from free length that still assures appropriate spring life. The cyclic condition maximum safe deflection is significantly less than the static application maximum safe deflection.