In spring design, there a a variety of parameters to consider. Some of the parameters are solely physical (wire diameter, coil diameter, length, etc.) and some are determined from the physical dimensions in combination with the material properties (spring constant, allowable stress, etc.).
One physical characteristic of a spring is whether or not it will experience set. This is a permanent deformation of the spring after it has been compressed. Whether or not set is an issue depends on the level of stress that the spring undergoes versus the material's tensile strength.
The common rule of thumb is that set is not a concern if the spring only experiences 40% or less of its tensile stress. If the spring experiences between 40% to 60% of its tensile stress, then the set can be compensated for during manufacturing. Above 60% of the tensile stress, set cannot be controlled.
The reason that spring set can be compensated for is that it occurs after only a few cycles and then the full amount of set has been reached.
Creep is a separate phenomenon from set. Set happens immediately, but creep happens with constant load over time. Since the metal is under stress, it should exhibit creep. Creep is also affected by temperature.
I want to know how a spring is affected by long-term constant load. I have not been able to find any information about creep in springs or how to calculate it during spring design.
How is the long-term creep of a spring determined?