How to Calculate Spring Creeping?

The solution to your problem is to use a pre-stressed spring.

This means that the spring is wound with higher pitch and compressed so that a small plastic deformation occurs. When the spring comes back to its expected free height the zone compressed over the elastic limit is in tension and will not, under load, come again in a compressive status which leads to a loss of tension.

An elasto-plastic FEA program or a computation based on a simplified material behaviour in the elastic and trans-elastic domains can help to define the increased pitch but without tests it is not possible to be sure due to mechanical properties dispersion.

This is used also for valves springs which have to offer same force over time.

The other way is to take a reserve and consider a loss of 5 to 10% and for such periods foresee a check every for instance 5 or 10 years !

Thanks,

I will look into pre-stressed springs.

Do you know how long it will take before any loss of tension occurs on a normal spring? (For the other option I will be fine with a 5 -10% loss over 10 years or longer).

Impossible to predict without knowing conditions and even in the case they are known there is a broad material properties dispersion which cannot be mastered.