Helical Spring Stress Relaxation
Helical Spring Design Resources – Spring Materials
The primary concern for elevated-temperature applications of helical springs is stress relaxation. Stress relaxation is the loss of load or available deflection that occurs when a spring is held or cycled under load. Temperature also affects modulus, tensile and fatigue strength. For a given spring, variables that affect stress relaxation are: stress, time and temperature, with increases in any parameter tending to increase the amount of relaxation. Stress and temperature are related exponentially to relaxation. Curves of relaxation vs. these parameters are concave upward. Other controllable factors affecting relaxation include:
- Alloy Type – more highly alloyed materials are generally more resistant at a given temperature or can be used at higher temperatures.
- Residual Stress – residual stresses remaining from forming operations are detrimental to relaxation resistance. Therefore, use of the highest practical stress-relief temperatures is beneficial. Shot peening is also detrimental to stress relaxation resistance.
- Heat Setting – various procedures can be employed to expose springs to stress and heat for varying times to prepare for subsequent exposures. Depending on the method used, the effect is to remove a usually large first-stage relaxation and/or to establish a residual stress system, which will lessen relaxation influences. In some cases, the latter approach can be so effective that, in application, compression springs may “grow” or exhibit negative relaxation. Increase in free length does not usually exceed one to two percent.
- Grain Size – coarse grain size promotes relaxation resistance. This phenomenon is used only in very high temperature applications.
It should be remembered that, if a material is used at its maximum temperature, a substantial reduction must be made in applied stress from that used at room temperature.
Content Copyright Spring Manufacturers Institute, Inc.
This information is attributed to, and provided courtesy of, the Spring Manufacturers Institute, Inc. (SMI). Newcomb Spring and SMI provide this as advisory information only, and disclaim any and all liability of any kind for the use, application or adaption of material published on this web site.