If a spring will only be operated for less than 10,000 cycles during its lifespan then fatigue does not need to be taken into account. Any spring that will be performing for more than 10,000 cycles can be affected by fatigue and this needs to be factored in during the design process.
The factors that can affect the fatigue performance in springs include working stress, material surface quality and wear.
When in operation, springs will generally work between two fixed positions. In order to predict the working life of the spring the working stress at these positions can simply be calculated.
A common cause of failure in an extension spring is when the loop of the spring breaks off in the area where the hook meets the body of the spring, this is the area of highest stress. Loops are subjected to bending and torsion stress, when they are formed in extension springs small tooling marks are unavoidable. These marks can increase the likelihood of a failure at this point, sometimes loops are formed using bends that are too small and a small radius can increase stress. Different fatigue performance can be caused by different types of end loop, this can be solved by using a loop with a transition radius between the spring body and the end loop of approximately the body radius.
When an extension spring is required to work dynamically, it must be noted that an extension springs can have around 20% lower performance with regard to fatigue than compression springs.
Material surface quality
The material surface quality is very important when trying to avoid the risk of spring failure, fatigue cracks generally originate from the surface of the material, so the greater the surface quality the more chance of improved fatigue performance.
There are a number of methods that can improve the surface quality, the most popular of these is shot-peening. This involves firing small rounded beads of material at the surface of the spring which will lead to a small residual compressive stress this then lowers the chance of a fatigue crack appearing.
The process of shot-peening is generally carried out only on compression springs and large leaf springs, otherwise the shot could become trapped in the coils of close wound torsion and extension springs. Also, the inside face of the coils would not be peened and this would eliminate the benefits of the process.
The wear of springs can be caused in a manner or ways, when operating it’s important that the maximum deflection of the spring should not exceed 85% of the available deflection. If a spring is working close to its solid (coil bound) length, the number of active coils will reduce due to coils coming into contact with each other and there is a chance that this could cause wear to the contacting faces.
Another cause of wear is when a spring works over a shaft or in a bore, if a spring is able to contact either the shaft or the wall of the bore then the wear can lead to early fatigue. If the inside diameter of the spring is worn this can increase the failure as this is where the working stresses are at their greatest.
Torsion springs have a lower fatigue performance in comparison to compression and extension springs, mainly due the friction and wear between the spring and that shaft. It’s hard to eliminate this with torsion springs however it can be reduced with an effective design.
There are other factors that can affect the fatigue performance such as corrosion, material cleanliness and speed of operation.