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Spring Selection
When our engineers validate a new spring drawing or are asked about an existing spring that may be used in a new environment, the first question they will ask is "Is the specified material appropriate?." Assuming the spring service conditions are fully understood and the material is readily obtainable, the four main criteria that the spring material must meet are:
A) Are the working stresses within the permitted design limits, or, put another way, is the material strong enough?
B) Will the spring be at risk of excessive load loss in service?
C) Will the spring be at risk of fatigue failure due to the number of cycles it must withstand?
D) Has the material, or its coating, sufficient corrosion resistance?
There are a few other technical requirements that have to be considered such as "must be non-magnetic" or "must make electrical contact", but these are relatively rare, and are beyond the scope of this article.
Strength
The most frequently used test for measuring the strength of spring materials is the tensile test. It measures accurately the tensile strength, but not the elastic limit, which is the value against which engineers wish to compare the spring operating stresses. They also have formulae for calculating the stresses in almost any spring together with a set of design rules expressed as operating (or solid) stress as a percentage of the tensile strength. If the operating stress is below the design rule – no problem. If it is not, then there are three possibilities: select a higher strength material, incorporate prestress into our spring production (if feasible), or advise the customer of the maximum spring deflection or load before the spring shape will become overstressed and its shape will be changed. Overstressed springs are not recommended, but sometimes this is the only feasible option.
Load Loss
All springs lose load or torque in service – this is called relaxation or low temperature creep. Usually the load loss is low and does not present a risk to the functionality of the spring, but this relaxation process occurs at temperatures as low as ambient. The relative performance of commonly used spring materials is best described by listing the maximum service temperature for each:
1. Plastics 40°C
2. Copper alloys 80°C
3. Carbon steels 120°C
4. SiCr steel 250°C
5. Stainless steels 300°C
6. Best nickel alloys 550°C
Note that low temperatures below 0°C are not a problem. Our engineers may advise prestressing in order to reduce the susceptibility to load loss.
Fatigue
Any spring that is subject to more than ten thousand cycles in its service life could be at risk of failure by a fatigue mechanism. Unfortunately, there is no correlation between the risk of relaxation, fatigue and corrosion, and some springs require resistance to all three of these so sometimes spring material selection will be the best compromise available. Carbon steels and SiCr have the best resistance to fatigue, but even these materials are significantly improved by the use of shot peening, a spring manufacturing process we apply to compression springs mostly, but it can be used for other spring types with less benefit. Glass bead peening is used for stainless steel springs because it improves fatigue resistance of this grade, which is not inherently as good as the aforementioned grades. Indeed, we have fatigue data for a great many spring materials, each with quantified performance with and without shot peening.
Corrosion
One of our previous Engineers Guides covers corrosion in detail - click here to view.
Conclusion
This article provides a logical approach to the selection of spring materials, whilst falling short of giving international specifications for those selected. It also makes clear that selection of spring materials often has to be considered alongside selection of spring manufacturing processes in order to be able to provide springs that are, as near as possible, 100% reliable. Advanex are happy to supply springs in accordance with almost any specification. If we offer a similar, more readily sourced grade as an alternative, this will almost certainly perform satisfactorily, but our teams will have considered each of the performance parameters described above.
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