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Material selection critical to spring life optimisation
Getting the choice of spring material wrong can not only impact on spring life, but can also present a hazard to health and safety, as Dan Goss of Goss Springs explains.
It can, of course, always be taken for granted that both customer and manufacturer want the maximum life for any given product. When it comes to the selection of the most appropriate spring for a given applications, the manufacturer has to consider the sheer extremes of the demands on that application which in turn will influence design of the component, the material selected and the manufacturing process.
For example, the small compression spring used in the actuation of an electromechanical switch will perhaps be expected to perform faultlessly for thousands of cycles. In contrast, the compression spring in an aircraft ejector seat mechanism, stored under compression, will be expected to perform to order only once.
The most widely used spring material is music wire – a material with high tensile strength that can withstand high stresses under repeated loading. In operation the material is subject to various degrees of stress and therefore must be highly tensile.
Where high temperature operation is required, we might consider standard stainless steel for the spring material, which will operate in conditions of up to 300°C. However some grades of stainless steel have restricted environmental operating conditions. But note that the basic music wire used for producing springs is available in different grades to meet different environmental requirements.
So when might stainless steel not be appropriate? Type 302 stainless should not be used in conditions where acids are present. This makes the material unsuitable in applications, for example, involving the processing citrus fruits. Instead type 316 should be used. These are examples of the detailed considerations of the most suitable material that need to take place.
Type 316 stainless steel is also suitable for in medical applications where contact with blood and tissue may occur or in low salt conditions, and this is another sector where material specification is a vital consideration. Other components used in the medical industry are often made from platinum, while iridium or gold may also be used. A readily worked alloy, platinum–iridium is much harder, stiffer, and more resistant to chemicals than pure platinum, which is relatively soft.
In the electrical industry, platinum–iridium is also very resistant to high-temperature electric sparks and is widely used for electrical contacts.
Extreme operational environments put quite different demands on the component. In the offshore industry, for example, Inconel is usually the material of choice because of the specific properties of Inconel alloys which render them oxidation and corrosion resistant, and so well suited for service in such environments and those which may be subject to pressure and heat. When heated, Inconel forms a thick, stable, oxide layer protecting the surface from further attack.
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