Industrial springs – more than meets the eye
21 December 2016
The industrial spring will not appear to most people as being a particularly complex engineered component.
Their applications are found every day, all over the place, but, and it is a big ‘but’, they are rarely seen, being hidden away and therefore many aspects of the design/material selection are overlooked. When you start to think about the basic function of a spring you begin to appreciate the layers of complexity involved in ensuring that the right design and material for the application are selected after careful consideration of all aspects of the operating requirements.
Nick Goss from Goss Springs puts it: “A spring either stores energy or releases it”. He goes on to illustrate the point with a couple of pertinent examples of applications at the extremes of this statement.
In ejector seat mechanisms the powerful compression spring is stored in the compressed state until required for use – perhaps once only – but the design and material used are crucial to ensuring that this fail safe component will work when required. At the other end of the scale, a very small spring used in the actuator mechanism of a microswitch to turn an appliance on or off may have to perform many thousands of times in the life of the product, and it has to work every time.
Material selection determines life expectancy
The type of application that the spring is to be used in will of course determine life expectancy, performance requirement and, depending on what the life expectancy might be, the type of material used. As a general rule of thumb says Nick Goss, “the more expensive the material we use, the longer the life of the component”. A typical application for a compression spring in an engine valve might involve 8,000 cycles per minute and the best material to guarantee this sort of performance is chrome silicon.
Special materials for arduous conditions
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, well suited for service in such environments which may be subject to pressure and heat. When heated, Inconel forms a thick, stable, oxide layer protecting the surface from further attack.
The material used isn’t always “special”
Whilst Goss Springs are experienced in the selection, production, heat treatment and finishing processes associated with the most expensive and complex materials, the performance limits of normal grade stainless steel are impressive.
For many standard duties a spring produced from range three music wire, or high tensile stainless steel is perfectly adequate. In operation the material is subject to various degrees of stress and therefore must be highly tensile.
A standard stainless steel will operate in conditions of up to 300°C. However, some grades of stainless steel have restricted environmental operating conditions. The basic “music wire” used for producing springs is however available in different grades.
Stainless steel not always appropriate
Here is a good example of the detailed consideration of the most suitable material that needs to take place: type 302 stainless should not be used in conditions where acids are present, making it unsuitable for applications processing citrus fruits where instead type 316 should be used – which is incidentally also suitable for in medical applications where contact with blood and tissue may occur or in low salt conditions.
Medical and electrical applications can require very special materials
Components used in the medical industry are often made from platinum; iridium or gold. A readily worked alloy, platinum–iridium is much harder, stiffer, and more resistant to chemicals than pure platinum, which is relatively soft. Platinum–iridium is also very resistant to high-temperature electric sparks and is widely used for electrical contacts.
The spring is a safety-critical piece of the jigsaw puzzle
Consider this point: if a spring fails in service because it has been working beyond the limits of its capacity then the product or component in which it has been used will also fail - perhaps with more costly consequences than the price of the spring. This is why, even for what appears at first to be a relatively simple component, it is critical that from the outset of a project, time is taken to ensure that the right product, material and specification for the application are carefully selected.
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