Mechanical springs Classification, stresses, materials and manufacturing processes

Springs are frequently used in mechanical systems and for a wide variety of applications. They are usually simple mechanical components (leaf or helical) whose design is sometimes overlooked in the preliminary design phase. Experience has shown that springs need to be carefully designed and manufactured to ensure the proper operation of the systems in which they are integrated. Historically speaking, the Institute of Spring Technology (IST) was set up in the UK during the Second World War to deal with the problem of weapons and vehicles being taken out of service due to broken springs. Today, the IST is still active and participates in the development of knowledge (with other laboratories around the world) on springs and associated materials.

Environmental issues are driving engineers to push back the limits of technology to find ever more energy-efficient systems, while guaranteeing their robustness and longevity in the interests of sustainable development. In this context, it is imperative to integrate good spring design so that springs do not become the critical elements of systems.

Springs are based on a simple physical phenomenon: any force applied to a mechanical part places it in a state of stress that deforms it. While this property often has detrimental effects on systems, mechanical components known as elastic links or, more commonly, springs, exploit this property in a positive way. Thus, the global displacement associated with the accumulation of local deformations is useful in several cases:

• to maintain a constant force or torque (in the case of fasteners) ;

• to signal or control the value of a force or torque (dynamometers);

• to store and then progressively restore motive energy (in the case of clocks);

• to store and then rapidly release motive energy (as in the case of electrical contacts, valve springs and the deployment of solar panels on satellites);

• to limit the force due to impact (case of buffers) ;

• to filter a vibration (in the case of suspensions).

The field of spring applications is therefore vast, and some of these applications are potentially critical when the mechanisms are high-tech (e.g. satellites) or involve human lives (e.g. car suspension springs).

The aim of this article is to help engineers better design springs for specific applications. To this end, the main mechanical laws governing spring operation are presented. This allows us to classify springs according to internal stresses (normal or tangential). Next, the various categories of materials that can be...