Durability of technical springs

Technical springs are spring steel components whose function is to deform elastically. Under load, they yield and absorb force, which they release again when they relax and return to their original shape. Insofar as this function is to work perfectly in the long term, the question of the service life and fatigue of a technical spring is of great interest to every user. A look at the factors that determine the durability of springs provides clarity in this respect.

The durability of a technical spring is defined by the ideal of its usefulness: if it shows no loss of force and remains stable over a certain spring travel neither breaks nor shortens under a given forceIf the material is not subjected to stress, i.e. its function is not disrupted or even rendered ineffective, it is considered durable. The conclusion to be drawn from this is that the stress in the material must never be higher than the strength of the material allows.

Whenever a force is applied to a piece of metal, it initially bends elastically. If the force is increased beyond a certain level, plastic deformation takes place. If the material returns to its original shape after the load is removed, elastic deflection has occurred. If it does not return to its original shape, this is referred to as plastic deformation. In the field of technical spring applications, elastic deflection is the rule. With these requirements Special moulded springs offer a high deformation capacity and reliable durability. The springs must have constant material properties under mostly extreme conditions.

This function of stress and deflection is calculated or represented in Hook's law. The elastic behaviour is given a specific value which, depending on the material thickness, provides information about the ultimate tensile strength, i.e. the degree of stress at which the metal breaks.

It is part of the physics of metals that a slow but steadily progressing plastic deformation takes place at stresses below the yield point of the material used. If the spring loses length under constant load, this phenomenon is known as "creep". The term "relaxation" is used when the spring loses load with constant compression.

The degree of creep and relaxation depends on different criteria:

• Tension in the metal
• Temperature
• Yield strength of the metal
• Duration

Temperature and voltage in particular have a significant influence on creep and relaxation.

The further phenomenon, fatigue, which can lead to material breakageoccurs with pulsating stress below the yield strength of the metal. The problem begins with the formation and development of a tiny fatigue crack that grows with pulsation. When the stress in the material reaches the ultimate yield point, the metal breaks.

Three factors determine the risk of such an event:

• Nominal tension value in the spring
• Amplitude of the pulsation
• Ultimate yield strength of the material

Materials with the same chemical analysis and the same strength can nevertheless exhibit different fatigue properties, as there are other material properties that have an influence on fatigue.

Corrosion and temperature also influence the fatigue strength of a spring. In order to be able to accurately assess the risk levels in relation to fractures, advanced fatigue tests are carried out and the results are compared with relevant statistics.

Of course, the durability of technical springs also plays a key role in finding a sustainable solution to a technical problem. The different criteria that need to be taken into account here make it clear that the solution with technical springs is always a matter of expertise and experience. Physical knowledge, material science and grown expertise are required to achieve good durability and the respective optimum. You are well advised to rely on specialists like Schaaf if you want the best current performance from technical springs.

Rely on the experience of the specialist for sustainably successful technical springs!