Technical springs are spring steel components whose function is to deform elastically. They give way under stress and absorb force, which they release again when relaxed and allow them to return to their original shape. Insofar as this function is to work flawlessly 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 regard.
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The durability of a technical spring is defined by the ideal of its use: If it shows no loss of strength and does not break or shorten during a certain spring deflection under a given force, i.e. its function is not disturbed or even disabled, 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 acts on a piece of metal, it initially bends elastically. If the force is increased beyond a certain level, plastic deformation takes place. When the material returns to its original shape after unloading, elastic deflection is present. In the event that the original shape is not regained, we are talking about plastic deformation. In the field of technical spring applications, elastic deflection is the norm. With these requirements, special shaped springs must offer high deformation capacity and reliable durability. Under mostly extreme conditions, the springs must have constant material properties.
This function of stress and deflection is calculated or represented in Hook's law. The elastic behavior is given a specific value that, 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 when the stress is below the yield point of the material used, a slow but steadily progressing plastic deformation takes place. When the spring loses length under constant load, this phenomenon is called "creep". We speak of "relaxation" when the spring loses load under constant compression.
The degree of creep and relaxation depends on different criteria:
In particular, temperature and stress affect creep and relaxation significantly.
The other phenomenon, fatigue, which can extend to fracture of the material, occurs with pulsating stress below the yield point of the metal. The problem begins with the inception 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:
Materials with the same chemical analysis and strength can still have different fatigue properties because there are other material properties that affect fatigue.
Corrosion and temperature also affect the fatigue strength of a spring. In order to be able to precisely assess the risk levels in relation to fractures, well-developed fatigue tests are carried out and the results compared with relevant statistics.
In order to solve a technical problem situation in the long term, the durability of technical springs also plays an important role. The different criteria that have to be considered here make it clear that the solution with technical springs is always a matter of competence and experience. Physical knowledge, material science and accumulated know-how are required to ensure good durability and the respective optimum. You are well advised to rely on specialists like Schaaf if you want the currently best performance of technical springs.
Rely on the experience of the specialist for sustainably successful technical springs!
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