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Relaxation and Fatigue With Spring Expert, Mark Hayes
Introduction
When our engineers receive an enquiry for a new spring, whether it be compression, extension, torsion or a wireform, there are a number of parameters that we consider to make sure that the spring will give the service performance our customers expect. For instance, the spring stresses and tolerances may be within accepted limits, but they would want to check and advise whether there were potential problems of relaxation or fatigue.
Relaxation
A spring, when supplied, will give a value of load or torque of X at a given length or rotational angle. However, during the service life of that spring the load X will diminish somewhat – that is to say the spring will relax. This happens to all springs and the extent of the relaxation is dependent upon the spring material selected, the operating temperature, stress and time. However, the % relaxation that will occur in service will also be influenced by the spring making processes employed in their manufacture.
If our engineers know that a new compression spring has a design life of 10 years operating at ambient temperature throughout, then an estimate of the relaxation can be derived from the extensive data resources available to us. If the spring is constantly stored at L2 and the relaxation estimate is less than 5%, then this would only be flagged as a concern if it was known that maintaining the full design load throughout the life of the spring was important. However, if the estimate was 20% or more, we know that this could seriously jeopardise the correct function of the spring. In these circumstances they would alert the customer to this long term performance, and advise on ways to reduce the relaxation.
For instance some springs would relax much less if made from stainless steel rather than carbon steel. They would also relax less if they were prestressed in manufacture, especially if that prestress process were carried out warm, at 200°C typically. On the other hand, the spring would relax more if it were shot peened, and so the need for this process would be carefully reviewed.
Fatigue
One reason why shot peening may have been specified is to reduce the chance of fatigue failure in service. Springs that operate over a wide stress range in service may be at risk of fatigue once the number of service cycles exceeds 10,000. As with relaxation, our engineers have an extensive database that enables them to predict the risk of fatigue, and the number of cycles at which that risk arises, which may be as little as thirty thousand or as much as many millions.
The spring material selected and the operating stresses will affect the fatigue risk, as will the springmaking processes used during manufacture. Shot peening is by far the most likely process to be used to eliminate the risk of fatigue, but the implications to relaxation would also have to be considered.
Conclusion
Relaxation and fatigue risks are undesirable in our products, and every effort is made to predict and advise about these, as springs that fail to meet their long term performance requirements damage the reputation of the whole supply chain.
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