Fail to prepare, prepare to fail
Published: 1 May 2019 - Victoria White
The failure of components can have serious consequences and can cause safety concerns and unnecessary costs associated with downtime. Understanding why components fail and knowing how to put clear design processes in place can minimise the risk of this happening. The Institute of Spring Technology is recognised as the industry’s most authoritative and influential voice and is renowned as a unique centre of excellence for spring technology, enjoying an unrivalled pedigree and reputation stretching back many decades.
The main aim of our failure investigation is not just to determine the root cause of failure but to suggest improvements that could be made to mitigate the risk of it occurring again. The most common way that a spring fails is through fatigue. Spring Fatigue is due to repeated operation between two (or more) loads causing a stress on the wire. This stress is below the ultimate tensile strength of the material but the repetitive action of more than ten thousand compressions causes a crack initiation which propagates under the influence of its repeated use.
A contributing factor to the life of the spring is the surface quality of the wire, particularly the presence of a defect that may serve as the initiation and catalyst for the breakage. Consideration must be given to the strength of the material and if decarburisation is present this can significantly reduce the life of the spring . Fatigue cracks can grow rapidly under the influence of residual tensile stresses so extra tensile stresses should be avoided. Corrosion and wear can also lead to damage to the surface of the wire creating an initiation point for fatigue. Therefore it is important not only to determine the failure mechanism but the root cause of failure.
The age old saying goes prevention is better than the cure and here at the Institute of Spring Technology we advise taking several steps to identify the risk of failure first so the life of the spring can be improved. Shot peening is a tried and tested method of accelerating a stream of particles at the component surface, putting the spring under residual compressive stress, increasing its resistance to fatigue crack formation. Another way of preventing failure is to specify a higher strength material or an improved surface quality so the life of the spring can be optimised. Finally, for springs that are too small to shot peen, optimised stress relief heat treatment can be applied and for high duty applications. Using unshaved or super clean raw material and electro polishing can be advantageous too. Tests of which can all be conducted at the Institute of Spring Technology on site at its unique test laboratory.
Other elements of failure can be due to corrosion, stress corrosion cracking, hydrogen embrittlement, liquid metal embrittlement, overloading the spring, or too much relaxation of the spring and general wear and cracking of the spring which contribute to breakages. All of which the Institute of Spring Technology have in depth research and experience at identifying these fields through failure investigations.
The design software allows design and validation work to be carried out right at the very start of the project on a range of spring types. The Spring Calculator Professional (SCP) software is a super user-friendly software which allows designs to be quickly evaluated, and the impact of changes in materials, design standards and spring parameters to be quickly evaluated, all of which help with saving time and money. The life of the designed spring at different operating positions can be predicted using fatigue and relaxation data based on The Institute of Spring Technology’s own research. Other key features included in the SCP software are that it provides Goodman diagrams, Load / Length, and other graphical tools to help evaluate the design. In addition, there are options to allow a design to be optimised to it in with real-world constraints, such as limited space for the spring in the finished equipment. Graphical and test based outputs are available, including engineering drawings, all with the ability to be customised with your company name and logo which adds to the prowess of your design projects. Choosing to use SCP design software can ensure that the spring is designed with the correct tolerances and therefore preventing some unnecessary failures. The Institute of Spring Technology have complete confidence in the Spring Calculator Professional Software and it is used by countless industry professionals.
Source: Industrial Compliance