Spring life is a very important factor to consider in the design and specification process because springs are often vital components within an assembly where failure may lead to significant consequences. Chris Petts at Lee Spring offers the following thoughts on the subject.
1. First, carefully consider the component design to minimise internal stresses, e.g. by avoiding small radii or complex shapes, ensuring the spring is correctly supported and operating well within its specified capacity. We all know how bad stress can be for us, but it is also the biggest threat to spring health too. You can help your springs live longer by selecting a larger wire diameter or use with a lower final load to relieve those unwanted high stress levels.
2. Optimise your material choices - preferably by consulting your spring manufacturer at an early stage, since material selection of a spring is key to good spring design and long life. Consultation at an early stage will determine if upgrading to a higher tensile range or quality grade material can improve cycle life for a particular application. Choosing the right material for a spring is key to a good, long-lasting performance. There are plenty of materials to choose from, including Super alloys, which offer excellent mechanical strength, maintain shape and structure in extreme temperatures and are non-corrosive and may be used in nuclear, offshore or medical industries, also Nimonic alloy for very high temperatures.
3. Consider the operating temperatures so as to keep within the optimal temperature range for the material chosen. Spring relaxation or loss in spring load occurs faster in high-temperature applications due to stress over time, conversely reducing operating temperatures can reduce the chances of spring relaxation.
4. Minimise the shock loading which occurs when the weight of a load is suddenly increased or speeded up - as in the case of a load being dropped from a height. The speed with which a load is dropped onto your springs will effectively increase the weight of the load, resulting in more damage to the spring; much like when you drop something small on your toe from great height - it hurts! Minimising this can save your springs from coil clash, non-axial forces, dynamic loading and consequential wear, which can all reduce the performance potential and longevity of the spring.
5. Surface treatments, e.g. shot peening for strength. Shot peening is the process of cold working with a machine that dimples the surface of the spring, making it stronger and more resistant to that deadly stress we spoke of earlier. Consider also coatings and surface treatments which can aid corrosion resistance or improve surface hardness or wear resistance. In addition to shot peening, some further options include electropolishing, tumbling, electroplating, electroless plating and chemical coatings such as phosphoric acid wash or paint finishes.
6. Reduce the possibility of resonance - whilst achieving the right frequency usually sounds like a good thing, with springs it actually isn't. When a spring's natural frequency matches the frequency of the operating speed, it will resonate, creating a low but powerful resonant vibration which can cause the spring to break - a lot like how singing opera at an exact note can smash a glass! To avoid this, keep the operating frequency to no more than one-thirteenth of the spring's natural frequency.
7. Pre-Stress for Less Stress - the act of pre-stressing can help springs to live that little bit longer. Pre-stressing is an additional manufacturing procedure which raises the elastic limit in torsion, allowing the use of higher solid stresses in your springs - so they'll be stronger for longer, and far less stressed.
To find out more about how to get the most out of your springs, experienced Lee Spring engineers offer expert assistance and technical support. Further information on Lee Spring products can be found on the website at www.leespring.co.uk.