Effect of Spring Unwinding on Fatigue

An agricultural tine, which is a relatively large double torsion spring with outer legs that are used to sweep through hay or other crops and turn them over, had failed. It was made hard-drawn carbon steel. Bending fatigue was revealed by visual examination to be almost certainly the cause of failure. The fatigue fracture origin was found on the inside surface of the legs at the point where they joined the coiled body of the spring. It was established that the tines after being wound up by loading with hay, sprung back through the neutral unloaded position and into the unwind direction. This movement into the unwind direction was concluded to be happening often enough to initiate fatigue. The stress relieving temperature was recommended to be increased to reduce the residual stresses from coiling and hence improve fatigue performance.