EFFECT OF GROUND SURFACE DEPTH IN HELICAL COMPRESSION SPRING STRESSES

摘要

A prevalent coal mill design uses stiff compression springs to force rolls onto a rotating bowl to grind coal prior to combustion in a boiler. The springs are usually plain end (constant pitch) with ground flats at each end. Preloads on these springs are typically 10,000lb to 40,000lb. These springs most often fail in fatigue near the ground flat at ends of the spring. Detailed finite element analyses show that stress concentrations are developed near the flat surface at each end of the spring. First, this paper presents some field experience of spring failures. Then, a parametric study of the depth of the ground of a typical spring showing the effect of this geometry change on spring wire stress concentrations is presented. Modeling the solid volume of the helical spring wire with a mechanical drawing program was developed. This geometry was imported into a finite element program for the rest of the modeling. A cutting plane was used to depict the ground flat. The resulting volume was then meshed with solid finite elements. A force was applied to the ground flat area of one end to stress the spring while the other spring end is constrained in all six degrees of freedom. Some typical spring failures that occurred in the field are presented showing agreement between the predicted location of the stress concentration and the failures.