Better Instrument Springs

Electrical measuring instruments play an important part in the generation, distribution, and sale of electrical power, and in the development and testing of electrical machinery. The accuracy of electrical measuring instruments depends as much on the quality of the control springs as on the design of the torque producing elements. Unstable effects found in the application of spiral springs to electrical instruments arise from aging in service and hereditary hysteresis in the spring material. There is little available information in the technical literature on these effects. In order to produce the most satisfactory spring controlled instruments for the electrical industry, there should be recorded in the technical literature a considerable body of detailed information on springs. Information is needed on such subjects as the effect of composition, condition of material, forming methods, stabilizing treatments, design details, residual stresses, service conditions, and temperature on the performance of spiral instrument springs. The information presented in this paper resulted from torsional pendulum tests, hardness tests, spring uncoiling tests, Various forming and stabilizing treatments, and measurements of hereditary hysteresis with the grid glow micrometer. The results of these tests are presented graphically, showing that moderate temperature heat treatment has an important effect on hereditary hysteresis, that the softening range of cold worked spring ribbon is very critical, that the forming temperature and forming time have a controlling effect on residual stresses, that a stress relief anneal reduces aging and hereditary hysteresis. The nature of the action of hereditary hysteresis is defined.