Abstract
This paper describes development work on springs for precision measuring instruments in which the various errors or deviations from desired behavior of the springs were studied separately. These errors were then compensated for by the design of a new-type helical spring, the combined error of which was reduced to well within 0.05 per cent.
Variations due to change in elastic modulus with changes in temperature were eliminated essentially by the development of a special alloy of a modified elinvar type containing 36 per cent nickel; 8 per cent or more chromium; 4 per cent of other elements including manganese, silicon, molybdenum, copper, and vanadium; and the remainder iron. By making use of a high degree of cold drawing followed by a moderate-temperature heat-treatment and by other special processing, safe working stresses were raised to 60,000 lb per sq in. in shear, while the error due to creep under working loads was reduced to less than 0.02 per cent and the error due to “back error” or hysteresis was reduced to less than 0.04 per cent.
Due to changes in pitch angle of wire and in radius of the coil as the load is applied, the load-deflection curve for a truly elastic helical spring of round wire deviates slightly from a straight line. Accurate formulas for the curve are developed in the paper. Using wires with the proper rectangular cross section, the resulting helical springs will theoretically have an exact straight-line rate. Actual behavior of these springs approximates very closely their theoretical behavior. An error results from the twisting of the springs under load. This has been corrected by using two springs in series, oppositely wound.
Various other means of compensating for the various errors were also studied, some of which are mentioned in the paper.
