An Investigation of Small Helical Torsion Springs

The object of the investigation was to provide easily handled data for the design of helical torsion springs from round patented plain carbon spring-steel wire. For this purpose, the validity of the curvature correction to the stress formula was investigated and the effects of variations in low-temperature heat treatment and wire size were explored.

It was found that the theoretically derived curvature correction for maximum stress (stress at intrados) was the dominating feature in fully heat-treated springs under all conditions, and for patented wire springs when the maximum stress was tensile. In patented wire springs, however, when the maximum stress was compressive it was found that, although with springs of small index the maximum stress was still the dominant feature, with those of large index yielding was due, not to the maximum compressive stress at the intrados, but to the smaller tensile stress at the extrados, and that a critical ratio existed where yielding occurred simultaneously at both extrados and intrados; a finding that has considerable practical value. It is suggested provisionally that the phenomenon may be due to the combined effect of the low-temperature heat treatment, which relieves textural stresses (present in the wire as the result of the drawing operation and the presence of cementite) and at the same time allows the strain-age-hardening effect to become evident.

Design charts are submitted incorporating the results of the above investigation.