Tool marks affecting the fatigue strength of wire springs

Abstract

There may be anxious about various kinds of tool marks caused on wire springs while they are being formed. From among those various tool marks affecting the fatingue strength, we picked up three kinds of tool marks caused on compression coil springs and two kinds of tool marks caused on tension coil springs and we investigated detrimental influence of those tool marks affecting the fatigue strength. We will hereunder summarize the investigation results.
Scratch defects on the first coil of compression coil spring caused by cut end of the wire do not affect the fatigue strength so much. But in case that the depth of tool mark caused on the inner and outer sides of coil spring is about 3% or more of the wire diameter and such a spring is not shot-peened, then the fatigue strength will be weakened by about 50% compared to that of the spring without tool mark. In this case, however, if the spring is shot-peened, the above percentage will be reduced to 10 to 30 in spite of the same depth of tool mark. Furthermore, the tool mark caused on the inner side of the spring will have greater influence to the fatigue strength than that of the outer side of the spring will do.
The tool mark caused on the inner side of hook of tension coil spring will weaken the fatigue strength remarkably, in case that the ratio of width of tool mark to its depth is small. In this investigation, however, no remarkable difference was noticed concerning the tool marks caused on the foot of hook.