Spring Material and Allowable Stress

Most springs work under variable stress, and their performance and service life largely depend on the characteristics of materials. The selection of spring material is mainly determined according to the working conditions of the spring, the type of load, whether it is subject to impact load and the allowable stress of the spring material. At the same time, the manufacturability of the spring shall also be considered. The material of spring must have high fatigue limit, yield point and sufficient impact toughness. In addition, the hot formed spring also requires the material to have good hardenability, low overheating sensitivity and not easy to decarburize.

See table 10-2-2 for common spring materials and their properties; See table 10-2-3 to table 10-2-5 for the tensile strength of some spring steel wires and bronze wires.

Cylindrical coil springs are divided into three categories according to the load conditions:
Class I — spring subjected to cyclic load for more than 1×106 times;
Class II – spring subject to impact load with the number of times of cyclic load within the range of 1×103~1×106;
Class III – spring subject to static load and cyclic load for less than 1×103 times.
Allowable shear stress TP and allowable bending stress of three types of springs σ The value of BP is selected from table 10-2-6.

When selecting materials and determining allowable stress, appropriate correction shall be made in case of the following conditions:

a) For important springs, if their damage has a significant impact on the whole machine, the allowable stress shall be properly reduced;
b) The spring treated with strong pressure can improve its fatigue limit, has obvious effect on improving the relaxation under load, and can appropriately increase the allowable stress;
c) The shot peened spring can also improve the fatigue strength or fatigue life, and its allowable stress can be increased by 20%;
d) When the working temperature exceeds 60 ℃, the shear modulus G shall be corrected, and the correction formula is
Gt=KtG, G is the shear modulus of F at normal temperature; G is the shear modulus at the operating temperature; KT is the temperature correction factor, and its value is taken from table 10-2-7.

Table 10-2-7

Spring Material