The Strength Characteristic of Helical Springs

Author:  Thurstoon, R. C. A.
Source:  Canadian Mining and Metallurgical Bulletin, v. 44 Oct 1951, p. 658-667
Doc ID:  1951008
Year of Publication:  1951
Abstract:  
A brief discussion of the design of the helical compression spring is given together with the method of manufacture, particularly in regard to the effect on strength characteristics. Design is based on three formulas which correlate the energy stored "U", the spring rate or applied load per inch deflection "S", and shear stress in a helical spring "q". The formula for shear stress neglects the effect of direct shear due to axial load, and the increase in shear stress due to difference in fiber length between the inside and the outside of the coil. This is corrected by applying Wahl's factor "K". Compression tests were run on truck bolster springs made of plain carbon, silico-manganese and chrome-vanadium steel. A 15,000 lb. load was applied in 1,000 lb. increments and the results plotted in the form of load-deflection and load-strain curves. In most cases the strains were less than the theoretical as calculated from the stress formula and Wahl's factor. However, Wahl's factor is a satisfactory basis for design. The steel bar stock generally used in spring manufacture is hot rolled. The surface of the bar is rough, decarburization is present, and seams occur frequently. This places surface condition as the controlling factor in spring life. Chemical composition and hardenability are secondary. Fatigue tests on the springs which had been previously static tested showed peened springs to have longer life than unpeened springs. Also, the tests indicate that plain carbon steel springs are at least comparable to alloy springs in fatigue. However, alloy springs are justified on the basis of higher hardenability, higher elastic limit and greater impact resistance. The two main methods for improving spring performance are shot peening and scragging. Shot peening affects surface tensile stresses by developing residual compressive stresses, thus increasing the endurance of the part. In scragging, the spring is initially coiled to a greater length than specified and then loaded beyond the yield point to produce set. After several loadings, no further permanent set takes place and the spring is elastic up to a higher load. In order to obtain best performance in hot wound springs, both for fatigue life and minimum setting in service, the spring should have negligible decarburization, should be accurately wound, and have evenly spaced coils, should be heat treated correctly and adequately shot peened and should be pre-set before service.