Machinery's Handbook, 31st Edition
310
Stresses in Springs
7) Loading — static, dynamic, or shock 8) Operating temperature 9) Design of spring — spring index, sharp bends, hooks.
Consideration should also be given to other factors that affect spring life: corrosion, buckling, friction, and hydrogen embrittlement decrease spring life; manufacturing operations such as high-heat stress-equalizing, presetting, and shot-peening increase spring life. Item 5, the type of service to which a spring is subjected, is a major factor in determining a safe working stress once consideration has been given to type of spring, kind and size of material, temperature, type of loading, and so on. The types of service are: Light Service: This includes springs subjected to static loads or small deflections and seldom-used springs, such as those in bomb fuses, projectiles, and safety devices. This service is for 1000 to 10,000 deflections. Average Service: This includes springs in general use in machine tools, mechanical products, and electrical components. Normal frequency of deflections not exceeding 18,000 per hour permit such springs to withstand 100,000 to 1,000,000 deflections. Severe Service: This includes springs subjected to rapid deflections over long periods of time and to shock loading, such as in pneumatic hammers, hydraulic controls and valves. This service is for 1,000,000 deflections, and above. Lowering the values 10 percent per mits 10,000,000 deflections. Fig. 1 through Fig. 6 show curves that relate the three types of service conditions to allowable working stresses and wire sizes for compression and extension springs, and safe values are provided. Fig. 7 through Fig. 10 provide similar information for helical torsion springs. In each chart, the values obtained from the curves may be increased by 20 percent (but not beyond the top curves on the charts if permanent set is to be avoided) for springs that are baked, and shot-peened, and compression springs that are pressed. Springs stressed slightly above the Light Service curves will take a permanent set. A curvature correction factor is included in all curves, and is used in spring design calculations (see examples beginning page 317 ). The curves may be used for materials other than those designated in Fig. 1 through Fig. 10, by applying multiplication factors as given in Table 1.
160 150 140 130 120 110 100
Hard Drawn Steel Wire QQ-W-428, Type II; ASTM A227, Class II
Light Service
Average Service
Severe Service
90 80 70
Wire Diameter (inch)
Fig. 1. Allowable Working Stresses for Compression Springs—Hard Drawn Steel Wire a
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