Machinery's Handbook, 31st Edition
Spring Design 329 Step 10: The large majority of hook breakage is due to high stress in bending and should be checked as follows: From Table 6, stress on hook in bending is: . . . , S IDd PD 5 0625 00625 5 9 06875 139 200pounds per square inch b 3 2 3 2 × × × = = = This result is less than the top curve value, Fig. 8, for 0.0625 inch diameter wire, and is therefore safe. Also see Note 5 that follows. Notes: The following points should be noted when designing extension springs: 1) All coils are active and thus AC = TC . 2) Each full hook deflection is approximately equal to 1 ∕ 2 coil. Therefore for 2 hooks, reduce the total coils by 1. (Each half hook deflection is nearly equal to 1 ∕ 10 of a coil.) 3) The distance from the body to the inside of a regular full hook equals 75 to 85 percent (90 percent maximum) of the ID. For a cross-over center hook, this distance equals the ID. 4) Some initial tension should usually be used to hold the spring together. Try not to exceed the maximum curve shown on Fig. 16. Without initial tension, a long spring with many coils will have a different length in the horizontal position than it will when hung vertically. 5) The hooks are stressed in bending, therefore their stress should be less than the maxi mum bending stress as used for torsion springs—use top fatigue strength curves Fig. 7 through Fig. 10. Method 2, using formulas: The sequence of steps for designing extension springs by for mulas is similar to that for compression springs. The formulas for this method are given in Table 3. Tolerances for Compression and Extension Springs.— Tolerances for coil diameter, free length, squareness, load, and the angle between loop planes for compression and exten- sion springs are given in Table 7 through Table 12. To meet the requirements of load, rate, free length, and solid height, it is necessary to vary the number of coils for compression springs by ± 5 percent. For extension springs, the tolerances on the numbers of coils are: for 3 to 5 coils, ± 20 percent; for 6 to 8 coils, ± 30 percent; for 9 to 12 coils, ± 40 percent. For each additional coil, a further 1 1 ∕ 2 percent tolerance is added to the extension spring values. Closer tolerances on the number of coils for either type of spring lead to the need for trimming after coiling, and manufacturing time and cost are increased. Fig. 18 shows deviations allowed on the ends of extension springs, and variations in end alignments. Table 7. Compression and Extension Spring Coil Diameter Tolerances Wire Diameter, Inch Spring Index 4 6 8 10 12 14 16 Tolerance, ± inch 0.015 0.002 0.002 0.003 0.004 0.005 0.006 0.007 0.023 0.002 0.003 0.004 0.006 0.007 0.008 0.010 0.035 0.002 0.004 0.006 0.007 0.009 0.011 0.013 0.051 0.003 0.005 0.007 0.010 0.012 0.015 0.017 0.076 0.004 0.007 0.010 0.013 0.016 0.019 0.022 0.114 0.006 0.009 0.013 0.018 0.021 0.025 0.029 0.171 0.008 0.012 0.017 0.023 0.028 0.033 0.038 0.250 0.011 0.015 0.021 0.028 0.035 0.042 0.049 0.375 0.016 0.020 0.026 0.037 0.046 0.054 0.064 0.500 0.021 0.030 0.040 0.062 0.080 0.100 0.125 Courtesy of the Spring Manufacturers Institute
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