Machinery's Handbook, 31st Edition
Spring Design
331
Table 10. Compression Spring Squareness Tolerances
Spring Index
Slenderness Ratio FL / D a
4
6
8
10
12
14
16
Squareness Tolerances ( ± degrees)
0.5 1.0 1.5 2.0 3.0 4.0 6.0 8.0
3.0 2.5 2.5 2.5 2.0 2.0 2.0 2.0 2.0 2.0
3.0 3.0 2.5 2.5 2.5 2.0 2.0 2.0 2.0 2.0
3.5 3.0 2.5 2.5 2.5 2.5 2.0 2.0 2.0 2.0
3.5 3.0 3.0 2.5 2.5 2.5 2.5 2.0 2.0 2.0
3.5 3.0 3.0 3.0 2.5 2.5 2.5 2.5 2.0 2.0
3.5 3.5 3.0 3.0 2.5 2.5 2.5 2.5 2.5 2.0
4.0 3.5 3.0 3.0 3.0 2.5 2.5 2.5 2.5 2.5
10.0 12.0
a Slenderness Ratio = FL D Springs with closed and ground ends, in the free position. Squareness tolerances closer than those shown require special process techniques which increase cost. Springs made from fine wire sizes, and with high spring indices, irregular shapes or long free lengths, require special attention in determining appropriate tolerance and feasibility of grinding ends. Table 11. Compression Spring Normal Load Tolerances Length Tolerance, ± inch Deflection (inch) a 0.05 0.10 0.15 0.20 0.25 0.30 0.40 0.50 0.75 1.00 1.50 2.00 3.00 4.00 6.00 Tolerance, ± Percent of Load 0.005 12 7 6 5 …………………………… 0.010 … 12 8.5 7 6.5 5.5 5 … … … … … … … … 0.020 … 22 15.5 12 10 8.5 7 6 5 … … … … … … 0.030 ……22171412 9.5 8 6 5 … … … … … 0.040 … … … 22 18 15.5 12 10 7.5 6 5 … … … … 0.050 … … … … 22 19 14.5 12 9 7 5.5 … … … … 0.060 …………2522171410 8 6 5 ……… 0.070 … … … … … 25 19.5 16 11 9 6.5 5.5 … … … 0.080 ………………2218 12.5 10 7.5 6 5 … … 0.090 ………………25201411 8 6 5 …… 0.100 … … … … … … … 22 15.5 12 8.5 7 5.5 … … 0.200 ………………………22 15.5 12 8.5 7 5.5 0.300 …………………………221712 9.5 7 0.400 ……………………………211512 8.5 0.500 ……………………………25 18.5 14.5 10.5 a From free length to loaded position. Torsion Spring Design.— Fig. 19 shows the types of ends most commonly used on tor sion springs. To produce them requires only limited tooling. The straight torsion end is the least expensive and should be used whenever possible. After determining the spring load or torque required and selecting the end formations, the designer usually estimates suitable space or size limitations. However, the space should be considered approximate until the wire size and number of coils have been determined. The wire size is dependent principally upon the torque. Design data can be developed with the aid of the tabular data, which is a simple method, or by calculation alone, as shown in the following sections. Many other factors affecting the design and operation of torsion springs are also covered in the section, Torsion Spring Design Recommendations on page 337 . Design formulas are shown in Table 13. Curvature correction: In addition to the stress obtained from the formulas for load or deflection, there is a direct shearing stress on the inside of the section due to curvature. Therefore, the stress obtained by the usual formulas should be multiplied by the factor K obtained from the curve in Fig. 20. The corrected stress thus obtained is used only for comparison with the allowable working stress (fatigue strength) curves to determine if it is a safe value, and should not be used in the formulas for deflection.
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