Machinery's Handbook, 31st Edition
304
Springs SPRINGS Introduction to Spring Design
Many advances have been made in the spring industry in recent years. For example: developments in materials permit longer fatigue life at higher stresses; simplified design procedures reduce the complexities of design, and improved methods of manufacture help to speed up some of the complicated fabricating procedures and increase production. New types of testing instruments and revised tolerances also permit higher standards of accuracy. Designers should also consider the possibility of using standard springs now available from stock. They can be obtained from spring manufacturing companies lo- cated in different areas, and small shipments usually can be made quickly. Designers of springs require information in the following order of precedence to sim- plify design procedures. 1) Spring materials and their applications 2) Allowable spring stresses 3) Spring design data with tables of spring characteristics, tables of formulas, and tolerances. Only the more commonly used types of springs are covered in detail here. Special types and designs rarely used such as torsion bars, volute springs, Belleville washers, constant force, ring and spiral springs and those made from rectangular wire are only described briefly. Belleville and disc springs are discussed in the section DISC SPRINGS starting on page 350 . Notation.— The following symbols are used in spring equations: AC = Active coils b = Widest width of rectangular wire, inches CL = Compressed length, inches D = Mean coil diameter, inches = OD − d d = Diameter of wire or side of square, inches E = Modulus of elasticity in tension, pounds per square inch F = Deflection, for N coils, inches F ° = Deflection, for N coils, rotary, degrees f = Deflection, for one active coil FL = Free length, unloaded spring, inches G = Modulus of elasticity in torsion, pounds per square inch IT = Initial tension, pounds K = Curvature stress correction factor L = Active length subject to deflection, inches N = Number of active coils, total
P = Load, pounds p = Pitch, inches R = Distance from load to central axis, inches S or S t = Stress, torsional, pounds per square inch S b = Stress, bending, pounds per square inch SH = Solid height S it = Stress, torsional, due to initial tension, pounds per square inch T = Torque = P 3 R , pound-inches TC = Total coils t = Thickness, inches U = Number of revolutions = F ° ⁄360 °
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