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HANDBOOK OF MECHANICAL DESIGN 



When the value of D/d, ratio of outside diameter of spring to diameter of wire, 

 has been determined, the chart below gives the maximum safe load, wire diameter, 

 and mean coil diameter for values of D/d, the spring index. 



MAXIMUM LOAD, WIRE DIAMETER, MEAN COIL DIAMETER, AND SPRING INDEX 



0.2 



0.3 



Q4 0.5 06 0.7 0.8 0.9 1.0 



2 3 4 



D-d = Mean Coil Diam.^ln. 



6 7 8 9 10 



15 2D 



For example, for a spring index of 6, find the wire diameter required if the spring 

 is to be loaded to 100 lb. maximum. Follow the diagonal for D/d = 6 (upper hori- 

 zontal scale) down untU it insects with the diagonal Pounds Load on Spring repre- 

 senting 100-lb. load. By dropping down vertically from this intersection point, the 

 bottom horizontal scale shows D — d = 0.9. On going horizontally from this 

 intersection point to scale on left, d = 0.175 (approx.). Or d can easUy be calculated 

 from knowing that D — d = 0.9 and D/d = 6, from which D = 6d, hence 

 Qd — d = 0.9 or d = 0.180 (exact). This chart is based on 50,000 lb. per sq. in. fiber 



