130 



HANDBOOK OF MECHANICAL DESIGN 



0.2 0.3 0.4 



Diam. of Wire in Inches 



Fig. 247. — Phosphor bronze wire, S.A.E. 81. Relation of wire diameter to physical properties. 



WAHL CORRECTION FACTOR 



As the spring index, i.e., the ratio of coU diameter to wire diameter, decreases, 

 the maximum stress developed becomes increasingly greater than that as calculated 

 by the conventional formulas. To compensate for this in the design calculations, 

 the Wahl correction factor must be applied. The accompanying chart (Fig. 248) 



2.0 

 1.9 

 1.8 



>- 

 fel.7 



-*- 

 U 



i£ 16 

 I 1.5 



O 



3 

 -13 



iL2 



II 

 1.0 



2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 

 D/b or D/d 



Fig. 248. — Wahl correction factor for different values of spring index. 



gives this correction factor. The optimum value for the spring index is 9, the mini- 

 mum value for practical purposes is 5. Below this figure, the stresses increase rapidly. 



TORSIONAL MODULI 



The torsional modulus of elasticity for Swedish steel, carbon steel, or vanadium 

 steel can be taken as 11, 500,000. For phosphor bronze and brass, a value of 6,000,000 

 can be taken. At high temperature, the value of G drops, as shown in Fig. 254 for steel. 



