128 



HANDBOOK OF MECHANICAL DESIGN 



2. Physical Properties 



This wire shall have a tensile strength of at least 100,000 lb. per sq. in. but should 

 be capable of being bent through an angle of 180 deg. around a ware of the same 

 diameter without breaking. 



3. Appearance 



The wire shall be uniform in quaUty and temper, cyhndrical in shape, and smooth 

 and free from injurious defects. 



4. Dimensional Tolerances 



The wire shall not vary from the specified diameter by more than the following: 

 Sizes over 0.050 in., by plus or minus 1 per cent 

 Sizes 0.050 to 0.025 in., by plus or minus 0.0005 in. 

 Sizes under 0.025 in. by plus or minus 0.00025 in. 



DESIGN CALCULATIONS 



Class I springs, i.e., springs subjected to rapid continuous deflections over a uni- 

 form stress range from zero to maximum or from an intermediate stress to maximum, 

 as in engine valve springs, must be designed on the basis of the endurance hmit of 

 the material. Class II and Class III springs, respectively, springs that operate only 

 intermittently or springs that are statically loaded are designed on the basis of the 

 static strength of the material. 



Because the static strength of wire of a given material increases with decreased 

 wire diameter, as shown in Figs. 243 to 247, a larger permissible stress can be used for 

 the smaller wires. The following table gives the maximum permissible working 

 stresses for springs for Class II and Class III service. 



