86 APPLIED MECHANICS 
rigidity of the wire is 11,000,000 lbs. per square inch and the spring has 20 
coils, how much will the spring extend under this pull, and how many ft.-lbs. 
of work must be done in producing this extension ? 
17. What is the stiffness, in lbs. per inch of axial deflection, of a close coiled 
helical spring having 10 coils whose mean diameter is 2 inches, diameter of 
wire 0:128 inch, and modulus of rigidity 11,000,000 Ibs. per square inch ? 
18. What length of wire 0°232 inch in diameter will be necessary to form a 
closely coiled helical spring which shall extend 0:05 inch per Ib. of axial load if 
the mean diameter of the coils is 3°5 inches, and the modulus of rigidity of the 
wire is 11,500,000 lbs. per square inch? 
19. It is required to design a close coiled helical spring which shall deflect 
1 inch under an axial load of 100 Ibs. with a shear stress of 50,000 Ibs. per 
square inch. The spring is to be made out of round steel wire having a modulus 
of rigidity of 11,000,000 Ibs. per square inch, and the mean diameter of the coils 
is to be 10 times the diameter of the wire. Find the diameter and length of 
the wire necessary to form the spring. 
