158 STEENGTH OF MATERIALS 



The resilience W of the spring is, therefore, 



If the spring is of rectangular cross section, which is the usual 

 form for plane spiral springs, the stress can be calculated 1>\ formulas 

 (99) and (101), Article 126. 



Problem 154. A steel clock spring \ in. wide and ^ in. thick is wound <n ;i 

 spindle T \ in. in diameter. With a factor of safety of 6, what is the maximum 

 moment available for running the mechanism ? 



Suggestion. The dangerous section occurs at the spindle where the moment is 

 greatest and the radius least. Therefore, in the present case, p = ft in., A = fi ' 4 in.. 



6 = 2 in.; and from Article 22, p max =,- - = 48,000 Ib./inA d can tlu-u ! 



6 

 calculated by fonnula (99), and M by formula (101). 



