ELECTRIC DISTRIBUTION AND WIRING. 



277 



pins which are strong enough to withstand the bending stresses to which they are sub- 

 jected are not perceptibly affected by these slight stresses of compression. 



The weight of wire and ice produces a bending stress in the cross-arms, and the 

 breadth, , and depth, d, of the cross-arms must be sufficient to sustain this bending 

 stress, the length of the cross-arms being determined by the number of wires and 

 their required distance apart. The simplest case is that shown in Fig 160, which 



w 



Fig. 160. 



shows a cross-arm carrying two wires. In this case the dimensions, b, d, and /, as 

 shown in the figure must satisfy the equation : 



607 



bd* 



S = 



(37) 



in which S is the permissible fiber stress of the cross-arm material in pounds per 

 square inch at the points, TT, Fig. 160, and Wis the total weight in pounds resting 

 on one pin. The dimensions, b, d, and /, are expressed in inches. 



A coating of ice one-eighth of an inch thick is seldom exceeded, and it is cheaper 

 to repair the line after an excessively severe sleet storm than it is to make it strong 

 enough to sustain much more than one- eighth of an inch of ice on the wires. 



The permissible values of S may be taken from the table of tensile strengths of 

 timber. 



(2) The stresses due to unbalanced tensions are the most important stresses to be 

 considered in pins and poles. Having given the value of the tension and the angle 

 turned at a corner, the side force, 0", Fig. 160, is easily determined, and the dimen- 

 sions, l f and d r (diameter of pin at base), Fig. 160, must satisfy the equation : 



S' = 



32 WV 



(38) 



