INSTALLATION OF TRENCHING 



called a cable-guard (Fig. 55), to prevent the cable from 

 leaving the groove or getting jammed between the block and 

 the sheave. 



The block of the pulley P (Fig. 56) is fastened by a small 

 chain n to the link of a large chain a b c, the extremities of 

 which are secured to the anchors A A' ; at each furrow the 

 point of traction n is displaced in the direction of b a dis- 

 tance equal to the width of the furrow. If the traction 

 cable C C/ is returned at a right angle on the pulley P, the 



effort f gives with 

 effort/' * the resultant 

 effort R which being 

 reported on the point 

 of traction n, may 

 be resolved into its 

 two components p and 

 P, which represent 

 the effort of traction 

 exerted on the anchors 

 A 7 and A. This dia- 

 gram shows that the 

 anchor A has to support an effort much greater than the 

 anchor A' and also much greater than the effort/" of the cable. 



Fig. 56. Diagram showing resultant of efforts of 

 traction. 



Fig. 57. Stationary Runner. 



Fig. 58. Stationary Runner used near the 

 Capstan. 



* The effort / is equal to effort/' (resistance of the plough and friction 

 of the cable on ground), plus the friction resistance of the pulley and the 

 resistance of the cable to bending round the pulley. Therefore, in reality, 

 the effort /is a. little greater than the effort /' and the resultant R is 

 the diagonal of a rectangle. 



