SEISMIC METHODS 



875 



introduced below the pistons, and the feed of the drive rod is upward. The 

 upward and downward feed or travel of the drive rod does not interfere 

 with and is entirely independent of its rotation. 



Where the ground to be drilled is comparatively soft and little pressure 

 on the bit is required to secure a satisfactory rate of penetration, a Kelly 

 or grief stem can be used to impart rotation to the drill rod. This is a hollow 



SECTION 



SHOWING 



GUIDE ROD 



Fig. 548. — Mechanism for imparting rotation and 

 feed to drill pipe of hydraulic feed type drill; (A) drill 

 chuck; (K) hydraulic feed cylinders; (L) drive rod; 

 (M) drive rod bushing; (N) beveled gearing; (O) set 

 screvk'; (P) yoke. 



steel rod approximately 14 feet long, the outside of which is either square 

 or round with external splines extending its entire length (Figure 549-N). 

 It is screwed to the top of the drill pipe and passes through a Kelly drive 

 bushing attached to the top of the drive rod on the drill. The drive bushing 

 fits the square sides of a square Kelly or the splines in a spHned Kelly, and 

 its rotation rotates the Kelly but at the same time permits it to slide down- 

 ward as the hole is advanced.* In soft drilling, the weight of the Kelly and 



* Use caution when working in the vicinity of the rotating Kelly to prevent loose 

 clothing from being caught. 



