The Sergeant Auxiliary Value-Drill (Fig. 11) is strictly speaking a drill for hard rock. It conbines an independent valve operated through an auxiliary valve, and contains a release rotation. These are the two features distinguishing; the Sergeant from other rock-drills. The valve is held in such a position that while the piston carrying the cutting-tool is moved toward the rock the exhaust remains open on one end, while the full pressure acts on the other end until the blow is struck, at which time the valve immediately reverses. The aux. iliary is the trigger to the main valve. It opens or closes the steam or air passages, releasing the pressure from one end or the other of the utain valve. A new rotating device, with a release movement, prevents twisting of the spiral bar or breaking of pawls and ratchets. When a rock-drill strikes a hard blow upon an uneven surface there is a tendency some times to twist the steel in the opposite direction to that in which it rotates. The effect of such a blow on the Sergeant drill is simply to turn the back-head around, overcoming the friction of the back-head springs, when with a rigid rotation it might twist the rifle-bar or break the pawls and ratchets.
The Gilhens this drill, designed by Mr. George M. Githens, of New York, a positive motion is retained for the valve, while at the same time all moving parts between the piston and the valve are done away with. As shown in Fig. 12, the valve 1' itself is placed in direct contact with the piston by which it is actuated. Midway, in the length of the piston, is a wide annular recess having a gen tle inclined plane at each end. The intervening annu lar space round the middle of the piston forms the chamber into which the steam or air is first admitted.
The valve V is over this middle portion, and is in the form of a segment of a circle, fitting accurately against a cylindrical face in the valve-ehest, the axis of this face being at right angles to that of the drill-cylinder. In the outer face of the valve is provided a pair of re cesses properly proportioned for admitting the air past the valve to the ends of the cylinder alternately. The air being admitted into the middle chamber of the cylinder, presses the valve outward and close up against its cylindrical face ; and the piston being at one end of its stroke, the other end of the valve has been raised by the inclined plane, and the valve has been rotated over its curved face, to a sufficient extent to open the port for the admission of the air to the end of the cylinder. The piston is thereby caused to make its stroke, and in so doing it reverses the valve by means of the other inclined plane. At the same time that the admission is taking place to one end of the eylinder, the opposite end is open to the ex haust E through the other recess in the back of the valve. It will tints be seen that the only moving pieces are the piston and the valve.
McCulloch's "Rio Tinto" this drill, shown in Figs. 13 and 1-1. the two pistons forged solid upon the piston-rod perform the double function of acting themselves as outlet or exhaust valves, and also of actuating the inlet slide-valve through a tappet T, struck by a swelling or spherical boss surrounding the piston-rod midway between them. The com pressed air or steam for working the drill is admitted to the valve-ehest, and is distributed by the slide-valve through ordinary ports and passages to the ends of the cylinder alternately.
The exhaust takes place direct from the cylinder through two sets of four holes E, which are alternately covered and uncovered by the pistons. The further extremities of the admission passages, just where they enter the cylinder, are each fitted with a rectangular valve I. which opens for admission into the cylinder, bnt closes against exit therefrom. hence, after the set of four exhaust-holes in front of either piston has been closed by the piston itself, the exhaust-air remaining in that end of the cylinder is compressed to the end of the stroke, thus forming a cushion, and preventing the piston from striking the cylinder-cover. In the forward-stroke, however, owing to the position of the exhaust-holes, the cushioning does not oiler any appreciable resistance to the force of the cutting blow, except when the piston is traveling too far, in consequence either of a soft place in the rock or of the drill not being kept in its proper working position. When the exhaust-air is being compressed in either end of the cylinder, it presses the non-return mushroom-valve /tighter upon its seating; whereas the corresponding valve at the other end of the cylinder is full open for the admission, the driving air pressure being greatly in excess of the strength of the light spring that tends to close the non-return valve. This action takes place alternately at each end of the cylinder. When the inlet slide-valve V' has been moved by the tappet to either end of its travel, so as to close one of the admission-ports and open the other, it is retained in that position by the air pressure acting upon its admitting end, and any movement is thereby prevented during the time that the boss on the piston-rod is not in contact with the tappet. The slide-valve is cylindrical, and the cylindrical casing or chest in which it slides is provided with an oil-hole on the outer side, and on the inner side has a longitudinal slot in which the arm of the tappet moves. At each extremity of its travel the slide-valve is pressed -by the tappet against a stop, consisting of a steel disk D with lndia-rubber backing. When de sired, both the valve and the tappet can lie re versed end for end, for equalizing their wear. During the inward or return stroke of the drill, it is caused to rotate through rather less than a quarter of a turn by means of a rifled spindle .8 fitted into the back-cylinder cover, and carrying a ratchet-wheel R with pawls held up by springs, which allows it to rotate in one direction only. On the spindle works a corresponding bush, fitted in the back end of the piston-rod, in which is also made a cavity long enough to receive the spindle when the piston-rod is at the extremity of its inward stroke. When the drill is making its forward or cutting stroke, the ratchet-wheel and rifled spindle are rotated freely by the bush in the piston-rod ; while in the return-stroke they are held by the pawls from rotating, and consequently the drill is now rotated by the bush through the extent of the turn FFr•ovided in the rifling of the spindle. The drill-cylinder is cast with V-shaped projections sliding in cor responding grooves in the cradle C in which it is mounted. The feed is given by a screw worked by hand. The cylinder is 34 in. diameter with a stroke of 5 in. ; and the weight of the drill unmounted is 808 lbs.