DRILLS, ROCK. 1. Dutta..s DRIVEN BY STEAM OR Ant.— 7'he Sergeant This machine has a positive vale. moved by direct cottac•t with the piston. It is used in quarry-work where the steam is wet, and where the rock is reasonably soft, such as slate, sand stone, oolitic limestone, etc. The valve is of rocker form, and is moved by shoulders on the piston. The valve and rocker are i 11 one piece.
The Rand Drill Cu,'s the invention and design of this chine the object was to obtain a better steam distribution than had before prevailed in chines of this class. The chief resulting differences between this machine and others are as follows: 1. In the so-called " tappet" machines the motion of the piston is arrested at the eonclu sion of the return or inboard stroke by a live-steam cushion. obtained by giving the valve a great degree of "lead." In this machine the piston is stopped (so far as is so to do) by an exhaust-steam cushion. obtained by closing the exhaust port soon after the return stroke has commeneed. and the steam thus compressed forms a portion of that used to effect the succeeding striking stroke. 2. In •tappet" machines the steam is used without expansion. In this machine expansion is introduced to any desired extent. 3. "Tappet " machines strike a cushioned blow. This machine strikes an uneashioned blow.
The cushioned blow is a necessity with " tappet"-valve gears—this necessity arising From the following circumstances: The length of stroke of a rock-drill is not constant. As the drill hole progresses in depth the cylinder must be correspondingly fed forward. but to effeet this feed with perfect regularity is fount) to be an impossibility. The effect of this irregular feed of the cylinder is to vary the point marking the end of the stroke of the piston—the ap proach of the piston to the lower cylinder-head varying from stroke to stroke. :Moreover, in starting a hole, and under certain circumstances, it is occasionally desirable to he able to feed the cylinder forward, so as to shorten the stroke still more than is actually necessary to ac emnmodate the usual irregularity of feed. In brief, the machine !mist be able to take strokes of considerably less than normal length without failure: to trip its valve. in order to continue in uninterrupted action. This circumstance has usually been provided for by simply giving the valve a great degree of lead at the lower end of the cylinder, tripping the valve at a point previously decided upon as the end of the shortest stroke to be allowed. and then submitting from necessity to the loss of power due to the cushion thus introduced into all strokes of usual length. In the maehine abort to be deserilied. provision has been made for this irregular feed and length or stroke, hut nevertheless, when fnli-length strokes are made, the valve does not move, nor is ste,ini admitted below 1110 piston. until the actual delivery of the blow.
Figs.:1, .1, and R are longitudinal sections taken on the }woken line A B D of Pig. 2, the and being shown in a number of successive positions. Fig. 2 is a cross-sec
tion on the line of Pig. 2.
lu Fig. :t 1Iu• piston has just completed its striking stroke and is ready to eommenee its return stroke. The steam which effected the preceding striking stroke has been exhausted the opening h, which forms only exhanst port for lhe upper or left-hand end of the cylinder. Stearn enters at the supply nozzle a, flows through the longitudinal groove b in the cylinder (seen also in Fig. 2) to the broad, shallow circumferential groove c in the piston, The longitudinal groove b is of such length as to maintain constant communication betweer the nozzle a and the circumfer ential groove c. Its aim is to lessen the otherwise inconvenient length of the circumferential groove c. This in turn dimin ishes the length of piston and cylinder, and hence weight of machine. This circumferential groove c forms, in effect, the steam chest of the machine, and from it the steam is distributed alternate ly to the opposite ends of the cylinder. Through the passage d steam pressure is maintained in the lower end of the valve-chest, firmly holding the valve in the position shown. Steam flows through the passage e e, and from this through the neck f of the valve to the passage gg, which in turn leads it to the lower end of the cylinder. The piston now starts upward, and presently takes the position shown in Fig. 4. In passing from the position of Fig. 3 to that of Fig. 4, the piston has closed the ports d e h and opened i j. Closing h confines the ex haust steam in the upper end of the cylinder, forming an exhaust cushion before the piston, 1111(1 ac complishes the first improvement named above. Closing- d merely isolates the steam already in the end of the valve-chest. Closing e cuts off the supply of steam to the lower end of the cylinder, and for that end effects the second im provement aimed at. Opening i has no effect, as its upper end is still closed by the valve. Opening j establishes coin inimical ion between t he lower ends of cylinder and and hence, as expansion goes on from the cut-off, the pressure acting on the end of the value will gradually fall. In Fig. !i the piston has ascended still farther, and uncovered the port, k, admitting steam through the passages / and a, respectively, to the upper end of the cylinder and valve-chest. The former completes the work of stopping the motion of the piston the latter, being opposed only by expanded steam at the lower end of the valve, as just ex plained. shifts the valve downward, thus establishing communication bet ween the port g g and the exhaust passage 0. The piston now commences its descent, and closes and opens the various ports in the reverse order to that just explained. Closing k has no effect, as l being now open, the steam can toss through it to the upper end of the cylinder. Closing i effects the cut-off for the upper end of the cylinder, exactly as closing e aid for the lower ma.