—The simplest kind of support is the “stretcher bar.” This consists essentially of a bar so constructed that it may be lengthened or shortened at pleasure, by means of a screw. It is fixed in position by screwing the ends into firm contact with the sides, or with the roof and the floor, of a heading. The machine is fixed to this bar by means of a clamp, which, when loosened, slides along the bar, and allows the drill to be placed in the required position, and to be directed at the required angle. The bar illustrated in [Fig. 26, Plate V.], is that which is used with the Darlington drill; in it, lightness and rigidity are combined in the highest possible degree by the adoption of the hollow section. The mode of setting the bar in a heading is shown in the drawing; the end claws are set against pieces of wood on the floor and the roof, and are tightened by turning the screw with a common bar.
The simple stretcher bar is frequently used in narrow drivings and in shafts of small diameter. But a more satisfactory support in drivings is afforded by a bar suitably mounted upon a carriage designed to run upon rails. The carriage consists simply of a trolly, to the fore part of which the bar is fixed usually by some kind of hinge-joint. It is obvious that the details of the construction of this support may be varied greatly, and numerous designs have been introduced and adopted. In [Figs. 27 and 28, Plate VI.], is shown a support of this character designed by J. Darlington. A single vertical bar is carried on the fore part of the trolly, and fixed, by the usual means, against the centre of the roof. This vertical bar carries an arm, which is capable of turning upon it, as upon a centre, and of sliding up and down it. This arm carries the drill. The central bar having been fixed in position, the arm is slid up to the highest position required, and fixed against the side of the heading. A row of holes are then bored from this arm. When these are completed, the arm is lowered the requisite distance, and another row of holes are bored. This is continued until all the holes are bored over one-half the face. The arm is then swung round, and fixed against the other side of the heading, and the holes are bored over that half the face in like manner. In this way, one-half the heading is kept clear to allow the operations of removing the dislodged rock to be carried on at the same time. If desired, two arms may be used. This arrangement gives undoubtedly great facilities for working the drill, and leaves the heading comparatively unencumbered.
In shaft sinking, the same support, slightly modified, is used without the trolly. The arrangement adopted in this case is shown in [Fig. 29, Plate VII.] The central bar is held firmly in its position by a cross stretcher bar set against the sides of the shaft. The arms are made to revolve upon this bar to allow the holes to be bored in the positions required. When all the holes have been bored, the support, with the machines, is hauled up, by means of a chain attached to the central bar, out of the way of the blast. With this support, the time of fixing, raising, and lowering is reduced to a minimum; while the facility with which the machines may be slid along and fixed to the arm, and the positions of the latter changed, allows the boring to be carried on rapidly.
For open work, as in quarrying, where the stretcher bar cannot be used, the tripod stand is adopted.
The Dubois-François Carriage.
—The support commonly used in France and in Belgium consists of a kind of carriage carrying bars upon which the drills are set. This carriage is used in drivings of all kinds; but it is particularly suitable for tunnelling. It has been adopted, with but slight modification, in the St. Gothard tunnel, and in several other important works of the like character.
A modification of the carriage is shown in [Figs. 30 and 31]. Being designed for ordinary mining operations, it carries but two machines; but it will be readily perceived that, by increasing the number of vertical screws, the same support may be made to carry a larger number. It consists essentially of a vertical frame of flat bar iron a b c d, 8 feet in length, and 4 feet 9 inches in height above the rails, the hinder portion of which rests upon a cast-iron plate e f g h, carried upon two wheels; on this are fixed the two uprights l, l′, which, being bound to the upper part by a transverse bar m m′, form a framing to serve as a support to the two vertical screws p′, q′. The front framing is formed of two longitudinals b c and b′ c′ and the uprights a, a′, and the vertical screws p, q, which are connected to the upper part by the single piece a d. This framing is supported below upon a small trolly with four wheels, connected to the two longitudinals of the framing by a pivot bolt n of T form, the bar of the T being inserted into the elongated openings o cut through the middle of the curved portion of the longitudinals. The cast-iron plate behind, the use of which is only to give stability to the carriage, carries above it, by means of the two curved pieces h, h′, a wrought-iron plate V, upon which the small tools needed for repairs are kept. Two screws, s, s′, carried by lugs cast upon the back of the plate, serve, by turning them down upon the rails, to fix the carriage, the latter being slightly lifted by the screws.
Each machine is supported at two points. Behind, the point of support is given by a cast-iron bracket t, having a projecting eye which enters between the two cheeks formed at the back end of the machine by the continuations of the two longitudinals of the framing. A pin bolt, carried by the machine, allows the latter to be fixed to the bracket, while leaving sufficient freedom of motion to allow of its being directed at the required angle. This bracket, shown in plan in [Fig. 33], is supported by a kind of nut, [Fig. 32], having two handles whereby it may be easily turned. By raising or lowering this, the hinder support of the drill may be brought to the requisite height. To prevent it turning upon the screw, a pin is passed through the hole o, which pin forms a stop for the handles aforementioned. The rotation of the bracket itself is rendered impossible by the form of the vertical screw upon which it is set, as shown in [Fig. 33]. In front, the support is a fork, the shank of which slides along in the piece U, [Figs. 30 and 31]. This support, which is not screwed on the inside, rests upon a nut of the same form as that already described, and the same means are employed to prevent rotation as in the case of the hinder supports.
Section III.—Appliances for Firing Blasting Charges.
In the foregoing sections, the machines and tools used in rock boring have been treated of. It now remains to describe those which are employed in firing the charges after they have been placed in the bore-holes. In this direction, too, great progress has been made in recent times. With the introduction of new explosive agents, arose the necessity for improved means of firing them. Attention being thus directed to the subject, its requirements were investigated and its conditions observed, the outcome being some important modifications of the old appliances and the introduction of others altogether new. Some of the improvements effected are scarcely less remarkable than the substitution of machine for hand boring.