Figs. 205-207.
The boring-head while at work may suddenly be jammed fast, either by breaking into a fissure, or in consequence of broken rock falling upon it from loose strata above. All the strain possible is then put upon the rope, either by the percussion cylinder or by the winding engine; and if the rope is an old one or rotten it breaks, leaving perhaps a long length in the hole. The claw grapnel, shown in [Fig. 199], is then attached to the rope remaining on the winding drum, and is lowered until it rests upon the slack broken rope in the bore-hole. The grapnel is made with three claws A A centred in a cylindrical block B, which slides vertically within the casing C, the tail ends of the claws fitting into inclined slots D in the casing. During the lowering of the grapnel, the claws are kept open, in consequence of the trigger E being held up in the position shown in [Fig. 199], by the long link F, which suspends the grapnel from the top rope. But as soon as the grapnel rests upon the broken rope below, the suspending link F continuing to descend allows the trigger E to fall out of it; and then in hauling up again, the grapnel is lifted only by the bow G of the internal block B, and the entire weight of the external casing C bears upon the inclined tail ends of the claws A, causing them to close in tight upon the broken rope and lay hold of it securely. The claws are made either hooked at the extremity or serrated. The grapnel is then hauled up sufficiently to pull the broken rope tight, and wrought-iron rods 1 inch square with hooks attached at the bottom are let down to catch the bow of the boring-head, which is readily accomplished. Two powerful screw-jacks are applied to the rods at the surface, by means of the step-ladder shown in [Fig. 201], in which the cross-pin H is inserted at any pair of the holes, so as to suit the height of the screw-jacks.
If the boring-head does not yield quickly to these efforts, the attempt to recover it is abandoned, and it is got out of the way by being broken up into pieces. For this purpose the broken rope in the bore-hole has first to be removed, and it is therefore caught hold of with a sharp hook and pulled tight in the hole, while the cutting grapnel, shown in [Fig. 200], is slipped over it and lowered by the rods to the bottom. This tool is made with a pair of sharp cutting jaws or knives I I opening upwards, which in lowering pass down freely over the rope; but when the rods are pulled up with considerable force, the jaws nipping the rope between them cut it through, and it is thus removed altogether from the bore-hole. The solid wrought-iron breaking-up bar, [Fig. 203], which weighs about a ton, is then lowered, and by means of the percussion cylinder it is made to pound away at the boring-head, until the latter is either driven out of the way into one side of the bore-hole, or broken up into such fragments as that, partly by the shell-pump and partly by the grapnels, the whole obstacle is removed. The boring is then proceeded with again, the same as before the accident.
The same mishap may occur with the shell-pump getting jammed fast in the bore-hole, as illustrated in [Fig. 208]; and the same means of removing the obstacle are then adopted. Experience has shown the danger of putting any greater strain upon the rope than the percussion cylinder can exert; and it is therefore usual to lower the grapnel rods at once, if the boring-head or pump gets fast, thus avoiding the risk of breaking the rope.
Fig. 208.
Shell-pump
Jammed in Bore-hole.
The breaking of a cutter in the boring-head is not an uncommon occurrence. If, however, the bucket grapnel, or the small screw grapnel, [Fig. 202], be employed for its recovery, the hole is readily cleared without any important delay. The screw grapnel, [Fig. 202], is applied by means of the iron grappling rods, so that by turning the rods the screw works itself round the cutter or other similar article in the bore-hole, and securely holds it while the rods are drawn up again to the surface. The bucket grapnel, [Fig. 206], is also employed for raising clay, as well as for the purpose of bringing up cores out of the bore-hole, where these are not raised by the boring-head itself in the manner already described. The action of this grapnel is nearly similar to that of the claw grapnel, [Fig. 199]; the three jaws A A, hinged to the bottom of the cylindrical casing C, and attached by connecting rods to the internal block B sliding within the casing C, are kept open during the lowering of the tool, the trigger E being held up in the position shown in [Fig. 206], by the long suspending link F. On reaching the bottom, the trigger is liberated by the further descent of the link F, which, in hauling up again, lifts only the bow G of the internal block B; so that the jaws A are made to close inwards upon the core, which is thus grasped firmly between them and brought up within the grapnel. Where there is clay or similar material at the bottom of the bore-hole, the weight of the heavy block B in the grapnel causes the sharp edges of the pointed jaws to penetrate to some depth into the material, a quantity of which is thus enclosed within them and brought up.
Another grapnel that is also used where a bore-hole passes through a bed of very stiff clay is shown in [Fig. 207], and consists of a long cast-iron cylinder H fitted with a sheet-iron mouthpiece K at the bottom, in which are hinged three conical steel jaws J J opening upwards. The weight of the tool forces it down into the clay with the jaws open; and then on raising it the jaws, having a tendency to fall, cut into the clay and enclose a quantity of it inside the mouthpiece, which on being brought up to the surface is detached from the cylinder H and cleaned out. A second mouthpiece is put on and sent down for working in the bore-hole while the first is being emptied, the attachment of the mouthpiece to the cylinder being made by a common bayonet-joint L, so as to admit of readily connecting and disconnecting it.