« ZurückWeiter »
the immersed portion of the hull presenting a planed wooden surface to the water. During experiments F and G this portion was covered with smooth rolled copper.
The speed of the vessel was determined, as stated, by the shore distances and by the taffrail log for each run over the base. The mean of the speeds of the different runs of the same experiment, by the shore niarks, is taken as the mean for the entire experiment according to that measurement. And, in the same manner, the mean of the speeds of the different runs of the same experiment by the taffrail log is taken as the mean for the entire experiment according to that measurement. These two measurements differed, and, with a slight irregullarity for the different experiments, the mean difference deduced from all the experiments showing, as might be expected, that the indications by the taffrail log were greater than those by the shore marks, the ratio being as 1:0572 to 1.0000. This ratio may be taken as a constant correction for the taflrail log, which, as its determinations were unaffected by current, can be considered, when thus corrected, as the most accurate measure of the vessel's speed. Accordingly, the speed by the log thus corrected is what is given in the following tables.
The mean number of revolutions made by the screw per minute during a run was ascertained from the time and the total number of revolutions given by the counter during that run; and the mean number of revolutions made per minute by the screw during an experiment is the mean of the means of those made during the runs composing that experiment.
(To be continued.)
Volatility of Sulphuric Acid.-Regnault has shown that mercury is volatile even at very low temperatures; but it has been generally thought that sulphuric acid is not volatile at ordinary temperatures. A writer in Il Progresso has found that in operating in an atmosphere that had been dried with sulphuric acid, litmus paper was soon discolored and gradually disorganized. This fact may explain the sulphur rays which are often observed in Geissler tubes. It may also be of great importance in chemical analyses, where the vapor of sulphuric acid under certain conditions might change the character of compounds without its presence being suspected.—Chron. Indust. C. WHOLE No. Vol. CXII.—(THIRD SERIES, Vol. Ixxxii.)
PERCUSSION ROCK DRILLS.
By ROBERT GRIMSHAW.
The name “ rock drill ” is in most cases a misnomer, as the devices used for perforating rock very seldom have a drilling action proper. The French word “perforatrice,” or perforator, is used with better reason. However, there is little more use in calling attention to this fact, than in stating our use of the word fore-plane is incorrect and the English use correct; the English meaning by fore-plane what we call a jack-plane, which is really the before plane.
As ordinarily known, a drill is a device which, by rotation and lengthwise advance, makes a round hole in some solid material, as wood, metal, ivory, etc.
In making holes in rock there are several ways; the oldest, by percussion only, accompanied by intermittent rotation of the drill bit. Very little rock is wrought by drills proper, having steady advance and continuous rotation, and breaking up all the material they remove. There is one type of rock drill which has continuous rotation and continuous advance, and which cuts out an annular channel, leaving in the centre a solid core which may, with care, be removed for inspection. The first and the third of these types--that is, the percussion drill and the ring or rock drill—are most generally in use. I shall in this paper treat of percussion drills only.
The drill proper is the drill bit, although usage sanctions the use of the word drill for the entire machine which guides or which drives the bit. Up to twenty years ago, there were very few drills driven except by hand power, the drill shank being either struck directly by hammer blows or raised by some machine and driven forward by a spring. We will speak of these two methods as the band drill and the handdrilling machine systems. So much more work can be done by handdrilling machines than by hand drilling pure and simple, that the next step was to introduce a machine to imitate hand percussion drilling, with much greater force than by hand direct or by hand power machines. Of course the use of steam and of compressed air in a cylinder, to effect percussion by a drill bit fastened to the piston rod of the cylinder, naturally suggests itself; the rotation of the drill bit on the back stroke and its advance lengthwise, to correspond with the depth of the cut taken at each stroke, being effected by many mechanical devices. Such a device constitutes the rock-perforating machine; or "rock drill" for short.
As ordinarily constructed, there is a double-acting steam engine, having a piston with a very long head, say 3 inches in diameter, the rod having the drill shank clamped to it direct. The valve motion of this machine, making as it does 300 strokes per minute and upwards, must be effective, simple, durable, compact and, if possible, economical of steam. Ordinarily the distributing valve is a slide valve, or its modification, the reciprocating piston valve; the cylinder having ports from each end of the steam chest in which the valve moves to and fro. The motion of the valve is effected by one of three systems; by lever, or tappets struck by the piston head, or by a boss or its equivalent upon the piston rod; by having the machine duplex; and by fluid transmission without any mechanical connection proper. Some machines have no separate external valve; the piston, or a sleeve thereon, performing the valve office.
The rotary motion is generally given by flutes upon the stem of the piston working in a fluted ratchet nut, although there are other methods; for instance, the stem may be in the back cylinder head and the ratchet nut in the piston head, etc. The forward feed of the drill bit may be by forward motion of the piston in the cylinder, the stroke getting longer each time; or by the stroke remaining the same in length, while changing its place in the cylinder bore; or the cylinder may be advanced. This feed may be either automatic, or by hand ; and if automatic, it may be regular and independent of the hardness of the rock being penetrated; or it may be controlled by which this depth of feed may be regulated by hand while the actual work is done automatically; or the forward feed may be determined by the depth of the last cut taken, so that if the drill strikes a hard place and does not cut as deep as when it is working in a soft place, the feed will be lessened. Generally the cylinder heads are cushioned with rubber protected with metal discs, or else there is very great cushion of the driving air or steam, in order to prevent the heads being knocked out by the drill bit coming suddenly into a soft place or a cavity.
As regards the mounting of the machine, this depends upon the
work to be done. If in quarrying or in other surface work, where the holes are all vertical or nearly so, the most common mounting is a tripod having adjustable legs, which are weighted down to keep the machine in place, and are often extensible to allow of more perfect and steady placing. The steam or compressed air is led to the neighborhood of the drill by steam pipes of iron, and from them by flexible hose to the steam chest. If the machine is for gallery work, in which all the holes are horizontal or nearly so, the machine is generally mounted upon a column having thrust screws which jam it against the top and bottom of the heading and hold it firm. In some cases this column is threaded, to allow the machine to be adjusted in height, in others the column is smooth and the machine is clamped at the desired height. In others this column is mounted upon a carriage and has transverse motion by slides upon the carriage frame. In all cases where a column is used, the machine has free adjustment in the horizontal plane and also swings in a vertical plane, so that it has free adjustment after the column is in place, in height, in the vertical angle and in the horizontal angle.
For that class of surface work where it is desirable to channel, or cut a number of holes in a line close together, there is an arrangement permitting this to be done. In some cases, for tunnel work, several drills are mounted upon a carriage and all are given lengthwise advance at the same time by this carriage, although each one may cut at any desired angle in the vertical or horizontal plane. For those situations where it is necessary to cut down close to the floor, a special type of frame must be used, and in some cases a special type of machine. I show on the screen several types of drills and several styles of mounting them. Some of the illustrations are from working drawings of the makers.
It can be said about the tappet system of working a valve, that, while giving a positive motion to the valve, there is liability to and there always has been trouble from breakage of the tappets and their slight working parts, which are struck so many times a minute with such great force. This trouble is increased in frosty weather and this brings the repair bill up very high. Some opponents of tappets think that because the piston has to strike the tappet on the working stroke, the full force of the blow is not delivered upon the rock; but it seems to me that this is but a trivial matter.
The duplex system is not in use in this country. Because of the very rough usage that the rock perforating machine has to undergo, it is well that there be as few exposed working portions as possible.
In reference to automatic feed; this is not generally applied to small machines, because these being used underground, light weight and simplicity is desirable. Where automatic feed is used, but one man is needed to work the machine.
In reference to steam or air cushion, those who oppose it do so on the ground that the motive fluid must be introduced by this means, in front of the advancing piston ; so that, although the head is preserved from being knocked out, the blow is lessened in force and deepness.
Rotation is for two purposes ; first to make a perfectly round hole, and second to make the bit work not only by mashing away the rock in front of it, but also by wedging off a portion of rock between the new cut and the last old one. To this a third reason may be added, that it preserves the edge of the bit, thereby lessening the wear and saving cost of sharpening.
The bits are of various shapes; the most common being in the form of an X. For loose, seamy rock, a Z shape is found to answer well.
The adjustability of the tripod may seem a very little thing, but it means a great deal. A machine properly mounted will work where two men cannot work with hammers upon the face of the cut. seam only two feet thick, like that at Port Henry, New York, the machine has to work nearly flat, with the legs spread apart.
By employing a lateral arm clamped to the column, the drill may be moved in or out on the arm, and the arm moved up or down or moved around, thus commanding a large portion of the breast without moving the column. The column should thrust against short pieces of timber at top and bottom. Columns over 8 feet long are apt to give give trouble from vibration. For a very large work, as tunnels, the column may be used to work out a drift of considerable size in extension of the line of the roof, and the tripods used to take up the bottom. This has the advantage of dividing the drilling ground and enabling more men to be used.
The disadvantage of the carriage is that the ground must be cleared before the carriage can be run up to the heading.
As regards the size of drills to use : 5-inch is used for submarine work, mounted upon a scow or frame; for deep heavy tunneling,