Part 4
This sheave is cheaper than a combined cast-iron and wrought-iron or steel sheave, and for many purposes it is entirely satisfactory. Its great weight is an objection, because it adds to the weight on the journals and also offers considerable resistance to being set in motion and stopped.
If a sheave is merely used to carry the rope or to deflect it only a little, the contact and pressure between the rope and the sheave is small; consequently, the power of the rope to turn the sheave will be slight. In such a case, when the rope starts or stops quickly, as it usually does in modern hoisting plants, the heavier the sheave the more will it lag behind the rope and the greater will be the wear on the rope due to slipping.
=3.= The sheave with a cast-iron hub and rim and wrought-iron or soft-steel spokes, Fig. 3, is an excellent and extensively used sheave, especially the larger diameters. The spokes are screwed into the hub and rim and are carried to the right and to the left of the hub alternately, as shown in Fig. 3 (_b_), so as to take hold of the opposite ends of the hub, thereby giving stiffness to the sheave against any side stress.
With a sheave having cast-iron arms, the load from the rope is transmitted to the shaft by a compressive stress through the arms directly under the load; that is, if a rope runs over the sheave, Fig. 2, putting a load on it from _j_ to _k_, this load will be transmitted as a compressive stress through the arms _l_ and _m_ to the hub and the shaft. Of course, a part of this load is carried around the rim to the lower arms and is supported by them in tension, but these lower arms are not considered in designing the sheave because cast-iron is of comparatively little value in tension, whereas it is of great value in compression. In the case of the sheave with wrought-iron arms, or spokes, Fig. 3, the load is transmitted around the rim to the side opposite its point of application and is carried from there to the hub and shaft by the tension of the spokes; in fact, from the method of construction, the spokes in this sheave act only by tension. The sheave is strong and rigid, and much lighter than a cast-iron sheave of the same strength, so that there is less wear between it and the rope due to any slipping action when it is started or stopped.
=4.= Sometimes, the spokes, instead of being radial as in Fig. 3, are made tangent at the center of the wheel, Fig. 4, to an imaginary circle, which is about 2 inches in diameter for a 10-foot sheave. Alternate pairs of spokes are made tangent to the opposite sides of the circle, so that they pull against each other, and this makes the sheave rigid in both directions. That is, spoke _A_ is tangent to the right side of the tangent circle and _A′_ to the left side, while spoke _B_ is tangent to the right side of the circle and _B′_ to the left side. The pair _B B′_ is joined to one end of the hub, while the pair _A A′_ is joined to the other end, thus giving lateral stiffness to the sheave. This arranges the spokes in groups of four, so that the total number must be some multiple of four. The tangential direction of the spokes is often necessary in very large sheaves carrying heavy loads, because with such a sheave it requires considerable force to turn the shaft in its bearings, and while radial spokes act only as long levers in turning the shaft, with tangential spokes there is also a direct pull to do it.
=5. Wood-Lined Sheaves.=--The rims of all sheaves are made either solid or with wooden lining, as shown in section in Fig. 5. One flange _a_ of the rim is a separate piece that is held on by bolts _b_. The wooden lining is in the form of blocks placed with the grain of the wood running radially and held securely by clamping together the two flanges with bolts, as shown. With such a sheave, there is much less wear on the rope than there is with one that has a plain cast-iron rim. The wear of the sheave proper is also avoided, because as the blocks wear down they are taken out and replaced by new ones.
=6. Diameter of Sheave.=--The size of a sheave about which a rope bends is determined generally by the size of the rope to be used, as explained under Wire Ropes in _Hoisting_, Part 2; but, if the rope is simply to be supported in a straight line, the space available for setting the sheave and its cost and weight usually determine the size used. The minimum allowable diameter of sheave should not be used unless it is necessary to do so, for the larger the sheave the less will be the wear of the rope due to the bending, and the longer the life of the rope, but the cost of the sheave, which increases with the size, puts a limit in the other direction.
=7. Rollers and Carrying Sheaves.=--Wooden or iron rollers are sometimes used for rope carriers or guides, instead of light sheaves, when the rope has merely to be supported and there is no bending of the rope, excepting the slight amount due to the sagging between the rollers. The diameter of the rollers is of little importance in such cases so far as the rope is concerned. If they are for use on a slope to keep the rope from dragging on the ground, they must be small, because the cars must run over them, and mine cars are usually made low because of restricted headroom in the mine. Rollers and carrying sheaves are fully described and illustrated in _Haulage_.
If a hoisting rope changes its course from a straight line, even if the deflection is only a small amount, a roller is not advisable and a sheave should be used, if possible.
CAGES
CAGES FOR VERTICAL SHAFTS
=8.= A =cage= is a carriage used for hoisting mine cars and their contents, men, timber, etc., in both vertical and inclined shafts. Cages are built of wood strengthened with iron or steel, or entirely of iron or steel.
=9.= The cage shown in Fig. 6 is much used in the anthracite region of Pennsylvania. It is made largely of oak strengthened with iron and the size varies to suit the shaft, being sometimes as large as 6 feet wide by 12 feet long. The general construction of the cage is evident from the figure, but several appliances that should be common to all cages in some form or other require detailed explanation.
A covering _a_, called a =bonnet=, protects persons on the cage from objects falling down the shaft, and is required by law in some States. This bonnet is made of steel plate with flanges or angle irons to stiffen it, and is usually inclined. To prevent objects of moderate size from wedging between the edge of the bonnet and the shaft lining, the former is sometimes made shorter than the cage, so that a space of about a foot is left between its lower edge and the shaft lining. A short bonnet of this character does not, however, fully protect persons on the cage. The upper part of the bonnet is fastened to the upper cross-bar of the cage by two hinges and is held up by rods _b_ that are attached to the bonnet and have sockets at their lower ends, which fit over pins bolted to the uprights of the cage. By raising the rods from the pins the bonnet can be lowered so that pipes or long timbers may be lowered on the cage.
=10. Safety catches= are intended to prevent a cage falling in case the hoisting rope breaks. A common form, shown at _c_, Fig. 6, and in detail in Fig. 7, consists of a pair of toothed cams _j_, Fig. 7, fastened on each side of the cage near the shaft guides. The drawbar _b_ to which the rope is attached extends through the top cross-piece of the cage and through the cylinder _d_, at the bottom of which is a plate _c_ supplied with lugs for the rods _f_ that connect it with the levers _g_. Inside the cylinder are three powerful rubber springs, which are in compression so long as the cage hangs from the rope, but are extended if the rope breaks, drawing the rods _f_ down and with them the ends of the levers _g_ to which they are attached; and, since the levers are pivoted, their other ends are moved upwards and with them the rods _k_. The cams _j_ are each attached to one end of the rods _k_ in such a manner that as the rods move upwards they rotate the cams inwards until they come in contact with the shaft guides. The teeth of the cams grasp the wooden shaft guides and stop the descent of the cage. The cams are provided with projections _a_ and _l_ that strike the guide and thus prevent the cams turning entirely around. Fig. 7 (_a_) shows the springs extended and the dogs _j_ just about to grasp the shaft guides, while Fig. 7 (_c_) shows the position of the dogs when the springs are compressed as they are when hoisting. At _e_ in cylinder _d_, Fig. 7 (_b_), there are slots for the lugs of plate _c_ to move up and down as the spring is compressed or extended. Instead of rubber springs, helical steel springs are sometimes used, and with a somewhat different design flat steel springs are used.
The cams, or dogs, may be placed at any point along the upright post of the cage, and in some cases two sets of cams are used on each side, one set at the top and another in the middle, both sets being connected by rods so that they work together. Practical tests of these catches, made by allowing the cage to drop, show that they are, as a rule, very efficient devices. The cams usually take hold at once, the cage dropping only a few inches, or, at most, a few feet if the guides are dry and free from oil. When the guides are very greasy or wet, the cage may drop several feet before the cams take a firm hold and stop it, and with ice-covered guides, instances are given where the cage has fallen 15 feet before the cams ploughed their way through the ice and took firm hold of the guides; but in so doing the momentum the cage acquired was so great that the guides were destroyed. Fortunately for the utility of safety catches, ropes are usually broken while a loaded cage is being raised, and the cage has an upward momentum; if a rope breaks when the cage is descending at a speed of 30 or 40 feet a second, its momentum is so great that either the catches or guides break. The catches generally hold and either the guides or cage suffer more or less injury under such circumstances. Instead of being placed near the top of the cage the dogs are frequently placed near the center, or near the bottom; in some cases two sets of dogs have been used, one set being at the top and the other at the bottom. Instead of being cam-shaped with a number of small teeth on the rim of the cam, as shown in Fig. 7, the dogs are now frequently made consisting of one or more strong straight teeth on each side of the guide. These teeth are operated similarly to those shown in Fig. 7, and are driven into the guides if the rope breaks, thus holding the cage more firmly than the cam-shaped guides, particularly where the guides are wet.
TABLE I
==================+=========================+=========+======== Platform | Guides | | -----------+------+--------+----------------+Safe Load| Weight Width |Length| Size |Distance Between| Pounds | Pounds ----+------+ Feet | Inches +------+---------+ | Feet|Inches| | | Feet | Inches | | ----+------+------+--------+------+---------+---------+-------- 4 | 3 | 6 | 6 × 6 | 4 | 6 | 5,000 | 2,000 6 | | 10 | 6 × 10 | 6 | 3 | 8,000 | 3,800 ====+======+======+========+======+=========+=========+========
=11. The Heavy Steel Cage.=--The cage that is shown in Fig. 8 is made of iron and steel except the wood flooring, which is laid in two courses, one lengthwise and one diagonal. The joints should not be driven too tightly, as the wood is likely to swell. The track is bolted to the floor, or =deck=, of the cage. The cast-steel safety dogs are operated by steel springs _a_, coiled about the bars _b_, which are connected to the drawbar _c_ by chains, as shown. The drawbar drops if the rope breaks and thus assists the action of the springs _a_. This cage is in use at both coal and iron mines, and is built to suit any size of shaft and guides. Standard sizes are given in Table I.
=12. The Light Steel Cage.=--Fig. 9 shows a light steel cage much used at gold and silver mines. It has a spring drawbar and steel safety dogs, operated by steel springs, as in Fig. 8, but the floor is of steel grating in order to give as little air pressure as possible against the cage. The openings _a_ in the side frames are provided so that through them the nuts can be tightened on the bolts that hold the shaft guides. The cage is provided with bails _b_ that swing down over each end of a car to hold it on the cage.
=13. Multiple-Deck Cages.=--Cages are sometimes built that have two or more decks or platforms one above the other, thus giving greater hoisting capacity to a shaft. A two-deck, safety, hoisting cage is shown in Fig. 10. The upper deck is heavier than in a single-deck cage of similar construction. The lower deck is suspended from the upper deck by means of pins so that it may be removed at any time. A double-deck cage may be used by first changing the car on the upper deck and then bringing the lower deck to the track level and changing the other car. Time can be saved by having two track levels, both at the loading and landing stations, enabling both decks to be loaded and unloaded at the same time.
=Multiple-deck cages= have been mainly used at ore mines in America and very few coal mines have been equipped with them. Cages are also built to accommodate two cars placed either side by side or end to end.
AUTOMATIC DUMPING CAGES
=14.= A =dumping cage= is a cage so constructed that at the proper place it can be automatically tipped sufficiently to dump the contents of a car that is on it and will then right itself for the down trip, thus avoiding the necessity of removing the car from the cage, and saving time at the head. The construction of the cage is such that the car is held firmly in place while dumping. The principle of the self-dumping cage is illustrated in Fig. 11, the cage being shown in its highest and lowest positions. The cage is made in two parts _a_ and _b_. The fixed frames _b_ slide on the guides _k_ and have attached to them the safety catches and hoisting gear. The movable part _a_ is united to the frame _b_ by the hinge _c_. The platform _d_, on which the car rests, is fastened to the movable part _a_ by the support _e_ and further secured by the braces _f_. At the top of _a_ is attached the wheel _g_ that runs along the rail _h_, keeping _a_ in an upright position until it reaches the dumping place _i_. Here the rail _h_ is bent as shown and the wheel _g_ is made to follow it by means of the guide _j_. This throws the top of _a_ over so as to incline the platform and dump the car that is on it. On lowering, the cage rights itself when _g_ passes below the point _i_. The part _b_ is kept in a vertical position by means of shoes that slide on the main guides _k_.
It is possible to dispense with the guide rail _h_ by attaching a flange to the top of _a_ at the back, to slide on the main guide _k_. This flange should be shorter than the shoe on _b_. The main guide is cut away at the point where this flange comes when the wheel _g_ enters the curved guide _j_, leaving an opening just large enough to allow the flange on _a_ to pass through. The shoe on _b_, being longer, completely spans the space and cannot pass through, but makes _b_ move straight up on the main guides.
The bottom of the cage in Fig. 11 has an interrupted track, and at the bottom of the shaft the track is also interrupted, as shown in the plan at the bottom of the figure, but in such a way that when the cage is resting at the bottom this portion of the track _n_ projects up through the bottom of the cage and makes a continuous track. When the cage is raised the wheels of the car drop into the spaces _n_ in the cage bottom, thus preventing the car from running off the cage during hoisting or dumping.
=15. Slope, or Inclined-Shaft, Hoisting.=--In a slope, or inclined shaft, the mine cars are attached directly to the hoisting rope and hoisted singly or in trains for inclinations less than 35°, at which inclination the material will begin to fall from the top of the car. For steeper slopes, it is customary to use a slope cage or carriage on which the mine car is hoisted, or else to dump one or more cars of the material into a gunboat, or skip, at the bottom of the slope or at some landing along the slope, and to then hoist the gunboat, or skip.
Fig. 12 shows a cage for use in a slope or steeply inclined shaft. It is made of steel with timber platform and differs from a vertical shaft cage mainly in having its upper frame inclined and in running on four wheels _a, b_. These wheels usually run on timber guides, so that the safety dogs _c_ will take hold of the guide in case the rope breaks. For slopes of variable inclination, the platform _d_ may be made adjustable by means of a hand lever so as to be always level.
=16.= A =slope carriage= is a frame so constructed that when rails are placed on the top and a mine car run on them the car will be practically horizontal. The carriage is mounted on wheels and axles in order to follow the slope tracks, and is supplied with a drawbar, or with hooks, as shown in Fig. 13, for attachment to the hoisting rope.
These carriages are sometimes built to run on a slope track of the same gauge as the mine cars, but to insure stability they have generally a broader gauge. The headroom necessary is governed not so much by the form of the carriage as by the length of the car and the inclination of the seam. This height is less when the cars are placed on the carriage with their length across the slope than when they are run on lengthwise; but this arrangement increases the width of the slope. When the inclination is very steep, the wheels are sometimes placed on the sides of the carriage and above its center of gravity and run between two tracks or guides, on each side of the slope.
The carriage, Fig. 13, is for use on slopes of a uniform inclination. It is made almost entirely of heavy timber, is stiff and simple of construction, and is easy to repair. Its details will be readily understood from the illustrations, except perhaps, the device for locking the car to prevent its running off during the hoist. The middle portion of the platform _a_ having a piece of the car track on it, may move vertically up or down. As shown in the side elevation, it is resting on the horizontal timbers _b_ of the carriage in a position ready for hoisting. At the end of the hoist, when the cage settles on the keeps _c_, shown in the end elevation, this platform reaches them first and is supported by them while the rest of the carriage descends still farther until the timbers _d_ rest on the keeps also. The track on the platform _a_ is then at the same level as that on _d_, and the car can be run off and replaced by another. When the empty car is on, the carriage is lifted from the keeps, but the platform _a_ remains until the timbers _b_ pick it up, when the keeps are swung back out of the way and the carriage is lowered.
Slope carriages usually have the tracks running crosswise so that the car is pushed on from the side instead of from the end.
SKIPS, OR GUNBOATS
=17. Skips= are self-dumping cars used for hoisting material from shafts or slopes. In a vertical shaft, they run in guide tracks; but in a slope they have wheels and run on a track like a car. In the anthracite region of Pennsylvania, skips are called =gunboats=.
As the skip is not detached from the hoisting rope, time is saved at the top over that needed to unhook and hook the cars to the rope or to remove and place the cars on the cage. But since dumping the material into the skip and again on the surface produces considerable fine material, skips, or gunboats, are seldom used for any material, such as coal, that is often lessened in value by being broken. The skip, or gunboat, shown in Fig. 14 is closed along the top _a_ and open at the end _b_, which is cut at about the angle of the slope in which it is to be used, so as to remain practically level during the hoist. It is made of sheet iron, the bottom, sides, and top being stiffened by angle or =T= irons, and the back stiffened and protected by 3-inch planks, backed by 3" × 6" timbers. The wheels of a skip are fixed on the axles, which run in journal boxes, thus insuring smoother running than is obtained with loose wheels. The details of the journal bearings, as shown in Fig. 15, consist of three castings, the bracket _a_, which is bolted or riveted to the gunboat, a pivot casting _b_, and the bearing proper _c_. The bearing _c_ rests on the axle and carries, by means of trunnions _d_, the pivot casting _b_, on the top of which is placed a rubber cushion _e_ to lessen the shocks between the casting and the bracket.
=18. Method of Loading Skips.=--In Fig. 16, a skip _a_ is shown in a slope standing immediately below a level where a car _b_ is ready to have its load dumped into the skip. Instead of dumping the mine car directly into the skip, a bin is frequently provided at the level station, or landing, into which the mine cars are dumped and from which the material is loaded into the skip through suitable chutes. The use of such bins makes the hoisting of material largely independent of the working conditions on the levels and the hoisting can be more systematically and satisfactorily carried on.
If the material comes to the slope as shown in Fig. 17, it is necessary to let down a bridge _a_, on which the car runs, in order to reach the skip. After the car is dumped, the bridge is lifted out of the way into the dotted position, so as to leave the slope unobstructed.
=19. Method of Dumping Skips.= To dump a skip at the surface, the tracks are extended above the slope mouth, as shown in Figs. 18 and 19, and are arranged so that the material may be dumped directly into a bin or into cars as desired.
In the arrangement shown in Fig. 18, the front wheel of the skip strikes a stop _a_ and, since the bail of the skip is pivoted far down toward the lower end, as the rope continues to pull, the rear of the skip is raised and the material is dumped. The objection to this method is that if the rope is slightly overwound the skip is pulled off the track and does not then right itself on the track when the rope is released.
In the Lake Superior iron and copper region, many of the dumps are built as shown in Fig. 19. In this dump, the rails of the main track _a_ are curved as shown at _b_; a short distance back of the beginning of this curve, another track _c_ begins outside the track _a_ and runs in a straight line parallel to the inclination of the hoist. The track _c_ is of a wider gauge than _a_, and the rear wheels of the skip have a wider tread than the front, so that they will run on _c_ while the front wheels take the curved track until they strike the stop _d_. The rear of the skip will thus be raised and the material dumped. There are but two tracks in the main part of the slope.