Part 5
In the method illustrated in Fig. 20, the rear and front wheels have the same tread, but the rear axle is longer than the front and has rollers _a_ on each side. These strike the track _b_, and while the front wheels follow the curved track _c_ these rollers run on the track _b_ and thus raise the rear end of the skip.
=20. Skip Cage.=--Where a self-dumping skip is to be used in a vertical or highly inclined shaft and it is desired to use safety catches, the skip _a_ is mounted in a cage or frame _b_, Fig. 21, similar to the self-dumping cage, Fig. 11. The skip being pivoted at _c_ one side of the center, and resting on the frame of the cage, tends to remain upright until it reaches the dump; but for safety it is sometimes locked in place by the latch _d_, which hooks over the pin _e_. When near the top, the roller _f_ on the end of the latch _d_ comes in contact with a bar that depresses the roller and thus unhooks the latch. The roller _g_ enters and travels along the guide rails _h_, tipping the skip. There are two rollers _g_, one on either side of the skip. The nose _i_ is temporarily caught on the roller _j_, thus stopping the movement of the skip sidewise and away from the upright guide.
BUCKETS
=21. Buckets=, such as are used for hoisting material during shaft sinking, are continued in use after mining begins when the amount of material to be hoisted is small.
CAR LOCKS
=22.= Several methods of keeping the car on the cage have already been illustrated: by chains, Fig. 8; by bails, Figs. 9, 10, and 12; by omitting sections of the rail under the car wheels, Fig. 11; and by dropping a portion of the platform, Fig. 13. A very common way is merely to put a pin through the hole in the drawbar and into the floor of the cage. Another common device consists of a brake block that fits between the wheels and can be thrown in from the side by a lever when the car is in place. Another device consists of a yoke, which, by means of a lever, is raised when the car is in place so that it passes about the axle and thus holds the car. A device frequently used on self-dumping cages is shown in Fig. 22.
The curved bars _a_ of iron, which just fit around the car wheels as shown, are attached to the loose bars _b_, on the ends of which are the weights _c_. When the cage is at the bottom, these weights strike on a cross-piece and are raised to the position shown by the dotted lines, throwing out the bars _b_, as shown by the dotted line, thus releasing the wheels. The devices shown in Figs. 11, 13, and 22 do not come into action until the cage leaves the landing and the cars must, therefore, be watched until that time.
CAGE GUIDES
=23. Guides= are used in all vertical shafts of any considerable depth and in many highly inclined shafts to keep the cage from swinging about and striking the sides of the shaft. They are made of wooden rails, iron rails, or wire ropes. In American mines, timber guides predominate, although some iron ones are used, and for small shafts at ore mines wire-rope guides are common. In English mines, wire ropes, called _conductors_, are very largely used. This difference in practice is probably due to the fact that in English mines the shafts are usually round and the cages are rectangular. In such a shaft, the wire-rope conductors hang from the head-frame without any cross-bracing, but they require a strong support, as both the weight of the ropes and the strain to give the necessary tension come on the head-frame. When both the shaft and the cage are rectangular, as in most American mines, timber guides are easily put in and they offer a good surface for the safety catches to grip.
Wooden guides are always rectangular in cross-section and in the United States are usually made of yellow pine or other long-grained wood that does not splinter easily; in some localities, oak or some of the other harder woods are used. There is no fixed size for cage guides, but 4" × 4", 6" × 8", 8" × 10", and 4¼" × 11" timbers are frequently used.
The guides are firmly fastened to the shaft buntons with lagscrews or with bolts countersunk into the guide so as to be clear of the shoes, and, to secure safety with speed in hoisting, the ends of the guides must be put together with joints that are not liable to displacement and that offer no projections to the shoes in passing. The buntons to which the guides are secured must be so firmly fastened that they cannot get out of place, and the guides must be set as nearly as possible in a straight line, because if they are crooked the cage is thrown back and forth as it travels along them and this not only increases the strain on the hoisting rope and engine, but sooner or later loosens and misplaces the guide. Fig. 23 shows a plan of a cage with the bunton _A_, guides _B_, and cage shoes _C_ in their normal positions.
LANDING FANS OR KEEPS
=24.= In order to take the strain off the hoisting rope while a cage or skip is being loaded or unloaded, a mechanism to support the cage is placed at the top and at any level of the mine where loading is done, excepting at the bottom level where all that is usually required are the cross-timbers for the cage to rest on. These supports have different names in various localities, being known as _fans_, _keeps_, _cage rests_, _landing dogs_, _landing chairs_, _wings_, etc. Their use increases the safety of caging.
=25.= A common form of keeps is shown in Fig. 24. The cage _a_ rests on four square bars of iron _b_, one under each corner of the cage. These bars have an eye or hub at the lower end and are keyed to the shafts _d_, which rest in cast-steel boxes. The levers _e_ and _f_, which are also keyed to the ends of the shafts _d_, are connected by a rod _g_. Chains _h_ prevent the fans from moving too far under the cage. When the cage is to be lowered, it is first lifted clear of the fans and the lever _e_ is moved into the dotted position, thus moving the fans _b_ out of the way and permitting the cage to be lowered. The inside of the fans have no projections, and the operating mechanism is such that no harm would come if they were left in the shaft and a hoist were made, as the cage would open out the fans and pass through them without any trouble. If, however, the fans are not drawn back at all the headings in the shaft when the cage is lowered, great damage results when the cage strikes the projecting fans. To avoid the possibility of such an accident, fans have been devised that fall back out of line of the shaft as soon as the weight of the cage is removed from them.
=26. Hydrostatic Fans.=--Most fans in use are built on the same principle as those just described, although the details of their construction may vary. An objection that can be raised against them is that, with large cages and heavy loads, the jar caused by letting the cage down on such a rigid support is very hard on the cage. All cages, particularly heavy ones, suffer much more wear from being landed too suddenly than from the strains of hoisting. For this reason, it is advisable to make the upper parts as light as compatible with strength and the side pieces stronger than needed for the actual strains to which they are subjected. Hydraulic fans, Fig. 25, have successfully overcome this trouble. The cylinder shown is one of four on which the cage rests. The eye at the lower end fits on a bar by means of which the cylinders are moved backwards and forwards similar to the motion of the fans _b_, Fig. 24. In Fig. 25 (_a_), the cage is shown as about to rest on the jaw _a_. As the cage settles, it pushes the plunger _b_ downwards, but this action is resisted by oil in the cylinder at _c_. At first, this resistance is very slight, because the =V=-shaped grooves _d_ in the plunger, which are of considerable size at the end of the plunger, allow the oil to escape freely into the upper chamber _e_. These grooves, however, taper down to nothing, so that the flow of oil through them decreases until none can pass except by leakage around the plunger. This allows the plunger with its load to settle slowly to the bottom, as shown in Fig. 25 (_b_).
If now the cage is lifted and the weight thus removed from the jaw _a_, the spring _g_ pushes the plunger _b_ outwards and allows the oil to run from _e_ back into _c_.
=27. Pneumatic Fans.=--A pneumatic fan, shown in section in Fig. 26, is one in which the shock of the landing is partially relieved by a cushion of compressed air. The fan is keyed at the bottom to the shaft _a_ that rotates it, as in Fig. 24. The cylinder _b_ contains the plunger _c_, which is kept at the top limit of its motion by the spring _d_. When the cage lands in the jaw _e_, the plunger descends, compressing the air in the cylinder _b_. The air escapes slowly through the ¹/₁₆-inch hole _f_, thus allowing the cage to settle into place with very little shock. These fans should be made of wrought-iron or cast-steel so as not to be easily broken.
=28. Cage Chairs.=--In the case of a cage required to stop at a large number of levels, it is expensive to provide fans at each level, and to obviate this a strong steel bar or dog may be used under each corner of the cage, all four bars being connected to a lever on the cage, by means of which they can be thrown out at will so as to rest on supports provided at each level. Fig. 27 shows Gray’s patent cage chair, which operates on this principle. The sliding bars _a_ are connected by the cross-bars _b_, which are pivoted at the center and operated by the bar _c_ through the links _d_. By moving the lever _e_ into the position shown, the bars _a_ are thrown out so as to rest in notches or on wall plates in the shaft. The springs _f_, through the cross-bars _b_, force the sliding bars _a_ back under the cage when the lever _e_ is released.
HEAD-FRAMES
=29.= A =head-frame= of wood, iron, or steel is built over a shaft or slope mouth to carry the sheaves over which the hoisting ropes are conducted from the mine to the drum of the hoisting engine; it also usually carries the upper portion of the cage guides or, in the case of a slope, the tracks for cars.
A head-frame must be strong enough to bear the strain brought on it due to the total load hoisted and the pull of the engine in hoisting this load; it must also be rigid in construction to withstand the severe vibration and shock to which it is subjected on account of the rapid hoisting and the jar due to the landing of the cages.
The amount and direction of stresses that a head-frame must resist are usually determined by applying the parallelogram of forces as follows: Fig. 28 is a simple head-frame at a slope; _a_ is the drum of the hoisting engine with the rope coming from its upper side and running over the head-sheave _b_ down to the slope cage _c_. Assuming that the angles _e_, _f_ made by the two portions of the rope with the horizontal are equal, and that the pull on each part of the rope is 20,000 pounds, to determine the amount and direction of the resultant of the two rope pulls, proceed as follows: Extend the rope lines to the point of intersection _g_ and from there lay off the two lines _g h_ and _g k_, to some definite scale, representing the pull of the rope. If a scale of 2,000 pounds to ⅒ inch is taken (⅒ inch = 2,000 pounds), _g h_ and _g k_ will each be 1 inch long. Complete the parallelogram by drawing _h l_ parallel to _g k_ and _k l_ parallel to _g h_. The diagonal _g l_ represents the direction and amount of the force acting on the head-frame due to the pull of the two portions of the rope. The diagonal, by measurement, is 1½ inches or ¹⁵/₁₀ inches long, and since each tenth inch equals 2,000 pounds, the stress on the head-frame in the line of the diagonal _g l_ is 2,000 × 15 = 30,000 pounds. The figure also shows that the direction of this force is vertical, hence there is no tendency for the frame to be pulled over to either side and, theoretically, side bracing is not needed.
=30.= Consider now the case of a vertical shaft, Fig. 29, in which, as before, _a_ is the drum, _b_ the head-sheave, _c_ the cage, and _d_ the head-frame, and assume the same pull of 20,000 pounds on each part of the rope. As before, extend the lines of the rope, which are the lines of force along which the pulls due to the engine and the load act, until they intersect at _g_. From this point lay off on these lines distances representing the stresses in the rope to any scale. Using the same scale as before, ⅒ inch = 2,000 pounds, the lines _g h_ and _g k_ representing the two forces will be each 1 inch long. Completing the parallelogram by drawing _h l_ parallel to _g k_, and _k l_ parallel to _g h_, and drawing the diagonal _g l_ through _g_, the resultant, _g l_ = ¹⁹/₁₀ inches, represents a stress of 38,000 pounds. The direction of the resultant is also determined, being in the line of the diagonal _g l_. If the head-frame shown in Fig. 28 were used for this case, it would be overturned by this resultant force, unless the leg on the opposite side of the shaft from the engine were securely anchored, so an inclined brace _m_ is added to resist this overturning action. The resultant of all forces acting on the head-frame should generally fall within the structure if the greatest stability is to be secured, but when this cannot be done it is necessary to resist the overturning pull by anchoring the head-frame to its foundations much more securely than is the case where the resultant falls within the structure.
The direction of the resultant force may be obtained by drawing a line through the intersection of the lines of action of the forces at _g_ and the center of the head-sheave _b_, as may be seen in Figs. 28 and 29.
=31.= In Figs. 28 and 29, the pull of one hoisting rope running from the top of the drum was considered, but in most cases it is necessary to consider the pull from two hoisting ropes, one running from the top and one from the bottom of the drum _f_, as shown in Fig. 30. _a b_ and _a′ b′_ represent the directions of action of the two forces acting on the hoisting ropes, while the two vertical forces _a c_ and _a′ c_ acting down the shaft are approximately equal to the two forces acting toward the drum. There are, therefore, two resultants _a d_ and _a′ d′_, the directions of which are determined by lines from _a_ and _a′_ through the center of the sheave _e_. The amounts of these resultant forces can be determined by the parallelogram of forces as shown in Figs. 28 and 29. A resultant that is a mean between _a d_ and _a′ d′_, both in position and amount, is sometimes taken, or the greater value as determined from _a d_ or _a′ d′_ and the greatest inclination as given by _a′ d′_ may be used, as being the worst theoretical conditions to which the frame may be subjected. A head-frame usually has a vertical post approximately parallel to the vertical pull of the rope in the shaft, and an inclined member _g h_ approximately parallel to the resultant determined by the parallelogram of forces. If _g h_, Fig. 30, is parallel to the resultant, the vertical leg _h i_ is under no strain and merely supports the end of _g h_. If the resultant falls between _g h_ and _h i_, both of these legs will be under compression. If the resultant falls outside of _g h_, the leg _g h_ will be under compression and _h i_ will be under tension. The head frame will be most stable when the resultant falls between _g h_ and _h i_, but this cannot always be accomplished in building the frame on account of the conditions at the head of the shaft; nor is it always advisable to do so from structural considerations.
=32.= Since wood is much better adapted to withstand compressive than tensile stresses and since steel is adapted to withstand either tensile or compressive stresses, it is much more important that the members of timber frame conform as closely as possible to the theoretical line worked out in Figs. 28, 29, and 30 than in the case of a steel frame. Take, for instance, the case shown in Fig. 31, where for some local reason it is impossible to put an inclined strut in or near the line of the resultant stress to withstand the pull that tends to overturn the head-frame. In a steel structure, _a_ can very easily be made a tension member by anchoring its lower end to a heavy foundation. This resists the tendency to overturn and makes a very stable structure. In practice, braces can generally be located parallel to the line of resultant strain, Fig. 29, or outside this line, as shown in Fig. 30, so that the strain due to the pull of the rope will come mainly on the inclined brace and not on the upright. To distribute the stress on the foot of the different parts of the frame, an inclined brace is usually set farther from the shaft than the parallelogram of forces locates it, and so placed that about two-thirds of the strain due to the pull of the rope comes on the brace and one-third on the upright parts of the frame. In order to give the frame a more stable base and because the base must be larger than the top of the frame to bring the foundations back from the shaft mouth, usually the members _h i_ are also slightly inclined.
Wherever permanency of head-frames is required, if steel is obtainable at a price at all comparable with wood, steel structures are being used, as timber frames rot.
TYPES OF HEAD-FRAMES
=33.= There are three types of head-frame construction--_the_ =A= _type_, the _square type without an inclined brace_, and the _square type with an inclined brace_.
=34. A Type of Head-Frame=.--Fig. 32 shows the construction of a triangular, or =A=-shaped, head-frame of which (_a_) is a side elevation and (_b_) an end view. This particular frame is largely used at anthracite mines, but the type is one quite commonly used for timber frames, though the details of construction vary in different localities. The height of the frame is from 30 to 50 feet, and with direct-acting engines this height should be sufficient to allow a play of at least two-thirds of a revolution between the cage landing and the overwinding point. The posts _a_ are parallel to the hoisting rope _b_ as it hangs down the shaft and the inclined brace _c_, which resists any thrust that would tend to rotate the head-frame, is parallel to the resultant pull of the two parts of this rope _b_; the inclined braces _d_ stiffen the frame and help support the cross-timbers _m_ that support the cage guides _e_. The sills _f_ are made of three pieces of timber 8 inches by 14 inches in cross-section. The posts _a_ rest in cast-iron shoes _g_ that are firmly bolted to the posts and sills. The inclined braces _c_, _d_ are fitted with cast-iron shoes _h_, _i_. The post _a_ and the two braces _c_, _d_ are held in place at the top of the frame by the casting _j_, which also supports the pillow-block _k_.
The posts _a_ and the brace _c_ are made up of two pieces of timber each 8 inches by 14 inches in cross-section. The brace _d_ consists of one piece of timber 8 inches by 14 inches in cross-section. The transverse braces _l_ consist of two pieces of timber 6 inches by 14 inches in cross-section, bolted through the timbers _a_ and _c_. The supports _m_ for the guides are single pieces of 8" × 8" timber. The center post, as shown in Fig. 32 (_b_), is braced by the two pieces _n_, _o_, which are supported by two timbers _p_, _q_ bolted to the two outside posts. The posts _a_ and the inclined braces _c_ are further braced by the tie-rods _r_, _s_, _t_, and _u_, all of which are fitted with turnbuckles, as shown at _v_. The different posts are firmly bolted together, the bolts being fitted with cast-iron washers.
Fig. 33 shows the construction of the ordinary timber gallows frame used at many ore mines.
Fig. 34 shows a steel =A= frame, of which the principal dimensions are as follows: height to sheave center 48 feet; base 33 feet 10 inches by 56 feet. Legs _a_ and _b_ are made of laced channels, as are also the central upright posts and cross-braces. The forward inclined legs are made of =I= beams. The weight of the frame is 98,000 pounds without the sheaves. The advantages claimed for this type of design are that it gives a very strongly braced frame while using a minimum of material. Also, in cases of overwinding, the cage goes over the top of the frame without injury to the frame, and should men be overwound they would fall only the height of the frame instead of being crushed against the top.
=35. Square Type Without Inclined Brace.=--Fig. 35 shows a steel frame in which the tendency to be overturned by the pull of the rope is resisted by a nearly vertical tension leg as explained in =Art. 32=. Each leg of the frame is built of channel bars connected by lattice bracing, as shown, and the legs are stiffened by horizontal channel cross-bars similarly braced and also by diagonal tie-rods, provided with turnbuckles.
Springs are sometimes placed under the journals of the head-sheaves to lessen the strain on the rope while starting the load; the 15-foot head-sheaves of the Robinson deep mine at Johannesburg have locomotive springs under the journal boxes, the actual load on each spring due to the weight of the sheave, rope, skip, and rock being equal to about 20,000 pounds; it was estimated that the sheave would thus be lowered by the load on it, about 3 inches, which would be equal to an action of a spring giving motion of 6 inches at the cage. Springs can often be used both on the rope and under the sheave in the same plant to advantage.
=36. Square Type With Inclined Brace.=--Fig. 36 shows a very substantial frame with square tower and inclined brace.
Its principal dimensions are as follows: height to sheave center 59 feet 6 inches; base of tower 15 feet 8 inches by 14 feet; distance of bottom of inclined leg from vertical post 48 feet. Each end post _a_ is composed of two channels, double-latticed. The horizontal members _b_ are =I= beams and each inclined member _c_ is made up of two angles. The inclined leg _d_ is trussed as shown and built of channel and angle beams, the main member being made of two channels, the incline and base members of the truss being made up of two angles, and the short vertical member of two channels. The center post of the tower is similar to the end posts, except that the uprights are =I= beams instead of channels. The frame is designed for a static weight of 16,000 pounds and for a maximum strain on the cable of 32,000 pounds.
Fig. 37 shows a frame of similar form, but in which the landing platform is placed at a height above the surface, so that the cars hoisted can be run off on a trestle and thus be delivered at the top of a car, breaker, tipple, or ore house. Its principal dimensions are as follows: height to sheave center 75 feet; base 40 feet 11¾ inches by 21 feet 8½ inches. The leg _a_ is made of two angles. The bracing leg _b_ is built of two angles. The diagonal braces _c_ are single angles. The horizontal braces are angles or channels of various sizes depending on the stresses.
=37.= The =head-sheave= is supported directly on top of the main frame, as shown in Figs. 32, 34, 36, and 37, or a small superstructure _a_ is built on top of the main frame, as shown in Fig. 38, so that the base of the sheave journals is perpendicular to the resultant pull on the frame, that is, to the theoretical direction of the inclined leg of the frame if one is used.