Part 3
=33. Band Friction Clutches.=--Fig. 18 shows a =band friction clutch= that is attached to and revolves with the shaft _a_. The winding drum runs loosely on the same shaft and has a driving-band ring or seat _b_ on one end; when the ring _c_ of the clutch is tightened by means of the mechanism shown, the clutch and driving band become practically one piece and the drum revolves with the clutch. The clutch is constructed as follows: The driving disk _d_ keyed to the driving shaft _a_ is connected to one end of the ring _c_ by a fixed arm _e_, which is bolted firmly to the disk _d_ and revolves with it; a movable arm _f_ that connects with the other end of the band _c_ turns on the pin _g_. When the band _c_ is loose, it can revolve about the seat _b_ without touching it, but the band can be tightened and made to clamp _b_ either when revolving or standing still, as follows: The sliding sleeve _h_ may be caused to slide about 6 inches along the hub of the disk _d_ by levers (not shown) that take hold of trunnions _i_ on a ring on the sliding sleeve; this sleeve is connected to the movable arm _f_ by a link _j_, and when the sleeve is on the end of the hub the link stands at an angle of about 60° with the shaft; by sliding the sleeve toward the disk _d_, the link is made to move the arm _f_ about 1½ inches at its outer end and to thus tighten the driving-band _c_, so that it grips the ring _b_. The adjusting nuts _k_ take up the wear of the wooden blocks with which the ring _c_ is lined. Band lifters _l_ hold the band clear of the ring when it is loose. The clutch shown is built to run in the direction indicated by the arrow, but such clutches may be built to run in either direction; they should always be run in the direction for which they are designed, so that the load may always come on the fixed arm. If the band be tightened slowly, there will be no sudden start or jerk on the rope, as the slip of the band will prevent the entire force of the grip taking effect at once; and after the drum reaches full speed, there is little or no slipping of the driving-band. It is best to keep the band only just tight enough to do the work, for should the car get off the track, or be overwound, or should a cage stick in the shaft for any reason, the band will slip and thus become a safety appliance, and not strain or break the rope, shaft timbering, or machinery, as would be the case if a positive clutch, Fig. 17, were used.
=34. The Beekman Friction Clutch.=--A simple friction clutch is shown in Fig. 19, in which _a_ is a section of the drum shell. The wooden blocks _b_ bolted to the side of the gear-wheel _c_ are made of suitable shape to conform to the =V=-shaped groove _d_ in the side of the drum. The steel spring _e_ between the two steel washers _f_, _f_ disengages the clutch, as soon as the pressure is relieved, by reversing the motion of the lever _g_ and screw _h_ from the opposite end of the drum. When the lever _g_ is turned, the screw _h_ is forced against the end of the pin _i_, which, in turn, presses the cross-key _j_ against the collar _k_, forcing the drum against the blocks _b_ and frictionally engaging the gear-wheel _c_. This drum shaft is prevented from moving endwise by means of the collar _l_ and the grooves _m_ in the babbitted pillow-block. The wide bearings of the drum on its shaft are lubricated by means of the pipes _n_.
A clutch is often used to change the length of the hoisting rope when hoisting from two or more lifts or levels. In this case the shaft carries two drums, one of which is fixed to the shaft, while the other is provided with a friction clutch. When it is desired to change the length of the rope, the cage attached to the loose drum is brought to, say, the upper landing. The cages both resting on the wings, the clutch is loosened and the other cage attached to the fixed drum is now brought to the desired level, when the clutch is again tightened and hoisting proceeds. The change is made in 2 or 3 minutes.
BRAKES
=35=. A =brake= is a device by means of which the motion of a hoisting drum may be retarded or stopped. This is accomplished by friction of the brake against the circumference of the brake wheel. There are three types of brakes, known as _block brakes_, _post brakes_, and _strap brakes_.
=36. The Block Brake.=--The =block brake=, Fig. 20, consists of one or more wooden blocks or shoes _b_ attached to a lever having a fulcrum at _d_, and connected by a rod to the lever _c_. Block brakes are objected to mainly because they throw a great load on the journals of the drum when they are applied; they cannot be relied on when there is a heavy load on the drum, and they require the application of great force to the lever _c_ for a given braking power. They are, however, cheap and easily applied to a drum, and the shoe is readily replaced when worn.
=37. The Post Brake.=--The =post brake=, Fig. 21, is composed practically of two block brakes applied at two places on the drum diametrically opposite each other, thus equalizing the pressure on the journals. The blocks are generally somewhat longer than in the block brake, or about one-quarter of the circumference of the drum on each side. In Fig. 21, _a_ is the drum; _b_ are wooden brake blocks; _c_ are the posts which in the brake shown are of massive, built-up, steel construction; _d_ are the fulcrums on the plates _e_, which plates are adjustable by means of the nuts _f_; by means of these nuts, the fulcrums may be brought closer together as the wooden blocks _b_ wear away; _g_ is a tension rod generally furnished with a turnbuckle to adjust its length as the wooden blocks wear away. Power is applied at the end of the bent lever _h_, as shown by the arrow.
The stops _i_ are adjusted so that the blocks _b_ on each side are equally distant from the drum when the brake is off. The fulcrums _d_ should be some distance below the drum and brake ring, for if they are too near the drum it will be difficult to swing the lower end of the wooden blocks far enough to clear the drum.
=38. Improved Post Brake.=--In order to have an equal clearance at top and bottom, and to have a more powerful leverage than in the ordinary post brake, the posts may be made movable at both top and bottom, Fig. 22. The tops of the posts _a a′_ are moved, as in Fig. 21, by the tension rod _b_ and the lever _c_, the latter being connected by rod _d_ to lever _e_. This lever is pivoted at _f_ and motion is transmitted to the fulcrums _j_ by the link _g_, the lever _h_, and the tension rod _i_. The back post _a_ is supported by the uprights _k_, which are pivoted at _l_ and swing backwards and forwards like a parallel ruler. The front post _a′_ is supported by the single upright _m_, pivoted at _n_. The setscrews _o_ regulate the motion of the bottom of the posts so as to give equal clearance to the bottom and top of the posts.
An objection to both the block and the post brake is the fact that, if the drum surface to which the brake is applied is not perfectly round, the resistance of the brake will not be uniform when applied while the drum is in motion.
=39. The Strap Brake.=--A =strap brake= consists of a wrought-iron band or strap that partly encircles the drum and is connected at its free ends to levers with which the band may be tightened on the brake wheel and the drum thus firmly held. The iron or steel band either lies directly against the wooden lagging of the drum or on wooden blocks bolted to the drum; or else it has bolted to it a lining of wooden blocks that bear on the drum when the band is tightened.
The most efficient forms of strap brakes are those in which the strap or straps are in contact with 270° or more of the circumference of the drum. The greater the arc of contact, the more securely is the drum held by the brake. A single strap is sometimes used, but this is only satisfactory with small drums, say 8 feet or less in diameter; on large drums two straps are generally used, each extending half way around the drum. The levers for transmitting the power from the hand lever or treadle to the brake strap are variously arranged. In some cases, the force is multiplied by several short levers; in others, by one long lever. The treadle or foot-lever, however, has been replaced almost entirely by the hand lever.
=40.= The simplest form of strap brake, Fig. 23, consists of a single strap _a_, with one end anchored at _b_ and the free end attached to the brake lever _c_. This brake acts on the same principle as the block brake and is open to the objection that it brings an undue load on the journals, but it is more efficient and holds the drum more firmly under a heavy load than a block brake.
Block brakes are usually run dry, but in band brakes and post brakes with ample surfaces and proper leverage the wood may be occasionally slightly oiled with black oil, which greatly adds to the durability of the blocks without unduly lessening the power of the brake.
=41.= A two-strap brake is shown in Fig. 24. One end of each strap _a_, _b_ is fastened to the pedestal _c_ by either of the methods shown in Fig. 24 (_a_), (_b_), and (_c_). In the method shown in Fig. 24 (_a_) and (_b_), the forgings _d_, _d′_, drawn out to the form of bolts, are riveted to the ends of the straps and passed through a casting _c_ that is secured to the foundation. The object in giving one bolt to one strap and two bolts to the other strap is to allow the straps to pass each other and yet have their lines of action intersect. The bolts are fastened to _c_ by four nuts on each bolt, i. e., two principal nuts and two locknuts. This gives a means of adjustment in the length of the strap to take up the wear.
A second method of securing or anchoring the back ends of the straps is shown at (_c_). In this case, a wrought-iron angular piece is riveted to each strap, and these pieces are passed over the bolt _e_ that takes the place of the casting of the former arrangement. Nuts are used, as shown, to adjust the straps for wear. The bolt should be short and stiff, so as to be well able to stand up to its work when the drum is moving or tending to move in the direction shown by the arrow.
When the brake is applied, the friction between the brake strap and the circumference of the brake wheel produces a great strain on the pedestal _c_, which must be securely anchored.
The front ends of the straps are worked into eyes, as shown at _f_, and by these eyes and suitable pins passing through them the ends are fastened to the brake lever _g_. This lever is supported on and rotates about a pin _h_, so that when the braking force is applied at _i_, in the direction of the arrow, the brake lever rotates, pulling down on strap _a_ and up on strap _b_; and, if the straps are held firmly at the back end, the more force that is applied at _i_ the tighter will the drum be gripped by _a_ and _b_.
The ends of the straps should be brought in as close to the drum as is practicable, both front and back, so as to give the greatest amount of contact between the drum and the straps and to get the best effect from the force applied. The springs _j_ are used with straps that are not stiff enough to clear the drum when the brake is released.
=42.= The rotation of the drum may assist or retard the action of the lever in applying the drop brake. For instance, if, in Fig. 23, the drum revolves in the direction indicated by the arrow, the pull of the drum at the brake strap is in the same direction as the pull of the lever when applying the brake and the action of the lever is then assisted by the motion of the drum. On the other hand, if the drum is revolving in the opposite direction, it opposes the action of the lever and a greater force must be applied to the lever to overcome this opposing pull of the drum. Hence, in the case of strap brakes, if possible, that end should be anchored which will cause the revolution of the drum to assist the lever in applying the brake and throw the strain on the anchor bolt instead of on the lever.
=43.= If a brake is required to work with the drum running in either direction, there are several ways of bringing the strain due to the load on the anchorage in whichever way the drum runs. One of the simplest of these is shown in Fig. 25, where _a_ is a drum with a strap brake _b_ embracing nearly the entire circumference; _c_ is a lever bar that is attached to the ends of the brake strap by pins _d_ and _e_, which work in the slots _f_ in the iron anchor plates _g_. One anchor plate is on each side of the lever, and both are bolted to the foundation. If the band is kept of the proper length, then, no matter which way the drum is turning, the pull of the drum will come on the anchorage, and the pull on the lever need be only sufficient to take up the slack end of the band. To illustrate: If the drum is turning in the direction indicated by the arrow, the pin _e_ holding the lower end of the band will be on the bottom of its slot and the pin _d_ will be free in its slot and engaged in tightening the slack end of the band through the motion of the lever _c_. Were the drum running the other way, the pin _d_ connected with the upper half of the band would move to the upper end of its slot and take the main load, while the pin _e_ at the lower end of the band would only have to take up the slack. The outer, or long, end of the lever moves downwards in all cases to tighten the band. Provision must be made to lift the band clear of the drum when slack, but no anchorage other than at _g_ should be attempted.
=44. The Differential Brake.=--The differential brake has both ends of the brake strap attached to short lever arms operated by the brake lever, but these arms are of different lengths and are so arranged that as the longer arm tightens the brake strap the shorter arm yields and loosens the strap. The tightening, however, is more than the loosening or yielding and, as a result, the brake band is tightened about the brake wheel. The form of the lever arm is immaterial so long as the differential principle is retained, that is, that the shorter arm yields when the longer pulls, when the brake is thrown into action. This principle is illustrated in Fig. 26. In this brake, no provision is made for anchoring either end of the brake strap, but the entire load is thrown on the lever arms _a_ and _b_. These lever arms are connected with the arm _c_, which revolves on the same shaft _d_ and is operated by the reach rod _e_. The revolution of the drum is thus resisted by the shaft _d_.
This brake is self-acting when the drum revolves so as to pull on the shorter arm, as indicated by the arrows; that is, the motion of the drum helps to set the brake when the latter is once applied. When, however, the drum revolves in the opposite direction, the action of the brake is opposed, instead of being assisted, by the motion of the drum. As a consequence, this particular form of brake is not adapted to hoisting drums that revolve in opposite directions at each alternate hoist. Differential brakes are not generally used.
=45. Power for Brakes.=--For small drums and light loads, the brakes are usually applied by hand power through suitable lever connections. The force that a man can exert can be multiplied indefinitely by levers and combinations of levers; but while the force is multiplied, the distance through which it can act is divided in the same ratio. A certain amount of motion is required to free the brake band from the drum, when the brake is off; this, then, limits the leverage that a man can use. Suppose, for instance, that with a strap brake the band moves from the drum ½ inch, thus increasing the diameter 1 inch, or the circumference about 3 inches. Then, supposing that a man can exert his force to advantage through 3 feet, or 36 inches, the available leverage is ³⁶/₃ = 12. That is, if a man can pull 50 pounds on his hand lever, he can exert 50 × 12 = 600 pounds circumferentially on the brake band, with simple levers. If any form of differential levers is used, the ratio by which the force applied at the hand lever can be increased will be considerably larger. A diagram will explain this more clearly.
=46.= In Fig. 27, _a_ is the hand lever, with a fulcrum at _b_ and a pin at _c_ by which it takes hold of a reach rod or connection _d_. This rod is connected to the end _h_ of the brake lever _e_, which is connected by pins at _f_, _g_ to the brake bands. If the leverage of the hand lever _a_ is made 6 to 1, that is, if
_ab_ 6 ----- = --- , _cb_ 1
and a force of 50 pounds is applied at _a_, a pull of 300 pounds will be exerted at the pin _c_ and, consequently, along the rod _d_ to the end of the brake lever _e_. Then, if the brake lever is made with a ratio of 4 to 1, that is, if
_eh_ 4 _eh_ ----- = --- = ---- , _eg_ 1 _ef_
a pull of 300 pounds × 4 = 1,200 pounds will be exerted at the pin _f_ or _g_. This total pull must be divided equally between the arms _eg_ and _ef_, giving 600 pounds pull on each. According to the principle of the lever, the distances through which these forces act are inversely proportional to the forces acting. It is assumed that the brakeman can exert the force of 50 pounds through 36 inches; if this is the motion of the end of the hand lever _a_, one-sixth of this, or 6 inches, will be the motion at _c_ and, therefore, at _h_; one-fourth of 6 inches or 1½ inches will be the motion at _f_ and _g_; that is, _f_ will increase its half of the brake band 1½ inches in circumference, and _g_ will do likewise with its half, making the total circumference 3 inches more, or the diameter 1 inch more, and thereby moving the band away from the drum ½ inch radially. The levers are all shown in mid-position to make the figure more simple, but the relative leverages remain the same at all points in the motion.
This is an example of simple levers, but the force applied at the hand lever may be increased in a much greater ratio by the use of a device known as a _differential lever_.
=47. The Differential Lever.=--The principle of the operation of the =differential lever= with which a constantly increasing force can be applied to the brake strap is illustrated in Fig. 28. Let _a o_ represent a straight lever whose fulcrum is at _o_; and let the reach rod be attached at _e_. In this position, if
_a o_ 6 ------ = --- , _e o_ 1
the effective lever is 6 to 1. If, now, the lever is moved through 30° to the position _b o_, the force applied at _a_ moves through the distance _a b_, and the reach rod through the horizontal distance _k f_, so that the effective leverage is increased a small amount _e k_ and the ratio of the arms becomes
_a o_ ------- . _k o_
When the lever is moved another 30° to the position _c o_, the reach rod moves a distance _i g_, which is less than _k f_, so that the effective leverage is increased by the amount _k l_ and the ratio of the arms becomes
_a o_ ------ . _l o_
Again, moving the lever 30° more to the position _d o_, the reach rod moves through the still shorter distance _j h_, which is less than _i g_, and the effective leverage becomes very great. It is evident from this that the farther the lever is moved toward _d_ the greater becomes the effective leverage. In practice, it would be impossible to move the lever through the entire quadrant to advantage, and there would also be more movement of the reach rod at the beginning of the stroke and less at the end than is needed to produce the desired effect.
From the principle just given, it is plain that, if _p o_, Fig. 28, represents a brake lever with the reach rod attached at _q_, a smaller pull will be exerted on the brake band if the lever is moved to the position _b o_ than would be exerted if a lever were moved through the same angle from _b o_ to _d o_. The movement from _p o_ to _b o_ is a convenient and easy one for the engineer to make, while the movement from _b o_ to _d o_ is inconvenient. To overcome the inconvenience and still to obtain the advantage of this latter movement, the differential lever shown in Fig. 29 is used. By means of an arm placed on the lever, the point of attaching the reach rod is at _l_ instead of _p_; hence, when the handle _r b_ is moved to the position _s b_, the point _l_ moves to _m_, thus securing a greater and gradually increasing pull with the easier movement of the handle.
A differential lever may be advantageously used in connection with any band or post brake and on a drum running in either direction. Such levers are considered by many preferable to the differential brake.
=48. Power Brakes.=--Large drums and heavily loaded drums cannot be controlled by hand-power brakes, and in such a case some other form of power, such as steam, compressed air, or water, must be used.
Fig. 30 shows, in outline, how such power is applied. The movements of the hand lever _A_, instead of being directly communicated to the lever operating the brake, merely control the valve _v_ connected with the cylinder _a_. By means of this valve, steam, compressed air, or water is admitted to either end of the cylinder and this moves the piston in the direction necessary to apply or release the brake. There are a number of varieties of such power brakes, differing in structural details, but the action of all is essentially the same. With steam or air power, the brake would be applied with its full force almost instantaneously, thus subjecting the various parts of the mechanism to very severe and objectionable strains, unless the valves were modified so as to regulate the admission of the steam or air. One method of controlling this action is the use of a valve that requires a long travel to give it a full opening. Such a valve can be opened a little, so as to allow the steam to leak through and thereby increase the pressure in the cylinder gradually. As the motion is difficult to regulate, a better method is by means of a floating valve, described in _Hoisting_, Part 1.
=49. Crank Brake.=--In addition to the brake applied to the drum and intended for use mainly in emergencies, many hoisting engines are also fitted with a strap brake applied to the crank-disk. In some states, crank-brakes are required by law. In order to give a large bearing surface, the crank-disk is made very large.
HOISTING (PART 4)
Serial 851D Edition 1
HOISTING APPLIANCES
SHEAVES
=1. Sheaves= are grooved iron or steel wheels used to carry or guide a rope. The general method of mounting them on a frame for hoisting light loads is shown in Fig. 1. The journal boxes are so constructed as to be easily taken apart for inspection or repair. For hoisting heavy loads, the timbers must be braced, as is explained under the heading Head-Frames in this Section. Sheaves are of two styles--those composed entirely of cast-iron and those with cast-iron hubs and rims and wrought-iron or soft-steel arms or spokes.
=2.= The =cast-iron sheave=, Fig. 2, has arms with a cross-section, as shown at _a b_, and with the flanges of the arms tapering from the hub to the rim; that is, _d_ is greater than _c_ and _f_ is greater than _e_. The bottom of the groove _g_ in the rim should be a circular arc, whose radius is a little larger than that of the rope used over the sheave, to allow for the angling of the rope due to its fleeting on the drum. The flanges _h_ are made quite deep to prevent the rope jumping off.