Hoisting Appliances

by International Correspondence Schools

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.

Fig. 20

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.

Fig. 21

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.

Fig. 22

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.

Fig. 23

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.

Fig. 24

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.

Fig. 25

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.

Fig. 26

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.

Fig. 27

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.

Fig. 28

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.

Fig. 29

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

[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