Hoisting Appliances

by International Correspondence Schools

ROPE WHEELS

Fig. 14

24. Koepe System.—In its lightest form, a drum requires a large amount of power to set it in motion, which power is absorbed by the brake and lost when it is brought to rest again. Furthermore, with deep shafts requiring long drums, the fleet, or angle that the rope makes with the head-sheave due to its traveling from one end of the drum to the other, is not only a disadvantage and possible cause of accident, but it is a source of wear. To overcome these objections and also the great cost of large cylindrical or conical drums, the Koepe system of hoisting, shown in [Fig. 14], was devised by Mr. Frederick Koepe. A single grooved driving sheave a is used in place of a drum. The winding rope b passes from one cage A up over a head-sheave, thence around the sheave a and back over another head-sheave, and down to a second cage B; it encircles a little over half the periphery of the driving sheave and is driven by the friction between the sheave and rope. A balance rope c beneath the cages and passing around the sheave d gives an endless-rope arrangement with the cages fixed at the proper points. The driving sheave is stronger than an ordinary carrying sheave, as it has to do the driving and is usually lined with hardwood, which is grooved to receive the winding rope, the depth of the groove being generally equal to twice the diameter of the rope. Instead of being placed parallel, the head-sheaves are placed at an angle with each other, each pointing to the groove in the driving sheave, thus reducing the side friction of the rope on the sheaves.

The system has been in successful operation since 1877, and experiments made on it have determined that, with a rope passing only one-half turn around the drum sheave, the coefficient of adhesion with clean ropes is about .3. If the ropes are oiled, the adhesion becomes less, and sometimes slippage occurs, producing not only wear of the driving sheave lining but giving an incorrect reading of the hoist indicator and thus possibly producing overwinding, unless the position of the cage is indicated by marks on the rope, or unless the engineer can see the cage.

At the end of the hoist, if the upper cage is allowed to rest on the keep, its weight and the weight of the tail-rope are taken from the hoisting rope, and there is then not enough pull on the hoisting rope to produce sufficient friction with the drum sheave to start the next hoist. To prevent this trouble, the keeps are dispensed with, or the rope is made continuous and independent of the cage. To do this, crossheads are placed above and below each cage and connected by ropes or chains outside of the cages. The bridle chains are then hung from the top crosshead, and when the cage rests on the keeps, the weight of the winding and tail-ropes remains on the driving sheaves.

25. Advantages and Disadvantages of the Koepe System.—With this system, only one driving sheave is necessary for the operation of two compartments, and it is light, inexpensive to build, and very narrow, admitting of a short sheave shaft and small foundations. This system permits a perfect balance of rope and cage, so that the work to be done by the engine is uniform, except for the acceleration, and consists only in lifting the material and overcoming the friction. There is no fleeting of the rope between the driving sheaves and the head-sheaves.

The system has the following disadvantages, which prevent its being used to any considerable extent: Liability to slippage of the rope on the drum; if the rope breaks, both cages may fall to the bottom; hoisting from different levels cannot be well done, for, since the cages are at fixed distances from each other, the length of the rope is such that when one cage A is at the top, the other cage B is at the bottom. If hoisting is to be done from the bottom, this is satisfactory, but if hoisting is to be done from some upper level, cage B, which is at the bottom, must be hoisted to that level to be loaded before it can go to the top. Then, when cage B goes to the top with its load, cage A must go to the bottom, wait there while cage B is being unloaded, and then be hoisted to the upper level to receive its load. For each trip, therefore, the time required for a cage to go from the bottom to the upper level and be loaded is lost; and two movements of the engines are necessary for a hoist instead of one.

26. The Whiting System.—This is a system of hoisting with round ropes, in which two rope wheels placed tandem are used in place of cylindrical or conical drums. As shown in [Fig. 15], for a two-compartment shaft the rope passes from one cage a up over a head-sheave c, down under a guide sheave d, and is then wound three times about the rope wheels e and f, to secure a good hold, then around a fleet sheave g, and back under another guide sheave h, up over another head-sheave i, and down to the other cage b. When the system is to be used for a single-compartment shaft, one end of the rope carries the cage and the other end carries a balance weight, which is run up and down in a corner of the shaft. A balance rope below the cages, as shown, is generally used, though it is not essential to the working of the system, as it is in the Koepe system. When sinking a shaft, a balance rope cannot be used as it interferes with the work at the bottom of the shaft.

Fig. 15

The drums or wheels e, f are light, inexpensive, and narrow, thus permitting short sheave shafts and small foundations. They are lined with hardwood blocks, each lining having three rope grooves turned in it. The main wheel e is driven by a hoisting engine, which may be either first or second motion. The following wheel f is coupled to the main wheel by a pair of parallel rods, one on each side, like the drivers of a locomotive. As the rope wraps about the wheels e, f three times, there are six semi-circumferences of driving contact with the rope, as compared with the one semi-circumference in the Koepe system, and there is no slipping of the rope on the wheels. The following wheel f is best tilted or inclined from the vertical an amount equal, in the diameter of the wheels, to the pitch of the rope on the wheel, so that the rope may not run out of its groove and may run straight from one wheel to the other without any chafing between the ropes and the sides of the grooves.

The capacity of the wheels e, f is unlimited, while grooved cylindrical drums, conical drums, and reels will hold only the fixed length of rope for which they are designed.

As shown by the dotted lines, the fleet sheave g is arranged to travel backwards and forwards, in order to change the working length of the rope from time to time to provide for an increased depth of shaft, and for the changes in the length of rope due to stretching and when the ends are cut off to resocket the rope. The fleet sheave g is moved a distance equal to half the change in the length of rope.

27. Hoisting from intermediate levels can be readily done with the Whiting system; for instance, if the cage a is at the top and cage b at the bottom, and hoisting is to be done from some upper level, it is only necessary to run the fleet sheave g out, and thus shorten the working length of the rope until cage b comes up to the upper level. It can then be loaded and go to the top. While cage b goes to the top, cage a descends to the same level, where it can be loaded while cage b is being unloaded, and can then go directly to the top without any of the lost time, as is the case in the Koepe system.

The system permits a perfect balance of rope and cage, so that the work to be done by the engines is uniform, except for the acceleration, and consists only in lifting the material and overcoming the friction.

There is no fleeting of the rope, so the rope wheels can be placed as close to the shaft as may be desired.

28. This system was tried as early as 1862 in Eastern Pennsylvania, but it was not used extensively because hoisting from great depths was not necessary, since, for depths of less than 1,000 feet, cylindrical and conical drums are quite satisfactory. In the Lake Superior copper region, there are now three Whiting hoists, two of which are probably the largest hoisting plants in the world. Each plant consists of a pair of triple-expansion, vertical, inverted-beam engines, driving direct a pair of 19-foot drums. The high-pressure cylinders are 20 inches in diameter, the intermediate cylinders 32 inches, and the low-pressure cylinders 50 inches, and all six of them have a 72-inch stroke. The rope used is a 2¼-inch plow-steel rope and hoists 10 tons of material at a trip, in one case from a depth of 4,980 feet, the deepest shaft in the world. Several plants on the Whiting system have been built in England, and two or more are working in South Africa.

29. Modified Whiting System.—A modification of the Whiting system is sometimes used in which a large drum keyed to the crank-shaft replaces the small tandem drums, and even the slight probability of the rope slipping in the Whiting system is thus obviated. One rope is fastened to one end of the drum, and the other rope to the other end in such a way that while one is winding on the other will be winding off the drum. One rope passes directly to the head-sheave while the other passes first around a fleet sheave, similar to that used for the Whiting system, but preferably placed horizontal, and thence to the head-sheave. This system possesses the same advantages as the Whiting system except that the depth of hoist is limited by the size of the drum, and that there is a fleet of the rope. Up to the limiting depth, as determined by the size of the drum, this system can be used with equal economy for any depth. This hoist, as well as the Whiting, is therefore especially suitable for a place where one mining company operates several mines, for it enables the company to select one size for all their permanent work, with all the advantages that come from duplicate machinery.