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.
ROPE FASTENINGS
Fig. 16
30. A common method of fastening a rope to a drum, [Fig. 16 (a)], is to pass the rope through a hole in the drum rim and then around the shaft, clamping the end to the rope between the shaft and shell, as shown. Care should be taken to make the radius of curvature of the hole at a as large as possible so that the rope will not be bent any sharper than is necessary. When an iron drum is used, the thickness of the rim does not afford enough depth in which to bend the rope and it is necessary to build in a pocket for the purpose, as shown at [Fig. 16 (b)]. It is well to make both sides of this pocket with a long radius to avoid damaging the rope in case all the rope is accidentally unwound and the drum backed so as to bring the rope against the other side of the pocket.
CLUTCHES
31. It is often desired to have the drum of a hoisting engine run loosely on the engine shaft, so that it may run independently of the engine. With such loose-running drums, the engine generally runs only in the direction required to hoist the load, while the cage is lowered entirely by means of the brake. In this way, one engine provided with several drums may be used for hoisting from several shafts or from several levels in the same shaft at the same time. Such a loose-running drum is connected to the engine shaft when a load is to be hoisted by means of a clutch, of which there are two forms commonly used for hoisting machinery: jaw or piston clutches and friction clutches.
Fig. 17
32. Jaw Clutch.—[Fig. 17] shows a jaw clutch, one-half a of which is shown ready to be bolted to a drum or flat rope reel, which is loose on the shaft b. The other half c of the clutch is moved back so that the jaws d are not in contact with the jaws e on the part a. The half c slides freely on a feather key f, which is driven tightly into a deep key seat in the shaft b; a collar g, fitting loosely in a groove in the hub of c, is provided with trunnions h on each side; levers i connect these trunnions with the lever j attached to a suitable handle, by means of which the clutch is made to slide endwise on the shaft so that the jaws d engage or disengage the jaws e and thus connect or disconnect the drum or reel from the clutch. There are generally four or six jaws d that engage the same number of jaws e on the drum, and it is necessary to have little or no play between d and e when the clutch is connected or there will be too much shock. The clutch is about 2 feet in diameter, and the jaws are 3 or 4 inches deep for the average 20" × 48" first-motion hoisting engine. Instead of the clutch being fastened to the shaft by feather keys, the shaft may be hexagonal where the clutch slides on it and the clutch is machined to match. Jaw clutches are made of either cast-iron or cast-steel, and should be in halves, for convenience of repair, and securely bolted together.