10. Safety catches are intended to prevent a cage falling in case the hoisting rope breaks. A common form, shown at c, [Fig. 6], and in detail in [Fig. 7], consists of a pair of toothed cams j, [Fig. 7], fastened on each side of the cage near the shaft guides. The drawbar b to which the rope is attached extends through the top cross-piece of the cage and through the cylinder d, at the bottom of which is a plate c supplied with lugs for the rods f that connect it with the levers g. Inside the cylinder are three powerful rubber springs, which are in compression so long as the cage hangs from the rope, but are extended if the rope breaks, drawing the rods f down and with them the ends of the levers g to which they are attached; and, since the levers are pivoted, their other ends are moved upwards and with them the rods k. The cams j are each attached to one end of the rods k in such a manner that as the rods move upwards they rotate the cams inwards until they come in contact with the shaft guides. The teeth of the cams grasp the wooden shaft guides and stop the descent of the cage. The cams are provided with projections a and l that strike the guide and thus prevent the cams turning entirely around. [Fig. 7 (a)] shows the springs extended and the dogs j just about to grasp the shaft guides, while [Fig. 7 (c)] shows the position of the dogs when the springs are compressed as they are when hoisting. At e in cylinder d, [Fig. 7 (b)], there are slots for the lugs of plate c to move up and down as the spring is compressed or extended. Instead of rubber springs, helical steel springs are sometimes used, and with a somewhat different design flat steel springs are used.
Fig. 7
Fig. 8
The cams, or dogs, may be placed at any point along the upright post of the cage, and in some cases two sets of cams are used on each side, one set at the top and another in the middle, both sets being connected by rods so that they work together. Practical tests of these catches, made by allowing the cage to drop, show that they are, as a rule, very efficient devices. The cams usually take hold at once, the cage dropping only a few inches, or, at most, a few feet if the guides are dry and free from oil. When the guides are very greasy or wet, the cage may drop several feet before the cams take a firm hold and stop it, and with ice-covered guides, instances are given where the cage has fallen 15 feet before the cams ploughed their way through the ice and took firm hold of the guides; but in so doing the momentum the cage acquired was so great that the guides were destroyed. Fortunately for the utility of safety catches, ropes are usually broken while a loaded cage is being raised, and the cage has an upward momentum; if a rope breaks when the cage is descending at a speed of 30 or 40 feet a second, its momentum is so great that either the catches or guides break. The catches generally hold and either the guides or cage suffer more or less injury under such circumstances. Instead of being placed near the top of the cage the dogs are frequently placed near the center, or near the bottom; in some cases two sets of dogs have been used, one set being at the top and the other at the bottom. Instead of being cam-shaped with a number of small teeth on the rim of the cam, as shown in [Fig. 7], the dogs are now frequently made consisting of one or more strong straight teeth on each side of the guide. These teeth are operated similarly to those shown in [Fig. 7], and are driven into the guides if the rope breaks, thus holding the cage more firmly than the cam-shaped guides, particularly where the guides are wet.
TABLE I
| Platform | Guides | Safe Load Pounds | Weight Pounds | ||||
|---|---|---|---|---|---|---|---|
| Width | Length Feet | Size Inches | Distance Between | ||||
| Feet | Inches | Feet | Inches | ||||
| 4 | 3 | 6 | 6 × 6 | 4 | 6 | 5,000 | 2,000 |
| 6 | 10 | 6 × 10 | 6 | 3 | 8,000 | 3,800 | |
11. The Heavy Steel Cage.—The cage that is shown in [Fig. 8] is made of iron and steel except the wood flooring, which is laid in two courses, one lengthwise and one diagonal. The joints should not be driven too tightly, as the wood is likely to swell. The track is bolted to the floor, or deck, of the cage. The cast-steel safety dogs are operated by steel springs a, coiled about the bars b, which are connected to the drawbar c by chains, as shown. The drawbar drops if the rope breaks and thus assists the action of the springs a. This cage is in use at both coal and iron mines, and is built to suit any size of shaft and guides. Standard sizes are given in [Table I].
12. The Light Steel Cage.—[Fig. 9] shows a light steel cage much used at gold and silver mines. It has a spring drawbar and steel safety dogs, operated by steel springs, as in [Fig. 8], but the floor is of steel grating in order to give as little air pressure as possible against the cage. The openings a in the side frames are provided so that through them the nuts can be tightened on the bolts that hold the shaft guides. The cage is provided with bails b that swing down over each end of a car to hold it on the cage.