[Illustration: 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.