Fig. 6
7. Where flat ropes are used or where round ropes wind on a conical drum, the length of rope wound or unwound is different for each turn of the drum. With all the indicators thus far described, while the speed with which the indicator moves is proportional to the speed at which the drum and the drum shaft revolve, it is not proportional to the speed of the rope when winding and unwinding on a conical drum or on a flat rope reel. [Fig. 6 (a) and (b)] shows two views of a compensating dial indicator. By means of the spiral form of sheave c, the hand d is made to move equal distances around the disk e for equal distances of cage movement in the shaft. The rope f passes about the spiral sheave and one end is attached at the small end g of the spiral, while the other end is fastened to the periphery of the sheave h, which takes its motion from the drum shaft or crank-shaft of the hoisting engine by means of the bevel gear i. Consequently, while the sheave h has a regular motion dependent directly on the revolution of the hoisting drum, the pointer d moves irregularly, depending on the position of the spiral sheave c; that is, whether a small or large diameter of the spiral is presented to the rope. The rope j carrying the counterweight k is attached to a small circular drum l that is on the same shaft as the spiral sheave. The purpose of this cord and counterweight is to keep the indicator line f taut and to bring the indicator back to position as the cord f unwinds from the sheave h.
8. In order that the pointer may not stand at exactly the same point on the dial when the cage is at the top and at the bottom, and so that the engineer may be able to distinguish between the top and the bottom positions of the cage by the pointer, the ratio of the gearing is usually increased by allowing one or two extra teeth on the worm-wheel. In the example in [Art. 5], assume a ratio of 27: 1; that is, if a worm-gear is used, the worm-wheel will have 27 teeth.
If the pitch of the teeth is ¾ inch, the circumference of the pitch circle will be ¾ × 27 = 20.25 inches and the diameter 6.44 inches.
The pitch of the worm will, of course, be the same as that of the wheel, and its diameter will be whatever is necessary to give sufficient strength outside of the shaft, since it bears no relation to the ratio of the gearing.
DRUMS AND REELS
9. The drum, or reel, of a hoisting engine is the part on which the rope winds. It is either keyed fast to the engine shaft or is connected to the shaft by means of a clutch, the shaft being made extra heavy to carry the strain due to the weight of the drum and the pull of the rope.
CYLINDRICAL DRUMS
10. The outer part, or shell, of a drum a, [Fig. 7], is supported on rims b, and these rims are connected by arms or spiders c with the hubs d. The brake rings e are for the band brakes, of which there may be one or two. The part a may be lagged with strips of wood bolted to the rims b, the heads of the bolts being countersunk. [Fig. 8] shows the detailed dimensions of a drum 8 feet in diameter having a 4-foot face designed to carry heavy loads and a large amount of rope. The shell is of boiler plate and the spiders of cast-steel.