Hoisting Appliances

by International Correspondence Schools

HOIST INDICATORS

1. The hoist indicator is a mechanism attached to the drum shaft of a hoisting engine to show the hoisting engineer the position of the cage or skip in the shaft throughout the time of hoisting. The use of such indicators is sometimes required by law, but there is a great diversity of opinion as to the advisability of using them. The objections to them are that they are liable to get out of order, and that in general the use of any automatic device that tends to relieve the hoisting engineer of responsibility and constant attention to his engine is not to be commended. A hoisting engineer, however, depends for his stopping point mainly on a mark made on the rope, or on the drum, or on both, and uses an indicator mostly as a guide for the position of the cage during the hoist.

TYPES OF INDICATORS

2. Column Indicators.—A very simple indicator, and one that was formerly very commonly used, is made by inserting a pin into the center of the end of the drum shaft and using this as a miniature drum on which to wind and unwind a chain or cord, which corresponds to the hoisting rope as the pin corresponds to the drum. This chain or cord is led over a pulley placed at the top of a pair of guides, representing the shaft, and carries at its end a weight, pointer, or gong, representing the cage or car, as shown in [Fig. 1].

Fig. 1

Fig. 2

The different landings in the shaft are marked on the guide; and as the pointer or gong rises and falls it indicates the position of the cage in the shaft. If a gong is used, pointer also may be added and the gong so arranged that it will ring at a point some distance before the landing is reached and thus attract the engineer’s attention. Indicators of this kind, though cheap and easily constructed, are not reliable, for the cord and chain may stretch or they may overlap in winding on the pin, or may bind in the pulley and thus indicate a wrong position of the cage.

3. An indicator should have a positive motion and be driven by gearing or by link belts. [Fig. 2] shows a column indicator that consists of a screw a working inside of a slotted pipe b, which may be of any length necessary. This screw is revolved by means of the gears c, which are rotated by the sprocket wheel d. A nut e travels up and down the screw a and the pointer f attached to the nut indicates the position of the cage in the shaft. The pipe standard b is usually painted a dead black and the different levels may be marked on it with chalk or white paint. Chalk marks are not safe, as they may be tampered with and the engineer thus misled.

Fig. 3

The pointer a, [Fig. 3], is moved by the rotation of the screw shaft b, which is revolved by the bevel gears c and d. This indicator also registers the number of hoists by means of the dials e, for at each hoist the lower end of the pointer a engages a ratchet wheel behind the two dial faces shown and thus registers on the dial.

Fig. 4

4. Dial Indicators.—[Fig. 4] shows a positive-motion indicator that is operated as follows: A worm a on the drum shaft b engages with the worm-wheel c on the small shaft d that is supported by the bearings e. The pointer f is rigidly attached to the shaft d and revolves in front of the properly marked dial g.

5. [Fig. 5] shows a dial indicator attached to drum hoists where the speed of rope is constant for each revolution. The wheel a of this indicator may be a worm-wheel working in a worm on the drum shaft, as described in connection with the indicator shown in [Fig. 4], or it may be a sprocket wheel driven by a link belt from a sprocket wheel on a drum, or it may be a gear-wheel driven directly from another gear-wheel on the drum. The gear-wheels b revolve a vertical shaft c fitted at the upper end with a worm d that drives the worm-wheel e placed on the end of the pointer spindle. The different levels from which hoisting is to be done may be painted on the dial, or better, they may be placed on movable targets that are clamped to the dial and can thus be moved as occasion requires.

Fig. 5

Example.—An indicator is desired for a shaft 800 feet deep at which the drum of the hoisting engine to be used is 10 feet in diameter; what ratio of gearing must be used so that the pointer will make one revolution during the hoist?

Solution.—The circumference of the drum is 31.42 ft. (πD = 10 × 3.1416 = 31.416 ft.); hence, the revolutions per hoist are 800 ÷ 31.42 = 25.46 revolutions. Then, if the pointer is to make one revolution per hoist, the ratio of the gearing will be 25.46 to 1. Ans.

6. Special Indicators.—One fault of nearly all indicators is that they give a regular movement throughout the winding, and the space over which the pointer travels is too small to enable the engineer to land the cage accurately. Indicators have been made with a differential motion to the pointer, the motion being greater at the time of landing and less during the middle of the hoist. They are also made with two pointers, one operating like the dial indicator above described and the other remaining stationary during all the hoist but the last few feet, when it moves around its circle.

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.