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.