In hoisting for mechanical filling, as apart from distribution (the latter being used in the technical sense of distributing the stock properly in the top of the furnace), there are two main systems in use to-day, and a third which is a combination of some of the features of each.

In the first system the stock is drawn from the bins into large round buckets, whose bottoms consist of a cone, or "bell," which is merely a large conical valve attached to a rod running up through the center to a hook at the top. This bucket is then transported to the bottom of the hoist and sent up, hanging by the hook at its top, to the top of the furnace, on which it is seated and then dumped by lowering the hook on the central rod carrying the bell. In this system the material does not break bulk from the bin to the furnace top.

Fig. 21. "Baker" suspension bin with hand-operated quadrant gates.

Different systems of rotating the buckets are in use, but this subject can more appropriately be discussed under the head of stock distribution. Various methods have also been used for lowering the bucket on to the furnace top after it reached the proper position. But while these differ from the one illustrated, they do not differ sufficiently in principle to make detailed description necessary.

In the other system a sort of tank, either rectangular, round or elliptical, which is called a skip, runs on an inclined plane, generally very steep, under the control of the hoisting rope and supported by wheels running on a track on the incline. This runs from a pit beneath the filling floor to a hopper above the furnace top, where it turns upside down and dumps, then returns to the bottom, its travel being thus confined entirely to the inclined track running to the furnace top, and known as the skipway.

This skip is usually filled from a motor-driven dumping car, with its body or dumping hopper mounted on a scale carried by the truck, called the scale car or larry. This car is traversed along in front of the bins and filled from them. It is then returned to a hopper or chute, which delivers into the skip when the latter is in its bottom position, and there dumped, returning for another load while the skip is hoisted to the furnace top.

In this system it will be seen the stock breaks bulk between the bins and the top of the furnace.

At some plants a combination of these systems is in use, in which the bucket with the bell bottom is never removed from the suspension truck on which it is carried, or only to a turntable serving it, and plies therefore only on the incline, being filled by a scale car exactly the same as a skip. The bucket in this case virtually becomes a skip which dumps through the bottom while retaining its upright position, instead of being turned upside down, as is done with the regular skip.

The first type of distribution is shown in Fig. 23, which shows the Ford-Parks modification of the original Neeland top.

The second of these styles of hoisting is well shown by Fig. 16, which is the design of Alexander E. Brown, and with which in connection with the distribution at the top of the furnace we shall deal more at length later.

Many variations of each of these systems are in use. Very frequently in the case of the skip hoist the coke bins are arranged on each side of the skipway, and coke is spouted directly into skips without the intervention of the scale car, the quantity of coke being measured by skipfuls instead of by weight. This is a very satisfactory system if the same coke is always used, but one liable to produce great derangement of the furnace if sudden changes are made from one coke to another of very different density.

Fig. 22. McKee-Baker suspended bin with continuous line of gates, hand operated.

The skip hoist is built with both single and double skips. The advantage of the double skip is that it is perfectly counterweighted, without extra mechanism for that purpose, and that it has a very great capacity. It, however, labors under the disadvantage that it discharges the stock at two points, one each side of the central plane, rather than at one point exactly in the central plane, as is done with the single skip.

The skips dump on opposite sides of the main central plane and on the same side of the central plane at right angles to it, this causes a lack of symmetry, and the irregularities of the distribution due to this, if not compensated, or eliminated by suitable means, are likely to cause very serious derangement of the working of the furnace. The elimination of these irregularities is made much easier if the skip dumps exactly in the central plane.

A double skip was put in use at the Pueblo plant of the Colorado Fuel and Iron Company a number of years ago, designed by Mr. E. G. Rust, in which the advantages of both the single and double skips were obtained by a most ingenious design. Both skips run in the central plane and occupy the same positions at top and bottom. Of course when one is at the top the other is at the bottom, and there is no fouling in those positions, but in order that they may pass each other the tracks starting from the top and bottom diverge from one another in the vertical plane so that they are separated vertically at the middle of the skip bridge by a sufficient distance to allow the under skip to pass beneath the upper. The gauge of the track for the upper skip is wider than that for the lower, which runs between the rails of the upper track. The deepening of the skip bridge at this point tends to give it additional strength at the place it is most needed.

This system has been in use at the Pueblo plant for a number of years, but for some unknown reason its use has never extended widely.