Pulleys should always be of ample breadth for the power they have to transmit; and it is more economical both in power and cost, to use broad single belting than the same strength in double. If the pulley will not take a belt broad enough for the work it has to do, a second belt may be made to run on the top of the first, and will do its share of the work. Belts should be washed occasionally with soap and tepid water, and oiled with cod-oil; but if of sufficient breadth, should not require the use of rosin, or adhesive materials, to make them grip the pulley. Makers of machines often err in constructing their driving pulleys too small both in breadth and diameter.
The horse-power which a belt is capable of transmitting obviously varies extremely with circumstances, but may be approximately calculated by the formula
| a · v | , |
| 66000 |
where a is the area of contact of the belt with the smallest pulley, and v its velocity in feet per minute. Another rule is, that at a velocity of 1000 ft. per min. each inch of breadth of belt should transmit 21/2 horse-power on metal pulleys, or 5 on wooden ones, on which the adhesion is greater. Adhesion may also be increased by covering the pulleys with leather or india-rubber. Both rules assume that the belt is of ample strength. One horse-power would be transmitted by a belt running 1000 ft. per min. with a pull of 33 lb. A good single belt should not break with a much less strain than 1000 lb. per inch of breadth, and should stand about 1/10 as much as a working strain.
Countershafting and high-speed machinery, such as disintegrators, striking machines of the Priestman type, &c., should run without material jar or vibration. If this occurs, it is generally a sign that the running part is not equally balanced. In this case, the shaft must be taken out of its bearings, and supported on two exactly horizontal straight-edges, when it will roll till the heaviest part is downwards; and weight must be taken off or added till it will lie in any position. In this way, the writer had recently to add fully 2 lb. of iron to the drum of a striking machine before equilibrium was secured, and a most troublesome vibration prevented. Of course all machinery should be supported as solidly as possible; and if circumstances permit, most machines are better on a ground-floor. In placing bark mills, however, it is frequently convenient to fix them in the top of a building, so that the ground material may be sent down shoots by its own weight to the required places. An alternative plan is to set the mill on the ground-floor, and to raise the ground material with a bucket-elevator. This may be done successfully by letting the material fall directly from the mill into the buckets; but otherwise it must be thrown in with a shovel, as buckets will not pick up ground bark, even from a hopper; and in any case such elevators are often troublesome. In a grinding plant designed by the writer, the unground material is filled on the basement floor into an iron barrow, which may be wheeled into an iron bow working between upright guide-rails. On pulling a brake-line, the barrow is raised to the top of the building, and its contents are tipped into a large hopper, after which the barrow rights itself, and descends for another load. In the bottom of the hopper is a sliding shover, which forces the material on to vibrating screens, by which it is guided either into a disintegrator, or crusher rolls, at pleasure. Both these discharge through iron spouts into large hoppers on the outside of a brick gable, from which, powdery materials like myrabolanes and valonia, can be run direct into barrows or trucks. It is very desirable that such hoppers should be separated from the main building by a fireproof partition. The writer is glad to say, he does not know of a case of fire from disintegrators grinding tanning materials, but he is informed that a Carter's disintegrator employed in grinding bones in a manure works has repeatedly set fire to the flannel bag into which the dust was allowed to escape. If this were to occur with a dry and dusty tanning material, it is not unlikely that it might result in an explosion such as sometimes happens in flour-mills from a similar cause. On the whole, however, mills of the coffee-mill type are probably more dangerous than disintegrators; since if they become partially choked, the heat caused by friction is very great.
For lubricating purposes, mineral oils of high density are not more dangerous than animal or vegetable, but rather the reverse; as, though they are possibly more inflammable, their mixture with cotton-waste and other porous vegetable materials is not spontaneously combustible, while vegetable and animal oils occasionally are. Heavy mineral oils should always be used as cylinder oils in high-pressure engines, in preference to other oils or tallow, since they are not decomposed by steam, and do no harm if blown into the feed-water, but serve to loosen and prevent scale and deposit. Ordinary oils and tallow, on the other hand, when submitted to the action of high-pressure steam, are separated into glycerin and fatty acids (see [p. 60]), and the latter corrode the valve faces and seatings, and in combination with temporary hardness in the boilers form a very dangerous porous deposit, which often leads to overheating of the tubes.
Next to the machinery, the pits demand special consideration. The chapter on the subject in Mr. Schultz's book on 'Leather Manufacture,' is well worth attentive study as giving American practice on the subject.
The old-fashioned method of sinking pits is to make them of wood, and carefully puddle them round with clay, which should be well worked up before use. Such pits, if made of good pine and kept in constant use, are very durable, some of the original pits at Lowlights Tannery, constructed in 1765, being still in use. Loam mixed with water to the consistence of thin mortar may also be employed, the pits being filled up with water, to keep them steady, at the same rate as the loam is run in. Probably the best materials for pit-sides are the large Yorkshire flagstones. Where these are not attainable, very durable pits may be made of brick, either built with Lias lime, and pointed with Portland cement, or built entirely with the latter. Common lime cannot be used, as it spoils both liquors and leather; and even cements with too large a percentage of lime are unsatisfactory. Brick and common mortar are, however, suitable for lime-pits.
The writer has constructed wooden pits in two ways. In the one case, after making the excavation, beams were laid in a well-puddled bed of clay; on these a floor of strong tongued and grooved deals was laid, and on this the pits were constructed of similar wood to the floor, and puddled round with clay. In the second case the pits were built like large boxes above ground, and when finished, lowered on to a bed of clay prepared for them, and then puddled both around and between. It may have been from defective workmanship in the first case, but those made on the last-named plan, which is that adopted from very early times, have certainly proved the tightest and most satisfactory. Mr. Schultz describes a plan as the Buffalo method, in which a floor is laid as just described, and grooves cut with a plane for the reception of the sides, which are formed of perpendicular planks, each end and side being finally tightened up by the insertion of a "wedge plank."
If bricks be used, great care must be taken that the cement is not merely laid so as to fill the joints towards the two surfaces of the wall, as is the habit of modern bricklayers, but actually floated into all the joints so as to make the wall a solid mass; or leaks can hardly be avoided. Cement pits are very good, and, though not particularly cheap in material, which must be of the best, are readily made by intelligent labourers under good supervision. The first step is to lay a level floor of good concrete, in which glazed pipes for emptying the pits may be embedded; care being also taken that all joints in these are thoroughly tight, since future repairs are impossible. The next step is to make frames, the exact length and breadth of the pits required, and perhaps 15 in. deep. These are arranged on the floor where the pits are to be, and the intervening spaces are filled with concrete of perhaps 1 of cement to 3 or 4 of crushed stone or brick. Rough stones and bricks may also be bedded in the concrete as the work goes on, to help to fill up. After the first layer has set, the frames may be raised and a second added, and so on. The work is generally finished by floating over it, while still damp, a little pure cement, to give a smooth surface. Before using, the cement should be tried on a small scale, to be sure that it does not discolour leather or liquors, and the pits should always be seasoned with old or cheap liquor before actual use.