In arranging shafting, moderate speeds, say 100-150 revolutions per minute, should be chosen for main lines, and when higher speeds are necessary, they should be got by light and well balanced counter-shafts, with wrought iron or wooden pulleys. (Cp. [p. 452].) In calculating speeds, it must be remembered that they vary inversely as the size of the pulleys. Thus a 3-feet pulley running at 100 revolutions will drive a 2-foot pulley at 150 revolutions, and a 12-inch one at 300. Of course the higher its speed, the more power any shaft will transmit, but increased friction and wear and tear soon limit this advantage. The velocity of a belt in feet per minute is obtained by multiplying the number of revolutions per minute by the girth of the pulley in feet or by its diameter multiplied by 31⁄7, or more accurately, 3·1416.
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, as suggested by Mr. J. Tullis, and will do its share of the work. Belts should be washed occasionally with soap and tepid water, and oiled with castor or neatsfoot oil; but if of sufficient breadth, should not require the use of rosin, or adhesive materials, to make them grip the pulley. Chrome-leather belts should be kept thoroughly oiled. They have a much greater adhesion than vegetable tannages, and this is increased by oiling. Good chrome belting is much stronger than bark-tanned; and is unaffected by damp or steam, but generally stretches somewhat more. 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 . v66000, 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 feet per minute, 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 indiarubber. Both rules assume that the belt is of ample strength. One horse-power would be transmitted by a belt running 1000 feet per minute with a pull of 33 lb. A good single belt should not break with a much less stress than 1000 lb. per inch of breadth, and should stand about 1⁄10 as much as a working stress.
The following table gives the experimental breaking stresses and extensions of some leathers. It may be noted that 1 square inch sectional area is equal to a belt 4 inches wide × 1⁄4 inch thick; and that kilos per cm2 × 14·22 = lb. per inch2.
Breaking Stresses of Leather.[186]
| — | Kilo per sq. centi- metre. | Lb. per sq. inch. | Stretch per cent. | ||
|---|---|---|---|---|---|
| Belting | leather, | layer system | 283 | 4,030 | 25·4 |
| „ | „ | Durio system | 298 | 4,240 | 21 |
| Well-tanned chrome leather | 740 | 10,500 | 32·5 | ||
| Over-tanned chrome leather | 234 | 3,330 | 23 | ||
| Stuffed alumed leather | 835 | 11,900 | 38·3 | ||
| Alumed “rawhide” | 921 | 13,100 | 31·4 | ||
[186] ‘Gerber,’ 1900, p. 73.
Good English tanned belting leather breaks at from 4500 to 5500 lb. per sq. inch sectional area.
Over-tanned leathers are less tough, whether of vegetable or mineral tannage, than those somewhat lightly tanned, and the tensile strength of leather varies considerably with the part of the hide from which it is taken, that from approximately over the kidneys being the strongest. Even thick and tough leather is easily torn if a cut or nick is once started, and all holes used in jointing belts should be carefully rounded. Glucose, and the use of acid in bleaching both lessen the toughness of belts, and they may also be rendered tender by the heat evolved in slipping on a pulley.
Countershafting and high-speed machinery, such as disintegrators, striking machines of the Priestman type, etc., 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 or spindle must be taken out of its bearings, and supported on two exactly horizontal straight-edges, on which it will roll till the heaviest part is downwards; and weight must then be taken off or added till it will lie in any position. In this way the writer has had to add fully 2 lb. of iron to balance 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 at a higher level, 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 over a pit, 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 apt to be troublesome. In a grinding plant designed by the writer, the unground material was filled on the basement floor into an iron barrow, which was wheeled into an iron sling working between upright guide-rails like a hoist. On pulling a brake line, the barrow was raised to the top of the building, and its contents were tipped into a large hopper, after which the barrow righted itself, and descended for another load. In the bottom of the hopper was a sliding shover, which forced the material on to vibrating screens, by which it was guided either into a disintegrator, or crusher-rolls, at pleasure. Both these discharged through iron spouts into large hoppers on the outside of a brick gable, from which powdery materials like myrobalans and valonia could 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. Fires may occur from hard substances getting into disintegrators along with the bark, etc. and if this occur with a dry and dusty tanning material, it is not unlikely that it may result in an explosion such as sometimes happens in flour mills, in which the fire is rapidly conveyed along spouts, and into chambers filled with dusty air. Insurance companies generally charge an extra rate for disintegrators, and it is very desirable to keep the mill-house structurally apart from other buildings, either by actual separation or by the introduction of brick gables dividing the roofs. On the whole, however, mills of the coffee-mill type are probably quite as dangerous as disintegrators; since if they become partially choked, the heat caused by friction is very great.