Referring for a moment to the above limitation, the weight of the shaft itself will cause it to deflect between the hanger bearings, and the amount of this deflection will depend upon the distance apart of the points of support, or, in other words, of the distance apart of the hanger bearings.
The second may be reduced to a minimum by so regulating the distance apart of the hanger bearings that the deflection of the shaft from the belt pressures shall not be sufficient to produce sensible irregularities in the axis of rotation of the shaft; by so connecting the bearings to the hangers that they shall be rigidly held, and yet capable as far as possible of automatically adjusting their bores to be true with the shaft axis, notwithstanding its deflection from any cause; by placing the pulleys transmitting the most power as near to the hanger bearings as practicable; by so disposing the driving belts as to deliver the power as near as possible equally on all sides of the shaft; and by having the shafting and the pulleys balanced so as to run true, so that the strains on the pulleys shall be equal at each point in the shaft rotation. From this it appears that the distance apart of the shafting hangers may vary according to the amount of power transmitted by a shaft of a given diameter. The following table (given by Francis) gives the greatest admissible distances between the bearings of continuous shafts subject to no transverse strain except from their own weight, as would be the case were the power given off from the shaft equally on all sides, and at an equal distance from the hanger bearing.
| Diameter of shaft in inches. | Distance between bearings, in feet. | |
| Wrought-iron shafts. | Steel shafts. | |
| 2 | 15.46 | 15.89 |
| 3 | 17.70 | 18.19 |
| 4 | 19.48 | 20.02 |
| 5 | 20.99 | 21.57 |
| 6 | 22.30 | 22.92 |
| 7 | 23.48 | 24.13 |
| 8 | 24.55 | 25.23 |
| 9 | 25.53 | 26.24 |
These conditions, however, do not usually obtain in the transmission of power by belts and pulleys, and the varying circumstances of each case render it impracticable to give any rule which would be of value for universal application.
For example, the theoretical requirements would demand that the bearings be nearer together on those sections of shafting where most power is delivered from the shaft, while considerations as to the location and desired contiguity of the driven machines may render it impracticable to separate the driving pulleys by the intervention of a hanger at the theoretically required location. The nearer together the bearings the less the deflection either from the shaft’s weight or from the belt stress, and since the friction of the shaft in its bearings is theoretically independent of the journal-bearing area, the closer the bearings the more perfect the theoretical conditions; but since it is impracticable to maintain the true alignment of the shaft, and as the friction due to an error in alignment would increase with the nearer proximity of the bearings, they are usually placed from about 7 to 12 feet apart, according to the facilities afforded in the location in which they are to be erected.
It is to be observed, however, that the nearer together the bearings are the less the diameter, and, therefore, the lighter the shafting may be to transmit a given amount of power, and hence the less the amount of power consumed in rotating the shafting in its bearings.
Cold-rolled Shafting—This is shafting made cylindrically round and parallel by means of cold rolling, which leaves a smooth and bright surface. The effects of cold rolling upon the metal have been determined by Major Wm. Wade, U.S.A., Sir William Fairbairn, C.E., and Professor Thurston, of the Stevens Institute, as follows:—
The experiments were made upon samples of cold-rolled shafting submitted by Messrs. Jones and Laughlins, of Pittsburgh, Pennsylvania.
SUMMARY OF THE RESULTS OBTAINED BY MAJOR WADE FROM NUMEROUS EXPERIMENTS WITH ORDINARY HOT-ROLLED BAR IRON, COMPARED WITH THE RESULTS OBTAINED FROM THE SAME KINDS OF IRON ROLLED AND POLISHED WHILE COLD BY LAUTH’S PATENT PROCESS.
| Iron rolled while | Ratio of increase by cold rolling. | Average rate per cent. of increase. | |||||||||||
| Hot. | Cold. | ||||||||||||
| Transverse.—Bars supported at both ends; load applied inthe middle; distance between the supports, 30 inches. Weight which gives a permanent set of one-tenth of an inch, viz. | - | 11⁄2 inch square bars | 3,100 | 10,700 | 3.451 | - | 162 | 1⁄2 | |||||
| Round bars, 2 | inch | diameter | 5,200 | 11,100 | 2.134 | ||||||||
| Round bars, 21⁄4 | „ | „ | 6,800 | 15,600 | 2.294 | ||||||||
| Torsion.—Weight which gives a permanent set of one degree,applied at 25 inches from centre of bars. Round bars, 13⁄4inch diameter, and 9 inches between the clamps | 750 | 1,725 | 2.300 | 130 | |||||||||
| Compression.—Weight which gives a depression, and a permanent set ofone-hundredth of an inch to columns 11⁄2 inches long and5⁄8 inch diameter | 13,000 | 34,000 | 2.615 | 161 | 1⁄2 | ||||||||
| Weight which bends and gives a permanent set to columns 8 inches long and3⁄4 inch diameter, viz. | - | Puddled iron | 21,000 | 31,000 | 1.476 | - | 64 | ||||||
| Charcoal bloom iron | 20,500 | 37,000 | 1.804 | ||||||||||
| Tension.—Weight per square inch, which caused rods3⁄4 inch diameter to stretch and take a permanent set, viz. | - | Puddled iron | 37,250 | 68,427 | 1.837 | - | 95 | ||||||
| Charcoal bloom iron | 42,439 | 87,396 | 2.059 | ||||||||||
| Weight per square inch, at which the same rods broke, viz. | - | Puddled iron | 55,760 | 83,156 | 1.491 | - | 72 | ||||||
| Charcoal bloom iron | 50,927 | 99,293 | 1.950 | ||||||||||
| Hardness.—Weight required to produce equal indentations | 5,000 | 7,500 | 1.500 | 50 | |||||||||
| Note.—Indentations made by equal weights,in the centre, and near the edges of the fresh cut ends of the bars, were equal; showing that the iron was as hard inthe centre of the bars as elsewhere. | |||||||||||||